Exciton dissociation mechanism and exciton density in organic semiconductors by modulated-photocurrent spectroscopy

Light excitation in organic semiconductors produces excitons, which are tightly bound electron-hole pairs that must be separated into mobile charge carriers to produce photocurrent in organic optoelectronic devices. The details of the exciton dissociation mechanism are required to control and improve the performance of these devices. Here it is demonstrated that modulated photocurrent spectrofscopy is sensitive to elucidating this mechanism. In this technique, a probe light beam of modulated intensity and a bias light beam of fixed intensity are combined to excite a two-terminal organic semiconductor device. The resulting modulated photocurrent is measured by a lock-in amplifier. In order to analyze the modulated photocurrent spectra, a model analysis is developed that incorporates in the photogeneration process different possible exciton disassociation mechanisms. Analytical expressions are derived which reveal the essential key spectral features that are the signature of each different exciton dissociation mechanism. Among the investigated mechanisms, exciton dissociation through electron transfer to donorlike traps in the lower part of the energy gap that creates single-type carriers (holes) photocurrent was found to dominate in the pristine small molecule organic semiconductors studied here. This process becomes evident by affecting the dynamics of the interactions of holes with the above traps which are captured by the modulated photocurrent spectroscopy, highlighting its sensitivity to this mechanism. Moreover, a very useful method is proposed to extract the steady-state exciton density.