Injection dependent minority carrier lifetime and defect configuration in thin film CdTe solar cells

Abstract

Although in CdS/CdTe thin film solar cells, experiments showed that the minority carrier lifetime depends on excess carrier concentration, it is not known that under steady state how the lifetime is affected by the defects. High defect concentration in CdS/CdTe solar cells violates the assumption needed to simplify of the Shockley Read Hall (SRH) recombination equation. Also, with high defect concentrations and thus short carrier lifetime, the excess carrier concentration under illumination can be equivalent to or even lower than the defect concentrations. By simplifying SRH equation considering high defect concentration, it is found that in neutral region, the lifetime is excess carrier dependent. By simultaneously solving the equations of charge neutrality, charge conservation and SRH recombination in the neutral region, the minority carrier lifetime as a function of generation rate can be calculated. The measured minority carrier diffusion length in a CdS/CdTe solar cell, as determined from the steady-state photo-generated carrier collection efficiency, shows the predicted transition of minority carrier lifetime versus optical injection level. A numerical fitting of the indirectly-measured minority carrier lifetime by assuming the minority carrier mobility gives a non-intuitive picture of the p-n junction with a low free hole concentration but a narrow depletion region width.