CZTS Layer Research Articles

Degradation and device physics modeling of TiO2/CZTS ultrathin film photovoltaics

A hybrid solar cell of a nonporous n-type-TiO2 nanolayer and a p-type semiconductor Cu2(Zn,Sn)Se4 (CZTS) ultrathin film has been numerically simulated using SCAPS-1D program. The device physics including carrier generation, charge collection and current–voltage characteristics are investigated and the degradation rate of the electrical parameters under the normal operation condition is considered through a time dependent approach. The simulation analyzes are based on the experimental data reported in literature. An ultrathin (d≤1μm) instead of a thin CZTS layer is considered due to high absorption coefficient of such materials which allows a sub-micron layer to be adequate for complete photoabsorption. The defect density/levels significantly reduce the efficiency over time. The results are interpreted with a device physics proposed in in literature on the tunneling recombination at the interface of such structure. The TiO2 layer was selected to be 50nm only.

Kesterite Cu2ZnSnS4 compounds via electrospinning: A facile route to mesoporous fibers and dense films

Kesterite Cu2ZnSnS4 (CZTS) layers composed of either mesoporous fibers or dense films were successfully synthesized by electrospinning following sulfurization at high temperature. CZTS layers were characterized using scanning electron microscopy (SEM), energy-dispersive spectroscopy (EDS), thermogravimetric analysis (TGA), Raman and X-ray photoelectronic spectroscopy (XPS), and X-ray diffraction (XRD). The optical properties were also recorded by UV–vis absorption spectroscopy. The results showed that, with the increasing of sulfurization temperature from 450 to 600°C, the electrospun precursor fibers evolved from isolated CZTS fibers to interconnected fibers, and finally forming a compact films composing of sub-micro crystal flakes, just by simply adjusting the solutes concentration and sulfurization parameters. All the synthesized CZTS samples had a single phase, good crystallinity and a stoichiometric composition. Moreover, the band gap evolved from 1.49eV to 1.44eV with the morphology changing from porous microfibers to compact films. This work puts forward a facile route to both CZTS fibers and dense films, and would be meaningful for exploiting CZTS-based solar cells.

A comparative study on the performance of Kesterite based thin film solar cells using SCAPS simulation program

A comparative study of thin film solar cells based on CZTS, CZTSe, and CZTSSe (Copper Zinc Tin Sulphur Selenium) absorbers layers were simulated with Cadmium Sulphide (CdS) as buffer layer and Zinc Oxide (ZnO) as window layer using a solar cell capacitance simulator (SCAPS). The influences of series resistance, band to band recombination, defects and interfaces, thickness of (CZTS|CZTSe|CZTSSe) absorber layer, (CdS) buffer layer and transparent conductive oxide layer (ZnO) on the photovoltaic cell parameters were studied in detail. Improvements in efficiency were achieved by changing the back contact metal work function (BMWF) and choosing the flat band option in SCAPS software. Based on the best possible optimisation, an efficiency (η) of 12.03%, 13.16% and 15.77% were obtained for CZTS, CZTSe, and CZTSSe respectively. The performance of thin film photovoltaic devices (TFPV), for Mo back contact before optimisation and the SCAPS simulated values (flat band) after optimisation were described in detail to have in-depth understanding for better design of experiments (DOE) to obtain high efficiency solar cells.

Cu2ZnSnS4 solar cells fabricated by short-term sulfurization of sputtered Sn/Zn/Cu precursors under an H2S atmosphere

Synthesis of Cu2ZnSnS4 (CZTS) thin films by short-term sulfurization of sputtered Sn/Zn/Cu precursors under ambient H2S is studied. The effect of the sulfurization processes on the film morphology, surface roughness, composition of the CZTS, and the crystallinity was investigated by using scanning electron microscopy, optical profiler, energy dispersive spectroscopy, and X-ray diffraction respectively. To further explore the CZTS layer, the following additional layers were deposited to complete the solar cells: CdS with chemical bath deposition; ZnO and Al2O3-doped ZnO with RF magnetron deposition; and, silver fingers as the front contact as the last layer. The optical and morphological properties of the CZTS films were investigated and compared. Subsequently, the electrical characteristics and the efficiencies of the regarding solar cells were analyzed. A maximum efficiency of 3.8% has been obtained for the fast sulfurization (30min at 580°C) and finally, the performance is compared with our best cell fabricated through the more common slow annealing.

