ISSN 19950780, Nanotechnologies in Russia, 2014, Vol. 9, Nos. 1–2, pp. 77–81. © Pleiades Publishing, Ltd., 2014. Original Russian Text © G.L. Pakhomov, V.V. Travkin, A.N. Tropanova, A.I. Mashin, A.A. Logunov, 2014, published in Rossiiskie Nanotekhnologii, 2014, Vol. 9, Nos. 1–2.
Organic Photovoltaic Cells on Polymeric Substrates with Buffer Nanolayers1 G. L. Pakhomova*, V. V. Travkina, A. N. Tropanovaa, A. I. Mashinb, and A. A. Logunovb a
The Institute for Physics of Microstructures of the Russian Academy of Sciences (IPM RAS), ul. Akademicheskaya 7, der. Afonino, Kstovskii raion, Nizhni Novgorod oblast, 607680 Russia b Lobachevsky Nizhni Novgorod State University, ul. Gagarina 23, str. 3, Nizhni Novgorod, 603950 Russia *email:
[email protected] Received August 12, 2013; in final form, October 10, 2013
Abstract—Prototypes of organic photovoltaic cells based on the planar “subphthalocyanine/fullerene” het erojunction were fabricated using flexible polymeric substrates. Currentvsvoltage characteristics in the dark and under illumination were recorded and basic photovoltaic parameters were derived. It was shown that introduction of the (top) barrier layer composed of Alq3 molecules under the cathode significantly lowers the parasitic resistances in the cells, so that fill factor rises up to 55%. If the (bottom) buffer InClPc layer is depos ited onto the anode (ITO), then the open circuit voltage of cells increases from 0.47 to 0.83 V. The highest power conversion efficiency was achieved in OPVC with both top and bottom interfacial layers (~1%). DOI: 10.1134/S199507801401011X
INTRODUCTION
top electrode (cathode) and preclude damage caused by thermal evaporation (barrier layer) [4, 5, 8]; (e) pro tect from penetration of contaminants [6] and (f) increase the amount of light captured inside the cell (optical spacer) [4, 5].
One of the important advantages of organic elec tronics consists in manufacturing feasibility of large area devices on webbased (polymeric) substrates. Most frequently, polyethylene terephthalate is used as substrate (polyimide is less common), covered with optically transparent conductive layer which serves as anode in organic photovoltaic cells (OPVC). The vast majority of commercially available flexible substrates are equipped with the anode made of double indium tin oxide In2O3:SnO2 (ITO), typically 70–100 nm thick, deposited by magnetron sputtering of the target. In the course of manufacturing, various thinfilms photovoltaic structures are formed on the surface of the anode by sequential deposition of polymeric [1] (from solution) or low molecular weight (by thermal evaporation) [2–12] compounds. In addition to these basic semiconducting layers of photoactive materials having typical thickness in the range of few tens nanometers and forming a heterojunction (planar, gradient or bulk), the scheme of OPVC may contain some else functional layers having typical thickness in the range of few nanometers [1–10]. The latter layers may perform multiple functions: (a) optimize growth surface before depositing basic semiconducting layer (i.e., act as buffer, BL) [2, 3, 9]; (b) prevent quenching of excitons at the metallic electrode (exciton blocking layer, EBL) [2–5, 9–12]; (c) improve injection and/or transport of charge carriers to respective electrodes (electron/hole transporting layer, ETL/HTL) [4, 6, 7]; (d) retrain unwanted diffusion of metal atoms from the
Finally, second (top) metallic electrode (cathode) and, if OPVC are expected to operate in atmospheric conditions, capping layer(s) are deposited. The latter process is called encapsulation [10]. Despite apparent complexity of such multilayer structures, there are indications of stability in charac teristics of polymeric OPVC, e.g., even if they are seri ously bent in various directions [1]. At the same time, data for OPVC based on low molecular weight semi conductors are insufficient [8]. In this work, laboratory prototypes of thinfilm OPVC on polymeric substrates with transparent ITO anode were fabricated and tested. OPVC employ a pla nar SubPc/C60 heterojunction (PHJ), where SubPc is boron chloride subphthalocyanine and C60 is buck minsterfullerene—Fig. 1a. Such PHJ is known as exemplary when studying structural effects and photo conversion mechanisms in low molecular weight semiconductors [9]. Then, the technological scheme of OPVC was modified using ultrathin layers of (a) indium chloride phthalocyanine InClPc, as buffer layer over the anode; and (b) tris–(8–hydroxy quinolynato)aluminum Alq3, as barrier layer under the cathode—Figs. 1a–1c. The effect of these inter layers on J–V characteristics of thus obtained devices in the dark and under illumination was analyzed.
