We investigate the use of simple nontoxic Cu 2 SnS 3 (CTS) nanoparticles (NPs) as low-cost dopant-free hole-transport materials (HTMs) a substitute for spiro-OMeTAD in an n-ip 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.

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Low-cost inorganic hole-transporting materials (HTMs) accompanied by a printable carbon electrode is an efficient approach to address the limitation of material cost of perovskite solar cells (PSCs) and get this technology closer to commercialization. The present work is focused on optimizing the Zn/Sn ratio of Cu 2 ZnSnS 4 /carbon hole collectors in n−i− p structured PSCs, where CuInS 2 /carbon is applied as the reference hole collector. This composition regulation is a solution to address the challenge of composition-related defects of the Cu 2 ZnSnS 4 (CZTS) material. The Zn/ Sn ratio was tuned by the initial proportion of the zinc precursor during the nanoparticle (NP) synthesis using a heating-up procedure. It was found that the enhancement of the Zn/Sn ratio leads to a gradual increase of the optical band gap. More importantly, an increased density of B-type defect clusters [2Zn Cu + Zn Sn ] is confirmed using Raman results. Additionally, results from the cyclic voltammetry measurement show that by increasing the Zn/Sn value, the highest occupied molecular orbital (HOMO) of HTM is pulled down. These data match the upward trend of photovoltage. CZTS HTM with an optimal Zn/Sn ratio of 1.5 has a compatible energy level, along with the features of uniform and smooth coverage. The best efficiency of about 14.86% was obtained for optimal CZTS/carbon-based PSCs, which reaches from 14.86 to 15.49% after 25 days of aging.

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