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Efficient tandem polymer photovoltaic cells using inorganic metal oxides as a transparent middle connection unit
Organic Electronics 11 (2010) 1230–1233
Contents lists available at ScienceDirect
Organic Electronics journal homepage: www.elsevier.com/locate/orgel
Efficient tandem polymer photovoltaic cells using inorganic metal oxides as a transparent middle connection unit Xiaoyang Guo, Fengmin Liu, Bin Meng, Zhiyuan Xie *, Lixiang Wang State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Graduate School of Chinese Academy of Sciences, Chinese Academy of Sciences, Changchun 130022, People’s Republic of China
a r t i c l e
i n f o
Article history: Received 8 March 2010 Received in revised form 19 April 2010 Accepted 1 May 2010 Available online 10 May 2010 Keywords: Polymer photovoltaic cells Tandem structure Open-circuit voltage
a b s t r a c t A kind of tandem polymer photovoltaic cell with inorganic metal oxides as a transparent middle connection unit to link the two subcells having different absorption characteristics is developed and its open-circuit voltage origin is investigated. The middle connection unit consists of an electron-transporting titanium oxide (TiOx) layer, a thin aluminum (Al) layer and a hole-transporting molybdenum oxide (MoO3) layer. The inserted thin Al layer between the TiOx and MoO3 layers not only acts as an efficient recombination site for electrons and holes photo-generated in the two subcells, but also realigns the electronic energy levels at TiOx/MoO3 interfaces to reduce the open-circuit voltage loss. The open-circuit voltage of the tandem cell is the summation of the two subcells. Ó 2010 Elsevier B.V. All rights reserved.
1. Introduction Photovoltaic (PV) cells based on organic molecules and conjugated polymers have drawn great research interests due to their potential advantages of light weight, flexibility and low-cost manufacturing [1,2]. The power conversion efficiency (PCE) of the bulk-heterojunction polymer PV cells has been increased above 6% in the past few years [3,4]. Although some narrow band-gap conjugated polymers have been developed to enhance spectral coverage to the solar spectrum, the narrow absorption range of conjugated polymers is still a drawback for polymer PV cells, which results in only a small fraction of the solar flux being harvested by the active layer in bulk-heterojunction polymer PV cell. One of the strategies to broaden the spectral coverage to solar spectrum is to make use of tandem architectures in which two or more cells with different absorption characteristics are linked in series or parallel connection [5–9]. The middle connection unit linking the subcells in tandem architectures plays an important role in determining the open-circuit voltage (VOC) and final * Corresponding author. Tel./fax: +86 431 85262819. E-mail address: [email protected] (Z. Xie). 1566-1199/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.orgel.2010.05.004
PCE of the PV cells. The middle connection unit should not only be highly transparent, but also act as an efficient recombination site for electrons and holes photogenerated in two subcells with low VOC loss. In addition, the middle connection structure should resist the bottom active layer from being destroyed while spin-coating the top active layer. Poly(3,4-ethylene dioxylenethiophene)– polystyrene sulfonic acid (PEDOT:PSS) are widely used as an anode buffer layer in polymer PV cells and as a middle connection unit in tandem polymer PV cells [10,11]. However, the absorption caused by PEDOT:PSS layer may reduce the number of photons absorbed by the active layers [12]. Inorganic metal oxides are potential candidates for the middle connection unit in tandem polymer PV cells, which have little absorption in visible region and high charge mobilities, and some inorganic metal oxides have been successfully used as buffer layers in polymer PV cells and organic light-emitting diodes (OLEDs) [5,9,13–19]. For example, wide band-gap titanium oxide (TiOx) has been utilized in polymer PV cells as an optical spacer [14], or electron-transporting/hole-blocking layer [15,16]. Molybdenum oxide (MoO3) is a kind of p-type semiconductor and has been employed as an anode buffer layer in OLEDs [17,18] and organic PV cells [19]. To date, there is no report
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using a combination of p-type and n-type inorganic metal oxides as a middle connection unit in tandem polymer PV cells and investigating the origin of VOC in tandem PV cell. Herein, we fabricate tandem polymer PV cells using a combination of TiOx and MoO3 as a middle connection unit to link the two subcells. It is found that inserting a thin Al layer between the TiOx and MoO3 layers realizes tandem PV cells and the VOC of the tandem cell is summation of the two subcells. The origin of VOC in tandem PV cells is discussed.
