Cd-free Buffer Layers Research Articles

Cd-Free Cu(In,Ga)(Se,S)2 Thin-Film Solar Cell With Record Efficiency of 23.35%

In this article, the excellent properties of state-of-the-art Cd-free Cu(In,Ga)(Se,S)2 (CIGSSe) solar cells with Zn(O,S,OH) x /Zn0.8Mg0.2O double buffer layers, deposited by a combination of chemical bath deposition and atomic layer deposition techniques, are presented. By the replacement of conventional CdS buffer layers with this double buffer layer, the open-circuit voltage ( V oc) deficit of the devices could be significantly reduced, and V oc increased by approximately 15 mV. In addition, the fill factor and short-circuit current were also improved, increasing the device efficiency by approximately 0.5 absolute percent compared with devices with CdS buffers. The Cd-free double buffer layer improved the device efficiency regardless of the bandgap of the CIGSSe absorber. The minority carrier lifetime ( τ ) measured via time-resolved photoluminescence became longer, indicating that carrier recombination is mitigated using the double buffer layer. Based on the device parameters extracted by fitting the Suns– V oc characteristics to the double-diode model, the longer τ could be attributed to the decreased recombination rate in the space-charge region, rather than in the bulk and at the interface. The best performing cell was evaluated by a reliable third party, the National Institute of Advanced Industrial Science and Technology; this cell achieved a new world record efficiency of 23.35% for 1-cm2-sized thin-film polycrystalline solar cells. The device parameters of this cell are also discussed in this article.

Energy level alignment of Cu(In,Ga)(S,Se)2 absorber compounds with In2S3 , NaIn5S8 , and CuIn5S8 Cd-free buffer materials

Motivated by environmental reasons, In2S3 is a promising candidate for a Cd-free buffer layer in Cu(In, Ga)(S, Se)(2) (CIGSSe)-based thin-film solar cells. For an impactful optimization of the In-2 S-3 alternative buffer layer, however, a comprehensive knowledge of its electronic properties across the absorber-buffer interface is of foremost importance. In this respect, finding a favorable band offset between the absorber and the buffer layers can effectively reduce the carrier recombination at the interface and improve open-circuit voltage and fill factor, leading to higher conversion efficiencies. In this study, we investigate the band alignment between the most common CIGSSe-based absorber compounds and In2S3. Furthermore, we consider two chemically modified indium sulfide layers, NaIn(5)S(8 )and CuIn5S8, and we discuss how the formation of these secondary phases influences band discontinuity across the interface. Our analysis is based on density functional theory calculations using hybrid functionals. The results suggest that Ga-based absorbers form a destructive clifflike conduction-band offset (CBO) with both pure and chemically modified buffer systems. For In-based absorbers, however, if the absorber layer is Cu-poor at the surface, a modest favorable spikelike CBO arises with NaIn5S8 and CuIn5S8.

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.

Theoretical analysis of minority carrier lifetime and Cd-free buffer layers on the CZTS based solar cell performances

In this report, one dimensional simulation program (SCAPS-1D) has been used to study the crucial effect of minority carrier lifetime on Cu2ZnSnS4 based solar cells. The results have led to a strong correlation between the minority lifetime and the Cu2ZnSnS4 absorber thickness. Then, the modeling and investigation of Cu2ZnSnS4 based solar cells with graded Zn1-xMgxO and ZnO1-ySy buffer layers revealed that, these buffer layers could be an alternative to the CdS layer if the Mg and S compositions are suitably adjusted. In the case of Zn1-xMgxO buffer layer, the device with Zn0.75Mg0.25O shows the best efficiency and produces an improvement of 4.8%, whereas cells with ZnO1-ySy show an enhancement above 5% once the S composition is larger than 0.3. These performances have ascribed to the favorable band alignment at the buffer layers/Cu2ZnSnS4 interface, a strong electric field at the p-n junction and great transparency of these alternative buffer layers. The simulations have also showed a high potential barrier at the ZnO-intrinsic/alternative buffer layers interface which harms the electrons’ transport.

