Efficiency enhancement of organic photovoltaics by addition of carbon nanotubes into both active and hole transport layer Minas M. Stylianakis and Emmanuel Kymakis Citation: Appl. Phys. Lett. 100, 093301 (2012); doi: 10.1063/1.3690056 View online: http://dx.doi.org/10.1063/1.3690056 View Table of Contents: http://apl.aip.org/resource/1/APPLAB/v100/i9 Published by the American Institute of Physics.
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APPLIED PHYSICS LETTERS 100, 093301 (2012)
Efficiency enhancement of organic photovoltaics by addition of carbon nanotubes into both active and hole transport layer Minas M. Stylianakis and Emmanuel Kymakisa) Center of Materials Technology and Photonics and Electrical Engineering Department, Technological Educational Institute (TEI) of Crete, Estavromenos, P.B. 1939, Heraklion, GR-71004 Crete, Greece
(Received 26 January 2012; accepted 9 February 2012; published online 27 February 2012) We investigate the effect of the incorporation of single walled carbon nanotubes (SWNTs) into both the active layer and the hole transport layer (HTL) on bulk heterojunction organic photovoltaic devices. The overall efficiency gain for such a device is 40%, compared with the pristine device. The observed efficiency gain is attributed to a more efficient exciton dissociation due to the SWNTs presence in the photoactive layer and an overall enhanced hole transport and collection through the SWNTs percolation paths, which are extended in both the active layer and C 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.3690056] the HTL. V
Organic photovoltaics (OPVs) have been a highly interesting field in recent years as they have a strong prospective to realize low cost power generation, which has the potential to be portable and deployable due to their flexibility, low cost, and light weight.1 To date, the state of the art OPVs are based on the bulk heterojunction concept, where a donor and an acceptor material are blended together in a nanoscale morphology.2 Compared with Si based photovoltaics, OPVs suffer from insufficient light absorption due to the thin active layer restricted by the short exciton diffusion length (10 nm) of the polymer and low carrier mobilities.3,4 A promising approach to tackle the charge carrier dissociation and transport drawbacks is the addition of one-dimensional nanostructures like carbon nanotubes (CNTs), which can act at the same time as exciton dissociating centers and ballistically conductive pathways with high carrier mobilities.5,6 CNTs have been widely utilized in OPVs as the electron acceptor material replacing the fullerenes,5 the transparent electrode replacing the ITO,7 and the hole transport layer (HTL) replacing the poly-(3,4-ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT:PSS)8 and have, likewise, been incorporated into the active layer as an additive.9 However, their proficient utilization is complex due to their large size in comparison with the thickness of the photoactive layer and their solubility problems. In this context, chemical modification is vital in order to induce the exfoliation of nanotubes bundles, reduce their length, and improve their solubility and processibility.10 In this work, chemical functionalized single-walled carbon nanotubes (SWNTs) have been blended into both the poly(3-hexylthiophene) (P3HT):phenyl-C61-butyric acid methyl ester (PCBM) photoactive layer and the PEDOT:PSS hole transport layer. The SWNTs incorporated into the active layer were modified with thiophene groups attached covalently to the edges and defects of the nanotubes, while for the incorporation into the PEDOT:PSS layer, aqueous dispersions of SWNTs made from charged surfactants were used. In this way, an improvement in power conversion efficiency by 40% compared to the pristine device without a)
