Exciplex-forming systems with extremely high RISC rates exceeding 107 s−1 for oxygen probing and white hybrid OLEDs

Abstract

A rare example of exciplex-forming systems with highly efficient thermally activated delayed fluorescence (TADF) exhibiting reverse intersystem crossing (RISC) yields of 97 and 96% and RISC rates of 5.1 × 107 and 3.6 × 107 s−1 at 300 K are developed for oxygen probing and organic light-emitting diodes (OLEDs). The exciplexes forming systems consist of a commercial acceptor 2,4,6-tris [3-(diphenylphosphinyl)phenyl]-1,3,5-triazine and a donor diphenyl bicarbazole substituted by electron withdrawing fluoro- or trifluoromethyl units. Affected by the donor modification, exciplex-forming system containing fluoro-substituted diphenyl bicarbazole demonstrated higher TADF efficiency due to its higher hole mobility (μh = 1.1·10−3 cm2/V·s at 1.6·105 V/cm) and lower activation energy (Ea) of 52 meV for exciplex formation in comparison to Ea of 127 meV observed for exciplex formation of trifluoromethyl-substituted diphenyl bicarbazole which is characterized by μh of 7.7·10−5 cm2/V at the same electric field. Due to extremely high RISC rates (one of the highest for TADF emitters observed up to now to the best of our knowledge), TADF was detected even under oxygen atmosphere. The developed exciplex emission based oxygen probes are characterised by non-linear oxygen sensitivity with the Stern–Volmer constants of ca. 3.27·10−3 and 4.7 ppm−1. In order to demonstrate possible OLED applications of the developed exciplex-forming systems, solution-processed white hybrid OLEDs with very high colour rendering index (CRI) of 92, colour temperature of 3655 K and CIE1931 coordinates of (0.384, 0.399) were fabricated using exciplex-forming emitters.

Keywords

• Thermally activated delayed fluorescence
• Organic-light-emitting diode
• Optical sensor
• Carbazole