Fabrication of Zn(O,S) thin films by ozone assisted photochemical deposition and its potential applications as buffer layers in kesterite-based thin film solar cells

Zn(O,S) thin films have attracted intensive attention for their potential application as Cd-free buffer layers in kesterite-based solar cells due to their desirable band alignment with Cu2ZnSnS4 (CZTS) or Cu2ZnSnSxSe4-x (CZTSSe) materials. In this work, Zn(O,S) thin films with controllable content of oxygen have been designed via ozone-assisted photochemical deposition processes. Results showed that the deposited Zn(O,S) thin films using various ozone flow rate are flat, compact and uniform with a thickness of ~ 40 nm, consisting of nanoparticles with a diameter of 30–50 nm. The introduction of ozone into synthesis processes can hardly impose any effect on the morphologies, but significantly modify and tune the oxygen contents in the deposited thin films. Moreover, optical absorption spectra indicate that the optical band-gap of Zn(O,S) films can be readily tuned from 3.3 to 3.7 eV by changing the ozone flow rates. Furthermore, it is confirmed that the solar cells with Zn(O,S)/CZTS structure has a higher conversion efficiency of 3.0% and a higher open circuit voltage of 0.516 V compared to those with ZnS/CZTS structure. Our research would open an easy and promising approach to prepare Cd-free buffer layer materials for kesterite-based thin film solar cells.

Fast Chemical Bath Deposition Process at Room Temperature of ZnS-Based Materials for Buffer Application in High-Efficiency Cu(In,Ga)Se2-Based Solar Cells

Chemical bath deposited (CBD) Zn(S,O) remains one of the most studied Cd-free buffer layers for Cu(In,Ga)Se2 -based (CIGSe) solar cells and has already been implemented at the industrial level for high-efficiency solar cells and modules. Exploring further routes of improvement is demanded and needed. One of the key routes is related to the high reactant concentration and high deposition temperature used for CBD-Zn(S,O). In this paper, we present a new family of reactants, based on hydroxylamine, presenting a high nucleophilic power and reactivity toward thiourea. We will show that the addition of these reactants allows to divide the concentration of thiourea by 3 and to perform Zn(S,O) deposition at room temperature. Efficiencies similar or even higher than the classical CBD cadmium sulfide are obtained due to higher J sc and FF of the CIGSe-based solar cells. Growth mechanism and impact of hydroxylamine on the composition of the Zn(S,O) and cell performances will be discussed.

Band alignment of Cd-free (Zn, Mg)O layer with Cu2ZnSn(S,Se)4 and its effect on the photovoltaic properties

Cu2ZnSn(S,Se)4 (CZTSSe) is an interesting absorber material for thin film solar cells. However, one of the key challenges for the kesterite-based solar cells is to improve the open-circuit voltage (Voc) deficit, which is resultant of recombination at the interface of buffer/absorber. In this work, Cd-free n-type buffer layers with two different Mg-doped ZnO layers (Mg0.26Zn0.74O, Mg0.30Zn0.70O) have been examined using ultraviolet photoelectron spectroscopy. The most important electronic properties which are essential for the band offset study, ie. fermi level location, valence and conduction band offsets at the interface in the CZTSSe substrate, have been determined. The conduction band offset values for Mg0.26Zn0.74O, Mg0.30Zn0.70O buffer layers has been calculated experimentally. We have also established the correlation between device parameters and performances for dual ion beam sputtered ZnO buffer/CZTSSe-based heterojunction solar cells as a function of conduction band offset and the energy distribution of interface defects, to gain deeper understanding about the Voc-deficit behavior from a high recombination rate at the buffer/kesterite interface using simulation study. From the simulation study, the values of the solar cell efficiency with Mg0.26Zn0.74O and Mg0.30Zn0.70O buffer layers are 10.18 and 10.25%, respectively, which are higher in comparison to those obtained by using conventional CdS buffer layer.

Numerical Optimization of Gradient Bandgap Structure for CIGS Solar Cell with ZnS Buffer Layer Using Technology Computer-Aided Design Simulation

The band structure characteristics of a copper indium gallium sulfur selenide (Cu(In1–xGax)SeS, CIGS) solar cell incorporating a cadmium-free zinc sulfide (ZnS) buffer layer were investigated using technology computer-aided design simulations. Considering the optical/electrical properties that depend on the Ga content, we numerically demonstrated that the front gradient bandgap enhanced the electron movement over the band-offset of the ZnS interface barrier, and the back gradient bandgap generated a back side field, improving electron transport in the CIGS layer; in addition, the short circuit current density (JSC) and open circuit voltage (VOC) improved. The simulation demonstrated that the conversion efficiency of a double graded bandgap cell is higher than with uniform or normal/reverse gradient cells, and VOC strongly correlated with the average bandgap in the space charge region (SCR) of CIGS. After selecting VOC from the SCR, we optimized the band structure of the CIGS cell with a Cd-free ZnS buffer by evaluating JSC and the fill factor. We demonstrated that the cell efficiency of the fabricated cell was more than 15%, which agrees well with the simulated results. Our numerical method can be used to design high-conversion efficiency CIGS cells with a gradient band structure and Cd-free buffer layer.

