Energy Fuels 2010, 24, 3739–3742 Published on Web 02/04/2010
: DOI:10.1021/ef901325e
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Effect of an Ultra-thin Molybdenum Trioxide Layer and Illumination Intensity on the Performance of Organic Photovoltaic Devices† Fujun Zhang,*,‡ Fengyong Sun,§ Yuzhu Shi,§ Zuliang Zhuo,‡ Lifang Lu,‡ Dewei Zhao, Zheng Xu,‡ and Yongsheng Wang‡
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‡ Key Laboratory of Luminescence and Optical Information, Beijing Jiaotong University, Ministry of Education, Beijing 100044, China, §Art and Science Park, School of Information Science and Technology, Liaoning University, Shenyang 110036, China, and School of Electrical and Electronic Engineering, Nanyang Technological University, Singapore 639798, Singapore
Received November 10, 2009. Revised Manuscript Received January 21, 2010
The effect of an ultra-thin molybdenum trioxide (MoO3) layer thickness inserted between the indium tin oxide (ITO) substrate and copper phthalocyanine (CuPc) layer on the performance of organic photovoltaic devices (OPVs) was studied. Experimental results demonstrate that the short-circuit current density (Jsc) was decreased slightly with the increase of MoO3 thickness; meanwhile, the fill factor (FF) was increased from 53.5 to 57.7%, respectively, leading to the improved power conversion efficiency with the optimal thickness of MoO3 (1 nm). The experimental results also reveal that the Ohmic contact is formed with the deposition of MoO3. Further, the effect of the MoO3 layer was checked from the variation of the performance of OPVs under different illumination intensities. It was found that the MoO3 layer could effectively prevent exciton quenching at the ITO anode side, resulting in the small variation of the FF for the devices with the MoO3 layer compared to the devices without the MoO3 layer under high illumination intensity.
materials, leading to a low fill factor (FF) because of the limited charge transporting.6 The FF is strongly determined by both series and shunt resistances. The series resistance originates from the bulk resistance of the materials and contact resistance at different interfaces. In addition, shunt resistance is due to the recombination of charge carriers near the dissociation site.7 Therefore, the donor/acceptor and organic/metal interfaces play an important role in the device performance.8 Krebs et al. reported a detailed understanding of the degradation mechanisms from the sides of interfacial energy level alignment, sheet resistance, light and thermal effects.9,10 To further improve the interface contact and charge transport efficiency, different functional buffer layers are introduced to modify both anode and cathode electrodes.11-14 However, there is still the lack of a detailed
Introduction The search for alternative sources to traditional energies is imperative because of the increasing demand for renewable and clean energies. Organic photovoltaic devices (OPVs) convert solar energy into electrical energy; therefore, the investigation on OPVs has been extensively focused because of their flexibility, lower cost, and environmental benignity in comparison to silicon-based solar cells.1-3 The operation of OPVs mainly involves (i) the light absorption by the active layer, (ii) the