- Email: [email protected]
Hybrid inverted organic photovoltaic cells based on nanoporous TiO2 films and organic small molecules
ARTICLE IN PRESS Solar Energy Materials & Solar Cells 93 (2009) 1613–1617
Contents lists available at ScienceDirect
Solar Energy Materials & Solar Cells journal homepage: www.elsevier.com/locate/solmat
Hybrid inverted organic photovoltaic cells based on nanoporous TiO2 films and organic small molecules M.N. Shan a,b, S.S. Wang b, Z.Q. Bian b,, J.P. Liu b, Y.L. Zhao a, a
The College of Chemistry and Chemical Engineering, Inner Mogolia University, Hohhot 010021, PR China Beijing National Laboratory for Molecular Sciences, State Key Laboratory of Rare Earth Materials Chemistry and Applications, College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, China b
a r t i c l e in fo
abstract
Article history: Received 7 January 2009 Received in revised form 27 April 2009 Accepted 28 April 2009 Available online 20 May 2009
Solar cells based on nanoporous TiO2 films with an inverted structure of indium tin oxide (ITO)/TiO2/ copper phthalocyanine (CuPc):fullerene (C60)/CuPc/poly(3,4-oxyethyleneoxythiophene):poly(styrene sulfonate) (PEDOT:PSS)/Au were fabricated. The best overall photovoltaic performance undergoing a series of device optimization was achieved with the device of ITO/dense TiO2 (30 nm)/nanoporous TiO2 (130 nm)/C60:CuPc (1:6 weight) (20 nm)/CuPc (20 nm)/PEDOT:PSS (50 nm)/Au (30 nm). The device using the nanoporous TiO2 films has better photovoltaic properties compared to those using dense TiO2 films. Higher photovoltaic performances were obtained by introducing a coevaporated layer of C60:CuPc between TiO2 and CuPc. The stability of inverted structure was better than that of the normal device, which gives a promising way for fabrication of solar cells with improved stability. & 2009 Elsevier B.V. All rights reserved.
Keywords: Photovoltaic cell TiO2 CuPc C60 Inverted structure
1. Introduction Organic photovoltaic cells have attracted considerable attention due to their potential for low-cost solar energy conversion [1–6]. Since a 1%-efficient thin-film organic photovoltaic cell based on a donor–acceptor (D–A) heterojunction was reported by C.W. Tang, the power conversion efficiency (PCE) has improved steadily [7,8]. However, the low carrier mobilities of organic materials limit the development of organic solar cells. The thicker organic film favors the ability of optical absorption, but is unfavorable to the carrier transportation due to the low carrier mobility [9,10]. To overcome this limitation, the inorganic functional nanostructures with high carrier mobility will benefit in increasing of the collection and transportation of the charges. The inorganic n-type semiconductor TiO2 is considered as an excellent material with good electron transport properties. Ordered nanoporous TiO2 can form a heterojunction with small organic molecule materials with large interface area. In addition, ordered nanoporous TiO2 has continuous ways for the carriers to contact the indium tin oxide (ITO) cathode, which will be beneficial in improving carrier collection efficiency. The copper phthalocyanine (CuPc) as organic p-type semiconductor has good optical absorption properties and a better energy band matching with TiO2; so much better photovoltaic performance of the
Corresponding authors. Tel./fax: +86 10 62757156.
E-mail address: [email protected] (Z.Q. Bian). 0927-0248/$ - see front matter & 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.solmat.2009.04.017
corresponding device might be expected. Moreover, this inverted device has better shelf stability compared to that of conventional structure of ITO/CuPc/fullerene (C60)/bathocuproine (BCP)/Al since the former could effectively protect C60 from the diffusion of oxygen [11]. In this article, we used organic small molecular material as optical absorption layer and nanoporous TiO2 as continuous charge transport paths to fabricate photovoltaic devices. We discussed the influences of the CuPc:C60 ratio, organic layer thickness, nanoporous TiO2 and coevaporated layer on the photovoltaic performance. By varying the fabrication parameter, we obtained the best optimized structure. Our experimental results show that the nanoporous TiO2 layer and coevaporated layer play an important role in the improvement of the photovoltaic performance. We also found that the inverted devices have better shelf stability than the normal device.
