9.0% efficient Cu2ZnSn(S,Se)4 solar cells from selenized nanoparticle inks

Thin‐film solar cells using Cu₂ZnSn(S,Se)₄ absorber materials continue to attract increasing attention. The synthesis of kesterite Cu₂ZnSnS₄ nanoparticles by a modified method of hot injection is explained. Characterization of the nanoparticles by energy dispersive X‐ray spectroscopy, X‐ray diffraction, Raman, and transmission electron microscopy is presented and discussed. When suspended in an ink, coated, and processed into a device, the nanoparticles obtained by this synthesis achieve a total area (active area) efficiency of 9.0% (9.8%) using AM 1.5 illumination and light soaking. This improvement over the previous efficiency of 7.2% is attributed to the modified synthesis approach, as well as fine‐tuned conditions for selenizing the coated nanoparticles into a dense absorber layer. Copyright © 2014 John Wiley & Sons, Ltd.     

High‐efficiency Cu2ZnSn(S,Se)4 solar cells fabricated through a low‐cost solution process and a two‐step heat treatment

A method for fabricating high‐efficiency Cu₂ZnSn(S,Se)₄ (CZTSSe) solar cells is presented, and it is based on a non‐explosive, low‐cost, and simple solution process followed by a two‐step heat treatment. 2‐Methoxyethanol was used as a solvent, and Cu, Zn, Sn, chloride salts, and thiourea were used as solutes. A CZTSSe absorber was prepared by sulfurising and then selenising an as‐coated Cu₂ZnSnS₄ (CZTS) film. Sulfurisation in a sulfur vapour filled furnace for a long time (2 h) enhanced the crystallisation of the as‐coated CZTS film and improved the stability of the CZTS precursor, and selenisation promoted further grain growth to yield a void‐free CZTSSe film. Segregation of Cu and S at the grain boundaries, the absence of a fine‐grain bottom layer, and the large grain size of the CZTSSe absorber were the main factors that enhanced the grain‐to‐grain transport of carriers and consequently the short‐circuit current (J_sc ) and efficiency. The efficiency of the CZTS solar cell was 5.0%, which increased to 10.1% after selenisation. For the 10.1% CZTSSe solar cell, the external quantum efficiency was approximately 80%, the open‐circuit voltage was 450 mV, the short‐circuit current was 36.5 mA/cm², and the fill factor was 61.9%. Copyright © 2016 John Wiley & Sons, Ltd.     

Cu2ZnSnS4 thin film solar cells by fast coevaporation

Solar cells based on kesterite‐type Cu₂ZnSnS₄ (CZTS) were fabricated on molybdenum coated soda lime glass by evaporation using ZnS, Sn, Cu, and S sources. The coevaporation process was performed at a nominal substrate temperature of 550°C and at a sulfur partial pressure of 2–3 × 10⁻³ Pa leading to polycrystalline CZTS thin films with promising electronic properties. The CZTS absorber layers were grown copper‐rich, requiring a KCN etch step to remove excess copper sulfide. The compositional ratios as determined by energy‐dispersive X‐ray spectroscopy (EDX) after the KCN etch are Cu/(Zn + Sn): 1.0 and Zn/Sn: 1.0. A solar cell with an efficiency of 4.1% and an open‐circuit voltage of 541 mV was obtained. Copyright © 2010 John Wiley & Sons, Ltd.     

Sodium Effects on the Diffusion,Phase, and Defect Characteristics of Kesterite Solar Cells and Flexible Cu2ZnSn(S,Se)4 with Greater than 11% Efficiency

Improving the efficiency of kesterite (Cu₂ZnSn(S,Se)₄; CZTSSe) solar cells requires understanding the effects of Na doping. This paper investigates these effects by applying a NaF layer at various positions within precursors. The NaF position is important because Na produces Na-related defects in the absorber and suppresses the formation of intrinsic defects. By investigating precursors with various NaF positions, the sulfo-selenization mechanism and the characteristics of defect formation are confirmed. Applying a NaF layer onto a Zn layer in a CZTSSe precursor limits Zn diffusion and suppresses Cu-Zn alloy formation, thus changing the sulfo-selenization mechanism. In addition, the surface NaF layer provides reactive Se and S to the absorber layer by generating Na₂Se_x and Na₂S_x liquid phases during sulfo-selenization, thus limiting the incorporation of Na into the absorber and reducing the Na effects. Efficiency values of 11.16% and 11.19% are obtained for a flexible CZTSSe solar cell by applying NaF between the Zn layer and back contact and between the Cu and Sn layers, respectively. This study presents methods for doping with alkali metals and improving the efficiency of photovoltaics.     

