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Solution-processed vanadium oxide as an anode interlayer for inverted polymer solar cells hybridized with ZnO nanorods
Organic Electronics 10 (2009) 1060–1065
Contents lists available at ScienceDirect
Organic Electronics journal homepage: www.elsevier.com/locate/orgel
Solution-processed vanadium oxide as an anode interlayer for inverted polymer solar cells hybridized with ZnO nanorods Jing-Shun Huang a, Chen-Yu Chou a, Meng-Yueh Liu a, Kao-Hua Tsai a, Wen-Han Lin a, Ching-Fuh Lin a,b,* a b
Institute of Photonics and Optoelectronics, National Taiwan University, Taipei, 10617 Taiwan, ROC Graduate Institute of Electronics Engineering and Department of Electrical Engineering, National Taiwan University, Taipei, 10617 Taiwan, ROC
a r t i c l e
i n f o
Article history: Received 17 March 2009 Received in revised form 16 May 2009 Accepted 19 May 2009 Available online 27 May 2009
PACS: 72.40.+w 72.80.Ga 81.05.Dz 81.07.-b 84.60.-h 85.60.-q
a b s t r a c t Solution-processed vanadium oxide (V2O5) as an anode interlayer is introduced between the organic layer and the Ag electrode for improving the performance of the low-cost inverted polymer solar cells hybridized with ZnO nanorods. Our investigations indicate that the solution-processed V2O5 interlayer as an electron-blocking layer can effectively prevent the leakage current at the organic/Ag interface. The power conversion efficiency is improved from 2.5% to 3.56% by the introduction of the V2O5 interlayer. The V2O5 interlayer also serves as an optical spacer to enhance light absorption, and thereby increases the photocurrent. Compared to the vacuum-deposited techniques, the fabrication of the solution-processed V2O5 interlayer is simple and effective. The solution-based approach makes it attractive for applications to mass production and potentially printed organic electronics. Ó 2009 Elsevier B.V. All rights reserved.
Keywords: Polymer solar cells Vanadium oxide Solution processing Interlayer Optical spacer ZnO nanorod
1. Introduction Polymer solar cells (PSCs) have attracted much attention due to their great potentials for large-area, lightweight, flexible, and low-cost devices [1–6]. Recently, bulk-heterojunction (BHJ) solar cells based on poly(3-hexylthiophene) (P3HT) and (6,6)-phenyl C61 butyric acid methyl ester (PCBM) with power conversion efficiency
* Corresponding author. Address: Institute of Photonics and Optoelectronics, National Taiwan University, Taipei, 10617 Taiwan, ROC. Tel.: +886 2 33663540; fax: +886 2 23642603. E-mail address: cfl[email protected] (C.-F. Lin). 1566-1199/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.orgel.2009.05.017
(PCE) of 4–5% have been reported [7–9]. However, control of the transportation of the charge carriers at interfaces is one of the most challenging issues in the improvement of PSCs. It has been reported that the insertion of an interlayer between the organic layer and the anode improves the device performance. To date, poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) [9,10] and transition metal oxides [11–14] have been employed for this purpose. However, only the deposition of PEDOT:PSS layer can be easily processed by solution-based coating techniques. Most transition metal oxides as the anode interlayers are deposited by the vacuum evaporation, which could detract from the advantage of the ease of PSC fabrication. Using solution-processed transition metal
J.-S. Huang et al. / Organic Electronics 10 (2009) 1060–1065
oxide as the anode interlayer for improving the PSC performance has seldom been reported. The aim of this work is to realize a low-cost and highefficiency inverted PSC hybridized with ZnO nanorod arrays by introduction of a solution-processed vanadium oxide (V2O5) as the anode interlayer. Our investigation shows that the photovoltaic device performance is improved by the introduction of the V2O5 interlayer due to the efficient suppression of the leakage currents at the organic/metal interface. Compared to the conventional BHJ structure (indium tin oxide (ITO)/PEDOT:PSS/active layer/ Al), the use of the inverted structure overcomes some obstacles such as the facile oxidation of Al [15] and the electrical inhomogeneities of PEDOT:PSS as well as its corrosion to ITO [16]. The inverted PSCs utilize an air-stable high work-function electrode as the back contact to collect holes and metal–oxide nanostructures at the ITO to collect electrons [17–19]. Furthermore, it has been reported that the ZnO nanorods have beneficial effects of collecting and transporting electrons in the inverted PSCs hybridized with the ZnO nanorods [20]. Our works combine these advantages of V2O5 interlayer and ZnO nanorods, which thereby suppress the leakage currents and improve the collection and transportation of the charge carriers, resulting in enhancements of PCE, open-circuit voltage (VOC), and fill factor (FF) of the devices. In addition, the V2O5 interlayer can serve as an optical spacer to increase light absorption, leading to an increased short-circuit density (JSC). Moreover, the V2O5 interlayer and ZnO nanorod arrays both are fabricated from simple solution-based processes, which are well-suited for use in high-throughput roll-toroll manufacturing. 2. Experimental The structure and the energy level diagram of the inverted PSCs of ITO/ZnO/P3HT:PCBM/V2O5/Ag are schematically presented in Fig. 1. Devices were fabricated on cleaned ITO-coated glass substrates (7 X/sq.). ZnO seed layer (50 nm) was spin-coated from a 0.5-mol solution of zinc acetate dihydrate in 2-methoxyethanol followed by annealing at 200 °C for an hour in air. Hydrothermal
Fig. 1. (a) Device structure of the photovoltaic cells. (b) Energy band diagram for the photovoltaic cells in this study. The work function value of Ag is referred to the Ref. [28].