2. Experimental details The photovoltaic devices were fabricated on precleaned glass substrates coated with a 60-nm-thick conducting ITO electrode. The TiO2 dense layer was dip-coated using an upgrade speed of 1 mm s 1 onto the ITO using the tetrabutyl titanate and petroleum ether solution with a volume ratio of 1:50. The samples were subsequently calcined using a heating rate of 1 1C min 1 from room temperature to 450 1C and held at 450 1C for 15 min. We
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attained a dense layer of TiO2 with a thickness of 30 nm, which was measured by a KLA-Tencor Alpha-Step surface Profiler. To make nanoporous TiO2 films, 1 g Pluronic P123 and 20 g ethanol were stirred into 3.2 g concentrated HCl with 3.6 g titanium ethoxide solution [12]. After approximately 10 min of stirring, the ITO-coated glass substrates with dense TiO2 were dipcoated using a coating speed of 1 mm s 1. The samples were aged at a room temperature for 48 h, and calcined using a heating rate of 1 1C min 1 to 450 1C and held at 450 1C for 15 min to remove the block copolymer and densify the TiO2. Then, a 130-nm-thick TiO2 anatase nanoporous film was produced. For the fabrication of solar cells, the CuPc (499%, by Aldrich), C60 (499.9% by Yongxin Technology CO. Ltd., China), poly (3,4-oxyethyleneoxythiophene):poly(styrene sulfonate) (PEDOT:PSS) (Baytron P, by H.C. Starck) were used as received. The BCP (by Acros Organics, 98%) was purified by vacuum gradient sublimation. The mixed C60:CuPc (1:6 weight) were grown by codeposition from independent organic evaporation sources, with the deposition rates monitored by a quartz-crystal microbalances. Then PEDOT:PSS was spin-coated with a speed of 5000 rpm in ambient conditions. The samples were annealed at 160 1C for 30 min in vacuum. Finally, a gold film was thermally evaporated to form a top electrode. The solar cells active area were 5 mm 1 mm. Current density–voltage (J–V) characteristics were measured with Keithley mode 4200 power source in the dark and under AM 1.5 G solar illuminations with an Oriel 300 W solar simulator at an intensity of 100 mW/cm2. The light intensity was measured with a photometer (International light, IL1400) corrected by a standard silicon solar cell. A field-emission-focused ion beam workstation (FIB, Strata DB235, FEI company) working at scanning electron microscopy (SEM) mode was employed to investigate the surface morphology of the TiO2 films.
3. Results and discussion
Fig. 1. SEM image of the nanoporous TiO2 layer. Table 1 Summary of photovoltaic performance for the solar cells with structures of ITO/ dense TiO2/nanoporous TiO2/C60:CuPc (20 nm)/CuPc(20 nm)/PEDOT:PSS(50 nm)/ Au(30 nm) with varying ratios of CuPc:C60. CuPc:C60
Voc (V)
Jsc (mA/cm2)
FF
PCE (%)
6:1 3:1 1:1 1:3 1:6
0.50 0.50 0.48 0.31 0.44
2.42 2.46 2.42 1.20 0.98
0.30 0.26 0.29 0.31 0.37
0.36 0.32 0.33 0.11 0.16
3.1. Characterization of nanoporous TiO2 films A dense TiO2 layer under the nanoporous TiO2 films is used to prevent organic layer infiltrating into nanoporous TiO2 and have a contact with ITO. Fig. 1 shows the SEM image of the nanoporous TiO2 layer. It can be seen that the pores on the surface are approximately 10 nm in diameter. The nanoporous TiO2 film is expected to form a good contact with the organic layer with large interface area, which would favor the charge transfer between the organic layer and the TiO2. 3.2. Influences of the coevaporated ratio on the photovoltaic performance We first investigated the influences of mixing ratio of CuPc:C60 on the photovoltaic performance, which was summarized in Table 1. The device structure was ITO/dense TiO2/nanoporous TiO2/C60: CuPc (20 nm)/CuPc (20 nm)/PEDOT: PSS (50 nm)/Au (30 nm). It was noticed that the photovoltaic performance dropped obviously due to the less light absorption of CuPc if the ratio of C60 in the mixtures was more than 50%. When the coevaportion ratio of CuPc:C60 was varied from 6:1, 3:1 to 1:1, the photovoltaic performance was not significantly improved. It can be inferred that the exciton dissociation occurs at the interface between the CuPc and the TiO2, then the electrons transport to the TiO2 and holes transport to the CuPc. The energy barrier between C60 and TiO2 might act as electron traps and the incident photo to current conversion efficiency spectra gave more convincing evidence from the previous publication [13]. When the ratio of
CuPc was more than 6:1, the mixing ratio was out of our control. Therefore, the best optimized photovoltaic performance is 0.36% when the ratio of CuPc:C60 is 6:1.
3.3. Influences of the organic layer thickness on the photovoltaic performance The schematic architecture of photovoltaic devices studied in this work was shown in Fig. 2. In order to optimize the performance of photovoltaic devices, devices 1–6 with different thicknesses of organic functional layers were fabricated. The influences of the thickness of organic functional layers on the photovoltaic properties were summarized in Table 2. Comparing devices 1 and 2, the short-circuit current density (Jsc) increased from 2.26 to 2.32 mA/cm2 when a 20-nm thickness CuPc layer was inserted between the CuPc:C60 coevaporated layer and PEDOT:PSS layer, which can be explained by increasing optical absorption. On the other hand, the Jsc decreased from 2.32 to 1.65 and 0.98 mA/cm2 when the thickness of the CuPc layer increased from 20 to 40 and 60 nm, respectively. This fact should be ascribed to the poor transportation property of CuPc resulting in the decrease in collection efficiency even the light absorption increased to some extents. Thus, we obtained an optimized value of CuPc layer thickness of 20 nm. Comparing the performance of devices 5, 2 and 6 in which CuPc layer was fixed at 20 nm, the Jsc and power conversion efficiency decreased simultaneously along with increase in thickness of CuPc:C60 coevaporation layer. Thus, the optimized value of CuPc:C60 layer thickness is 20 nm.
