CZTS Surface Research Articles

Fabrication of Cu2ZnSnS4 (CZTS) by co-electrodeposition of Cu-Zn-Sn alloys, and effect of chemical composition of CZTS on their photoelectrochemical water splitting

Sulfide kesterite Cu2ZnSnS4 (CZTS), an intrinsic p-type semiconductor, provides an attractive low-cost, environmentally friendly photoelectrochemical (PEC) water splitting to evolve hydrogen. The purpose of this study was to clarify the effect of the chemical composition of CZTS on the PEC properties and the correlation with their structures. The CZTS was fabricated on a Mo/glass by a non-vacuum process, i.e., co-electrodeposition (co-ED) of Cu-Zn-Sn (CZT) alloys, followed by sulfurization with solid sulfur. To optimize the chemical composition of CZTS, the bath concentrations, applied potentials and amounts of electric charge for preparation conditions of the CZT precursors were examined. The CZTS with Cu-poor and Sn-rich composition compared with stoichiometry was found to exhibit relatively effective PEC water splitting. It was suggested that the high PEC performance was attributed to efficient charge carrier transport due to the presence of a small amount of SnO2 phase on the CZTS surface.

Atomic Layer Grown Zinc–Tin Oxide as an Alternative Buffer Layer for Cu2ZnSnS4-Based Thin Film Solar Cells: Influence of Absorber Surface Treatment on Buffer Layer Growth

Zn1-x Sn x O y (ZTO) deposited by atomic layer deposition has shown promising results as a buffer layer material for kesterite Cu2ZnSnS4 (CZTS) thin film solar cells. Increased performance was observed when a ZTO buffer layer was used as compared to the traditional CdS buffer, and the performance was further increased after an air annealing treatment of the absorber. In this work, we study how CZTS absorber surface treatments may influence the chemical and electronic properties at the ZTO/CZTS interface and the reactions that may occur at the absorber surface prior to atomic layer deposition of the buffer layer. For this, we have used a combination of microscopy and synchrotron-based spectroscopies with variable information depths (X-ray photoelectron spectroscopy, high-energy X-ray photoelectron spectroscopy, and X-ray absorption spectroscopy), allowing for an in-depth analysis of the CZTS near-surface regions and bulk material properties. No significant ZTO buffer thickness variation is observed for the differently treated CZTS absorbers, and no differences are observed when comparing the bulk properties of the samples. However, the formation of SnO x and compositional changes observed toward the CZTS surface upon an air annealing treatment may be linked to the modified buffer layer growth. Further, the results indicate that the initial N2 annealing step integrated in the buffer layer growth by atomic layer deposition, which removes Na-CO x species from the CZTS surface, may be useful for the ZTO/CZTS device performance.

Insight into the role of air-annealing Cu2ZnSnS4 precursor films in improving efficiency of Cu2ZnSn(S,Se)4 solar cells

It is well known that low power conversion efficiency (PCE) has been key issue need resolve for Cu2ZnSn(S,Se)4 (CZTSSe) solar cells. In this work, we tried to improve the PCE by annealing Cu2ZnSnS4 (CZTS) precursor films in a temperature ranging from 100 oC to 500 oC in air before preparing CZTSSe absorber through selenization of the CZTS, and study influence mechanism of the air-annealing on PCE of CZTSSe solar cells by quantitative analysis. The results show that the PCE of the CZTSSe solar cell increases from 7.50% to 8.94% as the temperature increases from room temperature to 100 oC and decreases from 8.94% to 5.09% from 100 oC to 500 oC. By quantitative analysis of contribution degree of Voc, Jsc and FF to PCE and of J0, JL, Rs and Rsh to Voc, Jsc and FF, it is calculated that contribution degree of J0, Rs, JL and Rsh to the increased PCE is 50.47%, 38.43%, 9.80% and 1.30%, respectively, and to the decreased PCE is − 67.68%, − 22.38%, − 1.01% and − 8.93%, respectively. It is deduced by XRD, Raman and EDS measurements that the J0 is mainly determined by interfacial recombination induced by [2CuZn + SnZn] defect cluster near the surface of the CZTSSe, and the Rs is by crystal quality of CZTSSe and amount of Zn(S,Se) near surface of the CZTSSe. The change of the [2CuZn + SnZn], Zn(S,Se) and crystal quality should be attributed to that of O content near surface of CZTS induced by air-annealing.