Device characteristics of a 10.1% hydrazine‐processed Cu2ZnSn(Se,S)4 solar cell

A power conversion efficiency record of 10.1% was achieved for kesterite absorbers, using a Cu₂ZnSn(Se,S)₄ thin‐film solar cell made by hydrazine‐based solution processing. Key device characteristics were compiled, including light/dark J–V, quantum efficiency, temperature dependence of V_oc and series resistance, photoluminescence, and capacitance spectroscopy, providing important insight into how the devices compare with high‐performance Cu(In,Ga)Se₂. The record kesterite device was shown to be primarily limited by interface recombination, minority carrier lifetime, and series resistance. The new level of device performance points to the significant promise of the kesterites as an emerging and commercially interesting thin‐film technology. Copyright © 2011 John Wiley & Sons, Ltd.     

Impact of capping during the formation of Cu2ZnSn(S,Se)4 thin films

Major challenge for the fabrication of kesterite absorber thin films such as CZTSSe (Cu₂ZnSn(S,Se)₄) is the volatility of chalcogens. Material loss and poor morphology are two key issues during high temperature annealing, carried out for the formation of CZTSSe thin film absorber layers. The purpose of the present study is to investigate the influence of capping during the crystallization of precursor to CZTSSe films via annealing. In this work, initial precursor was synthesized from elemental constituents by ball milling. CZTSSe films were deposited by doctor's blade process. Annealing was carried out in two different atmosphere viz. vacuum and inert gas. Both sets of samples were annealed with and without capping. We found significant changes for different annealing atmospheres. Capping has a positive influence on the film properties, revealed by structural, morphological and compositional analysis. Capping reduced material loss of volatile constituents and resulted compact crystalline films.     

Systematic Efficiency Improvement for Cu2ZnSn(S,Se)4 Solar Cells By Double Cation Incorporation with Cd and Ge

The performance of kesterite Cu₂ZnSn(S,Se)₄ (CZTSSe) solar cell is known to be severely limited by the nonradiative recombination near the heterojunction interface and within the bulk of the CZTSSe absorber resulting from abundant recombination centers and limited carrier collection efficiency. Herein, nonradiative recombination is simultaneously reduced by incorporating small amounts of Ge and Cd into the CZTSSe absorber. Incorporation of Ge effectively increases the p-type doping, thus successfully improving the bulk conductance and reducing the recombination in the CZTSSe bulk via enhanced quasi-Fermi level splitting, while the incorporation of Cd greatly reduces defects near the junction region, enabling larger depletion region width and better carrier collection efficiency. The combined effects of Cd and Ge incorporation give rise to systematic improvement in open-circuit voltage (V_OC), short-circuit current density (J_SC), and fill factor (FF), enabling a high conversion efficiency of 11.6%. This study highlights the multiple cation incorporation strategy for systematically manipulating the opto-electronic properties of kesterite materials, which may also be applicable to other semiconductors.     

The Role of Sulfur in Solution‐Processed Cu2ZnSn(S,Se)4 and its Effect on Defect Properties

Understanding the electrically active defects in kesterite Cu₂ZnSn(S,Se)₄(CZTSSe) is critical for the continued development of solar cells based on this material, but challenging due to the complex nature of this polycrystalline multinary material. A comparative study of CZTSSe alloys with three different bandgaps, made by introducing different fractions of sulfur during the annealing process, is presented. Using admittance spectroscopy, drive level capacitance profiling, and capacitance‐voltage profiling, the dominant defect energy level present in the low sulfur content device is determined to be 0.134 eV above the valence band maximum, with a bulk defect density of 8 × 10¹⁴ cm^?3, while the high sulfur content device shows a deeper defect energy level of 0.183 eV and a higher bulk defect density, 8.2 × 10¹⁵ cm^?3. These findings are consistent with the current density–voltage characteristics of the resulting solar cells and their external quantum efficiency. It suggests that as the sulfur content increases, the bandgap of the absorber is enlarged, leading to an increasing open‐circuit voltage (V_oc), that is accompanied by stronger recombination due to the higher defect density of the sulfur‐rich absorber. This is reflected in large V_oc deficit and poor carrier collection of the high sulfur content device.     

