A Comparative Study via Photophysical and Electrical Characterizations on Interfacial and Bulk Exciplex-Forming Systems for Efficient Organic Light-Emitting Diodes

An efficient organic light-emitting diode based on the BCzPh:CN-T2T exciplex as an emitting layer (EML) has been fabricated by exploiting charge balance and favorable molecular orientations. To further understand the details of the exciplex-forming mechanism, time-resolved photoluminescence (TRPL), capacitance-voltage (CV), impedance spectroscopy (IS), and transient electroluminescence (EL) measurements were used to probe the photophysical and electrical characteristics of EL devices by incorporating interfacial (BCzPh/CN-T2T) and bulk (BCzPh:CN-T2T) exciplexes as the emitting layer. Interfacial- and bulk-exciplex devices exhibit a maximum external quantum efficiency (EQE) of 7.7 and 26.4%, respectively. The reason for different device performances was rationalized by comparing the accumulated amount of charge density at the EML's interface responsible for exciplex emission. In addition, the TRPL measurement monitored from short to long wavelengths was used to explore the harvest of nonradiative triplets back to singlets via reverse intersystem crossing and to examine the efficiency of delayed fluorescence. The bulk-exciplex system showed a distinct delayed fluorescence as compared to the interfacial one, which was also corroborated by the observation in the transient EL. The result indicates that the bulk exciplex can reduce the accumulated charge in the EML rapidly, resulting in improvement of EL efficiency. This assumption was further verified by CV and IS measurements. Our results reveal that the accumulated charge density and the bulk resistance of the bulk-exciplex device are much lower as compared to those of the interfacial counterpart device.