Efficient humidity sensor based on surfactant free Cu2ZnSnS4 nanoparticles

In addition to their proved potential in thin film photovoltaics, the nanoparticles of Cu2ZnSnS4 (CZTS) are also identified as one of the promising candidates for developing next generation humidity sensors. However, the surfactants generally used during their synthesis often lead to more organic residue and adversely affect the sensing behaviour. Herein, we report on the CZTS nanoparticles synthesized using two surfactant free routes and their improved humidity sensing behaviour. CZTS1 and CZTS2 nanoparticles made using the hydrothermal and molecular precursor aging method exhibited very high response of ∼3218.44 and ∼32115.07 respectively, as observed with 97%RH at room temperature. The exceptional response shown by the CZTS nanoparticles could be attributed to the relatively smaller crystalline sizes and larger number of good quality adsorption sites, as understood with the help of the density functional theory calculations. The calculations also indicate that, the slower response and recovery could be explained based on the stronger adsorption of water molecules on the CZTS surface. The results confirm the potential of the quaternary sulphide compound to be used in efficient humidity sensors and establish the role of the surfactant free synthesis method for achieving a significant improvement in the sensing ability.

Reduced recombination through CZTS/CdS interface engineering in monograin layer solar cells

The power conversion efficiency of Cu2ZnSnS4 (CZTS) solar cells is still limited by deep defects, low minority carrier lifetime and high recombination rates at the CZTS/CdS interface. The objective of this study was to find an effective method to reduce interface recombination of CZTS monograin layer solar cells. A two-step heterojunction formation process was applied by controlling the intermixing of Cd and Cu in the CZTS/CdS interface, which resulted in improved device efficiency of up to 11.7%. Surface analysis by x-ray photoelectron spectroscopy confirmed Cd diffusion into the surface of CZTS after CdS air-annealing by forming an ultra-thin Cu2Zn x Cd1−x SnS4 layer. Moreover, external quantum efficiency measurements showed that the absorption edge shifts to longer wavelengths with the addition of Cd into the CZTS surface layer. This surface modification and replacement of a CdS:Cu buffer layer by fresh CdS greatly reduced the interface recombination and improved the junction quality, contributing to an enhancement of J SC ∼3 mA cm−2 (from 20.5 to 23.6 mA cm−2) and fill factor ∼14% (from 59.4% to 67.7%). The serial resistance of the CZTS monograin layer solar cells was significantly reduced from 2.4 Ω cm2 to 0.67 Ω cm2. To understand the electrical behavior of the highest-efficiency CZTS monograin layer solar cell in more detail, the temperature-dependent current–voltage characteristics were analyzed.

Characterization of Cu2ZnSnS4 thin films prepared with and without thin Al2O3 barrier layer

We investigated the effect of thin Al2O3 barrier layer (∼20 nm) deposited by radio frequency magnetron sputtering technique on the structural, morphological, optical, and electrical properties of Cu2ZnSnS4 (CZTS) films. Major differences in the crystalline qualities of the films were not observed with the addition of the Al2O3 layer except for a small decrease in crystallite size. Raman spectra of the film with the Al2O3 layer exhibited the peaks belonging to ZnS and Cu2-xS secondary phases. X-ray photoelectron spectroscopy analysis showed that SO42- and CuSO4 compounds were formed on the surfaces of the films. The formation of the CuSO4 phase was associated with the bonding between CuO and S in an oxidized CZTS surface. The presence of the ZnS, Cu2-xS secondary phases, and CuSO4 on the surfaces of the films were also confirmed by scanning electron microscopy images. The film with the Al2O3 layer differed from the other films with its surface that have randomly distributed porous-structured agglomerations and a decrease in the ZnS secondary phases on its surface. All films exhibited different Na distributions in secondary ion mass spectroscopy depth profiles. The Al2O3 layer caused a slower gradient in Na diffusion with decreasing the accumulation of Na element at a distinct region. The activation energy for the thermally activated band conduction was calculated as 0.030 eV and attributed to the thermal release of the holes from the vacancy of copper (VCu) defect states. The results of temperature-dependent conductivity measurements indicated that there were different conduction mechanisms in the films.