Locating the electrical junctions in Cu(In,Ga)Se2 and Cu2ZnSnSe4 solar cells by scanning capacitance spectroscopy

We determined the electrical junction (EJ) locations in Cu(In,Ga)Se₂ (CIGS) and Cu₂ZnSnSe₄ (CZTS) solar cells with ~20‐nm accuracy by developing scanning capacitance spectroscopy (SCS) applicable to the thin‐film devices. Cross‐sectional sample preparation for the SCS measurement was developed by high‐energy ion milling at room temperature for polishing the cross section to make it flat, followed by low‐energy ion milling at liquid nitrogen temperature for removing the damaged layer and subsequent annealing for growing a native oxide layer. The SCS shows distinct p‐type, transitional, and n‐type spectra across the devices, and the spectral features change rapidly with location in the depletion region, which results in determining the EJ with ~20‐nm resolution. We found an n‐type CIGS in the region next to the CIGS/CdS interface; thus, the cell is a homojunction. The EJ is ~40 nm from the interface on the CIGS side. In contrast, such an n‐type CZTS was not found in the CZTS/CdS cells. The EJ is ~20 nm from the CZTS/CdS interface, which is consistent with asymmetrical carrier concentrations of the p‐CZTS and n‐CdS in a heterojunction cell. Our results of unambiguously determination of the junction locations contribute significantly to understanding the large open‐circuit voltage difference between CIGS and CZTS. Copyright © 2016 John Wiley & Sons, Ltd.     

Cu_2ZnSn(S,Se)_4薄膜太阳能电池的研究进展

锌黄锡矿结构的Cu_2Zn Sn(S,Se)_4(CZTSSe)材料,由于具有价格低廉、带隙合适、吸光系数高等优良光电性能,很适合作为新一代无机薄膜太阳能电池的吸光层材料,已受到各国科研人员的高度关注。国内外采用多种沉积薄膜技术来制备CZTSSe吸光层材料,主要包括真空和非真空方法。综述了最近CZTSSe太阳能电池制备技术所取得的一些进展,尤其对采用溶液法制备CZTSSe太阳能电池的发展现状做了重点阐述。展望了CZTSSe太阳能电池的发展趋势。     

Cu2ZnSnSe4 thin film solar cells produced by selenisation of magnetron sputtered precursors

Polycrystalline thin films of Cu₂ZnSnSe₄ (CZTSe) were produced by selenisation of Cu(Zn,Sn) magnetron sputtered metallic precursors for solar cell applications. The p‐type CZTSe absorber films were found to crystallize in the stannite structure (a = 5·684 Å and c = 11·353 Å) with an electronic bandgap of 0·9 eV. Solar cells with the indium tin oxide structure (ITO)/ZnO/CdS/CZTSe/Mo were fabricated with device efficiencies up to 3·2% measured under standard AM1·5 illumination. Copyright © 2009 John Wiley & Sons, Ltd.     

Research on structure of Cu2ZnSn(S, Se)4 thin films with high Sn-related phases

Cu2ZnSn(S, Se)4 (CZTSSe) thin films were deposited on flexible substrates by three evaporation processes at high temperature. The chemical compositions, microstructures and crystal phases of the CZTSSe thin films were respectively characterized by inductively coupled plasma optical emission spectrometer (ICP-OES), scanning electron microscopy (SEM), X-ray diffraction (XRD) and Raman scattering spectrum. The results show that the single-step evaporation method at high temperature yields CZTSSe thin films with nearly pure phase and high Sn-related phases. The elemental ratios of Cu/(Zn+Sn)=1.00 and Zn/Sn=1.03 are close to the characteristics of stoichiometric CZTSSe. There is the smooth and uniform crystalline at the surface and large grain size at the cross section for the films, and no other phases exist in the film by XRD and Raman shift measurement. The films are no more with the Sn-related phase deficiency.     

Quantum confinement and size effects in Cu2ZnSnS4 thin films produced using solution processed ultrafine nanoparticles

Copper–zinc–tin-sulfide (Cu₂ZnSnS₄, abbreviated as CZTS) is a direct band gap p-type semiconductor material with high absorption coefficient. Using oleylamine as solvent/stabilizing agent and metal chlorides and sulfur particles as chemical precursors, CZTS based nanoparticles were produced and subsequently deposited as thin films on glass substrates via spin coating of the nanoinks. The effect of temperature on crystallite size and phase composition was assessed after the solution mixture was undercooled by 30, 70 or 90 °C. Upon cooling the solution from 230 to 140 °C i.e. by 90 °C, maximum refinement in the nanoparticles size was noticed with average size on the order of few nanometers. The morphological and compositional studies of the nanoparticles were performed by means of scanning electron microscope, X-ray diffraction and Fourier transform infrared spectroscopy techniques. Phase-pure CZTS formation was confirmed from fast Fourier transform (FFT) patterns and lattice fringes observed during HR-TEM examination. Characterization of the annealed spin coated films, made from nanoink containing ultrafine nanoparticles, indicated morphological changes in the film surface during air annealing at 350°C that can be attributed to depression of CZTS phase decomposition temperature. Spectrophotometric studies of the annealed films suggested quantum confinement effect through an associated increase in the band gap value from 1.34 to 2.04 eV upon reduction in the nanoparticle size caused by increasing the degree of undercooling to 90 °C.     

