Postsurface Selenization for High Performance Sb2S3 Planar Thin Film Solar Cells

Sb2S3 has attracted great research interest very recently as a promising absorber material for thin film photovoltaics because of their unique optical and electrical properties, binary compound and easy synthesis. Sb2S3 planar solar cells from evaporation method without hole-transport layer (HTM) assistance suffer from sulfur deficit vacancy and high back contact barrier. Herein, we developed a postsurface selenization treatment to Sb2S3 thin film in order to improve the device performance. The XRD, Raman, and UV–vis spectra indicated the treated film kept the typical characters of Sb2S3. TEM/EELS mapping of treated Sb2S3 film revealed that only surface adjacent section was partly selenized and formed Sb2(SxSe1–x)3 alloy. In addition, XPS results further unfolded that there was trace selenium doping in the bulk of Sb2S3 film. The treated HTM-free Sb2S3 based solar cells were fabricated and an improved efficiency of 4.17% was obtained. The obtained VOC of 0.714 V was the highest and the power conversion efficiency also reached the top value among HTM-free planar Sb2S3 solar cells. The nonencapsulated device exhibited high stability. After storing in ambient air for up to 100 days, the device could maintain 90% efficiency. Systematic materials and device characterizations were implemented to investigate the improvement mechanism for postsurface selenization. The back alloying could suppress the rear contact barrier to improve the fill factor and carrier extraction capability. The bulk Se-doping helped to passivate the interface and bulk defects so as to improve the CdS/Sb2S3 heterojunction quality and enhance the long-wavelength photon quantum yield. The robust treatment method with multifunctional effect holds great potential for new chalcogenide thin film solar cell optimization.