Volume 11, Issue 9 e202300729
Review
Open Access

Current Status and Future Prospects of Kesterite Cu2ZnSn(S,Se)4 (CZTSSe) Thin Film Solar Cells Prepared via Electrochemical Deposition

Sun Kyung Hwang

Sun Kyung Hwang

Department of Materials Science and Engineering, Seoul National University, Seoul, 08826 Republic of Korea

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Joo Ho Yoon

Joo Ho Yoon

Department of Materials Science and Engineering, Seoul National University, Seoul, 08826 Republic of Korea

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Prof. Jin Young Kim

Corresponding Author

Prof. Jin Young Kim

Department of Materials Science and Engineering, Seoul National University, Seoul, 08826 Republic of Korea

Research Institute of Advanced Materials (RIAM), Seoul National University, Seoul, 08826 Republic of Korea

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First published: 09 February 2024

Graphical Abstract

This review explores strategies for improving Cu2ZnSn(S,Se)4 film quality and device performance, encompassing intrinsic control during the electrodeposition process and extrinsic treatment approaches. It also outlines prospects for electrochemically deposited Cu2ZnSn(S,Se)4 solar cells, emphasizing potential applications in tandem, flexible, and solar water-splitting devices. Technical insights into the electrochemical deposition process and future development perspectives are provided.

Abstract

The fabrication of kesterite Cu2ZnSn(S,Se)4 (CZTSSe) thin-film solar cells using the electrochemical deposition (ED), which is valued for its industrial feasibility, offers a cost-effective and environmentally friendly approach to the carbon-free and clean energy production. However, the reported power conversion efficiency of approximately 10 % for electrodeposited CZTSSe thin-film solar cells is lower compared to the alternative methods like sputtering and spin-coating, which is mainly attributed to the phase inhomogeneity and the rough morphology generated during the ED process. Ensuring the microscopic and macroscopic uniformity of the electrodeposited films is crucial for the improvement of the film quality and the device performances. In this review, strategies to address these challenges including the intrinsic film control such as the deposition mode, pH, concentration of metal ions, and complexing agents, as well as the extrinsic approaches such as doping, substitution of metal elements, and the introduction of interfacial layers. In addition, the prospects for the electrochemically deposited CZTSSe solar cells were presented, focusing on the promising applications in tandem, flexible, and solar water-splitting devices. Finally, this review will provide technical insights into the ED process for preparing CZTSSe solar cells, outlining a perspective for the future development of highly efficient CZTSSe thin film solar cells.

1 Introduction

To meet the increasing global energy demands to replace traditional fossil fuels, there is a rising interest in thin-film photovoltaic (PV) technology. Among various materials for the thin-film PVs, Cu2ZnSn(S,Se)4 (CZTSSe) is an earth-abundant and environmentally friendly version of CuInGaSe2 (CIGS), where expensive indium and gallium metal elements are substituted by cheap zinc and tin metal elements, respectively.1 Similar in structure to CIGS, CZTSSe is based on a device configuration of Mo/CZTSSe/CdS/i-ZnO/AZO/top contact electrode (Ni : Al). In Figure 1, material metrics of CZTSSe and CIGS indicate that kesterite CZTSSe exhibits significant potential for PV applications owing to its superior light absorption coefficient of approximately 104 cm−1, tunable direct bandgap, high stability, eco-friendliness, and low-cost.2 However, there is still a need for further research to enhance the performance of CZTSSe thin film solar cells due to the lower efficiency compared to CIGS.3

Details are in the caption following the image

(a) Schematic illustration of the typical CZTSSe device structure. Reproduced from ref.7b Copyright (2020), with permission from John Wiley & Sons, Ltd. (b) Unit cell of kesterite CZTS/Se structure. Blue, yellow, green, and orange balls represent Cu, Zn, Sn, and S/Se atoms, respectively. Reproduced from ref.2c Copyright (2018), with permission from American Physical Society. (c) Material metrics of CZTSSe and CIGS thin-film solar cells. Reproduced from ref.3b Copyright (2023), with permission from Springer Nature.

Various methods such as pulsed laser deposition,4 sputtering,5 thermal evaporation,6 electrochemical deposition (ED),7 sol-gel processing,8 and spin-coating9 have so far been employed to prepare a CZTSSe absorber material. However, some techniques are occasionally impractical in the case of scaling them up to industrial standards although they prove effective on a laboratory scale. On the other hand, ED method represents an industrially feasible process to grow Cu−Zn-Sn (CZT) precursor films.10 For example, CZTSSe fabricated via a solution-based ED process can further maximize their economic and environmental benefits over vacuum-based approaches due to the efficient utilization of materials (>95 %) and large-scale production potential.11 In most cases, ED process excludes the utilization of hazardous chemicals, and the electrolytic bath can be recycled over an extended duration. The detailed features of ED process for fabricating CZTSSe films are summarized through a SWOT analysis in Figure 2. The efficiency of CZTSSe thin-film solar cells reached an impressive record efficiency of 14.9 %, which was attributed to the precise control of the phase evolution in spin-coated Ag-alloyed CZTSSe. The resulting kesterite films exhibited high crystallinity with significantly reduced defects and minimized secondary phases.3a, 9c, 12 However, the highest reported power conversion efficiency (PCE) of the electrodeposited CZTSSe thin-film solar cells (approximately 10 %) is lower than that achieved by other processes,3a, 9c, 12 primarily due to the inhomogeneous phase distribution and the rough surfaces in the metal precursors prepared via the ED process.7a As a result, this leads to lower open-circuit voltage (VOC) and fill factor (FF), thereby restricting the performance of the electrodeposited CZTSSe thin-film solar cells.

Details are in the caption following the image

SWOT analysis of electrochemical deposition methods for fabricating CZTSSe precursor films.

The prerequisite for the ED process is to ensure the uniform lateral distribution of elements, maintaining consistency at both the microscopic aspect (the morphology of the electrodeposited materials) and the macroscopic aspect (thickness/composition across the substrate area).2b, 13 Microscopic uniformity is predominantly influenced by the applied deposition potential and interactions occurring between solution and substrate. Meanwhile, macroscopic uniformity can be enhanced by ensuring a uniform current through isotropic convection and improving mass transport via intrinsic/extrinsic treatment. Therefore, a precise optimization of deposition parameters (i. e., applied potential/current, pH, concentrations of metal ions, complexing agents, etc.) needs to be explored to ensure microscopic and macroscopic control during the electrodeposition process. In addition, various treatment methods, such as doping, substitution of metal elements, and the introduction of interfacial layers in the electrodeposited CZTSSe thin films, can contribute to addressing these challenges. This review paper aims to explain each parameter and its potential influence on the film growth. Moreover, the efficient CZTSSe thin film solar cells prepared by the ED process can be developed in diverse applications such as tandem, flexible, and solar water-splitting devices. Therefore, we will discuss the technical strategies for addressing the issues mentioned earlier in CZTSSe solar cells produced via ED and explore their applications. Finally, we will provide an outlook for the future development of highly efficient CZTSSe thin film solar cells using the ED process.

2 Principles of electrochemical deposition

Figure 3a illustrates a typical electrochemical cell system involving the nucleation and growth of films in aqueous electrolytes.14 The ED process takes place in the vicinity of the electrode surface, particularly within the nanoscale electric double layer (EDL). A diffusion layer, known as a mass transport boundary layer, is formed close to the electrode surface with its thickness depending on the viscosity of the solution and stirring rate. The movement of species towards the electrode surface is regulated by diffusion.15 In this process, cationic deposition species undergo reversible interconversion within both the diffusion layer and bulk electrolyte, establishing dynamic equilibrium conditions through hydrated complexes. Consequently, an EDL emerges at the electrode surface, where adsorbed water molecules and positively charged species are affixed. Within the EDL, metal complexes engage in electron transfer reactions, leading to discharge at the electrode surface. This results in the nucleation of species on the electrode surface, ultimately growing to deposit films.

