CZT Precursor Research Articles

Fabrication of Cu2ZnSnS4 (CZTS) by co-electrodeposition of Cu-Zn-Sn alloys, and effect of chemical composition of CZTS on their photoelectrochemical water splitting

Sulfide kesterite Cu2ZnSnS4 (CZTS), an intrinsic p-type semiconductor, provides an attractive low-cost, environmentally friendly photoelectrochemical (PEC) water splitting to evolve hydrogen. The purpose of this study was to clarify the effect of the chemical composition of CZTS on the PEC properties and the correlation with their structures. The CZTS was fabricated on a Mo/glass by a non-vacuum process, i.e., co-electrodeposition (co-ED) of Cu-Zn-Sn (CZT) alloys, followed by sulfurization with solid sulfur. To optimize the chemical composition of CZTS, the bath concentrations, applied potentials and amounts of electric charge for preparation conditions of the CZT precursors were examined. The CZTS with Cu-poor and Sn-rich composition compared with stoichiometry was found to exhibit relatively effective PEC water splitting. It was suggested that the high PEC performance was attributed to efficient charge carrier transport due to the presence of a small amount of SnO2 phase on the CZTS surface.

CZTS thin film solar cells on flexible Molybdenum foil by electrodeposition-annealing route

Earth-abundant and non-toxic Kesterite-based Cu2ZnSnS4 (CZTS) thin film solar cells are successfully fabricated on flexible Molybdenum (Mo) foil substrates by an electrodeposition-annealing route. A well-adherent, densely packed, homogeneous, compact, and mirror-like CZT precursor is initially produced through electrodeposition by using a rotating working electrode. Subsequently, the co-electrodeposited CuZnSn (CZT) precursor is sulfurized in quartz tube furnace at 550 °C for 2 h in N2 atmosphere with the presence of elemental sulfur in order to form CZTS. Different characterization techniques like XRD, SEM, HR-TEM, Raman, and Photoluminescence demonstrate that almost phase-pure CZTS formed after sulfurization. A flexible Al/Al-ZnO/i-ZnO/CdS/CZTS/Mo foil solar cell is produced, where CdS is deposited by chemical bath deposition and transparent conducting oxide (TCO) is deposited by DC sputtering. The CZTS solar device shows a 0.55% power conversion efficiency on flexible Mo foil substrate and it constitutes the first prototype of this kind of solar cell produced by electrodeposition-annealing route without any surface modification of the Mo substrate.Graphic abstract

Influence of co-electrodeposition parameters in the synthesis of kesterite thin films for photovoltaic

Abstract Electrodeposition is one of the most attractive techniques for the synthesis of thin films chalcogenides for photovoltaic applications, in particular for earth abundant kesterite. In this work, the influence of core parameters of this technique, such as the applied potential and deposition time, on the chemical composition of co-deposited Cu–Zn–Sn precursors is studied. Indeed, how the precursor composition affects the final composition of Cu2ZnSnSe4 after reactive annealing in Se atmosphere is also investigated, not only at the film level but also in solar cell devices. X-ray based techniques (XRF and XRD) reveal that the chemical and phase composition of electrodeposited precursors is highly sensitive to small modifications (25 mV) of the applied potential during electrodeposition. Only when the applied potential is −1.2 V (vs. Ag/AgCl) the precursor composition matches to what is required for operating devices. Multi-wavelength Raman spectroscopy of selenized precursors confirms that lower potential than −1.2 V favor the formation of Sn-rich secondary and ternary compounds after selenization, while a higher overpotential led to the formation of ZnSe in the film. On the other hand, the use of longer deposition times enhance the incorporation of zinc in the film, modifies the morphology of the precursor and promotes the formation ZnSe after selenization. Photovoltaic devices were only obtained at an electrodeposition potential of −1.2 V, and reaching a maximum efficiency close to 5% for a 10 min electrodeposited CZT precursor.

