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Title: Copper oxide thin films grown by pulsed laser deposition for photovoltaic applications
Author: Farhad, Syed Farid Uddin
ISNI:       0000 0004 5916 8748
Awarding Body: University of Bristol
Current Institution: University of Bristol
Date of Award: 2016
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Phase pure cuprous oxide (Cu2O) is desirable as an absorber material to realize efficient thin photovoltaic cells based entirely on 'cheap, environmentally benign and earth abundant metal oxides' for terawatt-level deployment. One of the major barriers to make efficient Cu2O based solar cells is the difficulty to grow stoichiometric as well as defect free interface with suitable photoanodes (for example, Zinc Oxide(ZnO)). This thesis mainly focuses on the growth of Cu(I) oxide thin films on variety of substrates produced by pulsed laser deposition (PLD) with varying growth temperature(Tsub: 25 °C - 400 °C), oxygen content(O2pp: 1-10 mTorr) and laser pulse energy(LP: 25-35 mJ), in order to control the structural, optical and electrical properties. The deposited films were characterized by variety of characterization techniques including, X-ray diffraction(XRD), Transmission electron microscopy(TEM), Raman spectroscopy and room temperature Photoluminescence (RT-PL), variable angle spectroscopic ellipsometry, UV-Vis-NIR spectroscopy, four point co-linear probe/Hall measurement and photoelectrochemical measurements. The structural analysis revealed that PLD films grown at 25 °C ≤ Tsub ≤ 200 °C with O2pp ≈ 3mTorr and LP ≈ 25mJ are single phase polycrystalline Cu2O while deposited on amorphous quartz substrate; and partially oriented pure and twinned Cu2O film with {001}Cu2O || {001}NaCl with < 110 > Cu2O || < 100 > NaCl epitaxy while grown on NaCl(100) substrate. All phase pure Cu2O films exhibited an optical bandgap in the range 2.15-2.40 eV with high absorption coefficient above 105 cm-1 near the band edge. The thin films grown at Tsub ≥ 300 °C with O2pp ≥ 3mTorr and LP ≈ 25mJ exhibited a predominant CuxOy species with the Cu2O structure, confirmed by XRD, TEM and Raman analysis, with a 2-3% lattice parameter expansion compared to the pure Cu2O. The RT-PL spectra exhibited an intense luminescence peak near 760 nm which has been attributed to a distinct Cu3O2 phase. The large bandgap observed in CuxOy species compared to pure Cu2O phase is attributed to the stoichiometric difference. It is concluded here that this is not a new phase, rather an oxygen-rich defect structure of Cu2O. Thin films grown at Tsub ≥ 200 °C and under more oxygen rich (≥5mTorr) conditions revealed mixtures of cupric (Cu (II)) and cuprous (Cu(I)) oxide phases. The electrical resistivity of Cu(I) oxide films grown at Tsub ≈ 25 °C(RT) and Tsub ≈ 200 °C(HT) on quartz substrate were found to vary in the ranges 3 mΩ.cm - 20 kΩ.cm and 80 mΩ.cm - 13 kΩ.cm respectively depending on the oxygen content. The Hall mobility (carrier concentration) of RT- and HT-grown films were estimated to be ~2.08 cm2 V-1s-1(~1019 cm-3) and ~22.21 cm2 V-1s-1 (~1013 cm-3) respectively where lower mobility is attributed to the scattering of holes in the grain boundaries of the nanocrystalline films. All Cu2O films demonstrated a p-type conductivity confirmed by positive Hall voltage as well as cathodic photocurrent nature. The external quantum efficiency of the best HT-grown found to be ~10 times higher compared to the best RT-grown film measured in a Photoelectrochemical cell. A preliminary study of a solid junction with a PLD-grown ZnO electrode demonstrated photoresponse but with very low efficiency, attributed to the presence of an amorphous layer at the Cu2O/ZnO interface.
Supervisor: Cherns, David Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available