ZnS Buffer Research Articles

Two stage modelling of solar photovoltaic cells based on Sb2S3 absorber with three distinct buffer combinations

Solar cell research has always been an attraction by virtue of its clean and green status. However, to overcome the implications of high cost and moderate efficiency, there has always been fierce competition to search alternative approach for designing efficient solar cells with optimal performance-cost ratio. Recently, antimony sulfide (Sb2S3) has received substantial attention as an absorber in thin film solar cells due to earth abundance, low cost, non-toxic property and high optical absorption. Still, its performance could not match Si based cells. In this work, we adopted two-stage simulation approach to design Sb2S3 absorber based heterojunction solar cell to enhance efficiency. Initial simulation for configuration optimization was done considering thickness, defect density, recombination (radiative, Auger) effect, carrier density of the Sb2S3 absorber layer. Buffer layer thickness and absorption coefficient optimization was taken up. Further, series and shunt resistance of the device as well as conduction band offset (CBO) at absorber/buffer interface was also optimized at initial stage only. In the next level of simulation, efficiency enhancement was achieved by optimizing optimal back contact metal work function, absorber layer band gap grading and temperature. The aforesaid two-stage optimization yielded efficiency ~24.81%, which is higher than conventional thin film solar cell. The optimal solar cell structure configuration, for Sb2S3 absorber solar cell, suggested a positive CBO of 0.26 eV (e.g.; ZnS buffer layer), a back contact metal work function of 5.1 eV (e.g.; Mo, Au) and band gap grading window ~1.31 to 1.62 eV.

Dilute Oxygen Alloys of ZnS as a Promising Toxic-Free Buffer Layer for Cu(In, Ga)Se2 Thin-Film Solar Cells

Using ZnS as a buffer layer in many thin-film solar cells, such as $\text {Cu}{(}\text {In}{,}\,\text {Ga}{)}\text {Se}_{{2}}$ (CIGS), has not been successful as it usually results in a high barrier that suppresses the flow of electrons to the designated contact. To tackle this issue, we analyze dilute oxygen (O) alloys of ZnS as a buffer layer. It exhibits an unusual energy bandgap ( ${E_{G}}$ ) bowing and a sharp increase in electron affinity energy. Such features commonly arise in anion-alloyed compositions due to band anticrossing (BAC) interactions between the introduced defect energy state of O and the extended conduction band edge (CBE) of ZnS that causes a downshift of the CBE. Besides the flexibility of tuning the CBE, this is important to avoid the toxicity of cadmium (Cd) and its compounds. In this article, the band edges of lightly alloyed ${\text {ZnS}_{{1}-{x}}\text {O}_{x}}$ are computed using an atomistic tight-binding (TB) BAC model. Then, a fitting energy band bowing (EBB) model is developed to capture efficiently the nonlinear variations of their ${E_{G}}$ and electron affinity energy. For O composition ranging between 0% and 5%, it is observed that the electron affinity energy sharply increases from 3.3 to 3.98 eV. Also, ${E_{G}}$ drastically reduces from 3.8 to 3.08 eV. Device-wise, by analyzing the effect of dilute O alloys and the doping density of the ZnS buffer layer, it is found that electron transport is remarkably improved with O composition. In ${\text {ZnS}_{{0.95}}\text {O}_{{0.05}}}$ alloy with doping density ${{1} \times {10}^{{18}}\,\,\text {cm}^{-{3}}}$ , the maximum power conversion efficiency (PCE) reaches approximately 23.82%.

An investigation on the photovoltaic performance of quantum dot solar cells sensitized by CdTe, CdSe and CdS having comparable size

Abstract Quantum dot sensitized solar cells (QDSSC) have been fabricated with CdTe, CdSe and CdS quantum dots (QDs) of nearly similar size as the photosensitizers. The optical and structural characteristics were studied using absorption spectroscopy, XRD and TEM. Efficiency of the fabricated solar cells was found to improve by the incorporation of an additional buffer layer of ZnS. The value of efficiencies for the QDSSCs corresponding to the sensitizers CdTe, CdSe and CdS are 0.22%, 0.25% and 0.20%. After treatment with ZnS using SILAR cycle, an enhacement in the efficiency to 1.41%, 1.98% and 0.94% was noticed for the QDSSC sensitized with CdTe, CdSe and CdS QDs respectively. The differences in efficiencies as well as other photovoltaic parameters for the solar cells have been explained using the differences in effective masses of charge carriers and carrier lifetime in the three different QD samples. CdSe is found to have high adsorption property towards ZnS compared to CdTe or CdS QDs. Also, the ZnS buffer layer acts as a good agent for reducing the back scattering of electrons thereby improving the efficiency of all the three QD solar cells.

