Effects of as-deposited CdTe microstructure on solar cell performance

In this study, two series of CdTe thin films were prepared by magnetron sputtering and analyzed in-situ using real time spectroscopic ellipsometry (RTSE). One series consists of CdTe thin films deposited at substrate temperatures ranging from 188°C to 304°C at a fixed Ar sputtering pressure of 18 mTorr, and the other consists of films deposited at pressures ranging from 2.5 to 50 mTorr at a fixed temperature of 230°C The RTSE data provide detailed information on nucleation and coalescence, as well as on the long term surface roughness development for these films. The results show that for films deposited at T_s ≥ 267°C and p_Ar<; 10 mTorr, initial growth occurs in the form of a thin bulk-like layer and is followed by clustering, whereas for films deposited at T_s ≤ 237°C and p_Ar> 10 mTorr, clustering occurs before any bulk-like layer forms. This behavior is consistent with trends observed at the end of the deposition to the extent that the final surface roughness is observed to decrease monotonically with temperature over the full range and to increase with pressure over the range from 2.5 to 18 mTorr. Trends identified by RTSE were confirmed through scanning electron and atomic force microscopies. In order to correlate the observed RTSE results with solar cell performance, devices fabricated by varying the Ar pressure for the CdTe layer were studied in detail. These devices were subjected to postdeposition CdCl₂ treatments at 387°C for different times ranging from 15 to 35 minutes. Overall, the solar cells deposited at p_Ar = 10 mTorr exhibited the best overall performance, independent of treatment time. A further study of the optical properties of CdTe thin films deposited as a function of substrate temperature was undertaken in order to study differences in strain and grain size before and after a post-deposition CdCl₂ t- - reatment. This study shows that a post-deposition CdCl₂ treatment leads to a shift in the critical point energies to values which are closer to those of single crystal CdTe, hence resulting in a strain relaxation in the network.