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3.4. Influences of the nanoporous TiO2 on the photovoltaic performance In order to check the influences of the nanoporous TiO2 layer on the performance of solar cells, two devices with nanoporous TiO2 and without nanoporous TiO2 were fabricated. The configurations are as follows: ITO/dense TiO2/nanoporous TiO2/C60:CuPc (1:6 weight) (20 nm)/CuPc (20 nm)/PEDOT:PSS (50 nm)/Au (30 nm) (device 5) and ITO/dense TiO2/C60:CuPc (1:6 weight) (20 nm)/CuPc (20 nm)/PEDOT:PSS (50 nm)/Au (30 nm) (device 7). Fig. 3 shows the J–V characteristics of these devices under illumination. Device 5 with the nanoporous TiO2 interlayer shows higher photovoltaic performance with an open circuit voltage (Voc) of 0.50 V, a Jsc of 2.42 mA/cm2, a fill factor (FF) of 0.30, and a PCE of 0.36% which is better than device 7 with Voc, Jsc, FF and PCE of 0.39 V, 0.38 mA/cm2, 0.17 and 0.025%, respectively. Incorporation of a nanoporous TiO2 layer, the only difference between devices 5 and 7, should thus be responsible for the photovoltaic performance improvement. The nanoporous TiO2 can enormously increase the dissociation areas between CuPc and TiO2, which results in the efficient exciton dissociation. On the other hand, the well-ordered structure of TiO2 can act as an efficient transport channel. With the help of the continuous nanoporous TiO2, the collection efficiency of electrons is greatly enhanced, leading to an evidently increase of photocurrent. 3.5. Influences of coevaporated layer of C60 and CuPc
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TiO2/CuPc (40 nm)/PEDOT/Au (device 8) were fabricated. The J–V characteristics of these devices are shown in Fig. 4. Device 5 with the coevaporated layer shows a higher PCE of 0.36% than device 8 with a PCE of 0.022%. Fig. 5 shows the energy diagram of this kind of photovoltaic cell, which suggests that the electrons in C60 cannot transport to the TiO2 layer due to the energy barrier between the LUMO of C60 and the TiO2. Therefore, the photovoltaic performance improvement by the addition of coevaporated layer might be attributed to the morphological change of CuPc reported by Forrest and coworkers [14]. The neat CuPc film has a corrugated surface, which has longer distance for most exciton diffusion to the dissociation interface, while the surface of CuPc:C60 mixed layer becomes smooth, which benefits the exciton diffusion and charge transport [13]. In Figs. 3 and 4, the J–V curves both exhibit an inflection point; it is normally indicative of a transport barrier and has been shown theoretically during degradation to be due to interface degradation and poor transport [15–18]. The efficiency is still very low, although photovoltaic performance was greatly improved by the addition of nanoporous TiO2. One of most important problems is the low pore-filling efficiency due to the small size of the nanopores and the low wetting ability. It is difficult to completely fill the nanopore by the organic material, which restrains the migration and separation of the excitons and results in a low efficiency [19]. Therefore, a nanoporous TiO2 layer with suitable diameter and modified surface may help to improve the performance of the solar cells.
For comparison, device with coevaporated layer adopting a configuration of ITO/dense TiO2/nanoporous TiO2/CuPc:C60 (20 nm)/CuPc (20 nm)/PEDOT/Au (device 5) and without coevaporated layer with a structure of ITO/dense TiO2/nanoporous
Fig. 2. Schematic structure of the solar cell based on ITO/dense TiO2/nanoporous TiO2/CuPc:C60/CuPc/PEDOT/Au.
Fig. 3. J-V characteristics of the photovoltaic devices without nanoporous TiO2 (ITO/dense TiO2/CuPc:C60/CuPc/PEDOT/Au, device 7) and with nanoporous TiO2 (ITO/dense TiO2/nanoporous TiO2/CuPc:C60/CuPc/PEDOT/Au, device 5) under illumination using an AM 1.5 solar simulator.