Performance Optimization of ZnS/CIGS Solar Cell with over 25% Efficiency Enabled by Using a CuIn3Se5 OVC Layer

Chalcopyrite CIGS solar cells have been demonstrated with the highest efficiency for thin-film solar cell families. Herein, the effects of thickness and bandgap of cadmium (Cd) free ZnS buffer layer grown on CIGS absorber with an ordered vacancy compound (OVC) at buffer/absorber interface in CIGS solar cell structure have been investigated using ADEPT 2.1 simulator. The ZnS buffer possesses a wider energy bandgap than the traditional CdS buffer layer, which substantiates the higher conversion efficiency of this solar cell, allowing the photons of lower wavelength into the CIGS layer. Besides, CuIn 3 Se 5 has been used as an OVC layer that assists to improve the conversion efficiency to an optimized level of over 25% by reducing the recombination rate. Moreover, the optimized short circuit current (J sc ) and corresponding open-circuit voltage (V oc ) yield a higher fill factor (FF) of 86.22%, which, therefore, results in an optimum efficiency of the CIGS cell estimated as 25.68% under AM 1.5 irradiance.

Role of low-cost non-toxic MgCl2 treatment on ZnS films: Optimization of physical properties for buffer layers

The polycrystalline thin films comprise grain boundaries which act as recombination centres that may be passivated by chloride treatment. The conventional CdS and CdCl2 are very famous as optical window and chloride treatment in solar cells respectively, while both are expensive and severely mutagenic. Therefore in this paper, we report MgCl2 treatment on ZnS films, which is a low-cost noncarcinogenic alternative. ZnS thin films were grown on glass and ITO substrates using e-beam evaporation method, treated with MgCl2 and further thermally annealed to optimize physical properties. Films are found amorphous in nature having transmittance of about 95% and band gap increased up to 3.95 eV with annealing. Surface roughness is increased and proper ohmic behaviour is found for 300 °C treated films and EDS pattern ensured the deposition of ZnS films. To minimize the environmental risk and to reduce cost, our results exhibit that CdS optical window may be easily substituted by MgCl2 treated ZnS buffer layer.

Improving the performance of cadmium telluride solar cell (CdTe) with different buffer layers

In this paper, the performance of the buffer layer of Cadmium Telluride (CdTe) thin film solar cell was optimized using SCAPS software. At first, five different buffer layers including CdS, In2S3, ZnO, ZnSe and ZnS with variable thicknesses from 10 to 100 nm have been replaced in the structure of the solar cell and it has been observed. As the thickness of the buffer layer is increased, the efficiency decreases.Due to the increase of thickness light absorption in the structure is increased and less light is absobed in CdTe layer. The reason behinf this incident is described physically. Then the performance of the solar cell with different buffer layers is compared, and the effect of each of the layers on the four main parameters of the solar cell has been investigated. Simulation results show that the ZnS buffer layer has a higher efficiency in all thicknesses than other materials which originates from the fact that the bandgap of this material is bigger than other materials.

The influence of ZnS buffer layer on the size dependent efficiency of CdTe quantum dot sensitized solar cell

Abstract We report the investigations on the quantum dot size dependence of efficiency of a CdTe sensitized solar cell. With an additional layer of ZnS over the QD layer, the efficiency of solar cell was found to vary according to the size of the synthesized CdTe QDs. The presence of trap states due to the surface imperfections of the QDs as a result of the colloidal growth mechanism was also found to affect the overall efficiency of the solar cell. By coating a buffer layer of ZnS over the CdTe QD layer with the help of SILAR method, the efficiency enhanced up to 2.16% by decreasing the charge recombination rate between QDs and metal oxide layer.

