Development of high efficiency, low degradation tandem solar cell using protocrystalline silicon (pc-Si:H) multilayer

Abstract

The purpose of this work is to develop thin film silicon solar cell that has a high conversion efficiency and low light-induced degradation ratio. In order to achieve this object, tandem solar cells were developed by using a protocrystalline silicon (pc-Si:H) multilayer solar cell. The pc-Si:H multilayer solar cell was proposed by our research group in 2000. It shows a fast light-induced metastbility with a low degradation ratio thanks to its layer by layer structure and nanosized silicon grains included in its highly hydrogen diluted sublayers. The tandem solar cell is a series stack of two solar cells. It improves the conversion efficiency by more efficient utilization of solar spectrum. Firstly, the pc-Si:H multilayer/pc-Si:H multilayer tandem solar cell was fabricated. As subtechnology to develop tandem solar cell, an n-p-p tunnel recombination junction was proposed. In this tunnel recombination junction structure, the hole injection is improved by valence band edge grading. The usefulness of it was verified by applying it to the tandem solar cell. By employing this tunnel recombination junction, tandem solar cell performance was improved by 16.0 %. Initial conversion efficiency of pc-Si:H multilayer/pc-Si:H multilayer tandem solar cell was 8.19 %. It was stabilized very fast within 1-hour light irradiation. It showed very low light-induced degradation ratio of 10.0 %. Secondly, a μc-Si:H solar cell was researched. It was fabricated by 13.56 MHz conventional RF-PECVD method at high process pressure condition. Electrical and microstructural characteristics of μc-Si:H film was researched depending on several experimental parameters such as rf-plasma source power, silace concentration and substrate temperature. Conversion efficiency of μc-Si:H single junction solar cell was 7.76 %. Thirdly, in order to achieve high conversion efficiency, the hybrid tandem solar cell was fabricated which stacks pc-Si:H multilayer solar cell and μc-Si:H solar cell in s...