“Flagella effect” of thermally activated delayed fluorescence emitter dominating the efficiency of non-doped solution-processed OLEDs

Abstract

Flexible chains have important research value in the field of organic optoelectronics and biomedicine because they can adjust the functionalization of organic molecules in the aggregated state. Herein, by regulating the length and mold of the flexible chain attached to a thermally activated delayed fluorescence (TADF) core, we propose the “flagella effect” of solution-processed emitters for non-doped organic light-emitting diodes. In the single-molecular state, the emission behavior of these molecules remained consistent because of the unified TADF core; however, the aggregated films showed significantly different emission characteristics. A longer alkyl chain improved the photoluminescence quantum yield (PLQY) of the pristine film, but blocked the carrier transport and balance. Alternatively, a shorter alkyl chain matched the carrier mobility but sacrificed the PLQY and color quality. Accordingly, the maximum external quantum efficiency (EQE_max) of the molecule with a hexane chain was 18.7%, which was about 2 times that of the molecule with an ethane chain and approaching 80 times that of the molecule with a dodecane chain. Besides, compared with homologous alkyl chains, the molecule with multi-oxygen chains only achieved an EQE_max of 2.1%, which is attributed to the large degree of crystallization and aggregation of the multi-oxygen chains in the pristine film, as confirmed by atomic force microscopy measurement.