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growth of the ZnO nanorod arrays (100 nm in length and 50 nm in diameter) was achieved by suspending the ZnO seed-coated substrates in an aqueous solution of 50-mM zinc nitrate at 90 °C in an oven. Subsequently, a solution containing 20-mg P3HT and 20-mg PCBM in 1ml o-dichlorobenzene (o-DCB) was spin-coated on top of the ZnO nanorods and dried slowly over the course of 40 min in air, forming the photoactive layer with a thickness of 300 nm. V2O5 powder (Riedel-de Haën, 99%) was homogeneously dispersed and suspended in isopropanol at different concentrations by using ultrasonic agitation. During the process of the ultrasonic agitation, it was observed that the V2O5 powder was pulverized to smaller particles. After the ultrasonic agitation, the color of the V2O5 colloidal solution is uniformly orange. Then the V2O5 colloidal solution was spin-casted in air on top of the photoactive layer. Finally, silver film (200 nm) was deposited on top in a vacuum of 2 106 torr. Devices were unencapsulated, stored in air, and illuminated at 100 mW/cm2 from a ThermoOriel 150 W solar simulator with AM 1.5G filters. The solar simulator was calibrated using a reference Si solar cell. All electrical measurements were carried out in air at room temperature. The active area of the device irradiated by the light was defined as 10 mm2 by using a photomask, so no extra current outside of the defined area was collected. Current density– voltage (J–V) curves were measured with a Keithley 2400 source measurement unit. The surface morphologies of the photoactive layers were measured by atomic force microscopy (AFM). The reflectance spectra of the devices were obtained using a Perkin–Elmer Lambda 35 UV–vis spectrophotometer. The transmission spectrum of the V2O5 layer was measured using the same UV–vis spectrophotometer. The crystallinity of V2O5 was analyzed at room temperature by X-ray diffraction (XRD) using Cu Ka radiation. 3. Results and discussion In order to avoid mixing or damaging the photoactive layer during the coating of V2O5, the selection of solvent
Fig. 2. The J–V curves of the photovoltaic devices with the V2O5 interlayer from various concentrations under 100 mW/cm2 AM 1.5G irradiation.
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Table 1 Photovoltaic parameters and efficiencies of inverted PSCs with the V2O5 interlayer from various concentrations (solvent:isopropanol). Concentration (lg/ml)
JSC (mA/cm2)
VOC (V)
FF (%)
PCE (%)
RS (X cm2)
RSH (X cm2)
No. V2O5 25 50 100 250 1000
10.21 10.49 10.61 10.75 11.16 8.55
0.50 0.51 0.51 0.55 0.52 0.50
49.36 49.91 53.96 60.21 51.35 46.78
2.52 2.67 2.92 3.56 2.98 2
2.24 2.12 2.10 1.35 3.4 6.3
394 570 579 620 431 250
for V2O5 is an important issue. That is, the solvent for V2O5 must be orthogonal to o-DCB. Several solvents including toluene, chlorobenzene, 2-methoxyethanol, dichlorometh-
ane, and isopropanol have been experimented for the V2O5. However, only the use of isopropanol as the solvent has negligible influences on the previously deposited photoactive layer due to the polar characteristics of these solvents. In general, the polarity of a solvent is related to its dielectric constant [21]. The relative dielectric constant (er) of isopropanol is 20, which is much larger than that of the P3HT/PCBM film (er 3.5) [22]. It is believed that the large dielectric constant qualifies isopropanol as an orthogonal solvent in this work. Fig. 2a shows the J–V characteristics of the devices with various concentrations of V2O5. The device without the V2O5 interlayer exhibits a JSC of 10.21 mA/cm2, a VOC of 0.5 V, and a FF of 49.36%, resulting in a PCE of 2.52%. The performance is similar to other reports on the same struc-
Fig. 3. (a) AFM images of the photoactive layers covered with and without the optimum V2O5 interlayer. AFM image scans are 5 5 lm. (b) Transmission spectrum of the V2O5 layer (from the 100 lg/ml V2O5 colloidal solution) on a glass substrate. (c) XRD spectrum of V2O5.
J.-S. Huang et al. / Organic Electronics 10 (2009) 1060–1065
ture without the V2O5 interlayer [16,19]. Introducing a 25lg/ml V2O5 interlayer, PCE is slightly improved to 2.67% with JSC of 10.49 mA/cm2, VOC of 0.51 V, and FF of 49.91%. When the concentration of V2O5 further increases to 100 lg/ml, the device has a significant improvement with JSC of 10.75 mA/cm2, VOC of 0.55 V, and FF of 60.21%. This results in a considerable improvement of PCE up to 3.56%, a 41% improvement. These results show that the V2O5 can act as a functional interlayer to enhance the photovoltaic performance. A series of V2O5 concentrations (25, 50, 100, 250, and 1000 lg/ml) is further investigated and summarized in Table 1. The JSC, VOC, and FF increase with the V2O5 concentration from 25 to 100 lg/ml. The highest PCE of 3.56% is achieved at the concentration of 100 lg/ml, showing that the optimum V2O5 interlayer is obtained. As a lowconcentration V2O5 interlayer is introduced (50 lg/ml or less), the improvement of device performance is not obvious. It is suspected that the concentration is too low to cover the photoactive layer completely. As a result, the leakage current will not be efficiently reduced. However, as the concentration of V2O5 increases to 1000 lg/ml, most V2O5 particles cluster together (2 lm in average), resulting in the increased contact resistance and thus leading to a low photocurrent (JSC 8.55 mA/cm2). Fig. 3a shows the AFM images of the photoactive layer covered with and without the optimum V2O5 interlayer. The V2O5 particles can be clearly observed. The rootmean-square roughness (rRMS) of the photoactive layer with the optimum V2O5 interlayer is 18.4 nm, which is about the 2 times of that without V2O5 (rRMS 10.6 nm). The AFM images clearly indicate that at the concentration of 100 lg/ml the photoactive layer is almost fully covered with V2O5. Fig. 3b shows the transmission spectrum of the V2O5 layer deposited from the 100 lg/ml V2O5 colloidal solution on a glass substrate. It shows that the V2O5 layer is almost transparent in the visible region (transmittance > 97%). Fig. 3c shows the XRD spectrum of V2O5. The diffraction peaks appears in the 2h range from 10° to 35° characterizing the orthorhombic crystalline structure of V2O5. Note that the performance of the device without the V2O5 interlayer in our experiments has similar performance to other reports [20,23]. By introducing the optimum V2O5 interlayer, the device has significant improvements in FF (from 0.49 to 0.6) and VOC (from 0.5 to 0.55 V). This indicates that V2O5 efficiently suppresses the leakage currents at the organic/Ag interface. Considering the device without V2O5, both P3HT and PCBM are in direct contact with Ag. It is possible for electrons to transfer from PCBM to Ag, thereby increasing the leakage currents. However, incorporating a V2O5 interlayer introduces two additional interfaces, organic/V2O5 and V2O5/Ag. As shown in Fig. 1b, the conduction band of V2O5 (2.4 eV) (Ref. [8]) is higher than the lowest unoccupied molecular orbital level of PCBM (3.7 eV), showing that V2O5 can block the reverse electron flow from PCBM to Ag. Thereby, V2O5 can effectively prevent the leakage current at the organic/Ag interface. In addition, the valence band of V2O5 (4.7 eV) (Ref. [8]) is close to the highest occupied molecular orbital level of P3HT (5.1 eV), revealing that V2O5 will help collecting holes.
