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Bulk heterojunction organic solar cells utilizing 1,4,8,11,15,18,22,25-octahexylphthalocyanine
Solar Energy Materials & Solar Cells 95 (2011) 3087–3092
Contents lists available at ScienceDirect
Solar Energy Materials & Solar Cells journal homepage: www.elsevier.com/locate/solmat
Bulk heterojunction organic solar cells utilizing 1,4,8,11,15,18,22,25-octahexylphthalocyanine Tetsuro Hori a, Naoki Fukuoka a, Tetsuya Masuda a, Yasuo Miyake a,b, Hiroyuki Yoshida a, Akihiko Fujii a,n, Yo Shimizu b, Masanori Ozaki a a
Division of Electrical, Electronic and Information Engineering, Graduate School of Engineering, Osaka University, 2-1 Yamada-oka, Suita, Osaka 565-0871, Japan Synthetic Nano-Function Materials Group, Research Institute for Ubiquitous Energy Devices, National Institute of Advanced Industrial Science and Technology (AIST), Kansai Centre, 1-8-31 Midorigaoka, Ikeda, Osaka 563-8577, Japan b
a r t i c l e i n f o
abstract
Article history: Received 6 April 2011 Received in revised form 22 June 2011 Accepted 24 June 2011 Available online 14 July 2011
Bulk heterojunction solar cells utilizing soluble phthalocyanine derivative, 1,4,8,11,15,18,22,25-octahexylphthalocyanine (C6PcH2) have been investigated. The active layer was fabricated by spin-coating the mixed solution of C6PcH2 and 1-(3-methoxy-carbonyl)-propyl-1-1-phenyl-(6,6)C61 (PCBM). The photovoltaic properties of the solar cell with bulk heterojunction of C6PcH2 and PCBM demonstrated the strong dependence of active layer thickness, and the optimized active layer thickness was clarified to be 120 nm. By inserting MoO3 hole transport buffer layer between the positive electrode and active layer, the FF and energy conversion efficiency were improved to be 0.50 and 3.2%, respectively. The tandem organic thin-film solar cell has also been studied by utilizing active layer materials of C6PcH2 and poly(3-hexylthiophene) and the interlayer of LiF/Al/MoO3 structure, and a high Voc of 1.27 V has been achieved. & 2011 Elsevier B.V. All rights reserved.
Keywords: Phthalocyanine Organic solar cells Photovoltaic cell Wet process Bulk heterojunction Tandem structure
1. Introduction Organic thin-film solar cells utilizing organic semiconductors have attracted as next-generation solar cells [1]. Organic semiconductors are classified into two types, p-conjugated polymers and low-weighted molecules. Organic thin-film solar cells based on p-conjugated polymers have been investigated since the discovery of fullerene doping effects to p-conjugated polymers, such as photoluminescence quenching [2], photoinduced charge transfer [3], and photoconduction enhancement [4]. Organic thinfilm solar cells with a bulk heterojunction active layer fabricated by a wet process, which is important for the large area fabrication [5,6], achieved high energy conversion efficiency [7,8]. On the other hand, some low-weighted molecular semiconductors demonstrate a high crystallinity and charge mobility due to p-conjugated intermolecular interactions; however, they are inappropriate for wet processes, because of their poor solubility in typical organic solvents. Therefore, in organic thin-film solar cells based on low-weighted molecular materials, a p–n junction structure [1] or p–i–n junction structure [9] was generally fabricated by thermal evaporation process under vacuum. Recently, some groups investigated solution processable organic thin-film solar cells utilizing low-weighted molecules [10]. n
Corresponding author. E-mail address: [email protected] (A. Fujii).
0927-0248/$ - see front matter & 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.solmat.2011.06.039
Recently, we have demonstrated high hole and electron drift mobilities of 1.4 and 0.5 cm2/Vs, respectively, in the crystalline phase of a mesogenic phthalocyanine derivative, 1,4,8,11,15,18,22,25octahexylphthalocyanine (C6PcH2) [11]. C6PcH2 could be spread uniformly on a substrate from the solution, and exhibits a mesogenic phase within a certain temperature range [12]. C6PcH2 could be mixed in a conventional solvent with a typical acceptor molecule, such as 1-(3-methoxy-carbonyl)-propyl-1-1-phenyl-(6,6)C61 (PCBM), and the films fabricated by a solution process are available as a bulk heterojunction active layer in organic solar cells. Recently, we reported on a simple organic solar cell with bulk heterojunction of C6PcH2 and PCBM fabricated by spin-coating, and demonstrated the high energy conversion efficiency of 3.1% and external quantum efficiency (EQE) higher than 70% at the Q-band region [13]. In the organic thin-film solar cells utilizing C6PcH2, high opencircuit voltage (Voc) of 0.81 V and short-circuit current density (Isc) of 9.6 mA/cm2 were demonstrated, however, the fill factor (FF) of the solar cell with C6PcH2 was 0.40, and lower than the conventional organic thin-film solar cell utilizing conducting polymers, such as, poly(3-hexylthiophene) (P3HT) [14]. Therefore, the improvement of FF is one of the important problems to be solved. Optimization of the active layer thickness and insertion of buffer layers are one of the ways to improve the FF. The photovoltaic properties of organic thin-film solar cells strongly depend on the thickness of active layer, indeed [14,15]. Moreover,
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hole transport materials, such as the poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) or molybdenum trioxide (VI) (MoO3) [16–18] are available as hole transport buffer layers between positive electrode and organic active layer, and must play an important role for improving the FF. Therefore, it is important to study the dependence of active layer thickness and the buffer layer effects in the solar cell utilizing C6PcH2 The absorption loss in the wavelength range of 400–600 nm is another problem in the organic thin-film solar cell utilizing C6PcH2, and should be interpolated by fabricating the tandem structure with a complementary material, such as, P3HT. Since the Voc increased about twice in a tandem organic thin-film solar cell by inserting the ultra-thin gold layer between two active layers [19], many tandem type organic thin-film solar cells were investigated, such as polymer based type [20], all solution processed type [21], polarity inverted type [22], and so on. The tandem organic thin-film solar cell utilizing C6PcH2 and P3HT is also attractive, because of wet process fabrication and wide-wavelength range absorption. In this paper, we report on the active layer thickness and hole transport buffer layer dependences of simple bulk heterojunction organic solar cells utilizing C6PcH2 and PCBM blend film. We also report on the tandem solar cells, the active layers of which are based on C6PcH2, P3HT and PCBM, with wide-wavelength range absorption.