Influence of pre-sulfurization temperature on properties of Cu2ZnSnS4 thin film in two-step sulfurization process

In this study, Cu2ZnSnS4 (CZTS) thin films were prepared by magnetron sputtering deposition of the Cu-Zn-Sn metallic precursor (CZT) on flexible molybdenum foils and a two-step sulfurization process. Low-temperature pre-sulfurization was performed before the final high-temperature sulfurization during the two-step sulfurization process to enhance the properties of CZTS. This paper presents a method to improve the properties of CZTS thin films, making them conducive to solar cell applications. The compositions and morphologies of CZTS were confirmed using energy dispersive spectroscopy and scanning electron microscopy, respectively. X-ray diffraction was conducted to investigate the structural properties of the CZTS thin films, and the results revealed that the CZTS phase in all the samples had a preferential plane along (112). Moreover, Raman spectroscopy helped distinguish the ZnS and Cu2SnS3 phases that are close to kesterite CZTS. Compared to the thin film prepared without pre-sulfurization treatment, a suitable increase in pre-sulfurization temperature could reduce the loss of Sn and enhance the crystallinity of CZTS. However, a higher pre-sulfurization temperature could result in the formation of secondary phases. The CZTS thin films exhibited good properties when the CZT precursor was pre-sulfurized at 150 °C. In addition, the CZTS thin films had a single phase and flat and dense surfaces with large grains. The optical parameters were calculated based on the reflectance and transmittance data obtained by UV–Vis spectroscopy. The absorption coefficient was larger than 1 × 104 cm−1, and the optical bandgap was about 1.55 eV; these are suitable properties for CZTS absorption layers in CZTS thin-film solar cells.

Cu2ZnSnS4 electrodeposition By Sulfurization of Metallic Precursors

Cu2ZnSnS4 (CZTS) is a kesterite-type material with promising properties among the materials suitable to be used as absorbers in solar cells. It is highly absorbent (α ≥ 105 cm-1), with a direct bandgap energy value (E GAP) suitable for solar light absorption (E GAP close to 1.5 eV) [1]. The constituting elements are not toxic, expensive or scarce, as is the case in competing family compounds such as chalcopyrites (CuInGaSe2). CZTS films are thus excellent candidates for low-cost, highly efficient and environmentally friendly solar cells [2]. There are different electroplating routes to obtain CZTS. In a previous work [3] we explored electrodeposition from a single bath containing the four elements and found it difficult to stabilize the sulfur source in the precursor mixture. Moreover, sulfur was found to incorporate into the material mainly during the sulfurization stage. In this work, the synthesis started with the deposition of a Cu-Zn-Sn precursor (CZT), to be continued by the incorporation of S during a heat treatment in the presence of vaporized sulfur that transforms the precursor into CZTS. The purpose of this investigation was to adjust the pH of the precursor bath to 5.9, so that CZTS could be deposited on top of ZnO, in order to prepare solar cells in superstrate configuration. The effect of electrodeposition time and annealing temperature were also explored. Thin films of CZTS were prepared by sulfurization of metallic precursors (CZT) previously electrodeposited during 5 to 10 min at -1.15 VSCE using conducting glass (FTO) as primary substrate. The precursor solution consisted of 100 mmol/L ZnSO4, 30 mmol/L CuSO4, 10 mmol/L SnSO4 and 100 mmol/L of sodium citrate as complexing agent. Citrates and hydroxyls concentrations were adjusted so that pH = 5.9. Electrodeposited CZT precursors underwent a reactive sulfurization annealing stage in a three-temperature zone thermal reactor. First, the samples were placed in the cold zone of the tube (room temperature). Then the second zone was heated to 500 °C and in the third zone the sulfur powder (0.3 g) was allowed to reach 350 °C (melting starts at around 150 °C). The vaporized sulfur reached the samples assisted by argon flux. After 30 to 60 minutes, the samples were left to cool down naturally. CZTS films were characterized by X-ray diffraction (XRD), Raman spectroscopy, scanning electronic microscopy (SEM), UV-Vis spectroscopy and photoelectrochemical techniques. SEM results showed good coverage and homogenous morphology. Also, cross-section images showed thickness values close to 1 micron and higher than expected, even for samples electrodeposited for just 10 min. Raman spectroscopy and XRD results confirmed the formation of crystalline CZTS after sulfurization, with no secondary phases detected in the film. Photocurrent analysis confirmed the p-type nature of the semiconductor. Direct energy gap values close to 1.5 eV were estimated for CZTS films using transmittance spectra. CZTS films obtained with short electrodeposition times are good-quality absorbers suitable to be used in kesterite thin films solar cells. Preliminary results from solar cells in superstrate configuration show a promising photovoltaic response.