formation of exciton and subsequent diffusion to the interface of donors and acceptors, (iii) the exciton dissociation into the electrons and holes, (iv) charge carrier transport in their individual pathway or layer, and (v) charge extraction by their corresponding electrodes. Finally, the electric current is produced when the cell is connected to a load. Recently, the power conversion efficiency (PCE) of both polymer OPVs has reached 5% or more.3-5 However, the overall efficiency is still limited by some factors. One limitation to the improvement of PCE is the narrow absorption range of the organic semi-conductor, which makes the large mismatch with the solar spectrum. Another limitation is considered as the low charge carrier mobility in organic
(6) Schilinsky, P.; Waldauf, C.; Huach, J.; Brabec, C. J. J. Appl. Phys. 2004, 95, 2816. (7) Jain, A.; Kapoor, A. Sol. Energy Mater. Sol. Cells 2005, 86, 197. (8) Zhang, F. J.; Vollmer, A.; Zhang, J.; Xu, Z.; Rabe, J. P.; Koch, N. Org. Electron. 2007, 8, 606. (9) (a) Krebs, F. C.; Spanggaard, H. Chem. Mater. 2005, 17, 5235. (b) Krebs, F. C.; Cruys-Bagger, N.; Anderson, M.; Lilliedal, M. R.; Hammond, M. A.; Hvidt, S. Sol. Energy Mater. Sol. Cells 2005, 86, 499. (c) Joergensen, M.; Norrman, K.; Krebs, F. C. Sol. Energy Mater. Sol. Cells 2008, 92, 686. (d) Krebs, F. C. Sol. Energy Mater. Sol. Cells 2009, 93, 394. (10) (a) Bundgaard, E.; Krebs, F. C. Sol. Energy Mater. Sol. Cells 2007, 91, 1019. (b) Krebs, F. C. Sol. Energy Mater. Sol. Cells 2006, 90, 3633. (c) Krebs, F. C.; Gevorgyan, S. A; Alstrup, J. J. Mater. Chem. 2009, 19, 5442. (d) Helgesen, M.; Soendergaard, R.; Krebs, F. C. J. Mater. Chem. 2010, 20, 36. (11) Na, S. I.; Oh, S. H.; Kim, S. S.; Kim, D. Y. Org. Electron. 2009, 10 (3), 496. (12) Kim, J. Y.; Kim, S. H.; Lee, H. H.; Lee, K.; Ma, W. L.; Gong, X.; Heeger, A. J. Adv. Mater. 2006, 18 (5), 572. (13) (a) Zhao, D. W.; Sun, X. W.; Jiang, C. Y.; Kyaw, A. K. K.; Lo, G. Q.; Kwong, D. L. Appl. Phys. Lett. 2008, 93, No. 083305. (b) Zhao, D. W.; Sun, X. W.; Jiang, C. Y.; Kyaw, A. K. K.; Lo, G. Q.; Kwong, D. L. IEEE Electron Device Lett. 2009, 30, 490. (14) Li, Y.; Zhou, W.; Xue, D.; Liu, J. W.; Peterson, E. D.; Nie, W. Y.; Carroll, D. Appl. Phys. Lett. 2009, 95, No. 203503.
† This paper has been designated for the Asia Pacific Conference on Sustainable Energy and Environmental Technologies (APCSEET) special section. *To whom correspondence should be addressed. Telephone: 0086-1051688605. E-mail:
[email protected]. (1) Brabec, C. J.; Sariciftci, N. S.; Hummelen, J. C. Adv. Funct. Mater. 2001, 11, 15. (2) G€ unes, S.; Neugebauer, H.; Sariciftci, N. S. Chem. Rev. 2007, 107, 1324. (3) Kim, J. Y.; Lee, K.; Coates, N. E.; Moses, D.; Nguyen, T. Q.; Dante, M.; Heeger, A. J. Science 2007, 317, 222. (4) Kim, K.; Liu, J.; Namboothiry, M. A. G.; Carroll, D. L. Appl. Phys. Lett. 2007, 90, No. 163511. (5) Chan, M. Y.; Lai, S. L.; Fung, M. K.; Lee, C. S.; Lee, S. T. Appl. Phys. Lett. 2007, 90, No. 023504.
r 2010 American Chemical Society
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Energy Fuels 2010, 24, 3739–3742
: DOI:10.1021/ef901325e
Zhang et al.
Figure 1. Chemical structures of (a) BPhen, (b) C60, and (c) CuPc, (d) device structure of OPVs, and (e) the schematic energy level diagram of the used materials before contact.