The C6PcH2:PCBM mixed solution was spin-coated onto the hole transport buffer layers at 500 rpm for 60 s in a glove box filled with argon gas. Aluminum (Al) layer as a counter electrode to the ITO was deposited onto the composite layers through shadow masks by thermal evaporation under a pressure of approximately 10–4 Pa. The evaporation speed and the thickness of the Al layer ˚ were approximately 5 A/s and 80 nm, respectively. The active area of the solar cell was 2 2 mm2. The fabrication of the tandem solar cell was carried out in the following manner. The PEDOT:PSS was deposited onto an ITO-coated quartz substrate by the above manner. The mixed chloroform solution of P3HT and PCBM, the composite concentration of which was 10 mg:8 mg/ml, was spin-coated onto PEDOT:PSS layer at 1000 rpm for 60 s in a glove box filled with nitrogen gas. The thickness of the P3HT:PCBM layer was estimated to be approximately 160 nm. Interlayer composed of 1.5 nm-thick lithium fluoride (LiF), 3 nm-thick Al and 25 nm-thick MoO3 were deposited onto the P3HT:PCBM active layer by thermal evaporation under a pressure of approximately 10–4 Pa. The evaporation speeds of the ˚ LiF, Al and MoO3 layers were approximately 0.1, 0.5 and 0.3 A/s, respectively. The C6PcH2:PCBM mixed chloroform solution, the
2. Experimental details C6PcH2 was synthesized according to the literature [23] with slight modifications and fully purified by column chromatography (silica-gel with toluene as eluent) followed by repetitive recrystallization from toluene–methanol (1:2) solution. Ragioregular P3HT and PCBM were purchased from Merck & Co., Inc. and Frontier Carbon Ltd., respectively. The molecular structures of the C6PcH2, P3HT and PCBM are shown in Fig. 1. PEDOT:PSS (Baytron P VP AI 4083) or MoO3 were used as hole transport buffer layers. PEDOT:PSS was spin-coated onto an indium-tin-oxide (ITO)-coated quartz substrate at 3000 rpm for 60 s using an aqueous solution diluted with the same volume of isopropanol, and dried at 100 1C for 10 min in an oven under atmospheric conditions. The thickness of the PEDOT:PSS layer was estimated to be approximately 30 nm. A MoO3 layer was fabricated by thermal evaporation onto an ITO-coated quartz substrate under a pressure of 10–4 Pa, and the evaporation speed ˚ of MoO3 was approximately 0.3 A/s. C6PcH2 and PCBM were dissolved in chloroform solvent, and the composite ratio of C6PcH2:PCBM was 2:1 by weight ratio [13].
C6H13 R
R
S S
R
N
N H
N R
N
N
n
R P3HT
C6H13
N H N
OMe N
R
R
R
O
R = C6H13
C6PcH2 PCBM Fig. 1. Molecular structures of C6PcH2, PAT6, and PCBM.
Fig. 2. Active layer thickness dependence of (a) absorbance spectra and (b) EQE spectra of the solar cells. (c) Absorbance versus thickness of active layer at a wavelength of 730 nm. (d) EQE versus thickness of active layer at a wavelength of 670 and 730 nm.
T. Hori et al. / Solar Energy Materials & Solar Cells 95 (2011) 3087–3092
composite concentration of which was 10 mg:5 mg/ml, was spincoated onto the interlayer at 500 rpm for 60 s in a glove box filled with nitrogen gas. The thickness of the C6PcH2:PCBM layer was estimated to be approximately 130 nm. 1.5 nm-thick LiF and 70 nmthick Al layers as a counter electrode to the ITO was deposited onto the tandem layers through shadow masks by thermal evaporation under a pressure of approximately 10–4 Pa. The evaporation speeds ˚ of the LiF and Al layers were approximately 0.1 and 5.0 A/s, respectively. Each process of P3HT:PCBM active layer, interlayer, C6PcH2:PCBM active layer and LiF/Al electrode fabrication were carried out without air exposure. The active area of the solar cell was 2 2 mm2. The structure of fabricated tandem solar cell is ITO/ PEDOT:PSS/P3HT:PCBM/LiF/Al/MoO3/C6PcH2:PCBM/LiF/Al (tandem cell). The solar cells with ITO/PEDOT:PSS/P3HT:PCBM/LiF/Al structure (P3HT single cell) and ITO/MoO3/C6PcH2:PCBM/LiF/Al structure (C6PcH2 single cell) were also fabricated for the comparison by the same manner. The absorbance spectra were measured using a spectrophotometer (Shimadzu UV-3150). The film thickness was evaluated using the atomic force microscope (AFM: KEYENCE VN-8000). The photocurrent spectra were measured with a programmable electrometer (Keithley 617S) using a xenon lamp light passing through a monochromator as a light source, and the external quantum efficiency (EQE) was estimated for each incident light wavelength using EQE (%)¼1240 I (A/cm2) 100/(l (nm) Pin (W/cm2)), where I is the photocurrent density and l is the wavelength of incident light. The current–voltage characteristics were measured with a high-voltage-source measurement unit (Keithley 237) under AM1.5G (100 mW/cm2) solar-illuminated conditions. From the current–voltage characteristics under AM1.5G, the FF and energy conversion efficiency (Ze) were estimated according to the following definitions: FF¼ImaxVmax/IscVoc
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and Ze ¼IscVocFF/Pin, where Imax and Vmax are the current density and voltage at the maximum output power, respectively, Isc is the short-circuit current density and Pin is the intensity of the incident light. The current–voltage characteristics and EQE spectra were measured in vacuum at room temperature.
