Proceedings of International Conference on Hybrid and Organic Photovoltaics (HOPV16)
Publication date: 28th March 2016
Formamidinium lead iodide (FAPI) has a small bandgap, which makes it a very promising light absorber for perosvkite photovoltaics. However, at room temperature its stable phase is the yellow δ phase, which is unsuitable for solar cell applications. Thus, it is of crucial importance to find a way to stabilize the lower band gap perovskite phase at room temperature. A possible idea on how to improve stability is mixing different kinds of perovskites, e.g. perovskites with different monovalent cations. The idea is that a modest mixing does not significantly affect the properties of FAPI but improves its stability. Concerning the stability, the key observation is that all perovskites have a similar structure, with the quasi-cubic inorganic framework hosting the monovalent cations in its cavities. On the contrary, depending on the type of cation, the δ phase of the compound may be structurally significantly different. This is the case, for example, of the δ phases of FAPI and CsPbI3 (CsPI). δ FAPI and CsPI have also a very different volume per stoichiometric unit, the second being smaller by ca. 10%, while the difference of volume in the perovskite phase is much smaller, in fact less than 1%. This suggests that mixing materials with different cations is energetically more convenient in the perovskite phase than in the corresponding δ phase. Following this idea, we computed the free energy balance of mixing cations in the δ and persovskite phases of CsxFA1-xPI. It was found that in the δ phase the energetic cost of mixing is not balanced by the mixing entropy. On the contrary, the perovskite system is energetically neutral with respect to the mixing, and the mixing entropy leads to a free energy gain of the order of kBT. In practice, cation mixing shifts the transition temperature from the δ to the perovskite phase by $ca. 300K.
This general design prinicple can be used for the exploration of mixed perovksites with improved optical and stability properties. Since a systematic exploration of all possible mixed cation-mixed halide systems is not feasible, we introduce an efficient search protocol based on evolutionary algorithms for a greatly accelerated computational scan of optimal mixed perovksite systems for solar cell applications.
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