New carboline‐based donors for green exciplex‐forming systems

Abstract

The carbazole of a model compound CPTBF was replaced by αand β-carboline to give donors α-CPTBF and β-CPTBF, respectively. The introduction of carboline leads the new donors to have deeper highest occupied molecular orbital (HOMO) energy levels. Different electron acceptors were tested, among them, a new acceptor, 3,4-CN, was found to give exciplexes with efficient green emissions that are blue-shifted as compared to that of model CPTBF:3,4-CN system. The exciplex formations of α-CPTBF:3,4-CN and β-CPTBF:3,4-CN blends were verified with the significantly red-shifted emissions different from those of constituent donor and acceptor together with the delayed fluorescent observed by time-resolved PL decay experiments. The organic light-emitting diode (OLED) devices with α-CPTBF:3,4-CN and β-CPTBF:3,4-CN blends as the emitting layer showed a maximum external quantum (EQE) of 7.57 and 7.34%, respectively, which is higher as compared to that (EQE = 6.87%) of the model device employing CPTBF:3,4-CN. These results were attributed to the higher exciplex photoluminescence quantum yields due to the higher delay fluorescence components, deeper HOMO, and higher triplet energy of the carboline donors. In addition, the β-CPTBF:3,4-CN exciplex-based OLED exhibited better efficiency roll-off at higher luminesce due to more charge balance with less polaron formation, which was analyzed with time-resolved EL.

Exciplex-forming cohost systems with 2,7-dicyanofluorene acceptors for high efficiency red and deep-red OLEDs

“…S2 , the ground state structures of 27-DCN and 27-tDCN exhibit the almost orthogonal configurations which are consistent with the observed X-ray structures. Compared to the HOMO of 27-DCN, which is mainly distributed on fluorene with limited contribution from dicyanofluorene branch (spiro-conjugation) 13 , the HOMO of 27-tDCN is delocalized over the two tert-butylbenzene and dicyanofluorene, illustrating the through space conjugation from tert -butylbenzene to dicyanofluorene 40 (Fig. 3 ), agreeing to the X-ray analyzed structures.…”