Direct observation of charge dynamics at electrodes and roles of buffer layers for organic devices via high-frequency impedance spectroscopy

Impedance spectroscopy (IS) measurements for frequencies up to 120 MHz were performed for a series of organic devices with various buffer layers (BLs) introduced into the electrode-organic interfaces. The fast charge dynamics observed in the high-frequency range at the interface provided clear information regarding the roles of BLs, which have not been the focus of many studies. Comparative studies on devices with two active layers and four different electrode configurations determined the origin of the signals observed via IS and clarified the charge and recombination dynamics specific to various electrodes. In a photovoltaic active layer, the suppression of charge recombination at the interface of an electrode under illumination was clearly observed owing to the BLs. In another hole-rich active layer that does not entail charge recombination, a model for charge dynamics of the interface state that can analytically characterize the BLs is proposed.