CZTS Absorber Layer Research Articles

Influence of hydrogen plasma treatment on secondary phases in CZTS thin films for energy harvesting

Cu2ZnSnS4 (CZTS) thin films deposited using direct current magnetron sputtering and sulfurized at argon atmosphere pressures of 950, 460, and 50 mbar are studied in a view of solar cell absorber fabrication. The main novelty in the work is the influence of the radio frequency (RF) electromagnetic field treatment on the Cu2−xS secondary phase. The treatment reduces the amount of the Cu2−xS secondary phase, which is confirmed by Raman spectroscopy. The RF effect is long-term, at least one year later. So, this treatment is a promising technique to achieve higher purities of CZTS absorber layers for solar cells.

Strain modulation for enhancing Cu–Zn ordering in CZTS absorber layer using seed layer assisted growth for efficient carrier transport

Cu2ZnSnS4 (CZTS) solar cells suffer from lower power conversion efficiency relative to its fellow copper indium gallium selenide thin-film technology, which have been asserted on the existence of non-stoichiometry and high degree of Cu–Zn disorder. Huge disparity among the lattice constants of Mo and CZTS is one of the causes of inducing strain in the film, which often creates defects in the CZTS structure. This work focused on investigating the effect of strain modulation using seed layer (SL) assisted growth on the structural and optoelectronic properties of CZTS films. The results indicate that SL growth of CZTS reduces strain in the film and improves the crystallinity and overall quality of the CZTS absorber, as indicated by SEM and x-ray diffraction studies. Raman shifts to higher wavenumber and photoluminescence (PL) energy shift corresponding to dominant band-to-band transition in SL CZTS correlate perfectly with the high value of order parameter. Bandgap enhancement and reduction in the Urbach energy of SL CZTS implicate higher ordering (reduction in Cu–Zn disorder) due to strain modulation. Consequently, substantial improvement from 2.13 to 13.5 cm2/V s in hole mobility is achieved. Finally, the faster response of the photodetector based on SL CZTS compared to without SL growth supports all the findings. Our results imply that SL assisted growth of CZTS could be critical to obtain a high-quality CZTS absorber layer.

Comparative study of the CZTS, CuSbS2 and CuSbSe2 solar photovoltaic cell with an earth-abundant non-toxic buffer layer

In the present article, modeling outcomes of novel and third-generation heterojunctions (3G) thin-film solar cells (TFSCs): quaternary semiconductor compound CZTS and ternary semiconductor compound CuSbS2 and CuSbSe2 have been presented. Solar cells have been studied and analysed to investigate optimized structure with good efficiency. CuSbS2 and CuSbSe2 are promising earth-abundant photovoltaic absorber materials which are chemically simpler than the widely studied Cu2ZnSnS4. However, CuSbS2 and CuSbSe2 photovoltaic (PV) devices currently have relatively low efficiency and poor reproducibility, often due to suboptimal material quality and insufficient optoelectronic properties. To overcome these concerns, we carried out numerical analysis using the Solar Cell Capacitance Simulator-1 dimensional software (SCAPS-1D). The band alignment and several parameters such as thickness and doping concentration have been investigated. Further C-V, C-f and J-V characteristics are also studied to compare and optimize the performance of CZTS, CuSbS2 and CuSbSe2 solar cell. Understanding the constraints imposed by the CZTS, CuSbS2, and CuSbSe2 absorber layers could help this technology to overcome the lower efficiency barrier.

Band alignment tuning at Mo/CZTS back contact interface through surface oxidation states control of Mo substrate

The molybdenum (Mo) back contact is one of the crucial factor that affects the performance of kesterite Cu2ZnSnS4 (CZTS) thin film solar cells. Thick MoS2 which is formed during sulfurization process of CZTS is known to be harmful for the device performance. MoOx on surface of Mo substrate with proper thickness can enhance the efficiency of CZTS solar cells through reducing the thickness of MoS2 layer and tailoring the work function of back contact. However, the role of oxidation states of Mo in MoOx on the performance of solar cells is unclear. Herein the effect of MoOx with different oxidation states on CZTS absorber and intermediate MoS2 layer were investigated. Our results shown that the efficiency of CZTS solar cells on Mo substrate with proper surface oxidation status (MoO2.03) can be as high as 2.86 times of the one fabricated on higher oxidized Mo (MoO2.21), although the thinnest MoS2 layer was present in the latter case. This work discloses the origin of the oxidation states of Mo for the efficiency enhancement of kesterite structured solar cells. The results will be also helpful to improve the performance of compound semiconductor by oxidation states control strategy.