2. Experimental details Fig. 1 shows the designed device structure of the tandem polymer PV cell and the chemical structures of active materials used in this study. The blends of poly(3-hexylthiophene) (P3HT)/[6,6]-phenyl C61-butyric acid methyl ester (PCBM) and of poly[2,6-(4,4-bis-(2-ethylhexyl)-4Hcyclopenta[2,1-b;3,4-b0 ]dithiophene)-alt-4,7-(2,1,3-benzothiadiazole)] (PCPDTBT)/[6,6]-phenyl C71-butyric acid methyl ester (PC70BM) are used as the active layers in subcells, respectively. PCPDTBT is a kind of low-band-gap conjugated polymer, and its lowest unoccupied molecular orbital (LUMO) and highest occupied molecular orbital (HOMO) levels are 3.6 and 5.3 eV respectively [20]. The dominant absorption of PCPDTBT is located at the region of 600–900 nm. The absorption of P3HT:PCBM (1:0.8 in weight ratio) and PCPDTBT:PC70BM (1:3 in weight ratio) blends are shown in Fig. 2. The absorption of P3HT:PCBM blend film dominates at the range of 500–650 nm, which is complementary to the absorption of PCPDTBT:PC70BM blend film. The indium tin oxide (ITO)-coated glass with a sheet resistance of 10 O/square was used as the substrate. A 40-nm-thick PEDOT:PSS (Baytron P AI 4083) layer was spin-coated onto the ITO glass substrate to serve as an anode buffer layer. The sample was then baked at 120 °C for 30 min in an oven. A P3HT:PCBM blend layer (1:0.8 in weight ratio, 100 nm) was spin-coated on PEDOT:PSS layer from chlorobenzene solution in glove box to serve as an active layer in bottom subcell. An electron-transporting layer of TiOx (9 nm) was deposited on top of the P3HT:PCBM layer by means of sol–gel chemistry from ethanol solution of tetrabutyl titanate (Ti[OC4H9]4) precursor. The film is subjected to hydrolysis to form a TiOx layer (the ratio of Ti to O is 1:1.74). The sample was then annealed at
Fig. 1. The tandem PV cell architecture and the active materials used in this study.
Fig. 2. Transmittance of the middle connection unit with a structure of TiOx (9 nm)/Al (2 nm)/MoO3 (10 nm), and absorbance of the P3HT:PCBM (1:0.8, 100 nm) and PCPDTBT:PC70BM (1:3, 100 nm) blend film.
150 °C for 10 min in glove box. The Al (2 nm)/MoO3 (10 nm) layers were thermally deposited onto the TiOx layer in vacuo at a pressure of about 4 104 Pa. A PCPDTBT:PC70BM blend (1:3 in weight ratio) layer was spin-coated on the MoO3 layer from chlorobenzene solution to serve as an active layer in the top subcell. Finally, the LiF (1 nm)/Al (100 nm) electrode was thermally deposited. The devices were annealed at 160 °C for 5 min inside glove box and encapsulated for measurement. The active area of the PV cells was about 0.12 cm2. Current density–voltage (J–V) characteristics of the PV cells were measured using a computer-controlled Keithley 236 source meter in the dark and under AM1.5G illumination from a calibrated solar simulator with an irradiation intensity of 100 mW/cm2. Ultraviolet photoelectron spectroscopy (UPS) was carried out using Thermo ESCALAB 250 surface analysis system equipped with a He-discharge lamp providing He–I photons of 21.22 eV. The absorption and transmission measurement was performed using Shimadzu UV-3600 spectrophotometer. 3. Results and discussion In order to investigate influence of the middle connection unit on the PV performance of tandem polymer PV cell, the tandem cells with P3HT:PCBM blend as the active layers in subcells are fabricated. For tandem polymer PV cells with two subcells being linked in series, the middle connection unit should act as a recombination site for electrons and holes photo-generated in bottom and top subcells. The PV performance of the tandem polymer PV cells with TiOx/MoO3 as a middle connection unit is summarized in Table 1. The TiOx layer in the middle connection unit serves as an electron-collecting layer in bottom cell since its LUMO level is 4.4 eV, providing an Ohmic contact with the LUMO of PCBM (4.3 eV) for electrons generated in the bottom subcell. The MoO3 with a HOMO level of 5.4 eV serves as a hole-collecting layer in top subcell. However, the tandem PV cell with TiOx (18 nm)/MoO3 (10 nm) as a middle connection unit merely shows a VOC of 0.51 eV, close to its single active layer PV cells. It indi-
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Table 1 PV performance of the tandem polymer PV cells based on P3HT:PCBM blend with different middle connection units. TiOx/Al/MoO3 (nm)
VOC (V)
JSC (mA/cm2)
FF
PCE (%)
RS (O cm2)
18/0/10 18/2/12 18/2/10 18/2/8 9/2/10 6/2/10
0.51 1.04 1.02 0.84 1.03 0.86
2.66 2.17 2.69 1.97 3.74 0.78
0.29 0.23 0.23 0.30 0.39 0.39
0.40 0.52 0.63 0.49 1.50 0.26
671 52 55 189 30 610