Thermal annealing induced modification on structural and optical properties of Cu2ZnSnS4 thin films for solar cell application

The influence of thermal annealing on sol-gel derived Cu2ZnSnS4 (CZTS) thin films was carried out in quartz tubular furnace at different temperature (300–600 °C) for 1 h in the presence of elemental sulfur. The structural studies confirm the formation of tetragonal kesterite CZTS structure along (112) plane and indicate the presence of impurity phases at lower sulfurization temperature. CZTS films revealed high dense morphology with large grain size at 500 °C. The optical band-gap energy decreased from 1.8 to 1.52 eV with the increase in sulfurization temperature. The electrical measurements show p-type conductivity of the films with carrier mobility of 13.12 cm2/V.s. The sulfurized CZTS thin film at 500 °C was used for superstrate solar cell structure with Cd-free buffer layer. The solar cell exhibited short-circuit current density of 4.8 mA/cm2, open-circuit voltage of 140 mV, fill factor of 0.31 and power conversion efficiency of 0.208% under 100 mW/cm2 illumination.

Wet Pretreatment-Induced Modification of Cu(In,Ga)Se2/Cd-Free ZnTiO Buffer Interface.

We report a novel Cd-free ZnTiO buffer layer deposited by atomic layer deposition for Cu(In,Ga)Se2 (CIGS) solar cells. Wet pretreatments of the CIGS absorbers with NH4OH, H2O, and/or aqueous solution of Cd2+ ions were explored to improve the quality of the CIGS/ZnTiO interface, and their effects on the chemical state of the absorber and the final performance of Cd-free CIGS devices were investigated. X-ray photoelectron spectroscopy (XPS) analysis revealed that the aqueous solution etched away sodium compounds accumulated on the CIGS surface, which was found to be detrimental for solar cell operation. Wet treatment with NH4OH solution led to a reduced photocurrent, which was attributed to the thinning (or removal) of an ordered vacancy compound (OVC) layer on the CIGS surface as evidenced by an increased Cu XPS peak intensity after the NH4OH treatment. However, the addition of Cd2+ ions to the NH4OH aqueous solution suppressed the etching of the OVC by NH4OH, explaining why such a negative effect of NH4OH is not present in the conventional chemical bath deposition of CdS. The band alignment at the CIGS/ZnTiO interface was quantified using XPS depth profile measurements. A small cliff-like conduction band offset of -0.11 eV was identified at the interface, which indicates room for further improvement of efficiency of the CIGS/ZnTiO solar cells once the band alignment is altered to a slight spike by inserting a passivation layer with a higher conduction band edge than ZnTiO. Combination of the small cliff conduction band offset at the interface, removal of the Na compound via water, and surface doping by Cd ions allowed the application of ZnTiO buffer to CIGS treated with Cd solutions, exhibiting an efficiency of 80% compared to that of a reference CIGS solar cell treated with the CdS.

Artificial twin-layer configurations of Zn(O,S) films by radio frequency sputtering in all dry processed eco-friendly Cu(In,Ga)Se2 solar cells

Cu(In,Ga)Se2 thin film solar cells are of great interest for research and industrial applications with their high conversion efficiencies, long-term stability and significant lifetimes. Such a solar cell of a p-n junction consists of p-type Cu(In,Ga)Se2 films as a light absorber and n-type CdS as a buffer layer, which often emerges with intrinsic ZnO. Aimed at eco-friendly fabrication protocols, a large number of strategies have been investigated to fabricate a Cd-free n-type buffer layer such as Zn(O,S) in Cu(In,Ga)Se2 solar cells. Also, if the Zn(O,S) films are prepared by coevaporation or sputtering, it will offer high compatibility with the preferred mass production. Here, we propose and optimize a dry method for Zn(O,S) deposition in a radio frequency sputtering. In particular, the strategy for the twin-layer configurations of Zn(O,S) films not only greatly improve their electrical conductance and suppress charge carrier recombination, but also avoid degradation of the Zn(O,S)/Cu(In,Ga)Se2 interfaces. Indeed, the high quality of such twin Zn(O,S) layers have been reflected in the similar conversion efficiencies of the complete solar cells as well as the large short-circuit current density, which exceeds the CdS reference device. In addition, Zn(O,S) twin layers have reduced the production time and materials by replacing the CdS/i-ZnO layers, which removes two fabrication steps in the multilayered thin film solar cells. Furthermore, the device physics for such improvements have been fully unveiled with both experimental current–voltage and capacitance–voltage spectroscopies and device simulations via wxAMPS program. Finally, the proposed twin-layer Zn(O,S)/Cu(In,Ga)Se2 interfaces account for the broadening of the depletion region of photogenerated charge carriers, which greatly suppress the carrier recombination at the space charge region, and eventually lead to the more efficient collection of charge carriers at both electrodes.

