Optimization and simplification of polymer-fullerene ... - CyberLeninka

Polymer 54 (2013) 5267e5298

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Feature article

Optimization and simplification of polymerefullerene solar cells through polymer and active layer design Petr P. Khlyabich, Beate Burkhart, Andrey E. Rudenko, Barry C. Thompson* University of Southern California, Department of Chemistry, Loker Hydrocarbon Research Institute, Los Angeles, CA 90089-1661, USA

a r t i c l e i n f o

a b s t r a c t

Article history: Received 14 June 2013 Received in revised form 15 July 2013 Accepted 20 July 2013 Available online 31 July 2013

Polymerefullerene bulk heterojunction (BHJ) solar cells have consistently been at the forefront of the growing field of organic photovoltaics (OPV). The enduring vision of OPV is the promise of combining a simple, low-cost approach with an efficient, flexible, lightweight platform. While efficiencies have improved remarkably over the last decade through advances in device design, mechanistic understanding, and evolving chemical structural motifs, steps forward have often been tied to a loss of simplicity and a deviation from the central vision of OPV. Within the context of active layer optimization, our focus is to target high efficiency while maintaining simplicity in polymer design and active layer processing. To highlight this strategy, this feature article focuses on our work on random poly(3hexylthiophene) (P3HT) analogs and their application in binary and ternary blend polymerefullerene solar cells. These random conjugated polymers are conceptually based on combining simple monomers strategically to influence polymer properties as opposed to the synthesis of highly tailored and synthetically complex monomers. The ternary blend approach further exemplifies the focus on device simplicity by targeting efficiencies that are competitive with complex tandem solar cells, but within the confines of a single active-layer processing step. These research directions are described within the broader context of recent progress in the field of polymerefullerene BHJ solar cells. Ó 2013 Elsevier Ltd. All rights reserved.

Keywords: Conjugated polymer Solar cell Fullerene

1. Introduction As sources of alternative energy are increasingly recognized to be more important in the 21st century, solar energy holds a special place as the only energy source that could single-handedly meet the ever-growing world energy demand [1e8]. Interest in thin-film photovoltaic technologies has grown out of the desire to find inexpensive and readily deployable solar technologies [9e12]. As a potential thin-film solar technology, organic photovoltaics (OPV) have been the subject of an enormous research effort and have seen the largest increase in peak efficiency of any solar technology over recent years [6,13e21]. The ultimate vision of OPV is that of a simple, low-cost platform capable of providing efficient solar energy conversion in a flexible and lightweight device [13,14,19] that offers straightforward integration into existing infrastructure [22e 27]. Recent reports of efficiencies approaching and exceeding 10% have continued to buoy interest in this research field [28e32]. A great number of approaches to OPV are currently being investigated [13,33e37]. In a broad sense these approaches can be

* Corresponding author. E-mail address: [email protected] (B.C. Thompson). 0032-3861/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.polymer.2013.07.053

classified based either on the film deposition method or the device architecture. Vapor deposition has been an effective strategy for producing primarily well-defined layers of organics with small molecule species [17,38,39], while solution processing has primarily been focused on polymer-based systems [13e15,40e44]. While a great number of specific device architectures have been explored, a loose classification divides OPV into single-junction [13,14,19,40,41,44] and multi-junction (tandem) solar cells [33,45e50]. Taking this great variety of device approaches into account, the solution processed single-junction cells are perhaps most consistent with the vision of a simple, low-cost OPV platform due to the demonstrated compatibility with high-speed roll-to-roll processing under ambient temperature and pressure with a minimal number of processing steps [22e26]. Among this class, the polymerefullerene bulk heterojunction (BHJ) solar cells are the most explored and most successful to date, with a number of examples of efficiencies exceeding 8% and even 9% now reported and certified [30,51e61]. A significant challenge for the maturing field of OPV is to push efficiencies to a level where they can become economically viable while maintaining the attractive simplicity in materials and processing that define the vision for this field [24,62e67]. It is within this context that efforts in our research group have been focused on

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general principles for the optimization of polymerefullerene BHJ solar cells (also referred to as binary blends) and the extension of this platform to the potential for higher efficiency with retention of simple device fabrication via the development of ternary blends. Our focus has been on the optimization of the active layer, specifically through new materials design, with an eye toward simplifying polymer synthesis and controlling polymer properties via a so-called “polymer approach” as opposed to a “monomer approach”, as will be described. Here our work is placed within the context of recent developments in the field and in order to highlight our efforts, focus is placed on random poly(3-hexylthiophene) (P3HT) analogs and their application in binary and ternary blend BHJ solar cells. 2. Polymerefullerene BHJ solar cells (binary blends) The basic device architecture of a BHJ solar cell is shown in Fig. 1, where the active layer (donoreacceptor blend) is defined by an interpenetrating network of donor and acceptor components [68e 72], which in turn is sandwiched between dissimilar electrodes [19,40,41,44]. The overall device architecture is classified as either the so-called standard cell (ITO/PEDOT:PSS/Active Layer/Al) [13,20,40e42,44] or the inverted cell. The inverted architecture has emerged as a potentially superior platform [73e75], especially in terms of long-term stability [76e79]. However, our focus here is on the composition of the active layer, so further discussions on device architecture are referred to a number of excellent recent reviews on the subject [13,14,17,20,40]. Within the active layer, soluble fullerene derivatives have become the ubiquitous acceptor component and appear to potentially be uniquely optimal for this role due to their high electron affinity and outstanding n-type mobility [80e82], although efforts to engineer non-fullerene acceptors have led to some promising results [83e88]. Among the donor components in these solution processed solar cells, conjugated polymers have been the most studied and most successful to date, providing excellent film forming properties and demonstrating potential for thermally stable morphology in polymerefullerene blends [13,71,72,78,79]. Discrete molecular donors are emerging as a potential alternative [89e98], but are significantly less explored and will not be treated here. Within the context of the polymerefullerene BHJ, manipulation of polymer structure and the corresponding optimization of processing conditions [68,71,73,99e101] has been the principal means of active layer optimization in solar cells and there are a number of recent reviews that highlight the great diversity of chemical structures that have led to high efficiency solar cells [16,102e115]. At a more basic level, polymer chemical structure has primarily been tailored for the purposes of achieving an optimal electronic

relationship with the fullerene acceptor [13,46,116], as opposed to a global optimization that also accounts for influences of polymer structure on bulk and interfacial morphology [103,105,108e 111,114]. Electronic structure optimization has been based on a constantly improving understanding of the mechanism of photoconversion [19,40e44,117e119], as illustrated in simplified schematic form in Fig. 2a and described below. To begin the charge photogeneration pathway, upon absorption of photons with energy greater than the optical band gap of the donor or acceptor material, excitons are created (Fig. 2a (i)) [120e123]. In organic materials the binding energy of excitons is significantly higher than the thermal energy available at room temperature [6,41,44,117,121,124,125], and thus excitons need to diffuse to the donoreacceptor (DeA) interface (Fig. 2a (ii)) for dissociation [126e135]. If the driving force is sufficient at the interface, excitons populate charge-transfer (CT) state or “hot” CT states (Fig. 2a (iii)), which can be described as a weakly bound pairing of a hole on the donor and electron on the acceptor moieties [44,125,136,137]. After that, through the manifold of “hot” charge separated (CS*) states and CS states [42,43] (Fig. 2a (iv) and (v)) free polarons (holes or electrons) are generated in the active layer (Fig. 2a (vi)), which then migrate through the corresponding donor and acceptor phases, respectively (Fig. 2a (vii)) and finally are extracted at the anode and cathode (Fig. 2a (viii)) and complete photocurrent generation [40,41,138]. The maximum achievable photocurrent or short-circuit current density (Jsc) for a given donoreacceptor pair is determined by the breadth of the absorption of the active layer and the degree of overlap with the photon flux from the sun [139], as illustrated in Fig. 2b. The importance of targeting a maximum photocurrent is that the Jsc is directly proportional to the efficiency (h) via h ¼ (Jsc Voc FF)/Pin, where Voc is open-circuit voltage [140], FF is the fill factor [141] and Pin is the incident power intensity of the sun. Nonetheless, a strategy of increasing the Jsc by simply lowering the polymer band gap is of limited value, as decreases in the donor band gap are also tied to decreases in Voc inasmuch as Voc is proportional to the offset between donor HOMO (HOMOD) and acceptor LUMO (LUMOA) [116,140] as shown in Fig. 2c. While the Voc is limited by the HOMOD LUMOA difference [116,140], the CT state, described as a hole on the donor and electron on the acceptor (as shown in Fig. 2d) more precisely represents the Voc in binary blends. This is specifically because the CT state represents the energy state at the interface [136,142e150]. The energy of the CT state (ECT) is defined as ECT ¼ ðHOMOD LUMOA Þ þ EBCT , where EBCT is the binding energy of the CT state [44,125]. The balance struck between decreasing the donor band gap to increase Jsc and lowering HOMOD to maximize Voc while retaining a driving force for charge generation (approximated by the LUMOD LUMOA offset) has led to predictions that the practical

Fig. 1. Device architectures and active layer structure of donoreacceptor bulk heterojunction solar cells.


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Fig. 2. (a) Simplified scheme of photocurrent generation in donoreacceptor BHJ solar cells. (b) Global total photon flux from the sun and maximum achievable integrated shortcircuit current density (Jsc), considering external quantum efficiency (EQE) of 100%. (c) Limit for the open-circuit voltage (Voc) in donoreacceptor BHJ solar cells. (d) Exciton and charge-transfer (CT) state in donoreacceptor solar cells.

efficiency limit for an ideal donor polymerefullerene acceptor pair will be 10e12% [14,40,42e44,116,150e155], although significantly higher efficiencies could potentially be achievable [37,40,41,154,156e159]. This limitation can be described as the Jsc Voc compromise and careful selection of donor and acceptor materials is necessary to reach the practical efficiency limit. 2.1. State-of-the-art Fig. 3 illustrates the chemical structures of the polymers and fullerenes that define the state-of-the-art in BHJ solar cells and the specific properties of the systems are tabulated in Table 1. As can be seen, all polymers that show the highest efficiencies to date in single layer BHJ solar cells are perfectly alternating donor/acceptor (D/A) polymers, including PTB70eF20, which despite the use of two acceptor monomers contains exclusively alternating electron rich and electron poor monomers. The use of the D/A approach [139] leads to relatively low band gaps, which range between 1.4 and 1.7 eV. As a result, in the BHJ solar cells short-circuit current densities (Jsc) exceeding 17 mA/cm2 are achieved when using PC71BM as an acceptor, which is used in order to increase the absorption in the visible part of the solar spectrum [80e82] as compensation for poor polymer absorption in the visible, as will be discussed below. Note also that from a light-harvesting perspective, none of these polymers absorb significantly beyond w850 nm. Furthermore, the amount of fullerene necessary for the optimal device performance exceeds that of the polymer in the majority of cases, which leads to a decrease in the impact of the polymer absorption in the red and near-infrared (near-IR), considering the generally finite thickness (100e200 nm) of the majority of polymer fullerene cells [59,160e 166] thus forcing reliance on PC71BM absorption in the visible.

This suppresses the upper limit of Jsc achievable for specific polymerefullerene combinations. Importantly though, high fill factors (FF) above 60% are reached for all the state-of-the-art systems due to thorough optimization of the BHJ morphology. High Jsc and FF translate into efficiencies exceeding 8% for majority of the polymers in Fig. 3. Finally, the Voc for all these polymerefullerene combinations is high based on optimized electronic relationships in the pairs. The polymers presented in Fig. 3 are examples of the most successful polymers to date used in the binary blend BHJ solar cells, but there are a vast number of other polymers which show efficiencies exceeding 7% [28,53,143,161,167e183]. In general, as in case of polymers from Fig. 3, the majority of these polymers are perfectly alternating D/A polymers with low band gaps, where PC71BM is used. The only exceptions among 7% efficient solar cells are BHJ devices where instead of PC71BM, indene-C60/70-bisadduct (ICBA) with higher-lying LUMO energy level is used as an acceptor, leading to enhanced Voc [184e186]. However, the scope of polymers that display enhanced efficiency with ICBA is rather limited at the present time [187e191]. 2.2. Limitations of current state-of-the-art systems While the state-of-the-art polymers described previously represent significant progress in the drive for higher efficiency, closer analysis also reveals a number of common features that point toward the necessity for further optimization, especially when considering the light-harvesting ability of the polymers. As a first point, consider the similarities in polymer design. All of the polymers in Fig. 3 are perfectly alternating copolymers with nominally electron-rich donor monomers alternating with electron-poor acceptor monomers. As such, the well-known donor/acceptor

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Fig. 3. Structures of the best performing polymers to date as well as the most commonly used fullerene acceptors.

effect [103,111,139] is the ubiquitous approach to give lower polymer band gaps to improve the long-wavelength photoresponse of the solar cells. As implied earlier, a clear limitation of such polymers is the resultant shift in the primary absorption band from the visible to the red and/or near-IR, with a loss of spectral coverage in the higher energy visible range [29,161,196e198]. Exemplary absorption spectra are shown in Fig. 4 to illustrate this point. The incorporation of acceptor units in the polymer backbone leads not to the true broadening of the absorption spectra but simple redshifting of the absorption profile, requiring the use of PC71BM as an acceptor to maintain high absorption in the visible [161,196,197], as illustrated in Fig. 4aec. From the device fabrication point of view, the majority of the state-of-the-art polymerefullerene pairs require solvent additives

Table 1 Performance of state-of-the-art binary blend BHJ solar cells. Polymer

Jsc Voc PolymerePC71BM Eg (nm/eV) (mA/cm2) (V)

FF h (%) (%)

PTB7 [30] PBDTTTeCeT [51] PDTGeTPD [52] PBDTTPD [53] PBDTeDTNT [54] PNNTe12HD [61] PDTPeDFBT [32] PBDTeTFQ [55] PTB7eF20 [192] PBDTDTTTeSeT [174] PBDTTTeCF [193] PDTSTPD [161] PBDTTeSeDPP [29] PCPDTBT [194] DTePDPP2TeTT [195]

1:1.5 1:1.5 1:1.5 1:1.5 1:1.5 1:2 1:2 1:1 1:1.5 1:1 1:1.5 1:2 1:2 1:4 1:3

70 63 67 70 61 64 63 58 67 66 67 68 62 69 70

713/1.7 785/1.6 734/1.7 720/1.7 838/1.5 738/1.7 899/1.4 718/1.7 770/1.6 780/1.6 775/1.6 717/1.7 899/1.4 689/1.8 920/1.4