Table 2 Summary of photovoltaic performance for the solar cells with structures of ITO/dense TiO2/nanoporous TiO2/C60:CuPc (1:6 weight)/CuPc/PEDOT:PSS(50 nm)/Au(30 nm) with varying thicknesses of CuPc layer and C60:CuPc layer. Device
CuPc:C60 layer thickness (nm)
CuPc layer thickness (nm)
Voc (V)
Jsc (mA/cm2)
FF
PCE (%)
1 2 3 4 5 6
40 40 40 40 20 60
0 20 40 60 20 20
0.45 0.52 0.54 0.52 0.50 0.49
2.26 2.32 1.65 0.98 2.42 1.37
0.28 0.26 0.29 0.29 0.30 0.27
0.28 0.32 0.26 0.15 0.36 0.18
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Table 3 Summary of the shelf stabilities of the inverted device (device 5) and normal device (device 9) kept in N2 glove box and in air with varying decay times. Sample and decay time
Voc (V)
Jsc (mA/cm2)
FF
PCE (%)
Device Device Device Device Device Device
0.52 0.52 0.52 0.49 0.49 0.49
2.30 2.30 2.25 3.73 3.34 2.49
0.29 0.29 0.28 0.55 0.55 0.41
0.35 0.35 0.33 1.00 0.90 0.50
5 5, 5, 9 9, 9,
256 h in N2 256 h in air 256 h in N2 256 h in air
4. Conclusion
Fig. 4. J-V characteristics of the photovoltaic devices without coevaporated layer (ITO/dense TiO2/nanoporous TiO2/CuPc (40 nm)/PEDOT/Au, device 8) and with coevaporated layer (ITO/dense TiO2/nanoporous TiO2/CuPc:C60 (20 nm)/CuPc (20 nm)/PEDOT/Au, device 5) under illumination using an AM 1.5 solar simulator.
In summary, the best overall photovoltaic performance undergoing a series of device optimization was achieved with the device of ITO/dense TiO2 (30 nm)/nanoporous TiO2 (130 nm)/C60:CuPc (1:6 weight) (20 nm)/CuPc (20 nm)/PEDOT:PSS (50 nm)/Au (30 nm). The photovoltaic performances of the solar cells with nanoporous TiO2 are greatly improved compared with that of device without nanoporous TiO2. The reason for the improvement of the photovoltaic performance might be the more exciton dissociation area and higher charge mobilities. A mixed layer of CuPc:C60 is also a critical factor to improve the photovoltaic performance of this kind of solar cells. Compared with the normal device, the inverted device shows a better shelf stability, which gives a promising way for fabrication of solar cells with improved stability.
Acknowledgements
Fig. 5. Schematic energy level diagram for the inverted device, with energy levels in eV relative to vacuum.
3.6. The stability of this photovoltaic device The stabilities of the inverted device and the normal device were also investigated. We measured the photovoltaic performance of the inverted device with a structure of ITO/dense TiO2/ nanoporous TiO2/C60:CuPc (20 nm)/CuPc (20 nm)/PEDOT:PSS (50 nm)/Au (30 nm) (device 5) and normal device with a structure of ITO/CuPc (40 nm)/C60 (68 nm)/BCP (7 nm)/Al (80 nm) (device 9). The photovoltaic performance of unencapsulated devices 5 and 9 kept in air and in N2 in dark with varying time is illustrated in Table 3. It can be seen that the stability of the inverted solar cell was greatly improved compared with that of the normal device. And the degradation ratios were remarkably reduced if the devices were kept in N2 glove box. It has been reported that there are examples of air-stable devices based on an inverted geometry [20]. In the previous studies, Eklund et al. [21] found the evidence of formation of the carbon–oxygen stretch modes in C60 by Fourier Transform Infrared Spectrometer measurement. In the normal device, after permeating through the Al electrode and the thin buffer layer, oxygen diffuses into C60 which accelerate the device degradation. But in the inverted structure solar cell, oxygen must permeate through the Au electrode, the buffer layer and the CuPc layer before it could reach the C60 layer, and thus the time for oxygen diffusing into C60 was delayed. However, the stability of the inverted device under illumination is poor, which might be caused by the photodegradation of organic materials by TiO2 nanomaterials in the presence of oxygen and light [22–24].
We thank the NNSFC (50772003, 20461002) and the Natural Science Foundation of Inner Mongolia (200711020203) for financial support.
Appendix A. Supporting Information Supplementary data associated with this article can be found in the online version at doi:10.1016/j.solmat.2009.04.017.
