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Title: The surface chemistry and interface engineering of lead sulphide colloidal quantum dots for photovoltaic applications
Author: Neo, Darren Chi Jin
ISNI:       0000 0004 6496 1628
Awarding Body: University of Oxford
Current Institution: University of Oxford
Date of Award: 2016
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This thesis examines the effect of lead sulphide (PbS) CQDs' surface chemistry and interfaces to their photovoltaic performance. Using PbS CQDs as the starting material, cation-exchange was utilised to form PbS/CdS core/shell CQDs, which were thoroughly characterised and the improved surface passivation was shown by increased photoluminescence yield and lifetime. The core/shell CQDs were incorporated into a ZnO/CQD heterojunction solar cell device and showed a substantial improvement of the mean open-circuit voltage (Voc), from 0.4 V to 0.6 V, over PbS reference devices. By optimising shell thickness and surface ligands, core/shell CQD devices with average device efficiency of 5.6 % were fabricated as compared to 3.0 % for unshelled PbS devices. The lower defect density due to better passivation confers lower carrier density in core/shell CQD film. To take advantage of low defect concentration and to aid charge extraction, a 3 dimensional quantum funnel concept was sought of by blending two populations of PbS/CdS CQDs of different sizes. By incorporating a blend component within a heterojunction device, even when the device thickness is beyond what is optimal for the depletion width and the diffusion length of the system, high Voc is still maintained. In a separate study, a p-i-n device strategy was examined, and with this approach, a maximum device efficiency of 6.4 % was achieved. Despite the improvements made to Voc by optimizing surface passivation, fill factors of the devices are low. By using poly(3-hexylthiophene-2,5-diyl) (P3HT) as a hole transport material (HTM), fill factor and the overall performance improved over a reference device without the HTM. Further studies showed that oxidation of the HTM material results in increased p-type characteristic, thus optimising hole transport. This beneficial oxidation process also makes the device air-stable. From this, devices of up to 8.1 % efficiency and devices with fill factor as high as 0.72 were fabricated.
Supervisor: Watt, Andrew ; Assender, Hazel Sponsor: ASTAR PhD National Science Scholarship
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available