Effect of Interface Traps on the Performance of Heterojunction aSi:H/cSi Thin Film Solar Cells

  • Aaesha Abdulla Alnuaimi

Student thesis: Master's Thesis

Abstract

One of the most attractive technologies for future thin film photovoltaics is a- Si:H/c-Si heterojunction (HIT) solar cells. This combination of crystalline and large band gap amorphous silicon attracts growing attention because of their potential for further efficiency improvement and cost reduction. These cells have recently demonstrated a very high efficiency of around 23%[1] However, at the a-Si:H/c-Si interface there exists interface traps (Dit). These traps are the result of unsaturated bonds (dangling bonds) at c-Si surface. The presence of interface traps (Dit) at the interface degrades the solar cell performance by enhancing generation and recombination of charge carriers. In the first part of the thesis, Physics Based TCAD simulation is used to investigate the effect of interface traps (Dit) at the a-Si:H/c-Si interface on the performance of a-Si:H(n+)/i-aSi:H/cSi(p) HIT solar cells. The simulation focuses on finding the relationship between interface trap (Dit) and the quality of c-Si absorber layer. The results show a stronger effect of Dit with increasing the quality of c-Si absorber layer. This highlights the tradeoff between material layer quality and interface traps in thin film a-Si:H/c-Si based solar cells and more significantly the importance of achieving a very high quality a-Si:H/c- Si interface. Furthermore, effective passivation of the c-Si surface is important to reduce the effect of Dit and improve the performance of aSi:H/cSi HIT solar cells. Forming Gas Annealing (FGA) is considered a highly compatible and inexpensive process to passivate the a-Si:H/c-Si interface thus reducing the Dit. In the second part of the thesis, aSi:H(n+)/i-aSi:H/cSi(p) HIT solar cells were fabricated to study the effect of Forming Gas Annealing on the performance of the cells. Fabricated solar cells were exposed to a mixed gas of N2:H2 = 95%:5% for 10, 20 and 25 min at 400°C. The effect of Forming Gas Annealing on the performance of thin film a-Si:H(n+)/c- Si(p)/c-Si(p+) heterojunction solar cells is investigated by measuring the IV characteristics before and after annealing. The results show a significant improvement in all key solar cells parameters due to the FGA annealing. In addition the longer the FGA times the larger the improvement. Finally, these results present a promising method of reducing Dit at the a-Si:H/c-Si interface used for future thin film HIT solar cells.
Date of AwardJun 2013
Original languageAmerican English
SupervisorMarcus Dahlem (Supervisor)

Keywords

  • Heterojunction; Silicon; Photovoltaic Cells; Amorphous Silicon.

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