Thermally Activated Delayed Fluorescence Triplet State Excitons for High-Performance Organic Photodetectors: A Novel Strategy

Photoinduced charge-trapping is a promising strategy for boosting the photosensitivity of organic photodetectors at the expense of their response time. In addition, they have a low carrier extraction yield due to the formation of low-energy triplet excitons through the recombination of a photogenerated hole–electron pair. Materials with thermally activated delayed fluorescence (TADF) exhibit a long-lived (≈µs) excited spin-triplet nearly iso-energetically aligned with that of an excited spin-singlet, which results in suppressed exciton losses and is widely used in organic light-emitting diodes. The extraction and population of triplet state excitons in TADFs is a sought-after but underexplored aspect of photoinduced gating in photodetectors. A 1,2,3,5-tetrakis(carbazol-9-yl)4,6-Dicyanobenzene (4CzIPN) TADF blend with a high-mobility Poly[2,5-bis(3-tetradecylthiophen-2-yl)thieno[3,2-b]thiophene] (PBTTT-C₁₄) polymer via bulk type-II offset can increase free carrier extraction yield (fast response) and retain trapped electrons in TADF triplet states (high gain). The PBTTT-C₁₄/4CzIPN ultraviolet photodetector device maintains a trade-off between high photogain (≈10³) and fast response time (79 ms) at 300 nm while operating at a shallow dark current (≈pA). Furthermore, the device shows high external quantum efficiency (≈10⁴ %) and detectivity (≈10¹¹ Jones) for low light power (