Tin perovskite/fullerene planar layer photovoltaics: improving the efficiency and stability of lead-free devicesElectronic supplementary information (ESI) available: Further characterisation data. See DOI: 10.1039/c5ta02950c

We report the first demonstration of orthorhombic CsSnI 3 films prepared from solution at room temperature that have defect densities low enough for use as the light harvesting semiconductor in photovoltaic devices even without using excess Sn in the preparative method, and demonstrate their utility in a model p-i-n photovoltaic device based on a CuI | CsSnI 3 | fullerene planar layer architecture. We also report an effective strategy for simultaneously improving both the efficiency and stability of these devices towards air exposure based on the use of excess of SnI 2 during CsSnI 3 synthesis from CsI and SnI 2 . A combination of photoelectron spectroscopy, contact potential measurements and device based studies are used to elucidate the basis for this improvement and role of the excess SnI 2 . The open-circuit voltage in these lead-free photovoltaic devices is shown to be strongly dependent on the degree of alignment between the perovskite conduction band edge and the lowest occupied molecular orbital (LUMO) in the fullerene electron transport layer. Furthermore, the energetics at the perovskite-fullerene interface are shown to be a function both of the LUMO energy of the fullerene and the nature of the interaction at the heterojunction which can give rise to a large abrupt vacuum level shift across the interface. A champion open-circuit voltage of ∼0.55 V is achieved using indene-C 60 bis-adduct as the electron extraction layer, which is twice that previously reported for a CsSnI 3 based PPV. The preparation of CsSnI 3 films at room temperature with defect densities low enough for light harvesting in photovoltaic devices is reported, along with a new strategy for improving device stability and evidence that the energetics at the perovskite/fullerene interface is a key determinant of the open-circuit voltage.