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SWNTs has been achieved. In order to get an insight of the enhancement mechanisms, the individual impact of the SWNTs in each polymer layer and in both layers was investigated. Three sets of devices based on the ITO/PEDOT:PSS/ P3HT:PCBM/Al structure have been fabricated and characterized. Sets 1, 2, and 3 have SWNTs incorporated into the P3HT:PCBM, the PEDOT:PSS, and in both layers, respectively. In all sets, the concentration by weight of SWNTs was altered and optimized depending on their respective photovoltaic device characteristics. The active layers were fabricated by mixing respective solutions of P3HT and PCBM at 1:1 ratio, in dichlorobenzene and spin-coated on PEDOT:PSS/ITO. The devices were post-annealed at 150 C for 5 min in a glove box under nitrogen atmosphere. Detailed information of the optimized fabrication and characterization conditions for the pristine ITO/PEDOT:PSS/P3HT:PCBM/ Al can be found elsewhere.8 Modified SWNTs by attaching covalently thiophenes via amide linkages were incorporated into the P3HT:PCBM layer. Briefly, carboxyl (COOH) groups were generated on the nanotube surface by acid treatment. Then, thionyl chloride and a catalytic amount of N,N-dimethyl-formamide (DMF) were reacted to convert the COOH to COCl. Finally, thiophene was nitrated to afford 2-nitrothiophene. The hydrogenation of the latter gave 2-aminothiophene that was used for amidation of SWNTs functionalized with carboxylic acid groups. The synthesis and the detailed characterization of the modified SWNTs are described in our previous report.11 For the PEDOT:PSS, SWNTs aqueous solutions were prepared with the aid of a surfactant sodium dodecylsulfate (SDS).12 The SDS micelles wrap around the individual SWNTs, overcoming the van der Waals forces among the nanotubes, and prohibit the aggregation of the nanotubes, thereby resulting in a stable and homogenous SWNTs aqueous suspension. PEDOT:PSS-SWNTs solutions were then formed by mixing the aqueous SWNTs solution with the PEDOT:PSS (Clevios P VP Al 4083) solutions in different wt. % ratios, followed by ultrasonication.13 The transmittance of the PEDOT:PSS-SWNTs with different SWNTs concentrations is shown in Fig. 1(a). The
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pristine PEDOT:PSS film gives a good optical transmittance (89.5% average) on the wavelength range from 500 to 800 nm. The transmittance of PEDOT:PSS film slightly decreases with increasing amount of SWNTs. With the SWNTs concentration increasing to 0.075 and 0.20 wt. %, the transmittance of composite film weakly drops by 1.45 and 3.35%, respectively. Figure 1(b) shows the absorption spectra of P3HT:PCBM blend films with varying SWNTs concentration. The addition of SWNTs does not have a significant impact on the absorption spectra for concentrations until 0.75%. The individual effect of the SWNTs concentration into the PEDOT:PSS and the P3HT:PCBM active layer on the devices photovoltaic characteristics is studied. The principal photovoltaic characteristics, such as the power conversion efficiency (g), the open-circuit voltage (Voc), the short-circuit current density (Jsc), and fill factor (FF) for the ITO/PEDOT:PSS/ P3HT:PCBM-SWNTs/Al devices with SWNTs wt. concentration in the P3HT:PCBM ranging from 0 to 1% are presented in Fig. 2(a). Figure 2(b) presents the photovoltaic characteristics for the ITO/PEDOT:PSS-SWNTs/P3HT:PCBM/Al devices with SWNTs wt. concentration in the PEDOT:PSS solution ranging from 0 to 0.2%. The pristine polymer-fullerene device has a Jsc of 8.56 mA/cm2, a Voc of 0.6 V, and a FF of 52%. Upon doping the active layer with SWNTs, the photocurrent attains a max-
FIG. 2. (Color online) Dependence of the short-circuit current density, the power conversion efficiency, the open circuit voltage, and the fill factor (a) on the SWNTs concentration on the P3HT:PCBM photoactive layer and (b) on the SWNTs concentration on the PEDOT:PSS HTL layer.
