Active selenium-driven confined crystallization and carrier dynamics in high-efficiency ultrathin semi-transparent Sb ₂ Se ₃ solar cells

To overcome the limited environmental stability of organic semi-transparent photovoltaic (STPV) devices, developing robust inorganic thin-film absorbers has become a critical research focus. Antimony selenide (Sb₂Se₃), with its Q1D crystal structure, excellent optoelectronic properties, and mechanical flexibility, is a promising candidate for STPV applications. However, thinning the absorber to enhance visible transmittance often degrades crystallinity, weakens preferred orientation, and increases defect density. Here, we present a dual-pathway device-engineering strategy that integrates KOH-assisted chemical-bath thinning of the CdS buffer layer with rapid thermal evaporation combined with magnetron sputtering of active selenium (Se). The introduced active Se atoms effectively reorganize molecular chains within a confined crystallization space, enhancing crystalline quality and promoting vertical [<i>hk</i>1] orientation. Simultaneously, Se doping passivates selenium vacancies and suppresses deep-level trap states, thereby improving carrier transport and reducing nonradiative recombination. Employing this strategy, we demonstrate for the first time an ultrathin Sb₂Se₃ STPV device with a total functional layer thickness of only 80 nm just one-seventh the thickness of conventional devices-achieving an efficiency of 7.02%, retaining 93% of that of thicker counterparts while maintaining an optical transparency exceeding 20%. This work establishes a practical route to simultaneously balance transparency and efficiency in inorganic STPV devices and underscores the potential of Sb₂Se₃ as a high-performance absorber for next-generation semi-transparent and tandem photovoltaics.