Simulation of a thin-film solar cell based in kesterite using Matlab

The development of thin-film solar cells has led the researches to evaluate different materials to improve the conversion efficiency, avoid the use of non toxic raw materials or reduce the manufacturing costs. One option is to use simulation tools, which allow the variation of solar cell materials, parameters, dimensions and other variables in order to evaluate performance indicators of the cell without wasting material. This paper introduces a mathematical model for kesterite based thin-film solar cells with both theoretical and experimental solar irradiance spectra. The model is implemented in Matlab and it is used to evaluate four performance indicators: open-circuit voltage, short-circuit current density, conversion efficiency and fill factor. Those indicators are evaluated for a ZnO/CZTSSe/buffer solar cell with buffer layers implemented with CdS and ZnS considering different thickness. The results show that all performance indicators are significantly improved with the ZnS buffer layer; moreover, the results also evidence that the irradiance model affects the performance indicators. Particularly, experimental based irradiance model improves the performance indicators for both evaluated layers. Finally, the thickness does not affect considerably the performance of the cell with ZnS buffer layer, while a higher thickness reduces the performance of the cell with CdS buffer layer.

Effect of Three Different Complexing Agents on the Deposition of ZnS Buffer Layer in Copper Indium Gallium Selenide Solar Cell

Effect of Three Different Complexing Agents on the Deposition of ZnS Buffer Layer in Copper Indium Gallium Selenide Solar Cell

Characterization of ZnS Thin Films Grown Using Chemical Bath Deposition with Three Different Complexing Agents.

Cadmium sulfide buffer layer was replaced with zinc sulfide thin film owing to toxicity. ZnS thin films were fabricated using chemical bath deposition. The inhibition of Zn(OH)2 and high uniformity are important factors for the deposition of ZnS. The characteristics of ZnS thin films were analyzed by adding ethylenediamine tetra-acetate acid (EDTA) and hexamethylene tetramine (HMTA). The morphological characteristics of the ZnS buffer layer are closely related to the use of a complexing agent which controls the concentration of Zn2+ ions and Zn(OH)2 in the deposition process. The addition of the complexing agent EDTA accelerated the cluster-cluster method but it exhibited lower uniformity and greater cracking phenomenon. HMTA can be effectively applied to increase the amount of Zn2+ ions forming ZnS. It can be easily found as Zn2 HMTA at high temperatures. The results of the experiment with the addition of HMTA revealed that the surface of the thin film did not change with the increase in the concentration of HMTA, but the thickness of the thin film increased gradually. HMTA promoted the ion-ion method to grow the thin film uniformly, but the speed was slow. Moreover, an experiment by using mixed EDTA and HMTA as the complexing agent was performed. The best ZnS thin film with a transmittance of 83% and a denser surface was prepared using HMTA complexing agent.

Development of flexible Cd-free Cu(In,Ga)Se2 solar cell on stainless steel substrate through multi-layer precursor method

Cu(In,Ga)Se2 (CIGS) absorbers on flexible stainless steel (SUS) substrates in thin-film solar cells are deposited by multi-layer precursor method with different [Ga]/([Ga]+[In]) grading profiles, thus yielding different band-gap energy (Eg) grading profiles. In Eg grading profiles, near-surface Eg is defined as average Eg within 200 nm from CIGS surface, and minimum Eg is the lowest Eg. It is disclosed that near-surface Eg and minimum Eg have impacts on open-circuit voltage and short-circuit current density, respectively. 17.6%-efficient CIGS solar cell on flexible SUS substrate using CdS buffer is obtained with the suitable Eg grading profile consisting of near-surface Eg and minimum Eg of 1.26 and 1.08 eV, respectively. Flexible Cd-free CIGS solar cell on SUS substrate using ZnS(O,OH) buffer is next fabricated with relative low conversion efficiency of 8.5%, attributed to a relatively low Zn diffusion into CIGS and/or the large conduction band offset (CBO) at ZnS(O,OH)/CIGS interface. However, all photovoltaic parameters of the Cd-free CIGS solar cell are improved after light-soaking process owing to improvement of the CBO at ZnS(O,OH)/CIGS interface. Ultimately, 14.8%-efficient Cd-free CIGS solar cell on flexible SUS substrate utilizing ZnS(O,OH) buffer is obtained, thereby demonstrating the ability for portable power source.

