CZTS Absorber Research Articles

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.

Oxide route for production of Cu2ZnSnS4 solar cells by pulsed laser deposition

In this work, we have investigated Cu2ZnSnS4 (CZTS) solar cells made from oxide, oxy-sulfide and sulfide precursors produced by pulsed laser deposition (PLD). Although sulfide precursors are widely used to fabricate CZTS solar cells, Sn loss is commonly observed due to the high volatility of SnxSy species during high temperature sulfurization. This can lead to a non-ideal absorber composition and a high density of detrimental Sn-related defects that severely affect the performance of the device. By using oxide precursors, we have shown that the Sn loss can be significantly reduced due to the higher thermal stability of SnxOy species when compared to their sulfide counterparts. However, the reaction mechanism for the oxide route results in rough CZTS films. We hypothesize that the SO2 gas that forms during the conversion from oxide to sulfide is trapped in the film during sulfurization, and can lead to grains with hollow cavities and thus increase the surface roughness. Therefore, we have developed an annealing route for the oxide precursors at lower annealing pressures, which leads to improved film morphology and device performance. As a result, we report a power conversion efficiency of 5.4% for solar cells made from oxide precursors. This is the highest value reported for a CZTS absorber produced by PLD.

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.

Investigations of carrier transport mechanism and junction formation in Si/CZTS dual absorber solar cell technology

We report on dual absorber n-silicon/p-Cu2ZnSnS4 (Si/CZTS) solar cell model with CZTS as top absorber, silicon as bottom absorber and ITO acting as transparent conducting oxide layer on top of CZTS material in order to exploit the advantages of well-established silicon technology and low-cost earth abundant CZTS materials. The valence band edge develops a triangular potential at the Si/CZTS interface, and hence, the drift–diffusion and thermionic carrier transport mechanisms are investigated using Silvaco ATLAS 2D simulator. The thermionic emission current component is suggested to improve the fill factor at the cost of short circuit current and open circuit voltage. The band discontinuities, built-in potential developed across the heterojunction, generation–recombination profile, electron and hole current densities are carefully examined to understand the transport phenomena of Si/CZTS structure for varying thickness, acceptor concentration, electron affinity and optical band gap of top CZTS absorber materials. Synthesis and deposition of CZTS thin films were carried out via sol-gel spin coating process with different preheating temperature range between 250 and 350 °C for 10 min, and absorption data obtained from experiment is used to explore the benefits of tunable band gap properties of CZTS absorbers in the Si/CZTS heterojunction solar cell device. The increased band gap for 350 °C preheated sample has minimized the electrical recombination losses and improved the infrared light absorption resulting in efficiency enhancement. Then the CZTS physical parameters are optimized for the Si/CZTS solar cell model and the conversion efficiency is improved from 8.97 to 14.74% by minimizing the electrical and optical losses of the materials.

Effect of solvent on the characteristic properties of nebulizer spray pyrolyzed Cu2ZnSnS4 absorber thin films for photovoltaic application

Cu2ZnSnS4 (CZTS) thin films are the most promising absorber layer for thin film solar photovoltaics. The nebulizer assisted spray pyrolysis technique was employed to deposit the CZTS absorber thin films on bare soda lime glass substrate at a temperature of 350°C without annealing and sulfurization. Different solvents such as Dimethyl sulfoxide (DMSO), Ethylene diamine, 2-Methoxy ethanol and Water were used. The influence of these solvents on the structural, morphological, compositional, optical and electrical properties were studied using an X-ray diffractometer, Raman spectrometer, Scanning electron microscope annexed with energy dispersive X-ray spectrometer, Atomic force microscope, UV-vis spectrophotometer and a digital current source meter. The X-ray diffraction pattern reveals the formation of CZTS in all the solvents with preferential peak along [112] direction and the size of the crystallites is found to vary from 21 nm for DMSO to 34 nm for water. Raman analysis exhibited a phase pure film for all the solvents except the film prepared from DMSO which reveals a peak for the presence of Cu2S. The morphological and compositional analysis shows better texture with uniform coverage on the glass substrate and off stoichiometric ratio for all the deposited films. From the optical analysis it is seen that the prepared films show a broad absorption spectra in the visible region and the band gap was found to lie in the range of 1.15 eV to 1.51 eV. The current voltage characterization in dark shows significant increase in current with the applied voltage for all the films with DMSO film recording the lowest. Hall measurements revealed a carrier concentration of 1.93 × 1018 cm−3, mobility of 13.67 cm2/V s and a conductivity of 42.27 × 10−2 Ω−1 cm−1 for water. From this study, we infer that the CZTS film deposited using precursor solution derived from water exhibits phase purity along with enhanced photovoltaic properties at a substrate temperature of 350 °C through nebulizer assisted spray pyrolysis technique.