CZTS layers for solar cells by an electrodeposition-annealing route

In the present work, Cu2ZnSnS4 (CZTS) thin films were successfully prepared using a co-electrodeposition–annealing route, in which Cu-Zn-Sn metal precursors were deposited by a novel electrolyte formula on Mo substrate, followed by annealing in elemental sulfur environment in quartz tube furnace with N2 atmosphere. Morphological, compositional and structural features of the films were investigated using scanning electron microscopy, energy dispersive X-ray spectroscopy and X-ray diffraction before and after sulfurization. Raman spectroscopy was performed after sulfurization to fully characterize the deposited films. Glow discharge optical emission spectroscopy (GDOES) has also been carried out to see how composition changes along the film thickness. Results confirmed that kesterite structure of CZTS has been well formed after sulfurization of the film. It was also observed that CZTS properties depend on annealing times and ramping rate during sulfurization.

Atom-probe tomographic study of interfaces of Cu2ZnSnS4 photovoltaic cells

The heterophase interfaces between the CdS buffer layer and the Cu2ZnSnS4 (CZTS) absorption layers are one of the main factors affecting photovoltaic performance of CZTS cells. We have studied the compositional distributions at heterophase interfaces in CZTS cells using three-dimensional atom-probe tomography. The results demonstrate: (a) diffusion of Cd into the CZTS layer; (b) segregation of Zn at the CdS/CZTS interface; and (c) a change of oxygen and hydrogen concentrations in the CdS layer depending on the heat treatment. Annealing at 573 K after deposition of CdS improves the photovoltaic properties of CZTS cells probably because of the formation of a heterophase epitaxial junction at the CdS/CZTS interface. Conversely, segregation of Zn at the CdS/CZTS interface after annealing at a higher temperature deteriorates the photovoltaic properties.

Synthesis of Cu2ZnSnS4film by air-stable molecular-precursor ink for constructing thin film solar cells

Cu2ZnSnS4 (CZTS) precursor ink routes have been demonstrated to open up new possible ways to fabricate low-cost and high-efficiency CZTS film solar cells, and a prerequisite is to investigate and develop precursor inks. Herein, we report the synthesis of air-stable molecular precursor ink for the construction of CZTS film solar cells. The precursor ink was prepared by dissolving Cu(acac)2, Zn(acac)2, and SnC2O4 and sulfur powder in pyridine, which yielded a viscous black solution. Subsequently, the precursor ink was spin-coated on a Mo-coated glass substrate, and the resulting film was heated in the air to burn off organics, producing an oxide film. The oxide film was subjected to the sulfurization process, in which the effects of sulfurization conditions on composition were investigated. CZTS film was obtained with Cu/(Zn + Sn) = 0.87 and Zn/Sn = 1.09, which is a composition that is close to the previously proposed composition for high-efficiency CZTS film solar cells. This CZTS thin film was used to construct a solar cell, and it exhibited an initial power conversion efficiency of 2.30%. Further improvement of the efficiency can be expected by the optimization of the ink composition, the composition/morphology/thickness/phase of the CZTS layer, and the device structure.

Effects of chlorine and carbon on Cu2ZnSnS4 thin film solar cells prepared by spray pyrolysis deposition

Cu2ZnSnS4 (CZTS) thin film solar cells were fabricated under non-vacuum conditions, wherein the absorption layer was prepared by the spray pyrolysis deposition method. By using a chlorine (Cl)-free solution instead of a Cl-containing solution, the solar cell efficiency was improved from 0.18% to 0.35%. On the CZTS surface deposited from the Cl-containing solution, Cl-containing precipitates with size of 10μm were present and were found to degrade the solar cell efficiency. The still low efficiency of 0.35% was caused by a layer containing small grains that existed between the CZTS layer and the Mo electrode, which included a significant amount of carbon. By optimizing the pre-annealing process, the small-grained layer became thinner and the efficiency was improved to 0.63%. Finally, by adjusting the chemical composition of the Cl-free solution and the CdS buffer layer deposition conditions, the cell efficiency was improved to 0.94%.

