Compact-Type Quasi-2D Perovskite Based on Two Conventional 3D Perovskites.

Quasi-2D perovskites are natural quantum well (QW) structures composed of insulating organic layers inserted between conducting [A n -1 Pb n X 3 n +1 ] 2- slabs. The presence of the bulky organic layer improves the stability but meanwhile sacrifices carrier transport performance. By utilizing two A-site cations of formamidinium (FA + ) and cesium (Cs + ), we synthesize unique compact-type quasi-2D perovskites CsPbBr 3 @FABr. Instead of the bulky organic cations, the FA + cation was employed to work as interlayer "spacer", while the smaller Cs + cation was chosen to occupy perovskite cages. Transient absorption reveals an energy transfer from small- n -value QWs to large- n -value QWs, enabling a photoluminescence quantum yield (PLQY) of 36.1%. After further promoting the formation of middle- n -value QWs, the homogeneous QW distribution provides a complete energy cascade to access more efficient energy transfer, leading to significant PLQY raise to 70.1%. We break the shackles to report the first case of compact-type quasi-2D perovskites, providing new guidelines for designing high-performance perovskite materials for optoelectronic devices.