Significant Efficiency Enhancement in Ultrathin CZTS Solar Cells by Combining Al Plasmonic Nanostructures Array and Antireflective Coatings

In the few past years, the economic and eco-friendly Cu2ZnSnS4 (CZTS) solar cells have caught lots of attentions. However, due to rather poor efficiency, identifying deficiencies and making improvements is necessary. In the present study, the performance improvement of ultrathin CZTS solar cells was achieved through (1) incorporation of anti-reflective coating (ARC) on the surface of cell and (2) embedding Al plasmonic nanostructures with different radius, periods, and vertical positions in the absorber layer. Various thicknesses of CZTS absorber layer were simulated optically and electrically using FDTD and DEVICE solver of Lumerical software. The reference solar cell consists of a 1.5-nm-thick CZTS absorber and exhibit an efficiency of up to 5.67%, short-circuit current density (Jsc) of 18.48 mA cm−2 and open circuit voltage of 0.58 V. Result showed a remarkable performance enhancement of the solar cell in spite of a very thin absorber layer. For a 500-μm-thick CZTS solar cell with the assistance of ARC and embedding Al plasmonic nanostructures, the efficiency is increased to 7.45% due to an increase in Jsc to 22.62 mA cm−2 with an open circuit voltage of 0.62 V.

Suppression of interfacial oxygen vacancies for efficient charge extraction at CZTS/TiO2 heterojunction

Earth abundant CZTS (Cu2ZnSnS4) absorber layers are promising for the development of cost-effective and large area photovoltaics; however, interfacial nonradiative recombination is a major obstruction to the pathways toward high performing CZTS devices. Elimination of interfacial recombination losses via interface engineering is paramount to obtain efficient CZTS solar cells. Herein, we report a systematic investigation of the influence of oxygen vacancies (OV) settled at the CZTS/TiO2 interface on the charge transfer rate in heterostructures. Modulation of OV by varying oxygen flow rate during TiO2 deposition was confirmed by x-ray photoelectron spectroscopy. Lower OV concentration shifted the conduction band offset from negative to positive at the CZTS/TiO2 heterojunction, which is essential for efficient charge transportation through the interface. Photoluminescence quenching of the CZTS/TiO2 heterojunction also showed a strong correlation between charge dynamics and OV at the interface. Finally, we found the fast decay response of photogenerated charge carriers for the CZTS/TiO2 device with lower OV strongly favors the suppression of carrier trapping at the interface. This work provides a critical insight into interface engineering in CZTS solar cells through regulating interfacial OV, particularly when an oxide electron transport layer is applied.

Simulation studies of CZTS thin film solar cell using different buffer layers

The most promising and powerful energy source among renewable energies is solar energy. It is a low cost, clean and environmentally friendly energy source. Different types of solar cells are CIGS, CZTS and perovskite solar cells. Here CZTS solar cell is considered. CZTS solar cells show a promising performance within the sector of sunlight-based energy production system. Performances of CZTS based solar cells depend heavily on the buffer layer parameters. In this work, numerical simulation is performed on structure based on CZTS absorber layer, ZnO (intrinsic) window layer and transparent conducting layer TiO2 with different buffer layers using SCAPS-1D software. It is also evaluated by another software called Spyder (python). This study also aims to optimize efficiency by varying layer depth and doping concentration. The optimized solar cell shows an efficiency of 10% under AM 1.5G spectrum. The electrical parameters like open circuit voltage, short circuit current, fill factor and efficiency has also been obtained for different buffer layers.

Modification of Back Contact in Cu2ZnSnS4 Solar Cell by Inserting Al-Doped ZnO Intermediate Layer.

