Light-induced Open-circuit Voltage Increase in Amorphous Silicon/Microcrystalline Silicon Tandem Solar Cells
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Light-induced Open-circuit Voltage Increase in Amorphous Silicon/Microcrystalline Silicon Tandem Solar Cells Xiaodan Zhang, Guanghong Wang, Shengzhi Xu, Shaozhen Xiong, Xinhua Geng and Ying Zhao Institute of Photo-electronic Thin Film Devices and Technology of Nankai University, Weijin Road 94#, Nankai District, Tianjin 300071, P.R.China ABSTRACT Light-induced metastability of amorphous/microcrystalline (micromorph) silicon tandem solar cell, in which the microcrystalline bottom cell was deposited in a single-chamber system, has been studied under a white light for more than 1000 hours. Two different light-induced metastable behaviors were observed. The first type was the conventional light-induced degradation, where the open-circuit voltage (Voc), fill factor (FF), and short-circuit current density (Jsc) were degraded, hence the efficiency was degraded as well. This phenomenon was observed mainly in the tandem cells with a bottom cell limited current mismatch. The second type was with a light-induced increase in Voc, which sometimes resulted in an increase in efficiency. The second type of light-induced metastability was observed in the tandem cells with a top cell limited current mismatch. The possible mechanisms for these phenomena are discussed. INTRODUCTION Hydrogenated amorphous silicon (a-Si: H)/microcrystalline (micromorph) silicon (ȝc-Si: H) tandem solar cell structure, which can utilize more solar spectrum [1], has been considered as one of the next generation thin film silicon based solar cell structures. In order to make this technique more cost effective and competitive than other techniques, one need to further improve the cell efficiency, decrease the equipment cost, and minimize the light-induced degradation in cell efficiency. Single-chamber plasma-enhanced-chemical-vapor deposition (PECVD) technique, even though it has a cross contamination problem [2], is considered to be a low cost technique compared to multi-chamber systems. Light-induced defect generation in a-Si: H, known as the Staebler-Wronski effect, causes the degradation in a-Si: H solar cells [3]. The metastable defects are not only induced by prolonged illumination [4, 5] but also by charge accumulation [6] or carrier injection [7, 8]. The light-induced defects reduce the mobility-lifetime product of the carriers, thereby degrading solar cell performance. The ȝc-Si: H solar cells have attracted a remarkable attention due to its ability to absorb long wavelength light. Previously, several groups reported that no light-induced degradation in ȝc-Si: H solar cells were observed. [9-11]. Recently, several groups reported that ȝc-Si: H solar cells also showed light-induced degradations, where the light-induced efficiency degradations are in the range of 2%-15% [12-17]. It was found that the light-induced degradation is dominated by the amount of amorphous component in the ȝc-Si: H intrinsic layer [16, 17]. However, it is interesting that Yue et al. [18] found that the volume fraction of the amorphous component is not necessarily the determini
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