Transmission electron microscopy analysis for the process of crystallization of film from sputtered Zn/CuSn precursor

The mechanism of crystallization of (CZTS) made by the sulfurization of a sputtered Zn/CuSn stack was studied by transmission electron microscopy. At , the Zn buried at the bottom of the metallic stack was found to be driven to the upper layer by alloying with Cu and reacting with S to form ZnS. At , CZTS was found to be formed and elemental Sn was observed in the vicinity of the back contact region, while large quantities of and ZnS were segregated at the film surface. At , was found, while all the elemental Sn had vanished. Upon extending the duration of the annealing to 10 min, at , all the secondary phases except ZnS were consumed and the CZTS formation was completed, while around 200 nm of was formed between the CZTS layer and Mo back contact. The finished solar cell exhibits an efficiency of 2.66%, an open-circuit voltage of 666.9 mV and a short-circuit current density of . The solar cell performance is possibly limited by the thick layer and the large density of voids in the back contact region due to the Sn loss at high sulfurization temperatures.

A 5.5% efficient co-electrodeposited ZnO/CdS/Cu2ZnSnS4/Mo thin film solar cell

Thin film solar cells with a structure of ZnO/CdS/Cu2ZnSnS4 (CZTS)/Mo were fabricated successfully by sulfurization of co-electrodeposited Cu–Zn–Sn–S precursors at 590°C for 15min. The best solar cell performance was achieved with an efficiency of 5.5% (VOC=673.8mV, JSC=18.7mAcm−2, FF=44%). The vibrational properties and phase identification of the absorber were studied by the polarised Raman and IR spectra. The CZTS absorber layer shows a bi-layered structure comprising of a well-crystallized photovoltaic layer with metallic ratios of Cu/Zn=1.95 and Zn/Sn=1.56 and a particulate-like bottom layer with a heavily Sn poor content. These particulates in the bottom CZTS layer feature with a high density of stacking faults along with ZnS phases and contribute to the low minority lifetime, the high recombination losses, and the increased series resistances in the device. In the well-crystallized CZTS absorber layer the trace of a twin defect on {112} plane is discovered. Dark electrical analysis indicates there is a relatively small energy barrier height of 44meV across the cell.

Stoichiometry effect on Cu2ZnSnS4 thin films morphological and optical properties

Thin films of Cu2ZnSnS4 (CZTS) were prepared by sulfurization of multilayered precursors of ZnS, Cu, and Sn, changing the relative amounts to obtain CZTS layers with different compositions. X-Ray Diffraction (XRD), Energy Dispersive X-Ray spectroscopy, and SEM were used for structural, compositional, and morphological analyses, respectively. XRD quantitative phase analysis provides the amount of spurious phases and information on Sn-site occupancy. The optical properties were investigated by spectrophotometric measurements and Photothermal Deflection Spectroscopy. These films show a clear dependence of the optical and microstructural properties on the tin content. As the tin content increases we found: (i) an increase in both crystalline domain and grain size, (ii) an abrupt increase of the energy gap of about 150 meV, from 1.48 to 1.63 eV, and (iii) a decrease of sub-gap absorption up to two orders of magnitude. The results are interpreted assuming the formation of additional defects as the tin content is reduced.

Epitaxial growth of kesterite Cu2ZnSnS4 on a Si(001) substrate by thermal co-evaporation

Using thermal co-evaporation we have prepared epitaxial Cu2ZnSnS4 (CZTS) films on Si(001) substrates. A substrate temperature as high as 370°C and proper substrate cleaning (HF-dip followed by thermal desorption of surface hydrogens) are found to be necessary for the epitaxial growth. Detailed transmission electron microscopy measurements and X-ray diffraction studies are used to reveal the orientation relation of the CZTS films with the underlying silicon substrate, and the formation of defects within the CZTS layer.