Development of high efficiency CZTS solar cell through buffer layer parameters optimization using SCAPS-1D

In this paper, numerical modelling and simulation using SCAPS-1D software have been carried out in order to investigate the effect of different buffer layers (CdS, CdSe and CdTe) on the efficiency of Cu2ZnSnS4 (CZTS) solar cells. The photovoltaic properties (open-circuit voltage (VOC), short-circuit current density (JSC), Fill Factor and Efficiency) have been studied at 300 K and under “AM 1.5 spectrum and one sun” by varying the thickness of the buffer and absorption layers, and the operating temperature. We found that the photovoltaic parameters of the solar cells are affected by the thickness change. Increasing the operating temperature has shown a negative effect on the solar cell performance, and the efficiency for all structures has been decreased with increasing temperature. At the optimum simulation parameters for all solar cell structures (50 nm buffer layer, 2000 nm CZTS absorber layer and operating temperature of 300 K), efficiency of 28.41%, 28.32% and 28.43% has been achieved for CdS, CdSe and CdTe, respectively. The present results can be considered as an important guideline for the fabrication of high-efficiency CZTS solar cell.

Numerical Simulation for Enhancement of the Output Performance of CZTS Based Thin Film Photovoltaic Cell

The performance of CZTS thin film photovoltaic cell has been simulated using SCAPS-1D (Solar cell capacitance simulator). The thickness of CZTS absorber layer, ZnO buffer layer and ZnO doped with Al window layer have been varied to optimize the overall output performance of CZTS based thin film photovoltaic cell. Simulation show the favorable result which can help to prove the feasibility of highly efficient CZTS thin film photovoltaic cell.

Improved solution-processed Cu2ZnSnS4 solar cells using a temporary Ag layer

A 25 nm thick Ag thin film was sputtered between the Cu2ZnSnS4 (CZTS) absorber and Mo electrode to improve the back contact in CZTS solar cells. The CZTS absorber was fabricated via a cost-effective N,N-dimethylformamide-based solution method. X-ray diffraction and Raman spectroscopy demonstrated that Ag diffused into the CZTS and partially substituted Cu, which resulted in an increased lattice constant a and grain size. Ag doping of CZTS (ACZTS) reduced both the Sn-loss and the presence of voids. An ACZTS photovoltaic device (ITO/i-ZnO/CdS/ACZTS/Mo) was fabricated and showed higher EQE than the CZTS device over the wavelength range of 450–800 nm. The open circuit voltage increased (Voc) from 543 to 631 mV, the short circuit current density (Jsc) increased from 10.36 to 14.37 mA, and the efficiency increased from 2.02 to 3.43% after sputtering the Ag layer. This is mainly because Ag functions as a temporary protective layer that made a tighter combination between Mo and the CZTS absorber.