Figure 2. I-V characteristics of the OPVs with different thicknesses of MoO3 (0, 1, 2, and 4 nm) under 100 mW/cm2.
study on the influence of the buffer layer on the performance of small-molecule OPVs. On the other hand, the performance of OPVs exhibits a dependence upon illumination intensity.15 Schilinsky et al. reported the simulation of light-intensity-dependent current characteristics of polymer OPVs. An extended replacement circuit describing the current-voltage characteristics of bulk heterojunction polymer OPVs at different illumination intensity was introduced and discussed.16 Therefore, it is worth investigating the dependence of small-molecule OPVs on the illumination intensity. In this paper, we investigate the effect of the molybdenum trioxide (MoO3) buffer layer and illumination intensity on the performance of copper phthalocyanine (CuPc)/C60 OPVs. The results indicate that the insertion of the MoO3 layer makes the formation of Ohmic contact between indium tin oxide (ITO) and CuPc, leading to the increase in FF. In addition, the performance of these devices are compared under different illuminations, exhibiting that the MoO3 could block the exciton quenching at the ITO/CuPc interfaces, contributing to the enhanced photocurrent and FF.
Figure 3. I-V characteristics of OPVs with different MoO3 thicknesses in the dark. Table 1. Summary of the Performance of the OPVs with Different Thicknesses of MoO3 (0, 1, 2, and 4 nm)
Experimental Section All cells were fabricated on ITO-coated glass substrates with a sheet resistance of 40 Ω/0. The substrates were cleaned in an ultrasonic bath with detergent, deionied water, acetone, and isopropyl alcohol successively for 15 min. After the ITO surfaces were dried in a laboratory oven, they were treated by oxygen plasma for 3 min. MoO3, CuPc, fullerene (C60), and 4,7-diphenyl1,10-phenanthroline (BPhen) were used as purchased and deposited sequentially by thermal evaporation in a high-vacuum chamber (8.0 10-5 Pa). The thickness of each layer was measured and monitored by a quartz oscillator crystal, located near the substrates. Finally, the Ag cathode was thermally evaporated, and the active area was about 0.1 cm2. All of the measurements were carried out under room temperature in air. The current-voltage (I-V) characteristics were recorded with a Keithley 2400 source meter in the dark and under different irradiation intensities. The chemical structures of the used materials, the device structure, and the schematic energy level diagram of the small molecule before contact are shown in Figure 1.
MoO3 thickness (nm)
Voc (V)
Jsc (mA/cm2)
FF (%)
PCE
0 1 2 4
0.43 0.42 0.42 0.42
4.16 4.15 3.98 3.87
53.5 56.6 57.7 57.6
0.96 1.01 0.99 0.94
Results and Discussion The photovoltaic characteristics are obtained under illumination. The current density obtained at zero applied voltage is the short-circuit current density (Jsc). The voltage at which the current becomes zero is the open-circuit voltage (Voc). The product of these two parameters gives the theoretical maximum power of OPVs. The FF is a measure of how far the I-V characteristics of actual devices differ from those of an ideal device. The calculated efficiency is the power conversion efficiency under illumination. The FF and PCE are defined as ðJVÞmax Voc Jsc FF and PCE ¼ FF ¼ Pin Jsc Voc
(15) Zhang, C. F.; Tong, S. W.; Zhu, C. X.; Jiang, C. Y.; Kang, E. T.; Chan, D. S. H. Appl. Phys. Lett. 2009, 94, No. 103305. (16) Schilinsky, P.; Waldauf, C.; Hauch, J.; Brabec, C. J. J. Appl. Phys. 2004, 95, 2816.
where J and V are the current density and voltage at the point of maximum power output of the solar cell, respectively, and Pin is the incident optical power density. 3740
Energy Fuels 2010, 24, 3739–3742
: DOI:10.1021/ef901325e
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Figure 4. I-V characteristics of OPVs with different thicknesses of the MoO3 layer (0, 1, 2, and 4 nm) in the dark and under different illumination intensities from 50 to 300 mW/cm2.
