CdTe nanowire structures for solar cells

Williams, Benjamin Luke CdTe nanowire structures for solar cells. Doctor of Philosophy thesis, University of Liverpool.

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Abstract

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 , or . 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 (

Item Type:	Thesis (Doctor of Philosophy)
Additional Information:	Date: 2013-07 (completed)
Uncontrolled Keywords:	Solar Cells, Photovoltaics, Thin-films, CdTe, II-VI Semiconductors, Nanotechnology, Nanowires
Subjects:	?? QC ??
Divisions:	Faculty of Science and Engineering > School of Physical Sciences
Depositing User:	Symplectic Admin
Date Deposited:	08 Aug 2014 09:33
Last Modified:	16 Dec 2022 04:40
DOI:	10.17638/00014653
Supervisors:	Durose, Ken Dhanak, Vin
URI:	https://livrepository.liverpool.ac.uk/id/eprint/14653