CdSeTe/CdTe thin film solar cell degradation induced by Se diffusion

The introduction of Se in CdTe to form CdSeTe/CdTe bilayer absorption film is a key progress to fabricate high-efficiency CdTe thin film solar cell. Se element exists in the front absorption layer with a relatively large concentration gradient, and therefore it would spontaneously diffuse into absorption layer, imposing long-term device stability problem. In this study, we fabricated CdSeTe/CdTe solar cells with different Se source reservoirs by thermal evaporation of CdSe layers with different thickness, followed by CdTe deposition using close-spaced sublimation (CSS) technique. It is found that insufficient amount of Se in the freshly fabricated solar cells, even though the cells demonstrated high efficiency, led to rapid device degradation. This was induced by the degraded junction of FTO/SnO 2 /CdSeTe, which was transformed from a high-quality rectifying diode to a heavily recombination-dominated non-rectifying junction. The diode performance change was induced by the Se diffusion from the front CdSeTe absorption layer deep into the absorption film. Se/Te inter-diffusion was highly vacancy-mediated, leading to formation of micro-voids at the front interface. This synergistic loss of Se passivation and presentation of micro-voids led to formation of a heavily defected FTO/SnO 2 /CdSeTe heterojunction. The solar cells with sufficient Se source reservoir were able to maintain the passivation of the deep-level defects at the SnO 2 /CdSeTe junction during/after Se diffusion, resulting in long-term device stability. This work clearly demonstrates that thermal stability of Se element gradient at the front CdSeTe absorption layer is one of the key factors to fabricate highly stable CdSeTe/CdTe thin film solar cells.