Although numerous metal halide perovskite materials have been investigated in the field of optoelectronic, the development of perovskite heterojunctions with exotic structures is still rare. Herein, we report the epitaxial growth of quasi-two-dimensional (Q-2D) perovskites on methylammonium lead iodide (MAPbI₃) single crystals to form perovskite heterojunctions with interfacial bonding. The MAPbI₃ adjacent to epitaxial Q-2D perovskite shows blue shifted photoluminescence with shortened lifetime, which becomes significant with the reduced layer number of the Q-2D perovskites. Our findings suggest the presence of an interfacial strain gradient leading to enhanced photocarrier separation. Accordingly, compared to the MAPbI₃ single crystal detector, the BA₂MAPb₂I₇/MAPbI₃ (BA: n-butylamine) heterojunction-based photodetector demonstrates a bandpass detecting property and exhibits 5 times enhanced external quantum efficiency and 83 times enhanced specific detectivity (D* = 3.26 × 10¹¹ Jones). Remarkably, the unencapsulated BA₂MAPb₂I₇/MAPbI₃ heterojunction is stable in ambient condition for > 300 days. The Q-2D/3D heterojunction shows suppressed ion inter-diffusion due to the presence of Q-2D phase.

Metal halide perovskite solar cell (PSC) has successfully distinguished itself in optoelectronic field by virtue of the sharp rise in power conversion efficiency over the past decade. The remarkable efficiency breakthrough at such a fast speed can be mainly attributed to the comprehensive study on film deposition techniques, especially the effective management of surface and interfacial defects in recent works. Herein, we summarized the current trends in performance enhancement for PSCs, with a focus on the generally applicable strategies in high-performance works, involving deposition methods, compositional engineering, additive engineering, crystallization manipulation, charge transport material selection, interfacial passivation, optical coupling effect and constructing tandem solar cells. Promising directions and perspectives are also provided.

Bandgap-graded materials present varying spectral responses at different positions, making them possible to be used as an alternative to photoactive materials array in multi-spectral responsive devices, thus miniaturizing the apparatus. However, the preparation of bandgap-graded materials usually requires complicated deposition process. Here we report a facile low-temperature solution process to make films with lateral bandgap gradients, which form spontaneously via self-spreading and interdiffusion of solutions. We show lead halide perovskite films with MAPbCl₃-MAPbBr₃ and MAPbBr₃-MAPbI₃ gradients, which exhibit light absorption onsets ranging from 410 to 781 nm. The bandgap-graded films were used to make self-powered multiband photodetectors, which show different spectral responses at different positions without applying bias voltage. Furthermore, self-powered spectrometers were made by using the multiband photodetectors.