EVALUATION AND IMPROVEMENT OF CARRIER COLLECTION EFFICIENCY IN AMORPHOUS SILICON-GERMANIUM SOLAR CELLS (PHOTOVOLTAIC, SPECTRAL RESPONSE, AFFINITY GRADIENT)

A new method is developed to extract spatial information from the spectral response or collection efficiency of a solar cell, without requiring knowledge of material or device transport parameters such as carrier diffusion length, carrier lifetime, internal electric field, etc. The new technique yields the probability of collection of an electron-hole pair as a function of the position in the solar cell where that pair is generated. Only the internal collection efficiency, the optical absorption coefficient, the thickness of the cell and the coefficient of reflection from the backwall are required as starting information. Regions within the solar cell which are responsible for transport losses are found quantitatively using the method.; Using the spectral response deconvolution technique to study amorphous silicon solar cells confirmed the existence of a large carrier transport loss due to the back diffusion of carriers near the front contacting layer. A new design of solar cell is proposed, using affinity gradients to minimize the losses due to back diffusion. The new design is tested using amorphous solar cells fabricated in the silicon-germanium alloy system. The desired affinity gradient is achieved by varying the composition of the alloy during deposition. The new design results in marked improvement in every photovoltaic parameter and an improvement in efficiency of 30% compared to an identical solar cell without the affinity gradient. The efficiency of the amorphous silicon-germanium graded affinity cell is also higher than that of the comparable a-Si solar cell.