Fabrication of p-i-n solar cells utilizing ZnInON by RF magnetron sputtering
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Fabrication of p-i-n solar cells utilizing ZnInON by RF magnetron sputtering Koichi Matsushima1, Ryota Shimizu1, Tomoaki Ide1, Daisuke Yamashita1, Hyunwoong Seo1, Kazunori Koga1, Masaharu Shiratani1, and Naho Itagaki1, 2 1 Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819-0385, Japan 2 PRESTO, Japan science and Technology Agency, Sanbancho 5, Chiyoda-ku, Tokyo 102-0075, Japan ABSTRACT We succeeded in photovoltaic power generation of p-i-n solar cells utilizing epitaxial ZnInON film with a wide band gap of 3.1 eV as the intrinsic layer, suitable for a top cell of tandem solar cells. The solar cell shows a high open circuit voltage (Voc) of 1.68 V under solar simulator light irradiation of 3.2 mW/cm2. The solar cell performance becomes worse under 100 mW/cm2, which is mainly attributed to the leakage current caused by crystal defects and grain boundaries. X-ray diffraction analysis reveals that the ZnInON film has rather large tilt and twist angles and a high dislocation density of 7.62×1010 cm-2. Such low crystallinity is a bottleneck for high performance of the solar cells. Our results demonstrate a potential of epitaxial ZnInON films as an intrinsic layer of wide band gap p-i-n solar cells with a high Voc. INTRODUCTION Building-Integrated Photovoltaics (BIPV) have attracted attention because BIPV can reduce the overall cost by using part of building facade [1, 2]. The transparent solar cells can be integrated into windows because the solar cells can absorb ultraviolet light without absorbing visible light [3]. This application requires low temperature fabrication of solar cells with wide band gap materials. It is commonly difficult to fabricate films with high crystal quality at low temperature. Recently, we have succeeded in fabricating ZnInON films with high crystal quality at low temperature of 250oC [4]. ZnInON semiconductor is a quasi-binary alloy of wurtzite ZnO and wurtzite InN [5, 6]. ZnInON has a direct and tunable band gap from 1.6 eV to 3.3 eV and a high absorption coefficient of 105 cm-1 [4, 7]. Compared with other tunable bandgap materials such as InGaN, which is generally grown by metal-organic chemical vapor deposition method, which is not suitable for large-area fabrication, ZnInON has an advantage that ZnInON can be fabricated by a conventional sputtering method, which enables low-cost and large-area fabrication. Therefore ZnInON is a candidate material for BIPV window application as well as a top cell of tandem solar cells after improving the photovoltaic performance. Further study is needed for such improvements. Here we demonstrate photovoltaic power generation of p-i-n solar cells using epitaxial ZnInON film with a wide band gap of 3.1 eV as the intrinsic layer and discuss bottleneck of the performance of the cells. EXPERIMENTAL DETAILS The solar cells were composed of n-type ZnO:Al (AZO), ZnInON, and p-type GaN template. ZnInON films of 300 nm in thickness were deposited on commercial p-GaN templates by RF magnetron sputtering in Ar-O2-N2 atmosphere. In this study, we used commercial p-typ
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