Chemical Processing of Mixed-Cation Hybrid Perovskites: Stabilizing Effects of Configurational Entropy

In the third-generation solar cells, especially considering cost-effectiveness and facile fabrication, incorporating perovskites as efficient absorbers enabled a breakthrough for emerging photovoltaics. In 2009, starting with power conversion efficiencies (PCEs) of 3.9%, they already show a record of 25.5%, reaching almost the power of conventional silicon solar cells. Ideally, perovskites can be represented by the simple building block AMX 3, where M is the metal cation, X is an oxide or halide anion, and A is a cation. The A-site cation is selected to equalize the total charge and to stabilize the lattice and can typically be Cs ⁺ or organic cations like CH ₃ NH ₃⁺ (MA) and NH ₂ CH=NH ₂⁺ (FA). The size of the organic cation and metal ion is an important parameter to modulate the crystal structure and electronic properties of perovskite materials. The first used single-cation hybrid perovskites (MAPbI ₃ and MAPbBr ₃) showed a lack of thermal and phase stability. Entropic stabilization could be achieved by compositional engineering, especially by combining different organic and inorganic cations at the A-site to obtain mixed-cation perovskites. In this chapter, we focus on the development of these perovskites considering the configurational stability, the tolerance factor concepts for evaluation of stable perovskite compositions, and the development of mixed/multiple-cation perovskite solar cells (PSCs).