3. Results and discussions 3.1. Thickness dependence The organic thin-film solar cells of ITO/PEDOT:PSS/C6PcH2:PCBM/ Al structure were fabricated to study the thickness dependence of active layers, and the absorbance spectra, EQE spectra and current– voltage characteristics under AM1.5G (100 mW/cm2) solar-illuminated conditions were measured. Thicknesses of active layers were changed 80–280 nm by controlling the concentration of C6PcH2:PCBM mixed solutions. The active layer thickness dependence of absorbance and EQE spectra in the solar cells are shown in Fig. 2. C6PcH2 exhibits the absorption of Q-band in the wavelength range of 600–800 nm, and the absorbance linearly increased depending on the film thickness. The EQE was also enhanced monotonically in the range of the active layer thickness of 80–140 nm, then saturated in the active layer thickness of more than 140 nm. Fig. 3 shows the active layer thickness dependence of current– voltage characteristics and photovoltaic properties of the solar cells under AM1.5G (100 mW/cm2) solar-illuminated conditions. The Voc was almost independent of the active layer thickness, and Isc linearly increased depending on the active layer thickness below 140 nm, and was saturated in the active layer thickness above 140 nm, that is corresponding with EQE, as shown in
Fig. 3. Active layer thickness dependence of (a) current–voltage characteristics, (b) Voc and Isc, and (c) FF and Ze of the solar cells with ITO/PEDOT:PSS/C6PcH2:PCBM/Al structures under AM1.5G (100 mW/cm2) solar-illuminated condition.
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Fig. 2(d). On the other hand, the FF decreased with increase in active layer thickness; therefore, the maximum of energy conversion efficiency was obtained to be 3.1% with Voc of 0.84 V, Isc of 8.4 mA/cm2 and FF of 0.44 at the active layer thickness of 120 nm. The Isc is not saturated and just decreases with increasing the active layer thickness in the case of typical P3HT:PCBM bulk heterojunction solar cells, as reported previously [11]. The amount of the absorption increases depending on the active layer thickness indeed; however, the amount of deactivated carriers and internal series resistance in the solar cells actually increases, because of the short carrier diffusion length in organic semiconductors. Since C6PcH2 possesses the high carrier mobility, the
Fig. 4. EQE spectra of the solar cells with ITO/buffer layer/C6PcH2:PCBM/Al structures.
generated carriers could be transported to the electrodes efficiently. Therefore, Isc is maintained even in the solar cells with thick active layer; however, FF decreased because of internal series resistance involving with the active layer thickness like P3HT:PCBM system [14].
3.2. Hole transport buffer layer dependence The organic thin-film solar cells of ITO/buffer layer/C6PcH2:PCBM/ Al structure were fabricated to study the hole transport buffer layer dependence, and the EQE spectra and current–voltage characteristics under AM1.5G (100 mW/cm2) solar-illuminated conditions were measured. The solar cells with no buffer layer, PEDOT:PSS buffer layer and MoO3 buffer layer were compared, and the photovoltaic properties were studied the thickness dependence of MoO3 buffer layer. The thickness of active layer was 120 nm. The EQE spectra of the solar cells with ITO/buffer layer/ C6PcH2:PCBM/Al are shown in Fig. 4. The solar cell with PEDOT:PSS buffer layer demonstrated higher EQE, and the solar cell with 6 nm-thick MoO3 buffer layer demonstrated the similar EQE with the solar cell without buffer layer at Q-band absorption region of C6PcH2. However, the EQE increased in the wavelength range of 450–500 nm in the solar cell with 6 nm-thick MoO3 buffer layer. Fig. 5 shows the hole transport buffer layer dependence of current–voltage characteristics and photovoltaic properties of the solar cells under AM1.5G (100 mW/cm2) solar-illuminated conditions. The energy conversion efficiency was improved because the Voc, Isc and FF were enhanced in the solar cells with hole transport buffer layers of PEDOT:PSS or MoO3 in comparison with the solar cells without buffer layer. Voc was improved to be about 0.8 V by inserting the MoO3, that is almost comparable to that in the case of PEDOT:PSS. Isc and FF strongly depended on the
Fig. 5. (a) Current–voltage characteristics of the solar cells with ITO/buffer layer/C6PcH2:PCBM/Al structures under AM1.5G (100 mW/cm2) solar-illuminated condition, and MoO3 thickness dependence of (b) Voc and Isc, and (c) FF and Ze.
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thickness of MoO3 buffer layer, and the maximum efficiency 3.2% with Voc of 0.79 V, Isc of 8.1 mA/cm2 and FF of 0.50 was obtained by inserting 6 nm-thick MoO3 buffer layer. It is noted that the improvement of the FF in organic thin-film solar cells with C6PcH2 was realized by the optimization of the active layer thickness and insertion of MoO3 buffer layer.