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[11] Q.L. Song, M.L. Wang, E.G. obbard, Degradation of small-molecule organic solar cells, Appl. Phys. Lett. 89 (2006) 251118-1–251118-3. [12] K.M. Coakley, Y. Liu, M.D. McGehee, K.L. Frindell, G.D. Stucky, Infiltrating semiconducting polymers into self-assembled mesoporous titania films for photovoltaic applications, Adv. Funct. Mater. 13 (2003) 301–306. [13] J.P. Liu, S.S. Wang, Inverted photovoltaic device based on ZnO and organic small molecule heterojunction, Chem. Phys. Lett. 470 (2009) 103–106. [14] B.P. Rand, J.G. Xue, S. Uchida, S.R. Forrest, Mixed donor–acceptor molecular heterojunctions for photovoltaic applicatons, J. Appl. Phys. 98 (2005) 1249021–124902-7. [15] M. Glatthaar, M. Riede, N. Keegan, Efficiency limiting factors of organic bulk heterojunction solar cells identified by electrical impedance spectroscopy, Sol. Energy Mater. Sol. Cells 91 (2007) 390–393. [16] M. Vogel, S. Doka, Ch. Breyer, On the function of a bathocuproine buffer layer in organic photovoltaic cells, Appl. Phys. Lett. 89 (2006) 163501-1–163501-3. [17] F.C. Krebs, K. Norrman, Analysis of the failure mechanism for a stable organic photovoltaic during 10000 h of testing, Prog. Photovolt: Res. Appl. 15 (2007) 697–712. [18] S.T. Zhang, Y.C. Zhou, J.M. Zhao, Y.Q. Zhan, Role of hole playing in improving performance of organic light-emitting devices with an Al2O3 layer inserted at
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Contents lists available at ScienceDirect
Solar Energy Materials & Solar Cells journal homepage: www.elsevier.com/locate/solmat
Hybrid inverted organic photovoltaic cells based on nanoporous TiO2 films and organic small molecules M.N. Shan a,b, S.S. Wang b, Z.Q. Bian b,, J.P. Liu b, Y.L. Zhao a, a
The College of Chemistry and Chemical Engineering, Inner Mogolia University, Hohhot 010021, PR China Beijing National Laboratory for Molecular Sciences, State Key Laboratory of Rare Earth Materials Chemistry and Applications, College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, China b
a r t i c l e in fo
abstract
Article history: Received 7 January 2009 Received in revised form 27 April 2009 Accepted 28 April 2009 Available online 20 May 2009
Solar cells based on nanoporous TiO2 films with an inverted structure of indium tin oxide (ITO)/TiO2/ copper phthalocyanine (CuPc):fullerene (C60)/CuPc/poly(3,4-oxyethyleneoxythiophene):poly(styrene sulfonate) (PEDOT:PSS)/Au were fabricated. The best overall photovoltaic performance undergoing a series of device optimization was achieved with the device of ITO/dense TiO2 (30 nm)/nanoporous TiO2 (130 nm)/C60:CuPc (1:6 weight) (20 nm)/CuPc (20 nm)/PEDOT:PSS (50 nm)/Au (30 nm). The device using the nanoporous TiO2 films has better photovoltaic properties compared to those using dense TiO2 films. Higher photovoltaic performances were obtained by introducing a coevaporated layer of C60:CuPc between TiO2 and CuPc. The stability of inverted structure was better than that of the normal device, which gives a promising way for fabrication of solar cells with improved stability. & 2009 Elsevier B.V. All rights reserved.
Keywords: Photovoltaic cell TiO2 CuPc C60 Inverted structure
1. Introduction Organic photovoltaic cells have attracted considerable attention due to their potential for low-cost solar energy conversion [1–6]. Since a 1%-efficient thin-film organic photovoltaic cell based on a donor–acceptor (D–A) heterojunction was reported by C.W. Tang, the power conversion efficiency (PCE) has improved steadily [7,8]. However, the low carrier mobilities of organic materials limit the development of organic solar cells. The thicker organic film favors the ability of optical absorption, but is unfavorable to the carrier transportation due to the low carrier mobility [9,10]. To overcome this limitation, the inorganic functional nanostructures with high carrier mobility will benefit in increasing of the collection and transportation of the charges. The inorganic n-type semiconductor TiO2 is considered as an excellent material with good electron transport properties. Ordered nanoporous TiO2 can form a heterojunction with small organic molecule materials with large interface area. In addition, ordered nanoporous TiO2 has continuous ways for the carriers to contact the indium tin oxide (ITO) cathode, which will be beneficial in improving carrier collection efficiency. The copper phthalocyanine (CuPc) as organic p-type semiconductor has good optical absorption properties and a better energy band matching with TiO2; so much better photovoltaic performance of the
Corresponding authors. Tel./fax: +86 10 62757156.
E-mail address: [email protected] (Z.Q. Bian). 0927-0248/$ - see front matter & 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.solmat.2009.04.017
corresponding device might be expected. Moreover, this inverted device has better shelf stability compared to that of conventional structure of ITO/CuPc/fullerene (C60)/bathocuproine (BCP)/Al since the former could effectively protect C60 from the diffusion of oxygen [11]. In this article, we used organic small molecular material as optical absorption layer and nanoporous TiO2 as continuous charge transport paths to fabricate photovoltaic devices. We discussed the influences of the CuPc:C60 ratio, organic layer thickness, nanoporous TiO2 and coevaporated layer on the photovoltaic performance. By varying the fabrication parameter, we obtained the best optimized structure. Our experimental results show that the nanoporous TiO2 layer and coevaporated layer play an important role in the improvement of the photovoltaic performance. We also found that the inverted devices have better shelf stability than the normal device.
