Investigation on light-induced storage of charges with capacitance/conductance-voltage and its frequency characteristics

Admittance analysis has been widely used to investigate the underlying mechanism of organic semiconductors. In this article, charges stored in organic devices with Alq3 and PMMA under light is investigated using capacitance/conductance-voltage (C/G-V) as well as its frequency characteristics. For the control device without PMMA, the capacitance has two peaks. Under dark condition, the first peak is depletion capacitance determined by built-in field and the second peak is diffusion capacitance used to inject carriers. Under illumination, however, the first peak is related to storage of space charges and the second peak is derived from both injected and light-induced carriers. By inserting 2 nm, 4 nm and 10 nm PMMA, the first capacitance peak under illumination ceased to behave as a peak gradually. As the thickness increased, after the first capacitance reached its maximum, it decreased slowly to thin PMMA but not up to 10 nm PMMA. Combined with C/G-V and its frequency characteristics, it evidently originated from the reversed field induced by interfacial accumulated charges. All the results emphasize the importance of the approach based on C/G-V and its frequency characteristics, paving a new way for the investigation of the storage of charges and the selection of blocked layer.