Ink formulation and low‐temperature incorporation of sodium to yield 12% efficient Cu(In,Ga)(S,Se)2 solar cells from sulfide nanocrystal inks

Solution phase deposition methods offer great potential for low‐cost photovoltaic device fabrication. We have previously developed a method for copper indium gallium disulfoselenide (CIGSSe) device fabrication based on drop‐casting copper indium gallium disulfide (CIGS) nanocrystals in a toluene or hexane‐based ink followed by chalcogen exchange in elemental selenium vapor at 500 °C. By starting with the chalcopyrite or sphaelerite phase of CIGS nanocrystals with controlled stoichiometry, superior composition uniformity can be achieved inherently. Here, we present a dramatic improvement in ink formulation using alkanethiol as the solvent, which enables the ability to create uniform nanocrystal coatings over large areas using a simple knife coating technique. In addition, we show a major improvement in device performance by a simple and low‐temperature method of incorporating sodium into the CIGSSe film based on soaking the films in aqueous NaCl solution. The addition of sodium plays an important role in improving the structural properties of the resulting CIGSSe films, where large and densely packed grain can be obtained. The improved film morphology significantly reduces recombination losses in the resulting device leading to a dramatically enhanced device performance. With the use of standard glass/Mo/CIGSSe/CdS/i‐ZnO/ITO device structure, photovoltaic devices yield total area power conversion efficiency as high as 12.0% under AM1.5 illumination without an anti‐reflection coating. Copyright © 2012 John Wiley & Sons, Ltd.     

Enhancement of photo‐conversion efficiency in Cu2ZnSn(S,Se)4 thin‐film solar cells by control of ZnS precursor‐layer thickness

CZTSSe thin‐film absorbers were grown by stacked ZnS/SnS/Cu sputtering with compound targets, and the precursors were annealed in a furnace with a Se atmosphere. We controlled the thickness of the ZnS precursor layer for the CZTSSe thin films in order to reduce the secondary phases and to improve the performance of the devices. The optimal value of the ZnS precursor thickness was determined for the CZTSSe absorbers, and this configuration showed an efficiency of up to 9.1%. In this study, we investigated the depth profiles of the samples in order to determine the presence of secondary phases in the CZTSSe thin films by Raman spectroscopy and Kelvin probe force microscopy. Cu₂SnSe₃, ZnSe, and MoSe₂ secondary phases appeared near the back contact, and the work function distribution of the CZTSSe thin‐film surface and the secondary phase distribution were different depending on the depths of the absorber layer. This phase characterization allows us to describe the effects that changes in the thickness of the ZnS precursor can have on the performance of the CZTSSe thin‐film solar cells. Although it is important to identify the phases, the effects of secondary phases and point defects are not yet fully understood, even in optimal devices. Therefore, phase identification that is based on the work function and the results obtained from the Raman spectra in terms of the depth profile are instrumental to improve the surface and interface of CZTSSe thin‐film solar cells. Copyright © 2015 John Wiley & Sons, Ltd.     

ZnO nanorod arrays as an antireflective coating for Cu(In,Ga)Se2 thin film solar cells

A ZnO nanorod antireflective coating has been prepared on Cu(In,Ga)Se₂ thin film solar cells. This coating leads to a decrease of the weighted global reflectance of the solar cells from 8.6 to 3.5%. It boosts the solar cells short‐circuit current up to 5.7% without significant effect on their open‐circuit voltage and fill factor (FF), which is comparable to a conventional optimized single layer MgF₂ antireflective coating. The ZnO nanorod antireflective coating was electrochemically prepared from an aqueous solution at 80°C. The antireflective capability of ZnO nanorod arrays (ZNAs) may be further improved by optimization of growth conditions and their geometry. Copyright © 2010 John Wiley & Sons, Ltd.     

