Investigation of cu2znsns4 (caus) nanostructured material system for energy conversion

Photovoltaic technology is a very crucial technology and it is gradually growing worldwide

as there is plenty of sunshine daily especially in African countries. The aim of photovoltaic

technology is to generate electricity from solar power using photovoltaic devices like solar

(photovoltaic) cells. Photovoltaic cells are mostly used to provide electricity in rural areas

which are not connected to the national grid like farms (cattle posts). In this work superstrate

CZTS solar cells (Cell-A and Cell-B) were fabricated from optimised CZTS absorber layers

and In2S3 buffer layer by a cost-effective spray pyrolysis technique. Firstly, CZTS absorber

layers were grown on borosilicate glass substrates from various precursor solutions and their

properties were studied through X-ray diffraction, Raman spectroscopy, UV-Vis

spectroscopic analysis, Hall measurement, and Scanning electron microscopy. X-ray

diffraction results revealed similar patterns for all samples that are three peaks: (1 1 2), (2 2

0), and (2 1 2) belonging to kesterite CZTS with tetragonal structure. All thin films were

growing along (1 1 2) plane. Results obtained via Raman spectroscopy revealed two wide

peaks at 248 cm-1

and 331 cm-1

in all thin films. Both peaks belonging to CZTS with

tetragonal structure. Both the X-ray diffraction and the Raman spectrometry results revealed

that samples prepared from solutions containing tin (IV) chloride were highly crystalline. The

maximum absorbance obtained for all thin films was between 1.5 and 4 in the visible and

near infrared region. Unlike crystallinity, the absorbance was high for samples that were

prepared from solutions containing tin (II) chloride as a tin source. As revealed by the Hall

measurement, the resistivity of the thin films was ranging from 2.84 x 10-2 Ωcm to 3.29 x 10-1

Ωcm and the sample with the lowest resistivity (labelled as CZTS003) was prepared from a

solution containing copper (II) chloride, zinc acetate, tin (IV) chloride and thiourea solutions.

The SEM micrographs showed well defined grains for all thin films except the one that was

prepared from a solution containing copper (II) chloride, zinc nitrate, tin (IV) chloride and

thiourea. A sample that had the lowest resistivity also exhibited the largest grains.

In the second part of this work, In2S3 buffer layers were also deposited on glass substrates

from a mixture of indium chloride solution and thiourea solution. The concentration of

indium chloride solution was held constant and the concentration of thiourea was varied

between 0.090 M and 0.105 M in steps of 0.005 M. After deposition, the thin films were

characterised via X-ray diffraction, Raman spectrometry, UV-Vis spectroscopy, Atomic

Force Microscopy, and Hall measurement. The results obtained by X-ray diffraction revealed

xv

polycrystalline β-In2S3, and the increased concentration of thiourea lead to decreased

crystallinity of the thin films. Similar Raman spectra of all thin films was obtained via Raman

spectroscopy. The two peaks found at 306 cm-1

and 365 cm-1

on the Raman spectra belong to

β-In2S3. All the thin films exhibited similar transmittance spectra, and the transmittance of the

thin films was lying between 60 % and 80 % in the visible and near infrared region of the

electromagnetic spectrum. Transmittance was highest in the visible region for the sample

prepared from solution containing 0.095 M of thiourea solution. In addition, the thin films

had wide optical band gaps lying between 2.75 eV and 3.0 eV. The electrical resistivity of

In2S3 thin-film layers increased from 5.6249 x 10-2 Ωcm to 4.0953 Ωcm when concentration

of thiourea solution was increased from 0.090 M to 0.100 M, however, when the

concentration was further increased to 0.105 M the resistivity of the In2S3 thin film layers

decreased.

The performance of fabricated solar cells (denoted as Cell-A prototype and Cell-B prototype)

was studied. Cell-A prototype performed better than Cell-B prototype, because the efficiency

at maximum power point of Cell-A was 0.158 % while the efficiency at the maximum power

point of Cell-B was 0.07 %. In addition, Cell-A exhibited higher open-circuit voltage and

short-circuit current density compared to Cell-B. The open-circuit voltage and short-circuit

current density of Cell-A were 200 mV and 2.26 mA/cm2

, respectively, while Cell-B had

shown open-circuit voltage of 80mV and short-circuit current density of 1.75 mA/cm2

.