The optimization of back contact interface is crucial to improve the performance of Cu2ZnSnS4 (CZTS) thin film solar cells. In this paper, we first employ Al-doped ZnO (AZO) as the intermediate layer into the Mo/CZTS interface to improve the quality of back contact region. This AZO intermediate layer, obtained from the sputtering method prior to the CZTS precursor deposition, initially blocks the direct contact of CZTS with the Mo layer and thus indeed suppresses the decomposition reaction between Mo and CZTS. Consequently, the generation of voids at the back contact region is obviously avoided. Besides, the AZO intermediate layer can inhibit the reaction between sulfur (S) and Mo during sulfurization process, and thus significantly reduce the thickness of MoS2. Meanwhile, the AZO intermediate layer with suitable thickness does not affect the crystal quality of CZTS absorber layer. Moreover, the effects of different thicknesses of predeposited AZO on the film morphology, composition, and corresponding device performance were systematically studied. After optimizing the thickness of the AZO layer, the efficiency of the resultant device has increased from 7.1% to 8.4% (the active area efficiency is 9.2%).

The investigation of the corrosion behavior of CZTS thin films prepared via electrodeposition

In this study, Cu2ZnSnS4 (CZTS) thin films were deposited on indium tin oxide (ITO)-coated glass substrate by single-step electrochemical deposition (ECD). CZTS absorber layers were obtained with −1.05 V cathodic potential and 2700 s deposition time for pH:5 and at room temperature. Trisodium citrate was used as a complexing agent for co-electrodeposition. After the deposition, the CZTS thin films were annealed at 580 °C in sulfur powder and N2 atmosphere for 60 min for the formation of stoichiometric kesterite structure. The structural, morphological and optical properties of the prepared thin films were investigated by X-ray diffraction (XRD) analysis, optical absorption techniques, scanning electron microscopy (SEM) observations and Raman spectroscopy measurements. Crystallisation of the films was confirmed by XRD measurements. The prepared films were also examined using Electrochemical Impedance Spectroscopy (EIS) and Tafel measurements. Nyquist, open circuit voltage (OCV) voltage(OCP) and Bode analysis were used to find out the structural changing and corrosion behavior of the CZTS films. The equivalent circuit parameters such as solution resistance (Rs), polarization resistance (Rp), a constant phase element (CPE), a CPE exponent (n) and diffusion potential were calculated as 253.8 Ω cm2, 944.2 Ω cm2, 1.547 × 10−3 Ω−1 s cm−2, 0.500 and 8.631 × 10−3 Ω−1 s1/2. cm−2, respectively. Also, with the help of Tafel measurements, the corrosion potential and current values were determined as −0.161 V and 1 × 10−6 A, respectively.

Modeling of ZnO/MoS2/CZTS photovoltaic solar cell through window, buffer and absorber layers optimization

In this work, we propose a feasible ZnO/MoS2/CZTS structure where the classical CdS buffer layer is replaced by a molybdenum disulfide buffer layer. This proposed solar cell is investigated thereafter using SCAPS code so as to determine the optimal value of each layer thickness constituting the cell of interest. The different photovoltaic parameters are determined versus each layer thickness. Our solar cell shows a conversion efficiency of about 23.69%. This corresponds to optimal thicknesses of 0.1 μm for ZnO window layer, 0.2 μm for MoS2 buffer layer and 1 μm for CZTS absorber layer and common doping concentrations of 1018 cm−3 for each layer. This conversion efficiency compares reasonably with those previously quoted in the literature.