Details are in the caption following the image

(a) Schematic of electrical double layer (EDL) and electrochemical deposition process. Reproduced from ref.14 Copyright (2021), with permission from John Wiley & Sons, Ltd. (b) Electrical circuit of a typical three-electrode cell system. (c) Photograph and SEM images of PbI2 layer with large area of 30.25 cm2 prepared by spin coating and electrodeposition methods. (d) Cell performance comparison between spin coating and electrodeposition. Reproduced from ref. [17] Copyright (2015), with permission from Elsevier. (e) Electrodeposition approach used to form kesterite precursors with stacked elemental layer (SEL) and co-electrodeposition, and (f) Cyclic voltammetry (CV) curve on Mo substrate in the CZT precursor solution. Reproduced from ref.7d Copyright (2013), with permission from Elsevier.

In the electrochemical process, a three-electrode cell configuration is commonly employed, allowing for accurate potential control of the working electrode (WE), where cationic metal species are reduced and deposited.16 Figure 3b illustrates the electrical circuit of a typical three-electrode cell system comprising a WE, counter electrode (CE), and reference electrode (RE). In this setup, the applied voltage of the WE are controlled relative to the RE, and the current between the WE and the CE is measured. The RE maintains a constant voltage, as current does not flow through it. On the contrary, the two-electrode cell configuration consists of the WE and the CE, with CE also serving the role of the RE. In this case, the current between two electrodes is measured while a voltage is applied. Therefore, the accuracy of applied voltage control is compromised due to the current flow through the CE.16

In recent, ED has been employed not only for CZTSSe but also for the fabrication of various thin-film devices (e. g., CdTe, CIGS, and perovskite). Chen et al. utilized the ED technique to produce high-quality perovskite thin films, comparing it with the spin coating process.17 As evident in Figures 3c and 3d, the ED process exhibits higher uniformity and yields films with superior crystallinity compared to spin coating. It is distinct on large-area substrates (5.5×5.5 cm2). Compared with the spin coating technique, the ED method demonstrates a higher average PCE and better reproducibility in PCE of large-area devices. Furthermore, the ED process is characterized by the absence of limitations related to substrate shapes or forms. Similarly, CZTSSe devices with an efficiency of 8.2 % were achieved using large-area electrodeposited metallic precursors on a pre-industrial scale.18 A stable metallic stack element layer (SEL), exhibiting impressive lateral uniformities over 15×15 cm2 area substrate, is deposited by high-speed stack electrodeposition followed by annealing under an air atmosphere. These studies highlight the significant potential of ED for a reproducible and industry-compatible process.

The SEL approach involves stacking individual metal materials (i. e., Cu, Zn, and Sn).18, 19 Since each metal is deposited separately, it is easy to control the stoichiometry of films, resulting in a high-quality and dense film.20 Therefore, the SEL approach has been employed in the fabrication process of large-area CZTSSe films.18 However, it requires multiple baths for each metal and is prone to in-depth inhomogeneity, necessitating procedures to address the interdiffusion or mixing of metal elements.21 Another approach used for fabricating metallic precursors is co-electrodeposition. All constituents can be deposited in a one-step process, which is conducted at constant deposition potential in a single bath containing Cu, Zn, Sn metal salts, and complexing agents.7a, 7b, 22 Since Co-electrodeposition has a complex chemistry, the need for accurate control of ion concentration is required.20a In addition, a large discrepancy in the standard reduction potentials and kinetics of each element poses a significant challenge to co-deposit a precursor film with a desired Cu-poor and Zn-rich stoichiometry. The reduction reactions for Cu, Zn, and Sn can be expressed as follows:23
(1)
(2)
(3)

Where, E, R, T, and F represent for the reduction potential, the ideal gas constant, temperature, and the Faraday constant, respectively. The reduction tendency occurs in the order of Cu, Sn, and Zn due to differences in reduction potentials. To decrease the large difference of reduction potentials in co-electrodeposition, a complexing agent is introduced and controlled. Further details on this will be explained below.

Cyclic voltammetry (CV) measurements are conducted to understand the specific redox system and to determine the appropriate potential for high-quality films. The process initiates at a specified potential V1 and progresses to another potential V2 at a consistent scan rate. Upon reaching potential V2, it reverses back to the initial potential V1. Throughout this potential sweeping, which can be iterated, the current is recorded. In a reversible system, the forward scan may induce electrolyte oxidation, while the reverse scan prompts the reverse reaction (reduction) or vice versa. The anticipated graph has a similar appearance in both forward and reverse directions. Figure 3f illustrates an example of a CV curve on Mo substrate in the CZT precursor solution, which is not perfectly symmetric in practical ED reactions since the material is consumed by depositing on the electrode.7d The dashed lines correspond to the reduction potential of Cu, Sn and Zn, respectively. It is worth noting that a potential exceeding −1.3 V can lead to hydrogen evolution. Therefore, a certain potential to reduce all metal elements in the electrolyte should be decided within a stable range.

3 Recent progress of the electrodeposited CZTSSe

Figure 4 and Table 1 present representative CZTS(Se) thin-film solar cells fabricated through the ED process, including sequential ED and co-electrodeposition. For the stacked metallic precursor films, secondary phases and non-uniform element distribution are significantly generated during the CZTSSe formation process due to the different diffusion rates and expansion coefficients of Cu, Zn, and Sn elements. Therefore, Ccontrolling the distribution of elements and minimizing secondary phases is crucial for enhancing the efficiency of electrodeposited PV devices. Qin et al. developed the strategy of controlling precursor from binary-ternary selenide to quaternary selenide in the sequentially electrodeposited CZTSSe solar cells.12 In this study, they used a metal sequence of Cu/Cu−Zn/Sn layer. Figure 4a shows the irregular morphology and inhomogeneous phase for the binary-ternary selenide precursor film (labeled as S-1-pre in Figure 4a). This binary-ternary selenide precursor includes SnSe, CuSe, ZnSe, and CTSe phases, while the quaternary selenide precursor consists of a pure CZTSe phase. The resulting CZTSSe film from the binary-ternary selenide precursor displays ZnSe secondary phases and CuSn deep defects. In contrast, the quaternary selenide precursor facilitates the formation of VCu defects, leading to improved photovoltaic performance in CZTSSe thin film solar cells. Finally, they achieved the highest efficiency of 10.03 % (with an active area efficiency of 10.9 %) by utilizing the quaternary selenide precursor film for the sequentially electrodeposited CZTSSe solar cells. Another group (Zhang et al.) introduced a Cu−Ge layer by electrodeposition at the bottom of the Cu/Sn/Zn metal stack, allowing for the doping of Ge into CZTSe films.24 The incorporation of Ge at the bottom of the film is observed to facilitate the downward diffusion of Sn. This leads to a more uniform distribution of Sn during the annealing process, preventing the formation of undesirable defect clusters and promoting a grain growth of CZTSe, as shown in Figure 4b. The Sn2+/Sn4+ peak ratio of the CZTSe films with Cu–Ge layer is decreased, which supports that the formation of SnZn2+ antisite defects is inhibited after Ge doping (Figure 4c). In addition, it influences the surface potential of CZTSe thin films, optimizing the band alignment at the CdS/CZTSe interface. The device performance of CZTSe thin-film solar cells with Ge doping is boosted from 6.74 % to 10.54 %.

