Exploring the synthesis of Cu2(Zn,Cd)SnS4 at high temperatures as a route for high-efficiency solar cells

Progress in Photovoltaics: Research and Applications, 2025; 0:1–16 https://doi.org/10.1002/pip.3899 1 of 16 Progress in Photovoltaics: Research and Applications RESEARCH ARTICLE OPEN ACCESS Exploring the Synthesis of Cu2(Zn,Cd)SnS4 at High Temperatures as a Route for HighEfficiency Solar Cells OutmanElKhouja1,2 | YuancaiGong3,4 | AlexJimenez-Arguijo3,4 | MaykelJimenezGuerra3,4 | AxelGonMedaille3,4 | RomainScaffidi5,6,7,8 | ArindamBasak9 | CristianRadu1,10 | DenisFlandre5,6,7,8 | BartVermang5,6,7,8 | SergioGiraldo3,4 | MarcelPlacidi3,4 | ZacharieJehlLi-Kao3,4 | AurelianCatalinGalca1,11 | EdgardoSaucedo3,4 1National Institute of Materials Physics, Magurele, Ilfov, Romania | 2Faculty of Science, Ibn Tofail University, Campus Universitaire, Kenitra, Morocco | 3Universitat Politècnica de Catalunya (UPC), Barcelona, Spain | 4Barcelona Centre for Multiscale Science & Engineering, Universitat Politècnica de Catalunya (UPC), Barcelona, Spain | 5IMO, Hasselt University, Diepenbeek, Belgium | 6IMOMEC, imec, Diepenbeek, Belgium | 7EnergyVille 2, Genk, Belgium | 8ICTEAM, UCLouvain, LouvainlaNeuve, Belgium | 9Thin Film Photovoltaic Lab, School of Electronics Engineering, KIITDeemed to Be University, Bhubaneswar, India | 10Faculty of Physics, University of Bucharest, Magurele, Ilfov, Romania | 11International Centre for Advanced Training and Research in Physics, Magurele, Ilfov,Romania Correspondence: Aurelian Catalin Galca ([email protected]) | Edgardo Saucedo ([email protected]) Received: 30 July 2024 | Revised: 12 December 2024 | Accepted: 4 February 2025 Funding: NIMP authors acknowledge funding from Ministerul Cercetării, Inovării și Digitalizării (Romanian Ministry of Research, Innovation and Digitalization) through the Core Programme PC3PN23080303 project, and from Unitatea Executivă pentru Finanțarea Învățământului Superior, a Cercetării, Dezvoltării și Inovării (UEFISCDI) through PN–III–P4IDPCE20200827 (Contract no. PCE74 09/02/2021) and ERANETM3ERANETLigthcell (Contract No. 19/15.03.2024) projects. Authors acknowledge the COST Action Research and International Networking project “Emerging Inorganic Chalcogenides for Photovoltaics (RENEWPV),” CA21148, supported by COST (European Cooperation in Science and Technology). This work received also funding from the European Union‘s Horizon 2020 research and innovation program under grant agreement number 952982 (CUSTOMART) and 866018 (SENSATE), and by the Science Ministry of Spain (Ministerio de Ciencia, Innovación y Universidades) projects number PID2020116719RBC41 (MATERONE) and TED2021130265BC21 (MIRACLE). A. J. A. thanks the European Social Fund+ for the FI fellowship. S.G. thanks the Juan de la Cierva grant IJC2020044716I funded by Ministerio de Ciencia, Innovación y Universidades (MCIN/ AEI/10.13039/501100011033) and by European Union‘s Horizon 2020 research and innovation program, NextGenerationEU/PRTR. E. S. is grateful to ICREA (Institució Catalana de Recerca i Estudis Avançats) Academia program. R. S. thanks Fonds Wetenschappelijk Onderzoek (FWO) for the funding through the Fundamental Research PhD Fellowship (1178024 N). A. B. thanks the grant SIR/2022/001011 by Science and Engineering Research Board (SERB) India. Horizon 2020 Framework Programme. Keywords: CZCTS films| photovoltaic optimization| structural characterization| sulfurization temperature tuning ABSTRACT The present research explores for the first time the intricate relationship between sulfurization temperature at unusual high temperatures (up to 700°C) and the structural/optoelectronic properties of Cu2(Zn,Cd)SnS4 (CZCTS) thin films, synthesized via a twostep sequential process involving the precursor film deposition using aprotic molecular ink followed by thermal treatment in sulfur atmosphere. Xray diffraction patterns confirms the tetragonal structure. Scanning Electron Micrographs revealed significant grain growth, with grain sizes increasing from ~0.3 μm at 620°C to ~1.5 μm at 680°C, effectively reducing grain boundary recombination. Energy dispersive Xray spectroscopy demonstrated a Cupoor and Znrich composition, with a