2. Experimental details The photovoltaic devices were fabricated on precleaned glass substrates coated with a 60-nm-thick conducting ITO electrode. The TiO2 dense layer was dip-coated using an upgrade speed of 1 mm s 1 onto the ITO using the tetrabutyl titanate and petroleum ether solution with a volume ratio of 1:50. The samples were subsequently calcined using a heating rate of 1 1C min 1 from room temperature to 450 1C and held at 450 1C for 15 min. We
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attained a dense layer of TiO2 with a thickness of 30 nm, which was measured by a KLA-Tencor Alpha-Step surface Profiler. To make nanoporous TiO2 films, 1 g Pluronic P123 and 20 g ethanol were stirred into 3.2 g concentrated HCl with 3.6 g titanium ethoxide solution [12]. After approximately 10 min of stirring, the ITO-coated glass substrates with dense TiO2 were dipcoated using a coating speed of 1 mm s 1. The samples were aged at a room temperature for 48 h, and calcined using a heating rate of 1 1C min 1 to 450 1C and held at 450 1C for 15 min to remove the block copolymer and densify the TiO2. Then, a 130-nm-thick TiO2 anatase nanoporous film was produced. For the fabrication of solar cells, the CuPc (499%, by Aldrich), C60 (499.9% by Yongxin Technology CO. Ltd., China), poly (3,4-oxyethyleneoxythiophene):poly(styrene sulfonate) (PEDOT:PSS) (Baytron P, by H.C. Starck) were used as received. The BCP (by Acros Organics, 98%) was purified by vacuum gradient sublimation. The mixed C60:CuPc (1:6 weight) were grown by codeposition from independent organic evaporation sources, with the deposition rates monitored by a quartz-crystal microbalances. Then PEDOT:PSS was spin-coated with a speed of 5000 rpm in ambient conditions. The samples were annealed at 160 1C for 30 min in vacuum. Finally, a gold film was thermally evaporated to form a top electrode. The solar cells active area were 5 mm 1 mm. Current density–voltage (J–V) characteristics were measured with Keithley mode 4200 power source in the dark and under AM 1.5 G solar illuminations with an Oriel 300 W solar simulator at an intensity of 100 mW/cm2. The light intensity was measured with a photometer (International light, IL1400) corrected by a standard silicon solar cell. A field-emission-focused ion beam workstation (FIB, Strata DB235, FEI company) working at scanning electron microscopy (SEM) mode was employed to investigate the surface morphology of the TiO2 films.
3. Results and discussion
Fig. 1. SEM image of the nanoporous TiO2 layer. Table 1 Summary of photovoltaic performance for the solar cells with structures of ITO/ dense TiO2/nanoporous TiO2/C60:CuPc (20 nm)/CuPc(20 nm)/PEDOT:PSS(50 nm)/ Au(30 nm) with varying ratios of CuPc:C60. CuPc:C60
Voc (V)
Jsc (mA/cm2)
FF
PCE (%)
6:1 3:1 1:1 1:3 1:6
0.50 0.50 0.48 0.31 0.44
2.42 2.46 2.42 1.20 0.98
0.30 0.26 0.29 0.31 0.37
0.36 0.32 0.33 0.11 0.16
3.1. Characterization of nanoporous TiO2 films A dense TiO2 layer under the nanoporous TiO2 films is used to prevent organic layer infiltrating into nanoporous TiO2 and have a contact with ITO. Fig. 1 shows the SEM image of the nanoporous TiO2 layer. It can be seen that the pores on the surface are approximately 10 nm in diameter. The nanoporous TiO2 film is expected to form a good contact with the organic layer with large interface area, which would favor the charge transfer between the organic layer and the TiO2. 3.2. Influences of the coevaporated ratio on the photovoltaic performance We first investigated the influences of mixing ratio of CuPc:C60 on the photovoltaic performance, which was summarized in Table 1. The device structure was ITO/dense TiO2/nanoporous TiO2/C60: CuPc (20 nm)/CuPc (20 nm)/PEDOT: PSS (50 nm)/Au (30 nm). It was noticed that the photovoltaic performance dropped obviously due to the less light absorption of CuPc if the ratio of C60 in the mixtures was more than 50%. When the coevaportion ratio of CuPc:C60 was varied from 6:1, 3:1 to 1:1, the photovoltaic performance was not significantly improved. It can be inferred that the exciton dissociation occurs at the interface between the CuPc and the TiO2, then the electrons transport to the TiO2 and holes transport to the CuPc. The energy barrier between C60 and TiO2 might act as electron traps and the incident photo to current conversion efficiency spectra gave more convincing evidence from the previous publication [13]. When the ratio of
CuPc was more than 6:1, the mixing ratio was out of our control. Therefore, the best optimized photovoltaic performance is 0.36% when the ratio of CuPc:C60 is 6:1.
