Stabilization of Organic Cations in Lead Halide Perovskite Solar Cells Using Phosphine Oxides Derivatives.

Preventing ion migration in perovskite photovoltaics is key to achieving stable and efficient devices. The activation energy for ion migration is affected by the chemical environment surrounding the ions. Thus, the migration of organic cations in lead halide perovskites can be mitigated by engineering their local interactions, for example through hydrogen bonding. Ion migration also leads to ionic losses via interfacial reactions. Undesirable reactivities of the organic cations can be eliminated by introducing protecting groups. In this work, we report bis(2-oxo-3-oxazolidinyl) phosphinic chloride (BOP-Cl) as a perovskite ink additive with the following benefits: (1) The phosphoryl and two oxo groups form six-membered intermolecular hydrogen-bonded rings with the formamidinium cation (FA), mitigating ion migrations. (2) The hydrogen bonding reduces the electrophilicity of the ammonium protons by donating electron density, therefore reducing its reactivity with the surface oxygen on the metal oxide. Furthermore, the molecule can react to form a protecting group on the nucleophilic oxygen at the tin oxide transport layer surface through the elimination of chlorine. As a result, we achieve perovskite solar cells with an efficiency of 25.0% and improved MPP stability T93 = 1200 h at 40-45 °C compared to a control device (T86 = 550 h). In addition, we show a negative correlation between the strength of hydrogen bonding of different phosphine oxide derivatives to the organic cations and the degree of metastable behavior (e.g., initial burn-in) of the device.