Synthesis and characterisation of copper zinc tin sulphide photovoltaic materials in bulk and single crystal forms

Abstract

The results presented in this thesis focus on the synthesis, structural, optoelectronic and spectroscopic characterisation on Cu2ZnSnS4 (CZTS) single crystals. Stoichiometric and off-stoichiometric CZTS single crystals with dimensions in submillimetre range were grown using chemical vapour transport method. The elemental composition of the crystals were confirmed with energy-dispersive X-ray spectroscopy, X-ray fluorescence spectroscopy, and atomic absorption spectroscopy. The phase purity were confirmed with powder X-ray diffraction and Raman spectroscopy, and no secondary phases were observed.

The external quantum efficiency (EQE) were measured by photocurrent spectroscopy using electrolyte contact, and approaches 100% close to the bandgap energy, indicating efficient carrier collection. The bandgap was found to be 1.64 – 1.68 eV. The room temperature photoluminescence spectroscopy of the crystals was attributed to radiative recombination via tail states, with lifetimes in the nanosecond range. Electrolyte electroreflectance spectra and spectra of the third derivative of the optical dielectric constant obtained from spectroscopic ellipsometry were fitted to two optical transitions at 1.71 and 1.81 eV in the bandgap region, suggesting a larger valence band splitting than predicted theoretically.

Single crystal devices were fabricated using Mo/CZTS/CdS/Al:ZnO/i-ZnO/Ni configuration and suffered from high series resistance or shunting. IV characteristic, EQE and impedance were measured and analysed.

Cu/Zn disorder were studied on samples annealed between 110 and 425 °C using resonance Raman spectroscopy, single crystal neutron diffraction and powder anomalous diffraction. Very high resonance Raman order parameter, Q, of up to 17 were observed in samples with low annealing temperatures after annealing for > 300 hours. The critical temperature observed is around 250 °C. The occupancy factors of Cu and Zn were successfully obtained using single crystal neutron diffraction, but not with powder anomalous diffraction due to strong correlation between the occupancy factors and the pseudosymmetry of kesterite structure.

Date of Award	2018
Original language	English
Awarding Institution	University of Bath
Supervisor	Mark Weller (Supervisor)