Nanoscale charge transport and local surface potential distribution to probe the defect passivation in Cr-substituted earth abundant CZTS absorber layer

The non-stochiometric deviations in CZTS often results in certain detrimental point defects that act as trap centers causing non-radiative recombination and potential fluctuations, eventually results in large open circuit voltage (VOC) deficit in CZTS solar cells. Cationic substitution in CZTS layer is one of the leading approaches to capture the great potential of kesterite solar cells for future cost-effective photovoltaic technology. Here, chromium incorporation in CZTS is studied for cationic substitution in CZTS layer, as it renders multiple chemical states including +1, +2 and + 4 of Cu, Zn and Sn cations in CZTS. The CZTS films determine to have good crystallinity and morphology with no significant disparity among pristine and Cr-doped CZTS films, apart from slight shift of XRD (112) peak indicating partial cation substitution by smaller Cr atoms. However, the Zn content is found to be decreased with increasing Cr content in CZTS layer. Substantial enhancement in the absorption of Cr-doped CZTS films indicate intermediate band (IB) within the band gap. Photoluminescence (PL) emission spectra strongly suggests reduced non-radiative recombination and potential fluctuations in Cr-CZTS films. X-ray photoelectron spectroscopy investigations reveal Zn substitution by Cr in CZTS crystal geometry. Using nanoscale Kelvin Probe Force Microscopy (KPFM) and Conducting Atomic Force Microscopy (CAFM), we verify the surface potential variation and nanoscale electrical conductivity in pristine and Cr-CZTS films. Additionally, Zn substitution by Cr eventually lead to suppression of ZnSn deep level defects segregated at grain boundaries (GBs). CAFM strongly confirms the GB passivation in Cr-CZTS films. This work widens the opportunity of exploring potential cationic substitution in CZTS for developing high efficiency CZTS solar cells.

Variation of different layer parameters in a CZTS based solar cell for optimum performance: A simulative approach

• Simulation of CZTS based solar cell using Wx AMPS. • The optimization of all the layers by varying different parameters viz. Electronics, Materials, optical are done. • The proposed structure is presented at room temperature. The importance of solar energy has increased tremendously in the past decade due to its many salient features. The Kesterite based solar cell (CZTS) that rely on earth abundant, non-toxic elements is used in this work. In the present simulative study, optimization of different layers like Al doped ZnO (front layer), i-ZnO, CdS (buffer layer), CZTS (absorber layer) have been analyzed by numerical simulation using wxAMPS. The basic objective of this study is to achieve the optimized layers parameters such as concentration (donor and acceptor), thickness, defect density of the solar cell which comprises of Al:ZnO/i-ZnO/CdS/CZTS/Mo from top to bottom. This paper explicitly deals with the idea to optimize output parameters which are represented as V OC = 0.754 Volt, J SC = 22.10 mA/cm 2 and efficiency of 14.17% of the cell.

A Status Review on Cu2ZnSn(S, Se)4-Based Thin-Film Solar Cells

Photovoltaics has become a significant branch of next-generation sustainable energy production. Kesterite Cu2ZnSn(S, Se)4 (copper-zinc-tin-(sulfur, selenium) or CZTS(Se)) is considered one of the most promising, earth-abundant, and nontoxic candidates for solar energy generation over the last decade. However, shallow phase stability of the quaternary phase and the presence of various secondary phases and defects are the main hindrances in achieving the target device performance. This paper summarizes various approaches to synthesize the CZTS absorber layer and the CdS n-type material layer. Besides, different CZTS solar cell device structures, as well as a comprehensive review of secondary phases and defects, have been illustrated and discussed. At last, this review is intended to highlight the current challenges and prospects of CZTS solar cells.

CZTS solar cell with non-toxic buffer layer: A study on the sulphurization temperature and absorber layer thickness

Cu2ZnSnS4 solar cell has been fabricated with a non-toxic buffer layer and Al doped ZnO transparent conducting layer. A detailed material study on each layer of the solar cell has been carried out independently. The precursor films prepared by spin coating were sulphurized at different temperatures to optimize the conditions to obtain phase pure CZTS films. X-ray diffraction, Raman spectroscopy and rietveld refinement studies confirmed the phase purity of the film sulphurized at 500 °C. The optimum CZTS properties like band gap of 1.45 eV, high absorption coefficient ~2 × 105 cm−1 and dense surface morphology were obtained for films sulphurized at 500 °C. Carrier concentration, mobility and resistivity of these films were ~1 × 1019 cm−3, 0.23 cm2 V−1 s−1 and 2.7 Ωcm, respectively. The efficiency measurements of the cells with device structure SLG/Mo/CZTS/ZnS/AZO/Ag were carried out using absorber films with three different thicknesses. The optimized CZTS absorber layer with thickness of ~1.8 µm exhibited solar cell conversion efficiency of 3.02% for an active area of 0.21 cm2 withopen-circuit voltageof 0.38 V,short-circuitcurrent density of 17.19 mA/cm2 and fill factor of 46%.