17.5 18.4 14.0 12.6 17.4 15.6 18.6 17.9 17.2 16.4 15.2 12.2 16.8 11.6 14.8

0.75 0.76 0.86 0.97 0.75 0.82 0.69 0.76 0.68 0.69 0.76 0.88 0.69 0.87 0.66

9.2 8.8 8.5 8.5 8.4 8.2 8.1 8.0 7.9 7.8 7.7 7.3 7.2 7.1 6.9

[199,200] for optimal device performance [30,51e 53,55,161,174,192,193]. Even though solvent additives tend to increase polymer’s crystallinity and improve the morphology of the BHJ binary blends [170,197,201e206], their use has a negative impact on the solar cell stability and increases degradation rate [207], and application in the roll-to-roll process is envisioned to be difficult due to high boiling point of the processing additive and hence complexities to remove them from the blend [25,26]. As indicated in Table 1 the polymerefullerene ratio in these solar cells is generally significantly weighted to the fullerene. The excess fullerene necessary poses several potential drawbacks to both broadband light harvesting and cost. In general, polymer fullerene solar cells work best across a narrow and finite range of active layer thickness (w100e200 nm) [59,160e163] as limited by the comparatively low charge carrier mobility of organics [208,209]. If a majority of the film is fullerene, regardless of how well the polymer absorbs in the red/near-IR, the capacity of such a small amount of polymer to effectively harvest this spectral region will be limited. A further downside to C70 fullerene is cost. While the cost of C60 has decreased dramatically in recent years, the cost of C70 is still quite high and is perhaps prohibitive in a practical sense [210]. Further drawbacks to C70 fullerenes relative to C60 have been identified and include lower electron mobility [211], deeperlying LUMO energy level limiting the Voc, [212e214], enhanced tendency toward aggregation [215], lower percolation [178,216e 220], higher solubility in organic solvents (e.g. chlorobenzene, odichlorobenzene, chloroform) [221e223] and as a result lower miscibility with polymers [223e225], and structural instability [226] with polymers in comparison to PC61BM. Additionally, high trap densities with PC71BM and the presence of deeper-lying trap states are limiting for device efficiency [227]. Finally, it has been found that polymerePC61BM solar cells provide a platform that is more amenable to moving toward thicker active layers, although


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Fig. 4. Absorption spectra of perfectly alternating D/A polymers. Reproduced with permission: (a) from Ref. [196], (b) from Ref. [197], (c) from Ref. [161], (d) from Ref. [195], (e) from Ref. [29].

requiring polymers that are more effective broadband absorbers to maximize light harvesting capacity [195]. Another element common to the state-of-the-art polymer structures is the lengthy monomer syntheses. As can be seen in Fig. 3, the most commonly used donor monomers for the D/A polymers utilized in the most efficient solar cells are benzo[1,2b:4,5-b0 ]dithiophene (BDT) and thieno[3,4-b]thiophene (TT), highlighted in blue and red respectively in PTB7. Despite, high power conversion efficiencies, these monomers require multistep synthesis. In the case of BDT, from 6 to 7 synthetic steps (depending on the side chains) are necessary to obtain 2,6-bis(trialkyltin)-BDT monomers from inexpensive commercially available precursors [228,229], while in case of TT, it requires from 5 to 9 steps (depending on the side chains and fluorination) to obtain 4,6dibromo-TT [62,230,231]. As a result, for example, PTB7 is synthesized in 16 steps, PBDTTTeCeT in 12 steps and PBDTeTFQ in 13 steps. Only recently, the use of novel donor and acceptor monomer units decreased the number of steps to 10 for PDTPeDFBT [232,233] and 8 for PDTGeTPD [234]. It is clear that in these state-of-the-art polymers the control of polymer electronic structure has been primarily targeted via fine-tuned manipulation of monomer structure enabled by lengthy and complex synthesis. This method of polymer design referred to here as the “monomer approach” also appears to diverge from the goal of targeting accessible and inexpensive materials. While great strides toward higher efficiency have been made with the polymers described in Fig. 3 and Table 1 and a great wealth of fundamental insight about the performance and potential of polymerefullerene solar cells has been gathered through their study, the common structural, electronic, and processing features discussed above argue that a subsequent level of optimization is needed to refocus efforts toward simple and potentially less expensive polymers and polymerefullerene combinations. In order to address this point, while still targeting high efficiency solar cells, our group has pursued a two-fold strategy. First, we have pursued a comprehensive polymer design strategy to target optimal electronic structure and morphology within the framework of short, simple syntheses. The aim is to control polymer properties through

the strategic combination of simple monomers through the socalled “polymer approach” as opposed to a monomer design approach referred to previously as the “monomer approach.” Our polymer approach has been centered on random and semi-random analogs of poly(3-hexylthiophene) (P3HT). The second element of our strategy has been to translate from the binary blend approach (polymerefullerene BHJ) to the ternary blend platform (polymere polymerefullerene). Growing evidence suggests that ternary blends offer the unique combination of simple device fabrication with the potential to move to enhanced device efficiencies. Further, we have found that random and semi-random P3HT analogs are especially well suited for the ternary blend approach. 3. Efforts toward optimization of polymer design for binary blends through the “polymer approach” Central to the operation of efficient polymerefullerene BHJ solar cells is an effective synergy between electronic processes and the bulk and interfacial morphology of the active layer [71,105,109,114]. A picture of an ideal polymerefullerene blend has been emerging over the last decade [14,42,44,116,150e153] and while a complete, definitive understanding of all optimal features has not yet been achieved, several desirable elements have been established. From an electronic point of view, the donor polymer should have a broad and intense absorption with an absorption coefficient on the order of 105 cm1 in order to maximize light absorption. The donor should have an effective energetic relationship with the acceptor (preferably a C60 fullerene) in order to ensure charge transfer and reduce geminate recombination while giving a maximum possible open-circuit voltage (see Fig. 2). The donor polymer should also be an effective hole transporter with a hole mobility that is suitably matched with the electron mobility of the fullerene acceptor. High hole mobility in polymers is strongly tied to ordering (crystallinity), which increases pep overlap. These optimal electronic properties have been discussed in greater detail in a number of excellent recent reviews [13,16,19,41,44,117,118,125,139,140,235]. While an understanding of the optimal electronic properties and relationships is becoming very mature [40e44], an

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understanding of the optimal morphology (both bulk and interfacial) is still developing. However, a few basic principles seem to hold. First, it is desirable to minimize fullerene loading in order to fully exploit the absorption potential of broadly absorbing polymer donors. This is necessary due to the very thin (100e200 nm) active layers [59,160e163] generally required for relatively low mobility organics [166,195,236e239]. If the polymer is only a minority of the active layer at this thickness, it does not matter how broadly or strongly the polymer can absorb light, contributions from the polymer extending beyond the fullerene absorption envelope in the red and near-IR will be minimal. Second, it is desired that the polymer and fullerene mix well enough to give a limited length scale of phase separation that would ideally be on an order of about two times the exciton diffusion length of the polymer and the fullerene [68,69,72,240]. Further, recently the role of diffuse interfaces of intermixed polymer and fullerene has emerged as potentially very important for the charge generation process [118,178,216e219,241e248], further emphasizing that good miscibility between polymer and fullerene is important. As such, an ideal donor can be qualitatively described as a polymer that has a broad and intense absorption, an effective energetic relationship with fullerene acceptors (for charge transfer and high Voc), a suitable level of ordering to give good hole mobility, and a good miscibility with C60-fullerenes that yields a percolated structure with a small length scale of phase separation at low fullerene loadings with the potential for a diffuse/intermixed interface. It is also proposed here that this ideal donor should be accessible through a short, simple synthetic procedure in order to retain the potential for low-cost in solar cell manufacture. It is the final point that is perhaps the most daunting considering how many functions need to be “programmed” into the polymer. It has been difficult to imagine a short, simple synthesis that can lead to optimization of all the necessary functions of a polymer in a BHJ solar cell. This has certainly led to the “monomer approach” in which monomer structure of both electron rich and electron poor monomers has undergone a fine-tuning in which function-specific structural elements have been added piece-by-piece to yield the structures that currently define the state-of-the-art. 3.1. Poly(3-hexylthiophene) (P3HT) In order to pursue the goal of an ideal polymer, we have focused on simplicity of structure and synthesis first, while attempting to build in the desired polymer properties by strategically combining a discrete set of monomers through the “polymer approach.” As a starting platform we selected P3HT for a number of reasons. First, it is among the simplest of all conjugated polymers based on a readily available and inexpensive single ring heterocycle monomer with an alkyl chain as the only functional group [249e251]. The predominantly regioregular (RR) placement of the alkyl chains controls ordering (crystallinity) [252e259], lowers the band gap relative to polythiophene [260,261], and engenders good solubility in a broad range of common organic solvents [222]. P3HT has a strong peak absorption coefficient (105 cm1) [257,262], high hole mobility (w2 104 cm2 V1 s1 in SCLC [263,264] or 0.1 cm2 V1 s1 in FET) [265], almost matching the electron mobility of PCBM [264,266e 268], limiting the space-charge buildup and enabling good fill factors of 0.65e0.7 [117,236,237,264,269e271]. With PC61BM as the acceptor efficiencies of up to 5% have been achieved in BHJ solar cells using P3HT as the donor material, whereas the average power conversion efficiency is 3% [249,272]. Higher efficiencies have been published more recently, using fullerene derivatives [80,81] such as indene-C60-bisadduct (IC60BA, 7.5%) and indene-C70-bisadduct (IC70BA, 7.4%) [184e186]. P3HT not only forms stable, small-scale phase separated morphology with PC61BM [71,72,238,240,272e

275], but charge transfer from the donor to acceptor is ultrafast and efficiently results in free charge carriers [80,126,128,129,276e 278], making P3HT an almost ideal candidate for BHJ solar cells. Further, the morphology of P3HTePC61BM solar cells is the most extensively studied among all polymerefullerene pairs and the length scale of phase separation is nearly ideal [71,249,254,272,279e282]. Importantly, P3HTePC61BM solar cells have been shown to generate thermally stable morphologies when the RR of P3HT is optimized at approximately 90% [263,283]. The environmental stability of P3HTePC61BM solar cells (while not the focus of this review) has also been demonstrated to be excellent giving lifetimes by accelerated testing of more than 3 years [284,285]. As a consequence, nearly every property of P3HT is seemingly ideal for polymerefullerene solar cells. However, several shortcomings of P3HT prevent it from achieving even higher efficiencies. The wide band gap (1.9 eV) of P3HT, resulting in an absorption onset at w650 nm and narrow absorption breadth (w400e 650 nm), inherently limits Jsc of the corresponding BHJ solar cell and thus the efficiency [14,139]. To a lesser extent the relatively high HOMO level of P3HT (5.2 eV) [44], which is reflected in a moderate Voc (w0.6 V with most common fullerene acceptors) [71,249] also limits the efficiency. Contrary to what is often implied by the term “fruit fly”, the prevalence of P3HT in the area of BHJ solar cells during the last decade can be explained by its exceptional set of attractive properties, which often do not translate into novel D/A copolymers [71,193,286e289], as well as the simplicity of its polymer structure. Our goal has been to transform P3HT into a low band gap broadband absorbing polymer while retaining all of the attractive properties and with minimal addition of synthetic complexity. Several successful design concepts resulting in P3HT analogs with improved polymer properties and solar cell efficiencies are discussed in detail in the following sections. 3.2. Design concept: random P3HT analogs One possible approach to improve the properties of P3HT solar cells is to randomize the polymer structure by decreasing the regioregularity or introducing alkylthiophene comonomers. Numerous literature examples demonstrate that random P3HT analogs are very promising materials for BHJ solar cells. An interesting study by Fréchet et al. showed that a certain degree of randomness enhances active layer stability and provides more stable photovoltaic performance [263,283]. Contrary to the common perception that the regioregularity of P3HT needs to be as high as possible (>95%) in order to achieve high efficiency, it was shown that P3HT with regioregularity as low as 86%, still has sufficient electronic properties for high efficiency in combination with improved thermal stability of the active layer, induced by the reduced driving force for long-range crystallization in the polymer [263]. On the other hand, it has also been shown that alkylthiophene based copolymers provide the opportunity to tune the frontier orbital energy levels. By decreasing the alkyl side chain density on the thiophene backbone using a long and branched alkyl chain on only every third thiophene ring Hou et al. were able to increase Voc considerably due to a decrease in the HOMO energy, which was attributed to the decrease in electron donating alkyl chains compared to P3HT [290]. A similar strategy was used by Ko et al. by only putting alkyl chains on the 3- and 4-position of every third thiophene ring [291]. In addition to an increased Voc they also observed increased efficiency compared to P3HT. This was attributed to both a decrease in the density of electron donating side chains as well as a slightly increased degree of twisting in the polymer backbone, which is beneficial for Voc but left Jsc almost unchanged [291,292].


To further investigate the influence alkyl side chains have on polymer properties as well as the performance of polymere fullerene BHJ solar cells, while also capitalizing on the fact that random P3HT-analogs have demonstrated better active-layer stability, we have developed a model system of random alkylthiophene copolymers [293]. More specifically, we studied the impact of increasing amounts of bulky, branched 2-ethylhexyl chains in rrpoly(3-hexylthiophene-co-3-(2-ethylhexyl)thiophene) copolymers on important properties such as the UVevis absorption, HOMO energy levels and solar cell performance. Five polymers were synthesized by copolymerizing hexylthiophene and 2ethylhexylthiophene at different ratios (m:n) using Stille coupling polymerization, as shown in Scheme 1 and resulting in P3HT, P3HT90-co-EHT10 (10% 2-ethylhexylthiophene), P3HT75-co-EHT25 (25% 2-ethylhexylthiophene), P3HT50-co-EHT50 (50% 2ethylhexylthiophene) and P3EHT (100% 2-ethylhexylthiophene). As shown in Fig. 5, polymers with 50% or less 2-ethylhexyl side chain content not only have the same band gap as well as almost the same absorption coefficient as P3HT, but also retain the characteristic vibronic feature of P3HT, indicating good ordering in the solid state [256]. The semicrystalline nature of all five polymers, indicated by the vibronic features in the UVevis was confirmed with both GIXRD and DSC measurements and is reflected in the high hole mobilities of the polymers (Table 2). In order to gain insight on the effect of the 2-ethylhexyl side chain content on the polymer HOMO levels, cyclic voltammetry (CV) [294] was measured both in solution and thin films. Whereas the HOMO levels of all five polymers in solution are almost identical, in the solid state an increase of the 2-ethylhexyl side chain content leads to a marked decrease of the HOMO energy (Fig. 6, Table 2) implying a solid state organization effect. This decrease in HOMO energy is directly reflected in the Voc measured in polymerePC61BM solar cells (Table 2), which increases with increasing 2-ethylhexyl side chain content indicating a strong correlation between the HOMODeLUMOA offset (Fig. 2b) and the Voc for this family of polymers. More detailed explanations, such as interfacial interactions between polymers and fullerenes, which other literature reports show to be relevant, are not necessary to explain the observed results [241,295]. As a direct consequence of the correlation between HOMO energies and Voc, the efficiency of BHJ solar cells with PC61BM for P3HT75-co-EHT25 (3.85%) is increased compared to P3HT (3.48%) due to higher Voc (0.69 V vs. 0.60 V) while Jsc remains on the same order as for P3HT (9.85 mA/cm2). More importantly though, the attractive properties of P3HT such as high absorption coefficient, semicrystallinity, high hole mobility and favorable mixing with PCBM are largely retained when small to moderate amounts of 2ethylhexylthiophene are introduced. This work also serves as a general demonstration of the potential for simple modularity and tunability in random alkylthiophene copolymers and the potential to pursue enhancement in polymer properties and photovoltaic device performance through random copolymerization.

Scheme 1. Synthesis of poly((3-hexylthiophene)-co-3-(2-ethylhexyl)thiophene).

