Unveiling the Impact of Bi³⁺ Heterovalent Doping on the Negative Capacitance and Ionic Conductivity of Perovskite Single Crystals: Implication in Neuromorphic Computing

The introduction of heterovalent metal ion doping in the lead (Pb) halide perovskites presents a novel opportunity to manipulate the electronic and ionic properties by introducing dopant charges and increasing the carrier concentration in single crystals. While previous studies have reported on the use of bismuth (Bi 3+ ) doping in methylammonium lead tribromide (MAPbBr 3 ) to adjust the optical properties, the comprehensive impact of Bi 3+ doping on the structural and electronic properties of MAPbBr 3 single crystals remains unexplored. This research, therefore, delves into the anomalous behavior of the structural, optical, and electrical properties of pristine and doped MAPbBr 3 single crystals through a combination of experimental and computational studies. The results reveal that Bi 3+ doping at the B-site of MAPbBr 3 single crystals induces lead vacancies (V Pb 2– ) and other cation vacancies near the Ag electrode interface. Moreover, bias-dependent electrochemical impedance spectroscopy measurements on the doped single crystals demonstrate a low-frequency semicircle and high-frequency recombination resistance, indicating improved ion transport properties due to Bi 3+ doping. This leads to low-frequency negative capacitance, particularly in low-field regimes, due to ion accumulation at the perovskite/electrode interface. As the voltage increases, a transition from negative to positive capacitance is observed around 0.5 V. These doped single crystals show promise for memristor applications due to their adjustable electronic and ionic conductivities, offering hope for future advancements in neuromorphic computing.