Impact of the secondary phase ZnS on CZTS performance solar cells using simulation program

In the present work ,Scaps 1d software is used to study the effect of ZnS secondary phase on the performance of solar cells. The standard CZTS-based devices are composed of a multilayer thin film formed on soda-lime glass (SLG), Mo/MoS2/CZTS/Cds/i-ZnO/ZnO Al. a metallic molybdenum (Mo) back electrode, MoS2 built between the absorber and the back contact; a p-CZTS absorber layer; an n-type buffer layer, typically Cds to form the junction of the solar cell, a TCO doped ZnO layer and a transparent ZnO:Al front contact. Ni/Al metal contact grids complete the cell. An ultrathin layer of ZnS is placed in the standard solar cell structure between the Cds buffer layer and the CZTS absorber layer to represent the second phase (SP) above CZTS [1].Cell characteristics such as ƞ, FF, Jsc , and Voc were studied. The formation of ZnS on the surface of CZTS has negative effects on the solar cell parameters where the conversion efficiency ƞ decreases.The impact of this layer on the performance of CZTS solar cells is also investigated through the results obtained from the analysis of the complex impedance (Z*) and the modulus (M*). An equivalent circuit of the solar cell consisting of series and parallel resistors has been evolved to study the electrical behavior around each interface of this structure.This new procedure has been very beneficial for the identity of the most important parameters of the diffusion and recombination process.

Electronic and optical properties of copper zinc tin sulphide (Cu2ZnSnS4) doped with calcium and barium

Harnessing solar energy with the help of cheaper, more abundant and environmentally-friendly materials is a major research thrust currently. Health hazards emanating from fossil fuel combustion, depletion of nonrenewable sources, global warming and environmental degradation are some of the push factors towards clean, non-toxic and environmentally friendly methods of producing electrical energy. To harvest solar energy, the Cu2ZnSnS4 (CZTS) semiconductor is currently a foremost candidate, which can supply encouraging amounts of energy. Some of its advantages are the optical direct band gap and high absorption coefficients. In this study, CZTS is investigated as a material for solar cells using first principles method. Thus, structural, electronic and optical properties of pure CZTS and doped CZTS (1 1 2) surface were investigated using the DFT as implemented in the CASTEP code. Alkaline earth metals, calcium (Ca) and barium (Ba) were adsorbed on the CZTS (1 1 2) surface using the adsorption locator module. Our results suggest that doping with barium rather than calcium could improve the photocatalytic activity of the device.

Engineering Cu2ZnSnS4 grain boundaries for enhanced photovoltage generation at the Cu2ZnSnS4/TiO2 heterojunction: A nanoscale investigation using Kelvin probe force microscopy

Over the past several years, kesterite Cu2ZnSnS4 (CZTS) absorber has been investigated comprehensively; however, the performance is still hampered by a large open-circuit voltage deficit associated with CZTS bulk defects and interface recombination. To overcome this trend, we report a facile approach to passivate both defect prone areas, i.e., bulk of CZTS and CZTS interface with a TiO2 buffer layer, simultaneously. The existence of oxygen ambient during TiO2 deposition has modulated the electrical properties of CZTS grain boundaries (GBs) not only inside the bulk but also at the surface of CZTS. The passivation of surface GBs is favorable for CZTS/TiO2 heterojunction electronic properties, whereas passivated bulk GBs improve the carrier transport inside the CZTS absorber. To directly probe the photovoltage generation at the CZTS/TiO2 heterojunction, Kelvin probe force microscopy is conducted in surface and junction modes. The acquired photovoltage map exhibits higher values at the GBs, which reveals an increment in downward band bending after oxygen diffusion inside the bulk of CZTS. In point of fact, the enhanced diffusion of oxygen accounts for the suppression of carrier recombination and reduction in dark current. Finally, current–voltage and capacitance–voltage measurements performed on the CZTS/TiO2 heterojunction further validate our outcomes. Our findings provide critical insight into the engineering of CZTS GBs to control electronic properties of CZTS and CZTS/TiO2 heterojunctions.