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Moreover, the series resistance (RS, defined from the J–V curves near 1.5 V under light illumination) is 1.35 X cm2 and 3.09 X cm2 for the device with and without the optimum V2O5 interlayer, respectively. The shunt resistance (RSH, defined from the J–V curves near 0 V under light illumination) is 610 X cm2 and 376 X cm2 for the device with and without the optimum V2O5 interlayer, respectively. It is known that the high RSH indicates less leakage current across the cell and contributes to the improved FF and VOC [24]. Another evidence for less leakage current is the rectification ratio (RR, defined as the current ratio at ±1.5 V from the J–V curves measured in the dark). The RR of the device without V2O5 is 4.37 102, while that of the device with the optimum V2O5 interlayer increases to 1.77 104. The high RR and elevated RSH both are strong evidences showing that the V2O5 interlayer can serve as an electron-blocking layer to effectively prevent the leakage currents, resulting in the dramatic improvement in PCE. Fig. 4a compares the incident photon-to-current conversion efficiency (IPCE) spectrum of the devices with and without the optimum V2O5 interlayer. The IPCE is defined as the number of photogenerated charge carriers
Fig. 4. (a) IPCE spectra for the devices with and without the optimum V2O5 interlayer. (b) The change in absorption spectrum [Da(k)] and the difference in IPCE spectrum [DIPCE(k)] resulting from the insertion of the optimum V2O5 interlayer. The inset is a schematic of the optical beam path in the both samples. The variables are defined in the text.
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contributing to the current per incident photon. The device with V2O5 interlayer shows the typical spectral response of P3HT:PCBM blend with a maximum IPCE of 69% at 515 nm, while for the device without V2O5 interlayer, the peak reaches 65% only. The IPCE spectra are consistent with the measured JSC in the devices. The insertion of V2O5 demonstrates a substantial enhancement of 6% at 515 nm in the IPCE. This enhancement agrees with the increase in JSC (5% increase in the device with V2O5). It indicates that the V2O5 interlayer also contributes to the increase in photocurrent. To further clarify the role of the V2O5 interlayer, we measured the reflectance (R) spectra of the devices with and without the optimum V2O5 interlayer. Since the two devices are identical except the addition of the V2O5 layer, comparison of the reflectance yields information on the additional absorption, Da(k), in the photoactive layer as a result of the spatial redistribution of the light intensity by the V2O5 interlayer (Fig. 4b). The Da(k) is given by [25]
Rwith V2 O5 ðkÞ 1 DaðkÞ pffiffiffi ln Rwithout V2 O5 ðkÞ 2 2d
where Rwith V2 O5 ðkÞ is the reflection from a device with the V2O5 interlayer, Rwithout V2 O5 ðkÞ is the reflection from an identical device without the V2O5 interlayer, and d is the thickness of the photoactive layer (d is 300 nm in both). The result shows a clear increase in absorption over the spectral region of the interband transitions. Since the spectral features of the P3HT absorption are evident in the Da spectrum, the increased absorption arises from a better match of the spatial distribution of the light intensity to the position of the photoactive layer. Fig. 4b also shows the difference in IPCE spectrum, DIPCE(k), between the devices with and without the optimum V2O5 interlayer. This spectrum reveals three peaks at 510, 540, and 600 nm, respectively. It implies that the contribution of the V2O5 interlayer in photocurrent is mainly at the three peaks which are vibronic features from the P3HT molecules [26]. Moreover, the feature of the DIPCE spectrum is analogous with Da spectrum, showing that the increased optical absorption is nearly transferred to the photocurrent. Evidently, the V2O5 interlayer functions as an optical spacer to increase the optical absorption by spatially redistributed the light intensity and thereby increase the photocurrent. Although PEDOT:PSS layer can be solution processed, its hygroscopic nature is likely to form insulating patches due to the water adsorption, thus degrading the devices [27]. In contrast, V2O5 is relatively insensitive to water and stable in air. The solution-processed V2O5 interlayer can serve as a barrier preventing oxygen or water from entering and degrading the photoactive layer. In addition, this approach does not need annealing treatment like PEDOT:PSS nor vacuum equipments, so it is simple, expeditious, and effective. This is very important for commercial realization of low-cost and large-area printed solar cells. 4. Conclusion In conclusion, we have demonstrated the use of the solution-processed V2O5 as the anode interlayer in in-
