Abstract
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.
The schematic diagram of interfacial accumulated charges formed in the device with (a) 2 nm PMMA and (b) 10 nm PMMA. The capacitance-voltage characteristics of the device ITO/ PEDOT: PSS/PMMA (0, 2, 4, 10 nm)/ Alq3 (80 nm)/LiF(0.6 nm)/Al (80 nm) under dark (a) and illumination (b) condition were detected in 2 kHz and normalized to the capacitance at low voltage. 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. [Display omitted]
•Two peaks in capacitance-voltage curves of ITO/PEDOT: PSS/PMMA/Alq3/LiF/Al were observed under dark and light conditions.•The dark capacitance for device with PMMA shows difference with respect to the control device (PMMA=0nm).•The first peak of 2, 4 and 10nm PMMA device located at 3, 3.5 and 5V, respectively were stronger than the control device.•Under illumination, the first capacitance changed more greatly with increased PMMA due to the storage of space charges.•The second peak under illumination was derived from injected carriers plus photo-carriers.