3.3. Tandem structured solar cells The organic thin-film solar cells of ITO/PEDOT:PSS/P3HT: PCBM/LiF/Al/MoO3/C6PcH2:PCBM/LiF/Al structure (tandem cell), ITO/PEDOT:PSS/P3HT:PCBM/LiF/Al structure (P3HT single cell) and ITO/MoO3/C6PcH2:PCBM/LiF/Al structure (C6PcH2 single cell) were fabricated, and the absorbance spectra and current–voltage characteristics under AM1.5G (100 mW/cm2) solar-illuminated conditions were measured. Fig. 6 shows the absorbance and EQE spectra of the tandem cell, P3HT single cell and C6PcH2 single cell. P3HT single cell absorbs from ultra-violet region till the wavelength of 650 nm, and C6PcH2 single cell absorbs ultra-violet region and near-infrared region in the wavelength range of 600–800 nm. In the tandem solar cells, few overlapping of the absorbance spectra for the active layers must be desired. Therefore, it is considered that C6PcH2 and P3HT are the effective combinations in tandem organic thin-film solar cells. The absorbance spectrum of the tandem cell was corresponding to the
Fig. 7. Current–voltage characteristics of the tandem cell, P3HT single cell and C6PcH2 single cell under AM1.5G (100 mW/cm2) solar-illuminated condition.
sum of the absorbance spectra of P3HT single cell and C6PcH2 single cell, resulting in the broad spectrum. The EQE of about 50% was observed in both P3HT single cell and C6PcH2 single cell at each peak wavelength, as shown in Fig. 6. It is considered that tandem cell corresponds to widewavelength range from ultra-violet region till near-infrared region. In C6PcH2 single cell, the EQE in an ultra-violet region decreased in comparison with the results in Fig. 5, because 25 nm-thick MoO3 buffer layer was used and an ultra-violet light was decayed during passing through the buffer layer. Fig. 7 shows the current–voltage characteristics of the tandem cell, P3HT single cell and C6PcH2 single cell under AM1.5G (100 mW/cm2) solar-illuminated conditions. The Voc of the tandem cell was 1.27 V. It is considered that the interlayer of LiF/Al/MoO3 effectively behaved as an intermediate electrode and the both active layers were connected with the series. The Voc of 1.27 V in the tandem cell indicates the small loss of 0.04 V from the sum of the Voc of the P3HT single cell and C6PcH2 single cell. At this stage, the Isc and FF of tandem cell were still low, because of the preliminary studies. However, it is considered that the solution processable tandem organic thin-film solar cells utilizing C6PcH2 with high efficiency could be fabricated by optimization of the fabricating conditions.
4. Conclusion
Fig. 6. (a) Absorbance spectra of the tandem cell, P3HT single cell and C6PcH2 single cell and (b) EQE spectra of the P3HT single cell and C6PcH2 single cell.
Bulk heterojunction solar cells utilizing soluble phthalocyanine derivative, C6PcH2 were investigated. The active layer was fabricated by spin-coating the mixed solution of C6PcH2 and PCBM, and active layer thickness dependence was investigated, and the optimized active layer thickness was clarified to be 120 nm. By inserting MoO3 hole transport buffer layer between the positive electrode and active layer, the FF and energy conversion efficiency were improved to be 0.50 and 3.2%, respectively. The tandem organic thin-film solar cell was also fabricated utilizing active layer materials of C6PcH2 and P3HT and the interlayer of LiF/Al/MoO3 structure, and a high Voc of 1.27 V was achieved.
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Acknowledgements This work was partly supported by Grants-in-Aid, for Young Scientists (A), and for the Japan Society for the Promotion of Science Fellows (No. 225630) from the Ministry of Education, Culture, Sports, Science and Technology, Japan, and by the Global Center of Excellence (Global COE) Program ‘‘Center for Electronic Devices Innovation’’ at Osaka University. References [1] C.W. Tang, Two-layer organic photovoltaic cell, Appl. Phys. Lett. 48 (1986) 183–185. [2] S. Morita, A.A. Zakhidov, K. Yoshino, Doping effect of buckminsterfullerene in conducting polymer: change of absorption spectrum and quenching of luminescence, Solid State Commun 82 (1992) 249–252. [3] N.S. Sariciftci, L. Smilowitz, A.J. Heeger, F. Wudl, Photoinduced electron transfer from a conducting polymer to buckminsterfullerene, Science 258 (1992) 1474–1476. [4] K. Yoshino, X.H. Yin, S. Morita, T. Kawai, A.A. Zakhidov, Enhanced photoconductivity of C60 doped poly(3-alkylthiophene), Solid State Commun. 85 (1993) 85–88. [5] F.C. Krebs, T. Tromholt, M. Jørgensen, Upscaling of polymer solar cell fabrication using full roll-to-roll processing, Nanoscale 2 (2010) 873–886. [6] F.C. Krebs, J. Fyenbo, M. Jørgensen, Product integration of compact roll-to-roll processed polymer solar cell modules: methods and manufacture using flexographic printing, slot-die coating and rotary screen printing, J. Mater. Chem. 20 (2010) 8994–9001. [7] G. Yu, J. Gao, J.C. Hummelen, F. Wudl, A.J. Heeger, Polymer photovoltaic cells: enhanced efficiencies via a network of internal donor–acceptor heterojunctions, Science 270 (1995) 1789–1791. [8] Y. Liang, Z. Xu, J. Xia, S.T. Tsai, Y. Wu, G. Li, C. Ray, L. Yu, For the Bright future—bulk heterojunction polymer solar cells with power conversion efficiency of 7.4%, Adv. Mater. 22 (2010) E135–E138. [9] M. Hiramoto, H. Fujiwara, M. Yokoyama, Three-layered organic solar cell with a photoactive interlayer of codeposited pigments, Appl. Phys. Lett 58 (1991) 1062–1064. [10] B. Walker, C. Kim, T. Quyen-Nguyen, Small molecule solution-processed bulk heterojunction solar cells, Chem. Mater. 23 (2011) 470–482.
