A sharp potential drop across the interface of the Pb-rich halide perovskites/TiO2 heterostructure is predicted from first-principles calculations, suggesting enhanced separation of photoinduced charge carriers in the perovskite-based photovoltaic solar cells. The potential drop appears to be associated with the charge accumulation at the polar interface. More importantly, on account of both the β phase structure of CH3NH3Sn(x)Pb(1-x)I3 for x < 0.5 and the α phase structure of CH3NH3Sn(x)Pb(1-x)I3 for x ≥ 0.5, the computed optical absorption spectra from time-dependent density functional theory (TD-DFT) are in very good agreement with the measured spectra from previous experiments. Our TD-DFT computation also confirms the experimental structures of the mixed Pb-Sn organometal halide perovskites. These computation results provide a highly sought answer to the question why the lead-based halide perovskites possess much higher power conversion efficiencies than the tin-based counterparts for solar-cell applications.

Motivated from the recent success in synthesizing bismuth-based double perovskites (J. Am. Chem. Soc. 2016, 138, 2138–2141), we perform a comprehensive study of interfacial properties of bismuth-based double perovskites Cs2AgBiX6 (X = Br, Cl) and TiO2 interfaces. The bismuth-based double perovskites possess desirable electronic and optical properties as excellent light absorber and thus may serve as lead-free alternatives to the organic–inorganic perovskites. On the basis of density functional theory computation, we systematically study the Cs2BBiX6 (B = Ag, Cu; X = Br, Cl)/TiO2 interfaces and analyze the trend of charge transfer across the interfaces. We find that the Cs4X4 (X = Br and Cl)/TiO2-mediated interfaces are prospective interfaces for charge extraction and separation due largely to the withdrawn trap states for the TiO2 part when in contact with the Cs4X4 termination. Moreover, the ionic interaction and charge redistribution across the specific interfaces can lead to the appropriate band alignment, reduced band gap for the rock-salt double perovskite part, and smooth gradient distribution for the locally projected density of states along the normal direction to the interfaces, further facilitating the charge transfer. Overall, we predict that bismuth-based double perovskites Cs2AgBiX6 (X = Br, Cl) and TiO2 interfaces are highly efficient for charge extraction, suggesting high potential for interfacial engineering optoelectronic devices.

Two-dimensional (2D) perovskites have been demonstrated great promise in x-ray detection application because of their stability, tunability, and the unique electronic properties. The centimeter-sized 2D perovskite (PMA)2PbI4 single crystal and the corresponding x-ray detector were fabricated. The Cu ion implanted device exhibits an excellent sensitivity of 283 μC Gyair−1 cm−2, the significantly enhanced mobility-lifetime (μτ) product of 8.05 × 10−3 cm2 V−1, and the lowest detectable dose rate of 2.13 μGyair s−1. Experimental observation combined with the DFT calculations shows that the improvement in Cu ion implanted x-ray detection is ascribed to the enhanced photoinduced charge carrier density and μτ product, and the increased carrier dissociation capability associated deeply with the decreased binding energy of exciton in the inorganic layer quasi-quantum well. The incorporation of the Cu interstitials by high-energy Cu ion implantation is able to introduce the donor and acceptor states with additional charge transfer channeling, resulting in the decreased exciton binding energy and fast dissociation of the exciton and the quick carrier extraction. Cu ion implantation regulating the dissociation of charge carriers in low-dimensional perovskites will motivate the application for 2D perovskite in high-performance x-ray detectors.

The photoluminescence (PL) variations of organic-inorganic hybrid lead halide perovskites in different atmospheres are well documented, while the fundamental mechanism still lacks comprehensive understandings. This study reports the reversible optical and electrical properties of methylammonium lead bromide (MAPbBr3 or CH3NH3PbBr3) single crystals caused by air infiltration. With the change in the surrounding atmosphere from air to vacuum, the PL intensity of perovskite single crystals decreases, while the conductivity increases. By means of first-principles computational studies, the shallow trap states are considered as key elements in PL and conductivity changes. These results have important implications for the characterization and application of organic-inorganic hybrid lead halide perovskites in vacuum.

Halide perovskite exhibits remarkable photovoltaic (PV) performance so far, however, its toxicity and instability hinder the commercialization. Researchers are devoted to explore nontoxic and stable solar cell alternatives with high efficiency. Considering the octahedra network and the high symmetry crystalline structure of prototype Mg3NF3 (Pm-3m), which is similar to the halide perovskites and tends to create high p-s band edge states transition probability associated closely with the excellent PV performance. We extensively screened the potential PV materials from 64 compounds in A3MX3 (A = Mg, Ca, Sr, Ba; M = N, P, As, Sb; X = F, Cl, Br, I) structure based on the bandgap, theoretical efficiency, band edge states transition, and thermal stability. Three Pb-free compounds, i.e., Ba3PI3, Ba3AsI3, and Ba3SbI3, are found to be ideal stable PV materials with the efficiency (25.9%) comparable to that of halide perovskites. The optical absorption of the best PV candidate, i.e., Ba3SbI3, has been improved further by low energy Cu ion implantation, and the occurrence of the threshold velocity in the electronic stopping power and the derived bandgap confirm again the high accuracy of our density functional theory results.