Establishing correlations between molecular precursor coordination and structure, opto-electronic properties, and photovoltaic performance of Cu2ZnSn(S,Se)4 thin-films

Abstract

Harvesting solar energy is an important tool for transitioning into a low carbon economy and
meeting net carbon zero commitments by 2050. To help slow detrimental rises in average
global temperature, a key area of industrial interest is the implementation of buildingintegrated photovoltaics (BIPVs), such as lightweight and flexible solar modules and
technologies that allow for bifacial solar illumination. More specifically, bifacial solar cells are
advantageous as they significantly reduce cost and more effectively utilise materials and space.
The research presented in this thesis aims to develop solution-processed Cu2ZnSn(S,Se)4
(CZTSSe) thin-film photovoltaic materials for bifacial solar harvesting applications. To
allow for simultaneous front and rear illumination, the direct deposition of CZTSSe on a
transparent conductive back contact, fluorine-doped tin oxide (F:SnO2 – FTO), has been
investigated.
By exploring the chosen solvent blend, precursor salt counterion, and dopants in the
CZTS precursor solution, the key link between the solution formulation and the optoelectronic properties of thin-film CZTSSe has been rationalised. Systematic solution studies
to understand changes to the coordination environment within the CZTS precursor ink
have been analysed using Fourier-transform infrared (FTIR) and Raman spectroscopy. For
example, varying the zinc precursor counterion was found to directly impact thiourea
complexation in solution, while the decomposition temperature of the zinc salt influenced
the rate of grain growth during annealing. Throughout this research, the effect of varying
the chosen molecular ink conditions on the CZTSSe thin-film surface has been inspected
with photoemission studies, namely X-ray photoelectron spectroscopy (XPS) and energyfiltered photoelectron emission microscopy (EF-PEEM). These results has provided new
information about the intricacies of the local electronic distribution of the CZTSSe surface
and the Sn distribution, indicating that Zn-rich surfaces increase the work function while
Sn-rich surfaces decrease the work function. Overall, the results shared in this thesis provide
detailed evaluations of different ways to control the structure, morphology, surface electronic
landscape, and solar cell performance of FTO-based CZTSSe thin-films by simply tuning
the molecular precursor solution.

Date of Award	17 Jun 2025
Original language	English
Awarding Institution	University of Bristol
Supervisor	David J Fermin (Supervisor) & Neil A Fox (Supervisor)