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Organic photovoltaic cell employing organic heterojunction as buffer layer
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Thin Solid Films 516 (2008) 3320 – 3323 www.elsevier.com/locate/tsf
Organic photovoltaic cell employing organic heterojunction as buffer layer Jiguang Dai, Xiaoxia Jiang, Haibo Wang, Donghang Yan ⁎ State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Graduate School of Chinese Academy of Sciences, Changchun 130022, People's Republic of China Received 8 August 2007; received in revised form 12 September 2007; accepted 24 September 2007 Available online 29 September 2007
Abstract Hexadecafluorophthalocyaninatocopper (F16CuPc)/zinc phthalocyanine (ZnPc) heterojunction layer has been used as buffer layer in organic photovoltaic (OPV) cells based on ZnPc and C60. The F16CuPc/ZnPc heterojunction with highly conductive property decreased the contact resistance between the indium-tin-oxide anode and the organic layer. As a result, the short-circuit current density and fill factor were increased, and the power-conversion efficiency was improved by over 60%. Therefore, the method provides an effective path to improve the performance of OPV cells. © 2007 Elsevier B.V. All rights reserved. Keywords: Organic photovoltaic cells; Contact resistance; Organic heterojunction
1. Introduction Organic photovoltaic (OPV) cells based on low-molecularweight and polymeric semiconducting materials have received considerable attention, due to their attractive advantages such as flexibility, easy fabrication and low-cost [1,2], etc. Recently, power-conversion efficiency (PCE) around 5% has been realized by using various approaches [3–6]. The choice of organic semiconductors materials [7–10], organic film morphology [11,12], and film thickness [13,14] is critical to improve OPV cells performance. However, in OPV devices, the contact between the organic materials and the electrodes is also a challenge since it strongly affects the charge-collection properties of the devices. Contrary to the case of silicon semiconductor, it is difficult to realize a good ohmic contact between an electrode and an organic semiconductor by heavy doping, and their contact usually forms a direct electrode-semiconductor junction if organic semiconductor is not doped. Thus, a Mott–Schottky barrier forms in the electrode–semiconductor interface, and the barrier can lead to poor efficiency of charge-collection at the electrode. Especially, indium-tin-oxide (ITO) is usually used as electrode in OPV device due to its transparence and high ⁎ Corresponding author. Tel.: +86 431 85262165; fax: +86 431 85262266. E-mail address: [email protected] (D. Yan). 0040-6090/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2007.09.043
conduction properties, and the contact between ITO and organic materials also usually shows a high contact resistance. Many methods has been proposed to reduce or eliminate the barrier, such as surface treatment of the ITO substrates by assembling a molecular monolayer [15–17] and UV-ozone [18], introduction of n-type layer [19,20] and high conductive organic materials [21] between electrode and the active layer used as buffer layer. In organic field-effect transistors, the contact resistance had been reduced by inserting a copper phthalocyanine (CuPc)/ Hexadecafluorophthalocyaninatocopper (F16CuPc) heterojunction layer as buffer layer between electrode and semiconductor [22], which took advantage of its high conductive property. In this work, we reduced the contact resistance, i. e. the series resistance of zinc phthalocyanine (ZnPc) and C60 cells, by inserting the F16CuPc/ZnPc organic heterojunction with high conductive property between ZnPc and ITO electrode. As a result, the fill factor (FF) was improved, and the PCE was increased by 67%. 2. Experimental details Fig. 1(a) and (b) illustrates the schematic cross sections of OPV cells with and without the F16CuPc/ZnPc heterojunction buffer layer, respectively. All devices were fabricated on glass substrates precoated with a 250 Å thick ITO anode (with a sheet resistance of 100 Ω/sq). The glass substrates were cleaned
J. Dai et al. / Thin Solid Films 516 (2008) 3320–3323
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Fig. 2. Absorption spectra of ZnPc (24 nm)/C60 (50 nm) film without (solid line) and with (dash line) 2 nm F16CuPc.
