Photon Upconversion in a 2D Inorganic–Organic Semiconductor Heterostructure

Van der Waals heterostructures (vdWHs) comprised of stacked 2D crystal monolayers can be used to elicit emergent electronic and photonic phenomena. Moreover, energy transfer processes may be engineered via vdWHs by taking advantage of the atomically-abrupt, Å-scale, and topologically tailorable interfaces within them. Here, we prepare heterostructures comprised of 2D WSe₂ monolayers interfaced with DBP-doped rubrene, an organic semiconductor capable of triplet fusion. We fabricate these films over large areas entirely through vapor deposition methods, thereby demonstrating a pathway toward device-scale heterostructures. Time-resolved and steady-state photoluminescence measurements reveal both quenching of WSe₂ emission by rubrene and emission from the DBP species at 612 nm, for excitation at 730 nm, which is clear evidence of photon upconversion. Through an analysis of the decay dynamics in the heterostructure we conclude that excitation transfer from WSe₂ to the triplet state of rubrene proceeds on sub-nanosecond timescales. The dependence of the upconversion emission on excitation intensity is consistent with a triplet fusion mechanism, and we achieve maximum efficiency (linear regime) of the upconversion emission at threshold intensities as low as 110 mW/cm², which is comparable to the solar irradiance. This study highlights the potential for advanced optoelectronic applications employing vdWHs which leverage strongly-bound excitons in monolayer TMDs and organic semiconductors.