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Title: CuSbS2 and related chalcogenides for sustainable photovoltaics
Author: Peccerillo, E.
ISNI:       0000 0004 6058 962X
Awarding Body: University of Liverpool
Current Institution: University of Liverpool
Date of Award: 2016
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This thesis presents a systematic investigation of some novel chalcogenides based on Earth abundant elements in order to test their suitability for sustainable photovoltaic (PV) applications. A comprehensive review of sulfo-salts in the family Cu-Sb-Bi-X (X = S, Se) is presented and indicates that CuSbS2 and Cu3BiS3 have potential as PV absorbers, showing optical band gaps in the desired range and the p-type conductivity required for PV devices. A systematic study about the formation of CuSbS2 and Cu3BiS3 thin films is presented, including of the experimental steps necessary to ensure the formation of stoichiometric films during the sulfurization of metallic precursor layers. Sulfurized CuSbS2 films (~1.2 µm thick) exhibited absorption coefficients of ~105 cm-1 and band gaps of ~1.5 eV. The films were p-type with mobilities of ~10 cm2 V-1 s-1 and resistivities in the range 10 - 1000 kΩ/□. CuSbS2 films were also deposited in a one-stage process by rf sputtering from a ternary target – they were generally more resistive than those deposited by sulfurization. Cu3BiS3 films deposited by sulfurization had band gaps of ~1.4 eV and p-type conductivity, with hole mobilities of ~3 cm2 V-1 s-1. The CuSbS2 films produced by both methods were tested in prototype CuSbS2/CdS heterojunction PV devices, and had efficiencies of ~0.1%. For these devices the dominant transport mechanism was multi-step tunnelling, suggesting that an improvement in the quality of the junction was necessary. Post-growth treatments were trialled, including impurity doping with NaF, Zn and In, and etching the absorber to remove the surface oxides identified by XPS. CuSbS2/CdS devices having efficiencies up to ~1% were achieved by doping the absorber with In and removing unwanted Sb2O3 from the layer surfaces with de-ionized water. Their Vocs and FFs were low and their Jscs were high, as reported by others. Alternative window layer materials were tested, including ZnS and ZnSe, which were expected to have an improved band alignment with CuSbS2 compared to CdS. ZnS gave Voc = 0.56 V, the highest yet reported for CuSbS2. However these alternative window layers had reduced photocurrents and their efficiencies were not greater than with CdS. Overall CuSbS2 and Cu3BiS3 films displayed the optical and electrical properties required for PV, but further developments will be essential in order to achieve high efficiency PV devices using these materials as absorbers.
Supervisor: Durose, K. Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral