How the absorber thickness affects the electrical and structural properties of Sb2Se3 solar cells

This work investigates how the absorber thickness and a mild post-annealing treatment (PAT) in air jointly affect the structural and electrical properties of thermally evaporated Sb2Se3 absorbers, and the performance of superstrate solar cells. Sb2Se3 layers with thicknesses of 400, 800, and 1200 nm were deposited on a CdSe buffer and integrated in glass/SnO2:F/SnO2/CdSe/Sb2Se3/Au devices. Contrary to the usual expectation for thin-film absorbers, the thinnest device (400 nm) yields the highest power conversion efficiency (PCE), increasing from 2.8 % to 3.7 % after annealing in air, while thicker absorbers (800 and 1200 nm) only reach 3.5 % and 3.3 %, respectively. These trends correlate with a higher hole concentration (≈4.6 × 1016 cm−3 before PAT and ≈1.3 × 1017 cm−3 after PAT) and reduced defect density in the thinnest absorber, as revealed by CV/DLCP analysis, which also shows a narrowing of the space-charge region that favors carrier collection in ultra-thin devices. Morphological and XRD analyses confirm conventional grain growth and only subtle changes in crystal orientation with thickness and annealing, indicating that microstructural evolution alone does not govern the efficiency trends. Instead, SIMS profiles show oxygen incorporation throughout the absorber after PAT, while PL measurements and the convergence of CV and DLCP profiles demonstrate suppression of deep defects. These observations point to oxygen-induced passivation of electrically active defects as the main mechanism behind the improved open-circuit voltage and fill factor. Overall, the results highlight that in Sb2Se3/CdSe solar cells with very high absorption coefficients, carrier transport, defect passivation, and space-charge region engineering are more critical than increasing the absorber thickness, enabling efficient devices with ultra-thin (400 nm) Sb2Se3 absorbers.