cates that the tandem architecture is not well realized and there exists large VOC loss. While the tandem PV cell with a structure of TiOx (18 nm)/Al (2 nm)/MoO3 (10 nm) as the middle connection unit demonstrates well tandem effect as shown in Table 1, which shows a VOC of 1.02 V, much higher than that of the tandem cell with TiOx (18 nm)/ MoO3 (10 nm) as the middle connection unit. The series resistance (RS) of the tandem PV cells is decreased from 671 to 55 O cm2 by inserting a thin layer of Al (2 nm) between the TiOx and MoO3 layers. It may be attributed that the thin Al layer acts as an efficient recombination site for holes and electrons generated in the two subcells. The tandem PV structure is well established with TiOx/Al/MoO3 as the middle connection unit since VOC of the tandem PV cell is double of VOCs in P3HT:PCBM blend PV cells [21]. The thicknesses of TiOx and MoO3 layers in the middle connection unit are optimized for achieving best PV performance and the results are also summarized in Table 1. The tandem PV cell with optimized TiOx (9 nm)/Al (2 nm)/MoO3 (10 nm) as the middle connection unit shows a VOC of 1.03 V, a short-circuit current (JSC) of 3.74 mA/cm2, and a fill factor (FF) of 0.39. The values give an overall PCE of 1.5%. The tandem PV cell with TiOx (9 nm)/Al (2 nm)/ MoO3 (10 nm) structure shows a smallest RS of 30 O cm2 among these PV cells. Further increasing the thicknesses of TiOx or MoO3 would increase RS of PV cells. However, decreasing their thicknesses also increases RS of the tandem PV cell, together with decreased VOC and JSC. This may be attributed to that the spin-coated 6-nm-thick TiOx layer after hydrolysis is not a continuous film, which may result in the active layer of the bottom cell is in contact with the MoO3 layer or the active layer of the top cell is directly in contact with the TiOx layer. Hence, the function of the TiOx/Al/MoO3 structure is weakened and the formed reverse current would reduce the PV performance of the tandem PV cell. Transmittance of the TiOx (9 nm)/Al (2 nm)/MoO3 (10 nm) structure is shown in Fig. 2. It can be seen that the TiOx (9 nm)/Al (2 nm)/MoO3 (10 nm) structure shows a transmittance higher than 95% in the range of 400–800 nm due to wide band-gap of TiOx and MoO3. The high transmittance of the middle connection unit may favor the incident light passing through it and being absorbed efficiently by the top subcell. As demonstrated above, the VOC of the tandem PV cells is dramatically increased by replacing the TiOx/Al/MoO3 structure for the TiOx/MoO3 structure. It is speculated that the energy level alignment at TiOx/MoO3 interface may play an important role in determining the final VOC of the
tandem PV cell. In order to explore the underlying physical mechanism, UPS measurements were conducted for the TiOx, TiOx/MoO3 and TiOx/Al/MoO3 films that were deposited on silicon substrate to determine the electronic structures at the TiOx/MoO3 and TiOx/Al/MoO3 interfaces. Since the TiOx layer is prepared via spin-coating and it is difficult to measure in situ the electronic structure at the TiOx/MoO3 and TiOx/Al/MoO3 interfaces via sequential deposition of each layer, individual samples of the TiOx, TiOx/MoO3 and TiOx/Al/MoO3 films are prepared and measured under same condition for comparison. The proposed energy level diagrams at the TiOx/MoO3 and TiOx/Al/MoO3 interfaces are shown in Fig. 3. The values of HOMO for TiOx and MoO3, the work function (EF) for Al, and the vacuum level for each material were extracted from the UPS spectra. The values of LUMO for TiOx and MoO3 were determined from UPS spectra together with their absorption spectra. Compared to the vacuum level of TiOx layer, the vacuum levels of MoO3 in the TiOx/MoO3 and TiOx/Al/MoO3 structure shift upwards 0.34 and 0.84 eV, respectively. The vacuum level shift results in the offset changes between the LUMO of TiOx and HOMO of MoO3. As shown in Fig. 3(a), there is an offset of 0.66 eV between the LUMO of TiOx and HOMO of MoO3 for the TiOx/MoO3 structure. Such a large offset would result in a large VOC loss during charge recombination for the tandem polymer PV cells. When a thin layer of Al is inserted between the TiOx and MoO3 layers, the electronic levels at the TiOx/MoO3 interface are realigned due to negative charge transfer from Al to MoO3 as shown in Fig. 2(b), and the offset between the LUMO of TiOx and HOMO of MoO3 is reduced to 0.16 eV. The small offset between the LUMO of TiOx and HOMO of MoO3 reduces energy loss when the photo-generated electrons and holes in the subcells recombine in the middle connection unit, and hence the VOC of the resulted tandem polymer PV cells is enhanced. The tandem polymer PV cells with the blends of P3HT:PCBM and PCPDTBT:PC70BM as the active layer in the two subcells are fabricated by using the TiOx (9 nm)/ Al (2 nm)/MoO3 (10 nm) structure as a middle connection unit. The complementary absorption of the P3HT:PCBM
Fig. 3. Proposed energy level diagrams at the TiOx/MoO3 (a) and TiOx/Al/ MoO3 (b) interfaces.