One Step Cu-Zn-Sn Alloy Deposition As a Precursor for CZTS Absorber in Photovoltaics

In recent years, research on thin film photovoltaic cells is focusing on the development of inexpensive, earth abundant and environmentally benign materials [1]. CZT(S,Se) (Cu2ZnSn(S,Se)4) thin films meet this unlike the present commercial photovoltaic thin films CIG(S,Se) (Cu(In,Ga)(S,Se)2) and CdTe. The crystal structure of CZT(S,Se) is obtained by replacing half of the indium and/or gallium in CIG(S,Se) by zinc and the other half by tin, which are both more earth-abundant materials than indium and gallium [2]. CZT(S,Se) has direct band gap of 1.0 eV (Cu2ZnSnSe4) to 1.5 eV (Cu2ZnSnS4) and a large absorption coefficient of over 104 cm-1. Up to now, many different techniques have been implemented to obtain CZTS thin films, featuring evaporation, sputtering, sol-gel deposition pulsed laser deposition, spray pyrolysis and many others. In order to reduce production costs and deployment of thin film PV-based technologies in mass scale, kesterite-based thin film solar cells technologies should be manufacturable at high throughput with low cost and it should exhibit good power conversion efficiency. Fabrication of kesterites on flexible substrate has high potential to decrease production cost due to roll-to-roll manufacturing on flexible substrate enables use of compact size deposition equipment with high throughput [3]. Recently electrodeposition has proven its utility as a solid route to obtain the CZT precursor both by plating a homogeneous Cu-Zn-Sn alloy or by subsequently depositing a stacked version of the layer; in both cases, reactive annealing is a mandatory step to introduce sulphur in the system, yielding kesterite crystals. In 2010, a one-step electrodeposition of copper-zinc-tin-sulphur precursors was described for the first time from a citrate/tartrate-based solution followed by an annealing step [4]. Although a one-step electrodeposition of the four elements would be ideal, the stoichiometric deposition of the quaternary coating is difficult to control and an additional annealing step to obtain the right ratio of the chalcogenide and to improve the crystallinity is still required. Furthermore, most researchers use thiosulfate as the sulphur source in acidic citrate/tartrate-based electrolytes, which is unstable and precipitates as sulphur. High working efficiencies have been obtained by using molybdenum coated soda lime glass as the back contact; as already said, the idea of implementing this technology at a larger production scale well suits the needing of flexible materials and low cost production techniques. In order to reduce production costs and deployment of thin film PV-based technologies in mass scale, Kesterite-based thin film solar cells technologies should be manufacturable at high throughput with low cost and it should exhibit good power conversion efficiency. Fabrication of Kesterites on flexible substrate has high potential to decrease production cost due to roll-to-roll manufacturing on flexible substrate enables use of compact size deposition equipment with high throughput. Many researchers report CZT precursor production starting from acidic baths; on the other side, even if alkaline baths for the many cited precursor deposition are known since a long time, the stability issues to which they are related make them less studied. Here we propose an alkaline electrolyte for one step deposition of the CZT precursor onto flexible molybdenum substrates. Electrochemical analyses are reported to describe the behaviour of the plating bath, as well as morphological inspection to assess the quality of the so obtained thin films. To test active layer activity, thermal annealing and reactive annealing were performed on the metallised molybdenum foil, as well as the sealing with conductive oxides via RF sputtering. After the evaporation of aluminium contacts, the efficiency of the cell was tested, yielding 0.1%. Quantum efficiency of the so obtained substrate is also reported to complete the photoactivity characterization. [1] K. Ito and T.Nakazawa, Electrical and optical properties of stannite-type quaternary semiconductor thin films, Jpn. J. Appl. Phys., 27 (1988) 2094 - 2097. [2] W. Wang, M.T. Winkler, O.Gunawan, T. Gokmen, T. K. Todorov, Y. Zhu and D.B. Mitzi, device characteristics of CZTSSe thin-film solar cells with 12.6% efficiency, Adv. Energy Mater., 4 (2014) 1301465. [3]. P. Reinhard, A. Chirila, P. Blosch, F. Pianezzi, S. Nishiwaki, S. Buecheler, A.N. Tiwari Review of progress toward 20% efficiency flexible CIGS solar cells and manufacturing issues of solar modules, IEEE J. Photovoltaics, 3 (2013) 572 - 580. [4] S.M. Pawar, B.S. Pawar, A.V. Moholkar, D.S. Choi, J.H. Yun, J.H. Moon, S.S. Kolekar, J.H. Kim, Single step electrosynthesis of Cu2ZnSnS4 (CZTS) thin films for solar cell application, Electrochim. Acta 55 (2010) 4057.