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Contents lists available at ScienceDirect
Solar Energy Materials & Solar Cells journal homepage: www.elsevier.com/locate/solmat
Bulk heterojunction organic solar cells utilizing 1,4,8,11,15,18,22,25-octahexylphthalocyanine Tetsuro Hori a, Naoki Fukuoka a, Tetsuya Masuda a, Yasuo Miyake a,b, Hiroyuki Yoshida a, Akihiko Fujii a,n, Yo Shimizu b, Masanori Ozaki a a
Division of Electrical, Electronic and Information Engineering, Graduate School of Engineering, Osaka University, 2-1 Yamada-oka, Suita, Osaka 565-0871, Japan Synthetic Nano-Function Materials Group, Research Institute for Ubiquitous Energy Devices, National Institute of Advanced Industrial Science and Technology (AIST), Kansai Centre, 1-8-31 Midorigaoka, Ikeda, Osaka 563-8577, Japan b
a r t i c l e i n f o
abstract
Article history: Received 6 April 2011 Received in revised form 22 June 2011 Accepted 24 June 2011 Available online 14 July 2011
Bulk heterojunction solar cells utilizing soluble phthalocyanine derivative, 1,4,8,11,15,18,22,25-octahexylphthalocyanine (C6PcH2) have been investigated. The active layer was fabricated by spin-coating the mixed solution of C6PcH2 and 1-(3-methoxy-carbonyl)-propyl-1-1-phenyl-(6,6)C61 (PCBM). The photovoltaic properties of the solar cell with bulk heterojunction of C6PcH2 and PCBM demonstrated the strong dependence of active layer thickness, and the optimized active layer thickness was clarified to be 120 nm. By inserting MoO3 hole transport buffer layer between the positive electrode and active layer, the FF and energy conversion efficiency were improved to be 0.50 and 3.2%, respectively. The tandem organic thin-film solar cell has also been studied by utilizing active layer materials of C6PcH2 and poly(3-hexylthiophene) and the interlayer of LiF/Al/MoO3 structure, and a high Voc of 1.27 V has been achieved. & 2011 Elsevier B.V. All rights reserved.
Keywords: Phthalocyanine Organic solar cells Photovoltaic cell Wet process Bulk heterojunction Tandem structure
1. Introduction Organic thin-film solar cells utilizing organic semiconductors have attracted as next-generation solar cells [1]. Organic semiconductors are classified into two types, p-conjugated polymers and low-weighted molecules. Organic thin-film solar cells based on p-conjugated polymers have been investigated since the discovery of fullerene doping effects to p-conjugated polymers, such as photoluminescence quenching [2], photoinduced charge transfer [3], and photoconduction enhancement [4]. Organic thinfilm solar cells with a bulk heterojunction active layer fabricated by a wet process, which is important for the large area fabrication [5,6], achieved high energy conversion efficiency [7,8]. On the other hand, some low-weighted molecular semiconductors demonstrate a high crystallinity and charge mobility due to p-conjugated intermolecular interactions; however, they are inappropriate for wet processes, because of their poor solubility in typical organic solvents. Therefore, in organic thin-film solar cells based on low-weighted molecular materials, a p–n junction structure [1] or p–i–n junction structure [9] was generally fabricated by thermal evaporation process under vacuum. Recently, some groups investigated solution processable organic thin-film solar cells utilizing low-weighted molecules [10]. n
Corresponding author. E-mail address: [email protected] (A. Fujii).
0927-0248/$ - see front matter & 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.solmat.2011.06.039
Recently, we have demonstrated high hole and electron drift mobilities of 1.4 and 0.5 cm2/Vs, respectively, in the crystalline phase of a mesogenic phthalocyanine derivative, 1,4,8,11,15,18,22,25octahexylphthalocyanine (C6PcH2) [11]. C6PcH2 could be spread uniformly on a substrate from the solution, and exhibits a mesogenic phase within a certain temperature range [12]. C6PcH2 could be mixed in a conventional solvent with a typical acceptor molecule, such as 1-(3-methoxy-carbonyl)-propyl-1-1-phenyl-(6,6)C61 (PCBM), and the films fabricated by a solution process are available as a bulk heterojunction active layer in organic solar cells. Recently, we reported on a simple organic solar cell with bulk heterojunction of C6PcH2 and PCBM fabricated by spin-coating, and demonstrated the high energy conversion efficiency of 3.1% and external quantum efficiency (EQE) higher than 70% at the Q-band region [13]. In the organic thin-film solar cells utilizing C6PcH2, high opencircuit voltage (Voc) of 0.81 V and short-circuit current density (Isc) of 9.6 mA/cm2 were demonstrated, however, the fill factor (FF) of the solar cell with C6PcH2 was 0.40, and lower than the conventional organic thin-film solar cell utilizing conducting polymers, such as, poly(3-hexylthiophene) (P3HT) [14]. Therefore, the improvement of FF is one of the important problems to be solved. Optimization of the active layer thickness and insertion of buffer layers are one of the ways to improve the FF. The photovoltaic properties of organic thin-film solar cells strongly depend on the thickness of active layer, indeed [14,15]. Moreover,
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hole transport materials, such as the poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) or molybdenum trioxide (VI) (MoO3) [16–18] are available as hole transport buffer layers between positive electrode and organic active layer, and must play an important role for improving the FF. Therefore, it is important to study the dependence of active layer thickness and the buffer layer effects in the solar cell utilizing C6PcH2 The absorption loss in the wavelength range of 400–600 nm is another problem in the organic thin-film solar cell utilizing C6PcH2, and should be interpolated by fabricating the tandem structure with a complementary material, such as, P3HT. Since the Voc increased about twice in a tandem organic thin-film solar cell by inserting the ultra-thin gold layer between two active layers [19], many tandem type organic thin-film solar cells were investigated, such as polymer based type [20], all solution processed type [21], polarity inverted type [22], and so on. The tandem organic thin-film solar cell utilizing C6PcH2 and P3HT is also attractive, because of wet process fabrication and wide-wavelength range absorption. In this paper, we report on the active layer thickness and hole transport buffer layer dependences of simple bulk heterojunction organic solar cells utilizing C6PcH2 and PCBM blend film. We also report on the tandem solar cells, the active layers of which are based on C6PcH2, P3HT and PCBM, with wide-wavelength range absorption.