Details are in the caption following the image

(a~c) Summary of high-efficiency CZTSSe solar cells fabricated via SEL ED process. (a) J-V curve of a 10.03 % efficient CZTSSe solar cells by modifying precursor and their top-view SEM images of two different kinds of precursor films (binary-ternary selenide vs. quaternary selenide). Reproduced from ref.12 Copyright (2022), with permission from Elsevier. (b) Effects of Ge doping on the film morphology and device performance in the electrodeposited CZTSSe thin film solar cells. (c) XPS spectra and schematic illustrations of the CZTSSe thin film showing the inhibition mechanism of SnZn deep level defects via Ge doping. Reproduced from ref.24 Copyright (2020), with permission from John Wiley & Sons, Ltd. (d, e) Representative CZTSSe solar cells fabricated via the Co-ED process. (d) J-V curve of a 9.9 % efficient CZTSSe solar cells via compositional and interfacial modification. (e) Cross-sectional TEM image and compositional depth profiles of the modified CZTSSe-based thin-film solar cell. Reproduced from ref.7c Copyright (2016), with permission from John Wiley & Sons, Ltd.

Table 1. Comparison of representative CZTSSe thin-film solar cells fabricated by the electrodeposition process.

Deposition method

Approach

JSC

(mA⋅cm−2)

VOC

(mV)

FF

(%)

Efficiency

(%)

SEL

Ge doping24

37.16

424

66.87

10.54[a]

SEL

Precursor phase

structure12

34.2

472

62.20

10.9[a]

10.03[b]

Co-ED

Band gap

engineering25

33.7

480

61.00

9.9[a]

Co-ED

Cs doping13

38.55

460

57.52

10.20[a]

  • [a] Active area. [b] Total area.

In the case of the co-electrodeposition process, Seo et al. demonstrated efficient CZTSSe thin film solar cells with an efficiency of 9.9 % via compositional and interfacial modifications by controlling the overall and directional composition of sulfur (S) and selenium (Se) in Figure 4d.7c The co-electrodeposition process was carried out from an aqueous solution containing Cu, Zn, and Sn cations along with tri-sodium citrate as a complexing agent, at a constant current density of −1.18 mA/cm2. The CZT precursor films obtained through co-deposition comprised Cu−Zn, Cu−Sn, and Sn phases. Subsequently, a consecutive thermal annealing process was performed in a mixed S/Se atmosphere, with varying ratios from 0 % to 31 %. Since the bandgap has a trade-off effect on the photovoltage and photocurrent, authors manipulate the CZTSSe thin film solar cells with an intermediate bandgap of 1.14 eV. Additionally, the device performance can be improved by modifying the top surface of the CZTSSe thin film (i. e., near the p-n junction area) with an additional supply of S ions (Figure 4e). This enhancement can be attributed to a reduction in dark current and interfacial recombination.

4 Deposition parameters

4.1 Electrodeposition mode

Figure 5a exhibits the polarization curve (j-E curve), which provide insights into metal deposition potentials and characterize the underlying electrochemical reactions. The polarization curve can be divided into four distinct regions. In the initial phase, the deposition of Cu cation occurs within the electrochemical potential range of −0.2 V to −0.4 V.7a The reduction of Sn commences at lower than the equilibrium potential of –0.65 V (vs. Ag/AgCl/KClsat), representing Cu+Snunderpotential deposition(UPD).7a, 26 Subsequently, within the potential range of –0.65 V to –1.1 V, the reduction of Sn2+ ions along with Cu2+ takes place. Upon applying a more negative potential, the reduction of Zn2+ takes place, initiating the deposition of the ternary Cu−Zn-Sn alloy. It is noticed that an abrupt increase of current has occurred as the potential becomes increasingly negative, which is due to the intensive hydrogen evolution reaction. This reaction manifests in the appearance of minuscule bubbles on the electrode surface, which affects the active area and film porosity, resulting in lateral inhomogeneity in the thickness and chemical composition of the CZT layer. Hence, meticulous consideration and control of the deposition potential are imperative for securing the macroscopic film morphology. Notably, the deposition potential must apply within the range of –1.1 V to –1.3 V to deposit the ternary CZT alloy.27 In addition, the composition of the CZT precursor can be tuned by varying the deposition potential. The increasing negative potentials within the potential window for the CZT precursor led to higher Zn content in CZT precursor films.

Figure 5b presents a comparative analysis of two distinct electrodeposition modes, namely chronopotentiometry and chronoamperometry, which are fundamental modes in electrodeposition processes.28 Chronopotentiometry, which can be described as galvanostatic mode, is a technique where the potential changes over time are observed while maintaining a constant current.[16] In the chronopotentiogram (V-t curve), distinct steps at relatively low potentials of approximately −0.75 V (vs. Ag/AgCl) was shown before stabilizing at the steady-state deposition potential of −1.2 V (vs. Ag/AgCl). This phenomenon is attributed to the varying reduction tendencies of individual metal cations, each with distinct reduction potentials, as a result of the continuous adjustment of the external potential to uphold a constant current during the deposition process. The CZT precursor film grown via galvanostatic mode exhibits numerous abnormally grown spherical protrusions with a diameter of approximately 1 μm, which is originated from the Sn out-growth. Sn electrodeposits often exhibit porous, coarse, non-adherent characteristics with the formation of structures such as whiskers, needles, and dendrites. Moreover, this uneven surface of the precursor film results in CZTSSe thin film with a highly rough surface after the annealing process.

Details are in the caption following the image

(a) Polarization curve of the Mo electrode in aqueous electrolyte solution with the composition of 23 mM CuSO4, 34 mM ZnSO4, 16 mM SnCl2, and 0.5 M Na3Cit, obtained by scan rate of 5 mV/s. (b) Comparison of two different kinds of electrodeposition modes (chronopotentiometry and chronoamperometry). Reproduced from ref.28 Copyright (2022), with permission from John Wiley & Sons, Ltd. (c) Schematic illustrations describing the formation mechanism of the sparse and dense nucleation. Reproduced from ref.7a Copyright (2019), with permission from American Chemical Society. (d) XRD patterns of precursor films prepared by galvanostatic and potentiostatic modes. Reproduced from ref.28 Copyright (2022), with permission from John Wiley & Sons, Ltd.

On the other hand, chronoamperometry, which can be described as potentiostatic mode, focuses on monitoring the current changes while maintaining a constant deposition potential throughout the electrodeposition. Upon applying a constant potential, a rapid change in the current density can be observed due to the diffusion-limited transport of metal cations from the electrolyte to the electrode surface. The CZT precursor films grown via the potentiostatic mode at −1.24 V (vs. Ag/AgCl) features uniform and even spherical alloys with smaller size. Given a sufficiently large fixed overpotential (−1.24 V vs. Ag/AgCl in this figure) for the reduction of all metal cation during the initial stage of deposition, nuclei are formed with the participation of all metal elements. Subsequently, growth proceeds on the existing electrodeposits, resulting in the formation of uniform and even precursor films. It is worth highlighting that this approach results in a smooth and compact surface for the CZTSSe thin films after the annealing process. Figure 5c illustrates that the consumed metal ions (i. e., Cu2+, Sn2+, and Zn2+) cause a concentration gradient in the initial step during electrodeposition, eventually influencing the roughness (i. e., sparse/dense) of precursor films. In Figure 5d, the X-ray diffraction (XRD) revealed that the precursor film grown via potentiostatic mode has smaller β-Sn peaks, which supports the reduced Sn-outgrowth. During the sulfo-selenization process, the electrodeposited metal alloy precursor films (i. e., Cu6Sn5, Sn metal, and Cu5Zn8) undergo a series of phase evolution reactions;5b, 29 The Zn(S1-xSex) (ZnSSe) phase initially forms typically at approximately 300 °C, which was followed by the formation of the Cu2Sn(S1-xSex)3 (CTSSe) phase from the reaction between Cu2(S1-xSex) and Sn(S1-xSex) phases at temperatures close to 400 °C. Then, the reaction occurs between CTSSe and ZnSSe at approximately 480~560 °C, leading to the formation of the final CZTSSe phase. It is noticeable that the morphological and/or compositional inhomogeneity of the initial precursor film has significant influence not only on the phase evolution during the annealing process, but also on the morphology of the final CZTSSe thin films as discussed in this section. This clearly demonstrates the ability to control the morphology of the film by using a proper deposition mode.28 Therefore, the potentiostatic mode was primarily employed to investigate the deposition mechanisms and morphology formation. Moreover, potentiostatic mode is more suitable for the co-electrodeposition.

