Copper-incorporation for polytypism and bandgap engineering of MAPbBr₃ perovskite thin films with enhanced near-Infrared photocurrent-response

The optoelectronic properties of lead-based halide perovskites can be enhanced through B-site engineering. Here, we studied the B-site alloying of MAPbBr₃ thin films with copper (Cu²⁺). The alloyed perovskite thin films were characterized by a dark color, enlarged average grain boundary, and lowering of the optical bandgap from 2.32 eV for pristine MAPbBr₃ to 1.85 eV for 50% Cu-substituted MAPbBr₃. Various characterization methods revealed that the Cu-incorporation leads to the appearance of a Cu-rich secondary phase. The conductivity increased over three orders of magnitude upon alloying. Temperature-dependent conductivity measurements at temperatures ranging from 110 K to 300 K revealed the occurrence of two phase-transitions in Cu-substituted perovskite, and only one transition in pristine MAPbBr₃. Photocurrent measurements of the alloyed perovskites showed that band-carrier generation occurred upon excitation in the near-infrared region. First-principles point defect calculation shows the likelihood of compensating Br vacancy formation with high Cu-substituting concentrations. Calculation of atomic orbital projected density of states (Cu_Pb + v_Br defect complex) revealed the presence of localized defect states within the pristine bandgap, explaining the observed sub-bandgap absorption. The results provide an insight into the alloying importance in phase-modulation and tailoring the optoelectronic properties of perovskites for a wide range of efficient optoelectronic devices.