Origin of p-Type Conduction in Amorphous CuI: A First-Principles Investigation

Recently, Jun et al. (Adv. Mater.2018,30, 1706573) reported Sn-doped amorphous phases of CuI (a-CuI) as a new class of transparent p-type semiconductors. It is surprising that high mobilities are found despite amorphous structures tend to localize carriers, particularly the directional p orbitals of anions. To reveal the microscopic origin of the new p-type amorphous semiconductors, density functional theory calculations are performed, and structural and electrical properties ofa-CuI are investigated. Amorphous models from melt-quench simulations consist of structural motifs that resemble local orders in crystalline polymorphs of CuI. Despite the absence of translational symmetry, states at valence band maximum (VBM) are substantially extended in linear ways due to the hybridization between I-5p states, explaining the high hole mobilities in the experiment. Sn-dopeda-CuI is also investigated and it is found that Sn atoms stabilize the amorphous structure without disturbing the hole transport. Furthermore, results on nonstoichiometric models show that Cu-deficiency increases the hole concentration ina-CuI and also enhances the delocalization of VBM states, which is consistent with the experimental observation that the hole mobility increases with doping concentrations. Herein, the origin of hole conduction in amorphous CuI is enlightened, which is believed to contribute in the development of high-performance p-type amorphous materials.