3.2. Photovoltaic performance of OPV cells Fig. 1. The device configuration of conventional OPV cells (a) and the cells with F16CuPc/ZnPc heterojunction (b).
ultrasonically in three sequential steps with acetone, methanol, and pure water in an ultrasonic bath. ZnPc and F16CuPc samples were purchased from Aldrich Company (USA), and were purified twice by thermal gradient sublimation prior to experiments. C60 sample was purchased from Acros Co., and was used as received without purification. Firstly, thin F16CuPc layer, 24 nm ZnPc layer and 50 nm C60 layers were deposited onto an ITO-coated glass substrate in series. Then, a 50 nm of Al electrode was evaporated onto the surface of C60 layer through a shadow mask. The organic materials and metal electrodes were deposited at about 10− 4 Pa, and active area of the OPV cells is 0.16 cm2. The PCE of the OPV cells were measured by using a Keithley 2400 and a Sciencetech solar simulator in air under 100 mW/cm2 simulated AM 1.5 illumination, and the intensity was monitored by using a standard silicon photodiode. The absorption spectra of organic films were recorded by the Jasco V-570 UV-VIS-NIR spectrophotometer, and the organic films were deposited on the precleaned quartz under the above conditions.
Fig. 3 shows typical J–V characteristics of ITO/ZnPc (24 nm)/C60 (50 nm)/Al and ITO/F16CuPc (2 nm)/ZnPc (24 nm)/C 60 (50 nm)/Al in the dark and under solar illumination. Comparing the OPV cells with and without F16CuPc, the open-circuit voltage (VOC) slightly increased by twenty millivolts (up to 0.50 V), the FF increased significantly
3. Results and discussion 3.1. Absorption spectrums of organic films The absorption spectrums of organic films with and without inserting 2 nm F16CuPc are shown in Fig. 2. The Q band absorption peak of the ZnPc/C60 film is located at a wavelength of 624 nm which comes from the absorption of ZnPc, and the absorption maximum of the film is located at 344 nm which is caused by C60. After inserting 2 nm F16CuPc, the band absorption and the peak intension do not show obvious change. The reason may be that the F16CuPc layer is too thin to affect the absorption of the whole organic active layer. Similar morphology of organic films with and without F16CuPc layer was also observed by atomic force microscopy (data not shown).
Fig. 3. Current density–voltage curves of ITO/ZnPc (24 nm)/C60 (50 nm)/Al and ITO/F16CuPc (2 nm)/ZnPc (24 nm)/C60 (50 nm)/Al devices: (a) in a linear plot and (b) a semi-logarithmic scale.
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from 0.36 to 0.53, and the short-circuit current density were similar (JSC = 3.6–4.0 mA/cm2), finally, the power conversion efficiency increased by ca. 60% to 1.05% for the cells using F16CuPc/ZnPc as buffer layer. The current density in the dark was also increased by a factor of 13 under + 1 V bias, which indicates that the introduction of the F16CuPc/ZnPc buffer layer reduced the injection barrier and improved the hole injection into the ZnPc layer. It is noted that the series resistance (RSA) of OPV cells decreased from 141 to 10 Ω·cm2. The RSA consists of the bulk resistance (RS,bulk) and the contact resistances (RS,contact). The RS,bulk comes from the bulk resistance of organic layers (F16CuPc, ZnPc and C60) and electrodes (ITO and cathode metals), and the RS,contact originates from the interface between the electrodes and the active layer. The RS,bulk are expected to be identical for all structures (noting that all the devices were prepared under the similar condition), therefore, the difference of total resistance comes from the decrease of RS,contact between ITO electrode and organic film due to the introduction of F16CuPc/ZnPc heterojunction buffer layer. In our previous works, the high conduction property of F16CuPc/CuPc heterojunction was observed at the zero gate voltage, and free charge carriers existed at both sides of the heterojunction interface between p-type CuPc and n-type F16CuPc [23,24]. The experiment results further revealed that the accumulation thickness of charge carriers was about 10 nm at the two sides of the heterojunction interface [24,25]. By inserting the high conductive organic heterojunction used as buffer layer into F16CuPc OTFT, Yan et al. effectively improved the contact between metal electrodes and organic semiconductors [22]. In this case, F16CuPc may form a highly conductive heterojunction with ZnPc, which is similar to the F16CuPc/ CuPc heterojunction. Because the F16CuPc layer (1∼3 nm) introduced to cover the whole surface of ITO electrode is very thin, and the latter also being thinner than the accumulation thickness of electrons, the ZnPc layer can directly contact the ITO electrode in some regions. Therefore, we believe a high conduction region will exist at the interface between the ITO and ZnPc. Abundant charge carriers at the organic heterojunction interface may reduce the width of electrode/organic contact potential barrier and improved the injection efficiency. So, the F16CuPc/ZnPc heterojunction as buffer layer was benefit to improve the hole injection and the RS,contact, further increased the PCE. Table 1 summarizes the photovoltaic parameters of the cells with various F16CuPc thicknesses. Under AM 1.5 100 mW/cm2 solar illumination, the normal ZnPc/C60 bi-layer OPV cell showed a low performance. When the F16CuPc layer was Table 1 The performance parameters of ITO/F16CuPc/ZnPc (24 nm)/C60 (50 nm)/Al devices with various F16CuPc thickness F16CuPc (nm)
VOC (V)
ISC (mA/cm2)
FF
η (%)
RSA (Ω·cm2)
0 1 2 3
0.48 0.50 0.50 0.50
3.6 3.7 4.0 3.9
0.36 0.45 0.53 0.40
0.62 0.84 1.05 0.77
141 20 10 9
Fig. 4. Typical current density–voltage curves of F16CuPc and ZnPc heterojunction diode under dark and illumination.