The C6PcH2:PCBM mixed solution was spin-coated onto the hole transport buffer layers at 500 rpm for 60 s in a glove box filled with argon gas. Aluminum (Al) layer as a counter electrode to the ITO was deposited onto the composite layers through shadow masks by thermal evaporation under a pressure of approximately 10–4 Pa. The evaporation speed and the thickness of the Al layer ˚ were approximately 5 A/s and 80 nm, respectively. The active area of the solar cell was 2 2 mm2. The fabrication of the tandem solar cell was carried out in the following manner. The PEDOT:PSS was deposited onto an ITO-coated quartz substrate by the above manner. The mixed chloroform solution of P3HT and PCBM, the composite concentration of which was 10 mg:8 mg/ml, was spin-coated onto PEDOT:PSS layer at 1000 rpm for 60 s in a glove box filled with nitrogen gas. The thickness of the P3HT:PCBM layer was estimated to be approximately 160 nm. Interlayer composed of 1.5 nm-thick lithium fluoride (LiF), 3 nm-thick Al and 25 nm-thick MoO3 were deposited onto the P3HT:PCBM active layer by thermal evaporation under a pressure of approximately 10–4 Pa. The evaporation speeds of the ˚ LiF, Al and MoO3 layers were approximately 0.1, 0.5 and 0.3 A/s, respectively. The C6PcH2:PCBM mixed chloroform solution, the
2. Experimental details C6PcH2 was synthesized according to the literature [23] with slight modifications and fully purified by column chromatography (silica-gel with toluene as eluent) followed by repetitive recrystallization from toluene–methanol (1:2) solution. Ragioregular P3HT and PCBM were purchased from Merck & Co., Inc. and Frontier Carbon Ltd., respectively. The molecular structures of the C6PcH2, P3HT and PCBM are shown in Fig. 1. PEDOT:PSS (Baytron P VP AI 4083) or MoO3 were used as hole transport buffer layers. PEDOT:PSS was spin-coated onto an indium-tin-oxide (ITO)-coated quartz substrate at 3000 rpm for 60 s using an aqueous solution diluted with the same volume of isopropanol, and dried at 100 1C for 10 min in an oven under atmospheric conditions. The thickness of the PEDOT:PSS layer was estimated to be approximately 30 nm. A MoO3 layer was fabricated by thermal evaporation onto an ITO-coated quartz substrate under a pressure of 10–4 Pa, and the evaporation speed ˚ of MoO3 was approximately 0.3 A/s. C6PcH2 and PCBM were dissolved in chloroform solvent, and the composite ratio of C6PcH2:PCBM was 2:1 by weight ratio [13].
C6H13 R
R
S S
R
N
N H
N R
N
N
n
R P3HT
C6H13
N H N
OMe N
R
R
R
O
R = C6H13
C6PcH2 PCBM Fig. 1. Molecular structures of C6PcH2, PAT6, and PCBM.
Fig. 2. Active layer thickness dependence of (a) absorbance spectra and (b) EQE spectra of the solar cells. (c) Absorbance versus thickness of active layer at a wavelength of 730 nm. (d) EQE versus thickness of active layer at a wavelength of 670 and 730 nm.
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composite concentration of which was 10 mg:5 mg/ml, was spincoated onto the interlayer at 500 rpm for 60 s in a glove box filled with nitrogen gas. The thickness of the C6PcH2:PCBM layer was estimated to be approximately 130 nm. 1.5 nm-thick LiF and 70 nmthick Al layers as a counter electrode to the ITO was deposited onto the tandem layers through shadow masks by thermal evaporation under a pressure of approximately 10–4 Pa. The evaporation speeds ˚ of the LiF and Al layers were approximately 0.1 and 5.0 A/s, respectively. Each process of P3HT:PCBM active layer, interlayer, C6PcH2:PCBM active layer and LiF/Al electrode fabrication were carried out without air exposure. The active area of the solar cell was 2 2 mm2. The structure of fabricated tandem solar cell is ITO/ PEDOT:PSS/P3HT:PCBM/LiF/Al/MoO3/C6PcH2:PCBM/LiF/Al (tandem cell). The solar cells with ITO/PEDOT:PSS/P3HT:PCBM/LiF/Al structure (P3HT single cell) and ITO/MoO3/C6PcH2:PCBM/LiF/Al structure (C6PcH2 single cell) were also fabricated for the comparison by the same manner. The absorbance spectra were measured using a spectrophotometer (Shimadzu UV-3150). The film thickness was evaluated using the atomic force microscope (AFM: KEYENCE VN-8000). The photocurrent spectra were measured with a programmable electrometer (Keithley 617S) using a xenon lamp light passing through a monochromator as a light source, and the external quantum efficiency (EQE) was estimated for each incident light wavelength using EQE (%)¼1240 I (A/cm2) 100/(l (nm) Pin (W/cm2)), where I is the photocurrent density and l is the wavelength of incident light. The current–voltage characteristics were measured with a high-voltage-source measurement unit (Keithley 237) under AM1.5G (100 mW/cm2) solar-illuminated conditions. From the current–voltage characteristics under AM1.5G, the FF and energy conversion efficiency (Ze) were estimated according to the following definitions: FF¼ImaxVmax/IscVoc
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and Ze ¼IscVocFF/Pin, where Imax and Vmax are the current density and voltage at the maximum output power, respectively, Isc is the short-circuit current density and Pin is the intensity of the incident light. The current–voltage characteristics and EQE spectra were measured in vacuum at room temperature.
