P -layer Optimization in High Performance a-Si:H Solar Cells
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P-LAYER OPTIMIZATION IN HIGH-PERFORMANCE A-SI:H SOLAR CELLS Yueqin Xu1, Bill Nemeth1, Falah Hasoon1, Lusheng Hong2, Anna Duda1, and Qi Wang1 1 National Renewable Energy Laboratory (NREL), Golden, CO 80401, USA 2 National Taiwan University of Science and Technology, Taipei, Taiwan
ABSTRACT We report our progress toward high-performance hydrogenated amorphous silicon (aSi:H) solar cells fabricated in NREL’s newly installed multi-chamber film Si deposition system. The a-Si:H layers are made by standard radio frequency plasma-enhanced chemical vapor deposition. This system produces a-Si:H p-i-n single-junction devices on Asahi U-type transparent conducting oxide glass with >10% initial efficiency. The importance of the p-layer to the cell is identified: it plays a critical role in further improving cell performance. Our optimization process involves changing p-layer parameters such as dopant levels, bandgap, and thickness in cells as well as applying a double p-layer. With the optimized p-layer, we are able to increase the fill factor of our cells to as high as 72% while maintaining high open-circuit voltage.
INTRODUCTION Hydrogenated amorphous silicon (a-Si:H) has long been studied as a unique thin-film material for a variety of applications, especially photovoltaics. Amorphous Si-based solar cells have evolved from single junction to tandem to advanced triple-junction structures. Despite the well-known Staebler-Wronski deterioration of cell performance [1], improvements have been made in the development of cells, and the best laboratory stable cell efficiency ranges up to 12.5% for multijunction configurations [2]. In addition, a-Si:H-based solar cell production has increased significantly in recent years along with rising global photovoltaic production. Despite the increasing interest in a-Si:H-based solar cells as a source for power, these devices still have many facets requiring further research and understanding to improve cell efficiency. These include minimizing light-induced degradation, managing light trapping [3], engineering bandgaps and interfaces, and improving p-layers. The p-layer in an a-Si:H solar cell serves to establish the built-in potential of a device electrically as well as act as a window layer for optical transmission of incident light into the device. In a superstrate configuration, the p-layer requires a transparent conducting substrate, which can be accomplished in many ways. Ideally, the p-layer will allow all usable light to pass into the absorbing intrinsic layer of the device and interact with the transparent conducting substrate to promote efficient hole extraction. Typical p-layers employ methods to minimize optical absorption by either incorporating carbon or hydrogen to widen the bandgap or create proto- or microcrystalline phases. Boron, a typical p-type dopant, can be found in several precursor gases used in plasma-enhanced chemical vapor deposition (PECVD) such as diborane (B2H6) and trimethylboron (TMB). Additionally, the deposition of the p-layer should not damage
