Patel, Adil; (2025) Organic Light-emitting Diodes: Computational studies of charge carriers balance and experimental characterisation of new materials. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

In this thesis, I have investigated the optimisation of the external quantum efficiency (EQE) of organic light-emitting diodes (OLEDs), using a combination of both computational and experimental approaches to deal with both energy levels alignment issues, and to verify the impact of certain materials design strategy on the everimportant luminescence quantum efficiency of the active layers. Firstly, the effect of interfacial energy-levels line-up and charge mobilities on the EQE was investigated by simulating a single-layer OLED using the software package called SETFOS 5.4 developed by Fluxim AG. I chose a device with a poly(9,9- dioctylfluorene-alt-benzothiadiazole) (F8BT) active layer and a calcium cathode. Both the zero-field electron mobility and anode work function were varied, with the aim of analysing the contribution of each variable to the device electron-hole balance and EQE. The zero-field electron mobilities were set between 10-3 to 10-1 cm2/Vs while the anode work function was systematically varied from 4.9 eV to 5.5 eV. When the hole injection barrier was smaller than 1.0 eV, the EQEs of the devices exhibited a dependence on both anode work function and electron mobility. Devices with comparatively low electron mobilities show the highest EQE for larger hole energy barriers, and the opposite is true for devices with high electron mobilities which sustain higher EQEs for smaller hole energy barriers. This signifies that low electron mobilities have a mitigating effect on large hole injection barriers. Secondly, a new class of near-infrared (NIR) carborane-containing polymers were measured for the presence of aggregation-induced emission (AIE). I measured the photoluminescence properties of three polymers in both solution and films: paracarborane indacenodithiophene (pCbT2-IDT), meta-carborane indacenodithiophene (mCbT2-IDT), and ortho-carborane indacenodithiophene (oCbT2-IDT). I found that pCbT2-IDT and mCbT2-IDT had a decreasing PLQY when they were more aggregated, whereas oCbT2-IDT showed the opposite, thus confirming the presence of AIE.

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