FIG. 1. (Color online) (a) Transmittance of PEDOT:PSS-SWNTs with different SWNTs concentration and (b) UV–Vis absorption spectra of P3HT:PCBM blend with different SWNTs concentration.
imum of 10.86 mA/cm2 at 0.5% and then decays to 8.34 mA/ cm2 at 1%. Further increasing the concentration of SWNTs has detrimental effect on device performances, with the efficiency significantly dropping. The Voc is constant at 0.6 V, while for higher concentrations, it decreases to 0.5 V. In contrast, the FF increases with increasing SWNTs concentration from 0.52 to 0.56. Consequently, the optimum device was obtained for a SWNTs wt. of 0.5%. The Jsc is increased from 8.56 to 10.68 mA/cm2 and FF from 0.52 to 0.55, which lead to an enhancement of PCE from 2.67 to 3.52%. The inclusion of modified SWCNTs with thiophene within the photoactive layer improves the device Jsc and FF, resulting in a commensurate increase in efficiency by 32% compared with the pristine device. The significant efficiency enhancement can be preliminary attributed to the creation of new percolation paths into the photoactive layer, resulting in a more efficient charge carrier transport.14 Upon doping the PEDOT:PSS with small concentration of SWNTs, a small change in Voc with increase in SWNTs
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TABLE I. Conductivity, roughness, and photovoltaic performance of the devices with different concentration of SWNTs in PEDOT:PSS layer. SWNTs (% wt.) 0 0.05 0.075 0.1 0.2
r (S/cm)
Tr (%)
rms (nm)
Jsc (mA/cm2)
Voc (V)
FF (%)
g (%)
5.25 104 7.2 103 2.3 103 2.7 103 4.1 102
89.5 88.9 88.2 87.9 86.5
1.2 1.3 1.7 2.1 2.6
8.56 8.65 9.75 9.52 7.8
0.6 0.6 0.59 0.58 0.55
0.52 0.48 0.44 0.43 0.38
2.67 2.49 2.53 2.37 1.63
concentration is observed for concentrations until 0.1%. However, for higher concentrations, the Voc reduces to 0.55 V. It can be concluded for low concentrations, the incorporation of SWNTs does not affect the work function of the PEDOT:PSS film. The Jsc increases from 8.56 mA/cm2 of the pristine device to 9.76 mA/cm2 of the device for 0.075% doping, after which it decreases to 7.8 mA/cm2 for 0.2% doping. Any concentration of SWNTs higher than 0.5 wt. % lead to short circuit, probably due to the occurrence of local shunts due to SWNTs directly bridging the ITO electrode. In order to elucidate the photovoltaic characteristics of the devices, electrical and optical characterization of the composites took place. The conductivity (r) of the PEDOT:PSS-SWNTs films, measured by the four-point probe technique, the transparency (Tr: transmittance at 520 nm) measured by UV-Vis spectroscopy, and the film root mean square (RMS) roughness measured by AFM are listed together with the respective photovoltaic characteristics in Table I. As expected, the SWNTs doping increases both the conductivity and surface roughness for even very low concentrations. The residual surfactants can form an insulating barrier between the nanotubes and decrease the bulk conductivity of the doped PEDOT:PSS layer. To study the influence of the surfactants on the HLT, a pristine PEDOT:PSS layer was doped with SDS in low concentrations. The conductivity of PEDOT:PSS was enhanced after the addition of SDS into the PEDOT:PSS aqueous solution, while the transparency of the PEDOT:PSS film did not change. This effect is in agreement with previous work, where a conductivity enhancement by a factor of 500 was observed. This enhancement was attributed to the effect of SDS on the conformation of the conductive PEDOT chains. The SDS replaces PSS as the counter anions to PEDOT in water, so that the distortion structure of the PEDOT chain disappears. This conformational change in the PEDOT chain results into the significant enhancement in the conductivity of the PEDOT:PSS film.15 Therefore, it can be concluded that the positive effect of the residual surfactant on the conductivity of PEDOT:PSS due to the anion exchange overcomes the decrease on the bulk conductivity. The optimum efficiency was obtained for 0.075 wt. %, where the conductivity increased to 2.3 103 from 5.25 104 S/ cm and the roughness increased to 1.7 from 1.2 nm for the pristine PEDOT:PSS film,. Therefore, it can be initially concluded that the improved photocurrent of the OPV devices with SWNTs on the PEDOT:PSS layer results from higher hole collection due to the higher conductivity and roughness of the HTL. The decrease in the photocurrent for concentrations higher than 0.1% can be explained by the observed
FIG. 3. (Color online) (a) J-V characteristics of the OPVs with SWNTs incorporated into different polymer layers, as described in Table II.
