Chalcogenide-based hole transport material for stable perovskite solar cells

To drive perovskite photovoltaic (PV) toward commercialization is necessary to develop large-area modules with high efficiency, enhance large-scale and low-cost production processes, and achieve long-term operational stability. The solar cells' stability depends upon many factors including the materials employed to produce the hole transporting layer (HTL). Besides, novel hole transport materials (HTMs) are required to achieve a large deployment of sustainable and cost-effective PV devices. Cu2ZnSnS4 (CZTS) can fulfil the targets of cost-effectiveness and sustainability, and here we investigate its impact on PV performance stability when employed as HTM. CZTS is a p-type semiconductor, commonly studied as a light absorber layer in heterojunction solar cells, but lately, it has shown promising results also as HTL in perovskite solar cells (PSCs). Here, we report on the synthesis of CZTS nanoparticles (NPs) employed as HTM in PSCs. The NPs have been synthesized by the hot-injection method, in an oxygen-free environment using a Schlenk line apparatus. CZTS NPs ink has been obtained by dispersing the undried nanoparticles in xylene. The ink was filtered and then spin-coated on the substrate. The resulting film was annealed in air on a hot plate. The resulting 50 nm thick HTL was almost transparent in the visible range of the solar spectrum, and it has been fully characterized by transmittance and scanning electron microscopies, UV-Vis, μ-Raman, and X-ray diffraction spectroscopies. Hole mobility and charge transport have been evaluated in plane and out of plane, ensuring the material feasibility to act as HTL. The preliminary results of the CZTS NPs-based HTM for PSCs in p-i-n architecture are discussed, focusing on the retention of the initial PV performances. The control device (organic-HTL / CH3NH3PbI3 / PC60BM-BCP / Ag) loses more than half of the initial efficiency in one month, but the devices employing the CZTS-NPs remain stable, and, in some cases, the PV performances improved with time. The PV parameters evolution with time has been monitored through periodical current/voltage and external quantum efficiency measurements, aided by admittance spectroscopy data analysis and scanning electron microscopy imaging. This work aims to promote a new path to control stability, employing an HTM able to prevent the degradation of the PV performance.