Spectral-resolved two-light beam photoresponse spectroscopy to resolve and investigate different photogeneration processes in organic semiconductors

Abstract

Abstract A photocurrent spectroscopy employing two-color light excitation beams scanned at different photon energies is proposed to resolve and to investigate different photogeneration processes involving exciton dissociation and above bandgap excitations in organic semiconductors. The basic concept of this spectroscopy is based on the phenomenon of photoresponse enhancement (PE), recently observed in pentacene films. In this phenomenon, the photocurrent, generated with modulated probe-light creating excitons, undergoes enhancement under additional above bandgap bias-light excitation, which acts to facilitate exciton dissociation. In the proposed photocurrent spectroscopy, the excitation energies of probe-light producing photocurrent undergoing PE and those of the bias-light which can induce PE are specified. This allows us to resolve photogeneration via exciton dissociation, which is facilitated by above bandgap excitations, and the onset of the above bandgap excitations, which facilitate exciton dissociation in pentacene films. It is found that light excitation in the region of 1.77–2.43 eV generates excitons diffusing with a diffusion length of 50–80 nm, which is compatible with that of triplet excitons, to the pentacene-substrate interface where dissociate via electron transfer to positive donor-like traps. For light excitation exceeding 2.43 eV, above bandgap excitations become strong competing triplet exciton creation by singlet fission. These results can be explained if the singlet fission and the above bandgap excitations are mediated by charge transfer states with energy-dependent electron-hole separations.