4.2 Electrodeposition overpotential

One of the primary parameters in electrodeposition is the applied deposition potential or current, which directly influences nucleation and growth rates, thereby affecting the morphology and chemical composition of films. This is because the rate of electron transfer from the electrode to electrodeposits is significantly related to the applied overpotential to the WE, which is defined as the difference between the reduction potential and the applied potential.14 Figure 6a shows the effects of nucleation and growth of films depending on deposition overpotentials. As selected reports, Cheon et al. demonstrated the strategy for controlling the CZT nucleation and film growth in galvanostatic mode by applying an initial nucleation stage with higher deposition current density prior to the steady-state deposition step with a lower current density.7a The deposited CZT precursor showed a decreasing size and was more compact as the duration of the nucleation step increased from 0 to 20 s (Figure 6b). Figure 6c illustrates the evolution mechanism of the rough surface in CZT precursor film without the initial nucleation time. At stage A, a dense Cu-rich thin layer consisting of Cu−Sn alloy such as Cu3Sn is deposited until the reaction rate is restricted by the diffusion of Cu2+ cations from the electrolyte to electrode surface. Subsequently, in stage B, a new Cu−Sn nuclei with higher Sn contents, such as Cu6Sn5 phase, was formed on top of the Cu-rich thin layer due to the low concentration of Cu2+ near electrode surface. It is the main reason for the rough and nonuniform morphology of precursor films since the newly formed Cu−Sn nuclei with high Sn composition exhibit large and sparse features and act as seeds for subsequent deposition growth (stage C). However, adopting the initial nucleation stage with a higher deposition current density during the electrodeposition process in galvanostatic mode suppressed the formation of abnormally grown large bumps. As shown in Figure 6d, improved photovoltaic performance is observed, especially in VOC and FF, in the CZTSSe thin film solar cells with reduced roughness due to the decreased interfacial recombination sites.

Details are in the caption following the image

(a) Voltage-time transient curves of ED process and schematic illustrations of the nucleation and growth of films depending on deposition overpotentials. Reproduced from ref.14 Copyright (2021), with permission from John Wiley & Sons, Ltd. (b) Top-view and cross-sectional SEM images of CZT precursor films with the initial nucleation time (ti) of 0, 10, and 20 s respectively. (c) Schematic illustration describing the evolution mechanism of the rough surface in CZT precursor film without the initial nucleation time, prepared by galvanostatic mode. (d) J-V curves of rough (i. e., ti=0 s) and smooth (ti=20s) CZTSSe devices. Reproduced from ref. [7a] Copyright (2019), with permission from American Chemical Society. (e) XRD spectra of CZT precursor films electrodeposited at different potentials ranging from −1.15 to −1.225 V. (f and g) Top-view SEM images of electrodeposited CZT precursors and CZTSSe thin films obtained at different potentials. Reproduced from ref.22e Copyright (2020), with permission from Elsevier.

Similar behavior was observed by Valdés et al., where the surface morphology of CZT precursor and CZTSSe films was dependent on the applied deposition potential.22e In Figure 6e, the CuSn bimetallic phase and Cu6Sn5 phase is mainly observed in the precursor films at low electrodeposition potentials of −1.15 and −1.175 V. In contrast, peak shifts to the Cu−Zn alloy, for example Cu5Zn8 phase, is observed in the precursor films at relative high potentials of −1.2 and −1.225 V while peak intensity of the CuSn bimetallic phase and Cu6Sn5 phase is reduced. Figure 6f and g display the top-view SEM images of electrodeposited CZT precursors and CZTSSe thin films obtained at different deposition potentials. The reduction in grain size can be attributed to the increasing cathode polarization, leading to an increase in the nucleation rate. CZTSSe films prepared at low potentials of −1.175 V have a lot of pinholes, which could be generated by the partial evaporation of volatile compounds such as SnSe2 during the annealing process. In contrast, the resulting CZTSSe films prepared at −1.2 V present a uniform and homogeneous grain size. Adjusting the deposition potential or current during the electrodeposition process allows for the control of the nucleation rate of the alloy, leading to electrodeposits with favorable adhesion, controlled morphology, and desired quantity.

4.3 Other parameters (pH, concentration of metal ions, and complexing agent)

The electrodeposition experiment offers the flexibility to manipulate various parameters apart from the deposition mode and potential, including pH value, material concentrations, and complexing agents. In this section, each parameter and its potential impact on the film growth will be briefly elucidated. The pH of the electrolyte can influence deposition conditions by leading to the formation of different metal complexes at various pH values. Moreover, as the pH of electrolytes changes, a shift in the rate of hydrogen evolution is observed at the WE. Since Zn2+ possesses the most negative reduction potential, the deposition of Zn is typically impeded by the hydrogen evolution reaction. Agasti et al. demonstrated the effect of electrolyte pH and hydrogen evolution on the morphology and composition of CZT precursor film and CZTS film prepared via co-electrodeposition.30 Figure. 7a shows j-t curve for samples prepared with electrolyte pH ranging from 4 to 8. Unlike curves for pH 6, 7, and 8, an irregular fluctuation in current density values is observed for pH 4 and 5. These fluctuations can be attributed to hydrogen evolution taking place at the working electrode. As hydrogen bubbles are released from the surface of the working electrode during electrodeposition, new nucleation sites are formed, leading to an instantaneous increase in current density.30 The intense hydrogen evolution results in some areas of the films at pH of 4 and 5 peeling off from the substrate, as evidenced in the SEM images in Fig. 7b. As a result, CZTS films deposited with electrolyte pH values of 6 and 7 exhibited the formation of dense and uniform kesterite CZTS thin films without any secondary phase (Figure 7b and c).

Details are in the caption following the image

(a) Chronoamperogram (j-t curve) with different pH conditions ranging from 4 to 8. (b) Analysis of CV curves and morphology control Effect of electrolyte pH during co-electrodeposition. (c) XRD patterns for CZTS films with electrolyte having pH 6~8. Reproduced from ref.30 Copyright (2017), with permission from Springer Nature.

In electrodeposition, the impact of the concentration of metal ions in the electrolyte is evident that the metal content within the film increases proportionally with the concentration of metal ions by influencing the mass transfer process. Hreid et al. demonstrated the correlation between the concentration of metal ions and the composition of alloy phases in the CZT precursor film prepared via the co-electrodeposition process.31 In Figure 8a, the j-t curves depending on different metal ion concentrations exhibit distinct changes in the surface properties, specifically in the interface between the electrolyte solution and the deposited metal film (i. e., WE) during electrodeposition. To indicate solutions with different concentrations, Cu2+, Zn2+, and Sn2+ were labeled as A, B, and C, respectively. When changing the concentration of Cu2+ ions (A1~A5), the j-t curves for all cases follow a similar pattern, with the cathodic current stabilizing after 50 s.

