Highly efficient organic light emitting diodes fabricated by solution process with new hole transport materials cross-linked at 120 °C

This work presents a rational design strategy for the synthesis of three thermal cross-linkable polystyrene-based hole transport copolymers (PPPIC-co-PBCB, PPIC-co-PBCB, PBIC-co-PBCB) by a simple free radical polymerization of planar structured indenocarbazole (IC)-based monomers having extended conjugated length and thermal cross-linkable benzocyclobutene (BCB). The materials show an extremely low cross-linking temperature of 120 °C and outstanding hole mobility of 1.01⨯10⁻² cm²V⁻¹s⁻¹ at 4.0⨯10⁵ Vcm⁻¹. This value is one of the highest mobilities reported for thermal cross-linked hole transport materials (x-HTMs) in the literature. The preferred horizontal orientation of transition dipole moment (TDM), which is hardly obtained in spin coated film, was successfully achieved, and used for charge transport analysis. Herein, we also confirmed that the cross-linking reaction in our x-HTMs did not have any negative effect on the mobility of the layer, a correct method for mobility calculation from the space-charge limited current (SCLC) model was suggested, and the extension of interface mixing was also estimated for the first time. Furthermore, solution-processed organic light-emitting diode (s-OLED) device employing PPPIC-co-PBCB as HTM shows ultra-efficient performance of 130.5 cdA⁻¹ and 37.3 % external quantum efficiency. To the best of our knowledge, this result is also one of the best efficiencies for phosphorescent s-OLEDs reported. The synthetic strategy for the polymers is also very simple with the cross-linkable monomers attached to the polystyrene backbone.