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Japanese Journal of Applied Physics 51 (2012) 10NE22 http://dx.doi.org/10.1143/JJAP.51.10NE22
Crystalline Silicon/Graphene Oxide Hybrid Junction Solar Cells Qiming Liu1 , Fumiya Wanatabe1 , Aya Hoshino1 , Ryo Ishikawa1 , Takuya Gotou2 , Keiji Ueno1 , and Hajime Shirai1 1 2
Graduate School of Science and Engineering, Saitama University, Saitama 338-8570, Japan Tokyo Research Laboratory, Mitsubishi Gas Chemical Co., Inc., Katsushika, Tokyo 125-8601, Japan
Received November 27, 2011; revised April 28, 2012; accepted May 11, 2012; published online October 22, 2012 Soluble graphene oxide (GO) and plasma-reduced (pr-) GO were investigated using crystalline silicon (c-Si) (100)/GO/pr-GO hybrid junction solar cells. Their photovoltaic performances were compared with those of c-Si/GO/pristine conductive poly(ethylene dioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) heterojunction and c-Si/PEDOT:PSS:GO composite devices. The c-Si/GO/pr-GO and conductive PEDOT:PSS/ Al heterojunction solar cells showed power conversion efficiencies of 6.5 and 8.2%, respectively, under illumination with AM 1.5 G 100 mW/cm2 simulated solar light. A higher performance of 10.7% was achieved using the PEDOT:PSS:GO (12.5 wt %) composite device. These findings imply that soluble GO, pr-GO, and the PEDOT:PSS:GO composite are promising materials as hole transport and transparent conductive layers for c-Si/ organic hybrid junction solar cells. # 2012 The Japan Society of Applied Physics
1. Introduction
Conjugated polymer thin-film solar cells have attracted considerable attention owing to their potential as a renewable, alternative source of electricity and their low cost, light weight, mechanical flexibility, and easy processing conditions as compared with inorganic devices such as crystalline Si (c-Si).1–3) Owing to great efforts to optimize the performance of organic solar cells (OSCs), a power conversion efficiency of 5–6% has been achieved using the bulk heterojunction (BHJ) structure.4,5) For hole collection in common OSCs, a buffer layer of poly(ethylene dioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) is often used to modify the indium–tin-oxide (ITO) electrode owing to the superior injection/collection properties of the PEDOT:PSS/ active layer interface. Several approaches have been developed to improve the conductivity of PEDOT:PSS including thermal treatment, secondary doping with inert solvents such as glycerol, sorbitol, dimethyl sulfoxide (DMSO), N,N0 -dimethyl formamide (DMF), tetrahydrofuran (THF), and polymers.6–10) Graphene consisting of one monolayer thick graphite sheet has also recently attracted a great deal of attention as a potential novel electronic material, owing to its unique transport properties with a high transparency, which is a better transparent conductive layer for photovoltaic devices.11–13) Several synthesis methods have been extensively studied, i.e., chemical vapor deposition (CVD) of hydrocarbons such as CH4 on a pure copper metal substrate or thermal decomposition of crystalline SiC at 1000 C.14,15) However, the subsequent detachment of graphene sheet from the copper substrate and a transfer process to another substrate are required to functionalize it as a transparent conductive electrode. As an alternative approach, the thermally and chemically reduced soluble GO (r-GO) is also possible as an improved transparent conductive electrode for OSCs and organic thinfilm transistors (OTFTs), although the average size of GO flake was in the range of 1–5 m.16) They can be directly spin-coated on a hydrophilic substrate such as glass and thermally grown (th-)SiO2 without any complicated processing.17) In addition, it is reported that soluble GO has a great potential as a hole-transporting and electron-blocking layer using poly(3-hexylthiophene):phenyl-C61 -butyric acid methyl ester (P3HT:PCBM) OSCs.15) Few studies, however,
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have been performed using GO and reduced GO (r-GO) as hole-transporting and transparent conductive layers for c-Si/ organic junction solar cells except c-Si/graphene Schottky junction devices.18,19) In this study, we first experimentally demonstrate the c-Si/soluble GO hybrid heterojunction solar cells with H2 /Ar plasma-reduced GO (pr-GO) or a conductive PEDOT:PSS composite as a transparent conductive layer. The photovoltaic performance of c-Si/PEDOT:PSS:GO composite junction solar cells was also demonstrated compared with those of c-Si/GO/pr-GO and c-Si/GO/ PEDOT:PSS heterojunction devices. 2. Experimental Procedure