1 The article was translated by the authors.
77
78
PAKHOMOV et al. (a)
NcVO
SubPc N N NB N N
InClPC
Cl N
N
N Cl N
N
In
N
N
N
N
N
N
N
V
N
N
N
N
O
N
Alq3 C60
N O
O Al N
N
O
In2O3:SnO2 (ITO) O
O C
C
O
CH2 O CH2
PET
n
ITOelectrode (anode)
(b)
(c)
3.2 eV
5.1 eV 5.6 eV
4.2 eV
PET
Alq3
4.5 eV
C60
4.8 eV
Substrate Al
SubPc
ITO
InClPc
3.5 eV 3.6 eV
Light source Al electrode (cathode)
5.7 eV
InClPc or NcVO
6.2 eV
SubPc + C60 (PHJ)
Alq3
Fig. 1. (a) Chemical structures of the compounds used in this work. (b) Diagram of energy levels of materials used in OPVC. (c) Schematic of multilayer OPVC. Thickness of layer: InClPc or NcVO = 2–3 nm, SubPc = 15 nm, C60 = 50 nm, Alq3 = 7–8 nm, Al = 80–100 nm. Active area of the devices was ~0.03 cm2.
EXPERIMENTAL Molecular structures of materials used in this study are shown in Fig. 1a; corresponding positions of fron tier molecular orbitals and electrode work functions are compared in Fig. 1b (data adopted from Refs. [2, 4, 5, 9–12] and references therein). The technological scheme of OPVC is sketched in Fig. 1c. All materials and methods are described earlier [3, 11]. Standard polyethylene terephthalate (PET) slides covered by indiumtin oxide (ITO) layer with 70–100 Ω/cm2 resistance were purchased from Ald rich. It should be noted that during the fabrication and testing procedures the samples, including completed OPVC prototypes, underwent some unintentional deformation (bending) without noticeable degrada tion of their photoelectric properties. Photoelectrical measurements were carried out in airfree conditions and in steadystate regime, as
reported previously [3, 11]. Additionally, power con version efficiency (η) in certain OPVC samples was estimated using ORIEL Sol3A solar simulator (New port) that provides calibrated illumination intensity of 1 sun (100 mW/cm2). RESULTS AND DISCUSSION J–V curves measured in the dark are shown in Fig. 2a. Their profiles display quite imperfect diode properties of initial OPVC (without interfacial layers) and reflect strong contribution of parasitic resistances. Rectification ratio RR (calculated as quotient of cur rent densities under forward and reverse bias at 1.5 Vdc) is only 7 × 101. Introduction of ultrathin Alq3 layer under the cathode markedly improve rectifying properties of cells: RR is equal to 4.6 × 104. Maximal value of RR (6.3 × 104) is achieved in the cells with
NANOTECHNOLOGIES IN RUSSIA
Vol. 9
Nos. 1–2
2014
ORGANIC PHOTOVOLTAIC CELLS ON POLYMERIC SUBSTRATES J, mA/cm2
10
J, mA/cm2 (а)
101
10–1
(b)
PHJ
PHJ PHJ/Alq3 InClPc/PHJ InClPc/PHJ/Alq3
0
79
101
PHJ/Alq3 InClPc/PHJ InClPc/PHJ/Alq3 PHJ = SubPc/C60
PHJ = SubPc/C60
100
–2
10
10–3 10–1
10–4 10–5
LIGHT OFF
LIGHT ON –2
10
10–6 10–7 –1.5
–1.0
–0.5
0
0.5
1.0
1.5 U, V
–1.5
–1.0
–0.5
0
0.5
1.0
1.5 U, V
Fig. 2. J–V curves for obtained OPVC prototypes (a) in the dark; and (b) under illumination; in semilogarithmic coordinates.