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Fig. 5. UVevis absorption of P3HTm-co-EHTn polymers in thin films (spin-coated from CB and annealed for 30 min under N2 at 150 C for (i), (ii) and (iii), 100 C for (iv), 40 C for (v)) where (i) is P3HT (purple line), (ii) is P3HT90-co-EHT10 (green line), (iii) is P3HT75-co-EHT25 (blue line), (iv) is P3HT50-co-EHT50 (red line) and (v) is P3EHT (orange line). Reproduced with permission from Ref. [293]. Copyright 2012 American Chemical Society.

3.3. Semi-random P3HT analogs Tuning the energy levels of P3HT through copolymerization and increasing the long term stability of polymerefullerene solar cells by introducing a degree of randomness are successful approaches to improve the already favorable properties of P3HT. It is clear though, that the primary limitation of P3HT, its large band gap and consequently narrow absorption breadth, cannot be addressed this way. On the other hand, it has been successfully shown that the donor/acceptor (D/A) approach in combination with a random copolymerization of electron-rich and electron-poor monomers not only results in polymers with low band gap, but also broad absorption due to the multichromophoric nature of the polymer backbone [296e298]. This broad absorption of random D/A copolymers is in strong contrast to perfectly alternating D/A copolymers, which even though they have lowered band gaps often suffer from narrow absorption breadths, thus limiting the achievable Jsc [168,287,299e301]. Efficiencies of over 5% have been achieved with random D/A copolymers underlining the promise of this class of polymers for the use in BHJ solar cells [289,302,303]. On the other hand, the hole mobilities of these polymers are generally low due to the large degree of disorder introduced by the random polymerization [296e298] and, contrary to what the UVevis absorption profile suggests, the photoresponse of the solar cells is often very low at longer wavelengths, limiting the achievable efficiencies. With the goal of combining the favorable properties of P3HT, such as semicrystallinity, high hole mobility and good miscibility with fullerenes at low polymerefullerene ratios, with the broad absorption of multichromophoric, random D/A copolymers, we introduced semi-random hexylthiophene based copolymers and Scheme 2 illustrates the first generation of semi-random polymers [304e308]. Semi-random copolymers are defined by a randomized polymer backbone, but due to the restricted linkage pattern of monomers, achieved by a careful choice of functional groups (Scheme 2b), a larger degree of structural order is retained than in completely random copolymers. Scheme 2a illustrates the concept of semi-random copolymers by showing a possible segment of a semi-random polymer. A central element of semi-random

5274


Table 2 Electronic and average photovoltaic properties of P3HTm-co-EHTn polymers [293]. PolymerePC61BM (ratio)

Eg (optical) (eV)

HOMO (eV) (solution)

HOMO (eV) (thin film)

SCLC hole mobility (cm2 V1 s1)f

P3HT (1:1)a P3HT90-co-EHT10 (1:0.8)b P3HT75-co-EHT25 (1:0.8)c P3HT50-co-EHT50 (1:3.5)d P3EHT (1:3.0)e

1.9 1.9 1.9 1.9 2.0

5.25 5.25 5.30 5.32 5.28

5.17 5.30 5.43 5.48 5.57

2.30 1.77 1.39 1.07 2.87

104 104 104 104 105

Jsc (mA/cm2)

Voc (V)

FF

PCE (%)

9.67 9.26 9.85 2.52 2.54

0.60 0.63 0.69 0.85 0.90

0.60 0.51 0.57 0.35 0.36

3.48 2.80 3.85 0.74 0.83

aee

All films were spin-coated from chlorobenzene (CB) and annealed under N2 at the specified conditions after aluminum deposition. 145 C for 60 min. b 110 C for 60 min. c 110 C for 30 min. d 110 C for 10 min. e Tested as-cast. f Measured for neat polymer films. a

polymers is that the polymer backbone is heavily dominated by head-to-tail coupled 3-hexylthiophene, with only a small percentage of acceptor monomer (shown as an example is benzothiadiazole (BTD)). As a consequence not only do these polymers retain a P3HT-like character but, depending on the effective conjugation length of the polymer, many different chromophores can be envisioned instead of just one. This is demonstrated conceptually in Scheme 2a, where chromophore I consists solely of head-to-tail coupled P3HT, and will thus absorb shorter wavelength photons, whereas chromophore II contains acceptor monomers (shown with BTD as an example) and is thus able to absorb longer wavelength photons due to the D/A effect, overall resulting in a broader absorption spectrum of the polymer. Additionally, semi-random copolymers are designed to favor shortrange crystallinity, over long-range crystallinity which, as explained above, is advantageous as it prevents large-scale phase separation between the polymer and fullerene but still allows for high hole mobility [263,283]. Scheme 2b shows an example for the established semi-random Stille polymerization using DMF as the solvent and Pd(PPh3)4 as the catalyst. Monomers for semi-random Stille polymerization are shown in Scheme 2b and are carefully chosen to inherently avoid sterically unfavorable linkages. The Stille polycondensation is used due to excellent functional group tolerance, allowing a wide variety of different comonomers to be incorporated into the semi-random architecture. Importantly, all monomers used for semi-random copolymers can be made in very few, simple steps, which is in

Fig. 6. HOMO levels of the P3HTm-co-EHTn polymers in the solid state (filled squares) and Voc (circles) of the optimized solar cells as a function of amount of 2-ethylhexyl side chains in the polymer backbone. Reproduced with permission from Ref. [293]. Copyright 2012 American Chemical Society.

strong contrast to the ever-increasing complexity of monomers used in perfectly alternating D/A polymers. Coupling of 2-bromo-5trimethyltin-hexylthiophene (1) monomers gives head-to-tail 3hexylthiophene linkages, which is desirable for efficient solar cells and is also illustrated in chromophore I in Scheme 2a. Scheme 2b also shows that acceptor monomers such as 4,7-dibromo-2,1,3benzothiadiazole (3) will react with donor monomers (1 or 2,5bis(trimethyltin)thiophene (2)) but not favor homocoupling, which avoids long (insoluble) segments of acceptor in polymers and enforces the generation of donoreacceptor interactions. The unsubstituted thiophene comonomer (2) is used not only to give stoichiometric balance in the polymerization, but also to further alleviate any unfavorable steric interactions. As mentioned, these restrictions in monomer connectivity help to overcome some of the limitations random copolymers have, namely an amorphous morphology and typically low hole mobilities as a consequence. As can be seen in Scheme 2c between 65 and 80% of 2-bromo-5trimethyltin-hexylthiophene (1) is used in the semi-random copolymers and only 10e17.5% acceptor monomer in order to retain the favorable properties of P3HT. The monomer incorporation ratio in the polymer backbone can be determined by careful integration of the 1H NMR spectra and is, for all presented semi-random copolymers, identical with the monomer feed ratio. Additionally, the semi-random polymers are observed to give similar molecular weights to P3HT (Mn z 15e25 kDa). Scheme 2c shows semi-random copolymers of the first generation, including P3HTTeBTD (10% benzothiadiazole (BTD) content), P3HTTeTP (10% thienopyrazine (TP) content) and P3HTTeTPeBTD (8.75% TP and 8.75% BTD content) where P3HTT stands for poly(3hexylthiophene-thiophene) as well as P3HTT and P3HT as reference polymers. All three semi-random polymers containing an acceptor have a lower band gap than P3HT (Table 3) and much broadened absorption profile (Fig. 7), with P3HTTeTPeBTD able to absorb almost twice as many photons as P3HT. As evidenced by peaks in the range of 5e7 (2q) in the grazingincidence X-ray diffraction (GIXRD) traces indexed as the (100) peaks in analogy to P3HT (Fig. 8), P3HTTeBTD and P3HTTeTPeBTD are semicrystalline, whereas P3HTTeTP is amorphous. It is surprising that P3HTTeTP does not show any signs of crystallinity considering that almost all other semi-random copolymers we reported are semicrystalline, including others containing TP [305,306,308]. A subtle, but important point illustrated in Fig. 8 is based on the peak positions for the various polymers. Specifically, note the shift to larger angles when comparing P3HT with P3HTTe TPeBTD, where the absence of a peak in the position analogous to P3HT indicates that ordering in these semi-random polymers is not simply the consequence of packing continuous sequences of 3hexylthiophene. It is a common misconception that these semirandom polymers are semi-crystalline as a consequence of such a


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Scheme 2. (a) Concept of multichromophoric semi-random copolymers. (b) A representative synthetic scheme. (c) Structures of the first generation of semi-random copolymers. Reproduced with permission from Ref. [304]. Copyright 2011 American Chemical Society.

large fraction of 3-hexylthiophene, which might result in crystallites that are rich in pure 3-hexylthiophene sequences. The absence of peaks in the region consistent with P3HT in the GIXRD establishes that a new packing motif influenced by the nature of the comonomer is present. Supportive of the effective ordering, hole mobilities, determined using space-charge limit current (SCLC) method [309], are extremely close to the hole mobility of P3HT (Table 3), which further validated the concept of semi-random polymers as P3HT analogs. The high hole mobilities were also confirmed for later generations of this family of polymers, as will be discussed. Even though the photovoltaic properties of this first generation of semi-random polymers (Table 3) were moderate, the unique combination of properties these polymers possess, namely a semicrystalline morphology and high hole mobility despite the

randomized polymerization together with broad and intense absorption of the solar spectrum, spurred an intensive investigation into this class of polymers. The choice of the diketopyrrolopyrrole (DPP) unit as the acceptor for the next generation of semi-random copolymers was influenced by a growing number of recent reports of high efficiency in BHJ solar cells with polymers and small molecules containing this unit [310e315]. The presence of one thiophene on each side of the DPP acceptor minimizes steric hindrance and induces planarity, which enhances chain packing and intermolecular pep interaction, making DPP a very attractive unit for incorporation into semirandom copolymers [312,316,317]. All three synthesized polymers, P3HTTeDPP-5%, P3HTTeDPP10% and P3HTTeDPP-15% (Scheme 3), contain DPP as the acceptor but in different quantities (5, 10 and 15% respectively) in order to

5276


Table 3 Optical band gaps, average SCLC mobilities and average solar cell performance (Jsc, Voc, FF and PCE) of first generation semi-random copolymers [304]. PolymerePC61BM (ratio)

Voc Eg (optical) SCLC hole Jsc (mA/cm2) (V) (eV)e mobility (cm2 V1 s1)f

P3HT (1:0.8)a P3HTT P3HTTeBTD (1:5)b P3HTTeTP (1:0.8)c P3HTTeTPeBTD (1:0.8)d

1.91 1.96 1.62 1.36 1.27

a b c d e f

2.30 8.21 2.06 2.50 2.35

104 105 104 104 104

10.22 e 2.87 3.22 3.04

0.59 e 0.79 0.44 0.39

FF

PCE (%)

0.64 e 0.33 0.50 0.37

3.89 e 0.75 0.71 0.43

Spin-coated from CB and annealed at 150 C for 30 min. Spin-coated from o-DCB and annealed at 100 C for 10 min. Spin-coated from o-DCB with 1.7% octanedithiol and tested as cast. Spin-coated from o-DCB and annealed at 100 C for 30 min. Determined from the onset of absorption in UVevis spectra in thin films. Measured for neat polymer films.

not only study the influence of the acceptor monomer on the polymer properties but also the effect of the total acceptor content [305]. In all three polymers, the introduction of DPP into the P3HT backbone is observed to significantly decrease the optical band gap and lead to the formation of a distinct dual band absorption in solutions and thin films (Fig. 9). This type of absorption profile is often ascribed to pep* (short wavelength band) and ICT (intramolecular charge transfer) transitions (long wavelength band) [235,318]. In the case of semi-random polymers, the dual band absorption could more specifically be assigned to pep* transitions of segments in the randomized polymer that are thiophene-rich (short wavelength band, chromophore I Scheme 2a) and ICT transitions in segments that are rich in D/A linkages (long wavelength band, chromophore II Scheme 2a). It is interesting to note, that as the DPP content increases (10e15%), the polymer absorption profile begins to converge toward that observed for perfectly alternating thiopheneeDPP polymers (as opposed to the broad absorption profile of random copolymers), which contain significantly higher contents (50%) of DPP acceptor [198,311,316,317]. Absorption coefficients in thin films of the ICT band of P3HTTeDPP-10% and especially P3HTTeDPP-15% are approaching 105 cm1 and are comparable to the peak value of P3HT. Another interesting feature in the thin film absorption spectra is the presence of the vibronic

features in the ICT band. The same vibrational shoulders were observed in the case of other DPP-based polymers and small molecules [310,319], and were ascribed to the high degree of ordering and strong intermolecular (pep) interactions [256]. The semicrystalline nature of all three P3HTTeDPP polymers was verified with GIXRD measurements and is again reflected in the high SCLC hole mobilities (Table 4), which are on the same order of magnitude as P3HT and reconfirm the P3HT-like character of semi-random polymers. The photovoltaic performance of P3HTTeDPP-5%, P3HTTeDPP10% and P3HTTeDPP-15% is summarized in Table 4. Importantly, the broad and intense absorption and high mobility of the DPPcontaining semi-random polymers translate into a strong and broad photoresponse (Fig. 10) of the polymerePC61BM solar cells and consequently high Jsc and efficiencies of close to 6%. These results confirm that semi-random P3HT analogs provide a simple and

Fig. 7. UVevis absorption of (i) P3HT, (ii) P3HTT, (iii) P3HTTeBTD, (iv) P3HTTeTP and (v) P3HTTeTPeBTD in thin film spin-coated from o-DCB and annealed for 30 min at 60 C under N2. Reproduced with permission from Ref. [304]. Copyright 2011 American Chemical Society.

Scheme 3. Synthesis and structures of P3HTTeDPP-5%, P3HTTeDPP-10% and P3HTTe DPP-15%. Reproduced with permission from Ref. [305]. Copyright 2011 American Chemical Society.

Fig. 8. Grazing-incidence X-ray diffraction of thin films spin-coated from chlorobenzene (CB) before and after annealing at 150 C for 30 min under N2. The inset shows the region around 2q ¼ 5e7 in greater detail. Polymers shown are P3HT ((i) a, (i) b), P3HTTeBTD ((iii) a) and P3HTTeTPeBTD ((v) a, (v) b) where b stands for before annealing and a for after annealing. As-cast P3HTTeBTD ((iii) b) is amorphous and not shown in the figure. Reproduced with permission from Ref. [304]. Copyright 2011 American Chemical Society.


