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Title: Photo-effects in organic MIS Devices and related structures
Author: Rostirolla, Bruno
ISNI:       0000 0004 7961 3634
Awarding Body: Bangor University
Current Institution: Bangor University
Date of Award: 2019
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The field of organic electronics has exponentially increased, especially in the last decade, with the discovery of numerous applications. In the first part of this work, the behaviour of poly(3-hexylthiophene):[6,6]-phenyl C61 butyric acid methyl ester (P3HT:PCBM) blends as active layer of a MIS capacitor was investigated. Electrical characterisation of devices was performed both in the dark and under illumination with light of controlled wavelength. When the device was exposed to light of different wavelengths, minimum capacitance shifts in the capacitance-voltage (C-V) plots were observed. Exposure to near-infrared (NIR) light also shifted the minimum capacitance, which was investigated with the use of different LUMO level PCBM-like molecules. The NIR response was likely to be related to the HOMO of P3HT and LUMO of PCBM energy difference. The device quickly recovered after light exposure and only a small anticlockwise hysteresis was observed (less than 0.5 V), indicating that a small quantity of electrons remained trapped at the semiconductor/insulator interface. A charge-injection-device with the blend as the active layer was also fabricated, however, electron transfer was not observed. The response of dinaphtho[2,3-b:2',3'-f]thieno[3,2-b]thiophene (DNTT)-based MIS capacitors and thin film transistors under different ambient conditions was also investigated. Analysing the C-V plots of DNTT MIS capacitors when exposed to light, bias stress, periods of vacuum and measurement frequency lead to the conclusion that exposure to air is needed in order to trigger light-related processes in the semiconducting layer. These processes however, tend to follow DNTT's absorption profile. A comparable feature was observed in the DNTT thin film transistors, where transistors characteristics significantly improved after 2.5 days of air exposure. Hole mobility, calculated both in the linear and saturation regimes, improved to ~0.3 cm2/Vs after the period of air exposure. A theoretical model that describes the features observed in the C-V plots of MIS capacitors was developed. In order to account for limited anticlockwise hysteresis, electron capture and emission rate from traps, described using Shockley-Read-Hall statistics, was introduced. The model correctly predicts the behaviour of MIS capacitors where electron trapping at the semiconductor/interface is not prominent.
Supervisor: Not available Sponsor: Merck Chemicals Limited
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: photocapacilance ; organic electronics