3. Results and discussions 3.1. Thickness dependence The organic thin-film solar cells of ITO/PEDOT:PSS/C6PcH2:PCBM/ Al structure were fabricated to study the thickness dependence of active layers, and the absorbance spectra, EQE spectra and current– voltage characteristics under AM1.5G (100 mW/cm2) solar-illuminated conditions were measured. Thicknesses of active layers were changed 80–280 nm by controlling the concentration of C6PcH2:PCBM mixed solutions. The active layer thickness dependence of absorbance and EQE spectra in the solar cells are shown in Fig. 2. C6PcH2 exhibits the absorption of Q-band in the wavelength range of 600–800 nm, and the absorbance linearly increased depending on the film thickness. The EQE was also enhanced monotonically in the range of the active layer thickness of 80–140 nm, then saturated in the active layer thickness of more than 140 nm. Fig. 3 shows the active layer thickness dependence of current– voltage characteristics and photovoltaic properties of the solar cells under AM1.5G (100 mW/cm2) solar-illuminated conditions. The Voc was almost independent of the active layer thickness, and Isc linearly increased depending on the active layer thickness below 140 nm, and was saturated in the active layer thickness above 140 nm, that is corresponding with EQE, as shown in
Fig. 3. Active layer thickness dependence of (a) current–voltage characteristics, (b) Voc and Isc, and (c) FF and Ze of the solar cells with ITO/PEDOT:PSS/C6PcH2:PCBM/Al structures under AM1.5G (100 mW/cm2) solar-illuminated condition.
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Fig. 2(d). On the other hand, the FF decreased with increase in active layer thickness; therefore, the maximum of energy conversion efficiency was obtained to be 3.1% with Voc of 0.84 V, Isc of 8.4 mA/cm2 and FF of 0.44 at the active layer thickness of 120 nm. The Isc is not saturated and just decreases with increasing the active layer thickness in the case of typical P3HT:PCBM bulk heterojunction solar cells, as reported previously [11]. The amount of the absorption increases depending on the active layer thickness indeed; however, the amount of deactivated carriers and internal series resistance in the solar cells actually increases, because of the short carrier diffusion length in organic semiconductors. Since C6PcH2 possesses the high carrier mobility, the
Fig. 4. EQE spectra of the solar cells with ITO/buffer layer/C6PcH2:PCBM/Al structures.
generated carriers could be transported to the electrodes efficiently. Therefore, Isc is maintained even in the solar cells with thick active layer; however, FF decreased because of internal series resistance involving with the active layer thickness like P3HT:PCBM system [14].
3.2. Hole transport buffer layer dependence The organic thin-film solar cells of ITO/buffer layer/C6PcH2:PCBM/ Al structure were fabricated to study the hole transport buffer layer dependence, and the EQE spectra and current–voltage characteristics under AM1.5G (100 mW/cm2) solar-illuminated conditions were measured. The solar cells with no buffer layer, PEDOT:PSS buffer layer and MoO3 buffer layer were compared, and the photovoltaic properties were studied the thickness dependence of MoO3 buffer layer. The thickness of active layer was 120 nm. The EQE spectra of the solar cells with ITO/buffer layer/ C6PcH2:PCBM/Al are shown in Fig. 4. The solar cell with PEDOT:PSS buffer layer demonstrated higher EQE, and the solar cell with 6 nm-thick MoO3 buffer layer demonstrated the similar EQE with the solar cell without buffer layer at Q-band absorption region of C6PcH2. However, the EQE increased in the wavelength range of 450–500 nm in the solar cell with 6 nm-thick MoO3 buffer layer. Fig. 5 shows the hole transport buffer layer dependence of current–voltage characteristics and photovoltaic properties of the solar cells under AM1.5G (100 mW/cm2) solar-illuminated conditions. The energy conversion efficiency was improved because the Voc, Isc and FF were enhanced in the solar cells with hole transport buffer layers of PEDOT:PSS or MoO3 in comparison with the solar cells without buffer layer. Voc was improved to be about 0.8 V by inserting the MoO3, that is almost comparable to that in the case of PEDOT:PSS. Isc and FF strongly depended on the
Fig. 5. (a) Current–voltage characteristics of the solar cells with ITO/buffer layer/C6PcH2:PCBM/Al structures under AM1.5G (100 mW/cm2) solar-illuminated condition, and MoO3 thickness dependence of (b) Voc and Isc, and (c) FF and Ze.
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thickness of MoO3 buffer layer, and the maximum efficiency 3.2% with Voc of 0.79 V, Isc of 8.1 mA/cm2 and FF of 0.50 was obtained by inserting 6 nm-thick MoO3 buffer layer. It is noted that the improvement of the FF in organic thin-film solar cells with C6PcH2 was realized by the optimization of the active layer thickness and insertion of MoO3 buffer layer.