Improving the efficiency of Cu2ZnSnS4 solar cells by promoting the homogeneous distribution of Sn element

Homogeneous element distribution of absorber, as well as high crystallinity, is very critical for the highly efficient Cu2ZnSnS4 (CZTS) thin-film solar cell. However, it is more difficult to prepare high quality CZTS absorber layer by post sulfurization of sputtered metallic stack precursors. Herein, we report a simple process, i.e., the optimization for heating rate during the sulfurization, to effectively improve both the uniformity of Sn element distribution and crystallinity of CZTS absorber. CZTS films are prepared by sulfurizing sputtered metallic stack precursors under S and SnS mixed atmosphere. The effects of heating rates on phase structure, morphology, composition distribution of CZTS films and resultant solar cells performance have been comprehensively investigated. CZTS film, sulfurized with a heating rate of 15 °C/min, demonstrates the uniformity of composition distribution, especially for Sn element, besides good crystallinity. Consequently, the series resistance of CZTS solar cells is reduced and the performance of solar cells is improved remarkably. A CZTS solar cell with efficiency of 7.02% (active area efficiency 7.69%) without MgF2 anti-reflection coating is obtained. The optimization for the heterojunction interface is expected to further improve efficiency.

Reduction of interface recombination current for higher performance of p+-CZTSxSe(1-x)/p-CZTS/n-CdS thin-film solar cells using Kesterite intermediate layers

There have been significant efforts to enhance the performance of CZTS thin-film solar cells. However, the increase in series resistance, formation of the unfavorable MoS2 between the absorber layer and back-contact, and recombination losses are some of the most effective factors for the low values of the efficiency and open-circuit voltage parameters. In this paper, five experimental CZTS solar cells with high efficiency of 8.4–11% have been mainly studied and discussed. Afterwards, two experimental works have been reproduced to validate our results. Additionally, with introducing the thin CZTSxSe(1-x) layer between the Mo back-contact and CZTS absorber layer, the efficiency and open-circuit voltage of solar cells have been notably improved. There is a significant decrease in recombination losses (less than 12 mA/cm2) after using CZTSxSe(1-x) intermediate layers between the absorber layer and back-contact. Using the CZTSe, and CZTS0.2Se0.8 thin layers and optimizing the structure as p+pn structure (p+-CZTSxSe(1-x)/p-CZTS/n-CdS), the following efficiencies (and open-circuit voltage) are achieved as: 15.98% (921 mV) and 17.81% (986 mV), respectively. Simulations demonstrate that at 0.8-V bias for p+-CZTS0.2Se0.8/p-CZTS/n-CdS structure, the recombination current total density is reduced to 6.47 mA/cm2. To obtain the highest performance in the proposed structures, the absorber layer carrier concentration should be 1 × 1016 cm−3 and the intermediate layer carrier concentration needs to be within the range of 1 × 1017 cm−3 to 1 × 1018 cm−3. Furthermore, the optimum thickness of CZTSe and CZTSSe intermediate layers in the proposed structures is 50 nm.

The impact of the carrier concentration and recombination current on the p+pn CZTS thin film solar cells

