The spectral response and efficiency of heavily doped emitters in silicon photovoltaic devices

Abstract

An analytical model of the spectral response and efficiency of an emitter is developed. The model is valid for an emitter of any thickness or any doping concentration and profiles. Explicit expression for the spectral response and efficiency of the emitter are derived. They take into account the space dependent mobility μ p , band gap narrowing ΔE g, lifetime and electric field. The expression reduces to simpler forms for quasi-transparent and transparent emitters. It is found that unlike a dark emitter, the effect of electric field on the behavior of the illuminated emitter is not negligible. The effect of change in ΔE g and μ p models on the spectral response and efficiency of the emitter is discussed. An increase in ΔE g degrades the performance of the emitter. An increase in μ p at high doping also degrades the behavior but only if the surface recombination velocity S p is large. If S p is very small, an increase of μ p improves the spectral response and the efficiency of the emitter. The results calculated theoretically are in good agreement with the experimental results of Ref. [25] both for thin and thick emitters except for λ ≤ 0.4 μm and λ > 1.0 μm. This discrepancy is attributed to the uncertainties in the experimental values of α at these wavelengths. It is shown that the value of μ p at high dopings can be determined from the spectral response measurements provided that the value of S p can be determined accurately from other independent measurements.