Abstract
The morphology and optoelectronic properties of wide-bandgap halide perovskites have profound influence on the performance of tandem solar cells in which these materials hold a great potential to push the efficiency beyond the maximum in single junction devices. Typically, alkali metals such as Cs+ are employed to overcome photo-instability in mixed-halide hybrid perovskites. However, while using the popular CsI in precursor solutions, small grain size (~250 nm) along with high densities of grain boundaries and their associated trap states are usually observed in polycrystalline films, leading to low photovoltaic performance. Here, we develop a non-stoichiometric one-step solution casting method to prepare wide-bandgap CsXMA1-XPb(I0·6Br0.4)3 films with reduced grain boundaries and high crystallinity using CsCl which plays a double role of Cs + source and crystallization controller. The resulting films exhibit improved carrier lifetime and mobility, and reduced carrier trap states. Elemental analysis revealed that some Cl ions remains in the final films, passivating the defect states. All these benefits led to high open circuit voltages and improved the power conversion efficiency (PCE) of Cs0.1MA0.9Pb(I0·6Br0.4)3 perovskite solar cells from 13.20% to 15.16%. Moreover, the developed method is also found to enhance the hydrophobicity and moisture resistance of CsXMA1-XPb(I0·6Br0.4)3 films, which enabled the improvement in the device ambient stability.
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