Mapping spectroscopic ellipsometry of CdTe solar cells for property-performance correlations

Mapping spectroscopic ellipsometry (M-SE) has been applied for optimization of polycrystalline CdS/CdTe solar cell fabrication on transparent conducting oxide (TCO) coated glass superstrates. During fabrication of these solar cells, the structure undergoes key processing steps after the sputter-deposition of the CdS/CdTe. These steps include CdCl₂ treatment of the CdTe layer and subsequent deposition of ultrathin Cu. Additional steps involve final metal back contact layer deposition and an anneal for Cu diffusion that completes the device. In this study, we have fabricated cells with variable absorber thickness, ranging from 0.5 to 2.5 μm, and variable CdCl2 treatment time, ranging from 5 to 30 min. Because both CdS window and Cu back contact layers are critical for determining device performance, an understanding of their deposition processes and process-property-performance relationships is important for device optimization. We have applied M-SE to map the effective thickness (volume/area) of the CdS and Cu films over 15 cm × 15 cm substrates prior to the fabrication of 16 × 16 arrays of dot cells. The model for M-SE analysis has been established using single-spot real time SE (RT-SE). We report correlations of cell performance parameters with the CdCl₂ treatment time and with the effective thicknesses from M-SE analysis. We demonstrate that correlations between optical/structural parameters extracted from M-SE analysis and device performance parameters facilitate process optimization.