Excited state and charge dynamics of hybrid organic/inorganic heterojunctions. II. Experiment

Abstract

In our companion paper (Paper I) [C. K. Renshaw and S. R. Forrest, Phys. Rev. B 90, 045302 (2014)], we developed a model for charge transport and photogeneration at hybrid organic/inorganic semiconductor heterojunctions (OI-HJs). Here we apply the model to two planar bilayer hybrid photovoltaic devices: the first using the wide-band gap $n\text{\ensuremath{-}}{\mathrm{TiO}}_{2}$ in combination with the hole transporting tetraphenyl-dibenzoperiflanthene (DBP), and the second based on the moderate-band gap $n$-InP and the hole transporting pentacene (PEN). We measure the external quantum efficiency (EQE) and current density vs voltage ($J\text{\ensuremath{-}}V$) characteristics of both devices as functions of temperature. The EQE spectra for both ${\mathrm{TiO}}_{2}$/DBP and InP/PEN provide convincing evidence that Frenkel states generated in the organic form hybrid charge transfer excitons (HCTEs) at the OI-HJ that are subsequently dissociated into free charges, and then collected at the opposing electrodes. The dissociation efficiency is found to be strongly influenced by the presence of surface states, particularly in the InP/PEN device. We further develop the $J\text{\ensuremath{-}}V$ model from Paper I to include an analytical expression for space-charge effects in the organic at high currents. Model fits to the $J\text{\ensuremath{-}}V$ data suggest that the temperature-dependent hole mobilities in both DBP and PEN result in increasing space-charge effects at low temperatures. Furthermore, we find that the $J\text{\ensuremath{-}}V$ characteristics of the ${\mathrm{TiO}}_{2}$/DBP device both in the dark and under illumination are governed by interface recombination. In contrast, the dark current in the InP/PEN device is governed by injection over the OI-HJ barrier, whereas the photocurrent is dominated by interface recombination. This work elucidates the role of the HCTE state in photogeneration, and the applicability of our model to a range of important optoelectronic devices.