Quantization of exciton charging in organic semiconductor

Excitons are widely found in semiconductors. In contrast to inorganic-based semiconductors, where the presence of excitons may need special morphonology, like low-dimensional layer structure, the excitons prevailingly exist in molecular organic semiconductors in a three-dimension solid state. The excitons can undergo a charging process to yield another kind of quasiparticles originating from the coupling of excitons and free charges. Herein, we investigated the interaction between excitons and free charges in a representative arylamine-derivative organic semiconductor(NPB), by the time-of-flight method. Distinctly different dynamics were observed for the NPB device with different free charge densities. It was proposed that it is the exciton charging that leads to the unique dynamic property of the device with electro-injected hole current. It was also observed that the exciton charging occurs in a quantized manner, which is dependent on the magnitude of the background stationary current. This work may suggest that the exciton charging effect is universally present in organic optoelectronic devices, however, it is only easily observed under certain dynamic conditions.