Fig. 9. UVevis absorption of polymers in thin film (spin-coated from o-DCB) where (i) is P3HT (black line) (annealed at 150 C for 30 min), (ii) is P3HTTeDPP-5% (red line) (as cast), (iii) is P3HTTeDPP-10% (green line) (as cast) and (iv) is P3HTTeDPP-15% (blue line) (as cast). Reproduced with permission from Ref. [305]. Copyright 2011 American Chemical Society.

effective route toward polymers with a broad photocurrent response in BHJ solar cells and emphasize the enormous potential of this family of copolymers. Even though efficiencies of solar cells based on DPP-containing semi-random copolymers are high, they are inherently limited due to high HOMO energies (5.2 eV for all three P3HTTeDPP polymers) and resulting moderate Voc. The acceptor thienopyrrolodione (TPD), on the other hand has recently gained significant attention and has been used in many D/A copolymers with high solar cell efficiencies generally showing large Voc and low HOMO levels and is thus a very interesting acceptor monomer for semi-random polymers [161,234,301,321e324]. The UVevis absorption profiles of P3HTTe TPD-10% and P3HTTeTPD-15% (see Scheme 4 for structures) in thin films are shown in Fig. 11 [306]. The absorption profiles of the TPDcontaining polymers are slightly broadened compared to P3HT but are much narrower than for DPP-containing semi-random polymers with a strong absorption in the visible, indicating that TPD is a weaker acceptor than DPP [107]. Even though the Voc of TPD-based semi-random copolymers is, as predicted, higher (0.72 V and 0.68 V) than for DPP-based polymers and the hole mobilities are also high, the solar cell efficiencies are moderate due to the narrow absorption breadth and consequently low Jsc (Table 5). The second generation semi-random polymers containing either DPP or TPD acceptors established the potential of this class of Table 4 Optical band gaps, SCLC mobilities and solar cell performance (Jsc, Voc, FF and PCE) of P3HTTeDPP-5%, P3HTTeDPP-10% and P3HTTeDPP-15% [305]. SCLC hole Jsc Voc PolymerePC61BM Eg (optical) mobility (mA/cm2)d (V) (ratio) b 2 1 1 c (eV) (cm V s ) P3HTTeDPP-5% (1:1)a P3HTTeDPP-10% (1:1.3)a P3HTTeDPP-15% (1:2.6)a

1.52

1.1 104

9.57

1.51

2.3 104

14.62

1.46

4

14.28

1.3 10

FF

PCEavg (PCEpeak) (%)

5277

Fig. 10. EQE of the BHJ solar cells based on P3HT (black squares), P3HTTeDPP-5% (red circles), P3HTTeDPP-10% (green triangles) and P3HTTeDPP-15% (blue stars) with PC61BM as the acceptor, under optimized conditions for device fabrication.

polymer for use in solar cells. However, among the semi-random polymers, P3HTTeTPeBTD (Scheme 2) containing two distinct acceptors (BTD and TP with a total acceptor content of 17.5%) is of special interest because of the unprecedented strong and uniform light absorption (Fig. 7), even though it has shown only moderate solar cell efficiency [304]. For comparison, only a small number of other random conjugated polymers with multiple acceptor monomers have been studied in polymer solar cells [302,325e327]. Even though the currents reported in such examples do not necessarily reflect the broad absorption profiles, and in some cases can be primarily attributed to absorption by PC71BM, strong solar cell performance in several cases gives reason to believe that this class of polymers, containing multiple distinct acceptors, as a whole is very promising. The complementary absorption profiles of P3HTTeDPP (both 10% and 15% acceptor content) and P3HTTeTPD (both 10% and 15% acceptor content) (see Figs. 9 and 11) as well as the higher Voc of P3HTTeTPD, make TPD and DPP a promising acceptor monomer combination for investigating semi-random two-acceptor polymers in more detail. As such, two-acceptor copolymers were made in an identical manner to Scheme 4 with the addition of DPP monomer [306]. The ratio of acceptors was varied and the total acceptor content was kept constant at 15% resulting in P3HTTeTPDeDPP (1:1) (7.5% TPD and 7.5% DPP), P3HTTeTPDeDPP (2:1) (10% TPD and 5% DPP) and P3HTTeTPDeDPP (1:2) (5% TPD and 10% DPP) (Scheme 5). The total acceptor monomer content was chosen as 15% in order to ensure good solubility of the resulting two-acceptor copolymers as well as to retain their P3HT-like character while providing a broad enough range over which to vary the relative content of the two acceptor monomers. As with the previously discussed examples of semi-random polymers

0.66 0.58 3.60 (3.73) 0.59 0.64 5.53 (5.73) 0.51 0.65 4.66 (4.72)

a Spin-coated from o-DCB and tested after 20 min under N2 before aluminum deposition. b Determined from the onset of absorption in UVevis spectra in thin films. c Measured for neat polymer films. d Mismatch corrected [320].

Scheme 4. Synthesis and structures of P3HTTeTPD-10% and P3HTTeTPD-15%.

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Fig. 11. UVevis absorption of polymers in thin film (spin-coated from o-DCB and solvent annealed for 20 min under N2) where (i) is P3HTTeTPD-10% (red line) and (ii) is P3HTTeTPD-15% (orange line).

containing one acceptor monomer [304,305,307], the two-acceptor polymers benefit from a highly reproducible synthesis and batchto-batch consistency of polymer properties. The UVevis spectra of these two-acceptor polymers in thin film are shown in Fig. 12. All three polymers show broad and intense absorption with absorption coefficients of up to 8 104 at 700 nm for P3HTTeTPDeDPP (1:2). The absorption peaks are observed to rise and fall according to the change in acceptor ratio and the overall absorption profiles mimic the weighted sum of the corresponding one-acceptor absorption profiles of P3HTTeDPP and P3HTTeTPD. The broad absorption profiles are reflected in the photocurrent response (Fig. 13) of the polymerePC61BM solar cells where P3HTTeTPDeDPP (1:1) and P3HTTeTPDeDPP (1:2) have peak EQE values of 61% and 68% at 680 nm, respectively, and at 800 nm show impressive EQE values of 29% and 40%, which is rarely achieved by conjugated polymers. Solar cell efficiencies are close to 5% (Table 6) due to a combination of very large Jsc and very high FF. Current densities exceed 16 mA/cm2, which is much higher than for previously published polymers containing multiple acceptors and, to the best of our knowledge, the highest current densities achieved for polymer BHJ solar cells with PC61BM as the acceptor [302,325,326]. The simple and modular nature of the semi-random polymerization leads to an almost infinite combination of structures that can be targeted, even with a relatively small subset of monomers. For example, we have also explored the influence of the simultaneous

Scheme 5. Structures of semi-random two-acceptor polymers containing TPD and DPP. Reproduced with permission from Ref. [306]. Copyright 2012 American Chemical Society.

incorporation of electron rich and electron poor monomers in a semi-random polymer [307] and we have further explored the influence of pairing two distinct acceptors in the polymer backbone [308]. However, the examples described above illustrate the potential of this class of polymer for application in polymerefullerene BHJ solar cells. Specifically, the semi-random polymers are based on a synthetic platform that is defined by simplicity. Reported polymers contain 65e90% 3-hexylthiophene and incorporate only 5e 17.5% of acceptor monomer. Acceptor monomers are not only used in a very small quantity, but are typically synthesized in only a few steps. The hallmark of semi-random polymers is that the attractive properties of P3HT are retained while the band gap and band energies are tuned via the content and identity of acceptor monomers

Table 5 Optical band gaps, SCLC mobilities and solar cell performance (Jsc, Voc, FF and PCE) of P3HTTeTPD-10% and P3HTTeTPD-15% [306]. SCLC hole Jsc Voc PolymerePC61BM Eg (optical) mobility (mA/cm2)d (V) (ratio) (eV)b (cm2 V1 s1)c P3HTTeTPD-10% (1:1.5)a P3HTTeTPD-15% (1:1.3)a a b c d

1.82

0.8 104

5.38

1.80

4

5.33

0.7 10

FF


0.72 0.58 2.22 (2.30) 0.68 0.56 2.02 (2.08)

Spin-coated from CB and tested as cast. Determined from the onset of absorption in UVevis spectra in thin films. Measured for neat polymer films. Mismatch corrected.

Fig. 12. UVevis absorption of polymers in thin film (spin-coated from o-DCB and solvent annealed for 20 min under N2) where (i) is P3HTTeTPDeDPP (2:1) (blue line), (ii) is P3HTTeTPDeDPP (1:1) (purple line) and (iii) is P3HTTeTPDeDPP (1:2) (green line). Reproduced with permission from Ref. [306]. Copyright 2012 American Chemical Society.


Fig. 13. EQE of the BHJ solar cells based on P3HTTeTPDeDPP (2:1) (blue squares), P3HTTeTPDeDPP (1:1) (purple triangles) and P3HTTeTPDeDPP (1:2) (green circles). Reproduced with permission from Ref. [306]. Copyright 2012 American Chemical Society.

in the highly modular synthesis. A randomized incorporation of acceptors also allows for unprecedented broadening of the absorption spectrum through the multichromophoric effect and alleviates the necessity of using C70 fullerenes as acceptors. With this platform we have demonstrated several examples of semi-random polymers that give Jsc > 14 mA/cm2 and even exceeding 16 mA/cm2, which is the highest ever reported value for a polymer solar cell based on a C60 acceptor. Importantly the FF of the solar cells have exceeded 0.6 and in several cases have even reached 0.65, despite being measured in air. As such, the semi-random polymers combine the tunability necessary to move toward higher efficiency while maintaining the simplicity in synthesis that is consistent with the vision of organic solar cells as an inexpensive platform for solar energy conversion. 3.4. Simplifying synthesis via direct arylation polymerization (DArP) A central element of the semi-random polymerization is simplification of polymer synthesis through a focus on controlling polymer properties via choice and composition of a small amount of acceptor monomer drawn from a subset of easily synthesized acceptors (“polymer approach”) as opposed to the highly specific tailoring of donor and acceptor monomer structures to control Table 6 Optical band gaps, SCLC mobilities and solar cell performance (Jsc, Voc, FF and PCE) of P3HTTeTPDeDPP (1:1), P3HTTeTPDeDPP (2:1) and P3HTTeTPDeDPP (1:2) [306]. SCLC hole Jsc Voc PolymerePC61BM Eg (optical) mobility (mA/cm2)d (V) (ratio) (eV)b (cm2 V1 s1)c P3HTTeTPDeDPP 1.48 (1:1) (1:1)a P3HTTeTPDeDPP 1.50 (2:1) (1:1.5)a P3HTTeTPDeDPP 1.47 (1:2) (1:2.0)a

1.5 104

15.26

4

11.67

1.9 104

16.37

2.6 10

FF


0.51 0.64 4.93 (5.03) 0.55 0.62 3.94 (4.11) 0.50 0.61 4.92 (4.97)

a Spin-coated from o-DCB and tested after 20 min under N2 before aluminum deposition. b Determined from the onset of absorption in UVevis spectra in thin films. c Measured for neat polymer films. d Mismatch corrected.

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electronic properties in perfectly alternating polymers (“monomer approach”). While the Stille polymerization has proven highly effective for synthesizing a broad range of semi-random polymers, this method also leaves room for improvements that can further simplify the synthesis and target more practical methods for large scale synthesis. Specifically, the Stille method requires monomers containing alkyltin functional groups, which are toxic and frequently unstable. Further, the synthesis of stannylated monomers typically requires a lithiationemetallation step under air-free cryogenic conditions. Eliminating these additional steps and the toxic tin byproducts of polymerization is highly desirable [328]. Along these lines our group has explored an emerging method of CeH activation polymerization, so-called Direct Arylation Polymerization (DArP) [329e332] for the synthesis of semi-random polymers. DArP is generally considered to be a green alternative to Stille polymerization since it does not require preparation of organotin derivatives of monomers, which eliminates toxic waste and reduces the number of synthetic steps. This is very advantageous since many stannylated monomers are unstable and difficult to purify [333]. While the direct arylation of small molecules has been known for several years [334], DArP has emerged only recently and it has been broadly applied to prepare a variety of conjugated polymers. For the synthesis of P3HT via DArP several conditions have been explored [335e339]. The most effective to date have been the conditions developed by Ozawa et al. [339]. The optimized procedure includes Herrmann’s catalyst, cesium carbonate, tris(o-(N,Ndimethyl)anilyl)phosphine and THF as a solvent. The reaction was conducted at 120 C in a pressurized vessel. These are also the conditions that have been used with modifications by Leclerc et al. to prepare a variety of alternating donoreacceptor conjugated copolymers [324,340e343]. In general, toluene was used as a solvent, tris(o-methoxyphenyl)phosphine was used as a ligand and pivalic acid was used as an additive. The reaction temperature was kept at 120 C and the reaction was performed in a pressurized vessel as well. These conditions are relatively complex since together with sophisticated catalysts and ligands, this method requires superheated solvent and a pressurized reaction vessel that may also complicate the scale up of such reactions. More attractive conditions for direct arylation were developed by Fagnou et al. [334,344,345]. The conditions include palladium (II) acetate (Pd(OAc)2) as a catalyst, N,N-dimethylacetamide (DMA) as a solvent, potassium carbonate (K2CO3) as an insoluble inorganic base, and a carboxylic acid that generates carboxylate anion in situ, which acts as a soluble organic base. These conditions allow the reaction to be performed at ambient pressure with inexpensive and bench stable reagents. Therefore, it is easy to conduct and scale up this reaction. The Fagnou conditions have been successfully applied to synthesize various conjugated polymers with high yields and high molecular weights [346e351], although not previously for the synthesis of P3HT. Recently, we have employed this protocol to synthesize P3HT and selected semi-random P3HT analogs and we have compared the properties of these polymers to previously reported Stille polymers [337]. The polymerizations are illustrated in Scheme 6 and the polymer properties are summarized in Table 7. As seen in Table 7, DArP yields polymers with molecular weights similar to those of Stille polymers in most cases. However the properties of DArP polymers were markedly different from Stille polymers. Specifically, the absorption onset and intensity were lower for DArP polymers, as illustrated in Fig. 14. Additionally, the melting and crystallization transitions were observed to be at lower temperatures for DArP P3HT and completely absent for DArP P3HTTeBTD and P3HTTeDPP. These differences are consistent with more disordered polymers characterized by defects induced by the

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Scheme 6. Synthetic routes to semi-random polymers via DArP and Stille methods: (a) P3HTTeBTD, (b) P3HTTeDPP, (c) P3HT, (d) carbon designations in 3-hexylthiophene.

polymerization method. Note that the RR of the P3HT produced by DArP is 88%, whereas Stille gives 93%, indicating an increase in unfavorable linkages in DArP. Furthermore, it is evident that the DArP method leads to so-called b-defects or linkages through the 4position of the thiophene (Scheme 6d). These defects prevent the chains from effective packing and ordering. The possibility of DArP producing b-defects has been previously postulated and corroborated by small-molecule modeling [346] and supported via 1H NMR

Table 7 Properties of polymers prepared by Stille and DArP methods [337]. Polymer

Method

Temperature, C

Mn, kDa

PDI

Yield, %

RR,a %

P3HTTeBTD P3HTTeBTD P3HTTeDPP P3HTTeDPP P3HT P3HT P3HT

DARP Stille DARP Stille DARP DARP Stille

70 95 70 95 70 95 95

19.2 20.5 6.8 13.7 14.3 15.5 15.2

2.4 2.6 3.2 2.6 2.9 2.9 2.6

47 67 53 66 44 39 74

e e e e 88 88 93

a

Regioregularity obtained by 1H NMR measurements.

in a recent report [352]. The presence of an insoluble and likely crosslinked fraction in these initial DArP polymers also supported the presence of b-defects. The formation of b-defects is consistent with the nature of the CeH activation mechanism of DArP. For P3HT and semi-random P3HT analogs there are distinct aromatic protons on the 2bromo-3-hexylthiophene monomer and the growing polymer chain e the more reactive a-proton and the less reactive b-proton (Scheme 6d). The abstraction of the a-proton leads to the favorable linear polymer chain, whereas the abstraction of the b-proton results in the formation of b-defects, as illustrated in Fig. 15. Additionally, with semi-random polymers other aromatic protons on thiophene or even on the DPP acceptor are available for activation. It is clear that the path to optimizing the synthesis of semi-random polymers via DArP requires control over the synthesis of P3HT and the improvement of selectivity of CeH activation. Toward this end we have focused on converging the properties of DArP P3HT and Stille P3HT through optimization of the synthetic protocol, while working to preserve the attractive simplicity of this method. As such, we centered our approach on using the reactivity difference between a- and b-protons of 3-hexylthiophene to minimize


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Fig. 15. Illustration of (a) fragment of P3HT chain without b-defects; (b) fragment of P3HT chain with a b-defect. Reproduced with permission from Ref. [337]. Copyright 2012 Wiley.

