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-NTol(2) 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-NTol(2), 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) (lambda(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 (lambda(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.