Kamaludin, Muhammad Shu’aib; (2019) Growth, modification and characterisation of organic semiconductor crystals. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

This thesis describes the measurement of the near-surface properties of organic semiconductors, and how these properties can be controllably modified, for use in future devices. Firstly however, a physical vapour transport organic single crystal growth system was built, and it was found that lower purity crystals (near-surface oxygen atomic content ~1 %) were more susceptible to oxidation than higher purity crystals (near-surface oxygen atomic content ~0.5 %). Steady-state photoluminescence spectra of the former show strong peaks sensitive to oxygen-related impurity presence. 6,6’-Dibromoindigo (Tyrian purple dye) single crystals were also grown and were, for the first time, characterised by atomic force microscopy and Raman spectroscopy. Nanopatterning of insulating oxide features on rubrene crystal surfaces was performed by local anodic oxidation, utilising an atomic force microscope tip. Oxide height increases with voltage bias and decreases with tip writing speed; 22 nm gaps at the surface between two parallel oxide lines were observed. Oxide depth was determined by conductance tomography, exposing subsurface layers without using chemical etching, whilst simultaneously mapping material conductance. Oxide depth exceeds its height; depth-to- height ratio is frequently above 1.6. Below an electric field of ~3×10⁶ V/cm oxide growth ceases, resulting in a maximum oxide vertical extent of ~60 nm at a voltage bias of ~20 V. Direct printing of 22 ± 3 μm wide electrical contacts on an organic semiconductor substrate was demonstrated, without requiring masking, lithography or surface treatment. Permanent photoconductivity reduction in rubrene two-terminal ‘coplanar’ devices was observed when devices were stored in ambient atmosphere after humidity exposure. Oxygen was localised near the surface. (75.2 ± 7.6) % of surface molecules are affected by oxygen incorporation and these may hinder exciton dissociation, correlated to the observed 60-90 % photocurrent reduction. Controllable mechanical scratching is used to study the flow of charge carriers in the near surface region of rubrene.

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