consistent Cd incorporation of ~3.7 at%. Raman spectroscopy showcases the homogeneity and purity of the CZCTS crystalline structure. Precise control of the sulfurization temperature plays a crucial role in determining the photovoltaic characteristics of CZCTSbased solar cells. By increasing the grain size and preventing the thermal decomposition of the CZTS phase, the photovoltaic performance peaked at a sulfurization temperature of 680°C, achieving a power conversion efficiency (PCE) of 10.4%, with an opencircuit voltage of 0.701 V, a shortcircuit current density of 24.3 mA/cm2 and a fill factor of 60.8%. External quantum efficiency reached a maximum of 83.3% at 580 nm. The bandgap of the CZCTS absorber was determined to be 1.48 eV, optimal for This is an open access article under the terms of the Creative Commons AttributionNonCommercialNoDerivs License, which permits use and distribution in any medium, provided the original work is properly cited, the use is noncommercial and no modifications or adaptations are made. © 2025 The Author(s). Progress in Photovoltaics: Research and Applications published by John Wiley & Sons Ltd. 2 of 16 Progress in Photovoltaics: Research and Applications, 2025 photovoltaic applications. However, further increasing the sulfurization temperature to 700°C resulted in a lower PCE of 8.5%, attributed to interface degradation and secondary phase formation. Temperaturedependent current–voltage measurements revealed a reduction in recombination losses, with an activation energy of 1.24 eV at the CZCTS/CdS interface, indicating effective defect passivation by Cd incorporation. The optimized films, sulfurized at 680°C, displayed an absorber thickness of ~1.2 μm after sulfurization, providing efficient light absorption and charge transport. The findings not only emphasize the critical role of sulfurization temperature in engineering CZCTS film and subsequently their functionality but also provide valuable insights for fine tuning their performance in the field of photovoltaic applications. 1 | Introduction Emerging renewable energy sources present a pioneering solution to mitigate the challenges posed by the energy crisis and environmental pollution. Among these, photovoltaic power generation technology emerges as a practical and vital cornerstone for realizing sustainable environmental conservation, energy efficiency, and emission reduction. This innovative approach not only addresses the pressing need for cleaner energy alternatives but also shows a significant leap toward fostering a green, sustainable future [1, 2]. Copper zinc tin sulfoselenide (CZTSSe) solar cells, based on the kesterite crystal structure, stand out as one of the most promising emerging technologies in the realm of photovoltaics. Their exceptional potential in the third generation of solar technologies comes from a combination of factors, including earthabundant materials, a tunable direct bandgap, high optical absorption coefficients, environmentally friendly characteristics, and a relatively costeffectiveness. This convergence of properties positions CZTSSe solar cells as a formidable candidate, charting a course toward a sustainable and economically viable future of photovoltaic technologies [3–7]. CZTSSe solar cells, evolved from 2nd generation Cu (In,Ga)(S,Se)2 (CIGS), mark a significant stride forward. It is noteworthy that the current pinnacle of power conversion efficiency (PCE) for CIGS devices stands at 23.35%, achieved through vacuum methods [8]. In contrast, CZTSSe devices have demonstrated competitive PCE values using solution processing techniques. The vacuum processes inherent to CIGS synthesis demand substantial energy inputs, considerable capital investment, and result in excessive operating costs. Furthermore, underlines the unique benefits of CZTSSe, which through solution processing not only yields encouraging results but also provides a more effective and economically viable alternative in the constantly changing field of solar cell technology [9]. Moreover, the translation of highperformance devices from laboratory settings to largescale commercial production encounters challenges like low yields, process intricacies, and