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Title: Quantum structures in photovoltaic devices
Author: Holder, Jenna Ka Ling
Awarding Body: University of Oxford
Current Institution: University of Oxford
Date of Award: 2013
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A study of three novel solar cells is presented, all of which incorporate a low-dimensional quantum confined component in a bid to enhance device performance. Firstly, intermediate band solar cells (IBSCs) based on InAs quantum dots (QDs) in a GaAs p-i-n structure are studied. The aim is to isolate the InAs QDs from the GaAs conduction band by surrounding them with wider band gap aluminium arsenide. An increase in open circuit voltage (VOC) and decrease in short circuit current (Jsc) is observed, causing no overall change in power conversion efficiency. Dark current - voltage measurements show that the increase in VOC is due to reduced recombination. Electroreflectance and external quantum efficiency measurements attribute the decrease in Jsc primarily to a reduction in InGaAs states between the InAs QD and GaAs which act as an extraction pathway for charges in the control device. A colloidal quantum dot (CQD) bulk heterojunction (BHJ) solar cell composed of a blend of PbS CQDs and ZnO nanoparticles is examined next. The aim of the BHJ is to increase charge separation by increasing the heterojunction interface. Different concentration ratios of each phase are tested and show no change in Jsc, due primarily to poor overall charge transport in the blend. VOC increases for a 30 wt% ZnO blend, and this is attributed largely to a reduction in shunt resistance in the BHJ devices. Finally, graphene is compared to indium tin oxide (ITO) as an alternative transparent electrode in squaraine/ C70 solar cells. Due to graphene’s high transparency, graphene devices have enhanced Jsc, however, its poor sheet resistance increases the series resistance through the device, leading to a poorer fill factor. VOC is raised by using MoO3 as a hole blocking layer. Absorption in the squaraine layer is found to be more conducive to current extraction than in the C70 layer. This is due to better matching of exciton diffusion length and layer thickness in the squaraine and to the minority carrier blocking layer adjacent to the squaraine being more effective than the one adjacent to the C70.
Supervisor: Watt, Andrew; Assender, Hazel Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: Optoelectronics ; Materials engineering ; Advanced materials ; Condensed Matter Physics ; Semiconductor devices ; Nanostructures ; Nanomaterials ; quantum dots ; lead sulphide ; zinc oxide ; gallium arsenide ; aluminium arsenide ; Indium Arsenide ; Graphene Electrode ; Squaraine ; Fullerenes ; Solar Cells ; Photovoltaics ; Advanced characterisation ; External quantum efficiency ; electroreflectance ; colloidal quantum dots