inserted, Voc was constant at 0.50 V, which indicates that the inserted F16CuPc did not reduce the built-in potential (the detailed reason will be discussed in Section 3.3). The FF was dramatically increased from 0.36 to 0.53 for the cell inserting 2 nm F16CuPc, and then was decreased to 0.40 for the cell inserting 3 nm F16CuPc. It is worthy to note that the RSA also showed a monotonic decrease from 141 to 9 Ω cm2. As a result of the steep increase in FF, the PCE also increases to a maximum of 1.05% for the cell inserting 2 nm F16CuPc. 3.3. Diodes of F16CuPc/ZnPc heterojunction To investigate the F16CuPc/ZnPc heterojunction property under illumination and dark, the J–V curves of F16CuPc (30 nm)/ZnPc (50 nm) diodes were measured, as shown in Fig. 4. The preparation process of the diodes was similar to that of the OPV cells based on ZnPc and C60 except the thick F16CuPc layer and the Au electrodes. The voltage was scanned from − 1 V to + 1 V and the dark J–V characteristics showed a reverse rectifying characteristic with a rectification ratio of 47, which is similar to the CuPc/F16CuPc heterojunction [24]. The diode under illumination showed an increase of the current density, however, the VOC was not observed in the F16CuPc/ ZnPc heterojunction diode. The lack of photovoltage in ZnPc/ F16CuPc diodes demonstrates that the interface between ZnPc and F16CuPc can not effectively dissociate excitons; therefore, the introduction of F16CuPc was not limiting to the transport of holes and did not change the VOC of OPV cells, which was consistent with the parameters listed in Table 1. 4. Conclusions Highly conductive organic heterojunction was used as a buffer layer to fabricate high performance small-molecule OPV cells. A large fill factor of 0.53 and an efficiency improved by 67% were obtained in the OPV devices of ZnPc and C60 by inserting F16CuPc/ZnPc heterojunction as buffer layer between the ITO electrode and the active layer. These improvements are attributed to the organic heterojunction layer with high conduction property, which lead to abundant charges at the
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interface between ITO and active layer that further decrease the RSA of the devices. Therefore, it is a simple and effective method for reducing the contact resistance in organic electronic devices by inserting a heterojunction layer with high conduction between semiconductor and electrode. Acknowledgments This work was supported by the National Natural Science Foundation of China (50773079, 20474064, 20621401) and the Special Funds for Major State Basic Research Projects (2002CB613400). References [1] D. Wöhrle, D. Meissner, Adv. Mater. 3 (1991) 129. [2] P. Peumans, A. Yakimov, S.R. Forrest, J. Appl. Phys. 93 (2003) 3693. [3] J. Xue, S. Uchida, B.P. Rand, S.R. Forrest, Appl. Phys. Lett. 85 (2004) 5757. [4] W. Ma, C. Yang, X. Gong, K. Lee, A.J. Heeger, Adv. Mater. 17 (2005) 1617. [5] G. Li, V. Shrotriya, J. Huang, Y. Yao, T. Moriarty, K. Emery, Y. Yang, Nat. Matters 4 (2005) 867. [6] F. Padinger, R.S. Rittberger, N.S. Sariciftci, Adv. Funct. Mater. 13 (2003) 85. [7] Q. Zhou, Q. Hou, L. Zheng, X. Deng, G. Yu, Y. Cao, Appl. Phys. Lett. 84 (2004) 1653. [8] X. Wang, E. Perzon, J.L. Delgado, P. Cruz, F. Zhang, F. Langa, M. Andersson, O. Inganäs, Appl. Phys. Lett. 85 (2004) 5081. [9] K. Colladet, S. Fourier, T.J. Cleij, L. Lutsen, J. Gelan, D.k. Vanderzande, L.H. Nguyen, H. Neugebauer, S. Sariciftci, A. Aguirre, G. Janssen, E. Goovaerts, Macromolecules 40 (2007) 65.