One-side polished single-crystal N-type Si(100) with resistivities of 3–5 cm and a thickness of 300 m was used as a substrate. The wafers were RCA cleaned in acetone, ethanol, and deionized water for 10 min each. They were then dipped in 5% hydrofluoric (HF) acid to remove any native oxide. A functionalized GO was prepared by a modified Hummers method; i.e., a chemical oxidation method as described elsewhere.20) GO is highly soluble in an aqueous solution; therefore, it is well dispersed in various solvents.19) The resultant GO solution was purified by dialysis to remove alkali and heavy metal ionic impurities in GO solution, especially Li, B, and Mn contained in graphite source materials and oxidizing agents. Prior to the spin coating of GO and PEDOT:PSS on a hydrophobic c-Si(100) wafer, UV oxidation was performed using a shadow mask with multiholes to promote a uniform coating. The GO flakes diluted in methanol solution (0.01, 0.5 wt %) was spin-coated on RCA-cleaned N-type c-Si(100) at 1000 rpm for 30 s followed by thermal annealing at 120 C for 30 min. Subsequently, for several samples, H2 /Ar plasma exposure was performed to reduce GO using a low-pressure remote-type microwave (MW) plasma source with the substrate temperature Ts , plasma exposure period, and H2 /Ar flow rate ratio as variables. The flow rates of H2 and Ar were 5–25 and 40 sccm, respectively, and the total gas flow rate was 60 sccm. The MW power was 200 W. The photovoltaic devices used have a structure of c-Si/GO/pr-GO (Fig. 1). The commercialized PEDOT:PSS (5% DMSO) (CLEVIOS1000) was also used as a hole-transporting layer for c-Si/GO/PEDOT:PSS heterojunction and c-Si/PEDOT:PSS:GO composite devices as a reference. Finally, silver metal paste was used as
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(a)
Ag
20
hν ν
18 Thickness (nm)
Pr-GO, PEDOT:PSS, PEDOT:PSS:GO GO
N-type c-Si(100)
16 14 12 10 8
(300 μm) ρ : 3-5Ω ·cm
6 0
50 100 150 200 250 Annealing temperature (°C)
Al
300
(b) 2
Fig. 1. (Color online) Schematic of photovoltaic device used in this
study.
k
1.5
a top electrode on the PEDOT:PSS layer with a device area of a 5 5 mm2 . Spin-coated GO, pr-GO, and PEDOT:PSS:GO films were characterized by atomic force microscopy (AFM; Seiko Instruments SPA-300/SPI-3800) and spectroscopic ellipsometry (SE; Horiba Jobin Yvon UVISEL). Sheet resistance was measured by the four-point probe method. The current density vs voltage (J–V ) characteristics were measured in the dark and under illumination with simulated solar light (AM 1.5G, 100 mW/cm2 , Bunkoukeiki CEP-25BX). The calculation of the power conversion efficiency was performed using the following equation: ¼
Voc Jsc FF ; Pin
where Voc is the open-circuit voltage, Jsc is the short-circuit current density, FF is the fill factor, and Pin is the incident light power. FF was determined using FF ¼ ðVm Jm Þ= ðVoc Jsc Þ, where Vm and Jm are the voltage and current density at the maximum power in the J–V curves in the fourth quadrant, respectively. 3. Results and Discussion
Figure 2 shows the changes in GO film thickness and the optical extinction coefficient k spectra in pr-GO thermally annealed at different temperatures. They were determined by SE characterization using a Tauc–Lorentz (TL) model combined with the effective medium approximation (EMA).21) Here, the probable structure was calculated by minimizing the mean square error 2 between the measured and calculated ellipsometric error parameters using a least linear regression method.22) The thickness of as-deposited GO films was 18 nm corresponding to the pile of 20–30 GO sheets with a high transparency in the entire energy region of 1–5 eV. The thickness decreased to 6–7 nm following the thermal annealing at >200 C for 30 min with darkening owing to the chemical reduction with the extraction of oxygen-related functional groups such as OH and C¼O. This procedure shortened the spacing of each GO flake sheet. However, the sheet resistance was 1 k/ and it was still 2 orders of magnitude higher than that of ITO. To further promote the reduction of GO, the H2 /Ar MW plasma
1
0.5 01
RT 100°C 200°C 300°C
2
3 4 Energy (eV)
5
6
Fig. 2. (Color online) Changes in (a) film thickness and (b) k spectra of spin-coated GOs thermally annealed at different temperatures.
