Characterization of minority carrier recombination in silicon concentrator solar cells

A critical analysis of the Small Signal Voltage Decay (SSVD) method of lifetime measurement, which is a simple variation of the commonly known Open-Circuit Voltage Decay (OCVD) method, was performed. It was found that for long-based diodes a complementary error function represents the voltage decay more accurately than the commonly assumed exponential decay. For short-based diodes the voltage decay is a summation of exponentially decaying modes. The precautions necessary for proper interpretation of experimental data from SSVD are discussed.;Using the SSVD technique an extensive study of minority carrier recombination in silicon concentrator solar cells was performed. It was generally found that the lifetime decreased with increasing illumination. Theoretical analyses and computer simulations were combined with experiments to isolate the causes of lifetime degradation. It was concluded that increases in emitter and back surface recombination with increasing base excess carrier concentration is responsible for lifetime degradation with illumination.;The difficulty associated with the theoretical analysis of SSVD and OCVD is the fact that the boundary conditions at the emitter-base junction are coupled. A closed-form solution to the emitter-base coupling problem has been obtained assuming a power-law type emitter doping and taking emitter band gap narrowing into account, thereby bringing out the exact dependence of the measured lifetime on factors such as base and emitter volume lifetimes, front and back surface recombination velocities, as well as band gap narrowing in the emitter.