p‐Type Transparent Quadruple Perovskite Halide Conductors: Fact or Fiction?

High‐performance p‐type transparent conductors (TCs) can be used for a variety of optoelectronic applications, particularly transparent electronics with complementary metal–oxide–semiconductor circuits. Recently, several quadruple perovskite halides, such as Cs4CdSb2Cl12 and Cs4CdBi2Cl12, have been theoretically predicted to be excellent p‐type TCs that surpass all the conventional p‐type TCs and hence attracted a lot of attention. In this work, experiments and hybrid density functional theory (DFT) calculations are combined to evaluate the p‐type dopability in these quadruple perovskite halides. The experimental results reveal that these materials are optically transparent but electrically insulating, which contradicts the previous prediction. The hybrid DFT calculations show that the difficulty in forming p‐type doping in these compounds is because of the easy formation of the compensating n‐type intrinsic defects in the p‐type region, which is resulted from too deep valence band maximum levels. It is further demonstrated that the bandgap underestimation associated with the semilocal DFT and the unintentional chemical boundary overestimation should be the origins of the p‐type conductivity predicted previously. The results highlight the importance of reasonable bandgap description and chemical boundary determination in predicting defect thermodynamics. Quadruple perovskite halides represented by Cs4CdSb2Cl12 were previously predicted to be the top p‐type transparent conductors. In this work, it is verified by a combined experimental and hybrid density functional theory study that these transparent quadruple perovskite halides can hardly be doped p‐type due to the too deep‐energy‐lying valence band maximums.