Improved Charge Recombination Efficiency in Organic Light-Emitting Transistors via Luminescent Radicals

Abstract

Luminescent radicals are attracting attention as emitters in electroluminescent devices thanks to the exploitation of doublet excitons. Recent studies reveal that exciton formation in radical organic light-emitting diodes (OLEDs) primarily occurs through a charge trapping mechanism. Although typically detrimental for OLEDs, this might be a key process to elucidate light emission in organic light-emitting transistors (OLETs). Here, a unipolar n-type architecture suitable for the implementation of radical emitters is introduced, designed based on computational calculations. The operation of the as-realized devices incorporating the newly synthesized [2,6-dichloro-4-(2,6-dimethoxyphenyl)phenyl](3,5-dichloro-4-pyridyl) (2,4,6-trichlorophenyl)methyl radical is investigated via transient electroluminescence measurements to demonstrate the occurrence of long-living emission ascribed to the charge trapping mechanisms. Moreover, a comprehensive understanding of the processes governing radical-OLET is obtained by recording complete 2D maps of both optical and electrical response of the device as a function of applied voltages. Notably, the trapping of electrons by radical moieties is demonstrated to generate a negative charge density in the emissive layer that facilitates holes to be injected: increasing the balance of opposite charge carriers, a tenfold enhancement of the external quantum efficiency (EQE) at the proper source-drain and source-gate voltage conditions is reported to reach a maximum EQE value of 0.2%.