Optimization of a-SiGe solar cells for tandem structures

This paper systematically studies the effect of the a-SiGe:H band gap on the solar cell performance. We found that the Ge incorporation efficiency drops with increasing the GeH₄/SiH₄ gas ratio. The Ge incorporation efficiency is the ratio between the Ge at.% to the GeH₄/SiH₄ gas ratio and it starts from 4 at 5% gas ratio and drops to 1% at 80 % gas ratio. Therefore, it is preferable to use small gas ratios to enhance the Ge incorporation efficiency as the germane gas is expensive. The experimentally fabricated solar cells are compared to a numerical device simulation performed by 6-1D computer software. Both experimental and simulated results have the same behavior. Both simulation and experiment show that the best cell performance is achieved at energy gap E_g = 1.4 eV, which is realized at 20% GeH₄/SiH₄ gas ratio and corresponds to 50% Ge at.%. The optimum deposition is achieved at a moderate Ge incorporation efficiency of 2.5. We introduce also a method to enhance the electronic quality of the layer without affecting its band gap by increasing the hydrogen dilution ratio during film growth. More dilution show lower band tails.