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Graphene-based and Low-gap Semiconductors for Electronic Applications

Li, P; (2012) Graphene-based and Low-gap Semiconductors for Electronic Applications. Doctoral thesis , UCL (University College London).

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Abstract

This thesis investigates electronic and optical properties of graphene, graphene oxide (GO) and low-gap polymers, and their applications in light-emitting diodes (LEDs) and photovoltaics (PVs). Charge transport is crucial for achieving high efficiency in polymer photovoltaics. Few-layer graphene (FLG) was synthesized via liquid-phase exfoliation and served as an intermediary in a ternary structure to facilitate charge transport rather than acting as an electron acceptor. To investigate the role GO can play, a bilayer photovoltaic device incorporation of poly (3-hexylthiophene-2,5-diyl) (P3HT) was fabricated which demonstrated that P3HT/GO heterojunctions could be created. The flexibility of GO is also discussed. Molecule interchain interaction can adversely affect optical properties such as photoluminescence (PL) and electroluminescence (EL). OLEDs incorporated with GO quantum dots (GOQDs) have been fabricated. PL quenching is observed with the addition of GOQDs, which is a result of a competition between energy transfer from polymer to GOQDs and prevention of interchain interaction by GOQDs. A blue-shift in PL as well as EL is observed. The addition of GOQDs could shift the value of light-on voltage due to a higher mobility of GOQDs compared with the host polymer. Also, application of GOQDs can balance charge carriers. Finally, a low-gap polymer with a broad absorption in the near infrared range has been studied both in PVs and LEDs. Binary blends have been investigated in terms of donor:acceptor ratio, annealing treatment, open circuit voltage (Voc) and fill factor (FF), showing different performance, especially with respect to thermal annealing and FF. Polymer-polymer PV with a high Voc has been achieved. For infrared LEDs, we have demonstrated ~ 1 μm EL emission from the pure polymer device and a blue-shift in the blended device, due to the reduction of polymer aggregation.

Type: Thesis (Doctoral)
Title: Graphene-based and Low-gap Semiconductors for Electronic Applications
Language: English
UCL classification: UCL > Provost and Vice Provost Offices > UCL BEAMS
UCL > Provost and Vice Provost Offices > UCL BEAMS > Faculty of Maths and Physical Sciences
URI: http://discovery.ucl.ac.uk/id/eprint/1362648
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