3.3. Influences of the organic layer thickness on the photovoltaic performance The schematic architecture of photovoltaic devices studied in this work was shown in Fig. 2. In order to optimize the performance of photovoltaic devices, devices 1–6 with different thicknesses of organic functional layers were fabricated. The influences of the thickness of organic functional layers on the photovoltaic properties were summarized in Table 2. Comparing devices 1 and 2, the short-circuit current density (Jsc) increased from 2.26 to 2.32 mA/cm2 when a 20-nm thickness CuPc layer was inserted between the CuPc:C60 coevaporated layer and PEDOT:PSS layer, which can be explained by increasing optical absorption. On the other hand, the Jsc decreased from 2.32 to 1.65 and 0.98 mA/cm2 when the thickness of the CuPc layer increased from 20 to 40 and 60 nm, respectively. This fact should be ascribed to the poor transportation property of CuPc resulting in the decrease in collection efficiency even the light absorption increased to some extents. Thus, we obtained an optimized value of CuPc layer thickness of 20 nm. Comparing the performance of devices 5, 2 and 6 in which CuPc layer was fixed at 20 nm, the Jsc and power conversion efficiency decreased simultaneously along with increase in thickness of CuPc:C60 coevaporation layer. Thus, the optimized value of CuPc:C60 layer thickness is 20 nm.
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3.4. Influences of the nanoporous TiO2 on the photovoltaic performance In order to check the influences of the nanoporous TiO2 layer on the performance of solar cells, two devices with nanoporous TiO2 and without nanoporous TiO2 were fabricated. The configurations are as follows: ITO/dense TiO2/nanoporous TiO2/C60:CuPc (1:6 weight) (20 nm)/CuPc (20 nm)/PEDOT:PSS (50 nm)/Au (30 nm) (device 5) and ITO/dense TiO2/C60:CuPc (1:6 weight) (20 nm)/CuPc (20 nm)/PEDOT:PSS (50 nm)/Au (30 nm) (device 7). Fig. 3 shows the J–V characteristics of these devices under illumination. Device 5 with the nanoporous TiO2 interlayer shows higher photovoltaic performance with an open circuit voltage (Voc) of 0.50 V, a Jsc of 2.42 mA/cm2, a fill factor (FF) of 0.30, and a PCE of 0.36% which is better than device 7 with Voc, Jsc, FF and PCE of 0.39 V, 0.38 mA/cm2, 0.17 and 0.025%, respectively. Incorporation of a nanoporous TiO2 layer, the only difference between devices 5 and 7, should thus be responsible for the photovoltaic performance improvement. The nanoporous TiO2 can enormously increase the dissociation areas between CuPc and TiO2, which results in the efficient exciton dissociation. On the other hand, the well-ordered structure of TiO2 can act as an efficient transport channel. With the help of the continuous nanoporous TiO2, the collection efficiency of electrons is greatly enhanced, leading to an evidently increase of photocurrent. 3.5. Influences of coevaporated layer of C60 and CuPc
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TiO2/CuPc (40 nm)/PEDOT/Au (device 8) were fabricated. The J–V characteristics of these devices are shown in Fig. 4. Device 5 with the coevaporated layer shows a higher PCE of 0.36% than device 8 with a PCE of 0.022%. Fig. 5 shows the energy diagram of this kind of photovoltaic cell, which suggests that the electrons in C60 cannot transport to the TiO2 layer due to the energy barrier between the LUMO of C60 and the TiO2. Therefore, the photovoltaic performance improvement by the addition of coevaporated layer might be attributed to the morphological change of CuPc reported by Forrest and coworkers [14]. The neat CuPc film has a corrugated surface, which has longer distance for most exciton diffusion to the dissociation interface, while the surface of CuPc:C60 mixed layer becomes smooth, which benefits the exciton diffusion and charge transport [13]. In Figs. 3 and 4, the J–V curves both exhibit an inflection point; it is normally indicative of a transport barrier and has been shown theoretically during degradation to be due to interface degradation and poor transport [15–18]. The efficiency is still very low, although photovoltaic performance was greatly improved by the addition of nanoporous TiO2. One of most important problems is the low pore-filling efficiency due to the small size of the nanopores and the low wetting ability. It is difficult to completely fill the nanopore by the organic material, which restrains the migration and separation of the excitons and results in a low efficiency [19]. Therefore, a nanoporous TiO2 layer with suitable diameter and modified surface may help to improve the performance of the solar cells.
For comparison, device with coevaporated layer adopting a configuration of ITO/dense TiO2/nanoporous TiO2/CuPc:C60 (20 nm)/CuPc (20 nm)/PEDOT/Au (device 5) and without coevaporated layer with a structure of ITO/dense TiO2/nanoporous
Fig. 2. Schematic structure of the solar cell based on ITO/dense TiO2/nanoporous TiO2/CuPc:C60/CuPc/PEDOT/Au.