Heterojunction post-heat treatment process driving high efficiency for Cu2ZnSnS4 solar cell

The quality of Cu2ZnSnS4 (CZTS) absorber layer, Mo/CZTS interface and heterojunction CZTS/CdS are the most important factors directly affecting the performance of CZTS solar cells. On the base of high quality absorber layer and back contact interface, the effects of post-heat treatment (PHT) process of CZTS/CdS on the quality of heterojunction and device performance have been comprehensively investigated. The PHT process can promote the elements in the heterojunction region interdiffuse between absorber and buffer layer, i.e., more Cd elements diffuse to CZTS, and Zn, Cu elements diffuse to CdS. Consequently, the elements interdiffusion affects the chemical composition of the absorber layer surface and the band alignment of the heterojunction. After applying the PHT process, the surface of CZTS becomes Cu-poorer in comparison to the CZTS bulk. Due to the diffusion of Cd2+into the CZTS layer, the free carrier density is increased. In addition, PHT process can also promote the recrystallization of CdS and thus greatly improve the crystallization quality of CdS. Owing to the more favorable band alignment at the CZTS/CdS interface as well as better crystallinity of CdS, higher quality heterojunction was obtained. Finally, at the optimized PHT temperature of 275 °C, a pure CZTS solar cell with active area efficiency of 9.75% (total area efficiency: 8.90%) has been fabricated. Based on the application of PHT process, reducing the deep level defects in CZTS bulk is expected to further improve the device performance.

Li-based selenized Cu2ZnSnS4 surface: Possible route to overcoming voc-deficit of kesterite solar cells

Usually, open-circuit voltage (Voc) of thin film solar cells significantly depends on the band bending at the front interface of an absorber/buffer. The failed band bending at a Cu2ZnSnS4/CdS (CZTS/CdS) interface severely hinders the increase in Voc due to the excessively high concentration of holes at the CZTS side. Alleviating the strong p-type or converting it to weak n-type at the CZTS surface is a credible idea to overcome the Voc− deficit. First-principles calculations show that the Li-based selenized CZTS surface presents the desired property with excellent advantages: (i) The greatly improved defects and band offset suppress carrier nonradiative recombination and facilitate electrons transmission, respectively. (ii) Its intrinsic weak n-type characteristic effectively promotes the large band bending at the interface.

Efficiency enhancement of CZTS solar cells using structural engineering

We numerically performs structural engineering for lowering interfacial recombination, and enhancing efficiency of CZTS solar cells. Structural modification of controlling carrier density profile at CZTS/CdS interfaces, CZTS absorber surface region carrier density, CZTS surface region bandgap modification, Fermi level pining and tunneling at the CZTS/CdS junction are studied for their impact device performance. We observed that the lowering hole concentration and widening bandgap of CZTS in vicinity of CZTS/CdS junction, lower interface recombination and improve performance. Fermi level pinning near to conduction band edge at CZTS/CdS interface showed lower recombination as compared to the case where Fermi level is pinned at midgap at CZTS/CdS interface. The donor defects from buffer layer pins the Fermi level near conduction band and acceptor defect from absorber CZTS layer pins Fermi level near mid gap at CZTS/CdS interface. Intra band tunneling through high conduction band offset barrier maintain performance for up to 0.5 eV spike barrier. High hole density profile at CZTS/metal lower back contact interfacial recombination. These structural engineering numerical performed here, present bright aspect for improving efficiency of CZTS solar cells. • Device engineering is performed for controlling interfacial recombination in CZTS solar cells. • Absorber surface layer parameters such as carrier density profile and bangap influence interfacial recombination. • Fermi level pinning near conduction band edge is advantageous than Fermi level pinned at midgap at CZTS/CdS interface. • High majority carrier concentration near back contact in CZTS minimize the effect of high Schottky barrier and high SRV.