exposure was performed with Ts , H2 /Ar flow rate ratio, and the electrode-substrate distance as variables. Figures 3(a) and 3(b) respectively show the optical transmittance and the line profile of pr-GO before and after the H2 /Ar MW plasma exposure at a Ts of 300 C for 20 min. The AFM images of the corresponding pr-GO films are also shown [Fig. 3(c)]. No significant changes in optical transmittance and sheet resistance were observed in the GO films exposed to H2 /Ar plasma for up to 1 h at a Ts below 300 C. The sheet resistance of as-deposited GO, however, decreased significantly from 150 Mcm to 325 cm after the H2 /Ar MW plasma exposure, although the optical transmittance decreased to 60–70% in the entire 300–900 nm region. The AFM observation also revealed that the RMS roughness value decreased significantly from 2.6 to 0.82 nm with the decrease in film thickness. These findings suggest that the film densification is promoted efficiently by the H2 /Ar plasma exposure. Figure 4 shows the J–V characteristic of Ag grid/pr-GO or conductive EDOT:PSS/GO/c-Si/Al hybrid junction solar cells under illumination with AM 1.5G 100 mW/cm2 simulated solar light. The pr-GO layer was formed by the reduction of 20-nm-thick spin-coated GO using H2 /Ar MW plasma treatment at a Ts of 300 C for 10 min. The details on the performance of several device structures, namely, c-Si/GO/pr-GO, c-Si/GO/PEDOT:PSS, and c-Si/ PEDOT:PSS:GO composite hybrid junctions are listed in Table I. The c-Si/GO/pr-GO heterojunction device showed a of 6.62% with a Jsc of 21.5 mA/cm2 , a Voc of 0.51 V, and a FF of 0.61. The device based on the c-Si/GO/conductive PEDOT:PSS buffer layer gave a of 8.09% with a Jsc of 26.4 mA/cm2 , a Voc of 0.52 V, and a FF of 0.59. The
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(a)
(c)
(b)
Fig. 3. (Color online) (a) Optical transmittance spectra and (b) the line profile of (pr-)GOs coated on glass before and after the H2 /Ar microwave plasma exposure at a Ts of 300 C for 20 min. (c) AFM images of the corresponding pr-GO films.
30
PEDOT:PSS/GO J (mA/cm2)
25 20
pr-GO/GO 15 10 5 0
0
0.1
0.2
0.3
0.4
0.5
0.6
Voltage (V) Fig. 4. (Color online) J–V curves of c-Si/GO/pr-GO and c-Si/GO/ PEDOT:PSS hybrid junction solar cells under illumination with a AM 1.5 G 100 mW/cm2 simulated solar light.
Table I. Performance details (Voc , Jsc , FF, and ) of the c-Si photovoltaic devices having different buffer layers.
Voc (V)
Jsc (mA/cm2 )
FF
(%)
pr-GO/GO
0.51
21.5
0.61
6.62
PEDOT:PSS/GO
0.52
26.4
0.59
8.09
PEDOT:PSS:GO composite
0.548
28.9
0.675
Buffer layer
10.7
100-nm-thick spin-coated conductive PEDOT:PSS was used as a hole transport layer, which was 100 times larger than that of pr-GO. Despite the several smaller nanometer thickness of pr-GO, its efficiency deteriorated compared with that of the conductive PEDOT:PSS device with lower Jsc and Voc , although their FF values were almost the same 0.59–0.61. These findings imply that GO is a possible
material as a hole transport layer of c-Si/GO junction solar cells. The lower Jsc and Voc of the pr-GO device is also due to the higher optical absorption and insufficiently low resistance of pr-GO. In addition, it is well known that the PEDOT:PSS shows a uniaxial optical anisotropic property, i.e., the ordinary and extraordinary optical components. The former shows a metallic characteristic, whereas the latter exhibits a more dielectric characteristic.23) The SE characterization revealed that these features were enhanced by the addition of a small amount of GO to PEDOT:PSS. As a result, increased to 10.2% with a Jsc of 28.7 mA/cm2 , a Voc of 0.548 V, and a FF of 0.675 in the PEDOT:PSS:GO composite device. The effect of GO addition to conductive PEDOT:PSS on the c-Si/PEDOT:PSS:GO composite is described elsewhere.24–26) These findings imply that the pr-GO/GO heterojunction has a high potential for OSCs as well as c-Si/organic hybrid junction solar cells as a holecollecting transparent conductive layer because of its easy processing. Further decreases in sheet resistance in pr-GO with a smaller thickness are required to further increase the performance of c-Si/GO/pr-GO hybrid junction solar cells. 4. Conclusions
Soluble GO and plasma-reduced (pr-)GO were respectively used as a hole-transporting buffer layer and transparent conductive layer of c-Si/organic hybrid junction solar cells. The sheet resistance of GO was decreased to 325 / with an optical transmittance of 60% by adjusting the H2 /Ar microwave plasma exposure condition, although it was still one or two orders of magnitude higher than that of ITO. The power conversion efficiencies of the pr-GO/GO/c-Si and conductive PEDOT:PSS/GO/c-Si hybrid junction solar cells were 6.5 and 8.2%, respectively. Furthermore, the efficiency increased to 10.7% when using the PEDOT:PSS:GO (12.5%) composite device. These findings imply that
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pr-GO and the conductive PEDOT:PSS composite are promising materials for c-Si/GO hybrid junction solar cells as a hole-collecting transparent conductive layer, because of their easy processing. Acknowledgements
This research was partially supported by a Japan Science and Technology Agency (JST) grant and a Grant-in-Aid for Scientific Research from the Ministry of Education, Culture, Sports, Science and Technology of Japan.