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[10] B.P. Rand, J. Xue, F. Yang, S.R. Forrest, Appl. Phys. Lett. 87 (2004) 233508. [11] C.J. Brabec, N.S. Sariciftci, J.C. Hummelen, Adv. Funct. Mater. 11 (2001) 15. [12] P. Peumans, S. Uchida, S.R. Forrest, Nature 425 (2003) 158. [13] T. Taima, M. Chikamatsu, Y. Yoshida, K. Saito, K. Yase, Appl. Phys. Lett. 85 (2004) 6412. [14] K. Suemori, T. Miyata, M. Yokoyama, M. Hiramotoa, Appl. Phys. Lett. 86 (2005) 063509. [15] N.R. Armstrong, C. Carter, C. Donley, A. Simmonds, P. Lee, M. Brumbach, B. Kippelen, B. Domercq, S. Yoo, Thin Solid Films 455 (2003) 342. [16] B. Johnev, M. Vogel, K. Fostiropoulos, B. Mertesacker, M. Rusu, M.C. Lux-Steiner, A. Weidinger, Thin Solid Films 488 (2005) 270. [17] S. Khodabakhsh, B.M. Sanderson, J. Nelson, T.S. Jones, Adv. Funct. Mater. 16 (2006) 95. [18] A.C. Arias, L.S. Roman, T. Kugler, R. Toniolo, M.S. Meruvia, I.A. Hummelgen, Thin Solid Films 371 (2000) 201. [19] M. Nonomura, I. Hiromitsua, S. Tanaka, Appl. Phys. Lett. 88 (2006) 042111. [20] D.Q. Gao, M.Y. Chan, S.W. Tong, F.L. Wong, S.L. Lai, C.S. Lee, S.T. Lee, Chem. Phys. Lett. 399 (2004) 337. [21] J. Wang, H.B. Wang, J. Zhang, X.J. Yan, D.H. Yan, J. Appl. Phys. 97 (2005) 026106. [22] X.J. Yan, J. Wang, H.B. Wang, H. Wang, D.H. Yan, Appl. Phys. Lett. 89 (2006) 053501. [23] J. Wang, H.B. Wang, X. Yan, H. Huang, D.H. Yan, Appl. Phys. Lett. 87 (2005) 093507. [24] H.B. Wang, J. Wang, H. Huang, X. Yan, D.H. Yan, Org. Electron. Spectr. Data 7 (2006) 369. [25] K.M. Lau, J.X. Tang, H.Y. Sun, C.S. Lee, S.T. Lee, D.H. Yan, Appl. Phys. Lett. 88 (2006) 173513.
