Si1-xGex Thin Film Heterojunction Solar Cells

  • Sabina Abdul Hadi

Student thesis: Master's Thesis

Abstract

Research into alternative energy is of vital interest due to global warming and the inevitable decline in current energy sources. Therefore, it is prudent to develop novel devices to extract alternative energy from renewable resources, and to recycle energy where possible. In solar cell research, there is a long-standing search to achieve cost parity with fossil fuels, maximizing energy conversion efficiency with minimal cost. One of the latest trends in solar industry is high quality crystalline thin film solar cells, which use fewer raw materials and have competitive efficiencies. Si cells do not absorb most of the sunlight spectrum, leaving space for improvement. Incorporating a semiconductor with a smaller band-gap such as Si1-xGex, in order to absorb more of the solar spectrum, is an attractive way to increase the output current and efficiency of thin film solar cells. The challenge in using Si1-xGex is the reduction in open circuit voltage (Voc) due to increased dark current from the smaller band-gap, and the formation of dislocations due to the lattice mismatch with Si. In this study a novel heterojunction emitter based solar cell (HIT) with large band gap (1.7 eV) amorphous Si (a-Si) emitter is used in Si1-xGex based cells in an effort to improve Voc. In this work, a-Si(n+)/c-Si1-xGex(p)/c-Si(p+) heterojunction solar cells with junction layers consisting of Si, Si0.75Ge0.25, Si0.59Ge0.41 and Si0.44Ge0.56 are fabricated and compared to study the effect of increasing Ge concentration and absorber layer thickness. The measured short-circuit current increases from ~14 mA/cm2 for Si to 21 mA/cm2 for Si0.44Ge0.56 cells, for one light pass and a 2 μm-thick absorber layer. The results show Voc of 0.61 V for Si cells, dropping to 0.32 V for Si0.44Ge0.56, consistent with the band-gap reduction. Experimental results combined with physics based TCAD simulation provide insight into the behavior of these cells and the required material and interface quality, enabling an estimate of the SiGe lifetime (~1 μs) and effective surface recombination velocity (~103 cm/s). The results indicate that SiGe with Ge fraction > 40% is promising for application to thin film solar cells.
Date of Award2012
Original languageAmerican English
SupervisorAmmar Nayfeh (Supervisor)

Keywords

  • Heterojunctions
  • Solar Cells

Cite this

'