Details are in the caption following the image

Effects of concentration of metal ions (Cu2+, Zn2+, and Sn2+) on deposition and morphology properties of CZT films (a) Chronoamperogram (j-t curves) and (b) Top-view SEM images with different concentrations of Cu2+, Zn2+, and Sn2+ (labeled as A, B, and C respectively). Reproduced from ref.31 Copyright (2015), with permission from RSC Publishing.

This suggests a consistent surface area of the films throughout the deposition, aligning with the similar morphologies observed in films with different concentrations of Cu2+ (Figure 8b). In addition, the concentration of Cu can influence the formation of Cu6Sn5 and Cu5Zn8 alloy phases and thus control their contents in the films. In the case of Zn metal ions, the slight increase of cathodic current in the plateau region is observed with increasing concentrations of Zn2+ ions, which suggests an expanded surface area of deposited alloy in precursor films. It is consistent with the slightly rougher film morphology observed in the top-view SEM image (B2 and B5 in Figure 8b). Interestingly, a convex peak in the early time between 30 s and 100 s is observed, and the intensity of the peak increases with the rise in Zn concentration. This phenomenon is explained by the requirement of sufficient Cu−Sn presence on the Mo surface for the reduction of Zn2+ to take place. In the case of Sn metal ions, the rapid increase of cathodic current at the plateau region with an increase in Sn2+ concentration is observed due to the increased surface area of the films by depositing Sn alloy with porous and large nucleation size. Consistent with earlier studies, Sn has a significant influence on the morphology of precursor films.7a

A complexing agent holds significance in the electrodeposition process as it works to reduce the variation in reduction potentials among different metal ions and enhance the solubility of metal ions. Moreover, it aids in decreasing the grain size in the deposits, ultimately improving the homogeneity and surface roughness of the precursor film.22a, 32 To investigate the effects of complexing agents in the electrodeposition process, Mkawi et al. used different complexing agents such as EDTA, tartaric acid, and trisodium citrate in an aqueous electrolyte, as illustrated in Figure 9.33 The discrepancy in reduction peaks between Zn and Cu is approximately 0.5 V in the absence of a complexing agent. With the addition of EDTA as a complexing agent, reduction peaks of Zn, Sn, and Cu shift to less negative potential, and the difference between the reduction potential of Zn and Cu is reduced to 0.35 V. When tartaric acid is utilized as a complexing agent, the difference between the reduction peaks of Zn and Cu is 0.45 V. In contrast, trisodium citrate complexing agents have the lowest difference of only 0.25 V and exhibit a distinct reduction peak intensity of each metal ion, suggesting a superior complexing effect compared to other complexing agents (EDTA and tartaric acid). The morphology of the CZTS thin film after annealing process is significantly improved in the CZTS thin film with trisodium citrate complexing agent (Figure 9b). Moreover, many other complexing agents including glutamic acid, glycine, potassium thiocyanate, sodium sulfate, and sodium thiocyanate have been explored in addition to sodium citrate, tartaric acid, and EDTA or in combination with them22a, 32, 34. Therefore, the proper addition of complexing agents in an electrolyte solution can effectively control the morphology by forming a homogenous and compact film.

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Effect of complexing agents on the electrodeposition for CZT precursor films (a) CV curves and (b) Top-view SEM images for co-electrodeposition without and with complexing agents of EDTA, tartaric acid, and trisodium citrate, respectively. Reproduced from ref.33 Copyright (2014), with permission from Elsevier.

5 Materials properties

5.1 Doping

The incorporation of extrinsic elements in small quantities into the kesterite matrix, known as doping, has the potential to improve the composition homogeneity and film crystallinity.24, 35 This enhancement is achieved through the promotion of grain growth and the simultaneous suppression of secondary phases and defects.36 The doping strategy has contributed to the development of various thin-film solar cells including CIGS, CdTe, and CZTSSe solar cells.37 In recent, the performance of CIGS thin-film solar cells has been consistently enhanced beyond 23 % through the doping of several metal elements.37a, 38 Therefore, numerous studies have been undertaken regarding doping in thin-film CZTSSe solar cells, even those produced using the electrodeposition method.13, 39 As selected reports, Mkawi et al. investigated the influence of metal dopants such as Li, Na, Cr, and Sb on CZTS thin films fabricated by the electrodeposition process.35b Figure 10a displays top-view SEM images of CZTS thin films doped with Li, Cr, Sb, and Na. It is evident that CZTS thin films with Na doping exhibit a superior morphology characterized by large grains and an absence of pinholes, in contrast to films doped with other metals. This is ascribed to the facilitated grain growth by the diffusion of sodium throughout the thin films. Moreover, Na dopants can distribute adequately and have a uniform impact throughout the depth of the film. Therefore, the improvement in crystallinity and photovoltaic performance was confirmed after doping, as shown in Figures 10b and c.

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(a) Top-view SEM images, (b) XRD patterns of CZTS thin films, and (c) J-V curves of CZTS thin film solar cells fabricated via electrodeposition doped with different metals of Li, Na, Sb, and Cr. Reproduced from ref.35b Copyright (2020), with permission from John Wiley & Sons, Ltd. (d) Top-view SEM images, (e) Surface contact potential difference (CPD) images and surface potential line profiles of CZTSe thin films with and without Ge doping. (f) Box plots of device performance of CZTSe devices with and without Ge doping. Reproduced from ref.24 Copyright (2020), with permission from John Wiley & Sons, Ltd. (g) Top-view and cross-sectional SEM images corresponding surface potential images of CZTSSe thin films with and without CsF treatment. (h) J-V curves of CZTSSe thin film solar cells with and without CsF treatment. (i) Schematic of tandem solar cells consisting of mechanically stacked 4-terminal perovskite/CZTSSe structures, with the CZTSSe device treated with CsF serving as the bottom cell. Reproduced with permission. Reproduced from ref.13 Copyright (2020), with permission from John Wiley & Sons, Ltd.

Another research group investigated the impact of germanium (Ge) doping in CZTSe thin-film solar cells (Figures 10d~f).24 The surface morphology exhibits a notable enhancement, displaying a compact structure and large grains. This improvement results from the facilitated grain growth and crystallization of CZTSe thin films through Ge doping, aligning with findings from previous studies.35d, 40 In addition, Ge doping promotes the diffusion of Sn element during the annealing process, leading to a homogeneous Sn element distribution and the suppression of undesirable Sn-related defects in films. In surface potential mapping images of CZTSe thin films with and without Ge doping (Figure 10e), there is a decrease in potential variations and work function observed in the CZTSe thin film with Ge doping. This indicates that the introduction of Ge into CZTSe thin films has an influence on the surface potential and work function of the samples, leading to the band alignment of the CdS/CZTSe interface.24 Combined with these effects, the device performance of CZTSe thin film solar cells with Ge doping is significantly enhanced, as can be seen in Figure 10f.

Hwang et al. observed a similar doping effect in CZTSSe thin-film solar cells, that utilized the CsF treatment in the electrodeposited CZT precursor (Figures 10g~i).13 The CZTSSe thin film with Cs doping has larger grains and fewer secondary phases compared to that without Cs doping. This can be attributed to the lower melting point of the Cs−Se phase. It helps other elements transport/diffusion and facilitates grain growth since Cs−Se acts as a fluxing agent during the annealing process. This results in reduced grain boundaries and carrier recombination, leading to the improved carrier collection and enhanced electronic device properties. In addition, Cs doping contributes to a more uniform distribution of surface potential, which indicates phase homogeneity by reducing the secondary phases and defects. In this study, a significant performance enhancement from 8.38 % to 10.20 % was achieved by employing Cs doping. Interestingly, authors applied the CZTSSe thin-film solar cells with Cs doping to the mechanically stacked perovskite/CZTSSe tandem solar cell by using a semi-transparent perovskite top cell (Figures 10i). The Cs doping shows comparable effects in the filtered CZTSSe bottom cell, suggesting that the Cs doping will likewise enhance the performance of the 4-terminal tandems. As a result, authors demonstrated the highly efficient mechanically stacked perovskite/CZTSSe tandem solar cell with a record efficiency of 23.01 %.

5.2 Substitution of metal elements

Another treatment method is the substitution of metal elements in the CZTS(Se) structure to suppress the formation of defect clusters and secondary phases.36, 41 The substitution of Sn with Ge, forming Sn-free Cu2ZnGeSe4 (CZGSe), was recently carried out by several groups.42 This substitution is aimed at eliminating adverse issues associated with Sn since Ge is more likely to exhibit a +4 oxidation state compared to Sn.35d, 42c This helps prevent the occurrence of potentially harmful +2 oxidation states. In addition, it can increase the optical bandgap and thus expand their application such as tandem structure. The preparation of CZGSe thin film solar cells by using electrodeposition process poses challenges due to the poor solubility of Ge-based salts. However, Liu et al. recently demonstrated CZGSe thin film solar cells by developing a synchronous strategy involving Ge incorporation and selenization, coupled with the electrodeposition process (Figures 11a~d).42a In detail, authors conducted two different approaches of two-step process (i. e., Ge incorporation and selenization) and one-step process (i. e., Co−Ge selenization), following the electrodeposition of a Cu−Zn layer. In this study, one-step process is more suitable for electrodeposited CZGSe thin films due to the uniform well-compact morphology and the better crystallinity, as shown in Figures 11a and b. In addition, CZGSe thin films prepared by Co−Ge selenization have an upward band bending at grain boundaries, which minimizes the recombination of collected electrons and predominating acceptor CuZn defects, leading to improved carrier separation (Figure 11c). With these positive effects, authors first achieved the highest efficiency of 3.69 % for the electrodeposited CZGSe solar cells (Figure 11d), which paved the way for the development of green electrodeposited CZGSe solar cells.

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(a) Schematic illustration for the fabrication process of CZGSe absorber involving electrodeposition and one/two-step annealing process, and SEM images of the resulting CZGSe thin films. (b) XRD patterns of CZGSe thin films with two-step (labeled as A) and one step (labeled as B) process. (c) Comparison of surface potential properties of CZGSe−A and B. (d) J-V curves of CZGSe−A and B. Reproduced from ref.42a Copyright (2023), with permission from RSC Publishing. (e) CMTS crystal structure. Reproduced from ref.43 Copyright (2016), with permission from Elsevier. (f) XRD patterns of CMTS thin films with different deposition time of Cu−Sn alloy. (g) J-V curves of CMTS thin film solar cells. Reproduced from ref.44 Copyright (2018), with permission from Elsevier.

The substitution of Zn with larger-sized Mn cations addresses issues associated with Cu/Zn disorders, such as CuZn and ZnCu antisite defects, and mitigates band tailing.41a Cu2MnSnS4 (CMTS) exhibits the stannite structure due to the lattice expansion (Figure 11e).43 Yu et al. presented an electrodeposited CMTS thin-film solar cells, where Zn is replaced by Mn, utilizing a two-step electrodeposition method.44 A two-step electrodeposition approach is for depositing Cu−Sn/MnO2 precursor films, involving the co-electrodeposition of a Cu−Sn alloy layer and the stacked electrodeposition of an MnO2 layer. A PCE of 0.76 % was achieved by optimizing the Cu−Sn alloy layer, showing the potential of an electrodeposited CMTS solar cells (Figures 11f and g).

5.3 Interface materials

In CZTSe thin films prepared by sequential electrodeposition with Cu/Sn/Zn metal stack, Cu is initially grown, and its morphology on the Mo surface directly influences the growth of subsequent stacked metal layers (Sn and Zn), impacting the overall quality of the CZTSe absorber layer.45 The dendritic-like clusters during the electrodeposition of the Cu layer on Mo are formed due to the 3D nucleation mechanism, leading to the undesirable diffusion of Cu elements (Figure 12a). This may result in issues such as uneven elemental distribution and significant interface recombination along with bulk recombination. Therefore, Han et al. introduced a Cu−Ge interfacial layer on the Mo substrate.45 The incorporation of a Cu−Ge interfacial layer results in the formation of a uniform Cu thin film without micro-sized clusters, contributing to improved CZTSe film quality characterized by low surface roughness, as can be seen in Figure 12b. This reduces activation energy and band gap difference, indicating lower recombination. Therefore, VOC-deficit decreases, resulting in a solar cell efficiency increased from 6.75 % to 9.54 % (Figure 12c). A functional Cu−Ge interfacial layer is crucial for high-quality CZTSe films, offering insights for electrodeposition of CZTSe thin films with stacked element layers. Introducing Ag interfacial layers has also been reported to contribute to the formation of flat and uniform Cu metal layers during the electrodeposition process,46 which was ascribed to the suppressed formation of dendritic Cu clusters.

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(a) Top-view SEM images of Mo/Cu and Mo/Cu−Ge/Cu layer. (b) SEM images and AFM images of CZTSe thin films with and without Cu−Ge buffer layer. (c) J-V curves of CZTSe thin film solar cells with and without Cu−Ge buffer layer. Reproduced from ref.45 Copyright (2021), with permission from American Chemical Society. (d) Top-view SEM images of CZTSSe, CZTSSe/CdS, and CZTSSe/In2S3/CdS thin films. (e) AFM images of CZTSSe thin films with different deposition time of In2S3 interfacial layer. (f) Schematic illustration of the evolution mechanism of the CdS layer on the CZTSSe thin films depending on the In2S3 interfacial layer. (g) J-V curves of CZTSSe thin film solar cells with and without the In2S3 interfacial layer. Reproduced from ref.47 Copyright (2021), with permission from John Wiley & Sons, Ltd.

Another approach is to incorporate an interfacial layer between the CZTSSe and CdS layers.47 Figure 12d illustrates the difference in the deposition results of CdS depending on the existence of an In2S3 interfacial layer. The CZTSSe thin films prepared via co-electrodeposition and subsequent annealing process have some physical defects such as pinholes and voids. The In2S3 interfacial layer contributes to the passivation of physical defects and the refined surface morphology by filling pinholes and voids. The nano-scale roughness on the CZTSSe grain surface generated by In2S3 deposition facilitates the formation of nuclei and increases the number of nucleation sites of CdS, potentially lowering the activation energy for heterogeneous nucleation (Figure 12e). This evolution mechanism of the CdS layer on CZTSSe thin films with and without the In2S3 interfacial layer is shown in Figure 12f. The incorporation of the In2S3 layer ensures the uniform growth of CdS layer, preventing the formation of macroscopic CdS agglomerates.47 This, in turn, improves the quality of the interface and reduces the overall device roughness. In Figure 12g, the improvement of CZTSSe device performance, especially VOC and FF, results from the introduction of the In2S3 interfacial layer. These findings hold significant promise for both individual CZTSSe solar cells and their integration into tandem solar cells.

