Analysis of a-Si:H/TCO contact resistance for the Si heterojunction back-contact solar cell Seung-Yoon Lee a,b , Hongsik Choi b , Hongmei Li a , Kwangsun Ji a , Seunghoon Nam b , Junghoon Choi a , Seh-Won Ahn a , Heon-Min Lee a , Byungwoo Park b,n a Device Materials and Component Laboratory, LG Electronics Advanced Research Institute, Seoul 137-724, Republic of Korea b WCU Hybrid Materials Program, Department of Materials Science and Engineering, Research Institute of Advanced Materials, Seoul National University, Seoul 151-744, Republic of Korea article info Available online 15 July 2013 Keywords: Amorphous Si Heterojunction back-contact solar cell Contact resistance abstract The contact resistance of amorphous Si (a-Si:H)/transparent-conducting oxide (TCO) is evaluated and analyzed in terms of the contribution to the series resistance (R s ) and fill factor (FF) in the Si heterojunction back-contact (HBC) solar cell. It is shown that p-a-Si:H (emitter) and n-a-Si:H (back surface field: BSF)/TCO contact resistance are of similar values (0.37–38 Ω cm 2 ) which are much higher than those of doped crystalline Si/metal contacts used in conventional Si solar cells. Of some factors affecting R s loss in the HBC solar cell, BSF/TCO contact is the most significant one when considering the contact area. By interleaving the n-type microcrystalline Si (n-μc-Si) between n-a-Si:H and TCO, 6-inch HBC solar cell with 20.5% efficiency is obtained, which was attributed to the reduced R s and improved FF. It is noteworthy that the variations of R s and FF are well estimated by measuring BSF-contact resistance, and are close to the empirical data: reduction in R s to 1.77 Ω cm 2 and the increase in FF by 6.0% compared to the cell without n-μc-Si interface layer. The results indicate that there is much room for higher efficiency by reducing the emitter- and BSF-contact resistance. Nonetheless, the method developed here can be a powerful tool to analyze the resistance component in HBC cell. & 2013 Elsevier B.V. All rights reserved. 1. Introduction Hydrogenated amorphous Si/crystalline Si (a-Si:H/c-Si) hetero- junction is one of the powerful scheme to enhance the open- circuit voltage (V oc ) and the conversion efficiency of the Si solar cell. Recently, a-Si:H heterojunction solar cell has shown very high efficiency potential over 24%, like Panasonic Corporation′s HIT (heterojunction with intrinsic thin layer) cell [1] even with practical-sized Czochralski-grown (CZ) wafer. The a-Si:H hetero- junction back-contact (HBC) solar cell is hybridized concept using both heterojunction and back-contact structure, which can take advantages from each cell, with high V oc from heterojunction and high J sc from no-front-shadowing back-contact structure [2]. Even though several institutes have achieved some improvements [3–7], HBC cell is still far from its expected efficiency potential over 25%. The main bottleneck for higher efficiency is usually lower fill factor (FF) and higher series resistance (R s ) [3–7], compared with the conventional Si solar cell with diffused emitter homojunction and metal–Si direct contacts. Considering the empirical relationship between V oc and FF, ideal FF should be higher in larger V oc cell [8]. However, a-Si:H/c-Si heterojunction cells usually have similar or less FF rather than the conventional Si solar cells. Therefore, it can be thought that FF loss by R s is more significant in heterojunction cells than the conven- tional solar cells. There have been several analytical and numerical researches on V oc and FF losses in various heterojunction solar cells: Schottky barriers between p-a-Si:H emitter and contact (TCO or metal) [9,10], minority-carrier transport barrier by the rear intrinsic a-Si:H buffer layer [4], resistive losses [5], etc. However, quantitative analysis of resistance loss and its effect on FF in the actual solar cells were hardly ever studied, especially in terms of n-type a-Si:H (n-a-Si:H) back-surface field (BSF) contact. In this study, series resistive components in both a-Si:H/c-Si heterojunc- tion emitter-contact and BSF-contact of the HBC cell were ana- lyzed and discussed. As previously reported, microcrystalline Si (μc-Si) layer between a-Si:H and TCO was effective for enhan- cing the FF of the HBC cell [7], and the effect of inserted microcrystalline interface layer was confirmed in a quantitative manner. Furthermore, the contributions of R s from both the emitter- and BSF-contact to the enhanced cell efficiency are quantitativly evaluated by considering the contact-area fraction, and finally, the FF loss by R s in the HBC cell is also discussed. Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/solmat Solar Energy Materials & Solar Cells 0927-0248/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.solmat.2013.06.026 n Corresponding author: Tel.: +82 2 880 8319; fax: +82 2 885 9671. E-mail address: byungwoo@snu.ac.kr (B. Park). Solar Energy Materials & Solar Cells 120 (2014) 412–416