13.3% efficient solution deposited Cu(In,Ga)Se2 solar cells processed with different sodium salt sources

In this work, we obtain the highest reported power conversion efficiency (13.3%) for sulfur‐free CIGSe solar cells by solution processing. Intentional sodium incorporation is achieved directly by dissolving sodium salts in the ink. The effect on solar cell properties of three sodium sources is investigated: NaCl, NaHCO₂, and NaSCN. A comparison is made with absorbers grown on soda‐lime glass substrates and in the absence of sodium sources. The incorporation via sodium salts yields significantly better results, which is attributed to enhanced‐sodium availability. A comparison with identically annealed sputtered metal precursor layers capable of delivering 15.1% module efficiency suggests that the cell results are limited by the selenization procedure. Copyright © 2015 John Wiley & Sons, Ltd.     

Cu-In-Ga-Se nanoparticle colloids as spray deposition precursors for Cu(In, Ga)Se2 solar cell materials

The use of nanoparticle colloids for spray deposition of Cu(In,Ga)Se₂ (CIGS) precursor films and subsequent fabrication of CIGS solar cells has been investigated. According to this approach, amorphous Cu-In-Ga-Se nanoparticle colloids were first prepared by reacting a mixture of CuI, InI₃, and GaI₃ in pyridine with Na₂Se in methanol at reduced temperature. Purified colloid was sprayed onto heated molybdenum-coated sodalime glass substrates to form Cu-In-Ga-Se precursor films. After thermal processing of the precursor films under a selenium ambient, CIGS solar cells were fabricated. Cu-In-Ga-Se colloids and films were characterized by inductively coupled plasma atomic emission spectroscopy, thermogravimetric analysis, transmission electron microscopy, x-ray diffraction, scanning electron microscopy, and Auger electron spectroscopy. Standard current-voltage characterization was performed on the CIGS solar cell devices with the best film exhibiting a solar conversion efficiency of 4.6%.     

基于纳米颗粒的铜锌锡硫硒薄膜制备

采用热注入法制备了Cu2ZnSnS4(CZTS)纳米颗粒,并形成高分散、稳定的"墨水",采用滴注方法形成CZTS前驱体薄膜。利用X射线衍射(XRD)、拉曼光谱(Raman)、透射电子显微镜(TEM)和紫外-可见光谱(UV-VIS)对CZTS纳米颗粒的晶体结构、表面形貌和带隙进行了表征。Raman数据显示合成的纳米颗粒为纯的CZTS,不存在ZnS和Cu2SnS3等杂相。傅里叶红外光谱(FTIR)和UV-VIS表明合成的CZTS纳米颗粒表面被油胺(OLA)包覆,并且其带隙为1.52 eV。对CZTS前驱体薄膜在硫化氢气氛和固态硒气氛中退火处理,得到铜锌锡硫硒(CZTSSe)薄膜。结果表明,经硫化氢处理后薄膜表面平整但CZTS晶粒并没长大,而经过固态硒处理后得到了结晶质量较好的CZTSSe薄膜。     

Defect studies in Cu2ZnSnSe4 and Cu2ZnSn(Se0.75S0.25)4 by admittance and photoluminescence spectroscopy

To achieve higher record efficiencies for solar cells containing Cu₂ZnSnSe₄ (CZTSe), Cu₂ZnSnS₄ (CZTS) or their solid solution Cu₂ZnSn(Se_xS_1?x)₄ (CZTSSe) as an absorber, it is necessary to obtain more knowledge about defect structure of these materials. In this work, admittance spectroscopy (AS) and low temperature photoluminescence spectroscopy (PL) were used for defect studies. Admittance spectroscopy in the frequency range from 20 Hz to 10 MHz was used for studies of CZTSe/CdS and CZTSSe/CdS monograin layer heterojunctions. The measurement temperature varied from 140 K to 245 K. Two defect states (labelled E_A1 and E_A2) were found in Cu₂ZnSnSe₄ and Cu₂ZnSn(Se_0.75S_0.25)₄. In different CZTSe/CdS heterojunctions the E_A2 state was present at 74 meV, but the second E_A1 defect state changed from 87 meV to 100 meV during time and had varying properties. In Cu₂ZnSn(Se_0.75S_0.25)₄ the E_A2 state was found at 25 meV. The E_A1 state at 154 meV showed the same properties as the two defect levels in CZTSe. In both cases the E_A2 defect state was attributed to an acceptor defect and the E_A1 state with changing properties to interface states. The detected PL bands were at 0.946 eV in CZTSe and at 1.028 eV in Cu₂ZnSn(Se_0.75S_0.25)₄. Obtained by PL measurements, defect states at 69 meV in CZTSe and at 39 meV in Cu₂ZnSn(Se_0.75S_0.25)₄ were attributed to the same acceptor defect that was found from the AS measurements.