simultaneously two interfacial layers, mostly due to a decrease of current density under the reverse bias— Fig. 2a. Under illumination, the current densities under the reverse bias increase by several orders of magnitude (from 2 to 5, depending on OPVC scheme) and reach nearly constant value—Fig. 2b. Using wellknown equations [13], common photovoltaic parameters of OPVC were calculated—table. As follows from table, insertion of the top Alq3 buffer layer increases efficiency η by 4.5 times, basi cally owing to the damping of parasitic resistances. So, shunt resistance Rsh increases by ~3 times, while series resistance Rs decreases by more than order of magni tude. Fourth quadrant of J–V plot in linear coordi nates is shown in Fig. 3 in detail. As seen from the fig ure, the profile of J–V curve for OPVC with Alq3 buffer is rather close to rectangular shape (angle of intersection of the curve with X or Yaxis approxi
mately corresponds to series or parallel resistance, respectively [13]). This determines fill factor FF equal to 0.55, the highest among studied OPVC—table. Although an additional layer has appeared in the current flow scheme (Fig. 1c), substantial decrease of Rs, is observed, whilst the value of short circuit cur rent Jsc increases faintly—table. Therefore, it is rea sonable to assume that Alq3 interlayer serves not so much as efficient exciton blocker [2, 4], but more likely reduces the barrier at C60/Al interface. Existence of such a barrier is known [2, 7], its formation is believed to be due to surface reactions of hot deposited metal atoms with acceptor fullerene [5, 6]. Notably, that the question of why Alq3 layer with thickness up to 8–9 nm does not hinder from efficient delivery of charge carriers (electrons) to cathode, as in the case of tunnel transparent Alq3 layers 2–3 nm thick, is still unanswered. Most likely explanation would come along with results of chemical analysis of
Photovoltaic parameters of OPVC measured in inert atmosphere under the white light illumination with incident intensity Pin = 30 mW/cm2 Parameter OPVC scheme PHJ PHJ/Alq3 InClPc/PHJ InClPc/PHJ/Alq3 NcVO/PHJ/Alq3
Uoc , V
Jsc , mA/cm2
Rs , kΩ/cm2
Rsh , kΩ/cm2
FF
η, %
0.47 0.51 0.83 0.84 0.55
0.29 0.32 0.26 0.29 0.39
1.8 0.15 7.1 2.3 1.2
4.0 11.7 6.7 14.5 4.6
0.21 0.55 0.24 0.39 0.32
0.10 0.45 0.18 0.48 0.23
NANOTECHNOLOGIES IN RUSSIA
Vol. 9
Nos. 1–2
2014
80
PAKHOMOV et al. J, mA/cm2 0
PHJ = SubPc/C60
–0.1
–0.2 PHJ PHJ/Alq3 InClPc/PHJ InClPc/PHJ/Alq3
–0.3 0
0.2
0.4
0.6
0.8 U, V
Fig. 3. IV quadrant of J–V plot (linear coordinates) for obtained OPVC prototypes under illumination.
Alq3 layer after the deposition: traces of metallic alu minum originating from the top deposited cathode were found in the layer bulk [2]. This however contra dicts other experiments: in inverted OPVC (where the reversed sequence of layers is implemented, i.e., ITO\Alq3\C60\Pc\Al [2, 7]) the role of Alq3 interlayer is retained, though it does not have direct contact with metallic electrode. Song et al. [7] also considered Alq3 as a buffer between aluminum and fullerene layers impeding the electron transfer from metal to C60, rather than as a widegap blocker of excitons. At the same time, such a buffer stabilizes the actual work function of the metal at the top interface, thereby holding the builtin potential in OPVC [7]. In recent work [6] authors con cluded that adding Alq3 under the cathode prevents from diffusion of traps into C60 layer, and hence slows down the degradation processes when OPVC is in use. Our preliminary data, taken from the secondary ion mass spectrometry with depth profiling, suggest that Alq3 may also work as a barrier for migration of oxy gencontaining impurities from the substrate material, through fullerene layer, to the top electrode. As follows from comparison of literature results, homogenous and highly resistive Alq3 layer also pro hibits from formation of the point shortcircuit defects arising, again, from the downward diffusion of metal (i.e., from the top electrode towards the bottom, dur ing and after thermal evaporation of metal), and retards subsequent structural reorganization in loose molecular layers [5, 6]. This raises shunt resistance Rsh markedly—table. Value of Uoc also slightly increases, so that resulting efficiency is getting higher. If the bottom interfacial InClPc layer is used, then the increase in η becomes not so important—table. It is mainly associated with the rise of the open circuit voltage Uoc from 0.47 to 0.83 V. Similar trend was