Thin Solid Films 516 (2008) 3320 – 3323 www.elsevier.com/locate/tsf
Organic photovoltaic cell employing organic heterojunction as buffer layer Jiguang Dai, Xiaoxia Jiang, Haibo Wang, Donghang Yan ⁎ State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Graduate School of Chinese Academy of Sciences, Changchun 130022, People's Republic of China Received 8 August 2007; received in revised form 12 September 2007; accepted 24 September 2007 Available online 29 September 2007
Abstract Hexadecafluorophthalocyaninatocopper (F16CuPc)/zinc phthalocyanine (ZnPc) heterojunction layer has been used as buffer layer in organic photovoltaic (OPV) cells based on ZnPc and C60. The F16CuPc/ZnPc heterojunction with highly conductive property decreased the contact resistance between the indium-tin-oxide anode and the organic layer. As a result, the short-circuit current density and fill factor were increased, and the power-conversion efficiency was improved by over 60%. Therefore, the method provides an effective path to improve the performance of OPV cells. © 2007 Elsevier B.V. All rights reserved. Keywords: Organic photovoltaic cells; Contact resistance; Organic heterojunction
1. Introduction Organic photovoltaic (OPV) cells based on low-molecularweight and polymeric semiconducting materials have received considerable attention, due to their attractive advantages such as flexibility, easy fabrication and low-cost [1,2], etc. Recently, power-conversion efficiency (PCE) around 5% has been realized by using various approaches [3–6]. The choice of organic semiconductors materials [7–10], organic film morphology [11,12], and film thickness [13,14] is critical to improve OPV cells performance. However, in OPV devices, the contact between the organic materials and the electrodes is also a challenge since it strongly affects the charge-collection properties of the devices. Contrary to the case of silicon semiconductor, it is difficult to realize a good ohmic contact between an electrode and an organic semiconductor by heavy doping, and their contact usually forms a direct electrode-semiconductor junction if organic semiconductor is not doped. Thus, a Mott–Schottky barrier forms in the electrode–semiconductor interface, and the barrier can lead to poor efficiency of charge-collection at the electrode. Especially, indium-tin-oxide (ITO) is usually used as electrode in OPV device due to its transparence and high ⁎ Corresponding author. Tel.: +86 431 85262165; fax: +86 431 85262266. E-mail address: [email protected] (D. Yan). 0040-6090/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2007.09.043
conduction properties, and the contact between ITO and organic materials also usually shows a high contact resistance. Many methods has been proposed to reduce or eliminate the barrier, such as surface treatment of the ITO substrates by assembling a molecular monolayer [15–17] and UV-ozone [18], introduction of n-type layer [19,20] and high conductive organic materials [21] between electrode and the active layer used as buffer layer. In organic field-effect transistors, the contact resistance had been reduced by inserting a copper phthalocyanine (CuPc)/ Hexadecafluorophthalocyaninatocopper (F16CuPc) heterojunction layer as buffer layer between electrode and semiconductor [22], which took advantage of its high conductive property. In this work, we reduced the contact resistance, i. e. the series resistance of zinc phthalocyanine (ZnPc) and C60 cells, by inserting the F16CuPc/ZnPc organic heterojunction with high conductive property between ZnPc and ITO electrode. As a result, the fill factor (FF) was improved, and the PCE was increased by 67%. 2. Experimental details Fig. 1(a) and (b) illustrates the schematic cross sections of OPV cells with and without the F16CuPc/ZnPc heterojunction buffer layer, respectively. All devices were fabricated on glass substrates precoated with a 250 Å thick ITO anode (with a sheet resistance of 100 Ω/sq). The glass substrates were cleaned
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Fig. 2. Absorption spectra of ZnPc (24 nm)/C60 (50 nm) film without (solid line) and with (dash line) 2 nm F16CuPc.
3.2. Photovoltaic performance of OPV cells Fig. 1. The device configuration of conventional OPV cells (a) and the cells with F16CuPc/ZnPc heterojunction (b).
ultrasonically in three sequential steps with acetone, methanol, and pure water in an ultrasonic bath. ZnPc and F16CuPc samples were purchased from Aldrich Company (USA), and were purified twice by thermal gradient sublimation prior to experiments. C60 sample was purchased from Acros Co., and was used as received without purification. Firstly, thin F16CuPc layer, 24 nm ZnPc layer and 50 nm C60 layers were deposited onto an ITO-coated glass substrate in series. Then, a 50 nm of Al electrode was evaporated onto the surface of C60 layer through a shadow mask. The organic materials and metal electrodes were deposited at about 10− 4 Pa, and active area of the OPV cells is 0.16 cm2. The PCE of the OPV cells were measured by using a Keithley 2400 and a Sciencetech solar simulator in air under 100 mW/cm2 simulated AM 1.5 illumination, and the intensity was monitored by using a standard silicon photodiode. The absorption spectra of organic films were recorded by the Jasco V-570 UV-VIS-NIR spectrophotometer, and the organic films were deposited on the precleaned quartz under the above conditions.