decrease in transparency, especially in the wavelengths around 500 nm, which has strong effects on the photon absorption yield and the charge-carrier-transport yield of the devices. The increase in the HTL conductivity should also lead to decrease on the device series resistance, leading to an improvement of the FF. However, the FF does not follow the same trend with the photocurrent, but it decreases from 0.52 to 0.38. This discrepancy may be due to a decrease in the device shunt resistance, arising from extra leakage current introduced through the device due to the presence of more defects in the doped PEDOT:PSS and by any SWNTs protruding towards the photoactive layer, forming additional current pathways through the device.16,17 In order to clarify this behavior, an extra ultrathin PEDOT:PSS layer was spin coated on top of the PEDOT:PSS-SWNTs film; in this way, the roughness of the film was similar to the pristine PEDOT:PSS, and no SWNTs protrude towards the photoactive layer. The addition of the extra PEDOT:PSS layer significantly alters the SWNTs role, and no improvement in the device performance was observed. Hence, it can be concluded that the improvement in the HTL conductivity is not high enough in order to facilitate a more efficient hole collection. The improvement in the hole collection can be mainly attributed to the increase in the HTL roughness, resulting in an increase in the interfacial area between the HTL and the photoactive layer. The J-V characteristics of the optimum OPV device structures with SWNTs incorporated into the P3HT:PCBM active area (0.5% wt.), with SWNTs incorporated into the PEDOT:PSS (0.075% wt.), and in the combined device, which the SWNTs incorporated in both polymer layers are shown in Fig. 3, while their photovoltaic parameters are listed in Table II. It can be clearly seen that the incorporation of SWNTs into the active layer improves both the photocurrent and the FF, resulting in an improvement in efficiency from 2.67 (pristine device) to 3.52%. How ever, the incorporation of SWNTs in the PEDOT:PSS layers improves the photocurrent, but there is a significant decrease in the FF, resulting in reduction in the efficiency from 2.67 (pristine) to 2.53%. Remarkably, when SWNTs are incorporated into both the PEDOT:PSS and the active layer, an outstanding
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TABLE II. Photovoltaic parameters of the P3HT/PCBM OPV devices without SWNTs, and SWNTs incorporated in the active layer, in the HTL, and in both polymer layers. Device structure ITO/PEDOT:PSS/P3HT:PCBM/Al Jsc (mA/cm2) Voc (V) FF (%) g (%) Reference device without SWNTs SWNTs in P3HT:PCBM only SWNTs in PEDOT:PSS only SWNTs in both P3HT:PCBM and PEDOT:PSS
8.56 10.68 9.76
0.6 0.6 0.59
52 55 44
2.67 3.52 2.53
11.76
0.6
53
3.74
increase in the photocurrent is observed from 8.56 to 11.76 mA/cm2. The Voc remains unchanged, while the FF slightly increases to 0.53. Thus, an enhancement in the efficiency from 2.67 (pristine device) to 3.74% by 40% is observed. In order to explain this dual effect enhancement, the SWNTs doping effect in the active and the hole transport layer on the OPV device performance is jointly discussed. The incorporation of SWNTs into the photoactive layer enhances the device performance since the exciton dissociation and charge transport and collection are improved. The SWNTs act as both efficient exciton dissociation centers and ballistic charge carrier pathways, significantly increasing the charge separation and transport compared with the pristine device, where the charge transfer takes place by slower hoping and tunneling among the phase separated clusters.6 However, the charge selectively of SWNTs network inside the photoactive layer dominates the enhancement characteristics of the OPV devices. The SWNTs can either act like bridges, enhancing electron transfer between the fullerene molecules, or enhance the hole transport in the P3HT. This issue is extensively discussed in the literature, where the efficiency improvement is attributed to either enhanced electron or hole transport by the SWNTs network, which can provide conducting pathways to the either the anode or the cathode electrode while, in both cases, maximizing the surface area for collection of charges.11,18–20 A recent direct kelvin probe force microscopy study on P3HT:PCBM/SWNTs devices demonstrated that photoinduced holes are transported to SWNTs while electrons are blocked because of the heterojunctions, and thus the SWNTs, which are a mixture of semiconducting