The Compromise Condition for High Performance of the Single Silicon Heterojunction Solar Cells

Hindawi Publishing Corporation International Journal of Photoenergy Volume 2012, Article ID 283872, 6 pages doi:10.1155/2012/283872 Research Article The Compromise Condition for High Performance of the Single Silicon Heterojunction Solar Cells Youngseok Lee,1 Vinh Ai Dao,2, 3 Sangho Kim,1 Sunbo Kim,3 Hyeongsik Park,3 Jaehyun Cho,3 Shihyun Ahn,3 and Junsin Yi1, 3 1 Department of Energy Science, Sungkyunkwan University, Suwon 440-746, Republic of Korea 2 College of Science, Faculty of Materials Science, Vietnam National University, 227 Nguyen Van Cu, Hochiminh, Vietnam 3 School of Information and Communication Engineering, Sungkyunkwan University, Suwon 440-746, Republic of Korea Correspondence should be addressed to Junsin Yi, Received 31 August 2011; Revised 14 November 2011; Accepted 14 November 2011 Academic Editor: C. W. Lan Copyright © 2012 Youngseok Lee et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. For optimum performance of the hydrogenated amorphous silicon/crystalline silicon (a-Si : H/c-Si) heterojunction solar cells, featuring a doping concentration, localized states, as well as thickness of emitter layer are crucial, since Fermi level, surface passivated quality, and light absorption have to be compromised themselves. For this purpose, the effect of both doping concentration and thickness of emitter layer was investigated. It was found that with gas phase doping concentration and emitter layer thickness of 3% and 7 nm, solar cell efficiency in excess of 14.6% can be achieved. For high gas phase doping concentration, the degradation of open-circuit voltage as well as cell efficiency was obtained due to the higher disorder in the emitter layer. The heavily doped along with thicker in thickness of emitter layer results in light absorption on short wavelength, then diminishing short-circuit current density. 1. Introduction Heterojunction solar cells consisting of crystalline silicon (cSi) and hydrogenated amorphous silicon (a-Si : H) offer a low cost and high efficiency energy conversion alternative to conventional crystalline silicon solar cells. Compared to conventional silicon solar cells with diffused n/p junction and back surface field layers (BSF), noteworthy cost reduction can be obtained due to a completely low temperature (∼ 200◦ C) formation process for both the n/p junction and BSF layer using hydrogenated amorphous silicon technology. Presently, Sanyo’s heterojunction with intrinsic thin layer (HIT) solar cells showed the world record efficiency of 23% for double-junction structure [1]. However, for the singlejunction HIT solar cell fabricated on polished wafers has reported approximately 13 ∼ 14% efficiency [2–6], in which the open-circuit voltage (Voc ) did not exceed 580 mV, and the fill factor 74% could be obtained. Beside Sanyo, most research groups have been working on single-junction HIT solar cell using p-type c-Si as a base substrate. When using a-Si : H and c-Si for junction formation, there are different aspects to be taken into account. Firstly to obtain high open circuit voltage (Voc ) and thus efficiency, the Fermi level in the emitter layer should be as close as to the nearest band as possible, which means that doping concentration is as high as it could be. The high doping concentration, nevertheless, also results in the high defect density in the films and leads to enhanced surface recombination [4]. The preferred doping concentration of emitter for HIT solar cell performance is still a matter of discussion. Sanyo’s group has held world record efficiency of 23%, despite of, the limitation outside of Sanyo because of improper deposition condition such as doping concentration and so on. E. Conrad et al. suggested an optimal doping concentration (B2 H6 /SiH4 ) of around 2000–3000 ppm [7]. Using simulation, N. Hernández-Como et al. proposed that the efficiency increases with increasing emitter doping concentration. Above a concentration of 3 × 1019 (cm−3 ), the solar cell efficiency reaches its saturation value [8]. Also the emitter thickness variation could determine the short-circuit current as well as built-in potential in case of very thin layer of a-Si : H(p). On raising the emitter thickness, a-Si : H(p) layer incorporated into solar cells acts as a “dead 2 International Journal of Photoenergy layer” and no electrons generated within the emitter layer are extracted due to intense carrier recombination within the defect emitter layer [9]. Reports on the optimum conditions varied in the literature and they can be classified roughly into two groups. Most research groups argue that 4 ∼ 5 nm is thick enough for good device performances [1, 9]. While, emitter thickness of around 15 nm is mentioned to be thin enough by another [10]. In this paper, the compromise conditions for doping concentration, as well as the thickness of emitter layer, were investigated to set up a baseline for single p/n heterojunction solar cells. ds (1.377 nm) a-Si:H(p) db (6.98 nm) SiO2 dSiO (0.002 nm) Si(100) 2. Experiment The commercial Czochralski-grown (CZ) c-Si(n) substrate with <100> orientation, resistivity of 1–10 Ω·cm, and 525 μm thickness has been used to fabricate the HIT solar cells. The crystalline Si substrates were treated by a sequence consisting of (1) acetone/methanol/DIW cleaning, (2) RCA cleaning. Native oxide was removed by a 1 min. dip in 1% hydrofluoric acid right before a-Si : H deposition. To change the doping concentration of the a-Si : H emitter, the gas phase doping concentration, B2 H6 /SiH4 , was varied in range of 2 to 10%, while the thickness of the a-Si : H emitter was fixed at 7 ± 0.05 nm. For the emitter thickness variation set, the gas phase doping concentration was 3%, the optimization condition in previous set, while emitter thickness varied in range of 3–15 nm. For the transparent conductive oxide (TCO), Indium Tin Oxide (ITO) thin film was deposited by rf magnetron sputtering at a substrate temperature of 200◦ C with thickness of about 80 ± 5 nm, followed by the deposition of silver/aluminum finger as the emitter contacts. Aluminum was evaporated on backside to create a good ohmic contact prior to area defining with mesa etching. As confirmed previously [11], the a-Si : H(p) layer thickness controlled by spectroscopy ellipsometry (SE) shows excellent agreement with one evaluated from transmission electron microscopy (TEM). Hence, ellipsometry spectra (ψ, Δ) were collected using a rotating-compensator instrument (J. A. Woollam, HR-190) in this study. For the analysis, we used an optical model consisting of ambient/surface roughness layer (a-Si : H(p))/bulk layer (aSi : H(p))/SiO2 /substrate (n-type c-Si), as shown in Figure 1. The dielectric function of the surface roughness layer was modeled as a 50/50 vol.% mix bulk layer material and voids [12]. The die (...truncated)