verted PSCs hybridized with ZnO nanorods. The optimum V2O5 interlayer is obtained at the concentration of 100 lg/ml, because the photoactive layer is almost completely covered with V2O5 at this condition. The highest PCE of 3.56% is achieved for the device with the optimum V2O5 interlayer, which is comparable to that of the devices with a vacuum-deposited V2O5 interlayer [28]. Our investigations show that the V2O5 interlayer can effectively prevent the leakage currents at the organic/Ag interface leading to improvements in VOC, FF, RS, and RSH. The optical absorption and IPCE are also improved by the optical spacer effect of the V2O5 interlayer, thus leading to the increased photocurrent. Compared to the vacuum-deposited techniques, this approach is simple, expeditious, and effective. It is also advantageous for potential applications to mass production of various large-area printed electronics with a very low cost. Acknowledgement This work was supported by the National Science Council, Taiwan, Republic of China, with Grant Nos. NSC96-2221-E-002-277-MY3, NSC97-2218-E-002-013, and NSC97-2221-E-002-039-MY3. References [1] F.C. Krebs, Polymer solar cell modules prepared using roll-to-roll methods: knife-over-edge coating, slot-die coating and screen printing, Sol. Energy Mater. Sol. Cells 93 (2009) 465–475. [2] F.C. Krebs, Fabrication and processing of polymer solar cells: a review of printing and coating techniques, Sol. Energy Mater. Sol. Cells 93 (2009) 394–412. [3] F.C. Krebs, M. Jørgensen, K. Norrman, O. Hagemann, J. Alstrup, T.D. Nielsen, J. Fyenbo, K. Larsen, J. Kristensen, A complete process for production of flexible large area polymer solar cells entirely using screen printing – first public demonstration, Sol. Energy Mater. Sol. Cells 93 (2009) 422–441. [4] C.J. Brabec, J.R. Durrant, Solution-processed organic solar cells, MRS Bull. 33 (2008) 670–675. [5] C.J. Brabec, J.A. Hauch, P. Schilinsky, C. Waldauf, Production aspects of organic photovoltaics and their impact on the commercialization of devices, MRS Bull. 30 (2005) 50–52. [6] C.J. Brabec, N.S. Sariciftci, J.C. Hummelen, Plastic solar cells, Adv. Funct. Mater. 11 (2001) 15–26. [7] G. Li, V. Shrotriya, J.S. Huang, Y. Yao, T. Moriarty, K. Emery, Y. Yang, High-efficiency solution processable polymer photovoltaic cells by self-organization of polymer blends, Nat. Mater. 4 (2005) 864–868. [8] A.J. Moule, K. Meerholz, Controlling morphology in polymer– fullerene mixtures, Adv. Mater. 20 (2008) 240–245. [9] B.C. Thompson, J.M.J. Fréchet, Organic photovoltaics – polymer– fullerene composite solar cells, Angew. Chem. Int. Ed. 47 (2008) 58– 77. [10] S. Günes, H. Neugebauer, N.S. Sariciftci, Conjugated polymer-based organic solar cells, Chem. Rev. 107 (2007) 1324–1338. [11] V. Shrotriya, G. Li, Y. Yao, C.W. Chu, Y. Yang, Transition metal oxides as the buffer layer for polymer photovoltaic cells, Appl. Phys. Lett. 88 (2006) 073508. [12] M.D. Irwin, B. Buchholz, A.W. Hains, R.P.H. Chang, T.J. Marks, P-type semiconducting nickel oxide as an efficiency-enhancing anode interfacial layer in polymer bulk-heterojunction solar cells, Proc. Nat. Acad. Sci. USA 105 (2008) 2783–2787. [13] C. Tao, S. Ruan, X. Zhang, G. Xie, L. Shen, X. Kong, W. Dong, C. Liu, W. Chen, Performance improvement of inverted polymer solar cells with different top electrodes by introducing a MoO3 buffer layer, Appl. Phys. Lett. 93 (2008) 193307. [14] C. Tao, S.P. Ruan, G.H. Xie, X.Z. Kong, L. Shen, F.X. Meng, C.X. Liu, X.D. Zhang, W. Dong, W.Y. Chen, Role of tungsten oxide in inverted polymer solar cells, Appl. Phys. Lett. 94 (2009) 043311. [15] M. Jorgensen, K. Norrman, F.C. Krebs, Stability/degradation of polymer solar cells, Sol. Energy Mater. Sol. Cells 92 (2008) 686–714.
J.-S. Huang et al. / Organic Electronics 10 (2009) 1060–1065 [16] M.P. de Jong, L.J. van Ijzendoorn, M.J.A. de Voigt, Stability of the interface between indium–tin–oxide and poly(3,4ethylenedioxythiophene)/poly(styrenesulfonate) in polymer lightemitting diodes, Appl. Phys. Lett. 77 (2000) 2255–2257. [17] M.S. White, D.C. Olson, S.E. Shaheen, N. Kopidakis, D.S. Ginley, Inverted bulk-heterojunction organic photovoltaic device using a solution-derived ZnO underlayer, Appl. Phys. Lett. 89 (2006) 143517. [18] F.C. Krebs, Air stable polymer photovoltaics based on a process free from vacuum steps and fullerenes, Sol. Energy Mater. Sol. Cells 92 (2008) 715–726. [19] F.C. Krebs, Y. Thomann, R. Thomann, J.W. Andreasen, A simple nanostructured polymer/ZnO hybrid solar cell-preparation and operation in air, Nanotechnology 19 (2008) 424013. [20] K. Takanezawa, K. Hirota, Q.S. Wei, K. Tajima, K. Hashimoto, Efficient charge collection with ZnO nanorod array in hybrid photovoltaic devices, J. Phys. Chem. C 111 (2007) 7218–7223. [21] T.H. Lowery, K.S. Richardson, Mechanism and Theory in Organic Chemistry, third ed., Harper Collins Publishers, New York, 1987. [22] L.J.A. Koster, V.D. Mihailetchi, P.W.M. Blom, Ultimate efficiency of polymer/fullerene bulk heterojunction solar cells, Appl. Phys. Lett. 88 (2006) 093511.
