Copper migration in CdTe heterojunction solar cells

CdTe solar cells were fabricated by depositing a Au/Cu contact with Cu thickness in the range of 50 to 150 Angstrom on polycrystalline CdTe/CdS/SnO2/glass structures. The increase in Cu thickness improves ohmic contact and reduces series resistance (R(s)), but the excess Cu tends to diffuse into CdTe and lower shunt resistance (R(sh)) and cell performance. Light I-V and secondary ion mass spectroscopy (SIMS) measurements were performed to understand the correlations between the Cu contact thickness, the extent of Cu incorporation in the CdTe cells, and its impact on the cell performance. The CdTe/CdS/SnO2/glass, CdTe/CdS/GaAs, and CdTe/GaAs structures were prepared in an attempt to achieve CdTe films with different degrees of crystallinity and grain size. A large grain polycrystalline CdTe thin film solar cell was obtained for the first time by selective etching the GaAs substrate coupled with the film transfer onto a glass substrate. SIMS measurement showed that poor crystallinity and smaller grain size of the CdTe film promotes Cu diffusion and decreases the cell performance. Therefore, grain boundaries are the main conduits for Cu migration and larger CdTe grain size or alternate method of contact formation can mitigate the adverse effect of Cu and improve the cell performance.