Characterization of a CZTS thin film solar cell grown by sputtering method

We report the performance of Cu2ZnSnS4 (CZTS) thin film solar cell that showed efficiency in the range of 6.2% without an anti-reflection coating. Initially, the CZTS precursor film was co-sputtered using three different targets; copper (Cu), tin sulfide (SnS) and zinc sulfide (ZnS). The Cu target was subjected to DC power, and RF power was used for the SnS and ZnS targets. The as-grown CZTS film was sulfurized in a H2S/N2 environment at 525°C, which re-crystalized the film with grain sizes in the range of 1μm. Cadmium sulfide (CdS) was used as the n-type layer. Current–voltage (I–V), quantum efficiency (QE) and capacitance–voltage (C–V) measurements were used to characterize the cell device. The modeling and analysis of QE and CV data showed that a significant portion of the CZTS layer did not contribute to the photo-generation. Optimizing CZTS phase purity, improving QE in the broader wavelength region, and increasing minority carrier lifetime are necessary steps to further improve CZTS device performance.

Effects of sulfurization temperature on CZTS thin film solar cell performances

Synthesis of Cu2ZnSnS4 thin film solar cells by sulfurization of sputtered Sn/Zn/Cu precursors is studied. The sulfurization temperatures were varied, and the morphology, cross section, and composition of the CZTS were investigated by scanning electron microscopy, energy dispersive spectroscopy, X-ray diffraction and Raman scattering. To further explore the CZTS layer, the following additional layers were deposited to complete the solar cells: CdS with chemical bath deposition; ZnO and AZO with RF magnetron deposition; and, finally, silver fingers as the front contact. The efficiency and characteristics of the thin film solar cells were measured and a detailed comparison is reported. Sulfurization at 550°C yields a maximum efficiency of 5.75% without any anti-reflective layers.

Toward a high Cu2ZnSnS4 solar cell efficiency processed by spray pyrolysis method

In this work, a review about the influence of the growth parameters on the chemical and physical properties of Cu2ZnSnS4 (CZTS) deposited by pneumatic spray pyrolysis technique and its impact on the thin film solar cells is presented and analyzed in order to identify the major drawbacks of this technique and the possibility to improve the device efficiency. Our best solar cell using sprayed CZTS shows an open-circuit voltage of 361 mV, a short-circuit current density of 7.5 mA/cm2, a fill factor of 0.37, and an efficiency of 1% under irradiation of AM 1.5 and 100 mW/cm2. Some of the key mechanisms related to the properties of sprayed CZTS layers, as well as those concerning the solar cells mechanisms that limit the cell performance, are also analyzed.

CZTS stoichiometry effects on the band gap energy

Abstract The considerable spread of Cu2ZnSnS4 (CZTS) optical properties reported in the literature is discussed in terms of material stoichiometry. To this purpose, kesterite thin films were prepared by sulfurization of multilayered precursors of ZnS, Cu and Sn, changing the relative amounts to obtain CZTS layers with different compositions. X-Ray Diffraction (XRD), Energy Dispersive X-Ray (EDX) spectroscopy, X-Ray Photoelectron Spectroscopy (XPS) and Raman spectroscopy were used for structural and compositional analysis. XRD quantitative phase analysis provides the amount of spurious phases and information on Sn-site occupancy. The optical properties were investigated by spectrophotometric and Photothermal Deflection Spectroscopy (PDS) measurements to assess the absorption coefficient of samples with different compositions. The PDS data show an increase of the sub-band absorption as the Sn content decreases. The results are interpreted assuming the formation of additional defects as the tin content is reduced. Those defects can also be responsible for the decrease of the band gap energy value as the Sn/Cu ratio is decreased.

Effects of Back Contact Instability on Cu2ZnSnS4 Devices and Processes

Cu2ZnSnS4 (CZTS) is a promising material for thin film solar cells based on sustainable resources. This paper explores some consequences of the chemical instability between CZTS and the standard Mo “back contact” layer used in the solar cell. Chemical passivation of the back contact interface using titanium nitride (TiN) diffusion barriers, combined with variations in the CZTS annealing process, enables us to isolate the effects of back contact chemistry on the electrical properties of the CZTS layer that result from the synthesis, as determined by measurements on completed solar cells. It is found that instability in the back contact is responsible for large current losses in the finished solar cell, which can be distinguished from other losses that arise from instabilities in the surface of the CZTS layer during annealing. The TiN-passivated back contact is an effective barrier to sulfur atoms and therefore prevents reactions between CZTS and Mo. However, it also results in a high series resistance and ...

