Molecular Triplet Sensitization of Monolayer Semiconductors in 2D Organic/Inorganic Hybrid Heterostructures

Hybrid heterostructures (HSs) comprising organic and two-dimensional (2D) monolayer semiconductors hold great promise for optoelectronic applications. So far, research efforts on organic/2D HSs have exclusively focused on coupling directly photoexcited singlets to monolayer semiconductors. It remains unexplored whether and how the optically dark triplets in organic semiconductors with intriguing properties (e.g., long lifetime) can be implemented for modulating light–matter interactions of hybrid HSs. Herein, we investigate the triplet sensitization of monolayer semiconductors by time-resolved spectroscopic studies on Pd-octaethylporphyrin (PdOEP)/WSe2 and PdOEP/WS2 HSs with type I and type II band alignment, respectively. We show that PdOEP triplets formed in ∼5 ps from intersystem crossing can transfer energy or charge to WSe2 or WS2 monolayers, respectively, leading to a significant photoluminescence enhancement (180%) in WSe2 or long-lived charge separation (>2 ns) in WS2. The triplet transfer occurs in ∼100 ns, which is more than 3 orders of magnitude slower than singlet and can be attributed to its tightly localized nature. Further study of thickness dependence reveals the dictating role of triplet diffusion for triplet sensitization in organic/2D HSs. This study shows the great promise of much less explored molecular triplets on sensitizing 2D monolayer semiconductors and provides the guidance to achieve long-range light harvesting and energy migration in organic/2D HSs for enhanced optoelectronic applications.