The influence of Cu2ZnSnS4 thin films with characteristics of treatment conditions on spray pyrolysis technique for solar cells applications

Generally, thin-film solar cells are used in CdTe or CIGS based on light-absorbing materials that it is non-toxic and cost. It is best on the materials for CZTS cost-effective, the high absorption coefficient of over 104 cm−1. CZTS based on electrical (light) conversion efficiency of 0.6% of the solar cell is an important growth in this research area was reached in the last five years. Cu2ZnSnS4 thin films have been prepared by spray pyrolysis technique for different substrate temperatures of 300 °C, 350 °C, 375 °C, 400 °C, and 450 °C with varying precursor concentrations. The prepared Cu2ZnSnS4 thin films for various temperatures were characterized for various parameters like its structure using X-ray diffraction (XRD), phase constituents with energy dispersive analysis (EDS), morphology by scanning electron microscope (SEM), and optical properties UV–vis spectrophotometry technique. The XRD and EDS results are embarked as a peak at a 2θ position of 28.5° corresponding to (112) planes. The excellent polycrystalline kieserite structure is obtained from XRD for the Cu2ZnSnS4 films. The organized harvests also exposed the morphology with size ranging from the formation of smooth, compact, and even surface of CZTS. The composition of the Cu2ZnSnS4 thin film fundamentally agrees with the theoretical value. The absorption coefficient in the order of 104 cm−1 is obtained. The experimental bandgap of 1.5 eV corresponds well with the substance Cu2ZnSnS4 materials analysis of SEM, and EDAX is making them excellent aspirants as absorbent materials for the thermal solar cell process.

Kinetic Processes and Surfactant Design of Group I Elements on the CZTS (1̅1̅2̅) Surface

Cu2ZnSnS4 (CZTS) is a promising thin-film solar cell material consisting of earth-abundant and nontoxic elements. Yet there exists a fundamental bottleneck that hinders the performance of the devic...

How different are CZTS grain boundaries?

No, defects are not always bad. Vacancies, antisites, or impurities are in general trapped at the grain boundaries and contribute to scattering (of charge carriers). In general, grain boundaries reduce the performance (efficiency of the solar cell). However, there are certain materials where the defects enhance performance. CdTe, CuInSe2 (CIS), Cu2ZnSnS4 (CZTS) are a few examples. The present work involves CAFM studies on CZTS films deposited to Si and soda-lime glass substrate to explore the underlying physics behind unique grain boundary physics in CZTS. Discussions are made in the light of the reactions that take place on CZTS surface and corresponding energy band diagrams are drawn

Significantly Improving the Crystal Growth of a Cu2ZnSn(S,Se)4 Absorber Layer by Air-Annealing a Cu2ZnSnS4 Precursor Thin Film.

The crystal quality of a Cu2ZnSn(S,Se)4 (CZTSSe) thin film is crucially important to a high-performance CZTSSe solar cell. After selenization, a bilayer CZTSSe thin film consisting of a large-grain top layer and a small-particle bottom layer is usually observed according to the literature. In this work, a facile air-annealing pretreatment is conducted for a Cu2ZnSnS4 precursor thin film prior to selenization, which can lead to sodium diffusion into the CZTS precursor thin film and surface oxidization of the CZTS thin film. Our experimental results revealed that the Na prediffusion and the surface oxidation of the CZTS precursor thin film can significantly promote the crystal growth of the CZTSSe thin film, which can completely remove the small-particle bottom layer and form a large-grain-spanned CZTSSe thin film. As a result, a photoelectric conversion efficiency of 9.80% was achieved by this method.

Performance of different anti-reflection coating and TCO layers for kesterite based thin film photovoltaic devices using Essential Macleod simulation program

Abstract Optical losses at the front stack interface of thin film photovoltaic (PV) devices is responsible for ~4% loss of light reaching the active layer. The primary aim of this simulation studies is to obtain the least possible reflectance at the front surface of kesterite CZTS and CZTSe based thin film devices, considering its effects with and without the antireflection coatings. Essential Macleod simulation software was employed to theoretically refine the optical thicknesses of Zinc oxide (ZnO) window layers and different transparent conductive oxide (TCO) layers like aluminium doped ZnO (ZnO:Al) and Indium doped Tin oxide (ITO). The Minimum reflectance values within a wide wavelength range from 300 to 800 nm were recorded. Moreover, to obtain complete transmittance from UV to visible region, low refractive index materials such as MgF2, Al2O3 and CeF3 were taken as different antireflective layers (ARC) and their corresponding reflectance and transmittance were simulated. The optical thicknesses of ZnO with different TCO and ARC layers were optimised to achieve a low reflectance of less than 5% for the better performance of the thin film devices. Based on the best possible optimisation, Al2O3 antireflection layer with ZnO:Al (transparent conductive layer) and ZnO (window layer) exhibited a reflectance of less than 1% within the mid-wavelength range from 500 to 800 nm with the optimised thicknesses suitable for thin film devices.