observed earlier in [3] for OPVC prototypes on glass substrates and can be explained by several factors. First, the structure of SubPc layer grown over the InClPc buffer is specific: SubPc partially crystallizes, in contrast to the SubPc layer grown on the bare ITO surface, which is amorphous under the conditions used. This would change the positions of molecular levels of the donoracceptor pair (Fig. 1b), whose mutual shift affects the value of Uoc [13]. Second, InClPc is a ptype molecular semiconductor, which can be used, per se, as photoactive layer in OPVC [12]. Taking into account that optical absorption peak (so called Qband) of SubPc and InClPc is allocated in different parts of visible spectrum [3, 11, 12], contri bution of InClPc layer into harvesting of light energy by OPVC can be admitted (in contrast to almost trans parent Alq3) [5]. However, since the thickness of InClPc interlayer in our case is very small, the second reason appears to be less probable. More probably is that ultrathin InClPc overlayer modifies the actual work function of ITO, thus energetic structure of the anode/SubPc interface will be altered—Fig. 1b. Nonetheless, in the cells with InClPc buffer a series resistance Rs increases, and hence the fill factor FF decreases (table). This obviously should be attributed to the increased losses after insertion of additional resistive component (layer) in the OPVC scheme (Fig. 1c). Short circuit current Jsc decreases, too— table. As seen from Fig. 3, the J–V curve for OPVC with InClPc has S–shaped profile, as well as the J–V curve for initial OPVC. This is socalled “kink” effect illustrating the large contribution of parasitic resis tances. Such flaws are practically missing in OPVC proto types with simultaneously two interfacial layers, Alq3 above and InClPc below the basic PHJ—Fig. 1c. Bot tom InClPc layer is responsible for maintaining high value of Uoc, whereas top barrier Alq3 layer allows to compensate for losses in Jsc and, partly, in Rs values and to increase parallel resistance Rsh—table. Conse quently, fillfactor FF for OPVC with two interfacial layers is more than one and half times greater than that for initial OPVC (kinkeffect is suppressed, as seen from Fig. 3), and efficiency η is the highest. These samples (without additional optimization) were then tested using the simulated sunlight, the measured η value was 0.93%. To better prove the role of the bottom InIcPc buffer layer, we have fabricated two series of OPVC proto types within the same technological process: in InClPc/PHJ/Alq3/Al and in NcVO/PHJ/Alq3/Al configuration. Here, NcVO is a vanadyl naphthalocy anine, a fourleaf complex of phthalocyanine type (Fig. 1a), which is also ptype molecular semiconduc tor [14]. Similarly to PcVO [3], InClPc and SubPc, this complex has a nonplanar shape and forms continu ous ultrathin layers that have, however, somewhat unlike morphology and rather more conductive [15].
NANOTECHNOLOGIES IN RUSSIA
Vol. 9
Nos. 1–2
2014
ORGANIC PHOTOVOLTAIC CELLS ON POLYMERIC SUBSTRATES
The deposition and measurement conditions were strictly identical for both series of samples. The profiles of J–V curves for OPVC with NcVO buffer layer did not suffer from kinkeffect (not shown here) due to the presence of (top) Alq3 interlayer, as in the case of OPVC with bottom InClPc buffer. Yet, introduction of NcVO has a tiny effect on Uoc as com pared with introduction of InClPc—table. The rise of Jsc is likely connected with a lower resistivity of NcVO layer in comparison with InClPc. Since the parasitic resistances in NcVO/PHJ/Alq3/Al structures are still important, resulting efficiency remains by about 2 times worse then in their InClPc/PHJ/Alq3/Al ana logs—table. This fully supports the above conclusions on the role of InClPc buffer.
4.
5.
6.
7.
CONCLUSION In this work we show how additional functional molecular layers modify output characteristics of OPVC on flexible PET substrates with a planar “SubPc/C60” heterojunction. These interlayers (few nm’s thick) were inserted at either anode/player interface or nlayer/cathode interface, or at both. By varying the material (i.e., function) of the layer, the cell’s photovoltaic parameters, mostly Uoc and FF, can vastly be improved. In the issue, the power conversion efficiency increases by five times (up to ~1%). Evi dently, by corresponding optimization of the material, thickness and morphology of the layers composing the device, a further magnification of the overall perfor mance in possible.