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[23] D.C. Olson, J. Piris, R.T. Collins, S.E. Shaheen, D.S. Ginley, Hybrid photovoltaic devices of polymer and ZnO nanofiber composites, Thin Solid Films 496 (2006) 26–29. [24] A. Moliton, J.M. Nunzi, How to model the behaviour of organic photovoltaic cells, Polym. Int. 55 (2006) 583–600. [25] J.K. Lee, N.E. Coates, S. Cho, N.S. Cho, D. Moses, G.C. Bazan, K. Lee, A.J. Heeger, Efficacy of TiOx optical spacer in bulk-heterojunction solar cells processed with 1,8-octanedithiol, Appl. Phys. Lett. 92 (2008) 243308. [26] Y. Kim, S. Cook, S.M. Tuladhar, S.A. Choulis, J. Nelson, J.R. Durrant, D.D.C. Bradley, M. Giles, I. McCulloch, C.S. Ha, M. Ree, A strong regioregularity effect in self-organizing conjugated polymer films and high-efficiency polythiophene: fullerene solar cells, Nat. Mater. 5 (2006) 197–203. [27] K. Kawano, R. Pacios, D. Poplavskyy, J. Nelson, D.D.C. Bradley, J.R. Durrant, Degradation of organic solar cells due to air exposure, Sol. Energy Mater. Sol. Cells 90 (2006) 3520–3530. [28] K. Takanezawa, K. Tajima, K. Hashimoto, Efficiency enhancement of polymer photovoltaic devices hybridized with ZnO nanorod arrays by the introduction of a vanadium oxide buffer layer, Appl. Phys. Lett. 93 (2008) 063308.
Contents lists available at ScienceDirect
Organic Electronics journal homepage: www.elsevier.com/locate/orgel
Solution-processed vanadium oxide as an anode interlayer for inverted polymer solar cells hybridized with ZnO nanorods Jing-Shun Huang a, Chen-Yu Chou a, Meng-Yueh Liu a, Kao-Hua Tsai a, Wen-Han Lin a, Ching-Fuh Lin a,b,* a b
Institute of Photonics and Optoelectronics, National Taiwan University, Taipei, 10617 Taiwan, ROC Graduate Institute of Electronics Engineering and Department of Electrical Engineering, National Taiwan University, Taipei, 10617 Taiwan, ROC
a r t i c l e
i n f o
Article history: Received 17 March 2009 Received in revised form 16 May 2009 Accepted 19 May 2009 Available online 27 May 2009
PACS: 72.40.+w 72.80.Ga 81.05.Dz 81.07.-b 84.60.-h 85.60.-q
a b s t r a c t Solution-processed vanadium oxide (V2O5) as an anode interlayer is introduced between the organic layer and the Ag electrode for improving the performance of the low-cost inverted polymer solar cells hybridized with ZnO nanorods. Our investigations indicate that the solution-processed V2O5 interlayer as an electron-blocking layer can effectively prevent the leakage current at the organic/Ag interface. The power conversion efficiency is improved from 2.5% to 3.56% by the introduction of the V2O5 interlayer. The V2O5 interlayer also serves as an optical spacer to enhance light absorption, and thereby increases the photocurrent. Compared to the vacuum-deposited techniques, the fabrication of the solution-processed V2O5 interlayer is simple and effective. The solution-based approach makes it attractive for applications to mass production and potentially printed organic electronics. Ó 2009 Elsevier B.V. All rights reserved.
Keywords: Polymer solar cells Vanadium oxide Solution processing Interlayer Optical spacer ZnO nanorod
1. Introduction Polymer solar cells (PSCs) have attracted much attention due to their great potentials for large-area, lightweight, flexible, and low-cost devices [1–6]. Recently, bulk-heterojunction (BHJ) solar cells based on poly(3-hexylthiophene) (P3HT) and (6,6)-phenyl C61 butyric acid methyl ester (PCBM) with power conversion efficiency
* Corresponding author. Address: Institute of Photonics and Optoelectronics, National Taiwan University, Taipei, 10617 Taiwan, ROC. Tel.: +886 2 33663540; fax: +886 2 23642603. E-mail address: cfl[email protected] (C.-F. Lin). 1566-1199/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.orgel.2009.05.017
(PCE) of 4–5% have been reported [7–9]. However, control of the transportation of the charge carriers at interfaces is one of the most challenging issues in the improvement of PSCs. It has been reported that the insertion of an interlayer between the organic layer and the anode improves the device performance. To date, poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) [9,10] and transition metal oxides [11–14] have been employed for this purpose. However, only the deposition of PEDOT:PSS layer can be easily processed by solution-based coating techniques. Most transition metal oxides as the anode interlayers are deposited by the vacuum evaporation, which could detract from the advantage of the ease of PSC fabrication. Using solution-processed transition metal
J.-S. Huang et al. / Organic Electronics 10 (2009) 1060–1065
oxide as the anode interlayer for improving the PSC performance has seldom been reported. The aim of this work is to realize a low-cost and highefficiency inverted PSC hybridized with ZnO nanorod arrays by introduction of a solution-processed vanadium oxide (V2O5) as the anode interlayer. Our investigation shows that the photovoltaic device performance is improved by the introduction of the V2O5 interlayer due to the efficient suppression of the leakage currents at the organic/metal interface. Compared to the conventional BHJ structure (indium tin oxide (ITO)/PEDOT:PSS/active layer/ Al), the use of the inverted structure overcomes some obstacles such as the facile oxidation of Al [15] and the electrical inhomogeneities of PEDOT:PSS as well as its corrosion to ITO [16]. The inverted PSCs utilize an air-stable high work-function electrode as the back contact to collect holes and metal–oxide nanostructures at the ITO to collect electrons [17–19]. Furthermore, it has been reported that the ZnO nanorods have beneficial effects of collecting and transporting electrons in the inverted PSCs hybridized with the ZnO nanorods [20]. Our works combine these advantages of V2O5 interlayer and ZnO nanorods, which thereby suppress the leakage currents and improve the collection and transportation of the charge carriers, resulting in enhancements of PCE, open-circuit voltage (VOC), and fill factor (FF) of the devices. In addition, the V2O5 interlayer can serve as an optical spacer to increase light absorption, leading to an increased short-circuit density (JSC). Moreover, the V2O5 interlayer and ZnO nanorod arrays both are fabricated from simple solution-based processes, which are well-suited for use in high-throughput roll-toroll manufacturing. 2. Experimental The structure and the energy level diagram of the inverted PSCs of ITO/ZnO/P3HT:PCBM/V2O5/Ag are schematically presented in Fig. 1. Devices were fabricated on cleaned ITO-coated glass substrates (7 X/sq.). ZnO seed layer (50 nm) was spin-coated from a 0.5-mol solution of zinc acetate dihydrate in 2-methoxyethanol followed by annealing at 200 °C for an hour in air. Hydrothermal
Fig. 1. (a) Device structure of the photovoltaic cells. (b) Energy band diagram for the photovoltaic cells in this study. The work function value of Ag is referred to the Ref. [28].
