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Liewald, C.; Strohmair, S.; Hecht, H.; Glowacki, E. D.; Nickel, B. (2018): Scanning photocurrent microscopy of electrons and holes in the pigment semiconductor epindolidione. In: Organic Electronics, Vol. 60: pp. 51-56
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Photocurrent microscopy is used to characterize the kinetics of electrons and holes in organic field-effect transistors (FETs) with the hydrogen-bonded pigment epindolidione as active layer. The method relies on electrons and holes, generated on local illumination, which are provided after exciton splitting, to probe charge trapping. In the dark, hole conduction is observed for negative gate voltage while no electron conduction is observed for positive gate voltage. However, under illumination, a fast displacement current with 60 mu s onset time and 1 ms exponential decay occurs for positive gate voltage, which can be explained by exciton splitting underneath the semitransparent top contact followed by subsequent electron trapping and hole extraction. Afterward, trapped electrons hop via further trap states within the film to the insulator into interface traps (13 ms exponential decay) which induce a positive threshold voltage shift in the FET transfer curves for hole transport. Photocurrent microscopy confirms that the displacement current occurs only for illumination under and near the semitransparent source/drain contacts, which act here as metal-insulator-semiconductor (MIS) diodes. For negative gate voltage instead, the photocurrent comprises an enhanced hole current in the FET channel between the contacts. In the channel region, the detrapping of holes at the interface with the insulator (3 ms time constant) enhances the transistor current at low frequencies < 1 kHz, whereas the displacement current between the contacts and the gate is observed only at frequencies > 10 kHz. Thus, we show here that photocurrent microscopy allows to identify the kinetics of electrons and holes in traps close to the contacts and in the FET channel of pigment transistors.