ZnS Buffer Layers Grown by Modified Chemical Bath Deposition for CIGS Solar Cells

ZnS thin films were prepared by the chemical bath deposition method using disodium ethylene-diaminetetraacetic acid and hexamethylenetetramine as complexing agents in acidic conditions. The film prepared using a preheated S-ion source showed full surface coverage, but some clusters were found that were generated by the cluster-by-cluster reaction mechanism. On the other hand, the film prepared without this source had a uniform, dense, and smooth surface and showed fewer clusters than the film prepared using a preheated S-ion source. The x-ray photoelectron spectroscopy spectra showed the energy core levels of Zn, O, and S components, and Zn-OH bonding decreased on the film using the preheated S-ion source. Especially, various binding energy peaks were found in the Zn 2p3/2 spectrum by Gaussian function fitting, and no peak corresponding to Zn-OH bonding was found for the film prepared using a preheated S-ion source. Moreover, the x-ray diffraction spectrum of the ZnS thin film using a non-preheated S-ion source showed amorphous or nanoscale crystallinity, but the emission peaks indicated that the structure of the film using preheated S-ion source was zincblende.

The fabrication of Cd-free Cu2ZnSnS4-Ag2ZnSnS4 heterojunction photovoltaic devices

Cu2ZnSnS4 (CZTS) is a low cost, non-toxic material composed of earth-abundant elements with a large optical absorption coefficient. However, in CZTS solar cells, CdS buffer layers are widely used, which contain toxic elements and lead to lattice mismatch with CZTS thin films. In this paper, we have systematically investigated a I-II-IV-VI semiconductor material to replace the CdS buffer layer with a non-toxic material that offers good lattice matching with the CZTS film. For the first time, Cd-free Cu2ZnSnS4-Ag2ZnSnS4 heterojunction photovoltaic device is fabricated, where Cu2ZnSnS4 and Ag2ZnSnS4 layers are obtained by co-sputtering method. The cell exhibits a 4.51% efficiency which is higher than those of CZTS solar cells with ZnS and In2S3 buffer layers. This work offers the possibility to realize environmentally benign, scalable, low-cost, and high-efficiency solar cells.

Optimization by simulation of the nature of the buffer, the gap profile of the absorber and the thickness of the various layers in CZTSSe solar cells

Performances of ZnO/ZnS/CZTSSe polycrystalline thin film solar cells (Copper Zinc Tin Sulphur Selenium-solar cell) were simulated for different thicknesses of the absorber and ZnS buffer layers. Simulations were performed with SCAPS (Solar Cell Capacitance Simulator) software, starting with actual parameters available from industrial data for commercial cells processing. The influences of the thickness of the various layers in the structure of the solar cell and the gap profile of the CZTSSe absorber layer on the performance of the solar cell were studied in detail. Through considerations of recent works, we discuss possible routes to enhance the performance of CZTSSe solar cells towards a higher efficiency level. Thus, we found that for one specific thickness of the absorber layer, the efficiency of the CZTSSe solar cell can be increased when a ZnS layer replaces the usual CdS buffer layer. On the other hand, the efficiency of the solar cell can be also improved when the absorber layer presents a grad-gap. In this case, the maximum efficiency for the CZTSSe cell was found equal to 13.73%.

Chemical bath deposited ZnS buffer layer for Cu(In,Ga)Se2 thin film solar cell

The dependence of Zn precursors using zinc sulfate (ZnSO4), zinc acetate (Zn(CH3COO)2), and zinc chloride (ZnCl2) on the characteristics of the chemical bath deposited ZnS thin film used as a buffer layer of Cu(In,Ga)Se2 (CIGS) thin film solar cell was studied. It is found that the ZnS film deposition rate increases with higher stability constant during decomplexation reaction of zinc ligands, which affects the crack formation and the amount of sulfur and oxygen contents within the film. The band gap energies of all deposited films are in the range of 3.40–3.49eV, which is lower than that of the bulk ZnS film due to oxygen contents within the films. Among the CIGS solar cells having ZnS buffer layers prepared by different Zn precursors, the best cell efficiency with 9.4% was attained using Zn(CH3COO)2 precursor due to increased Voc mainly. This result suggests that [Zn(NH3)4]2+ complex formation should be well controlled to attain the high quality ZnS thin films.

