Enhanced performance via partial lead replacement with calcium for a CsPbl3 perovskite solar cell exceeding 13% power conversion efficiency

Cesium metal halides are potential light-harvesting materials for use in the top cells of multi-junction devices due to their suitable bandgaps and good thermal stabilities. In particular, CsPbl(3) has a bandgap of 1.7 eV, which is suitable for perovskite/Si tandem cells. However, the desirable black phase for CsPbl(3) is not stable because Cs is too small to support the Pbl(6) octahedra. Also, there is room for improvement in terms of cell performance. Herein, we partially replace Pb2+ with Ca2+ in the CsPbl(3) precursor, producingmultiple benefits. Firstly, more uniform films with larger grains are produced from CsPbl(3) with Ca2+, due to the reduction in the size of the colloids in the precursor solution with Ca2+. This morphology improvement provides better contact at the interface between the perovskite and the hole transport layer. In addition, it is found that the surface of the film is modified by the formation of a Ca rich oxide layer, providing a surface passivation effect. Finally, incorporation of Ca increases the band gap, leading to an increase in output voltage. The best CsPbl(3) solar cell using 5% Ca2+ substitution in the precursor achieves a stabilised efficiency of 13.3%, andmaintains 85% of its initial efficiency for over 2 months with encapsulation.