Cu2ZnSnS4 thin films prepared by sulfurization of co-electrodeposited metallic precursors

Cu2ZnSnS4 (CZTS) thin films were prepared by thermal sulfurization of co-electrodeposited CuZnSn (CZT) metal precursors. Electrodeposition times between 10 and 40 min were used to study the influence of this parameter on the composition, structure, thickness, and morphology of the sulfurized films. CZT precursors and CZTS films were characterized by X-ray diffraction (XRD), energy-dispersive spectroscopy (EDS), and Raman spectroscopy. The morphologies were evaluated by electronic microscopy. The electrodeposition time was found to influence the chemical composition of the metal precursor, especially the Cu and Sn content. Furthermore, XRD results showed the formation of Cu5Zn8, while Sn seemed to be present in an amorphous state. A cauliflower-like morphology was observed in the precursors, especially at long deposition times, which can be related to an electrodeposition mechanism controlled by mass transfer. A significant increase of the film thickness was observed after sulfurization. The morphology changed to round particles and presented a bi-layered structure with an internal compact layer of nanometer size particles and an external layer formed by micrometer-size particles. Raman spectroscopy, XRD, and EDS measurements confirmed the formation of crystalline CZTS after sulfurization. In addition, a disperse Cu2S secondary phase co-existed in the film. Increasing the precursor deposition time increased the amount of secondary phases in the film after sulfurization. Direct energy gap values close to 1.5 eV were estimated for CZTS films using transmittance spectra in the infrared region. CZTS films obtained with short electrodeposition times are promising as absorbers in kesterite thin films solar cells.

Co-Electrodeposition of Metallic Precursors for the Fabrication of CZTSe Thin Films Solar Cells on Flexible Mo Foil

Earth abundant Cu2ZnSnSe4 (CZTSe) thin films solar cells were fabricated on flexible molybdenum (Mo) foil substrate through an electrochemical approach. After co-electroplating from single electrolyte, Cu-Zn-Sn (CZT) precursor was selenized in quartz tube furnace at 550°C for 15 minutes in Ar atmosphere with the presence of elemental selenium in order to form CZTSe compound. Before selenization, CZT precursor was soft-annealed at 310°C for 150 minutes in Ar atmosphere in order to improve intermixing of the elements and to reduce roughness. The formation of Kesterite CZTSe on selenized film was confirmed by X-ray diffraction (XRD) and Raman spectroscopy. SEM imaging and EDS line profile on the cross-section of CZTSe layer showed the well-formed CZTSe along depth of the film also. Opto-electrical characteristics of the film by photoluminescence spectroscopy confirmed the suitability of the absorber layer to make solar cell. A flexible solar cell with Al/Al-ZnO/i-ZnO/CdS/CZTSe/Mo-foil configuration, which showed 0.1% power conversion efficiency at first attempt, has been fabricated, in which buffer layer CdS has been deposited through chemical bath deposition and transparent conducting oxides (i-ZnO, Al-ZnO) have been deposited by DC pulsed sputtering.