Fig. 3 shows typical J–V characteristics of ITO/ZnPc (24 nm)/C60 (50 nm)/Al and ITO/F16CuPc (2 nm)/ZnPc (24 nm)/C 60 (50 nm)/Al in the dark and under solar illumination. Comparing the OPV cells with and without F16CuPc, the open-circuit voltage (VOC) slightly increased by twenty millivolts (up to 0.50 V), the FF increased significantly
3. Results and discussion 3.1. Absorption spectrums of organic films The absorption spectrums of organic films with and without inserting 2 nm F16CuPc are shown in Fig. 2. The Q band absorption peak of the ZnPc/C60 film is located at a wavelength of 624 nm which comes from the absorption of ZnPc, and the absorption maximum of the film is located at 344 nm which is caused by C60. After inserting 2 nm F16CuPc, the band absorption and the peak intension do not show obvious change. The reason may be that the F16CuPc layer is too thin to affect the absorption of the whole organic active layer. Similar morphology of organic films with and without F16CuPc layer was also observed by atomic force microscopy (data not shown).
Fig. 3. Current density–voltage curves of ITO/ZnPc (24 nm)/C60 (50 nm)/Al and ITO/F16CuPc (2 nm)/ZnPc (24 nm)/C60 (50 nm)/Al devices: (a) in a linear plot and (b) a semi-logarithmic scale.
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from 0.36 to 0.53, and the short-circuit current density were similar (JSC = 3.6–4.0 mA/cm2), finally, the power conversion efficiency increased by ca. 60% to 1.05% for the cells using F16CuPc/ZnPc as buffer layer. The current density in the dark was also increased by a factor of 13 under + 1 V bias, which indicates that the introduction of the F16CuPc/ZnPc buffer layer reduced the injection barrier and improved the hole injection into the ZnPc layer. It is noted that the series resistance (RSA) of OPV cells decreased from 141 to 10 Ω·cm2. The RSA consists of the bulk resistance (RS,bulk) and the contact resistances (RS,contact). The RS,bulk comes from the bulk resistance of organic layers (F16CuPc, ZnPc and C60) and electrodes (ITO and cathode metals), and the RS,contact originates from the interface between the electrodes and the active layer. The RS,bulk are expected to be identical for all structures (noting that all the devices were prepared under the similar condition), therefore, the difference of total resistance comes from the decrease of RS,contact between ITO electrode and organic film due to the introduction of F16CuPc/ZnPc heterojunction buffer layer. In our previous works, the high conduction property of F16CuPc/CuPc heterojunction was observed at the zero gate voltage, and free charge carriers existed at both sides of the heterojunction interface between p-type CuPc and n-type F16CuPc [23,24]. The experiment results further revealed that the accumulation thickness of charge carriers was about 10 nm at the two sides of the heterojunction interface [24,25]. By inserting the high conductive organic heterojunction used as buffer layer into F16CuPc OTFT, Yan et al. effectively improved the contact between metal electrodes and organic semiconductors [22]. In this case, F16CuPc may form a highly conductive heterojunction with ZnPc, which is similar to the F16CuPc/ CuPc heterojunction. Because the F16CuPc layer (1∼3 nm) introduced to cover the whole surface of ITO electrode is very thin, and the latter also being thinner than the accumulation thickness of electrons, the ZnPc layer can directly contact the ITO electrode in some regions. Therefore, we believe a high conduction region will exist at the interface between the ITO and ZnPc. Abundant charge carriers at the organic heterojunction interface may reduce the width of electrode/organic contact potential barrier and improved the injection efficiency. So, the F16CuPc/ZnPc heterojunction as buffer layer was benefit to improve the hole injection and the RS,contact, further increased the PCE. Table 1 summarizes the photovoltaic parameters of the cells with various F16CuPc thicknesses. Under AM 1.5 100 mW/cm2 solar illumination, the normal ZnPc/C60 bi-layer OPV cell showed a low performance. When the F16CuPc layer was Table 1 The performance parameters of ITO/F16CuPc/ZnPc (24 nm)/C60 (50 nm)/Al devices with various F16CuPc thickness F16CuPc (nm)
VOC (V)
ISC (mA/cm2)
FF
η (%)
RSA (Ω·cm2)
0 1 2 3
0.48 0.50 0.50 0.50
3.6 3.7 4.0 3.9
0.36 0.45 0.53 0.40
0.62 0.84 1.05 0.77
141 20 10 9
Fig. 4. Typical current density–voltage curves of F16CuPc and ZnPc heterojunction diode under dark and illumination.