6 New applications

6.1 Tandem devices

Multi-junction (tandem) solar cells use multiple semiconductors with different bandgaps to mitigate losses caused by carrier thermalization.28, 48 A top cell with a large bandgap in a double-junction tandem configuration absorbs high-energy photons and produces a high voltage while letting lower-energy photons pass through to be absorbed in a bottom cell with a smaller bandgap.49 Therefore, tandem solar cells can achieve higher efficiency compared to a single solar cell due to the effective utilization of photons.48b, 50 Figure 13a depicts plots of theoretical PCEs for double-junction tandem solar cells.48a The relationship between bandgaps of the bottom and top cell reveals that the maximum performance of approximately 44 % can be achieved by pairing a bottom cell with a bandgap of around 1.1 eV and a top cell with a bandgap ranging from 1.6 to 1.75 eV. CZTSSe is an appropriate photoactive material as a bottom cell of a double-junction tandem due to an adjustable bandgap in the low range of 1.0 to 1.5 eV.51 Furthermore, when combined with perovskite solar cells, CZTSSe thin-film solar cells provide advantages of having the lowest environmental effect and low-cost of power generation in comparison to other tandem combinations, as shown in Figure 13b.52 This encourages the development of perovskite/CZTSSe tandem solar cells.13, 53

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(a) Calculated theoretical efficiency limit for 2-terminal tandem solar cells. Reproduced from ref.48a Copyright (2018), with permission from Springer Nature. (b) Normalized environmental impact to produce different perovskite-based tandem solar cells. Reproduced from ref.52 Copyright (2018), with permission from Springer Nature. (c) Cross-sectional SEM image of the first demonstrated monolithic perovskite/CZTSSe tandem solar cells. Reproduced from ref.54 Copyright (2014), with permission from AIP Publishing. (d) Key challenges in the electrochemically deposited CZTSSe thin film solar cells for realizing the perovskite/CZTSSe tandem solar cell. (e) 3D AFM images of CZT precursor films and CZTSSe thin films, prepared by galvanostatic mode and potentiostatic mode, (f) Schematic illustration of roughness control strategy by using ion-milling process, and SEM images of CZTSSe solar cells before and after ion-milling. (g) Cross-sectional SEM images and (h) J-V curves of monolithic perovskite/CZTSSe tandem solar cells with a record efficiency of 17.5 %. Reproduced from ref.28 Copyright (2022), with permission from John Wiley & Sons, Ltd.

The first perovskite/CZTSSe monolithic tandem solar cells are demonstrated by Todorov et al. in 2014 (Figure 13c).54 However, it has an insufficient tandem performance of 4.6 %, which is much lower than those of individual CZTSSe or transparent sub-cells due to the unreliable contact between two sub-cells resulting from the surface roughness of the bottom cell. Ensuring a reliable electrical contact between two sub-cells is crucial for realizing the efficient tandem solar cells. To address this challenge, Hwang et al. developed the roughness-controlled strategy for the electrochemically deposited CZTSSe solar cells (Figures 13d~h).28 Authors confirmed that the rough surface of the CZTSSe bottom cell, which is the main reason for the short failure of perovskite/CZTSSe tandem, is attributed to the inherent roughness of CZTSSe film itself and the randomly generated CdS clusters with macro-scale size, as can be seen in Figure 13d. The surface roughness of the CZTSSe bottom cell is significantly decreased by adopting the roughness control strategy consisting of the potentiostatic mode and the ion-milling process. In Figure 13e, the film surface of the CZT precursor and CZTSSe became even and uniform, when prepared by the potentiostatic method. Moreover, the surface roughness of the CZTSSe bottom cell (ITO/i-ZnO/CdS/CZTSSe/Mo) can be reduced significantly by ion-milling process, which was uniform to prepare the solution processed perovskite top cells. Consequently, the perovskite/CZTSSe monolithic tandem has achieved an efficiency of 17.5 %, marking a new record for the highest efficiency of perovskite/CZTSSe monolithic tandem solar cells. In addition, this surpasses the record efficiency of single-junction CZTSSe solar cells. This result presents a significant breakthrough in the advancement of perovskite/CZTSSe monolithic tandem solar cells.

6.2 Flexible devices

Flexible CZTSSe thin film solar cells are one of the most promising future designs.3b, 6, 39b, 55 The electrodeposition can conveniently prepare large-area devices based on flexible metal substrates in a nonvacuum condition, which makes it possible to develop roll-to-roll growth lines.55c, 56 In order to obtain a precursor film on the flexible substrate, it is vertically placed as a cathode while the suitable deposition potential is applied to the electrodes to reduce metal cations (Figure 14a).55a In addition, the stacked element layer of Cu/Sn/Zn, prepared by sequentially electrodeposition process, is more preferred due to the ease of fabrication and control of the film composition. However, achieving high-quality absorber layers on flexible substrate through the electrodeposition method poses several challenges such as ionic adsorption or unexpected reactions, along with difficulties related to severe defects, uncontrollable grain growth, and unoptimized interfaces. In 2021, Liu et al. achieved a device performance of 6.33 % for flexible CZTSe solar cells fabricated using the electrodeposition process. This was accomplished through the implementation of an in situ electrochemical treatment (ET) during the electrodeposition process. In detail, it was designed to introduce a MoOX layer as an effective intermediate layer to suppress the thick MoSe2 layer by electrochemically oxidizing the Mo foils in pulse chronocoulometric mode. In Figure 14a, top-view and cross-sectional SEM images depict the improved film quality of CZTSe without undesirable defects/secondary phases and the significantly decreased MoSe2 layer in the CZTSe thin films with the ET process.55a The photographs of Mo foil, electrodeposited CZT precursor film on Mo foil, and flexible CZTSe solar cells are shown, indicating that the ED process can be successfully conducted on a flexible substrate. With these effects, the efficiency of flexible CZTSe thin film solar cells increased from 4.21 % to 6.33 %, which results pave the way for developing the flexible CZTSe thin film solar cells prepared by the electrodeposition process.

Details are in the caption following the image

(a) Flexible CZTSSe thin-film solar cells prepared by green electrodeposition process with and without in situ ET process on Mo foil. Reproduced from ref.55a Copyright (2021), with permission from American Chemical Society. (b) Schematic illustration and Photo images of the electrodeposition process on Mo foil and the prepared bifacial stacked Cu/Sn/Zn precursor films on Mo foil. (c) SEM images for the front and back side of CZTSe thin films on Mo foils and Photo image of the fabricated bifacial CZTSe thin-film solar cells. (d) J-V curves for both sides of bifacial CZTSe thin-film solar cells. Reproduced from ref.57 Copyright (2023), with permission from Royal Society of Chemistry. (e) Photo images for application of flexible CZTSSe thin-film solar cells prepared by sputtering method on various substrates such as PET film, paper, and cloth. Reproduced from ref.59 Copyright (2020), with permission from American Chemical Society. (f) Photo image of CZTS transparent module produced by low-cost roll-to-roll module production, which is developed by crystalsol company. Reproduced from ref.60 Copyright (2020), with permission from IOP Sciences.