Conduction band offset engineering in wide-bandgap Ag(In,Ga)Se2 solar cells by hybrid buffer layer

Ag(In,Ga)Se2 (AIGS) is one of the promising candidates for the top cell absorber in the tandem structure. However, the conversion efficiency of AIGS solar cells is still lower than that required for the top cell. In this study, to improve the conversion efficiency of AIGS solar cells, we controlled the conduction band offset (CBO) at the buffer layer/ZnO and buffer layer/AIGS interfaces. The reduction in interface recombination at the CdS buffer layer/AIGS interface was achieved by introducing a ZnS(O,OH) buffer layer instead of a CdS buffer layer, although the fill factor (FF) decreased markedly because the CBO at the ZnS(O,OH)/ZnO interface prevented the electron flow under a forward bias. We found that the introduction of a CdS/ZnS(O,OH) hybrid buffer layer is efficient in controlling the CBO at both the buffer layer/AIGS and buffer layer/ZnO interfaces and improving the solar cell conversion efficiency.

Highly transparent ZnS buffer layer prepared by ultrasonic spray pyrolysis for photovoltaic applications

Zinc sulfide thin films are deposited by ultrasonic spray pyrolysis. The substrate temperature and sulfur concentrations are varied to attain suitable property for the application as window layer in solar cells. Films prepared at lower temperature have traces of ZnO phase together with the dominant cubic ZnS phase. At 400°C with Zn/S ratio of 1/1.5, sample exhibited higher transmittance (∼86%) consistently in the visible region. With an increase in the sulfur concentration transmittance reduced to ∼50%. Band gap varied from 3.5 to 3.8eV in the films. The existence of single phase ZnS is confirmed through XPS analysis of the optimized sample. 0.5% efficiency is obtained for AgInS2/ZnS solar cells with the optimized ZnS film.

Study of CZTS and CZTSSe solar cells for buffer layers selection

Abstract The output characteristics, of a thin film solar cell, are affected by several parameters related to the hetero-structure that is taken into account. In this paper, nontoxic thin film solar cells were studied. CdS/CZTS, CdS/CZTSSe, ZnS/CZTS and ZnS/CZTSSe hetero-junctions were numerically simulated using the Solar Cell Capacitance Simulator (SCAPS). According to the simulation results, we found that CdS buffer was suitable for the CZTSSe absorber and an efficiency of 9.02% was obtained. The ZnS buffer was selected for CZTS absorber and an efficiency of 8.02% has been reached. By optimizing the absorber film parameters (thickness and carrier density), we obtained conversion efficiencies as high as 9.47% and 10.00% for CdS/CZTSSe and ZnS/CZTS based solar cells respectively.

Numerical Modeling and Simulation of CIGS-Based Solar Cells with ZnS Buffer Layer

Usually a buffer layer of cadmium sulphide is used in high efficiency solar cells based on Cu(In,Ga)Se2(CIGS). Because of cadmium toxicity, many in-vestigations have been conducted to use Cd-free buffer layers. Our work focuses on this type of CIGS-based solar cells where CdS is replaced by a ZnS buffer layer. In this contribution, AFORS-HET software is used to simulate n-ZnO: Al/i-ZnO/n-ZnS/p-CIGS/Mo polycrystalline thin-film solar cell where the key parts are p-CIGS absorber layer and n-ZnS buffer layer. The characteristics of these key parts: thickness and Ga-content of the absorber layer, thickness of the buffer layer and doping concentrations of absorber and buffer layers have been investigated to optimize the conversion efficiency. We find a maximum conversion efficiency of 26% with a short-circuit current of 36.9 mA/cm2, an open circuit voltage of 824 mV, and a fill factor of 85.5%.

Effect of the Concentration of Complexing Agent on the Formation of ZnS Buffer Layer by CBD Method

Effect of the Concentration of Complexing Agent on the Formation of ZnS Buffer Layer by CBD Method