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Functional heterointerfaces via electromodulation spectroscopy 10.17863/CAM.16560 Khong, Siong-Hee Functional heterojunctions in organic electronic devices are interfaces formed either between a conducting electrode and an organic semiconductor or between two different organic semiconductors in blended and multilayered structures. This thesis is primarily concerned with the energy level alignment and the interfacial electronic structures at functional heterojunctions encountered in electronic devices made with solution-processable semiconducting polymers. Investigations on the electronic structures across these heterointerfaces are performed with the combined use of electromodulation and photoemission spectroscopic techniques. Electromodulation and ultraviolet photoemission spectroscopic techniques enable direct determination of the surface work functions of electrodes at the electrode/semiconducting polymer interfaces. We overcame the inherent problems faced by electromodulation spectroscopy, which undermine accurate determination of interfacial electronic structures, by performing electroabsorption (EA) measurements at reduced temperatures. We showed in this thesis that low-temperature EA spectroscopy is a surface sensitive technique that can determine the interface electronic structures in electrode/polymer semiconductor/electrode diodes. Using this technique, we demonstrated that the energy level alignments in these solution-processed organic electronic devices are determined by the surface work functions of passivated metals rather than by those of clean metals encountered in ultrahigh vacuum. This thesis also discloses our studies on the electronic structures in polymeric diodes with type II donor-acceptor heterojunctions using the EA spectroscopy. We showed that minimising meausurement temperature and attenuating EA illumination intensity enable accurate determinations of the electronic structures in these devices. We demonstrated that the electronic structures and the performance characteristics of multilayered polymer light-emitting diodes are also determined by the surface work functions of passivated metals. Our investigations confirm that electronic doping of the organic active layers, rather than minimisation of the Schottky barriers at electrode/polymer contacts, holds the key in realising high-performance organic light-emitting devices. University of Cambridge 2010-07-06 Thesis Doctoral Doctor of Philosophy (PhD) eng http://www.dspace.cam.ac.uk/handle/1810/226748 https://www.repository.cam.ac.uk/handle/1810/226748 https://apollo8-f-pro.lib.cam.ac.uk/bitstreams/d75d1e38-93c6-407a-a1db-aaddb4882762/download f05c9cb7f1f9b08ffd9932de6e98725d https://apollo8-f-pro.lib.cam.ac.uk/bitstreams/725e7558-000f-4837-8431-361de57b79aa/download 0b1a12d4eb169023d6d65183e2d7aac9 https://www.rioxx.net/licenses/all-rights-reserved/ Organic electronics Energy level alignments Electromodulation Spectroscopy Electroabsorption Stark effect Heterojunctions Push back effects Interface dipole