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Title: Understanding the open-circuit voltage in small molecule heterojunction photovoltaic devices
Author: Chauhan, Virendra
ISNI:       0000 0004 2703 1675
Awarding Body: University of Warwick
Current Institution: University of Warwick
Date of Award: 2010
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Organic photovoltaic devices continue to attract attention as they offer the prospect of low-cost energy production on flexible substrates. However, there remains a need to understand factors which affect device performance at a more fundamental level. One particular area of focus has been related to the device open-circuit voltage (Voc) whose origins are still the subject of debate. This thesis therefore sets out to try and understand and correlate the electronic properties of materials with critical device parameters, such as the Voc. The experimental determination of the work function, ionisation potential and electron affinities are important for predicting and understanding the energetics at the interface formed when two solids are combined. The offsets at the interface between different oxide / phthalocyanine / fullerene heterojunctions were characterised. It was found that the upper limit to Voc is governed by the offsets between the HOMO state in the donor and the LUMO state of the acceptor (the effective bandgap). Furthermore, by understanding the electronic structure and characterising device performance in the limit of low temperature, it was found that the experimental Voc is limited by the reverse saturation current and the exciton binding energy - a bottleneck for organic semiconductors. An understanding of the benefits of incorporating molybdenum oxide into a device architecture was achieved by conducting a thorough investigation on its electronic structure. The conduction mechanism is believed to be dictated by a distribution of near Fermi level defect states caused by non-stoichiometry. Finally, the effects of tuning the molecular orientation on the device performance were investigated with a flat lying perylene derivative. Optimised device architectures based on the photoactive ClAlPc / C60 heterojunction displayed an efficiency of 3.0 %.
Supervisor: Not available Sponsor: Engineering and Physical Sciences Research Council (EPSRC) ; Excitonic Solar Cell Supergen Consortium
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: QD Chemistry ; TK Electrical engineering. Electronics Nuclear engineering