Santarelli, Luca; (2018) Organic Semiconductors-Based Devices Electrical Reliability to Environmental Stress. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

In this thesis, I report on the characterisation of the response of organic semiconductor based devices, namely organic light-emitting diodes (OLEDs), organic field-effect transistors (OFETs) and organic photovoltaic diodes (OPVDs) to environmental stress factors such as electrostatic discharge (ESD) and neutrons irradiation. The ESD stress was obtained by means of a transmission line-pulsing (TLP), responsible to generate current pulses with an increasing amplitude and a duration of few tens of nanoseconds. The exposure to neutron irradiation was obtained in the pulsed neutron and muon source at ISIS part of the Rutherford Appleton Laboratories (RAL). The tested devices were: P3HT (poly(3-hexylthiophene)):PCBM ([6,6]- phenyl C61 butyric acid methyl ester) bulk heterojunction solar cells; PBTTT (poly(2,5-bis(3-hexadecylthiophen-2-yl)thieno[3,2- b]thiophene) and P3HT OFETs; F8BT (poly(9,9'-dioctylfluorene-alt-benzothiadiazole)) OLEDs. An analysis of both electrical (IV and JV curves, Electroluminescence (EL)) and optical (photoluminescence (PL), Raman Spectroscopy) characteristics of tested devices prior and following the exposure to various degrees of ESD, neutron irradiation or both is reported. For each tested device I obtained the respective TLP parameters (the leakage current (ILEAK), the TLP current (ITLP), the TLP voltage (VTLP), the TLP resistance (RTLP)) and the correlation of these with parameters extracted by means of their electrical/optical characterisation, namely: (i) the charge mobility, the threshold voltage (VTH) and the on/off ratio of OFETs; (ii) the current density (Jsc), the open-circuit voltage (Voc), the fill factor (FF) and the power conversion efficiency (η) of OPVs; (iii) the turn-on voltage (Von), the external quantum efficiency (EQE) and the EL maximum wavelength emission (λmax) of OLEDs. Importantly, the activity carried out in this thesis gives novel insights about the response of conjugated polymer-based devices with respect to the stressing stimuli (ESD events, cosmic rays) they are exposed to in their most suitable application fields (space, medicine, robotics), such as the energy necessary to cause a total or partial failure during ESD events, the requirements necessary to design electrical protections, the expected loss of device figures after a decade of exposure to cosmic rays. Interestingly, the results in this thesis reported point out, in most of the cases, an excellent robustness of these devices to both ESD and cosmic rays stress. In fact, whilst technology silicon-based is found to suffer a permanent failure in most of the cases for an applied TLP power lower than 400 W, polymer-based technology was found to withstand up to 800 W (OPVs and OLEDs) without suffering permanent damages. As regards the stress correlated to the same dose of neutrons irradiation, optoelectronic devices based on inorganic semiconductors suffer of a 90% reduction of their figures of merit (JSC, h), whilst the same figures are reduced of only 20% in polymer-based devices. Although previous works are reported in literature, the work reported in this thesis, at the best of my knowledge, is the first work reporting a systematic quantitative TLP characterisation of organic devices along with a qualitative description of the effects on the organic materials within these devices because of the conditions imposed by the TLP test (high-frequency, high-voltage). Therefore, this thesis opens a new scenario proposing an investigating tool aimed both at measuring parameters useful for the design of the devices and at highlighting organic materials properties that can lead organic electronics to gain its definitive momentum.

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