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4. Conclusion In summary, we developed a kind of highly transparent middle connection unit for tandem polymer PV cells, which consists of an electron-transporting TiOx, a thin Al layer and a hole-transporting MoO3 layer. The open-circuit voltage origin of the resulted tandem PV cells is demonstrated. It is found that the thin Al layer inserted between the TiOx and MoO3 layers not only provides an efficient recombination site for electrons and holes photogenerated in the two subcells, but also realigns the electronic energy levels at the TiOx/MoO3 interface to increase the open-circuit voltage of the tandem PV cell. Acknowledgments Fig. 4. J–V characteristics of the bottom and top PV cells, and tandem PV cell under 100 mW/cm2 AM 1.5G illumination.
Table 2 PV performance of the bottom and top PV cells, and the tandem PV cell. Device
VOC (V)
JSC (mA/cm2)
FF
PCE (%)
Bottom cell Top cell Tandem cell
0.56 0.60 1.12
8.48 8.83 5.51
0.50 0.37 0.47
2.37 1.97 2.92
and PCPDTBT:PC70BM blends covers the whole visible region. The single active layer PV cells with structures of ITO/PEDOT (40 nm)/P3HT:PCBM (100 nm)/TiOx (9 nm)/ Al (100 nm) and ITO/MoO3 (10 nm)/PCPDTBT:PC70BM (100 nm)/LiF (1 nm)/Al (100 nm) were also fabricated for comparison studies. The illuminated J–V characteristics of the tandem PV cell and the two single active layer PV cells under 100 mW/cm2 AM 1.5G illumination are shown in Fig. 4, and their PV parameters are summarized in Table 2. It can be seen that the VOC of the tandem PV cell is 1.12 V, which is nearly the sum of the VOCs of the two subcells (0.56 and 0.60 V), indicating that the TiOx (9 nm)/Al (2 nm)/MoO3 (10 nm) structure is a kind of good candidate for the middle connection unit in tandem polymer PV cells. The JSC of the tandem PV cell is about 5.51 mA/cm2, lower than each single PV cell based on different active layers. This may be attributed to that the partial overlapped absorption of the P3HT:PCBM and PCPDTBT:PC70BM films at the visible region and the optical field redistribution in the tandem structure when the two cells are stacked. The overall PCE of the tandem PV cell is 2.92%, higher than those of the single active layer PV cells (2.37% and 1.97%). The PCE of the tandem PV cell is expected to be further enhanced if the morphology of the P3HT:PCBM and PCPDTBT:PC70BM blends is well optimized and the photocurrent of the two subcells is well matched.
The authors acknowledge the financial support from the National Natural Science Foundation of China (Nos. 50873100, 20834005 and 20921061), 973 project (2009CB623602 and 2009CB930603) and the Chinese Academy of Sciences (KJCX2-YW-M11). References [1] G. Yu, J. Gao, J.C. Hummelen, F. Wudl, A.J. Heeger, Science 270 (1995) 1789. [2] C.J. Brabec, N.S. Sariciftci, J.C. Hummelen, Adv. Funct. Mater. 11 (2001) 15. [3] J. Peet, J.Y. Kim, N.E. Coates, W.L. Ma, D. Moses, A.J. Heeger, G.C. Bazan, Nat. Mater. 6 (2007) 497. [4] H.-Y. Chen, J. Hou, S. Zhang, Y. Liang, G. Yang, Y. Yang, L. Yu, Y. Wu, G. Li, Nat. Photon. 3 (2009) 649. [5] J.Y. Kim, K. Lee, N.E. Coates, D. Moses, T.-Q. Nguyen, M. Dante, A.J. Heeger, Science 317 (2007) 222. [6] G. Dennler, H.-J. Prall, R. Koeppe, M. Egginger, R. Autengruber, N.S. Sariciftci, Appl. Phys. Lett. 89 (2006) 073502. [7] A. Hadipour, B. de Boer, J. Wildeman, F.B. Kooistra, J.C. Hummelen, M.G.R. Turbiez, M.M. Wienk, R.A.J. Janssen, P.W.M. Blom, Adv. Funct. Mater. 16 (2006) 1897. [8] V. Shrotriya, E.H. Wu, G. Li, Y. Yao, Y. Yang, Appl. Phys. Lett. 88 (2006) 064104. [9] X. Guo, F. Liu, W. Yue, Z. Xie, Y. Geng, L. Wang, Org. Electron. 10 (2009) 1174. [10] Y. Zhao, Z. Xie, Y. Qu, Y. Geng, L. Wang, Appl. Phys. Lett. 90 (2007) 043504. [11] J. Gilot, M.M. Wienk, R.A.J. Janssen, Appl. Phys. Lett. 90 (2007) 143512. [12] T. Ameri, G. Dennler, C. Waldauf, P. Denk, K. Forberich, M.C. Scharber, C.J. Brabec, K. Hingerl, J. Appl. Phys. 103 (2008) 084506. [13] A.G.F. Janssen, T. Riedl, S. Hamwi, H.-H. Johannes, W. Kowalsky, Appl. Phys. Lett. 91 (2007) 073519. [14] J.Y. Kim, S.H. Kim, H.-H. Lee, K. Lee, W. Ma, X. Gong, A.J. Heeger, Adv. Mater. 18 (2006) 572. [15] C. Waldauf, M. Morana, P. Denk, P. Schilinsky, K. Coakley, S.A. Choulis, C.J. Brabec, Appl. Phys. Lett. 89 (2006) 233517. [16] A. Hayakawa, O. Yoshikawa, T. Fujieda, K. Uehara, S. Yoshikawa, Appl. Phys. Lett. 90 (2007) 163517. [17] S. Tokito, K. Noda, Y. Taga, J. Phys. D Appl. Phys. 29 (1996) 2750. [18] H. You, Y. Dai, Z. Zhang, D. Ma, J. Appl. Phys. 101 (2007) 026105. [19] V. Shrotriya, G. Li, Y. Yao, C.-W. Chu, Y. Yang, Appl. Phys. Lett. 88 (2006) 073508. [20] D. Mühlbacher, M. Scharber, M. Morana, Z. Zhu, D. Waller, R. Gaudiana, C. Brabec, Adv. Mater. 18 (2006) 2884. [21] F. Padinger, R.S. Rittverger, N.S. Sariciftci, Adv. Funct. Mater. 13 (2003) 85.
