Substitution effect on carbazole-centered donors for tuning exciplex systems as cohost for highly efficient yellow and red OLEDs

Abstract

Three new carbazole-based hole-transporting materials PTCz-9′Cz, PTCz-3′TCz, and PTCz-NTol₂ were synthesized, characterized, and examined for the propensity of exciplex formation with the electron-transporting material PO-T2T. Through enhancing the electron-donating ability of the substituents, the HOMO energy of the donors, PTCz-9′Cz, PTCz-3′TCz, and PTCz-NTol₂, increased from −5.59 eV, −5.40 eV to −5.02 eV, respectively. Consequently, the photoluminescence of the corresponding exciplex is red-shifted, giving blue, green and orange emissions. Among these three donor : acceptor blends, the device utilizing PTCz-9′Cz : PO-T2T (2 : 1) as the emitting layer rendered the best device efficiency with a maximum external quantum efficiency (EQE_max) of 10.9%. Then, the two D–A–D-configurated fluorescent emitters, iCzPNT and iCzBTh2CN, with different electron-accepting cores were introduced as the emitting guests of this exciplex-forming cohost system to explore the possibility of high efficiency OLEDs with all organic materials. The device with 10 wt% iCzPNT as the dopant was found to give yellow electroluminescence (EL) (λ_EL= 578 nm and CIE (0.53, 0.46)) with an EQE_max of 7.8%, whereas the device with 10 wt% iCzBTh2CN as the dopant gave red EL (λ_EL = 618 nm and CIE (0.63, 0.37)) with an EQE_max of 8.0%. Thanks to the TADF feature of the exciplex-forming system, the energy transfer between the exciplex host and fluorescent guest proved to be a promising strategy to efficiently harvest the electro-generated singlet and triplet excitons for giving high efficiency OLED devices. This work highlights the versatility of introducing a substitution effect onto the electron donor to manipulate the emission wavelength of the exciplex and facilitate the identification of a good exciplex-forming system for serving as the cohost of tailor-made fluorescent emitters.