Fig. 14. UVevis absorption spectra of DArP and Stille polymers. (a) P3HTTeBTD; (b) P3HTTeDPP; (c) P3HT, where the designations DARP95 and DARP70 refer to the temperature at which the polymerization was conducted (Table 7). Reproduced with permission from Ref. [337]. Copyright 2012 Wiley.

the activation of the b-position, while keeping the a-position active enough to produce a high molecular weight polymer in a reasonable yield. For this purpose, the influence of the reaction temperature and catalyst loading on the properties of P3HT (such as melting and crystallization points, molecular weight, yield and regioregularity) was studied. It was found that lowering the

temperature of the reaction mixture from 120 C to 20 C results in a steady increase of the melting and crystallization points and regioregularity of P3HT, at the same time, however, the molecular weight experienced a decrease [338]. The same effect was observed for the lowering of the catalyst loading from 2 mol% to 0.25 mol%. The best balance of properties was found for P3HT prepared at 70 C with 0.25 mol% of Pd(OAc)2 and 48 h of reaction time, which resulted in 50% yield and an Mn of 16 kDa (PDI ¼ 2.3), an RR of 93% with a Tm of 207 C and a Tc of 178 C. In order to match the properties of DArP P3HT with those of Stille P3HT more closely, we further minimized the reactivity of the sterically hindered b-protons through increasing the bulk of the catalytic system, as illustrated in Scheme 7a. This has been achieved by using a bulkier branched neodecanoic acid (NDA) [353] in place of pivalic acid (PivOH) (Scheme 7b). The optimized conditions and the properties of P3HT are summarized in Scheme 8. As a result of the extensive optimization of DArP parameters, the properties of DArP and Stille P3HT were closely matched. As such, both methods produced P3HT with reasonable yields (w60e70%), molecular weights (w20 kDa), PDIs (2.7e2.8) and regioregularities (w93%). Furthermore, the melting and crystallization points of the optimized DArP P3HT have been measured to be 217 C and 187 C respectively. These values are similar to those of Stille P3HT, which melts at 214e221 C and crystallizes at 185e186 C [293,304,305,307]. The convergence of the optical properties of DArP and Stille P3HT was also observed. As illustrated by the UVe vis spectra of DArP and Stille P3HT in Fig. 16, the absorption onset and intensity of the optimized DArP P3HT are remarkably similar to those of Stille P3HT, which themselves differ greatly from those of unoptimized DArP P3HT (DArP95). These results demonstrate the potential of simple and attractive DArP conditions for the synthesis of alkylthiophene-based conjugated polymers with high quality. It is clear that DArP opens an avenue to further simplifying the synthesis of semi-random polymers. Taken as a whole, the efforts toward simple polymer design discussed in the preceding sections demonstrate that the “polymer approach” to optimizing the performance of BHJ solar cells offers great potential and maintains consistency with the central vision of OPV as a simple and inexpensive technology.

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Scheme 7. (a) Hypothetical steric hindrance of b-protons of P3HT chain; (b) chemical structures of pivalic acid (PivOH) and neodecanoic acid (NDA). Reproduced with permission from Ref. [338]. Copyright 2013 Wiley.

4. Ternary blend bulk heterojunction solar cells: a simple architecture with great potential 4.1. Motivation, scope, and background Simple, practical synthesis of conjugated polymers is motivated by the desire to maintain consistency with the vision of OPV as a low-cost platform for solar energy conversion. However, this approach must also be balanced with the necessity of moving to higher efficiencies that are required for a viable technology. As noted in Section 2, the very best polymerefullerene solar cells reported to date are only 9.2% efficient [30]. By most measures, this is still too low for practical application [13]. Tandem cells [33,45e50] provide a clear path to higher efficiency, with efficiencies of 10e12% already demonstrated for various OPV technologies [28,32,50,354,355]. Specifically, polymerefullerene based tandem cells are reported to achieve 10.6% efficiency [28]. The increase in efficiency from 9.2% to 10.6% when moving from a single junction polymerefullerene BHJ solar cell to a tandem cell is however accompanied by a significant increase in the complexity of

Scheme 8. Reaction conditions and properties of P3HT: (a) unoptimized DArP P3HT (DArP95); (b) optimized DArP P3HT; (c) Stille P3HT.

Fig. 16. The UVevis spectra of unoptimized DArP95 P3HT (black curve), optimized DArP P3HT (red curve), and Stille P3HT (blue curve).

device design and fabrication. Fig. 17a illustrates typical tandem cell architectures. One obvious drawback of tandem cells is the inability to process the active layer in a single processing step as in the case of binary blend BHJ solar cells [33,45e50]. Furthermore, an intermediate recombination layer, which is deposited in yet another additional processing step, requires a specific set of electro-optical properties in order to efficiently recombine and transport charges [356,357]. Nonetheless, tandem cells offer the potential to move toward a practical efficiency limit of 14e15% [45,46,151,152,154,155,358], while the practical efficiency of single junction polymerefullerene cells is only 10e12%, as dictated by the JsceVoc compromise as detailed in Section 2 [14,40,42e44,116,150e 155]. The balance of potential for higher efficiency with the cost of increased complexity of device fabrication with tandem cells presents a strategic challenge in the field of OPV. A route promising efficiency competitive with tandem cells that does not sacrifice the attractive simplicity of single active-layer devices is highly desirable. When trying to conform the concept of tandem cells to the attractive simplicity of single layer solar cells, it is useful to understand the benefits of tandem cells. First, tandem cells provide a broader, more effective harvesting of the solar spectrum when two (or more) layers with complementary absorption are used [28,31,33,45e50]. Further, when the sub-cells are connected in series (as is most common) the Voc can approach the sum of the Voc values for the individual sub-cells [28,31,33,45e50]. As such, an enhancement in photon harvesting and Voc leads to the potential for higher efficiencies than with single-junction cells. As a potential alternative to tandem cells, ternary blends have been explored by many groups over the last decade. A ternary blend device is a single active layer device illustrated schematically in Fig. 17b and analogous to the typical BHJ solar cell shown in Fig. 1. In this context, a ternary blend is composed of a combination of three distinct electron donor and acceptor species in which at least two of the components combine to provide a synergistic broadening of the absorption spectrum. The pairing of components can take on a variety of identities with either two donors and an acceptor [359e 383], two acceptors and a donor [359,384e402], or a donor, acceptor, and intermediary species [199,359,383,403e438]. Additionally, polymers, fullerenes, discrete molecules, and inorganic nanocrystals have been used in all combinations and in many cases a broadening of the absorption spectrum has been observed to translate into a broadened EQE response and higher Jsc than in any limiting binary blend combination of the constituent components [360,362e364,367,368,376,380,381,388,401,421,422,439,440].


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Fig. 17. (a) Tandem solar cells in standard and inverted configuration. (b) Ternary blend BHJ solar cells. (c) The proposed origin of the open-circuit voltage pinning in ternary blend BHJ solar cells. Reproduced with permission: (a) from Ref. [50], (c) from Ref. [359].

However, in most reported ternary blends the FF is observed to be highly composition dependent, where only small amounts of the third component causes sharp decreases in the FF [381,401,403,407,411,421]. Further, a generally (but not exclusively) observed phenomenon is that the Voc of ternary blend solar cells is pinned to the lesser of possible values based on the limiting binary blends [363,364,369,370,376,388,404e406,414,417,420]. The Voc pinning can be rationalized with a simple HOMOeLUMO energy diagram, illustrated schematically in Fig. 17c for the case of a system based on two donors and one acceptor. Conceptually, after exciton dissociation, holes formed in either donor HOMO will ultimately come to reside in the highest-lying HOMO via an energetically favored charge transfer between the two donors. As a consequence, the highest-lying donor HOMO energy level becomes a Voc determining level, pinning the Voc to the smallest of the two limiting potential values [364]. The latter feature concerning the Voc presented the strongest barrier for ternary blends as a potentially simplified analog of tandem cells. If the voltage is necessarily pinned to the lesser value, diminishing voltages with increasing absorption breadth (decreasing polymer band gap) would render little advantage over binary blends. However, a sufficient number of scattered reports existed prior to 2011 in which ternary cells displayed intermediate values of the Voc relative to the limiting binary blends to warrant further investigation into the potential to move beyond the smallest Voc while still broadening the photocurrent response [368,390,395,403,411]. Despite the historical limitations of ternary blends, the platform does offer the enticing potential of retaining the simplicity in device fabrication of BHJ solar cells (binary blends) while introducing the potential of multiple donoreacceptor interactions found in tandem cells to improve light harvesting and achieve values of the Voc higher than that of the lowest limiting value. In order to validate the potential of this platform our goal has been to understand the nature of the Voc in ternary blends.

if and how the Voc could be modulated by mixing of multiple electronically complementary components without negatively influencing charge transport. Toward this end, we designed a three component system in order to probe the composition dependence of the Voc in ternary blend BHJ solar cells [384]. The model system is based on P3HT as the donor and two soluble fullerene acceptors, PC61BM and ICBA (Fig. 18). This model system was chosen for several reasons. First, the P3HTePC61BM binary blend is the most widely studied polymerefullerene pair [249]. Second, the P3HTe ICBA BHJ solar cell can be optimized under the same conditions of solvent, concentration, polymerefullerene ratio, and annealing conditions as for P3HTePC61BM to give solar cells with equally high FF [184,272]. This enables the fabrication of ternary blends across the entire composition regime where the only variable is the ratio of the two acceptors. Importantly, the difference in the LUMO energies of the two fullerenes [44,441,442] is sufficient to give nearly 250 mV difference in the Voc of the limiting binary blends [184,272].

4.2. Tuning the open-circuit voltage (Voc) in ternary blend solar cells 4.2.1. Polymer:fullerene:fullerene ternary blends Prior to 2011 it had not been established if it was possible to generate ternary blend solar cells with intermediate Voc and high FF across a broad composition regime. Further, it was not understood

Fig. 18. Structures and corresponding HOMO and LUMO energy levels of P3HT, ICBA and PC61BM. Reproduced with permission from Ref. [384]. Copyright 2011 American Chemical Society.

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To test the model system, the overall polymerefullerene ratio was kept constant at 1:1, while the ratio between PC61BM and ICBA was varied with a 10% increment. Each P3HT:PC61BM:ICBA ternary blend solar cell was processed with individually optimized annealing time, but with a constant temperature of 150 C with the purpose of achieving optimal device performance while the active layer thickness was kept constant at w100 nm for all three component compositions. The data for the series of solar cells is shown in Table 8. The most significant observations from Table 8 are that the Voc is regularly increasing from 0.605 V to 0.844 V as the amount of the ICBA in the ternary blend BHJ solar cell is increased (illustrated in Fig. 19) and that the FF remains above 0.57 for all compositions despite measurement in air. This signifies the first evidence of the composition-dependent tunability of the Voc in ternary blend BHJ solar cells, and furthermore establishes that high FF can be achieved across the entire composition range of a ternary system. This proves that the Voc is not necessarily pinned to the smallest Voc of the corresponding binary blends and that with careful selection of donor and acceptor materials efficient ternary blend BHJ solar cells are possible. Importantly, the high FF (above 0.57) obtained for each P3HT:PC61BM:ICBA ternary blend solar cell, is attributed to a balanced and trap free charge transport through the bulk [141,236,237,270,443,444], and favorable morphology [69e72] and was supported by transmission electron microscopy (TEM) images. As such, charge separation and transport do not appear to be hindered in the ternary blend solar cells. While the Voc is composition dependent and the FF is consistently high, the Jsc of the ternary blend system decreases as the amount of ICBA in the ternary blend solar cell is increased. This is explained based on the significantly lower absorption coefficient of ICBA in the visible part of the solar spectrum with respect to PC61BM and since the active layer thicknesses were kept constant for all ternary blend solar cells, the photoresponse (EQE values) decreases as the ICBA amount in the three component system is increased, as represented in Fig. 20. To verify the possibility of achieving higher Jsc in the case of high contents of ICBA, increased film thicknesses of P3HT:PC61BM:ICBA photovoltaic devices at 1:0.5:0.5 (137 nm) and 1:0:1 (174 nm) ratios showed improved Jsc, FF and h of 9.82 mA/cm2, 0.59, 3.92% and 9.23 mA/cm2, 0.59, 4.55%, respectively, with essentially no change in the Voc at 0.682 V and 0.839 V, relative to the devices reported in Table 8, thus proving the Table 8 Photovoltaic properties of P3HT:PC61BM:ICBA ternary blend BHJ solar cells at different fullerene ratiosa [384]. P3HT:PC61BM:ICBA c

1:1:0 1:0.9:0.1d 1:0.8:0.2d 1:0.7:0.3e 1:0.6:0.4f 1:0.5:0.5g 1:0.4:0.6e 1:0.3:0.7h 1:0.2:0.8d 1:0.1:0.9d 1:0:1h

Jsc (mA/cm2)

Voc (V)b

FF

h (%)

9.90 9.22 9.11 8.58 8.31 8.27 8.18 8.14 8.19 8.18 8.23

0.605 0.618 0.631 0.649 0.669 0.688 0.709 0.741 0.769 0.804 0.844

0.60 0.59 0.57 0.58 0.58 0.57 0.57 0.57 0.59 0.60 0.58

3.57 3.29 3.28 3.22 3.11 3.18 3.22 3.34 3.69 3.91 3.98

a Devices were spin-coated from CB and after aluminum deposition annealed at 150 C under N2 for the specified times. b Standard deviations of less than 0.005 were observed in all cases for averages over eight pixels. c 60 min. d 20 min. e 40 min. f 30 min. g 50 min. h 10 min.