Contents lists available at ScienceDirect
Organic Electronics journal homepage: www.elsevier.com/locate/orgel
Efficient tandem polymer photovoltaic cells using inorganic metal oxides as a transparent middle connection unit Xiaoyang Guo, Fengmin Liu, Bin Meng, Zhiyuan Xie *, Lixiang Wang State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Graduate School of Chinese Academy of Sciences, Chinese Academy of Sciences, Changchun 130022, People’s Republic of China
a r t i c l e
i n f o
Article history: Received 8 March 2010 Received in revised form 19 April 2010 Accepted 1 May 2010 Available online 10 May 2010 Keywords: Polymer photovoltaic cells Tandem structure Open-circuit voltage
a b s t r a c t A kind of tandem polymer photovoltaic cell with inorganic metal oxides as a transparent middle connection unit to link the two subcells having different absorption characteristics is developed and its open-circuit voltage origin is investigated. The middle connection unit consists of an electron-transporting titanium oxide (TiOx) layer, a thin aluminum (Al) layer and a hole-transporting molybdenum oxide (MoO3) layer. The inserted thin Al layer between the TiOx and MoO3 layers not only acts as an efficient recombination site for electrons and holes photo-generated in the two subcells, but also realigns the electronic energy levels at TiOx/MoO3 interfaces to reduce the open-circuit voltage loss. The open-circuit voltage of the tandem cell is the summation of the two subcells. Ó 2010 Elsevier B.V. All rights reserved.
1. Introduction Photovoltaic (PV) cells based on organic molecules and conjugated polymers have drawn great research interests due to their potential advantages of light weight, flexibility and low-cost manufacturing [1,2]. The power conversion efficiency (PCE) of the bulk-heterojunction polymer PV cells has been increased above 6% in the past few years [3,4]. Although some narrow band-gap conjugated polymers have been developed to enhance spectral coverage to the solar spectrum, the narrow absorption range of conjugated polymers is still a drawback for polymer PV cells, which results in only a small fraction of the solar flux being harvested by the active layer in bulk-heterojunction polymer PV cell. One of the strategies to broaden the spectral coverage to solar spectrum is to make use of tandem architectures in which two or more cells with different absorption characteristics are linked in series or parallel connection [5–9]. The middle connection unit linking the subcells in tandem architectures plays an important role in determining the open-circuit voltage (VOC) and final * Corresponding author. Tel./fax: +86 431 85262819. E-mail address: [email protected] (Z. Xie). 1566-1199/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.orgel.2010.05.004
PCE of the PV cells. The middle connection unit should not only be highly transparent, but also act as an efficient recombination site for electrons and holes photogenerated in two subcells with low VOC loss. In addition, the middle connection structure should resist the bottom active layer from being destroyed while spin-coating the top active layer. Poly(3,4-ethylene dioxylenethiophene)– polystyrene sulfonic acid (PEDOT:PSS) are widely used as an anode buffer layer in polymer PV cells and as a middle connection unit in tandem polymer PV cells [10,11]. However, the absorption caused by PEDOT:PSS layer may reduce the number of photons absorbed by the active layers [12]. Inorganic metal oxides are potential candidates for the middle connection unit in tandem polymer PV cells, which have little absorption in visible region and high charge mobilities, and some inorganic metal oxides have been successfully used as buffer layers in polymer PV cells and organic light-emitting diodes (OLEDs) [5,9,13–19]. For example, wide band-gap titanium oxide (TiOx) has been utilized in polymer PV cells as an optical spacer [14], or electron-transporting/hole-blocking layer [15,16]. Molybdenum oxide (MoO3) is a kind of p-type semiconductor and has been employed as an anode buffer layer in OLEDs [17,18] and organic PV cells [19]. To date, there is no report
X. Guo et al. / Organic Electronics 11 (2010) 1230–1233
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using a combination of p-type and n-type inorganic metal oxides as a middle connection unit in tandem polymer PV cells and investigating the origin of VOC in tandem PV cell. Herein, we fabricate tandem polymer PV cells using a combination of TiOx and MoO3 as a middle connection unit to link the two subcells. It is found that inserting a thin Al layer between the TiOx and MoO3 layers realizes tandem PV cells and the VOC of the tandem cell is summation of the two subcells. The origin of VOC in tandem PV cells is discussed.