and metallic nanotubes, work as donors.21 The efficiency enhancement is attributed to increased hole mobility and not electron mobility. Consequently, if the hypothesis that the performance enhancement is due to the improvement in hole transport is valid, the observed dual substantial enhancement observed in the simultaneously doped devices can be postulated to be mainly due to a more efficient hole transport through the nanotube percolation paths, which are extended in both the active layer and the HTL. In the pristine device, the photogenerated excitons are dissociated at the P3HT-PCBM interfaces, and the electrons and the holes are transported to the Al and to the ITO, respectively. The presence of a SWNTs network in both the photoactive layer and the hole transport layer provides additional exciton dissociation centers and a direct pathway for hole collection to the ITO electrode, respectively. The SWNTs inside the photoactive layer
enhance the exciton dissociation and suppress the recombination ratio since the photogenerated excitons can be dissociated at the additional fullerene/nanotube and the polymer/ nanotube interfaces.14 However, the SWNTs inside the PEDOT:PSS can provide a direct ballistic link for the holes in the active layer to be effectively transported through the nanotubes to the ITO electrode. Furthermore, it has been reported that both semiconducting and metallic SWNTs function as efficient hole acceptors in P3HT/SWNTs heterojunctions. The P3HT p-dopes both types of SWNTs, and the work function difference between the nanotube and P3HT leads to a built-in voltage driving the efficient exciton dissociation and hole transfer.22 This observation further supports the above hypothesis. The above argument can be also be supported by the fact that the device with only PEDOT:PSS doped exhibits an increase in the photocurrent, but a significant reduction in the FF compared with the pristine device. This low FF is mainly due to SWNTs protrude towards the active layer. However, for the device with both the polymer layers doped, improvement in both the photocurrent and the FF is observed, indicating that the SWNTs that may protrude from the HTL do not limit the hole transport but enhance it. From the above discussion, an enhancement in the hole mobility of the complete OPV devices is postulated to be the dominant enhancement factor. In order to validate this presumption, hole-only devices with the following structure: ITO/PEDOT:PSS/P3HT:PCBM/Au were fabricated for all the devices, described at Table II. All three devices with doped SWNTs exhibit more current density than that of the pristine device, confirming the enhancement of hole mobility. The hole mobility was estimated from the J–V characteristics at low voltage region, where the current is described by the Mott–Gurney square law JSCLC ¼ (9/8)eoerl(V2/L3), where eoer is the permittivity of the polymer, l is the charge carrier mobility, and L is the film thickness.23 It should be noted that the obtained hole mobilities are not for the active layer but for the complete device including the active and the hole transport layers. The hole mobilities of the device prepared with the SWNTs incorporated into P3HT:PCBM, into the PEDOT:PSS, and in both polymer layers are calculated to be 1.52 103, 1.35 103, and 1.6 103 cm2 V1 s1, respectively. However, the hole mobility for the pristine device is found to be 1.2 103 cm2 V1 s1. The hole mobility increase for the devices with the SWNTs doped in both polymer layers allows more balance charge transport in the active layer, thus improving both the Jsc and the FF of the devices. The increase in the hole mobility is not analogous with the increase in the photocurrent, indicating that the presence of residual surfactants may have a negative effect. In conclusion, we have demonstrated a 40% enhancement in efficiency by incorporating SWNTs into both the active and the hole transport layer of OPVs. The dual substantial enhancement is postulated to be due to a more efficient exciton dissociation and hole transport and collection through the nanotube percolation paths. The SWNT network inside the photoactive layer enhances the exciton dissociation, suppress the recombination ratio, and improves the hole mobility while the SWNTs inside the HTL provide a direct ballistic link for the holes in the active layer to be effectively
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transported through the nanotubes to the respective electrode. 1
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