Impact of CdSe thickness on voids and defects in CdSe1-xTex/CdTe solar cells

CdTe thin film solar cells have evolved to incorporate selenium and form a graded bandgap CdSe 1–x Te x absorber layer. While this approach has been reported to passivate electrically active defects through selenium out-diffusion along grain boundaries, its impact on electrically active defects remains incompletely understood. Here, we investigate the effect of increasing Se content, controlled through systematic variation of the CdSe diffusion-layer thickness, on device performance and defect properties. Deep-level transient spectroscopy (DLTS) reveals a progressive deepening of the dominant defect level with increasing Se incorporation, with the trap energy shifting from 230 ± 7 meV in Se-free devices to 355 ± 4 meV in devices with the highest Se content. At high selenium incorporation, significant broadening of the DLTS peak is observed, consistent with the formation of extended defect states. Cross-sectional SEM identifies Kirkendall void formation near the front interface, driven by CdSe/CdTe interdiffusion, which correlates with pronounced photocurrent losses and reduced device efficiency. Contrary to expectations of defect passivation, increased Se incorporation is found to worsen the measurable electrically active defect content, primarily through the introduction of microstructural defect regions. However, opposing trends in device V oc imply secondary effects such as improvement in interfacial recombination may be simultaneously occurring.