Fig. 19. Open-circuit voltage (Voc) of the P3HT:PC61BM:ICBA ternary blend BHJ solar cells as a function of the amount of ICBA in the blends. Reproduced with permission from Ref. [384]. Copyright 2011 American Chemical Society.

possibility of simultaneously achieving high Jsc and FF and tunable Voc in the ternary blend BHJ solar cells. Another important observation made from this study was that important properties of P3HT remained the same at all P3HT:PC61BM:ICBA ratios. As can be seen in Fig. 21a, the vibronic shoulder at 600 nm in the absorption spectrum, which is attributed to the interchain vibrational absorption induced by a high degree of ordering and strong interchain interaction [256,257,445], is present for all P3HT:PC61BM:ICBA blends. Furthermore, the degree of P3HT crystallinity, measured using GIXRD (Fig. 21b), was found to be similar with the corresponding interchain distance (100) for P3HT in the range of 16.4e16.7 A. As such, the ability to retain semicrystallinity in P3HT is not hindered in the ternary blends. The results with this model ternary system established several important concepts, indicating that the Voc in ternary blend BHJ solar cells is not limited to the smallest Voc of the corresponding binary blend solar cells, but can be varied between the extreme Voc values without detriment to the FF or Jsc. By extension, these results suggest a route to enhance the efficiency of single layer BHJ solar cells through the judicious selection of donor and acceptor components with complementary absorption characteristics and suitably aligned energy levels to allow broad photon harvesting (high Jsc) with a Voc larger than that dictated by the component with the smallest band gap. As such, a higher Voc should be achievable for a given Jsc than with binary blends, giving a path to alleviate the JsceVoc compromise that limits the efficiency of polymerefullerene BHJ solar cells. 4.2.2. Polymer:polymer:fullerene ternary blends Despite the conceptual step forward achieved for ternary blend BHJ solar cells, the model system described above, based on a polymer donor and two fullerene acceptors, does not enable an overall efficiency increase of the ternary blend solar cells beyond the corresponding binary blend solar cells due to the lack of increase of the Jsc, since all three components absorb the same part of the solar spectrum (Fig. 20a) [384]. Toward overcoming this limitation, a transition to ternary systems based on a single fullerene acceptor and two mutually effective donor polymers exhibiting complementary absorption properties and energy levels was pursued [360]. For this purpose, the model ternary blend system (Fig. 22) of two donor polymers, high band gap poly(3-hexylthiophene-co-3(2-ethylhexyl)thiophene) (P3HT75-co-EHT25) [293] and low band gap poly(3-hexylthiophene-thiopheneediketopyrrolopyrrole)


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Fig. 20. (a) UVevis absorption spectra of thin films spin-coated from chlorobenzene (CB) and annealed at 150 C under N2 for 20 min, where (i) is P3HT (black line), (ii) is PC61BM (red line) and (iii) is ICBA (blue line). (b) External quantum efficiency of the ternary blend BHJ solar cells based on P3HT:PC61BM:ICBA at different ratios: 1:1:0 (red squares), 1:0.5:0.5 (green circles), 1:0:1 (purple triangles) at thicknesses 95e105 nm presented in Table 8. Reproduced with permission from Ref. [384]. Copyright 2011 American Chemical Society.

Fig. 21. (a) UVevis absorption spectra of thin films spin-coated from chlorobenzene (CB) and annealed at 150 C under N2 for 20 min with P3HT:PC61BM:ICBA ratios: (i) is 1:1:0 (red line), (ii) is 1:0.8:0.2 (blue line), (iii) is 1:0.5:0.5 (green line), (iv) is 1:0.2:0.8 (black line) and (v) is 1:0:1 (purple line). (b) Grazing-incidence X-ray diffraction of thin films of P3HT:PC61BM:ICBA spin-coated from chlorobenzene (CB) and annealed at 150 C under N2 for 20 min, where (i) is 1:1:0 (red line), (ii) is 1:0.2:0.8 (blue line), (iii) is 1:0.5:0.5 (green line), (iv) is 1:0.8:0.2 (black line) and (v) is 1:0:1 (purple line). Reproduced with permission from Ref. [384]. Copyright 2011 American Chemical Society.

(P3HTTeDPP-10%) [305], and PC61BM as an acceptor was used. The selected donor polymers together with PC61BM provide broad and uniformly strong absorption from 300 to 830 nm. Importantly, the HOMO levels of the two polymers are different, with the larger band gap P3HT75-co-EHT25 showing a lower HOMO of 5.4 eV [293] to enable Voc tuning and target a larger Voc in the ternary blend than is achievable with the lower band gap polymer P3HTTeDPP-10% (HOMO ¼ 5.2 eV) [305] alone. The differences in HOMO energy of the two polymers lead to a Voc of 0.675 V and 0.574 V in the corresponding binary blends as seen in Table 9. Furthermore, high efficiencies with high FF were obtained in both binary blend polymerePC61BM solar cells [293,305]. Finally, the similarity in chemical compositions of both, the semi-random [304e308] P3HTTeDPP-10% and the random [293] P3HT75-co-EHT25, which are P3HT analogs containing 80% and 75% of 3-hexylthiophene repeat units, respectively, ensures a consistency of properties ideal for analyzing ternary blend behavior in the context of a “P3HTePC61BM” model system and allows the use of similar processing conditions at each composition. As seen in Table 9, P3HTTe DPP-10%ePC61BM is found to be optimal at 1:1.3, while P3HT75-co-

EHT25 is optimal at 1:0.8. As such, another opportunity of this system was to investigate the dependence of the Voc on composition in a ternary blend system where the limiting binary blends do not give optimal performance at the same polymerefullerene ratio. Importantly, the polymers used for this study were synthesized based on the principles of the “polymer approach” and provide simple components for a simple device architecture. Ternary blend BHJ solar cells were optimized at each polymere polymer ratio to obtain the highest efficiencies. Optimal processing conditions include slow solvent evaporation (solvent annealing) from the P3HTTeDPP-10%:P3HT75-co-EHT25:PC61BM blends after spin-coating and prior to aluminum deposition. Since ternary blend solar cells contain more variables that can be tuned, the optimization of the three component system included an increase in the number of cells needed to achieve optimal device performance while keeping the same optimization steps as for the binary blend solar cells [22,25,26,446]. In this study, the thickness of the active layer of the ternary cells was also kept constant at w85e90 nm, and thus efforts were not made to increase Jsc beyond the complementary properties of the components.

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P.P. Khlyabich et al. / Polymer 54 (2013) 5267e5298 Table 9 Photovoltaic properties of P3HTTeDPP-10%:P3HT75-co-EHT25:PC61BM ternary blend BHJ solar cells at optimized ratios [360]. P3HTTeDPP-10%:P3HT75-co-EHT25:PC61BM Jsc (mA/cm2)e Voc (V)f FF

h (%)

1:0:1.3a 0.9:0.1:1.1b 0.8:0.2:1.0b 0.7:0.3:1.0c 0.6:0.4:1.0c 0.5:0.5:0.9a 0.4:0.6:0.9a 0.3:0.7:0.8a 0.2:0.8:0.8a 0.1:0.9:0.9a 0:1:0.8d

5.07 5.51 5.37 4.15 4.12 4.00 3.74 3.64 3.27 3.10 3.16

14.38 15.05 14.60 11.54 11.19 10.89 10.19 9.77 8.57 8.25 7.96

0.574 0.603 0.608 0.614 0.619 0.622 0.626 0.633 0.639 0.646 0.675

0.62 0.61 0.61 0.59 0.59 0.59 0.59 0.59 0.60 0.59 0.59

All devices were spin-coated from o-dichlorobenzene (o-DCB) and placed under N2 before aluminum deposition for: a 30 min. b 60 min. c 45 min. d 20 min. e Mismatch corrected [320]. f Standard deviations of less than 0.005 were observed in all cases for averages over eight pixels.

Fig. 22. Structures, corresponding HOMO energy levels and absorption profiles of PC61BM (black line), P3HT75-co-EHT25 (red line) and P3HTTeDPP-10% (green line). Reproduced with permission from Ref. [360]. Copyright 2012 American Chemical Society.

The data in Table 9 demonstrates that the Voc is composition dependent and that the FF is uniformly high across the entire composition range at 0.59. Complementary absorption of the two polymers leads to an increase in Jsc relative to the binary blends for 0.9:0.1:1.1 and 0.8:0.2:1.0 ratios and efficiencies reach 5.51% and 5.37%, respectively, which is an increase by 0.44% and 0.30% over 5.07% with P3HTTeDPP-10%ePC61BM. Furthermore, at all other three component combinations, except 0.1:0.9:0.9 (where the Jsc increase is smaller than the Voc decrease, relative to 0:1:0.8), power conversion efficiencies are higher than for the binary P3HT75-coEHT25ePC61BM solar cell. This observation supports ternary blend BHJ solar cells as an effective way to overcome the efficiency limits of the corresponding binary blend solar cells. Another important observation from Table 9 is that the amount of PC61BM necessary for optimal device performance decreases as the amount of P3HT75-co-EHT25 in the three component system is increased and the overall polymerefullerene ratio of the individually optimized ternary blend solar cells tracks regularly with the P3HTTeDPP-10%:P3HT75-co-EHT25 ratio. Further analysis of the Voc evolution in these optimized ternary blends reveals a different trend of the composition dependence than observed in the earlier studies with one donor polymer and two acceptors, as can be see by comparing Figs. 19 and 23a [384,439]. In the present case, upon introduction of the second polymer component into either limiting polymerefullerene binary blend, the Voc rapidly changes by 29 mV. After this, for the ternary blend composition regime, the Voc monotonically increases from 0.603 V to 0.646 V as the amount of P3HT75-co-EHT25 is increased. Fig. 23b highlights the importance of individual optimization of ternary blends at each polymerepolymer ratio through a comparison of the data set subjected to individual optimization of overall

fullerene content at each polymerepolymer ratio with two data sets where the overall polymerefullerene ratio is kept constant at 1:1.1 and 1:1.0. A significant decrease of the Voc and deviation from linearity in the ternary blend composition regime is observed in the cases of fixed ratio. Specifically, in the case of fixed overall polymerePC61BM ratio at 1:1.1, while a linear dependency of the Voc on the composition is observed across most of the ternary regime, a deviation is observed for the case of 10% of P3HT75-co-EHT25, which is an optimal overall polymerefullerene ratio. Across the remaining composition range, the Voc values are significantly lower than for individually optimized ternary blends. For the fixed overall polymerePC61BM ratio of 1:1.0, non-linear behavior is observed for the Voc across a more significant portion of the ternary regime. Here, linear behavior at P3HT75-co-EHT25 loadings below 50% is followed by a saturation regime with almost constant Voc up to 80%, before an increase at higher P3HT75-co-EHT25 loadings. Furthermore, the Voc decrease and deviation from linearity in the case of not individually optimized ternary blend solar cells is accompanied by a significant decrease of Jsc, FF and overall efficiency [360]. This demonstrates that individual optimization of the overall composition at each polymerepolymer ratio is necessary for achieving maximum efficiency in ternary blend solar cells and upon the introduction of a donor polymer with lower-lying HOMO, a significant increase in the Voc can be achieved even at small amounts if the solar cell parameters are optimized. Referring to Fig. 23a, the Jsc of the individually optimized ternary blend solar cells increases as the amount of P3HT75-co-EHT25 in the three-component system increases up to 20% followed by steep decrease upon further loadings. This is explained based on the complementary absorption of the ternary blend systems as can be seen in Fig. 24a, while maintaining a constant film thickness. The dilution of P3HTTeDPP-10% polymer leads to the decrease of the absorption intensity in the near-IR, while absorption in the visible part of the solar spectrum increases. As a result, the threecomponent system absorbs fewer photons when the amount of P3HT75-co-EHT25 in the system is above 30%, thus decreasing Jsc. The same behavior is mirrored in the EQE response of the ternary blend BHJ solar cells shown in Fig. 24b. Significantly, the introduction of only 10% of P3HTTeDPP-10% in the three component system leads to the noticeable photoresponse in the 700e800 nm range. High FF values were obtained at all ternary blend BHJ solar cell compositions and are attributed to a balanced and trap free charge


Fig. 23. (a) Open-circuit voltage (Voc) (black squares e left axis) and short-circuit current density (Jsc) (red circles e right axis) of the individually optimized ternary blend BHJ solar cells from Table 9 with different fraction of the polymer P3HT75-coEHT25 component in the blends. (b) Open-circuit voltage (Voc) of the individually optimized ternary blend BHJ solar cells (open squares), with fixed overall polymere PC61BM ratio at 1:1.1 (blue stars) and with fixed overall polymerePC61BM ratio at 1:1.0 (green triangles). Reproduced with permission from Ref. [360]. Copyright 2012 American Chemical Society.

transport through the bulk [236,237,270,443,444] and favorable morphology [69e72]. Hole mobilities of the three component systems measured using the SCLC technique [309] were found to be on the same order of magnitude as for binary blend systems with a constant increase as the amount of P3HTTeDPP-10% in the ternary system was increased as shown in Table 10. An important point about the mobility is that either polymer alone displays a hole mobility on the order of 1e2 104 cm2 V1 s1, while all fullerene blends across the ternary regime show values in the range of 1e 3 103 cm2 V1 s1. The increase in hole mobility upon blending with fullerenes is a highly attractive quality of conjugated polymers in that fullerenes do not disrupt the ability of the polymers to effectively transport charge [117,218,220,447]. Taken as a whole, high FF and hole mobilities suggest no increase in recombination or charge trapping in the ternary blends. TEM images in Fig. 25 show similar bicontinuous blends with nanometer length-scale phase separation at different P3HTTeDPP-10%:P3HT75-co-EHT25:PC61BM ratios. As a result, introduction of the third component in the blend does not change the qualitative nature of the overall polymere fullerene morphology, suggesting no negative impact on charge transport through the ternary blends. As in the case of P3HT in P3HT:PC61BM:ICBA ternary blends [384], many properties of P3HT75-co-EHT25 and P3HTTeDPP-10% are preserved in the ternary blend systems. From the UVevis absorption in Fig. 24a, it can be seen that the absorption spectrum at

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Fig. 24. (a) UVevis absorption spectra of thin films spin-coated from o-dichlorobenzene (o-DCB) and placed under N2 for 30 min with P3HTTeDPP-10%:P3HT75-coEHT25:PC61BM ratios, where (i) is 1:0:1.3 (red line), (ii) is 0.9:0.1:1.1 (green line), (iii) is 0.8:0.2:1.0 (blue line), (iv) is 0.7:0.3:1.0 (cyan line), (v) is 0.5:0.5:0.9 (wine-red line), (vi) is 0.3:0.7:0.8 (dark yellow line), (vii) is 0.1:0.9:0.9 (yellow line) and (viii) is 0:1:0.8 (black line). (b) External quantum efficiency of ternary blend BHJ solar cells, where (i) is 1:0:1.3 (red line), (ii) is 0.9:0.1:1.1 (green line), (iii) is 0.8:0.2:1.0 (blue line), (iv) is 0.7:0.3:1.0 (cyan line), (v) is 0.6:0.4:1.0 (magenta line), (vi) is 0.5:0.5:0.9 (wine-red line), (vii) is 0.4:0.6:0.9 (olive line), (viii) is 0.3:0.7:0.8 (dark yellow line), (ix) is 0.2:0.8:0.8 (purple line), (x) is 0.1:0.9:0.9 (yellow line) and (xi) is 0:1:0.8 (black line). Reproduced with permission from Ref. [360]. Copyright 2012 American Chemical Society.

each ternary composition reflects a simple weighted sum of the absorption spectra of the individual components. Moreover, vibronic shoulders [256,257,445] for both polymers are present in the UVevis absorption for the P3HTTeDPP-10%:P3HT75-coEHT25:PC61BM ternary blends, supporting the retention of crystallinity in both polymer components, which has been verified via GIXRD. Clearly the polymers retain their semi-crystalline nature in binary and ternary blends as seen in Fig. 26 and for a specific case of the 0.8:0.2:1.0 ratio, two peaks are present in the GIXRD profile. The interchain distances for P3HTTeDPP-10% and P3HT75-co-EHT25 change in opposite directions as the amount of the P3HT75-coEHT25 in the ternary blends increases. The interchain distance for P3HTTeDPP-10% increased from 15.27 A to 15.41 A going from 1:0:1.3 to 0.8:0.2:1.0, while P3HT75-co-EHT25 packs more tightly with interchain distances decreasing from 16.69 A to 16.02 A for 0:1:0.8 and 0.8:0.2:1.0. The ability of both polymers to remain semi-crystalline in the ternary blends should be a contributing factor for the high Jsc observed in the ternary blend solar cells.

