Evaluating Cu₂SnS₃ Nanoparticle Layers as Hole-Transporting Materials in Perovskite Solar Cells

We investigate the use of simple nontoxic Cu2SnS3 (CTS) nanoparticles (NPs) as low-cost dopant-free hole-transport materials (HTMs) a substitute for spiro-OMeTAD in an n-i-p mesoscopic architecture of perovskite solar cells (PSCs). Besides, this work confirms the critical role of the crystalline phase of CTS NPs on the performance of the device. Using a facile one-pot heating-up procedure, pure zincblende and wurtzite structures of CTS NPs were obtained by sulfur element and thiourea as the sulfur source, respectively, and were dispersed in chloroform to make very stable nonpolar ink that is compatible with the perovskite. Nanoparticles with the wurtzite crystal phase showed much better photovoltaic performance compared to the zincblende phase with efficiencies of 13.01 and 7.87%, respectively. The efficiency of the reference solar cell using spiro-OMeTAD was 16.01%. Results from impedance spectroscopy and external quantum efficiency show that by switching from wurtzite-CTS to zincblende-CTS, the resistance of charge transfer at the perovskite/HTM interface is increased, which matches with the descending trend of fill factor of the corresponding cells. According to the morphological characteristics and electrical properties of HTM layers, the better performance of wurtzite-CTS in comparison with zincblende-CTS is due to two factors: (i) enhancement of the band-gap energy and alignment of the valance band maximum; and (ii) uniform smooth coverage of the perovskite film by monodispersed wurtzite-CTS NPs.