3048 IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 58, NO. 9, SEPTEMBER 2011 Amorphous Si Rear Schottky Junction Solar Cell With a LiF/Al Back Electrode Liang Fang, Seung Jae Baik, Sooyeon Lim, Seunghyup Yoo, and Koeng Su Lim Abstract—Amorphous Si (a-Si) rear Schottky junction solar cells with a LiF/Al back electrode are proposed as an alternative prototype for high-efficiency thin-film photovoltaics. This device is free from absorption losses occurring at the rear n-type a-Si layer, and thus, overall power conversion efficiency was improved by 13% compared with a conventional p-i-n type solar cell. An ultrathin LiF layer between the absorber and the rear electrode reduces shunt leakage, as well as series resistance; this, in turn, suppresses degradation of the open-circuit voltage and the fill fac- tor while enhancing photocarrier collection in the long-wavelength regime. Index Terms—Absorption loss, amorphous Si (a-Si) solar cell, LiF, Schottky junction. I. I NTRODUCTION S INCE the historical demonstration of gas-phase doping of amorphous Si (a-Si) thin films [1], it has been commonly accepted that the p-i-n configuration is the optimal implemen- tation of a-Si solar cells. In addition, due to their simplicity of fabrication, Schottky junction solar cells have been imple- mented for the assessment of emerging photovoltaic materials [2], as they have played a role in the history of a-Si solar cell development [3]. One of the critical drawbacks of the p-i-n configuration is that doped layers do not directly contribute to power conversion. However, it has been recently pointed out that power conversion of the light absorption in the p-type window layer is feasible by tailoring band alignment of the front electrode junction [4], and power loss from the absorption at the n-type layer could be minimized by employing wideband gap materials such as silicon oxides [5]. In rear Schottky junction solar cells, the n-type layer is no longer necessary for the generation of an internal electric field, and thus, the power loss originating from Manuscript received January 27, 2011; accepted June 13, 2011. Date of current version August 24, 2011. This work was supported in part by the Korea Science and Engineering Foundation under Grant 2008-0062241, by the Ministry of Education, Science and Technology and the Ministry of Knowledge Economy of the Korean Government under New Renewable Energy Project 2008-373, and by the School of Information Technology, Korea Advanced Institute for Science and Technology under Brain Korea 21 Project in 2010. The work of S. Yoo and S. Lim was supported by Korea Energy Management Corporation under New and Renewable Energy R&D Grant 2008-N-PV08-02. The review of this paper was arranged by Editor S. A. Ringel. L. Fang, S. J. Baik, S. Yoo, and K. S. Lim are with the Department of Electrical Engineering, Korea Advanced Institute of Science and Technology, Daejeon 305-701, Korea (e-mail: solar100@kaist.ac.kr). S. Lim is with Samsung Electronics Company, Gyeonggi-Do 445-701, Korea. Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TED.2011.2160267 the n-type a-Si (n-a-Si) layer can be completely eliminated. Moreover, this is a more appropriate implementation than classical front Schottky junction a-Si solar cells, which show significant absorption loss due to the thin layer of the front metal [3]. On the other hand, in forming a Schottky junction, Fermi-level pinning caused by metal or disorder-induced gap states [6], [7] cause degradation of the open circuit voltage V oc and the fill factor FF of solar cells. To circumvent these problems, separating the a-Si absorber and the Al rear electrode is necessary, which could be realized by inserting a dielectric layer between the absorber and the rear electrode. Nevertheless, the insertion of an insulating dielectric would cause a drastic increase in series resistance. It has been well documented in organic electron device literature that LiF has a high dipole moment in a molecular scale [8] and that the work function of Al is effectively reduced once LiF is coupled to Al [9]. Accordingly, a LiF/Al electrode has been widely adopted for the cathode in organic light-emitting diodes [10] and organic solar cells [11]. This dipole–electrode combination could be also applied to inorganic devices [12] in order to attain a metal/insulator dielectric/semiconductor Schottky junction with a sizable reduction of series resistance. In this paper, a-Si solar cells with a rear Schottky junction are fabricated using a LiF/Al back electrode to achieve carrier collection enhancement in the long-wavelength regime, and the implications of the proposed device scheme are discussed in terms of attaining further improvement of the power conversion efficiency of a-Si solar cells. II. EXPERIMENTAL Commercially available textured fluorine-doped SnO 2 (SnO 2 : F) on glass (Asahi U-type) was used as the front electrode of the fabricated solar cells. Photochemical vapor deposition was used for the deposition of p-type amorphous SiC (p-a-SiC) and n-a-Si, and plasma-enhanced chemical vapor deposition was used for the deposition of intrinsic a-Si. Prior to the deposition of LiF/Al by thermal evaporation, the surface of intrinsic a-Si is exposed to air ambient for several minutes. LiF with various thicknesses ranging from 0.4 to 2 nm is deposited on the air-exposed intrinsic a-Si to investigate the optimal thickness of LiF. Meanwhile, n-a-Si with various thicknesses ranging from 10 to 40 nm is deposited on the intrinsic a-Si for reference samples. Two different types of reference p-i-n solar cells were fabricated, i.e., one type was exposed to air for several minutes before n-a-Si deposition, and the other was fabricated in a continuous vacuum process. The thickness of thermally evaporated LiF was measured by 0018-9383/$26.00 © 2011 IEEE