Improving Performance of Amorphous Silicon Solar Cells Using Tungsten Oxide as a Novel Buffer Layer between the SnO 2 /p
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1245-A07-05
Improving Performance of Amorphous Silicon Solar Cells Using Tungsten Oxide as a Novel Buffer Layer between the SnO2/p-a-SiC Interface Liang Fang, Seung Jae Baik, Koeng Su Lim , Seung Hyup Yoo, Myung Soo Seo and Sang Jung Kang Department of Electrical Engineering, KAIST, 335 Gwahak-ro, Yuseong-gu, Daejeon 305-701, Republic of Korea ABSTRACT A thermally evaporated p-type amorphous tungsten oxide (p-a-WO3) film was introduced as a novel buffer layer between SnO2 and p-type amorphous silicon carbide (p-a-SiC) of pin-type amorphous silicon (a-Si) based solar cells. By using this film, a-Si solar cells with a p-a-WO3/pa-SiC double p-layer structure were fabricated and the cell photovoltaic characteristics were investigated as a function of p-a-WO3 layer thickness. By inserting a 2 nm-thick p-a-WO3 layer between SnO2 and a 6 nm-thick p-a-SiC layer, the short circuit current density increased from 9.73 to 10.57 mA/cm2, and the conversion efficiency was enhanced from 5.17 % to 5.98 %. INTRODUCTION One of the strategies to improve the efficiency of pin-type amorphous silicon (a-Si) based solar cells is to insert a buffer layer between the p-type layer and transparent conducting oxide (TCO) electrode, or between the p-type and active layers. With a suitable material and configuration, the buffer layer can benefit a-Si:H solar cells by preventing recombination loss at the p/i interface [1-3], the carbon-alloyed amorphous silicon carbide (a-SiC:H) buffer layer has been developed to solve the heterojunction interface problem between the p-type a-SiC:H (p-aSiC:H) window and intrinsic amorphous silicon (i-a-Si:H) active layers in a-Si:H based pin type thin film solar cell. Because the structure misfit at the p-a-SiC:H/i-a-Si:H interface could be reduced effectively by using this buffer layer, the open circuit voltage (Voc) and short wavelength response could be dramatically enhanced, and thus a high efficiency of 11.2 % was obtained (by using an Ag back electrode) [3]; or by enhancing hole collection via lowering of the interface potential barrier [4]. However, the high defective p-a-SiC:H window and a-DLC:H buffer layers with a short carrier life time, and newly formed interface between the window and buffer layers still generate the considerable recombination loss of photo-generated carriers. In addition, the serious resistance of the cell, originated from the low dark conductivity (σd) value of the buffer layer, severely limits the overall cell performance. Recently, an amorphous tungsten oxide (aWO3) has been successfully used as a buffer layer for an organic solar cell [5] and organic light emitting diode (OLED) [6]. In this letter, we investigate the effects of inserting an a-WO3 buffer
layer between SnO2 and p-type a-SiC (p-a-SiC) on the a-Si solar cell performance. Furthermore, in order to gain insight into the buffering effect of the a-WO3 layer, a transport analysis based on the Schottky barrier model was performed. EXPERIMENTAL WO3 films were thermally deposited on bare glass (Corning 7059) and surface te
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