inserted, Voc was constant at 0.50 V, which indicates that the inserted F16CuPc did not reduce the built-in potential (the detailed reason will be discussed in Section 3.3). The FF was dramatically increased from 0.36 to 0.53 for the cell inserting 2 nm F16CuPc, and then was decreased to 0.40 for the cell inserting 3 nm F16CuPc. It is worthy to note that the RSA also showed a monotonic decrease from 141 to 9 Ω cm2. As a result of the steep increase in FF, the PCE also increases to a maximum of 1.05% for the cell inserting 2 nm F16CuPc. 3.3. Diodes of F16CuPc/ZnPc heterojunction To investigate the F16CuPc/ZnPc heterojunction property under illumination and dark, the J–V curves of F16CuPc (30 nm)/ZnPc (50 nm) diodes were measured, as shown in Fig. 4. The preparation process of the diodes was similar to that of the OPV cells based on ZnPc and C60 except the thick F16CuPc layer and the Au electrodes. The voltage was scanned from − 1 V to + 1 V and the dark J–V characteristics showed a reverse rectifying characteristic with a rectification ratio of 47, which is similar to the CuPc/F16CuPc heterojunction [24]. The diode under illumination showed an increase of the current density, however, the VOC was not observed in the F16CuPc/ ZnPc heterojunction diode. The lack of photovoltage in ZnPc/ F16CuPc diodes demonstrates that the interface between ZnPc and F16CuPc can not effectively dissociate excitons; therefore, the introduction of F16CuPc was not limiting to the transport of holes and did not change the VOC of OPV cells, which was consistent with the parameters listed in Table 1. 4. Conclusions Highly conductive organic heterojunction was used as a buffer layer to fabricate high performance small-molecule OPV cells. A large fill factor of 0.53 and an efficiency improved by 67% were obtained in the OPV devices of ZnPc and C60 by inserting F16CuPc/ZnPc heterojunction as buffer layer between the ITO electrode and the active layer. These improvements are attributed to the organic heterojunction layer with high conduction property, which lead to abundant charges at the
J. Dai et al. / Thin Solid Films 516 (2008) 3320–3323
interface between ITO and active layer that further decrease the RSA of the devices. Therefore, it is a simple and effective method for reducing the contact resistance in organic electronic devices by inserting a heterojunction layer with high conduction between semiconductor and electrode. Acknowledgments This work was supported by the National Natural Science Foundation of China (50773079, 20474064, 20621401) and the Special Funds for Major State Basic Research Projects (2002CB613400). References [1] D. Wöhrle, D. Meissner, Adv. Mater. 3 (1991) 129. [2] P. Peumans, A. Yakimov, S.R. Forrest, J. Appl. Phys. 93 (2003) 3693. [3] J. Xue, S. Uchida, B.P. Rand, S.R. Forrest, Appl. Phys. Lett. 85 (2004) 5757. [4] W. Ma, C. Yang, X. Gong, K. Lee, A.J. Heeger, Adv. Mater. 17 (2005) 1617. [5] G. Li, V. Shrotriya, J. Huang, Y. Yao, T. Moriarty, K. Emery, Y. Yang, Nat. Matters 4 (2005) 867. [6] F. Padinger, R.S. Rittberger, N.S. Sariciftci, Adv. Funct. Mater. 13 (2003) 85. [7] Q. Zhou, Q. Hou, L. Zheng, X. Deng, G. Yu, Y. Cao, Appl. Phys. Lett. 84 (2004) 1653. [8] X. Wang, E. Perzon, J.L. Delgado, P. Cruz, F. Zhang, F. Langa, M. Andersson, O. Inganäs, Appl. Phys. Lett. 85 (2004) 5081. [9] K. Colladet, S. Fourier, T.J. Cleij, L. Lutsen, J. Gelan, D.k. Vanderzande, L.H. Nguyen, H. Neugebauer, S. Sariciftci, A. Aguirre, G. Janssen, E. Goovaerts, Macromolecules 40 (2007) 65.
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