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growth of the ZnO nanorod arrays (100 nm in length and 50 nm in diameter) was achieved by suspending the ZnO seed-coated substrates in an aqueous solution of 50-mM zinc nitrate at 90 °C in an oven. Subsequently, a solution containing 20-mg P3HT and 20-mg PCBM in 1ml o-dichlorobenzene (o-DCB) was spin-coated on top of the ZnO nanorods and dried slowly over the course of 40 min in air, forming the photoactive layer with a thickness of 300 nm. V2O5 powder (Riedel-de Haën, 99%) was homogeneously dispersed and suspended in isopropanol at different concentrations by using ultrasonic agitation. During the process of the ultrasonic agitation, it was observed that the V2O5 powder was pulverized to smaller particles. After the ultrasonic agitation, the color of the V2O5 colloidal solution is uniformly orange. Then the V2O5 colloidal solution was spin-casted in air on top of the photoactive layer. Finally, silver film (200 nm) was deposited on top in a vacuum of 2 106 torr. Devices were unencapsulated, stored in air, and illuminated at 100 mW/cm2 from a ThermoOriel 150 W solar simulator with AM 1.5G filters. The solar simulator was calibrated using a reference Si solar cell. All electrical measurements were carried out in air at room temperature. The active area of the device irradiated by the light was defined as 10 mm2 by using a photomask, so no extra current outside of the defined area was collected. Current density– voltage (J–V) curves were measured with a Keithley 2400 source measurement unit. The surface morphologies of the photoactive layers were measured by atomic force microscopy (AFM). The reflectance spectra of the devices were obtained using a Perkin–Elmer Lambda 35 UV–vis spectrophotometer. The transmission spectrum of the V2O5 layer was measured using the same UV–vis spectrophotometer. The crystallinity of V2O5 was analyzed at room temperature by X-ray diffraction (XRD) using Cu Ka radiation. 3. Results and discussion In order to avoid mixing or damaging the photoactive layer during the coating of V2O5, the selection of solvent
Fig. 2. The J–V curves of the photovoltaic devices with the V2O5 interlayer from various concentrations under 100 mW/cm2 AM 1.5G irradiation.
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Table 1 Photovoltaic parameters and efficiencies of inverted PSCs with the V2O5 interlayer from various concentrations (solvent:isopropanol). Concentration (lg/ml)
JSC (mA/cm2)
VOC (V)
FF (%)
PCE (%)
RS (X cm2)
RSH (X cm2)
No. V2O5 25 50 100 250 1000
10.21 10.49 10.61 10.75 11.16 8.55
0.50 0.51 0.51 0.55 0.52 0.50
49.36 49.91 53.96 60.21 51.35 46.78
2.52 2.67 2.92 3.56 2.98 2
2.24 2.12 2.10 1.35 3.4 6.3
394 570 579 620 431 250
for V2O5 is an important issue. That is, the solvent for V2O5 must be orthogonal to o-DCB. Several solvents including toluene, chlorobenzene, 2-methoxyethanol, dichlorometh-
ane, and isopropanol have been experimented for the V2O5. However, only the use of isopropanol as the solvent has negligible influences on the previously deposited photoactive layer due to the polar characteristics of these solvents. In general, the polarity of a solvent is related to its dielectric constant [21]. The relative dielectric constant (er) of isopropanol is 20, which is much larger than that of the P3HT/PCBM film (er 3.5) [22]. It is believed that the large dielectric constant qualifies isopropanol as an orthogonal solvent in this work. Fig. 2a shows the J–V characteristics of the devices with various concentrations of V2O5. The device without the V2O5 interlayer exhibits a JSC of 10.21 mA/cm2, a VOC of 0.5 V, and a FF of 49.36%, resulting in a PCE of 2.52%. The performance is similar to other reports on the same struc-
Fig. 3. (a) AFM images of the photoactive layers covered with and without the optimum V2O5 interlayer. AFM image scans are 5 5 lm. (b) Transmission spectrum of the V2O5 layer (from the 100 lg/ml V2O5 colloidal solution) on a glass substrate. (c) XRD spectrum of V2O5.