Fig. 3. J-V characteristics of the photovoltaic devices without nanoporous TiO2 (ITO/dense TiO2/CuPc:C60/CuPc/PEDOT/Au, device 7) and with nanoporous TiO2 (ITO/dense TiO2/nanoporous TiO2/CuPc:C60/CuPc/PEDOT/Au, device 5) under illumination using an AM 1.5 solar simulator.
Table 2 Summary of photovoltaic performance for the solar cells with structures of ITO/dense TiO2/nanoporous TiO2/C60:CuPc (1:6 weight)/CuPc/PEDOT:PSS(50 nm)/Au(30 nm) with varying thicknesses of CuPc layer and C60:CuPc layer. Device
CuPc:C60 layer thickness (nm)
CuPc layer thickness (nm)
Voc (V)
Jsc (mA/cm2)
FF
PCE (%)
1 2 3 4 5 6
40 40 40 40 20 60
0 20 40 60 20 20
0.45 0.52 0.54 0.52 0.50 0.49
2.26 2.32 1.65 0.98 2.42 1.37
0.28 0.26 0.29 0.29 0.30 0.27
0.28 0.32 0.26 0.15 0.36 0.18
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Table 3 Summary of the shelf stabilities of the inverted device (device 5) and normal device (device 9) kept in N2 glove box and in air with varying decay times. Sample and decay time
Voc (V)
Jsc (mA/cm2)
FF
PCE (%)
Device Device Device Device Device Device
0.52 0.52 0.52 0.49 0.49 0.49
2.30 2.30 2.25 3.73 3.34 2.49
0.29 0.29 0.28 0.55 0.55 0.41
0.35 0.35 0.33 1.00 0.90 0.50
5 5, 5, 9 9, 9,
256 h in N2 256 h in air 256 h in N2 256 h in air
4. Conclusion
Fig. 4. J-V characteristics of the photovoltaic devices without coevaporated layer (ITO/dense TiO2/nanoporous TiO2/CuPc (40 nm)/PEDOT/Au, device 8) and with coevaporated layer (ITO/dense TiO2/nanoporous TiO2/CuPc:C60 (20 nm)/CuPc (20 nm)/PEDOT/Au, device 5) under illumination using an AM 1.5 solar simulator.
In summary, the best overall photovoltaic performance undergoing a series of device optimization was achieved with the device of ITO/dense TiO2 (30 nm)/nanoporous TiO2 (130 nm)/C60:CuPc (1:6 weight) (20 nm)/CuPc (20 nm)/PEDOT:PSS (50 nm)/Au (30 nm). The photovoltaic performances of the solar cells with nanoporous TiO2 are greatly improved compared with that of device without nanoporous TiO2. The reason for the improvement of the photovoltaic performance might be the more exciton dissociation area and higher charge mobilities. A mixed layer of CuPc:C60 is also a critical factor to improve the photovoltaic performance of this kind of solar cells. Compared with the normal device, the inverted device shows a better shelf stability, which gives a promising way for fabrication of solar cells with improved stability.
Acknowledgements
Fig. 5. Schematic energy level diagram for the inverted device, with energy levels in eV relative to vacuum.
3.6. The stability of this photovoltaic device The stabilities of the inverted device and the normal device were also investigated. We measured the photovoltaic performance of the inverted device with a structure of ITO/dense TiO2/ nanoporous TiO2/C60:CuPc (20 nm)/CuPc (20 nm)/PEDOT:PSS (50 nm)/Au (30 nm) (device 5) and normal device with a structure of ITO/CuPc (40 nm)/C60 (68 nm)/BCP (7 nm)/Al (80 nm) (device 9). The photovoltaic performance of unencapsulated devices 5 and 9 kept in air and in N2 in dark with varying time is illustrated in Table 3. It can be seen that the stability of the inverted solar cell was greatly improved compared with that of the normal device. And the degradation ratios were remarkably reduced if the devices were kept in N2 glove box. It has been reported that there are examples of air-stable devices based on an inverted geometry [20]. In the previous studies, Eklund et al. [21] found the evidence of formation of the carbon–oxygen stretch modes in C60 by Fourier Transform Infrared Spectrometer measurement. In the normal device, after permeating through the Al electrode and the thin buffer layer, oxygen diffuses into C60 which accelerate the device degradation. But in the inverted structure solar cell, oxygen must permeate through the Au electrode, the buffer layer and the CuPc layer before it could reach the C60 layer, and thus the time for oxygen diffusing into C60 was delayed. However, the stability of the inverted device under illumination is poor, which might be caused by the photodegradation of organic materials by TiO2 nanomaterials in the presence of oxygen and light [22–24].
We thank the NNSFC (50772003, 20461002) and the Natural Science Foundation of Inner Mongolia (200711020203) for financial support.
Appendix A. Supporting Information Supplementary data associated with this article can be found in the online version at doi:10.1016/j.solmat.2009.04.017.
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