Study of Kesterite Cu2ZnSnS4 Thin Films prepared
from Cu2ZnSnS4 Nanoparticle Inks

Abstract

Due to the recent rapid industrial and population growth, there has been an
increase in the demand for energy which has led to a strong reliance on non-renewable energy sources which has negative environmental effects. Due to
its unlimited sources and absence of environmentally harmful green house gas
emissions, solar energy has emerged as an effective candidate for renewable
energy. Cu2ZnSnS4 (CZTS) has been widely investigated as an absorber for
thin film solar cells. It can replace Cu(In, Ga)Se2 (CIGS), and CdTe since
it consists of non-toxic, abundant, and cheap elements. Its high absorption
coefficient, optimal band gap, and naturally abundant non-toxic elemental
constituents give it several advantages over most thin film absorber materials. However, the present performance of CZTS solar cells is still below their
theoretical limit. Recently, there has been a rise in interest in improving the
performance and lowering the production cost of solar cells based on CZTS.
In this study, CZTS nanocrystals were synthesized by one of the non-vacuum
techniques: the CZTS absorber is synthesized by using the hot injection technique, and it is responsible for synthesizing the crystalline layer that performs
as the solar cell device absorber. CZTS nanoparticles were synthesized using
a chemical synthesis process, with sulphur being injected into a solution of hot
metallic precursor ions. It was revealed that the reaction temperature and time
greatly influenced their composition, structure, and optical properties. In this
research, a method was proposed to fabricate nonstoichiometric CZTS thin
films. Using soda lime glass and molybdenum substrates, the CZTS thin films
were deposited on the different substrates by drop casting and spin coating
methods, followed by preheating in air and post-deposition annealing treatments, in two different atmospheres, H2S and N2. The CZTS thin films were
annealed at different annealing temperatures. The morphology, composition, structure, and optical properties of the CZTS thin films were characterized using scanning electron microscopy, energy dispersive X-ray spectroscopy, X-ray
diffraction, Raman spectroscopy, and UV-vis spectroscopy. The influence of
the annealing on the morphology, composition, and structure of the films has
been studied. From the XRD patterns of the CZTS thin films, the peaks of all
CZTS thin films are indexed to tetragonal CZTS (PDF 026-0575) and can be
attributed to the (112), (220), and (312) planes of kesterite CZTS, respectively.
The Raman spectra of the samples give evidence that in all spectra the dominant peaks are located at 330–338 cm−1
, indicating that these samples contain
peaks associated with the Cu2ZnSnS4 phase. A significant influence on the
energy bandgap of CZTS thin films was observed during the annealing of thin
films. In addition, the fabrication of CZTS/CdS heterojunctions was explored
to gain a better understanding of the properties of the interface reactions that
take place between the Mo/CZTS and CZTS/CdS layers. the effect of high
annealing temperature on a Mo/CZTS/CdS heterojunction was carried out at
temperatures ranging from 450°C to 550°C at an annealing time of one hour
in H2S and N2 atmosphere . Annealing promotes cadmium diffusion from the
n-type semiconductor to the absorber. According to various studies, Cd diffusion improved the performance of thin-film solar cells. However, at annealing
temperatures of 500 and 550 °C, the presence of a Cu-related secondary phase
was observed, which is detrimental because it introduces shunting routes that
lower the devices’ FF due to its metallic nature and low resistivity profile. In
addition, Mo combines with S at the CZTS/Mo interface to form MoS2, which
leads to losses of Voc, Jsc, and FF in CZTS devices. The findings presented
here indicate that there is significant interdiffusion of the elements from the
different layers in a Mo/CZTS/CdS heterojunction, which occurs at the temperatures commonly used to fabricate CZTS devices. Further improvements
in CZTS devices will require strategies to control this interdiffusion.