demonstrated. The J-V characteristic dependence upon the thickness of MoO3 is shown in Figure 3. Zhang et al.15 also found these similar dark J-V characteristics of P3HT and PCBM single-layer devices with e-beam-deposited Al cathode as fabricated and after annealing at 160 °C for 10 min. On the other hand, FF is enhanced from 53.5% for the device without MoO3 to around 57.0% with the MoO3 layer deposited. It indicates that the contact resistance is reduced by incorporating the MoO3 layer, resulting from the improvement of the contact interface. It is attributed to the formation of the Ohmic contact at the ITO/CuPc interface, benefiting the hole transport and extraction to the ITO anode. Therefore, the large improvement of FF is obtained with the insertion of the MoO3 ultra-thin layer. Overall, the maximum PCE for the device with 1 nm MoO3 reaches 1.01% because of the balance of the photocurrent and the FF. Now, we turn to the effect of illumination intensity on Voc and Jsc of OPVs. It is known that Voc is determined by the difference between the highest occupied molecular orbital (HOMO) of the donor and the lowest unoccupied molecular orbital (LUMO) of the acceptor based on a single-diode model.17,18 However, Voc is usually smaller than the difference
To study the influence of an ultra-thin buffer layer on the performance of CuPc/C60-based OPVs, different thicknesses of MoO3 layers with 0, 1, 2, and 4 nm were inserted between ITO and CuPc in the standard device with a structure of ITO/ CuPc (20 nm)/C60 (40 nm)/Bphen (8 nm)/Ag (100 nm). The I-V characteristics of these OPVs under 100 mW/cm2 are shown in Figure 2. Their corresponding parameters are summarized in Table 1. It can be observed that Jsc decreases with the increase of the MoO3 thickness. Jsc is defined as the current density obtained when the applied voltage is zero. At this time, the photogenerated charge carriers are driven by the potential difference between the internal built-in voltages. Because of the valence band of MoO3 (5.3 eV) and the work function of ITO (4.8 eV), the built-in potential is increased with the insertion of the MoO3 layer between the ITO and CuPc layers. The increased built-in electric field should benefit the transporting of the charge carriers induced by exciton dissociation. However, these holes should overcome the energy barrier between CuPc and MoO3 to reach the ITO anode. Therefore, the competition between the hole transporting in the CuPc layer and the hole tunneling from the CuPc layer into the ITO anode is correlated with the thickness of the MoO3 layer, determining Jsc. Additionally, the decreased Jsc might also be attributed to the shift of effective optical field distribution across the CuPc/C60 interface because of the insertion of the MoO3 layer. As a result, the Jsc decreases from 4.16 to 3.87 mA/cm2 for the device with 0 and 4 nm MoO3 layers, respectively. From the dark J-V characteristics of OPVs with different MoO3 thickness, the effect of the interfacial state on charge carrier transporting and tunneling could be further
(17) Brabec, C.; Cravino, A.; Meissner, D.; Sariciftci, N. S.; Fromhertz, T.; Rispens, M. T.; Sanchez, L.; Hummelen, J. C. Adv. Funct. Mater. 2001, 11, 374. (18) Brabec, C.; Cravino, A.; Meissner, D.; Sariciftci, N. S.; Rispens, M. T.; Sanchez, L. Thin Solid Films 2002, 368, 403. (19) Shen, L.; Zhu, G. H.; Guo, W. B.; Tao, C.; Zhang, X. D. Appl. Phys. Lett. 2008, 92, No. 073307. (20) Oku, T.; Nagaoka, S.; Suzuki, A.; Kikuchi, K.; Hayashi, Y.; Inukai, H.; Sakuragi, H.; Soga, T. J. Phys. Chem. Solids 2008, 69, 1279.
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intensity, Voc should be expressed as follows Voc
Egap KT ð1 -PÞγNc 2 - ln ¼ q q PG
!
where Egap is the energy difference between the HOMO of CuPc and the LUMO of C60, P is the dissociation probability of a bound electron-hole pair into free charge carriers, G is the generation rate of bound electron-hole pairs, γ is the Langevin recombination constant, and Nc is the effective density of states, equal to 2.5 1025 m-3.25 Voc increases with the increase of the incident illumination intensity, which should be attributed to the increased generation rate of bound electron-hole pairs (G) with the increased incident illumination intensity. It is worth mentioning that FF of the OPVs with MoO3 has a small change with the incident illumination intensity; however, FF of the device without MoO3 exhibits an obvious decrease. That could be explained by the fact that, in the absence of the MoO3 layer, more photogenerated excitons are quenched at the ITO/CuPc interface under high illumination intensity. However, the insertion of the MoO3 layer at such an interface can effectively prevent exciton quenching at the ITO anode side. As a consequence, there is a slight variation of FF for the devices with MoO3.