9) B. Yin, Q. Liu, L. Yang, X. Wu, Z. Liu, Y. Hua, S. Yin, and Y. Chen:
J. Nanosci. Nanotechnol. 10 (2010) 1934. 10) S.-I. Na, G. Wang, S.-S. Kim, T.-W. Kim, S.-H. Oh, B.-K. Yu, T. Lee, and
D.-Y. Kim: J. Mater. Chem. 19 (2009) 9045. 11) K. S. Novoselov, A. K. Gein, S. V. Morozov, D. Jiang, Y. Zhang, and S. V.
Dubonos: Science 306 (2004) 666.
12) A. K. Geim: Science 324 (2009) 1530. 13) H. Bai, C. Li, and G. Shi: Adv. Mater. 23 (2011) 1089. 14) R. Reina, X. Jia, J. Ho, D. Nezich, H. Son, V. Bulovic, M. S. Dresselhaus,
and J. Kong: Nano Lett. 9 (2009) 30. 15) X. Li, G. Zhang, X. Bai, X. Sun, X. Wang, E. Wang, and H. Dai: Nat.
Nanotechnol. 3 (2008) 538.
16) K. Suganuma, S. Watanabe, T. Gotou, and K. Ueno: Appl. Phys. Express 4
(2011) 021603. 17) K. Ueno: Mon. Disp. 18 [1] (2012) 67. 18) X. Li, H. Zhu, K. Wang, A. Cao, J. Wei, C. Li, Y. Zhen, X. Li, and D. Wu:
Adv. Mater. 22 (2010) 2743.
1) A. C. Mayer, S. R. Scully, B. E. Hardin, M. W. Rowell, and M. D. 2) 3) 4) 5) 6)
7) 8)
McGehee: Mater. Today 10 [11] (2007) 28. M. C. Scharber, D. Mu¨hlbacher, M. Koppe, P. Denk, C. Waldauf, A. J. Heeger, and C. J. Brabec: J. Adv. Mater. 18 (2006) 789. Y. He, H. Y. Chen, J. Hou, and Y. Li: J. Am. Chem. Soc. 132 (2010) 1377. S. R. Forrest: MRS Bull. 30 [1] (2005) 28. B. C. Thompson and J. M. J. Fre´chet: Angew. Chem., Int. Ed. 47 (2008) 58. S. K. M. Jo¨nsson, J. Birgerson, X. Crispin, G. Greczynski, W. Osikowicz, A. W. Denier van der Gon, W. R. Salaneck, and M. Fahlman: Synth. Met. 139 (2003) 1. C. Gravalidis, A. Laskarakis, and S. Logothetidis: Eur. Phys. J.: Appl. Phys. 46 (2009) 12505. B. L. Groenendaal, F. Jonas, D. Freitag, H. Pielartzik, and J. R. Reynolds: Adv. Mater. 12 (2000) 481.
19) G. Fan, H. Zhu, K. Wang, J. Wei, X. Li, Q. Shu, N. Guo, and D. Wu: ACS
Appl. Mater. Interfaces 3 (2011) 721. W. S. Hummers, Jr. and R. E. Offeman: J. Am. Chem. Soc. 80 (1958) 1339. G. E. Jellison, Jr.: Opt. Mater. 1 (1992) 41. G. E. Jellison, Jr. and F. A. Modine: Appl. Phys. Lett. 69 (1996) 2137. L. A. A. Pettersson, F. Carlsson, O. Ingana¨s, and H. Arwin: Thin Solid Films 313–314 (1998) 356. 24) M. Ono, Z. Tang, T. Gotou, K. Ueno, and H. Shirai: Appl. Phys. Express 5 (2012) 032301. 25) Q. Liu, Z. Tang, I. Khatri, R. Ishikawa, K. Ueno, and H. Shirai: to be published in Phys. Status Solidi (2012). 26) Z. Tang, Q. Liu, M. Ono, R. Ishikawa, K. Ueno, and H. Shirai: to be published in J. Appl. Phys. 20) 21) 22) 23)
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