2. Experimental details Fig. 1 shows the designed device structure of the tandem polymer PV cell and the chemical structures of active materials used in this study. The blends of poly(3-hexylthiophene) (P3HT)/[6,6]-phenyl C61-butyric acid methyl ester (PCBM) and of poly[2,6-(4,4-bis-(2-ethylhexyl)-4Hcyclopenta[2,1-b;3,4-b0 ]dithiophene)-alt-4,7-(2,1,3-benzothiadiazole)] (PCPDTBT)/[6,6]-phenyl C71-butyric acid methyl ester (PC70BM) are used as the active layers in subcells, respectively. PCPDTBT is a kind of low-band-gap conjugated polymer, and its lowest unoccupied molecular orbital (LUMO) and highest occupied molecular orbital (HOMO) levels are 3.6 and 5.3 eV respectively [20]. The dominant absorption of PCPDTBT is located at the region of 600–900 nm. The absorption of P3HT:PCBM (1:0.8 in weight ratio) and PCPDTBT:PC70BM (1:3 in weight ratio) blends are shown in Fig. 2. The absorption of P3HT:PCBM blend film dominates at the range of 500–650 nm, which is complementary to the absorption of PCPDTBT:PC70BM blend film. The indium tin oxide (ITO)-coated glass with a sheet resistance of 10 O/square was used as the substrate. A 40-nm-thick PEDOT:PSS (Baytron P AI 4083) layer was spin-coated onto the ITO glass substrate to serve as an anode buffer layer. The sample was then baked at 120 °C for 30 min in an oven. A P3HT:PCBM blend layer (1:0.8 in weight ratio, 100 nm) was spin-coated on PEDOT:PSS layer from chlorobenzene solution in glove box to serve as an active layer in bottom subcell. An electron-transporting layer of TiOx (9 nm) was deposited on top of the P3HT:PCBM layer by means of sol–gel chemistry from ethanol solution of tetrabutyl titanate (Ti[OC4H9]4) precursor. The film is subjected to hydrolysis to form a TiOx layer (the ratio of Ti to O is 1:1.74). The sample was then annealed at
Fig. 1. The tandem PV cell architecture and the active materials used in this study.
Fig. 2. Transmittance of the middle connection unit with a structure of TiOx (9 nm)/Al (2 nm)/MoO3 (10 nm), and absorbance of the P3HT:PCBM (1:0.8, 100 nm) and PCPDTBT:PC70BM (1:3, 100 nm) blend film.
