Sn4+-free, stable tin perovskite films for lead-free perovskite solar cells

Organic-inorganic hybrid perovskite solar cells (PSCs) achieved already more than 25% power conversion efficiency making them one of the fastest-growing solar cell technologies. However, most state-of-art PSCs employ lead (Pb) as a "B" cation in the ABX3 type crystalline absorbers. Sn has been considered to be a potential replacement for Pb. However, chemically processed Sn-perovskite films often exhibit a high number of Sn vacancies and a high density of Sn4+ species, leading to poor stability and low power conversion efficiency of the tin-based PSCs. Herein we report co-evaporation of CH3NH3I and SnI2 to obtain Sn4+-free CH3NH3SnI3 perovskite films with excellent optoelectronic properties and improved stability. XRD measurements confirm the polycrystalline films, mainly oriented in the (100) plane of a cubic crystal. XPS analysis confirms that the films consist of Sn only in the 2+ oxidation state. AFM and KPFM analysis reveal smooth topology and uniform surface workfunction. The films show very good stability under heating and photodegradation confirmed by XRD and photoluminescence (PL) degradation measurements. The co-evaporated CH3NH3SnI3 films exhibit PL quantum yields up to 9x10-4 translating in a quasi Fermi-level splitting of 844 meV under one sun equivalent conditions. Extracted low values of Urbach energies (16 meV) suggest that the films exhibit a low number of defects near the band edges showing high promise in developing lead-free, high efficiency, and stable PSCs.