Figure 5. Dependence of Voc and Jsc of CuPc/C60-based OPVs on the incident illumination intensity from 50 to 300 mW/cm2.
between the HOMO of the donor and the LUMO of the acceptor.19-21 Therefore, Voc should also be dependent upon the other factors, such as temperature and illumination light intensity. Katz et al. and Chirvase et al. reported the temperature dependence of Voc of polymer/fullerence bulk heterojunction polymer OPVs.22,23 Koster et al. described a new model, reasonably explaining the influence of illumination intensity on Voc based on the notion that the quasi-Fermi levels are constant throughout the entire active layers, including both drift and diffusion of charge carriers.24 To understand the influence of the illumination intensity on the performance of small-molecule OPVs, different incident illumination intensities are illuminated on our devices with different MoO3 thicknesses. To decrease the effect of the temperature on the performance of OPVs, each measurement was finished in 15 s. Figure 4 shows the J-V characteristics of these OPVs in the dark and under different illumination intensities from 50 to 300 mW/cm2. It can be clearly observed that Voc and Jsc of all devices have similar behavior to the increase of the illumination intensity. The dependence of Voc and Jsc on illumination intensity is summarized in Figure 5. The enhancement of Jsc dependence upon illumination intensity should be attributed to the increased absorption of the active layer, which results in more excitons dissociated into charge carriers, contributing to the enhanced Jsc. Considering the change of illumination
Conclusion The influence of the thickness of the MoO3 buffer layer on the performance of CuPc/C60 OPVs was investigated. With the insertion of the MoO3 layer, FF of the devices is improved from 53.5 to 57.7% because of the Ohmic contact between ITO and CuPc, leading to the reduction of the contact resistance. However, Jsc slightly decreases because of the possible reasons of a little increased energy barrier between CuPc and MoO3 and the redistribution of the effective optical field. Therefore, the optimal thickness of the MoO3 layer is 1 nm. In addition, the incident illumination intensity has a strong impact on the performance of the OPVs with and without MoO3. Under high illumination, the devices with the MoO3 layer have a much higher FF than those without the MoO3 layer likely because of the prevention of exciton quenching at the ITO anode by the MoO3 layer. Hence, the ultra-thin MoO3 layer has a potential application in the enhancement of OPVs. Acknowledgment. The authors express their thanks for the financial support from the National Natural Science Foundation of China (Grants 10804006 and 10774013), National Natural Science Funds for Distinguished Young Scholar (Grant 60825407), Major State Basic Research Development Program of China (Grant 2010CB327704), Natural Science Foundation of Beijing (Grant 1102028), and the 111 Project (Grant B08002).
(21) Hu, T.; Zhang, F. J.; Xu, Z.; Zhao, S. L. Synth. Met. 2009, 159, 754. (22) Katz, E. A.; Faiman, D.; Tuladhar, S. M.; Kroon, J. M.; Wienk, M. M.; Fromherz, T.; Padinger, F.; Brabec, C. J.; Sariciftci, N. S. J. Appl. Phys. 2001, 90, 5343. (23) Chirvase, D.; Chiguvare, Z.; Knipper, M.; Parisi, J.; Dyakonov, V.; Hummelen, J. C. J. Appl. Phys. 2003, 93, 3376. (24) Koster, L. J. A.; Mihailetchi, V. D.; Ramaker, R.; Blom, P. W. M. Appl. Phys. Lett. 2005, 86, No. 123509.
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