Diffusion length in CdTe by measurement of photovoltage spectra in CdS/CdTe solar cells

Modified method of surface photovoltage (SPV), spectral response measuremen t, and constant photocurrent method (CPM) were applied to thin film CdS/CdT e solar cells with the aim of finding diffusion length of minority carriers (L) in the CdTe material. The SPV signal was theoretically calculated with out constraints of absorption coefficients for the incident radiation and t hickness of the sample assuming one space charge region (SCR) located on th e CdS/CdTe interface. In addition to the diffusion length, the SPV is a fun ction of the surface recombination velocity and the parameters of the SCR, which complicates the evaluation. Illuminating the back side of the solar c ell (without ohmic contact) we obtain a photovoltage spectrum predominantly influenced by the diffusion length. On the other hand, the standard measur ement using light penetrating from the CdS side strongly depends on the thi ckness of the SCR. The small signal approximation model presented here succ essfully explains both measured spectra and permits extraction of the diffu sion length of minority carriers and thickness of the SCR in CdTe absorber. The CPM is used for determination of absorption coefficients in the CdTe l ayer. The absorption of this material depends on its preparation and must b e known for correct evaluation of experimental data. (C) 2001 American Inst itute of Physics.