J.-S. Huang et al. / Organic Electronics 10 (2009) 1060–1065
ture without the V2O5 interlayer [16,19]. Introducing a 25lg/ml V2O5 interlayer, PCE is slightly improved to 2.67% with JSC of 10.49 mA/cm2, VOC of 0.51 V, and FF of 49.91%. When the concentration of V2O5 further increases to 100 lg/ml, the device has a significant improvement with JSC of 10.75 mA/cm2, VOC of 0.55 V, and FF of 60.21%. This results in a considerable improvement of PCE up to 3.56%, a 41% improvement. These results show that the V2O5 can act as a functional interlayer to enhance the photovoltaic performance. A series of V2O5 concentrations (25, 50, 100, 250, and 1000 lg/ml) is further investigated and summarized in Table 1. The JSC, VOC, and FF increase with the V2O5 concentration from 25 to 100 lg/ml. The highest PCE of 3.56% is achieved at the concentration of 100 lg/ml, showing that the optimum V2O5 interlayer is obtained. As a lowconcentration V2O5 interlayer is introduced (50 lg/ml or less), the improvement of device performance is not obvious. It is suspected that the concentration is too low to cover the photoactive layer completely. As a result, the leakage current will not be efficiently reduced. However, as the concentration of V2O5 increases to 1000 lg/ml, most V2O5 particles cluster together (2 lm in average), resulting in the increased contact resistance and thus leading to a low photocurrent (JSC 8.55 mA/cm2). Fig. 3a shows the AFM images of the photoactive layer covered with and without the optimum V2O5 interlayer. The V2O5 particles can be clearly observed. The rootmean-square roughness (rRMS) of the photoactive layer with the optimum V2O5 interlayer is 18.4 nm, which is about the 2 times of that without V2O5 (rRMS 10.6 nm). The AFM images clearly indicate that at the concentration of 100 lg/ml the photoactive layer is almost fully covered with V2O5. Fig. 3b shows the transmission spectrum of the V2O5 layer deposited from the 100 lg/ml V2O5 colloidal solution on a glass substrate. It shows that the V2O5 layer is almost transparent in the visible region (transmittance > 97%). Fig. 3c shows the XRD spectrum of V2O5. The diffraction peaks appears in the 2h range from 10° to 35° characterizing the orthorhombic crystalline structure of V2O5. Note that the performance of the device without the V2O5 interlayer in our experiments has similar performance to other reports [20,23]. By introducing the optimum V2O5 interlayer, the device has significant improvements in FF (from 0.49 to 0.6) and VOC (from 0.5 to 0.55 V). This indicates that V2O5 efficiently suppresses the leakage currents at the organic/Ag interface. Considering the device without V2O5, both P3HT and PCBM are in direct contact with Ag. It is possible for electrons to transfer from PCBM to Ag, thereby increasing the leakage currents. However, incorporating a V2O5 interlayer introduces two additional interfaces, organic/V2O5 and V2O5/Ag. As shown in Fig. 1b, the conduction band of V2O5 (2.4 eV) (Ref. [8]) is higher than the lowest unoccupied molecular orbital level of PCBM (3.7 eV), showing that V2O5 can block the reverse electron flow from PCBM to Ag. Thereby, V2O5 can effectively prevent the leakage current at the organic/Ag interface. In addition, the valence band of V2O5 (4.7 eV) (Ref. [8]) is close to the highest occupied molecular orbital level of P3HT (5.1 eV), revealing that V2O5 will help collecting holes.
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Moreover, the series resistance (RS, defined from the J–V curves near 1.5 V under light illumination) is 1.35 X cm2 and 3.09 X cm2 for the device with and without the optimum V2O5 interlayer, respectively. The shunt resistance (RSH, defined from the J–V curves near 0 V under light illumination) is 610 X cm2 and 376 X cm2 for the device with and without the optimum V2O5 interlayer, respectively. It is known that the high RSH indicates less leakage current across the cell and contributes to the improved FF and VOC [24]. Another evidence for less leakage current is the rectification ratio (RR, defined as the current ratio at ±1.5 V from the J–V curves measured in the dark). The RR of the device without V2O5 is 4.37 102, while that of the device with the optimum V2O5 interlayer increases to 1.77 104. The high RR and elevated RSH both are strong evidences showing that the V2O5 interlayer can serve as an electron-blocking layer to effectively prevent the leakage currents, resulting in the dramatic improvement in PCE. Fig. 4a compares the incident photon-to-current conversion efficiency (IPCE) spectrum of the devices with and without the optimum V2O5 interlayer. The IPCE is defined as the number of photogenerated charge carriers
Fig. 4. (a) IPCE spectra for the devices with and without the optimum V2O5 interlayer. (b) The change in absorption spectrum [Da(k)] and the difference in IPCE spectrum [DIPCE(k)] resulting from the insertion of the optimum V2O5 interlayer. The inset is a schematic of the optical beam path in the both samples. The variables are defined in the text.