Antimony‐Doped Tin Oxide as Transparent Back Contact in Cu2ZnSnS4 Thin‐Film Solar Cells

Antimony‐doped tin oxide (Sn2O3:Sb, ATO) is investigated as a transparent back contact for Cu2ZnSnS4 (CZTS) thin‐film solar cells. The stability of the ATO under different anneal conditions and the effect from ATO on CZTS absorber growth are studied. It is found that ATO directly exposed to sulfurizing anneal atmosphere reacts with S, but when covered by CZTS, it does not deteriorate when annealed at T < 550 °C. The electrical properties of ATO are even found to improve when CZTS is annealed at T = 534 °C. At T = 580 °C, it is found that ATO reacts with S and degrades. Analysis shows repeatedly that ATO affects the absorber growth as large amounts of Sn−S secondary compounds are found on the absorber surfaces. Time‐resolved anneal series show that these compounds form early during anneal and evaporate with time to leave pinholes behind. Device performance can be improved by addition of Na prior to annealing. The best CZTS device on ATO back contact herein has an efficiency of 2.6%. As compared with a reference on a Mo back contact, a similar open‐circuit voltage and short‐circuit current density are achieved, but a lower fill factor is measured.

Mechanism study of Zn/Sn ratio on the MoSe2 formation in Zn-rich Cu2ZnSnSe4 absorber layer

Controlling the thickness of the interfacial MoSe2 layer in Cu2ZnSnSe4 (CZTS) thin film solar cells can have a profound impact on device performance. The Zn/Sn ratio in the absorber layer of CZTS solar cells significantly affects the thickness of MoSe2 after selenization; however, the underlying mechanism remains unknown. In this study, we employed a fast-cooling selenization system to study the phase transformation and analyzed the back side of the CZTS absorber layer to characterize the mechanism by which Zn/Sn ratio affects the thickness of the MoSe2 absorber layer. Experiments revealed the formation of various metal-selenide phases at the bottom of the CZTS absorber layer during selenization. At temperatures below 400 °C, this mainly involves Sn- and CuSn-selenides; above 450 °C, this mainly involves Cu- and Zn- selenides. Since MoSe2 grows significantly above 450 °C, Cu- and Zn-selenides are the main reasons for reducing the thickness of MoSe2. The fact that MoSe2 growth generally occurs above 450 °C means that a higher Zn/Sn ratio leads to the formation of more Zn-selenides, such that there is less Se available for the formation of MoSe2, resulting in a thinner layer.

Two-stage production and characterization of Cu-poor kesterite CZTS absorber layers

The two-stage production and characterization of single phase CZTS films with controlled band gap and good crystallinity have been reported. Obtaining Cu poor stoichiometry and preventing Sn loss have been aimed in this study. Cu poor structures have been formed and pure kesterite CZTS samples have been obtained by sulfurizing the precursor layers above 500 °C. Optical band gap value of the sample obtained at 550 °C has been found as 1.48 eV which is suitable for solar cell applications. Despite the improvement of film surfaces with increased sulfurization temperature, further efforts are required to enhance the surface morphology of the films to use them with a heterojunction partner in solar cell applications.

Cadmium Free Cu2ZnSnS4 Solar Cells with 9.7% Efficiency

AbstractCu2ZnSnS4(CZTS) thin‐film solar cell absorbers with different bandgaps can be produced by parameter variation during thermal treatments. Here, the effects of varied annealing time in a sulfur atmosphere and an ordering treatment of the absorber are compared. Chemical changes in the surface due to ordering are examined, and a downshift of the valence band edge is observed. With the goal to obtain different band alignments, these CZTS absorbers are combined with Zn1−xSnxOy (ZTO) or CdS buffer layers to produce complete devices. A high open circuit voltage of 809 mV is obtained for an ordered CZTS absorber with CdS buffer layer, while a 9.7% device is obtained utilizing a Cd free ZTO buffer layer. The best performing devices are produced with a very rapid 1 min sulfurization, resulting in very small grains.