Use of a New Organic Complexing and Buffer Agent for Zn(S,O) Deposition for High-Efficiency Cu(In,Ga)Se2-Based Solar Cells

Chemical-bath-deposited (CBD) Zn(S,O) is one of the most studied and promising Cd-free buffer layers for Cu(In,Ga)Se2 (CIGSe) -based solar cells and has already demonstrated its potential to lead to high-efficiency solar cells. However, CBD Zn(S,O) presents some inconveniences compared with a classic CBD cadmium sulfide (CdS) buffer. Indeed, Zn(S,O) deposition time, important ammonia concentration, and metastable behavior of the final devices are obstacle to its generalized use. A new complexing agent, i.e., morpholine, has been mixed to ammonia as a buffer and complexing agent. From a theoretical study, the mix of these leads to the formation of a majority zinc-morpholine complex instead of the classic zinc-ammonia one. In situ studies have shown an increase in the deposition rate, allowing us to reduce the concentration of the other reactants. Scanning electron microscopy and X-ray photoelectron spectroscopy measurements have shown that the deposited material presents equivalent covering properties and sulfur incorporation ([S]/([S] + [O]) ratio) to the one determined using standard formulations. Finally, CIGSe-based devices have been realized using the optimized CBD Zn(S,O) as a buffer layer. Higher efficiencies than CdS-buffered reference are measured, and no metastability is observed.

Fabrication of Cd-free CuInSe2 solar cells using wet processes

CuInSe2 has become an attractive material for use in the production of thin-film solar cells. Recently, energy conversion efficiencies over 20% have been achieved by CuInSe2 solar cells created using vacuum processes. To reduce manufacturing costs, more recent efforts have focused on non-vacuum processes. In this study, we fabricated solar cells exclusively by wet processes. CuInSe2 absorption layers and In2S3 buffer layers were electrochemically or chemically deposited, and ZnO window layers were formed using solution-based chemical methods. The deposited CuInSe2 absorption layers were heat-treated to improve crystallinity. We deposited a Cd-free buffer layer of In2S3 using a chemical bath method on the CuInSe2 absorption layers, and then ZnO window layers were deposited by a solution-based process. With this system, we successfully fabricated CuInSe2 solar cells and obtained a cell efficiency of 2.38%.

From 20.9 to 22.3% Cu(In,Ga)(S,Se)2 solar cell: Reduced recombination rate at the heterojunction and the depletion region due to K-treatment

Certified efficiency of 22.3% has been achieved for Cu(In,Ga)(Se,S)2 solar cell. Compared to our previous record cell with 20.9% efficiency, the major breakthrough is due to the increased Voc, benefited from potassium treatment. A lower reverse saturation current and a longer carrier collection length deduced from electron-beam induced current indicate that the degree of carrier recombination at the heterojunction and depletion region for the 22.3% cell is lower. Further characterizations (capacitance–voltage profiling, temperature-dependent Voc, Suns–Voc) and analysis indicate that the recombination coefficients at all regions were reduced, especially for the interface and depletion regions. Device simulation was performed assuming varying defect densities to model the current–voltage curve for the 22.3% cell. The best model was also used to estimate the achievable Voc if defect densities were further reduced. Furthermore, by using higher bandgap Cd-free buffer layers, a higher Jsc was achieved which gives an in-house solar cell efficiency of 22.8%. Recombination analysis on the 22.8% cell indicates that the interface recombination is further reduced, but the recombination coefficients at the depletion region was higher, pointing out that further improvement on the depletion region recombination could help to achieve a higher Voc and therefore an efficiency beyond 23%.

Tunable optical and structural properties of Zn(S,O) thin films as Cd-free buffer layer in solar cells

Zn(S,O) thin films were grown by three step process: (1) the deposition of Zn layer on glass substrates by vacuum thermal evaporation, (2) the oxidation of the as-deposited Zn layer in air atmosphere and (3) the sulfurization of the oxidized samples in N2 atmosphere. The samples were analyzed for their microstructural, morphological and optical properties. XRD analysis revealed that the films were polycrystalline with a highly c-axis orientation. EDX measurements suggested that the sulfur content of the films could be inflected by the sulfurization temperature. The optical constants of the films were calculated from the analysis of the transmittance and reflectance data. The band gap energy varied from 3.27 to 3.08eV depending on the sulfur content. The surface morphological studies revealed that the films had an average roughness between 13.1 and 16.9nm. Zn(S,O) thin films exhibited n-type electrical conductivity.

