Interplay between cation composition and charge transport characteristics in GAxFAyMA1-x-yPbI3 halide perovskites

Halide perovskites (HPs) are a well-known class of mixed electronic and ionic conductors with diverse applications in optoelectronic devices. The simultaneous transport of ionic and electronic carriers has beneficial and detrimental effects depending on the intended applications. There is an extensive understanding of the charge transport characteristics in HPs since the phenomenon is of applied relevance. However, considering that several applications use compositions containing mixed cations, a deeper understanding of how the degree of substitution and the characteristics of the substituent cations affect the charge transport characteristics is needed. To this end, we experimentally studied the ionic conductivity (σion), current–voltage hysteresis (J–E hysteresis), mobility (μe) and density (ne) of electronic carriers, and bandgap energies (Eg) of up to 24 compositions of methylammonium lead iodide partially substituted with guanidinium and formamidinium. The results indicate that σion, J–E hysteresis, and μe decrease with the degree of substitution, with the J–E hysteresis being smaller the larger size of the substituent cation. At the same time, σion appears to be lower in compositions with equimolar substituents, in which the entropy of mixing is maximum. On the other hand, a slight increase in ne was observed with the substitution degree, showing highest values for FA+-rich compositions, where Eg is the lowest. The results advance the understanding of how it is possible to customize charge transport properties through the rational design of compositions in HPs.