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Table 10 Average hole mobilities of neat P3HTTeDPP-10%, P3HT75-co-EHT25 and P3HTTe DPP-10%:P3HT75-co-EHT25:PC61BM blends at different ratios in thin films spincoated from o-DCB [360]. P3HTTeDPP-10%:P3HT75-co-EHT25:PC61BM

Mobility (cm2 V1 s1))

P3HTTeDPP-10% 1:0:1.3 0.9:0.1:1.1 0.8:0.2:1.0 0.7:0.3:1.0 0.5:0.5:0.9 0.3:0.7:0.8 0.1:0.9:0.9 0:1:0.8 P3HT75-co-EHT25

2.30 3.23 2.38 2.37 2.19 2.13 2.01 1.92 1.11 1.39

104 103 103 103 103 103 103 103 103 104

As a result of thorough optimization of ternary blend solar cells, efficiencies higher than for corresponding binary blend solar cells were obtained in this system. The results of this work support the growing evidence that ternary blend BHJ solar cells are an effective strategy toward more efficient organic photovoltaics manufactured in a single active layer processing step with the possibility to exceed power conversion efficiency limits for binary blend solar cells. The results presented here also demonstrate that judicious choice of paired components must be accompanied by careful optimization of film composition and processing if the potentially paradigmshifting nature of this platform is to be realized. Research on ternary blends has increased dramatically since 2011, with a number of groups focusing on a variety of ternary systems. Spectral broadening of the photoreseponse has been observed in a number of cases [362,363,376e378,380,381,420e 422] and a tunable Voc has been reported in a lesser number of cases [388,393,397,398,401,402]. Within the context of polymere fullerene based ternary systems, the work of Brabec et al. [363,380,381] and You et al. [362] has been especially noteworthy. Brabec et al. has explored a range of P3HTePCBM based ternary blends containing low band gap polymers as sensitizers [363,380,381]. An increase in the breadth of the photoresponse has indicated great promise, but a constant Voc and diminishing FF with increasing sensitizer content has limited a push toward higher efficiency. On the other hand, You et al. [362] have achieved the highest efficiency to date in ternary blend solar cells with efficiencies exceeding 7% using two polymers with complementary absorption and PC61BM as the acceptor, as will be described in more detail in the following sections.

Fig. 26. Grazing-incidence X-ray diffraction of thin films where (i) is 1:0:1.3 (red line), (ii) is 0.9:0.1:1.1 (green line), (iii) is 0.8:0.2:1.0 (blue line), (iv) is 0.7:0.3:1.0 (cyan line), (v) is 0.5:0.5:0.9 (wine-red line), (vi) is 0.3:0.7:0.8 (dark yellow line), (vii) is 0.1:0.9:0.9 (yellow line) and (viii) is 0:1:0.8 (black line). Reproduced with permission from Ref. [360]. Copyright 2012 American Chemical Society.

While a strong focus is certainly on targeting higher efficiencies with ternary blend systems, a central focus has become the disambiguation of the mechanism or mechanisms of operation in these devices. A deeper insight into the mechanism will allow more tailored design of components and a realization of the potential of this platform. 4.3. A mechanistic model for the tunable Voc in ternary blend solar cells A number of conceptual models for the operation of ternary blend organic solar cells have been proposed in recent years [362,364,448]. Most of the models are highly qualitative and are based on conceptual schemes as opposed to rigorous proof that unites an electronic and morphological description of the devices. It is challenging to develop a comprehensive model of ternary blend solar cells for several reasons. First, the model must incorporate the electronic interactions between all three components and account for their role in the charge photogeneration pathway. Second, the

Fig. 25. TEM images of P3HTTeDPP-10%:P3HT75-co-EHT25:PC61BM at (a) 1:0:1.3, (b) 0.9:0.1:1.1, (c) 0.8:0.2:1.0, (d) 0.5:0.5:0.9, (e) 0.2:0.8:0.8 and (f) 0:1:0.8 for BHJ solar cells presented in Table 9 (scale bar is 50 nm). Reproduced with permission from Ref. [360]. Copyright 2012 American Chemical Society.


mechanism must be able to account for a composition-dependent Voc and must be able to explain cases where the Voc changes regularly with composition and where the Voc does not change. Finally, different mechanisms could operate in different systems. To date, three main models have emerged, including the sensitization/ cascade model (or simply cascade model) [364,383], the parallellike model [362,382], and the organic alloy model [448]. The sensitization/cascade model has been described at length in a recent review [383] and a number of recent publications [363,380,381,449] and is illustrated schematically in Fig. 27a. The cascade model assumes that the HOMO and LUMO energy levels of the three components in the blend must be positioned in a way to allow cascade exciton dissociation and charge transfer. Energy transfer from the large band gap donor to the smaller band gap donor can operate as a means of exciton cascade. Also, depending on the specifics of the system, an exciton can dissociate at either donor:acceptor interface or the interface of the two donors. The created charges then can either migrate through the corresponding donor or acceptor phase or transfer to another donor if the energy level alignment is suitable. While this mechanism may be operational in certain systems [363,380,381], the model not only fails to account for a tunable Voc, but indeed predicts that the Voc should be invariant and pinned to the lowest possible value, as described previously relative to Fig. 17c [364,383]. It is evident that a tunable Voc is the key, enabling element driving ternary blends toward

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higher efficiencies than binary blends. As such, this model and systems that potentially operate under this model will not be considered further here. The latter two models both account for a tunable Voc and are thus most consistent with the properties that render ternary blends most attractive. The parallel-like model (Fig. 27b) proposed by You et al. [362,382], describes charge transport in ternary blend solar cells as if distinct polymer phases are connected in parallel within a blend. This model dictates that the two polymers must be totally isolated from one another in the ternary blend, must both form an effective and continuous interface with the common fullerene acceptor, and must both form effective contacts with the same hole-collecting anode. This geometry is proposed to lead to two distinct hole-collecting pathways in which charge or energy transfer between the two polymers is prohibited. This model is attractive in its conceptual simplicity and in the ease with which it is viewed analogously to a tandem cell [33,45e50], in which two types of sub-cells can be envisioned for both platforms. Not surprisingly, several reports use this model to establish equivalentcircuit models for ternary blend solar cells [398,400,450]. However, this model has several shortcomings. First, there is absolutely no proof of the morphological structure that is proposed as necessary for this mechanism of operation. Second, the polymer pairs (DTffBT/DTPyT and TAZ/DTBT) that were used by You et al. [362] to advance this model are of remarkably similar chemical structure

Fig. 27. (a) Sensitization/cascade model. (b) Parallel-like model. (c) Polymer pairs (DTffBT/DTPyT and TAZ/DTBT) used in the ternary blend BHJ solar cells proposed to operate under a parallel-like mechanism. Reproduced with permission: (a) from Ref. [382], (b) from Ref. [383].

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(Fig. 27c). While the mixing of polymers is generally thermodynamically unfavorable [451], it is hard to imagine that in the kinetically trapped morphology of a polymerefullerene blend that these pairs of polymers would be totally immiscible and totally lacking in shared interface. As a case-in-point, P3HT and PCPDTBT (Fig. 4b) studied by Brabec et al. [364] in ternary blends with PC61BM displayed evidence of direct polymerepolymer electronic interaction (charge transfer) [452], despite structures that are significantly more dissimilar than either pair of polymers in Fig. 27c. As such, it seems likely that the parallel-like model is not more than a conceptualization that fails to unite the physical morphology and the electronic output of the solar cells. However, further detailed characterization is absolutely required to probe and unite the morphology and electronics of these devices. The third model proposed for the mechanism of operation in ternary blends is the so-called alloy model. This model was proposed by Street et al. [448] after analysis of the compositiondependent Voc for the model system (Fig. 18) presented in Fig. 19. This model is named based on an analogy to inorganic semiconductor alloys in which the band gaps (as well as the valence band and conduction band energies) are known to vary regularly as a function of composition in the alloy [453e457]. Consider the example of In1xGaxN [453], shown in Fig. 28a, where the band gap is observed to decrease regularly as the ratio of In and Ga is varied. The band gaps of inorganic alloys (A1xBx or A1xBxC) are known to evolve with composition (x) according to the general quadratic expression in Equation (1), which is an extension of Vegard’s law where E(x) is the energy of the alloy band gap, EA and EB are the band gaps of the unalloyed individual materials and b is the socalled bowing parameter [453,456,458,459].

EðxÞ ¼ ð1 xÞEA þ xEB bxð1 xÞ

(1)

Fig. 28. (a) Evolution of the band gap of In1xGaxN alloy with composition. (b) Valence and conduction band energies as a function of In1xGaxN alloy composition (band alignments are expressed relative to GaN as a function of indium content). Reproduced with permission from Ref. [453]. Copyright 2010 American Institute of Physics.

The evolution of the alloy band gap can be traced to regular variations in the valence band and conduction band energies, as illustrated in Fig. 28b for In1xGaxN. The individual evolution of valence band and conduction band energies can also be described as a function of composition in the manner of Equation (1) [457]. The origin of the analogy with ternary blends is based on closer examination of the implications of Figs. 18 and 19. In Fig. 19, it is evident that the Voc for the binary blend P3HT:ICBA is the highest among all the data points. In the simple picture for Voc (Fig. 2), this is rationalized as due to the large HOMODeLUMOA offset. After addition of 10% of PC61BM to give a 90:10 mixture of the two acceptors, the Voc decreases. From the simple HOMODeLUMOA offset model of the Voc, this implies that effectively, the LUMOA has decreased due to the addition of 10% of PC61BM with its lower lying LUMO. As more PC61BM is added, the Voc decreases regularly and proportionally to the amount of PC61BM added. This implies conceptually that the combination of the two acceptors and their relative ratio determines the effective LUMOA for the acceptor combination. Analogous to inorganic alloys, this suggests that the composition of PC61BM1xICBAx determines the position of LUMOA of the fullerene alloy (analogous to the conduction band edge) and thus the Voc considering the constant donor and HOMOD of P3HT. The alloy model therefore implies that the Voc in ternary blends is tunable due to the synergistic function of the two acceptors in the case of the donor:acceptor:acceptor system or the two donors in the case of donor:donor:acceptor systems. The alloy model earns its name as an analogy to the electronic properties of inorganic semiconductor alloys, but does not imply any type of structural analogy as the two types of materials systems are clearly distinct. While the variation in electronic structure with composition is well-established and firmly rooted in structural variation for inorganic alloys [453e457], the concept initially appears counterintuitive for organics. In the case of organic molecular mixtures, the separate energy levels of the two species are generally expected to be preserved in the blend. Indeed, polymer blends generally have an optical absorption that is the weighted sum of the two materials rather than characteristic of a uniform material of average composition [360,362,363,404,414,417]. To probe this concept of organic alloy formation the first aim was to study this phenomenon at a deeper level than simply via the Voc of the solar cells. At an electronic level, the model implies the LUMOA in the two-acceptor system will be the compositionweighted average of the two individual acceptor LUMOs. As such, the energy of the CT state (Fig. 2d) should vary as the acceptor composition is varied in blends with a constant donor. In order to measure the energy of the CT state directly, the photocurrent spectral response (PSR) technique [460] was employed to explore the electronic states of the ternary blend solar cells. PSR measures the optical absorption of those transitions that result in the generation of mobile carriers in the solar cell and therefore provides information about the electronic states. Previous studies have shown that the PSR spectrum at energies below the optical band gap of the separate materials arises from direct excitation at the interface from the donor HOMO to the acceptor LUMO and measures the optical absorption of the heterojunction interface (the interface band gap or CT state) [460,461]. The excitation corresponds to the direct population of the CT state [44,125,136,137,462e 465]. Fig. 29a shows PSR spectra of P3HT:PC61BM:ICBA ternary blends and Fig. 29b shows details of the spectrum at 1.6e1.8 eV with the background subtracted. As can be seen from Fig. 29a, the energy of the CT state, which is extracted between the limits of the pair of dashed lines, increases continuously with composition as the amount of the ICBA in the three component mixture increases. Furthermore, the CT state energy tracks in exactly the same fashion


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Fig. 30. Plot of the estimated CT state energy defined by PSR at photocurrent (PC) values of 0.1 and 0.01 from the data of Fig. 29a (closed symbols) compared to the values of the Voc (open triangles) for P3HT:PC61BM:ICBA ternary blend solar cells. The solid lines are the model of Equation (1) with the same bowing parameter b. Reproduced with permission from Ref. [448]. Copyright 2013 American Chemical Society.

Fig. 29. (a) Photocurrent spectral response (PSR) data for the P3HT:PC61BM:ICBA ternary blend solar cells plotted as a function of ICBA fraction in PC61BM:ICBA pair. The inset indicates the CT transition or interface band gap that is being measured and the pair of dashed lines indicates the range over which the CT state energy is extracted. (b) Expanded plot of the peaks near 1.7 eV in the PSR spectra of (a), with the background subtracted. The peak centered above 1.7 eV corresponds to PC61BM absorption and the peak centered below 1.7 eV corresponds to ICBA. Reproduced with permission from Ref. [448]. Copyright 2013 American Chemical Society.

as the Voc in ternary blend BHJ solar cells as depicted in Fig. 30. Importantly, the composition dependence (PC61BM1xICBAx) of both the Voc (EVoc(x)) and the CT (ECT(x)) state energy can be fit effectively with the same quadratic dependence as represented in the general form of Equation (1), using the same bowing parameter (b) of 0.18. By extension, the origin of the CT state evolution is ascribed to a compositionally dependent LUMOA of the fullerene alloy (PC61BM1xICBAx) that varies according to the same quadratic dependence. This observation also highlights the fact that the Voc in ternary blend solar cells is also CT state energy dependent as is the case for well-known binary polymerefullerene BHJ solar cells [136,142e150]. Moreover, the qVoc is 0.55 eV lower than CT state energy, where q is elementary charge, which correlates well with previous empirical relationships between Voc and the CT state energy for binary blend BHJ solar cells [136,142e150,292,460, 466e470]. The above PSR results provide strong evidence for the alloy model, in which the compositionally dependent LUMOA of the organic alloy PC61BM1xICBAx determines the CT state energy and ultimately the Voc in P3HT:PC61BM:ICBA ternary blends. A direct analogy to the inorganic alloy model would also imply that the band gap of the PC61BM1xICBAx alloy should also vary with composition. However, the presence of two distinct peaks in the

PSR data for the P3HT:PC61BM:ICBA system presented in Fig. 29b, where the absorption peak positions of PC61BM and ICBA differ by about 0.045 eV with ICBA having the lower energy peak, confirms that molecular excitations (optical band gap) in the ternary blend are the weighted sum of the two peaks of the pure materials, rather than a single peak at a compositionally averaged energy. Thus, the alloy model of Equation (1) does not apply to the optical excitation of the bulk acceptor material. The apparent contradiction of compositionally averaged (alloyed) frontier orbital energies and the independent optical band gaps of components is also observed in the polymer:polymer:fullerene model system of P3HTTeDPP10%:P3HT75-co-EHT25:PC61BM (Fig. 22) as described below. It is proposed that this dichotomy has a physical origin and is indeed the key element that enables simultaneous spectral broadening and Voc tuning in ternary blend organic solar cells, as will be described below. Toward the development of this point, the same set of PSR measurements and analysis were applied to the P3HTTeDPP10%:P3HT75-co-EHT25:PC61BM ternary blend solar cells and the results are presented in Figs. 31 and 32. Once again, the CT state energy of the ternary blend system is increased as the amount of P3HT75-co-EHT25 in the three component system is increased and this increase tracks regularly with ternary blend composition, implying a regular variation in the HOMOD of the alloy, with a constant LUMOA of PC61BM. Also, the qVoc is 0.55 eV lower than CT state energy as was found in case of P3HT:PC61BM:ICBA [384]. Using Equation (1) with zero bowing (b ¼ 0), the energy of the CT state and the Voc were fit analogously as shown in Fig. 32. The subtleties in the PSR data, such as deviation in the endpoints, are likely ascribed to an alloy-to-dopant transition at very low concentrations of one of the alloy component, analogous to observations for dopants in earlier studies [396]. Additionally, it was also observed in this case that the optical excitations cannot be described according to the alloy model, as the PSR data shows the photocurrent of the ternary blend system is the weighted sum of the photoresponse of the two polymers as shown in Fig. 31b as opposed to a weighted average. This is demonstrated when looking at the low energy peaks marked E1 and E2, which correspond to P3HTTeDPP-10% and the higher energy peaks which are a combination of P3HTTeDPP-10% and P3HT75-co-EHT25, as well as PC61BM. It is observed that these marked peaks change

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Fig. 32. Plot of the estimated CT state energy defined by PSR at photocurrent (PC) values of 0.1 and 0.01 from the data of Fig. 31a (closed symbols) compared to the values of the Voc (open triangles), as a function of P3HT75-co-EHT25 fraction for the mixed donor system. Solid lines are linear fit to the data with the same slope (zero bowing factor). Reproduced with permission from Ref. [448]. Copyright 2013 American Chemical Society.