Investigation of ZnO:Al window layer of Cu2ZnSnS4 thin film solar cells prepared by non‐vacuum processing

AbstractCu2ZnSnS4 (CZTS) thin film solar cells have been fabricated using sol‐gel sulfurizing method under non‐vacuum processing. The solar cells structure is Al/ZnO:Al/CdS/ CZTS/Mo/Soda Lime Glass(SLG) substrate. In our previous reports, the surface of CZTS got rougher as the particle diameter increased. In addition, there were many spots where CZTS layer was not coated enough by ZnO:Al window layer. In this report, in order to improve conversion efficiency of the CZTS solar cells, we focus on the ZnO:Al layer and the effect of an increase in the number of ZnO:Al coating repetition and the dependence on drying time and temperature for ZnO:Al were investigated. Optimized conversion efficiency of 3.61% was achieved with 15 coating repetition of ZnO:Al layer coated at 350 °C and dried for 3 min. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Effects of sulphurization time on Cu2ZnSnS4 absorbers and thin films solar cells obtained from metallic precursors

We report the results of a study of the sulphurization time effects on Cu2ZnSnS4 absorbers and thin film solar cells prepared from dc-sputtered stacked metallic precursors. Three different time intervals, 10min, 30min and 60min, at maximum sulphurization temperature were considered. The effects of this parameter' change were studied both on the absorber layer properties and on the final solar cell performance. The composition, structure, morphology and thicknesses of the CZTS layers were analyzed. The electrical characterization of the absorber layer was carried out by measuring the transversal electrical resistance of the samples as a function of temperature. This study shows an increase of the conductivity activation energy from 10meV to 54meV for increasing sulphurization time from 10min to 60min. The solar cells were built with the following structure: SLG/Mo/CZTS/CdS/i-ZnO/ZnO:Al/Ni:Al grid. Several ac response equivalent circuit models were tested to fit impedance measurements. The best results were used to extract the device series and shunt resistances and capacitances. Absorber layer's electronic properties were also determined using the Mott–Schottky method. The results show a decrease of the average acceptor doping density and built-in voltage, from 2.0×1017cm−3 to 6.5×1015cm−3 and from 0.71V to 0.51V, respectively, with increasing sulphurization time. These results also show an increase of the depletion region width from approximately 90nm–250nm.

Solution-processed copper zinc tin sulfide thin films from metal xanthate precursors

The quaternary semiconductor copper zinc tin sulfide (Cu2ZnSnS4, CZTS) is one of the most promising alternatives to Ga and In based semiconductors for thin film solar cells. It consists of non-toxic, cheap, and abundant elements and displays highly beneficial optical as well as electronic properties for photovoltaic applications. In this work we present a solution-based preparation method for CZTS thin films using exclusively metal xanthates as precursor materials. The introduction of branched alkyl side chains (3,3-dimethyl-2-butyl) into the metal xanthates leads to highly soluble precursors with low decomposition temperatures. In addition, these precursors already contain the sulfur needed for the formation of the metal sulfide. Therefore, no external sulfur source such as thiourea, thioacetamide, or elemental sulfur is necessary. For the preparation of CZTS thin films, solutions containing these metal xanthates were used to coat precursor layers, which were subsequently annealed at temperatures between 180 and 350 °C. Depending on the temperature, nanocrystalline films with primary crystallite sizes ranging from 3 nm (180 °C) up to approximately 43 nm (350 °C) were obtained. A combined X-ray diffraction, Raman spectroscopy, and TEM-EDX study showed that a precursor solution with a Cu/(Zn + Sn) ratio of 0.6 has to be used to obtain CZTS films, which show high optical absorption (>2 × 105 cm−1) and an optical band gap of approximately 1.31 eV. First experiments concerning photovoltaic activity of the solution processed CZTS layers were carried out.