150 °C for 10 min in glove box. The Al (2 nm)/MoO3 (10 nm) layers were thermally deposited onto the TiOx layer in vacuo at a pressure of about 4 104 Pa. A PCPDTBT:PC70BM blend (1:3 in weight ratio) layer was spin-coated on the MoO3 layer from chlorobenzene solution to serve as an active layer in the top subcell. Finally, the LiF (1 nm)/Al (100 nm) electrode was thermally deposited. The devices were annealed at 160 °C for 5 min inside glove box and encapsulated for measurement. The active area of the PV cells was about 0.12 cm2. Current density–voltage (J–V) characteristics of the PV cells were measured using a computer-controlled Keithley 236 source meter in the dark and under AM1.5G illumination from a calibrated solar simulator with an irradiation intensity of 100 mW/cm2. Ultraviolet photoelectron spectroscopy (UPS) was carried out using Thermo ESCALAB 250 surface analysis system equipped with a He-discharge lamp providing He–I photons of 21.22 eV. The absorption and transmission measurement was performed using Shimadzu UV-3600 spectrophotometer. 3. Results and discussion In order to investigate influence of the middle connection unit on the PV performance of tandem polymer PV cell, the tandem cells with P3HT:PCBM blend as the active layers in subcells are fabricated. For tandem polymer PV cells with two subcells being linked in series, the middle connection unit should act as a recombination site for electrons and holes photo-generated in bottom and top subcells. The PV performance of the tandem polymer PV cells with TiOx/MoO3 as a middle connection unit is summarized in Table 1. The TiOx layer in the middle connection unit serves as an electron-collecting layer in bottom cell since its LUMO level is 4.4 eV, providing an Ohmic contact with the LUMO of PCBM (4.3 eV) for electrons generated in the bottom subcell. The MoO3 with a HOMO level of 5.4 eV serves as a hole-collecting layer in top subcell. However, the tandem PV cell with TiOx (18 nm)/MoO3 (10 nm) as a middle connection unit merely shows a VOC of 0.51 eV, close to its single active layer PV cells. It indi-
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Table 1 PV performance of the tandem polymer PV cells based on P3HT:PCBM blend with different middle connection units. TiOx/Al/MoO3 (nm)
VOC (V)
JSC (mA/cm2)
FF
PCE (%)
RS (O cm2)
18/0/10 18/2/12 18/2/10 18/2/8 9/2/10 6/2/10
0.51 1.04 1.02 0.84 1.03 0.86
2.66 2.17 2.69 1.97 3.74 0.78
0.29 0.23 0.23 0.30 0.39 0.39
0.40 0.52 0.63 0.49 1.50 0.26
671 52 55 189 30 610
cates that the tandem architecture is not well realized and there exists large VOC loss. While the tandem PV cell with a structure of TiOx (18 nm)/Al (2 nm)/MoO3 (10 nm) as the middle connection unit demonstrates well tandem effect as shown in Table 1, which shows a VOC of 1.02 V, much higher than that of the tandem cell with TiOx (18 nm)/ MoO3 (10 nm) as the middle connection unit. The series resistance (RS) of the tandem PV cells is decreased from 671 to 55 O cm2 by inserting a thin layer of Al (2 nm) between the TiOx and MoO3 layers. It may be attributed that the thin Al layer acts as an efficient recombination site for holes and electrons generated in the two subcells. The tandem PV structure is well established with TiOx/Al/MoO3 as the middle connection unit since VOC of the tandem PV cell is double of VOCs in P3HT:PCBM blend PV cells [21]. The thicknesses of TiOx and MoO3 layers in the middle connection unit are optimized for achieving best PV performance and the results are also summarized in Table 1. The tandem PV cell with optimized TiOx (9 nm)/Al (2 nm)/MoO3 (10 nm) as the middle connection unit shows a VOC of 1.03 V, a short-circuit current (JSC) of 3.74 mA/cm2, and a fill factor (FF) of 0.39. The values give an overall PCE of 1.5%. The tandem PV cell with TiOx (9 nm)/Al (2 nm)/ MoO3 (10 nm) structure shows a smallest RS of 30 O cm2 among these PV cells. Further increasing the thicknesses of TiOx or MoO3 would increase RS of PV cells. However, decreasing their thicknesses also increases RS of the tandem PV cell, together with decreased VOC and JSC. This may be attributed to that the spin-coated 6-nm-thick TiOx layer after hydrolysis is not a continuous film, which may result in the active layer of the bottom cell is in contact with the MoO3 layer or the active layer of the top cell is directly in contact with the TiOx layer. Hence, the function of the TiOx/Al/MoO3 structure is weakened and the formed reverse current would reduce the PV performance of the tandem PV cell. Transmittance of the TiOx (9 nm)/Al (2 nm)/MoO3 (10 nm) structure is shown in Fig. 2. It can be seen that the TiOx (9 nm)/Al (2 nm)/MoO3 (10 nm) structure shows a transmittance higher than 95% in the range of 400–800 nm due to wide band-gap of TiOx and MoO3. The high transmittance of the middle connection unit may favor the incident light passing through it and being absorbed efficiently by the top subcell. As demonstrated above, the VOC of the tandem PV cells is dramatically increased by replacing the TiOx/Al/MoO3 structure for the TiOx/MoO3 structure. It is speculated that the energy level alignment at TiOx/MoO3 interface may play an important role in determining the final VOC of the