New Route for Fabrication of High-Quality Zn(S,O) Buffer Layer at High Deposition Temperature on Cu(In,Ga)Se$_2$ Solar Cells

Employing Cd-free buffer layer for Cu(In,Ga)Se2 (CIGS) solar cells is very important for improving the device power conversion efficiency and for solving the environmental issues of Cd for mass production. Zn(S,O) buffer layer as one of the most promising alternatives to CdS have shown higher short-circuit current density JSC but much lower open-circuit voltage V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">OC</sub> due to the improper energy alignment with CIGS. In this study, we developed a novel chemical bath deposition process that effectively removes the stationary bubbles and enables the growth of high-quality Zn(S,O) thin film at high temperatures. Devices with Zn(S,O) buffer layer grown at 95 °C have shown V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">OC</sub> comparable to that of devices with CdS buffer layer. Systematic characterization results suggest that our method leads to Zn(S,O) buffer layers with larger oxygen concentration and thus better band alignment with CIGS layer, which well explains the improved V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">OC</sub> and power conversion efficiency.

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%.

Progress Toward 1000-mV Open-Circuit Voltage on Chalcopyrite Solar Cells

Previously, an open-circuit voltage of 960 mV was reported on Se-free Cu(In,Ga)S2 (CIGS) solar cell with a CdS buffer layer. In this paper, we report our latest progress toward 1000 mV on a Se-free Cu(In,Ga)S 2 solar cell with a Cd-free buffer layer. The highest open-circuit voltage of 973 mV was demonstrated by rapid thermal annealing and Zn1- x Mg x O buffer layer application.

Buffer layer optimization for high efficiency CIGS solar cells

This work presents a study concerning the numerical optimization of a buffer layer for high efficiency CIGS solar cells. The dependence of the solar cell properties on the buffer layer material, the layer thickness, the type and density of defects within the same layer were numerically investigated and analysed. Promising results were obtained with alternative Cd-free buffer layers (ZnSnO, InS and ZnS) in place of the standard CdS.

Atomic Layer Deposition of Zn(O, S) Buffer Layer on CIGS Solar Cell

Deposition of Zn(O, S) thin film as the Cd-free buffer layer is an important topic in Copper Indium Gallium Diselenide (CIGS) solar cells since it offers the potential enhancements for low cost and good for our environment. For CIGS buffer layer deposition, chemical bath deposition is the most common method in use. However, the incompatibility with in-line vacuum-based production methods is an issue of concerned. In this study, the buffer layer of Zn(O, S) thin films were fabricated by atomic layer deposition (ALD) method with alternately depositing of ZnS and ZnO thin films, using diethyl Zinc((C2H5)2Zn, DEZ), dimethyl sulfide((CH3)2S, DMS), and H2O as the precursors. As the alternative layers were deposited, atomic layer deposition technique exhibits good step-coverage, uniformity and accurate control of layer thickness. By changing the cycle ratios (m/n) of ZnS and ZnO, the oxygen (O)/sulfide(S) composition can be controlled in the Zn(O, S) buffer layer thin film as shown in Fig. 1. Various O/S composition ratios of Zn(O, S) thin films were deposited on the CIGS absorber. The optical properties, microstructure and chemical analyses of the deposited Zn(O, S) buffer layer thin films were carried out by ellipsometry, scanning electron microscopy (SEM), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). The CIGS device performances were measured by current-voltage (I-V) measurements. Keywords: CIGS, Zn(O, S) buffer layer, atomic layer deposition (ALD) References [1] U. P. Singh, S. P. Patra. International Journal of Photoenergy, 2010, 311–330.[2] N. Naghavi, S. Spiering, M. Powalla, B. Cavana, D. Lincot. Progress in Photovoltaics: Research and Applications, 2003, 11, 437-443[3] U. Zimmermann, M. Ruth, M. Edoff. Proceedings of the 21st European Photovoltaic Solar Energy Conference, Dresden, 2006, 1831–1834.[4] A. Hultqvist, C. Platzer-Björkman, J. Pettersson, T.Törndahl, M. Edoff. Thin Solid Films, 2009, 517, 2305–2308.[5]J. Lindahl, J. T. Wätjen, A. Hultqvist, T. Ericson, M. Edoff and T. Törndahl. Progress in Photovoltaics: Research and Applications, 2013, 1588-1597. Figure 1