How CDTE Solar Cells Operate: Determining Collection using Bifacial Device Characterization

The fundamental mechanisms governing carrier transport and in CdTe solar cells are not well established. Effects of diffusion length (L), depletion width (W), primary heterojunction vs. back junction are not well correlated with the CdTe thickness (t), or back contact. Bifacial analysis provides quantitative insight into CdTe device operation by separating the effects of front and back junction. Back spectral response (SR_B) was analyzed to evaluate L and W. Front spectral response (SR_F) is nearly unaffected by L or W. SR _B and back J_sc are higher for thinner cells as SR _B is limited by diffusion across the field free region that is smaller for thinner cells. Bifacial characterization results indicate a photosensitive back barrier. There is no evidence of a back junction and we conclude that a single junction determines recombination current. These results establish that performance for AM1.5 light through front contact is determined primarily by voltage dependent collection, not diffusion length