In 2023, Liu et al. first demonstrated bifacial flexible CZTSSe solar cells prepared via a facile electrodeposition process for advanced applications such as building-integrated photovoltaics (Figures 14b~d).57 Bifacial flexible CZTSSe solar cells are considerably attractive due to their advantage of high energy generation per area and multidirectional light irradiation, however, its technological manufacturing process is challenging to prepare high-quality films of both sides.55c To address this challenge, authors propose a symmetrical electrodeposition process for bifacial CZTSSe thin film solar cells. Utilizing the double-sided conductive properties of Mo foil, simultaneous electrochemical deposition on both sides was achieved. The deposition followed a sequence, with Cu, Sn, and Zn deposited sequentially at constant potentials of −0.35 V, −0.95 V, and −1.2 V, respectively. Ultimately, the double-sided metal stacked layers (Zn/Sn/Cu/Mo foil/Cu/Sn/Zn) were successfully fabricated, as shown in Figure 14b. To form double-sided absorber layer of CZTSe/Mo foil/CZTSe, the metal stacked precursor films were vertically placed in the graphite box and conducted selenization annealing process. In Figure 15c, a schematic illustration of bifacial absorber layers and SEM images for the front and back side of CZTSe thin films on Mo foils exhibit no distinct macroscopic difference (i. e., morphology and thickness) between both sides and densely compacted surface with a grain size exceeding 1 μm, indicating a high crystallinity of CZTSe films. This approach ensures the uniformity of bifacial flexible CZTSe solar cells while significantly reducing costs and time. The noteworthy achievement of high efficiencies, specifically 6.43 % and 6.20 % for both sides, marks a significant breakthrough in the development of electrodeposited bifacial flexible CZTSSe solar cells, presenting an economical approach for eco-friendly material production (Figure 14d). In comparison to solution-based methods, the identified interface-suppressed VOC that limits efficiency provides a clear perspective for further enhancement. Therefore, the flexible CZTSe thin film solar cells prepared by the electrodeposition process need to be further explored through studies of flexible substrate properties, residual stress, electrodeposition parameters, and physics because the current record efficiency of Mo foil-based flexible CZTSSe thin film solar cells stands at 11.19 %.58 Moreover, various attempts should be made towards the practical application of flexible CZTSSe thin film solar cells, as shown in Figure 14e.59 Figure 14f displays the photo image of the CZTS transparent module produced by low-cost roll-to-roll module production, which is developed by Crystalsol company.60 This commercial application will expand the potential for future advanced flexible applications.

Details are in the caption following the image

Electrodeposited CZTS solar cells for water splitting system (a) Schematic illustration, energy band diagram, H2 evolution of Pt-In2S3/CZTS photoelectrochemical solar water splitting system, Reproduced from ref.62a Copyright (2023), with permission from Elsevier. (b) Cross-sectional SEM images of Pt/CdS/CZTS and Pt/In2S3/CdS/CZTS photocathodes before and after 3 h durability test. H2 and O2 evolution test of Pt/In2S3/CdS/CZTS-BiVO4 two electrode photoelectrochemical solar water splitting device. Reproduced from ref.62b Copyright (2015), with permission from American Chemical Society. (c) CZTS solar cell/BiVO4 tandem devices for unbiased solar water splitting application. Reproduced from ref.27 Copyright (2018), with permission from John Wiley & Sons, Ltd.

6.3 Photoelectrochemical water-splitting devices

Another possible future application for CZTS thin film solar cells could be as electrodes in photoelectrochemical water-splitting systems.61 Utilizing CZTS photoactive materials, which are abundant and cost-effective, for the photoelectrochemical water-splitting system could help the cost down, particularly when considering large-scale hydrogen production.27, 62 To demonstrate the CZTS solar cells′ effectiveness as a photocathode, many investigations of the electrodeposited CZTS solar cells for a water-splitting system were carried out, as shown in Figure 15. Tanaka et al. employed a Pt-In2S3/CZTS photoelectrode and validated the effective charge separation (h+ and e) on the CZTS under photo-irradiation, especially with the introduction of a thin SnO2 layer.62a In addition, the presence of Pt as a co-catalyst reduced the over-potential for H2 evolution, resulting in an enhancement in the activity for photoelectrochemical water splitting (Figure 15a). Jiang et al. used the electrodeposited CZTS thin films with a modified In2S3/CdS double layer and Pt deposits (Pt/In2S3/CdS/CZTS) as a photocathode for hydrogen production in water splitting system (Figure 15b).62b The Pt/In2S3/CdS/CZTS electrode exhibited a good coverage of film and a significantly elevated cathodic photocurrent. Furthermore, the incorporation of the In2S3 layer proved to be effective in stabilizing against degradation caused by the photo-corrosion of the CdS layer. A solar-to-hydrogen (STH) conversion efficiency of 0.28 % was successfully achieved using a simple two-electrode cell comprising the Pt/In2S3/CdS/CZTS photocathode and a BiVO4 photoanode. In recent, Jiang and co-workers advanced the design of the unbiased photoelectrochemical water-splitting system by developing the electrodeposited CZTS solar-BiVO4 tandem device for the first time (Figure 15c).27 The tandem cell demonstrates a stable STH conversion efficiency exceeding 1.46 % over extended reaction periods in an aqueous solution, maintaining stability against photo-corrosion. Following comprehensive optimizations and various attempts, CZTS solar cells as photoelectrodes will balance between cost, environmental considerations, and operational performance, making them suitable for practical applications in photoelectrochemical solar water splitting in the future.61h, 63

7 Summary and Outlook

The electrodeposited CZTSSe solar cells hold promising prospects for the future of solar energy technology and its practical applications. Continued advancements in electrodeposition techniques and materials engineering are expected to lead to the significant enhancement in the efficiency, stability, and cost-effectiveness of CZTSSe solar cells. Moreover, we should further explore ways to improve scalability and environmental sustainability in the electrodeposition processes, aiming to make CZTSSe solar cells more competitive within the renewable energy industry. Optimizations in film quality and interface engineering represent crucial focal points for further improvement. The development of methods to achieve high-quality absorber layers, mitigate defects, control grain growth, reduce surface morphology, and optimize interfaces will likely contribute to boosting the overall performance of CZTSSe solar cells. Furthermore, the various treatment strategies involving doping, substitution of metal cations, and introduction of interfacial layer for passivating defects and improving film phase homogeneity have been suggested in recent years, which have contributed to the rapid advancement of electrodeposited CZTSSe thin film solar cells.64 In the coming years, the anticipated future designs for electrodeposited CZTSSe solar cells, including multi-junction tandems, flexible, and utilization as photocathodes in water-splitting systems, highlight the considerable commercial promise of these solar cells in diverse applications. Via the effective optimization of the above-addressed challenges and additional investigations, the electrodeposited CZTSSe solar cells are expected to catch up with the record efficiency of CZTSSe and, potentially be commercialized like CIGS.

Acknowledgments

This work was supported by the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT (MSIT), Republic of Korea (No. 2021M3H4A1A03057403 and 2022M3J1A1063226).

    Conflict of interests

    The authors declare no conflict of interest.

    Biographical Information

    Sun Kyung Hwang is a Ph.D. candidate at Seoul National University, majoring in Materials Science and Engineering. She received her B.S. degree in the Department of Environmental Science from Hankuk University of Foreign Studies in 2018. Her research focuses on the development and characterization of thin-film solar cells including CZTSSe and tandem solar cells under the supervision of Prof. Jin Young Kim.

    Biographical Information

    Joo Ho Yoon is currently a Ph.D. candidate in Materials Science and Engineering in Seoul National University. He received a B.S. degree in School of Civil, Environmental and Architectural Engineering in Korea University in 2022. His research focuses on development of CZTSSe thin film solar cells and tandem solar cells under the supervision of Prof. Jin Young Kim.

    Biographical Information

    Jin Young Kim is an associate professor in the Department of Materials Science and Engineering at Seoul National University, South Korea. He received his B.S. (2000), M.S. (2002), and Ph.D. (2006) in Materials Science at Seoul National University. Before joining SNU, he worked at the National Renewable Energy Laboratory, USA (2007–2011), and Korea Institute of Science and Technology, Korea (2011–2015), on next-generation thin film solar cells. His current research interests include the fabrication of nanostructured electrodes via various approaches including electrochemical approaches and their applications in solar energy utilization including thin-film solar cells (e. g., CIGS, CZTS, perovskite, and tandem solar cells) and solar fuel generation (e. g., H2 production and CO2 reduction).