tandem PV cell. In order to explore the underlying physical mechanism, UPS measurements were conducted for the TiOx, TiOx/MoO3 and TiOx/Al/MoO3 films that were deposited on silicon substrate to determine the electronic structures at the TiOx/MoO3 and TiOx/Al/MoO3 interfaces. Since the TiOx layer is prepared via spin-coating and it is difficult to measure in situ the electronic structure at the TiOx/MoO3 and TiOx/Al/MoO3 interfaces via sequential deposition of each layer, individual samples of the TiOx, TiOx/MoO3 and TiOx/Al/MoO3 films are prepared and measured under same condition for comparison. The proposed energy level diagrams at the TiOx/MoO3 and TiOx/Al/MoO3 interfaces are shown in Fig. 3. The values of HOMO for TiOx and MoO3, the work function (EF) for Al, and the vacuum level for each material were extracted from the UPS spectra. The values of LUMO for TiOx and MoO3 were determined from UPS spectra together with their absorption spectra. Compared to the vacuum level of TiOx layer, the vacuum levels of MoO3 in the TiOx/MoO3 and TiOx/Al/MoO3 structure shift upwards 0.34 and 0.84 eV, respectively. The vacuum level shift results in the offset changes between the LUMO of TiOx and HOMO of MoO3. As shown in Fig. 3(a), there is an offset of 0.66 eV between the LUMO of TiOx and HOMO of MoO3 for the TiOx/MoO3 structure. Such a large offset would result in a large VOC loss during charge recombination for the tandem polymer PV cells. When a thin layer of Al is inserted between the TiOx and MoO3 layers, the electronic levels at the TiOx/MoO3 interface are realigned due to negative charge transfer from Al to MoO3 as shown in Fig. 2(b), and the offset between the LUMO of TiOx and HOMO of MoO3 is reduced to 0.16 eV. The small offset between the LUMO of TiOx and HOMO of MoO3 reduces energy loss when the photo-generated electrons and holes in the subcells recombine in the middle connection unit, and hence the VOC of the resulted tandem polymer PV cells is enhanced. The tandem polymer PV cells with the blends of P3HT:PCBM and PCPDTBT:PC70BM as the active layer in the two subcells are fabricated by using the TiOx (9 nm)/ Al (2 nm)/MoO3 (10 nm) structure as a middle connection unit. The complementary absorption of the P3HT:PCBM
Fig. 3. Proposed energy level diagrams at the TiOx/MoO3 (a) and TiOx/Al/ MoO3 (b) interfaces.
X. Guo et al. / Organic Electronics 11 (2010) 1230–1233
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4. Conclusion In summary, we developed a kind of highly transparent middle connection unit for tandem polymer PV cells, which consists of an electron-transporting TiOx, a thin Al layer and a hole-transporting MoO3 layer. The open-circuit voltage origin of the resulted tandem PV cells is demonstrated. It is found that the thin Al layer inserted between the TiOx and MoO3 layers not only provides an efficient recombination site for electrons and holes photogenerated in the two subcells, but also realigns the electronic energy levels at the TiOx/MoO3 interface to increase the open-circuit voltage of the tandem PV cell. Acknowledgments Fig. 4. J–V characteristics of the bottom and top PV cells, and tandem PV cell under 100 mW/cm2 AM 1.5G illumination.
Table 2 PV performance of the bottom and top PV cells, and the tandem PV cell. Device
VOC (V)
JSC (mA/cm2)
FF
PCE (%)
Bottom cell Top cell Tandem cell
0.56 0.60 1.12
8.48 8.83 5.51
0.50 0.37 0.47
2.37 1.97 2.92
and PCPDTBT:PC70BM blends covers the whole visible region. The single active layer PV cells with structures of ITO/PEDOT (40 nm)/P3HT:PCBM (100 nm)/TiOx (9 nm)/ Al (100 nm) and ITO/MoO3 (10 nm)/PCPDTBT:PC70BM (100 nm)/LiF (1 nm)/Al (100 nm) were also fabricated for comparison studies. The illuminated J–V characteristics of the tandem PV cell and the two single active layer PV cells under 100 mW/cm2 AM 1.5G illumination are shown in Fig. 4, and their PV parameters are summarized in Table 2. It can be seen that the VOC of the tandem PV cell is 1.12 V, which is nearly the sum of the VOCs of the two subcells (0.56 and 0.60 V), indicating that the TiOx (9 nm)/Al (2 nm)/MoO3 (10 nm) structure is a kind of good candidate for the middle connection unit in tandem polymer PV cells. The JSC of the tandem PV cell is about 5.51 mA/cm2, lower than each single PV cell based on different active layers. This may be attributed to that the partial overlapped absorption of the P3HT:PCBM and PCPDTBT:PC70BM films at the visible region and the optical field redistribution in the tandem structure when the two cells are stacked. The overall PCE of the tandem PV cell is 2.92%, higher than those of the single active layer PV cells (2.37% and 1.97%). The PCE of the tandem PV cell is expected to be further enhanced if the morphology of the P3HT:PCBM and PCPDTBT:PC70BM blends is well optimized and the photocurrent of the two subcells is well matched.
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