Fig. 31. (a) Photocurrent spectral response (PSR) data for the P3HTTeDPP-10%:P3HT75co-EHT25:PC61BM ternary blend solar cells plotted as a function of P3HT75-co-EHT25 fraction in the polymer donor pair. The pair of dashed lines indicates the range over which the CT state energy is extracted. The inset shows the photocurrent signal at 1.6 eV as a function of P3HT75-co-EHT25 fraction in the polymer donor pair. (b) High energy PSR data from (a) plotted on a linear scale to show the exciton peaks from the donor mixture. Solid lines are fits to the E1, E2 and E3 peaks. The P3HT75-co-EHT25 fraction is indicated for each data set. Reproduced with permission from Ref. [448]. Copyright 2013 American Chemical Society.

intensity with composition but do not change in energy, proving that the composition dependence of the high energy absorption spectrum again has a different behavior from the continuously changing CT state. The retention of molecular optical absorption properties is also supported by the absorption spectra and the EQE response in Fig. 24. It is clear from the preceding data that the CT state energy is dependent on the composition of the components in the ternary blends for the two examples that have been discussed. This implies an alloying effect in which the LUMO of the two-acceptor component and the HOMO of the two-donor component takes on a materials’ averaged value. However, it is also clear that the optical response of all components remain independent in the blends, implying an invariance in the optical band gaps of the

components. The following explanation has been proposed for the apparent contradiction between the excitation and electronic behavior in ternary blend BHJ solar cells. When considering light absorption at energies greater than the optical band gap of components in organic solar cells, excitons are formed. The complete overlap of the electron and hole wavefunctions and the strong Coulomb interaction of the exciton leads to a highly localized state which is therefore confined within a single molecule (or localized segment of a polymer chain) [126,131,133,471e477] and is not subject to a variation based on blending of components, as in an alloy. Hence the exciton states are characteristic of the different individual components rather than the average material composition. On the other hand, the CT state is a bound species comprising a hole in the donor and an electron in the acceptor. Due to the phase separated BHJ structure, the electron and hole are physically apart from each other and therefore have a much smaller Coulomb interaction and consequently are more delocalized [132,472,473,478e482]. The delocalized wavefunction of the CT state extends over more than a single molecule and therefore is determined by the average composition (i.e., the averaged frontier orbital energies) of the donor and acceptor phases at the interface rather than by a single molecule of either type. Therefore the CT state energy (and thus the Voc) is observed to vary as the composition of either the donor or acceptor is changed. The variation in CT energy is regular and described well by an alloy model, as developed here. It is indeed this dichotomy of molecular excitions and composition averaged CT state energies made apparent in the alloy model that drives the potential of ternary blend solar cells. With judicious pairing of polymer donors, a broad photoresponse and high Jsc can be targeted, exceeding that of any binary polymerefullerene blend and alleviating the need to use C70 fullerenes for light harvesting. Further, a higher Voc can be achieved in the ternary blend than is possible with the lower band gap polymer alone. This provides the route to overcome the JsceVoc compromise for binary blend polymerefullerene solar cells. We have also demonstrated that FF does not necessarily suffer in ternary blends and high FF combined with high Jsc and Voc can lead to efficiencies in ternary blends exceeding those of binary blends [360,384]. Most importantly, we have demonstrated that simple random and semi-random polymers can


be used effectively in this platform, providing consistency with the vision of a low-cost and easily fabricated solar cell platform. While significant further testing is required to validate and expand the alloy model, it does provide insight into potentially valuable polymer design principles for ternary blends. While the exact structural nature of a polymer (or fullerene) alloy is not clear, the materials averaged electronic properties imply that the components are subject to intimate and uniform electronic interaction and the delocalization of states should be extensive, leading to continuous changes in HOMO or LUMO energies with composition. The measurements of CT state energy via PSR reflects that the interfacial composition of the donoreacceptor heterojunction is a function of the overall composition of the ternary blend. The precise origin and nature of this interfacial morphology as well as the nature of the bulk alloy structure are still a subject of current investigation. The model certainly does imply that the two alloy components should have a strong physical interaction. Note that the highly similar chemical structures of P3HTTeDPP-10% (80% 3hexylthiophene) and P3HT75-co-EHT25 (75% 3-hexylthiophene) are consistent with a picture of strongly interacting polymers, suggesting that random and semi-random P3HT analogs are indeed especially well-suited for the ternary blend approach. Furthermore, observation of tunable Voc and high efficiency in ternary blends with the highly similar polymer pairs reported by You et al. [362] (Fig. 27c) also suggests a likely strong interaction of polymer donors and consistency with the alloy model. At the current time, the alloy model thus predicts a charge photogeneration pathway in ternary blends very similar to that of binary blends. In the case of a donor:donor:acceptor blend, the two donors are predicted to effectively form a single phase donor alloy that is percolated by an acceptor phase. Light is absorbed independently by both donor components, but the CT state energy (and Voc) is determined by the material averaged composition at the interface. For systems in which the Voc is not observed to be composition dependent, but rather pinned to the lesser of possible values, it is argued that alloy formation does not occur (likely due to an incompatibility of polymer components) and a cascade-type mechanism (Fig. 27a) among the three distinct components is likely operational. Furthermore, in systems where the Voc is not pinned, but does not vary in a regular fashion with composition, our focus on optimal processing conditions (Fig. 23b) suggests that the device processing simply may not be optimized to give the most desirable morphology [360]. The nature of this desirable morphology [383,422,483e485], the disambiguation of the operational mechanism [364,381,383,416,486,487], and the ultimate potential for efficiency of ternary blends are clearly the focus as research in this area moves forward. 5. Conclusions and outlook Tremendous progress has been made in recent years in OPV. Champion efficiencies exceed 10% and a deep understanding of the fundamental principles of device operation is emerging. As the field moves toward commercial application, the importance of moving to even higher efficiencies is clear. This drive to high efficiency must however be coupled with efforts to increase the long-term stability of solar cells and also to maintain the focus on the central vision of OPV as a low-cost approach to an efficient, flexible, and lightweight solar energy conversion platform. The polymerefullerene BHJ solar cell appears to be most consistent with this vision, although many recent advances in polymer and active layer design have been at the expense of the attractive simplicity that defines the BHJ solar cell. Our focus has been to generate conjugated polymers for BHJ solar cells that can be synthesized in a minimum number of steps from simple, commercially available building blocks with the goal

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of controlling polymer properties through the strategic combination of monomers in a “polymer approach” as opposed to highly tailored syntheses of complex monomers in the “monomer approach.” Semi-random P3HT analogs developed in our group and described here have embodied this “polymer approach” and have given rise to efficient (5e6%) solar cells and have achieved the highest reported photocurrents (>16 mA/cm2) for BHJ solar cells with C60-fullerene acceptors. The versatility and modularity of this class of polymers is only just being realized and promises significant potential for future development. Moving to efficiencies that promise to transform OPV into a viable technology will however require a paradigm shift beyond simple polymerefullerene (binary) BHJ solar cells. Tandem cells hold great promise for higher efficiency, but at the cost of increased complexity in device fabrication. Ternary blends are emerging as an alternative to tandem cells, promising efficiencies greater than binary blends, but without the necessity of moving to more complex device architectures. The central element of ternary blends is the ability to broaden the absorption of the active layer without limiting the Voc by incorporating red/near-IR-absorbing polymers. The ability to tune the Voc in ternary blends without negatively influencing FF has been a major step forward. Importantly, blends of random and semi-random polymers have proven effective and well-suited for this platform. A continuing focus on developing an understanding of the mechanism of operation in ternary blend solar cells and elucidating the design principles for optimal polymer structures provides an exciting avenue of future research and a direction that promises a move to higher efficiency within the context of a simple device fabrication. Acknowledgments This material is based upon work supported as part of the Center for Energy Nanoscience, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Award DE-SC0001013, specifically for partial support of P.P.K., B.B., A.E.R. and B.C.T. References [1] Razykov TM, Ferekides CS, Morel D, Stefanakos E, Ullal HS, Upadhyaya HM. Sol Energy 2011;85:1580e608. [2] Devabhaktuni V, Alam M, Shekara Sreenadh Reddy Depuru S, Green RC, Nims D, Near C. Renew Sustain Energ Rev 2013;19:555e64. [3] Avrutin V, Izyumskaya N, Morkoç H. Superlattices Microstruct 2011;49: 337e64. [4] Jacobson MZ, Delucchi MA. Energ Policy 2011;39:1154e69. [5] Armaroli N, Balzani V. Energy Environ Sci 2011;4:3193e222. [6] Schlenker CW, Thompson ME. Top Curr Chem 2012;312:175e212. [7] Jäger-Waldau A. Green 2011;1:277e90. [8] Po R, Maggini M, Camaioni N. J Phys Chem C 2010;114:695e706. [9] Kirchartz T, Rau U. Introduction to thin-film photovoltaics. In: Abou-Ras D, Kirchartz T, Rau U, editors. Advanced characterization techniques for thin film solar cells. Weinheim, Germany: Wiley-VCH Verlag GmbH & Co. KGaA; 2011. p. 1e32. [10] Hegedus S. WIREs Energy Environ 2013;2:218e33. [11] Jäger-Waldau A. Int J Photoenergy 2012:1e6. [12] Schmidtke J. Opt Express 2010;18:A477e86. [13] Brabec CJ, Gowrisanker S, Halls JJM, Laird D, Jia S, Williams SP. Adv Mater 2010;22:3839e56. [14] Dennler G, Scharber MC, Brabec CJ. Adv Mater 2009;21:1323e38. [15] Li G, Zhu R, Yang Y. Nat Photonics 2012;6:153e61. [16] Beaujuge PM, Fréchet JMJ. J Am Chem Soc 2011;133:20009e29. [17] Kumar P, Chand S. Prog Photovolt Res Appl 2012;20:377e415. [18] Nelson J. Mater Today 2011;14:462e70. [19] Thompson BC, Fréchet JMJ. Angew Chem Int Ed 2008;47:58e77. [20] Su Y-W, Lan S-C, Wei K-H. Mater Today 2012;15:554e62. [21] Singh RP, Kushwaha OS. Macromol Symp 2013;327:128e49. [22] Krebs FC. Sol Energy Mater Sol Cells 2009;93:394e412. [23] Krebs FC, Tromholt T, Jørgensen M. Nanoscale 2010;2:873e86. [24] Espinosa N, Hösel M, Angmo D, Krebs FC. Energy Environ Sci 2012;5: 5117e32.

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Petr P. Khlyabich was born in Saint-Petersburg, Russia in 1985. He entered the Department of Physics at St. Petersburg State University in 2002 and defended his bachelor’s of science thesis in 2006 and was subsequently admitted to the master’s program at St. Petersburg State University in the Department of Polymers and Liquid Crystals Physics. There he focused on the characterization of hydrodynamic and electro-optical properties of fullerene derivatives and silicon-containing polymers for gas separation membranes under the guidance of Prof. Natalia Yevlampieva. Petr earned his master’s of science degree in Physics in 2009. In 2008 Petr was admitted to the Ph.D. program in the Department of Chemistry at the University of Southern California where he joined Prof. Barry C. Thompson’s group. His research has focused on materials and device design for solution processable binary and ternary blend organic solar cells.

Beate Burkhart was born in Goeppingen, Germany in 1982 and moved to Dresden, Germany in 2002 to study Chemistry at the Technical University of Dresden where she worked with Prof. Peter Metz focusing her work on the synthesis of a model system of Zaragozic acids. She defended her master’s thesis in spring 2008 and as part of her degree she studied for two years in Rennes, France at the Ecole Nationale Superieure de Chimie de Rennes where she earned a master’s degree in fall 2007. Beate started her PhD studies at USC in the fall of 2008 and joined Prof. Barry Thompson’s research group. Beate completed her PhD in fall 2012 and her thesis work focused on the synthesis of random and semi-random polymers. Since January 2013 Beate is employed with Merck in Darmstadt, Germany.

Andrey E. Rudenko was born in Crimea, Ukraine in 1987. After finishing high school in 2004 he moved to SaintPetersburg, Russia to study chemistry at Saint-Petersburg State University. From 2004 to 2008 his research was focused on the transformation of substituted aziridines. In 2008 he joined the research group of Prof. Victor V. Sokolov where he participated in developing a novel approach to the synthesis of aromatic cyclopentylamines. After receiving his Diploma in Chemistry with honors, Andrey started his doctoral work at USC and joined the research group of Prof. Barry C. Thompson in fall 2009. His research has focused on developing the direct arylation polymerization (DArP) method for the synthesis of poly(3-hexylthiophene) and semi-random P3HT analogs. Andrey is also focusing on the synthesis of novel conjugated polymers.

Barry C. Thompson was born in Milwaukee, Wisconsin in 1977. Barry then attended the University of Rio Grande in Rio Grande, Ohio, where he majored in Chemistry and Physics. After completing his undergraduate studies, Barry moved to the University of Florida to pursue a Ph.D. in Chemistry with Prof. John R. Reynolds as an NSF Graduate Research Fellow, focusing on conjugated polymers for electrochromic and photovoltaic applications. Upon completion of his Ph.D. in 2005, Barry moved to Prof. Jean Fréchet’s lab at UC Berkeley to further pursue his interests in polymer-based photovoltaics as an ACS-PRF Postdoctoral Fellow. After a three-year stay at Berkeley, Barry moved to the University of Southern California, Department of Chemistry and Loker Hydrocarbon Research Institute in 2008 as an Assistant Professor of Chemistry. Barry serves on the editorial advisory board for three journals and is a member of the Center for Energy Nanoscience (a DOE Energy Frontier Research Center at USC). His current research interests are focused on the design, synthesis, and application of novel conjugated polymers for organic photovoltaics.