Electrochemical studies on the CZT precursor deposition for CZTS solar cell application

Cu-Zn-Sn-(Se,S)(CZTS) is one of the most promising thin-film solar cells. The electrochemical methods to fabricate CZT deposits from one bath were investigated. Citrate was used to form a complex with each metallic compound. The proper potential and pH were determined from cyclic voltammetry and stability diagrams, respectively. The selected range of pH was from 4.7 to 6.2, and that of the voltage was from −0.85 V to −1.15 V (vs. Ag/AgCl). The theoretical value of the Cu/(Zn+Sn) ratio should be between 0.7 and 1.0 to shows proper photovoltaic effect. The working condition to produce such a ratio were −0.85 V at pH 4.7 and −1.15 V at pH 5.7 based on an energy dispersive spectrometer analysis of the deposits. Deposits at −1.15 V and pH 5.7 were denser than those at −0.85 V and pH 4.7. Finally, −1.15 V and pH 5.7 was selected optimum condition for the electrodeposition of the CZT precursor.

Composition controlled preparation of Cu–Zn–Sn precursor films for Cu2ZnSnS4 solar cells using pulsed electrodeposition

A pulsed electrodeposition technique is developed to prepare Cu–Zn–Sn (CZT) precursor films for the Cu2ZnSnS4 (CZTS) solar cells. The CZT precursor films are co-deposited on Mo-coated substrate using a cyanide-free electrolyte containing Zn (II) and Sn (II) salts. During the deposition, CuSO4 solution is supplied at controlled rate using a peristaltic pump to effectively regulate Cu2+ concentration. In addition, C6H5Na3O7 is used as a coordination ligand to further balance activities of the Cu2+, Sn2+ and Zn2+. The CZTS films are then prepared using a sulfurization process to convert the electrodeposited CZT precursors at 580 °C in a sulphur atmosphere. The annealed thin films are characterized by X-ray diffraction (XRD), Raman spectroscopy, field emission scanning electron microscopy (FE-SEM), EDAX and X-ray photoelectron spectroscopy (XPS) techniques for their structural, morphological, compositional and chemical properties. It is found that the addition rate of Cu (II) has significant effects on the properties of the CZTS thin films. The CZTS film prepared using the optimized copper addition rate (0.15 ml/min) shows pure kesterite phase, Cu-poor and Zn-rich composition, compact morphology and good band gap ∼1.45 eV. Solar cells using the structure glass/Mo/CZTS/CdS/i-ZnO/ZnO:Al achieves a respectable external quantum efficiency and solar cell efficiency.

A 5.1% efficient kesterite Cu2ZnSnS4 (CZTS) thin film solar cell prepared using modified sulfurization process

AbstractCu2ZnSnS4 (CZTS) absorber thin films are prepared by sulfurization of sputtered Cu/Sn/Zn (CZT) stacked metallic precursor. The modified sulfurization process is adapted to prepare photovoltaic quality CZTS films. Specifically, sputtered CZT precursor films are sulfurized in sulfur powder contained graphite box using rapid thermal processing furnace at 580 °C for 10 min, in N2(95%) + H2S (5%) atmosphere. The Cu‐poor CZTS films with various Cu/(Zn+Sn) ratio are prepared by varying Cu layer deposition time. The effect of Cu/(Zn+Sn) ratio on the properties of CZTS films is investigated. The CZTS thin film solar cells with Cu/(Zn+Sn)=0.76 shows best conversion efficiency of 5.1% (Voc: 573 mV, Jsc: 18.38 mA/cm2, FF: 0.48%, and active area: 0.31 cm2). (© 2015 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Characterization of Cu2ZnSnSe4 solar cells prepared from electrochemically co-deposited Cu–Zn–Sn alloy

Cu–Zn–Sn (CZT) precursors for Cu2ZnSnSe4 (CZTSe) solar cell were prepared by electrochemical co-deposition method with different metals composition. CZT precursors were preliminary annealed in three different atmospheres in order to obtain homogenous, without pores Cu–Zn–Sn layers. A high crystalline quality CZTSe absorber was synthesised as has been determined by X-ray diffraction and Raman spectroscopy methods. CZTSe based solar cells were fabricated and champion cell demonstrated 2.7% efficiency and reached as high as 70% of external quantum efficiency. Scanning electron microscope investigations of CZTSe solar cells cross-section revealed that Mo/CZTSe interface exhibits large voids and local delamination from Mo layer. The back contact issues are detrimental to solar cell performance reducing shunt resistance and open circuit voltage.