material utilization complexities. Fortunately, the solution method emerges as a promising way with vast developmental potential, characterized by its costeffectiveness, straightforward largearea preparation, and reproducible PCE [10, 11]. In 2021, Xin's group achieved a remarkable PCE of 13.0% utilizing a dimethyl sulfoxide (DMSO)- based processing approach [12]. Meng etal. [13] demonstrated further improvements, achieving an efficiency of 13.6% through innovative solution methods, recently being reported an updated value of approximately 15% [14]. Significantly, the certified efficiency of CZTSSe solar cells fabricated using the solution method exceeded the highest efficiency of 12.62%, previously set by Kim etal. [15] using vacuum methods. This highlights the pivotal role of solution processing, not only in achieving competitive efficiencies but also in surpassing benchmarks established by traditional vacuum methods. Currently, the production of CZTSSe absorbers through solution processing typically involves the following steps: (i) deposit of a precursor layer to the substrate and (ii) heat treatment of the precursor layer under sulfur and/or an inert atmosphere to produce the final polycrystalline absorbers [16]. While the highest efficiency recorded for CZTSSe solar cells manufactured through solution processing is at an impressive 14.9%, it is noteworthy that this figure still significantly lags behind the theoretical limit predicted by the Shockley–Queisser model, which stands at approximately 32%. The disparity highlights the existing gap between realworld achievements and the theoretical upper bounds within the field of CZTSSe solar cell efficiency. Subsequently, it emphasizes the significance of ongoing research and developments in fabrication techniques in order to bridge this gap and get closer to attaining the maximal efficiency potential predicted by the Shockley–Queisser model. The key to producing highefficiency CZTSSe solar cells is getting toptier absorber films [17, 18]. The utilization of a hydrazinebased precursor solution stands as the most successful method for CZTSSe solar cell preparation within the solution processing paradigm. This innovative approach, pioneered by IBM in 2010, achieved a groundbreaking certified conversion efficiency of 12.6% in 2013, a record efficiency that has set the standard for several years [19, 20]. Nevertheless, hydrazine solvents pose inherent challenges due to their highly toxic and explosive nature, coupled with explicit transportation and application restrictions in certain countries and regions. These limitations have catalyzed a rapid shift toward the development and widespread adoption of solution methods employing nonhydrazine solvent systems. Presently, an array of environmentally friendly, stable, and highefficiency solvent systems has emerged. Notable examples include dimethyl sulfoxide (DMSO), N,Ndimethylformamide (DMF), ethylene glycol methyl ether (EGME), thioglycolic acid and ammonia (TGA), and the ethylenediamine/ethanedithiol system (EN/EDT). This transformative shift not only addresses safety concerns but also signifies a pivotal advancement in the field of CZTSSe solar cell fabrication, promoting sustainability and wider accessibility [21]. Table1 displays a selected overview of the experimental settings and device performances of CZTS/CZTSSe based solar cells employing various solventbased solutiondeposited absorbers. Reducing the significant Voc losses found in kesterite solar cells has mainly depended on improving absorber qualities through techniques such controlling CuZn disorder, reducing surface and grain boundary imperfections, and controlling absorber grain development [58–60]. Several studies have exhibited strategies to improve the absorber characteristics in kesterite solar cells. Extended lowtemperature annealing enhances 1099159x, 0, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/pip.3899 by Readcube (Labtiva Inc.), Wiley Online Library on [21/03/2025]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License 3 of 16 TABLE 1 | Overview of the experimental settings and device performances of CZTS/CZTSSe based solar cells employing various solventbased solutiondeposited absorbers. Solar cell Molecular ink solvent Dissolution condition Heat treatments Voc Jsc FF PCE Ref. CZTSSe N2H4N2, glove box 500°C 513.4 35.2 69.8 12.6 Wang etal. (2014) [20] CZTSSe N2H4N2, glove box 500°C 471 37.1 70.3 12.7 Kim etal. (2014) [22] CZTS MOE and MEA Stirring (50°C) 580°C 664 14.8 58 5.7 Zhang etal. (2014) [23] CZTSSe C₆H₁₅N/C3H8S N2, glove box 500°C 382 34.4 60.1 7.86 K. W. Brew etal. (2015) [24] CZTS MOE Stirring (50°C) 250°C + 580°C 581 24.1 66 9.82 Z. Su etal. (2015) [25] CZTSSe DMSO Stirring 540°C 449 38.8 68.1 11.8 Xin etal. (2015) [26] CZTSSe MOE Stirring 560°C 446 32.22 55.97 8.04 Y.