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contributing to the current per incident photon. The device with V2O5 interlayer shows the typical spectral response of P3HT:PCBM blend with a maximum IPCE of 69% at 515 nm, while for the device without V2O5 interlayer, the peak reaches 65% only. The IPCE spectra are consistent with the measured JSC in the devices. The insertion of V2O5 demonstrates a substantial enhancement of 6% at 515 nm in the IPCE. This enhancement agrees with the increase in JSC (5% increase in the device with V2O5). It indicates that the V2O5 interlayer also contributes to the increase in photocurrent. To further clarify the role of the V2O5 interlayer, we measured the reflectance (R) spectra of the devices with and without the optimum V2O5 interlayer. Since the two devices are identical except the addition of the V2O5 layer, comparison of the reflectance yields information on the additional absorption, Da(k), in the photoactive layer as a result of the spatial redistribution of the light intensity by the V2O5 interlayer (Fig. 4b). The Da(k) is given by [25]
Rwith V2 O5 ðkÞ 1 DaðkÞ pffiffiffi ln Rwithout V2 O5 ðkÞ 2 2d
where Rwith V2 O5 ðkÞ is the reflection from a device with the V2O5 interlayer, Rwithout V2 O5 ðkÞ is the reflection from an identical device without the V2O5 interlayer, and d is the thickness of the photoactive layer (d is 300 nm in both). The result shows a clear increase in absorption over the spectral region of the interband transitions. Since the spectral features of the P3HT absorption are evident in the Da spectrum, the increased absorption arises from a better match of the spatial distribution of the light intensity to the position of the photoactive layer. Fig. 4b also shows the difference in IPCE spectrum, DIPCE(k), between the devices with and without the optimum V2O5 interlayer. This spectrum reveals three peaks at 510, 540, and 600 nm, respectively. It implies that the contribution of the V2O5 interlayer in photocurrent is mainly at the three peaks which are vibronic features from the P3HT molecules [26]. Moreover, the feature of the DIPCE spectrum is analogous with Da spectrum, showing that the increased optical absorption is nearly transferred to the photocurrent. Evidently, the V2O5 interlayer functions as an optical spacer to increase the optical absorption by spatially redistributed the light intensity and thereby increase the photocurrent. Although PEDOT:PSS layer can be solution processed, its hygroscopic nature is likely to form insulating patches due to the water adsorption, thus degrading the devices [27]. In contrast, V2O5 is relatively insensitive to water and stable in air. The solution-processed V2O5 interlayer can serve as a barrier preventing oxygen or water from entering and degrading the photoactive layer. In addition, this approach does not need annealing treatment like PEDOT:PSS nor vacuum equipments, so it is simple, expeditious, and effective. This is very important for commercial realization of low-cost and large-area printed solar cells. 4. Conclusion In conclusion, we have demonstrated the use of the solution-processed V2O5 as the anode interlayer in in-
verted PSCs hybridized with ZnO nanorods. The optimum V2O5 interlayer is obtained at the concentration of 100 lg/ml, because the photoactive layer is almost completely covered with V2O5 at this condition. The highest PCE of 3.56% is achieved for the device with the optimum V2O5 interlayer, which is comparable to that of the devices with a vacuum-deposited V2O5 interlayer [28]. Our investigations show that the V2O5 interlayer can effectively prevent the leakage currents at the organic/Ag interface leading to improvements in VOC, FF, RS, and RSH. The optical absorption and IPCE are also improved by the optical spacer effect of the V2O5 interlayer, thus leading to the increased photocurrent. Compared to the vacuum-deposited techniques, this approach is simple, expeditious, and effective. It is also advantageous for potential applications to mass production of various large-area printed electronics with a very low cost. Acknowledgement This work was supported by the National Science Council, Taiwan, Republic of China, with Grant Nos. NSC96-2221-E-002-277-MY3, NSC97-2218-E-002-013, and NSC97-2221-E-002-039-MY3. References [1] F.C. Krebs, Polymer solar cell modules prepared using roll-to-roll methods: knife-over-edge coating, slot-die coating and screen printing, Sol. Energy Mater. Sol. Cells 93 (2009) 465–475. [2] F.C. Krebs, Fabrication and processing of polymer solar cells: a review of printing and coating techniques, Sol. Energy Mater. Sol. Cells 93 (2009) 394–412. [3] F.C. Krebs, M. Jørgensen, K. Norrman, O. Hagemann, J. Alstrup, T.D. Nielsen, J. Fyenbo, K. Larsen, J. Kristensen, A complete process for production of flexible large area polymer solar cells entirely using screen printing – first public demonstration, Sol. Energy Mater. Sol. Cells 93 (2009) 422–441. [4] C.J. Brabec, J.R. Durrant, Solution-processed organic solar cells, MRS Bull. 33 (2008) 670–675. [5] C.J. Brabec, J.A. Hauch, P. Schilinsky, C. Waldauf, Production aspects of organic photovoltaics and their impact on the commercialization of devices, MRS Bull. 30 (2005) 50–52. [6] C.J. Brabec, N.S. Sariciftci, J.C. Hummelen, Plastic solar cells, Adv. Funct. Mater. 11 (2001) 15–26. [7] G. Li, V. Shrotriya, J.S. Huang, Y. Yao, T. Moriarty, K. Emery, Y. Yang, High-efficiency solution processable polymer photovoltaic cells by self-organization of polymer blends, Nat. Mater. 4 (2005) 864–868. [8] A.J. Moule, K. Meerholz, Controlling morphology in polymer– fullerene mixtures, Adv. Mater. 20 (2008) 240–245. [9] B.C. Thompson, J.M.J. Fréchet, Organic photovoltaics – polymer– fullerene composite solar cells, Angew. Chem. Int. Ed. 47 (2008) 58– 77. [10] S. Günes, H. Neugebauer, N.S. Sariciftci, Conjugated polymer-based organic solar cells, Chem. Rev. 107 (2007) 1324–1338. [11] V. Shrotriya, G. Li, Y. Yao, C.W. Chu, Y. Yang, Transition metal oxides as the buffer layer for polymer photovoltaic cells, Appl. Phys. Lett. 88 (2006) 073508. [12] M.D. Irwin, B. Buchholz, A.W. Hains, R.P.H. Chang, T.J. Marks, P-type semiconducting nickel oxide as an efficiency-enhancing anode interfacial layer in polymer bulk-heterojunction solar cells, Proc. Nat. Acad. Sci. USA 105 (2008) 2783–2787. [13] C. Tao, S. Ruan, X. Zhang, G. Xie, L. Shen, X. Kong, W. Dong, C. Liu, W. Chen, Performance improvement of inverted polymer solar cells with different top electrodes by introducing a MoO3 buffer layer, Appl. Phys. Lett. 93 (2008) 193307. [14] C. Tao, S.P. Ruan, G.H. Xie, X.Z. Kong, L. Shen, F.X. Meng, C.X. Liu, X.D. Zhang, W. Dong, W.Y. Chen, Role of tungsten oxide in inverted polymer solar cells, Appl. Phys. Lett. 94 (2009) 043311. [15] M. Jorgensen, K. Norrman, F.C. Krebs, Stability/degradation of polymer solar cells, Sol. Energy Mater. Sol. Cells 92 (2008) 686–714.
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