Free and trapped hybrid charge transfer excitons at a ZnO/small-molecule heterojunction

We study the temperature-dependent electrical and optical properties of hybrid charge transfer excitons (HCTEs) at a ZnO/4,4{'}-bis(N-carbazolyl)-1,1{'}-biphenyl (CBP) organic/inorganic semiconductor heterojunction (OI-HJ). The electroluminescence spectrum of the HCTE formed by recombination of injected charge undergoes a hypsochromic shift from 1.65 ±{} 0.01 eV to 2.05 ±{} 0.01 eV with decreasing temperature from T=3000.16em0exK to 30 K at a current density of J=1000.16em0exmA0.16em0excm^-2, and with increasing voltage from V=1.50.16em0exV to 4.5 V. We observe an external quantum efficiency of 6.0 ±{} 0.2% at a wavelength of 332 nm for excitons generated in CBP, indicating exciton to charge conversion via HCTEs. However, no HCTE photoluminescence is observed at temperatures as low as T=250.16em0exK. A quantum mechanical model based on the coexistence of both free and trapped singlet HCTE states at the OI-HJ describes these observations. The free HCTE consists of a hole in CPB Coulombically bound with an energy of 9 meV to a delocalized electron in ZnO, and it is the precursor to photocurrent generation via CBP Frenkel excitons. The observed electroluminescence is due to electron injection into localized defect states at the ZnO surface that are bound to holes in CBP forming a trapped HCTE with binding energies of 60--430 meV and oscillator strength that is four orders of magnitude higher compared to the free HCTE.