Modulation of delayed fluorescence pathways via rational molecular engineering
Sanchari Debnath, Pria Ramkissoon, Ulrike Salzner, Christopher R. Hall, Naitik A. Panjwani, Woojae Kim, Trevor A. Smith, Satish Patil
Abstract
One of the key challenges in developing efficient organic light-emitting diodes (OLEDs) is overcoming the loss channel of triplet excitons. A common approach to mitigate these losses to enhance the external quantum efficiency of OLEDs is employing emitter molecules optimized for thermally activated delayed fluorescence (TADF) or triplet-triplet annihilation (TTA). However, achieving both in the solid state from the same organic chromophore poses a formidable challenge due to energetic and structural requirements needing to be met simultaneously. Here, we demonstrate TADF and TTA in donor-acceptor phthalimide derivatives by employing triphenylamine (TPA) or phenyl carbazole (PhCz) as a donor. Thin films of the TPA-substituted phthalimides doped in the poly(methyl methacrylate) matrix exhibit TADF emission from the singlet charge-transfer (CT) state. On the contrary, PhCz-substituted emitters display dominant TTA-induced delayed fluorescence in the neat film due to long-range molecular ordering that facilitates efficient triplet diffusion. The present study provides insight into how dual TADF-TTA delayed fluorescence can be realized in thin films of molecular semiconductors via rational molecular design. Achieving both thermally activated delayed fluorescence and triplet-triplet annihilation has potential for the development of organic light-emitting diodes with high efficiency but is challenging. Here, the authors report a donor-acceptor chromophoric design to achieve both in thin films.