Polyimide passivation-enabled high-work function graphene transparent electrode for organic light-emitting diodes with enhanced reliability

Abstract

Chemical vapor deposition (CVD)-gown graphene has tremendous potential as a transparent electrode for the next generation of flexible optoelectronics such as organic light-emitting diodes (OLEDs). A semiconductor coating is critical to improve the work function but usually makes graphene rougher and more conductive, which increases leakage, and then significantly restrict device efficiency improvement and worsens reliability. Here an insulating polyimide bearing carbazole-substituted triphenylamine units and bis(trifluoromethyl)phenyl groups (CzTPA PI/2CF₃) with high thermal stability is synthesized to passivate graphene. The similar surface free energy allows the uniform coating of CzTPA PI/2CF₃/N-methylpyrrolidone on graphene. Despite of a slight decrease in conductivity, CzTPA PI/2CF₃ passivation enables a substantial reduction in surface roughness and improvement in work function. By using such CzTPA PI/2CF₃-passivated graphene as anode, a flexible green OLED is demonstrated with a maximum current, power, and external quantum efficiencies of 88.4 cd A⁻¹, 115.7 lm W⁻¹, and 24.8%, respectively, which are among the best of the reported results. Moreover, the CzTPA PI/2CF₃ passivation enhances the device reliability with extending half-life and reducing dispersion coefficient of efficiency. The study promotes the practical use of graphene transparent electrodes for flexible optoelectronics.