Fabrication of solar cells based on Cu2ZnSnS4 films grown with optimized chemical composition homogeneity

Se fabricaron celdas solares con estructura Mo/CZTS/ZnS/ZnO usando como capa absorbente películas de Cu2ZnSnS4 (CZTS) crecidas por evaporación simultánea de sus precursores desde una fuente de evaporación coaxial construida con un diseño avanzado que permitió mejorar significativamente la homogeneidad en la composición química en todo el volumen. Mediante mediciones con difracción de rayos X (XRD) se verificó que bajo condiciones de crecimiento optimizado era posible obtener películas en la sola fase Cu2ZnSnS4; los análisis de perfiles de profundidad con espectroscopía de fotoelectrones de rayos X (XPS) permitieron confirmar que las muestras preparadas desde una fuente coaxial crecieron solamente en la fase CZTS y que su composición química presentaba una mejor homogeneidad en todo el volumen que aquellas depositadas desde tres fuentes de evaporación separadas. Asimismo, se encontró que las celdas solares fabricadas con una capa absorbente CZTS crecidas desde una fuente de evaporación coaxial presentaban eficiencias de conversión significativamente mayores que las celdas fabricadas con capas CZTS preparadas desde fuentes de evaporación separadas. Se obtuvieron eficiencias de conversión de 5,6%, una corriente de cortocircuito de 18,3 mA/cm2 y un voltaje de circuito abierto de 0,52 V. © 2019. Acad. Colomb. Cienc. Ex. Fis. Nat.

Properties of phase transition and interfaces of Cu2ZnSnS4 with hetero-phase junctions

Over the past few years, quaternary Cu2ZnSnS4 (CZTS) has attracted a great deal of attention as the most promising photovoltaic absorber layer, because it is composed of abundant and non-toxic elements. CZTS has two fundamental crystal structures under conventional preparation conditions: kesterite (KS) and stannite (ST). Because of the extremely similar crystal structure and the extremely small crystal binding energy, the phase transition between these two crystal structures is very easy to occur. Therefore, in the actual preparation and application process, it is very likely that there is a phase junction between KS and ST in CZTS material. In this article, based on density functional theory calculations, we first discussed the phase transition process caused by stress in details, which is one of the common phase transitions in experiments. The transition pressure exists at around 31 GPa and the phase transition is most likely to appear along the [100] crystal orientation. On this basis, we calculated and analyzed the structure and properties of five kinds of hetero-phase junctions which are composed of low index crystal planes and have very little interfacial stress. The hetero-phase junctions that composed by (101), (110), and (100) crystal planes present the type-II band alignment, which is beneficial for the photoelectric conversion. While, the hetero-phase junctions that composed by (112) and (001) crystal planes present the type-I band alignment, which offer larger energy barrier for the photo-generated carriers and is disadvantageous for improving photoelectric conversion. These findings can contribute to research on the relevant properties of CZTS absorber layer in the future.

Towards high-efficiency CZTS solar cell through buffer layer optimization

Cu2ZnSnS4 (CZTS)-based solar cells show a promising performance in the field of sunlight-based energy production system. To increase the performance of CZTS-based solar cell, buffer layer optimization is still an obstacle. In this work, numerical simulations were performed on structures based on CZTS absorber layer, ZnO window layer, and transparent conducting layer n-ITO with different buffer layers using SCAPS-1D software to find a suitable buffer layer. Cadmium sulfide (CdS), zinc sulfide (ZnS) and their alloy cadmium zinc sulfide (Cd0.4Zn0.6S) were used as potential buffer layers to investigate the effect of buffer thickness, absorber thickness and temperature on open-circuit voltage (Voc), short-circuit current (Jsc), fill factor (FF) and efficiency (η) of the solar cell. The optimum efficiencies using these three buffer layers are around 11.20%. Among these three buffers, Cd0.4Zn0.6S is more preferable as CdS suffers from toxicity problem and ZnS shows drastic change in performance parameters. The simulation results can give important guideline for the fabrication of high-efficiency CZTS solar cell.