Identifying the Factors That Lead to PLQY Enhancement in Diluted TADF Exciplexes Based on Carbazole Donors

Here, we compare the effects of solid-state dilution on the photophysical and electrical performance of three thermal-activated delayed fluorescence exciplexes based on carbazole-containing donors with a high-performance acceptor PO-T2T. We observe intrinsically different photophysical behaviors between the exciplexes, which is explained by the presence of a heterogeneous distribution of triplet energies in one of the donor molecules arising from a distribution of planar and highly twisted biphenyl bridges. This study of different donors demonstrates that the recently discovered efficiency enhancement of solid-state diluted exciplexes is independent of the separation-associated spectral blueshift, with the latter being universal while the former is only found for donors with low structural rigidity. Organic light-emitting diodes produced from these carbazole-based exciplexes show decreased electrical performance likely due to the relatively low conductivity of the donors and host, suggesting that functional conductive hosts may, in some cases, be required to translate optical efficiency enhancements into improved electrical performance.