Mitigating Singlet Exciton Back‐Transfer using 2D Spacer Layers for Perovskite‐Sensitised Upconversion

Abstract Photon upconversion has potential applications in light–emitting diodes, photocatalysis, bio‐imaging, microscopy, 3D printing, and photovoltaics. Bulk lead‐halide perovskites have emerged as promising sensitisers for solid‐state photon upconversion via triplet–triplet annihilation due to their excellent optoelectronic properties. In this system, a perovskite‐sensitiser absorbs photons and subsequently generates triplet excitons in an adjacent emitter material, where triplet–triplet annihilation can occur, allowing for the emission of higher energy photons. However, a major loss pathway in perovskite‐sensitised upconversion is the back‐transfer of singlet excitons from the emitter to the sensitiser via Förster Resonance Energy Transfer. In this investigation, a 2D perovskite spacer layer is introduced between the bulk perovskite‐sensitiser and rubrene emitter to mitigate back‐transfer of singlet excitons. This modification reveals the inherent balance between efficient triplet exciton transfer across the interface with a potential barrier vs the mitigation of near‐field back‐transfer by increasing the distance between the sensitiser and singlet excitons in the emitter. Notably, the introduction of this spacer layer enhances the relative upconversion efficiency at lower excitation power densities while also sustaining performance over extended timescales. This work represents significant progress toward the practical applications of perovskite‐sensitised photon upconversion.