Correlation of recombination and open circuit voltage in planar heterojunction perovskite solar cells

Mixed-cation and halide perovskite solar cells have shown superior photovoltaic performance compared to mono-cation based representatives. Their remarkable photovoltaic performance is the effect of superior compositional engineering along with their unique optoelectronic properties. However, there is still a lack of understanding of the working principles of multi-cation based perovskite solar cells. In this study, we employ the state-of-the-art multiple-cation perovskites FA(0.83)MA(0.17)Pb(I0.83Br0.17)(3) and Cs-5(MA(0.17)FA(0.83))(95)Pb(I0.83Br0.17)(3) as absorber layers and investigate their intrinsic and interfacial dynamics in planar TiO2-based PSCs. We demonstrate that the incorporation of Cs+ in FA(0.83)MA(0.17)Pb(I0.83Br0.17)(3) leads to an increase in power conversion efficiency, improved perovskite phase stability and reduced recombination. However, these devices along with these excellent characteristics still suffer from lower open circuit voltage (V-oc) as compared to FA(0.83)MA(0.17)Pb(I0.83Br0.17)(3) based devices. By analysing the morphological properties and capacitance versus frequency responses, we show that the higher grain size and higher accumulation of photogenerated charges lead to the high open circuit voltage in FA(0.83)MA(0.17)Pb(I0.83Br0.17)(3) based devices. Our results demonstrate that the values of V-oc not only depend on the recombination, but also on charge accumulation phenomena.