Effect of Na doping on the performance and the band alignment of CZTS/CdS thin film solar cell

Alkali doping can suppress deleterious antisite defects in kesterite Cu2ZnSnS4 (CZTS) and improve the open-circuit voltage. In this study, the effects of light Na-doping on the performance and the band alignment of CZTS/CdS thin-film solar cells were investigated. CZTS:Na thin films were fabricated by the spin coating with 10% Na doping on the surface of CZTS. The Na-doping led to the narrower FWHM and larger grain size. The hole concentration and the conductivity were improved due to the NaZn shallow acceptor defects. In addition, Na-doping can improve the band alignment of absorber/buffer interface and inhibit SRH recombination by the Na passivation effect and the suppression of SnZn defects. The typical cliff-like conduction band offset (CBO) was reduced from 0.25 eV in CZTS:Na/CdS to 0.1 eV in CZTS/CdS heterojunction. CZTS:Na device exhibited a higher Voc of 653 mV than that of CZTS/CdS device. The maximum conversion efficiency reached 7.46%, increased by 44% after Na-doping. These results clarify the effect of Na-doping on the band structure of the heterojunction in CZTS solar cells and support a new aspect that synthesis of a surface-doping CZTS:Na absorber has great potential for future research.

Photovoltaic materials: Cu2ZnSnS4 (CZTS) nanocrystals synthesized via industrially scalable, green, one‐step mechanochemical process

AbstractThe present study demonstrates a scalable approach towards the pure Cu2ZnSnS4 (CZTS) phase starting from microsized Cu, Zn, Sn, and S mixture or a combination of nanosized CuS, SnS plus microsized Zn and S. A set of mechanochemical reactions was carried out in an industrial vibratory mill. The pure CZTS phase was obtained after 360 minutes of milling with an average crystallite size of 15 nm for both mixtures. The kinetics of CZTS formation was investigated following the X‐ray diffractometry (XRD) patterns of the reaction mixtures milled for various times. The morphology of particles and their elemental composition was studied by scanning electron microscopy (SEM)/transmission electron spectroscopy (TEM) and EDX analysis. Photoelectron spectroscopy (XPS) analysis showed the presence of Cu+, Zn2+, Sn4+, and S2− species on the surface of CZTS with only minor oxidation. Thermal stability of the synthesized materials was followed by thermogravimetric analysis. Based on UV‐VIS measurements, the calculated bandgap Eg = 1.44 to 1.50 eV of the synthesized CZTS samples is acceptable for photovoltaic application. Moreover, the applied mechanochemical approach can produce 100 to 500 g of CZTS in a batch mode experiment that is 10 to 50 times higher amount as the throughput of laboratory planetary mills. Hypothetically, in the applied industrial eccentric vibratory mill, the amount can be increased up to 50 kg per one batch.

Ageing of kesterite solar cells 2: Impact on photocurrent generation

As yet results of ageing experiments of Kesterite solar cells have not been published. However, at least in crystalsol detailed investigations of monograin-based solar cells have been performed proving very different stabilities depending on the exact composition of the Kesterites. Completely stable solar cells even without any sealing have been demonstrated internally. However in this paper we report on ageing effects observed for internally called “highly unstable” Cu2ZnSnS4 (CZTS) solar cells aged for 8.5 and 95.5 h at 105 °C in air, already also reported in part 1 of these investigations. During these times the initial efficiency of 5.4% decreased by 76% and another 88%, respectively. The photocurrent generation was investigated in detail by light beam induced current (LBIC) measurements of the front side and electron beam induced current (EBIC) measurements of cross sections of the CZTS/CdS/ZnO active junctions. Besides a pronounced decrease of the LBIC and the EBIC signal also a shift of the EBIC signal much further into the Kesterite material was observed, which can be explained by the in part 1 already found diffusion of copper from the surface of CZTS into the CdS buffer layer acting finally as a strong charge carrier recombination center. Thereby the EBIC signal first observed mainly close to the interface has vanished, only a small EBIC signal in a depth of around 400 nm still remains visible, a depth, from which only minority carriers can still reach this main recombination center in the interface.