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Title: CdTe nanowire structures for solar cells
Author: Williams, Benjamin Luke
Awarding Body: University of Liverpool
Current Institution: University of Liverpool
Date of Award: 2013
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This thesis investigates the growth and characterisation of CdTe and core-double shell ITO/CdS/CdTe nanowires (NWs), ITO/ZnO/CdS/CdTe/Mo thin-film solar cells and ITO/ZnO/CdS/CdTe(NW)/CdTe/Mo core-triple shell NW solar cells. First, the generation of Au-catalysed CdTe NWs arrays on Mo foils is reported, with CdTe being deposited by close-space sublimation. NWs were up to 20 μm long, had diameters in the range 5 – 500 nm and densities in the range 106 – 107 cm-2. A vapour-liquid-solid mechanism of NW growth, mediated by a Au-Te liquid catalyst, which accounts for the initial delay to NW nucleation (and the observed film growth) is postulated. By fitting a theoretical model of NW growth to experimental NW radius-time data, values for the sidewall diffusion length (2 μm) and sticking probability (0.61) were obtained. NW growth was also achieved on CdTe/Mo, with higher NW densities (108 cm-2) achieved compared to growth directly on Mo. NW defects were evaluated by TEM and photoluminescence (PL). TEM analysis showed low-energy Σ = 3 grain boundaries present in the NWs, their incidence being dependent on the growth axis, which was either <110>, <111> or <112>. No high energy grain boundaries were observed. Their low-temperature PL spectra were dominated by excitonic emission with rarely observed above-gap emission also being recorded. PL also provided evidence of a deep level due to Au. UV-VIS-IR measurements showed that above-gap reflectance of NW arrays (~ 0.1%) was lower than for films of equivalent material (~ 10%). Methods of coating the NWs with conformal CdS and ITO shells were explored, with RF sputtering proving most suitable. HRTEM imaging demonstrated that the CdS/CdTe interface was epitaxial, the quality of the core-shell interface being thought to be critical for NW solar cells. Low temperature PL spectra of CdS/CdTe NWs showed CdS luminescence typical of single-crystal material and also implied that interdiffusion at the core-shell interface occurred. Above-gap reflectance for core-double shell ITO/CdS/CdTe NWs was ~ 0.1%. The fabrication procedures for thin-film CdTe devices on Mo foils were investigated so that understanding could be transferred to NW device fabrication. For thin-film devices, a peak efficiency of 8.0% was achieved upon incorporation of a double-anneal treatment process and a highly-resistive transparent ZnO layer. The main limitation to performance was the rectifying back contact, having a barrier height of = 0.51 eV. Methods to improve the quality of the contact are suggested based on device modelling. Simulations comparing the operation of thin-film and NW solar cells predict that when minority carrier diffusion lengths are short (< 0.7 μm), JSC enhancements can be achieved by adopting the NW device’s radial junction configuration. For the full NW device, efficiencies of 2.5% were obtained.
Supervisor: Durose, Ken; Dhanak, Vin Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: QC Physics