Growth and characterization of Cu2ZnSnS4 photovoltaic thin films by electrodeposition and sulfurization

Cu2ZnSnS4 (CZTS) thin films have been successfully prepared by sulfurization of Cu–Zn–Sn (CZT) alloy precursors. The CZT precursors were co-deposited on Mo-coated glass substrates via a one-step electrodeposition process. A comparative study of sulfurization process was carried out to optimize the kesterite CZTS phase quality. Morphological and structural analyses reveal a large grain microstructure and less secondary phases in the close-zone sulfurized thin film. The solar cell derived from the close-zone thin film shows the better conversion efficiency of 3.62%. Amounts of voids at the CZTS/Mo interface and the thick MoS2 layer is thought to be the root cause of the lower performance.

Electrosynthesis of CZTS films by sulfurization of CZT precursor: Effect of soft annealing treatment

In the present work, CZTS thin films have been prepared by sulfurization of electrodeposited Cu–Zn–Sn (CZT) precursor. Prior to sulfurization, as-deposited CZT precursors have been soft annealed in Ar atmosphere at different temperatures (250–350°C). The structural, morphological, compositional and optical properties of the films have been investigated in detail. It is found that, soft annealing temperature has a significant impact on the properties of CZTS thin films. The systematic study on the improvement in the properties of CZTS films using soft annealing route has been studied and discussed.

Annealing Time Dependence of Crystallization in Cu2ZnSnSe4Films

Cu2ZnSnSe4(CZTSe) films were formulated by the selenization of Cu(Zn, Sn) (CZT) metal precursors. The CZT precursor was prepared by depositing zinc onto a Cu-Sn layer co-sputtered on molybdenum-coated soda-lime glass substrates. Selenium was evaporated thermally onto the CZT precursor. The resulting CZTSe film was then annealed at 450 °C for various annealing times (1, 3, 5, 10 and 15 min) using a rapid thermal process (RTP). Crystallization of the CZTSe film as a function of the annealing time was observed by X-ray diffraction (XRD). The XRD patterns of the films annealed over 3 min were well matched to those of a CZTSe single crystal. The lattice parameters of the CZTSe films annealed over 5 min were a = 5.639 Å and c = 11.356 Å. These values were reasonably close to the lattice parameters of a single crystal of Cu2ZnSnSe4(a = 5.693 Å and c = 11.333 Å). The energy gap of the CZTSe films was 1.14 eV.

Characteristics of Cu2ZnSnSe4 Film Formed by Using Co-sputtered Precursors and Selenization

A Cu2ZnSnSe4 (CZTSe) film was formed by the selenization of Cu(Zn, Sn) (CZT) alloy precursors. The CZT precursor was prepared by depositing zinc onto a layer of Cu-Sn co-sputtered on molybdenum-coated soda-lime glass. Selenium was evaporated on the CZT precursor. The CZTSe film was then annealed for a minute at various substrate temperatures ranging from 350°C to 650°C in steps of 50°C in a rapid thermal process using tungsten halogen lamps. The lattice parameters of the CZTSe annealed at temperatures over 500°C were found to be a = 5.709 Å and c = 11.351 Å. We also found that the energy gap of the CZTSe was 1.137 eV, which was independent of the annealing temperature.

A two-step approach for electrochemical deposition of Cu–Zn–Sn and Se precursors for CZTSe solar cells

A two-step electrochemical route for fabrication of Cu–Zn–Sn (CZT) and Se precursors' layers for a thin film Cu2ZnSnSe4 (CZTSe) absorber for solar cells is demonstrated. The CZT was electrochemically co-deposited in a citrate solution and after that an appropriate amount of Se was electrodeposited on the top. The CZT+Se were annealed in Ar atmosphere using a slow or fast increase in temperature up to 500°C. SEM with EDX, XRD and Raman spectroscopy examinations of precursors and of the manufactured CZTSe thin films were carried out. The XRD measurements indicated that the CZT precursor contained η-Cu6.26Sn5, Sn and γ-CuZn5 phases. The electrodeposited Se was polycrystalline with a hexagonal structure. The manufactured CZTSe in all the cases presented Cu2ZnSnSe4 with kesterite or partially disordered kesterite structure; however a purer CZTSe phase was formed using the fast increase in the annealing temperature.