8.
9.
10.
11.
ACKNOWLEDGMENTS The work was supported in part by Presidium of RAS (Program no. 8) and RFBR (grants nos. 1208 31482, 120201106).
12.
REFERENCES 1. M. Kaltenbrunner, M. S. White, E. D. G l owacki, T. Sekitani, T. Someya, N. S. Sariciftci, and S. Bauer, “Ultrathin and lightweight organic solar cells with high flexibility,” Nature Commun. 3, 770 (2012). DOI: 10.1038/ncomms1772. 2. M. Morsli, J. C. Bernède, L. Cattin, F. Dahou, and A. Khelil, “On the exciton blocking layer at the inter face organic/cathode in multiheterojunction organic solar cells,” Energy Procedia 31, 74 (2012). 3. G. L. Pakhomov, V. V. Travkin, A. N. Tropanova, E. Yu. Gudkov, and Yu. N. Drozdov, “Prototypes of
NANOTECHNOLOGIES IN RUSSIA
Vol. 9
Nos. 1–2
13. 14.
15.
81
photovoltaic cells based on subphthalocyanine with a lower buffer layer,” Semiconductors 46 (11), 1381 (2012). P. Peumans, V. Bulovic, and S. R. Forrest, “Efficient photon harvesting at high optical intensities in ultrathin organic doubleheterostructure photovoltaic diodes,” Appl. Phys. Lett. 76, 2650 (2000). N.N. Wang, J.S. Yu, Y. Zang, and Y.D. Jiang, “Pho tocurrent analysis of organic photovoltaic cells based on Cu/C60 with Alq3 as a buffer layer,” Chinese Phys. B 19 (3), 038602 (2010). P. Chen, J. Huang, Z. Xiong, and F. Li, “The roles of traps in the energy loss for aged organic solar cells: a transient photovoltage study,” Organ. Electron. 14, 621 (2013). Q. L. Song, C. M. Li, M. L. Wang, X. Y. Sun, and X. Y. Hou, “Role of buffer in organic solar cells using, C60 as an acceptor,” Appl. Phys. Lett. 90, 071109 (2007). T. T. Luong, Z. X. Chen, and H. W. Zhu, “Flexible solar cells based on copper phthalocyanine and buckmin sterfullerene,” Solar Energy Mater. Solar Cells 94, 1059 (2010). R. Pandey, A. A. Gunawan, K. A. Mkhoyan, and R. J. Holmes, “Efficient organic photovoltaic cells based on nanocrystalline mixtures of boron subphtha locyanine chloride and C60,” Adv. Funct. Mater. 22, 617 (2012). C.F. Lin, S.W. Liu, C.C. Lee, J.C. Huang, W.C. Su, T.L. Chiu, C.T. Chen, and J.H. Lee, “Opencircuit voltage and efficiency improvement of subphthalocya ninebased organic photovoltaic device through depo sition rate control,” Solar Energy Mater. Solar Cells 103, 69 (2012). P. A. Stuzhin, M. S. Mikhailov, V. V. Travkin, E. Yu. Gud kov, and G. L. Pakhomov, “Multilayer photovoltaic structures based on tetrathiadiazoloporphyrazine / sub phthalocyanine heterojunction,” MacroHeteroCycles 5 (2), 162 (2012). N. M. Bamsey, A. P. Yuen, A.M. Hor, R. Klenkler, J. S. Preston, and R. O. Loutfy, “Heteromorphic chlor oindium phthalocyanine films for improved photovol taic performance,” Solar Energy Mater. Solar Cells 95, 2937 (2011). J.M. Nunzi, “Organic photovoltaic materials and devices,” C. R. Phys. 3, 523 (2002). H. Yanagi, Y. Kobayashi, D. Schlettwein, D. Woehrle, and N. R. Armstrong, “Photoelectrochemical investi gations on naphthalocyanine derivatives in thin films,” J. Phys. Chem. 98 (17), 4760 (1994). E. Orti, M. C. Piqueras, R. Crespo, and J. L. Bredas, “Influence of annelation on the electronic properties of phthalocyanine macrocycles,” Chem. Mater. 2 (2), 110 (1990).
2014
