Structural exciton localization drives efficient solid-state sensitized triplet fusion upconversion

Abstract Photochemical upconversion has several potential applications in optoelectronics. However, there is yet to be demonstrated a rational approach to high efficiencies in nanoscale solid-state devices. Here we demonstrate that the liquid triplet fusion medium, 9,10-bis( n -octyl-diisopropylsilylethynyl)anthracene (NODIPS-An), which behaves as a solid on excitonic timescales, can achieve high-efficiency upconversion on the nanoscale. Owing to its amorphous structure, there are highly-coupled sites that trap upconverted states and prevent the back transfer that plagues nanoscale upconversion systems. With NODIPS-An filling the pores of a sensitized alumina nano-scaffold, we achieve an absolute photon upconversion efficiency of 8.2(4)%. The majority of efficiency losses are attributed to fluorescence and triplet energy transfer yields, and thus the self-trapping of excitons within the triplet fusion medium is demonstrated to effectively prevent the back transfer of excitons to the sensitizer. Strategies are proposed to improve the fluorescence yield and triplet energy transfer to pursue higher efficiencies in nanoscale solid-state photochemical upconversion devices.