3.3. Tandem structured solar cells The organic thin-film solar cells of ITO/PEDOT:PSS/P3HT: PCBM/LiF/Al/MoO3/C6PcH2:PCBM/LiF/Al structure (tandem cell), ITO/PEDOT:PSS/P3HT:PCBM/LiF/Al structure (P3HT single cell) and ITO/MoO3/C6PcH2:PCBM/LiF/Al structure (C6PcH2 single cell) were fabricated, and the absorbance spectra and current–voltage characteristics under AM1.5G (100 mW/cm2) solar-illuminated conditions were measured. Fig. 6 shows the absorbance and EQE spectra of the tandem cell, P3HT single cell and C6PcH2 single cell. P3HT single cell absorbs from ultra-violet region till the wavelength of 650 nm, and C6PcH2 single cell absorbs ultra-violet region and near-infrared region in the wavelength range of 600–800 nm. In the tandem solar cells, few overlapping of the absorbance spectra for the active layers must be desired. Therefore, it is considered that C6PcH2 and P3HT are the effective combinations in tandem organic thin-film solar cells. The absorbance spectrum of the tandem cell was corresponding to the
Fig. 7. Current–voltage characteristics of the tandem cell, P3HT single cell and C6PcH2 single cell under AM1.5G (100 mW/cm2) solar-illuminated condition.
sum of the absorbance spectra of P3HT single cell and C6PcH2 single cell, resulting in the broad spectrum. The EQE of about 50% was observed in both P3HT single cell and C6PcH2 single cell at each peak wavelength, as shown in Fig. 6. It is considered that tandem cell corresponds to widewavelength range from ultra-violet region till near-infrared region. In C6PcH2 single cell, the EQE in an ultra-violet region decreased in comparison with the results in Fig. 5, because 25 nm-thick MoO3 buffer layer was used and an ultra-violet light was decayed during passing through the buffer layer. Fig. 7 shows the current–voltage characteristics of the tandem cell, P3HT single cell and C6PcH2 single cell under AM1.5G (100 mW/cm2) solar-illuminated conditions. The Voc of the tandem cell was 1.27 V. It is considered that the interlayer of LiF/Al/MoO3 effectively behaved as an intermediate electrode and the both active layers were connected with the series. The Voc of 1.27 V in the tandem cell indicates the small loss of 0.04 V from the sum of the Voc of the P3HT single cell and C6PcH2 single cell. At this stage, the Isc and FF of tandem cell were still low, because of the preliminary studies. However, it is considered that the solution processable tandem organic thin-film solar cells utilizing C6PcH2 with high efficiency could be fabricated by optimization of the fabricating conditions.
4. Conclusion
Fig. 6. (a) Absorbance spectra of the tandem cell, P3HT single cell and C6PcH2 single cell and (b) EQE spectra of the P3HT single cell and C6PcH2 single cell.
Bulk heterojunction solar cells utilizing soluble phthalocyanine derivative, C6PcH2 were investigated. The active layer was fabricated by spin-coating the mixed solution of C6PcH2 and PCBM, and active layer thickness dependence was investigated, and the optimized active layer thickness was clarified to be 120 nm. By inserting MoO3 hole transport buffer layer between the positive electrode and active layer, the FF and energy conversion efficiency were improved to be 0.50 and 3.2%, respectively. The tandem organic thin-film solar cell was also fabricated utilizing active layer materials of C6PcH2 and P3HT and the interlayer of LiF/Al/MoO3 structure, and a high Voc of 1.27 V was achieved.
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Acknowledgements This work was partly supported by Grants-in-Aid, for Young Scientists (A), and for the Japan Society for the Promotion of Science Fellows (No. 225630) from the Ministry of Education, Culture, Sports, Science and Technology, Japan, and by the Global Center of Excellence (Global COE) Program ‘‘Center for Electronic Devices Innovation’’ at Osaka University. References [1] C.W. Tang, Two-layer organic photovoltaic cell, Appl. Phys. Lett. 48 (1986) 183–185. [2] S. Morita, A.A. Zakhidov, K. Yoshino, Doping effect of buckminsterfullerene in conducting polymer: change of absorption spectrum and quenching of luminescence, Solid State Commun 82 (1992) 249–252. [3] N.S. Sariciftci, L. Smilowitz, A.J. Heeger, F. Wudl, Photoinduced electron transfer from a conducting polymer to buckminsterfullerene, Science 258 (1992) 1474–1476. [4] K. Yoshino, X.H. Yin, S. Morita, T. Kawai, A.A. Zakhidov, Enhanced photoconductivity of C60 doped poly(3-alkylthiophene), Solid State Commun. 85 (1993) 85–88. [5] F.C. Krebs, T. Tromholt, M. Jørgensen, Upscaling of polymer solar cell fabrication using full roll-to-roll processing, Nanoscale 2 (2010) 873–886. [6] F.C. Krebs, J. Fyenbo, M. Jørgensen, Product integration of compact roll-to-roll processed polymer solar cell modules: methods and manufacture using flexographic printing, slot-die coating and rotary screen printing, J. Mater. Chem. 20 (2010) 8994–9001. [7] G. Yu, J. Gao, J.C. Hummelen, F. Wudl, A.J. Heeger, Polymer photovoltaic cells: enhanced efficiencies via a network of internal donor–acceptor heterojunctions, Science 270 (1995) 1789–1791. [8] Y. Liang, Z. Xu, J. Xia, S.T. Tsai, Y. Wu, G. Li, C. Ray, L. Yu, For the Bright future—bulk heterojunction polymer solar cells with power conversion efficiency of 7.4%, Adv. Mater. 22 (2010) E135–E138. [9] M. Hiramoto, H. Fujiwara, M. Yokoyama, Three-layered organic solar cell with a photoactive interlayer of codeposited pigments, Appl. Phys. Lett 58 (1991) 1062–1064. [10] B. Walker, C. Kim, T. Quyen-Nguyen, Small molecule solution-processed bulk heterojunction solar cells, Chem. Mater. 23 (2011) 470–482.
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