One of the main causes of low performance of Cu2ZnSnS4 (CZTS) based thin film solar cells is its high recombination current. In this paper, the effects of the carrier concentration and recombination current in the CZTS thin film solar cells have been carefully scrutinized. In continue, for comparison and validation, two simulated structures based on the CZTS absorber layer have been reproduced and the influence of the absorber layer defects and carrier concentration has been analyzed. In this work, two different structures using p+-CZTS intermediate layer (between the CZTS absorber layer and Mo back-contact) and dual CZTS layer have been proposed and optimized to decrease recombination and extract the desired photovoltaic parameters. By using and optimizing these structures as p+pn junction, 15.62% and 19.08% efficiencies have been achieved in p+-CZTS intermediate layer and p+pn CZTS dual layer structures, respectively. The highest efficiency of dual absorber based solar cell is achieved when the carrier concentration of the p+-CZTS and p-CZTS absorber layers is 1 × 1018 cm−3 and 5 × 1015 cm−3, respectively. Moreover, the optimum thickness of the p+-CZTS and p-CZTS absorber layers in p+pn CZTS dual layer structure is 1 and 1.5 µm, respectively. Finally, the total recombination current density is reduced to 2.05 mA/cm2.

Investigations on copper zinc tin sulfide thin films grown through nebulizer assisted spray pyrolysis technique

An attempt is made in this work to synthesize the nontoxic Cu2ZnSnS4 (CZTS) thin film on FTO coated glass substrates through aerosol assisted nebulizer spray pyrolysis technique at different annealing temperatures for photovoltaic energy conversion. The deposited thin film is further used to form a heterojunction interface with a cadmium sulfide layer to fabricate a solar cell with Glass/FTO/CdS/CZTS/Ag superstrate structure to reveal its photovoltaic application. Various characterization techniques are utilized to study its inherent properties. X-ray diffraction (XRD) is employed to investigate the structural parameters such as crystallite size, microstrain and dislocation density which shows a preferential peak for (112) plane around 28.5° confirming the formation of kesterite CZTS. Raman measurements establish the peak for CZTS at 336 cm−1 and confirm the absence of parasitic secondary phases for an excitation wavelength of 488 nm. Scanning electron microscope and atomic force microscope used to examine the surface morphology and roughness of the films reveal a good surface morphology with a grain size of 555.9 nm and roughness of 123.7 nm for an annealing temperature of 350°C. The bandgap of the deposited CZTS films is found to be around 1.5 eV. The I-V characteristics of CZTS seem to be better for the 350°C annealed film. The performance of the optimized CZTS absorber layer is tested by forming a solar cell structure. The devised solar cell exhibited an open circuit voltage of 213 mV and a short circuit current density of 490 μA cm−2 with a conversion efficiency of 0.68% substantiating the usage of the prepared film as an absorber for photovoltaic conversion.

Effect of Cu and Sn concentration on the performance of all-sprayed CZTS solar cell

Cu2ZnSnS4/In2S3 thin film solar cell was prepared using chemical spray pyrolysis. Influence of variation in copper concentration in precursor solution on the properties of CZTS thin films was monitored. Concentration of copper was found to have essential role in adjusting composition and crystallinity of CZTS thin films. Junction was fabricated using CZTS as absorber layer and spray pyrolysed In2S3 as buffer layer. Two sets of devices were prepared, one by varying copper concentration and the other by varying tin concentration in the precursor solution for the deposition of CZTS thin films. For both sets, In2S3 thin films were deposited as buffer layer and Ag electrodes were used for taking contacts. All the performance parameters were found to have dependence on the concentration of Cu and Sn. For getting maximum conversion efficiency, the sprayed CZTS absorber layer should have elemental ratio of Cu:Zn:Sn:S=2.5:1:0.7:12 in the precursor solution. Maximum efficiency obtained was 1.85 %.

Influence of boric acid concentration on the properties of electrodeposited CZTS absorber layers

This work involves the synthesis and characterization of (CZTS) layers. The films were prepared on Mo/glass substrates by single-step electrodeposition method followed by sulfurization at 500 °C under argon flow. The effect of boric acid concentration on the crystallographic structure, compositional and morphological properties of CZTS films was investigated, with the objective to understand the growth behavior and to enhance the film properties. Cyclic Voltammetry was used in order to estimate the adequate deposition potential for the CZT alloy. The x-ray diffraction analysis showed the formation of the kesterite phase in all the samples. The Raman and x-ray photoelectron spectroscopy studies confirmed the existence of the CZTS phase. The scanning electron microscopy was employed to inspect the films structure. The results indicated that increasing the concentration of boric acid affects the physico-chemical properties of the films.