- T. Hsieh (2016) [27] CZTSSe TGA, CH3NH2Stirred (45°C) 510°C 378 28.17 65.4 6.96 Y. Yang etal. (2016) [28] CZTSSe C2H4(SH)2. and C2H4(NH2)2 Stirring (60°C) 480°C 448 35.19 65.66 10.36 Guchhait etal. (2017) [29] CZTS MOE Stirring (50°C) 600°C 650 25 66.2 10.8 S. H. Hadke etal. (2018) [30] CZTSSe DMSO + Tu Stirring 550°C 463 33.4 65.5 10.1 C. M. SutterFella etal. (2018) [31] CZTSSe DMSO Stirring 300°C, 500°C, 550°C 531 33.7 64.8 11.6 A. CabasVidani etal. (2018) [32] CZTSSe EGME Stirring 540°C + 520°C 578 30.5 63 11.1 S.- H. Wu etal. (2018) [33] CZTSSe C2H4(SH)2. and C2H4(NH2)2 Stirring 550°C 386 29.36 57.26 6.49 Q. Yan etal. (2019) [34] CZGTS DMF N2, glove box 540°C 583 33.60 55.9 11.0 J. A. Clark etal. (2019) [35] CZTSSe DMSO Stirring 560°C 460 32.2 58.2 8.6 S. Ge etal. (2019) [36] CZTS MOE Stirring (60°C) 200°C + 580°C 640 27.8 71 12.6 Z. Su etal. (2020) [37] CZTS MOE Stirring (50°C) 600°C 670 21.5 57.22 8.24 A. Ibrahim etal. (2020) [38] CZTSSe MOE Stirring (60°C) 550°C 465.1 39.33 66.51 12.18 X. G. Zhao etal. (2020) [39] CZTSSe MOE Stirring (60°C) 555°C 507 33.97 65.36 11.1 G.- X. Liang etal. (2021) [40] CZTSSe 1 C2H4(SH)2. and C2H4(NH2)2 + stabilizer Stirring (70°C) 550°C 495 37.07 66.26 12.16 X. Chang etal. (2021) [41] (Continues) 1099159x, 0, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/pip.3899 by Readcube (Labtiva Inc.), Wiley Online Library on [21/03/2025]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License 4 of 16 Progress in Photovoltaics: Research and Applications, 2025 CuZn disorder and increases Voc of the device. SnOx is created by heat treatment in air, which passivates flaws at the grain boundaries and absorber surface and can achieve efficiencies of up to 12.6%. Grain growth is managed by carefully controlled sulfoselenization processes, which improves device efficiency [15, 20, 61]. Cd alloying pure sulfide kesterite has an array of benefits, including stabilizing the conduction band offset, reducing CuZn antisite defects, and positively modulating the bandgap [21, 37]. Through the use of several deposition processes, including coevaporation and solutionbased procedures, the incorporation of Cd has resulted in efficiency gains of up to 10% in CZTS solar cells. Notable improvements, such as over 12% efficiency in CZCTS solar cells, have been reported; these were made possible by interface defect elimination and postannealing [37, 62]. Highefficiency CZCTS solar cells often require hightemperature postannealing or CZTS/CdS heterojunction heat treatment [63]. The full potential of Cd alloying in CZCTS remains untapped. Recent findings reveal that defect cluster formation, impacting kesterite band tailing, is related to absorber grain growth mechanisms. Directphase transformation grain growth, facilitated by Cu+- Sn4+- based DMSO solutions, shows promise in suppressing defect formation [64]. This approach, successful in CZTSSe solar cells, holds the potential for enhancing CZCTS solar cell efficiency [12, 50]. This study explored, for the first time, the complex relationship between sulfurization at unusually high temperatures (up to 700°C) and the structural and optoelectronic characteristics Solar cell Molecular ink solvent Dissolution condition Heat treatments Voc Jsc FF PCE Ref. CZTSSe DMSO N2, glove box 550°C 540 32.1 72.3 12.5 Y. Gong etal. (2021) [42] CZTSSe C2H4(SH)2. and C2H4(NH2)2 Stirring (70°C) 300°C + 550°C 463 35.65 62.47 10.24 W. Xie etal. (2022) [43] CZTSSe DMF/DMSO binary solvents Stirring 530°C 493.5 33.89 73.30 12.26 Y. Sun etal. (2022) [44] CZTSSe DMF Stirring 550°C 501 35.36 66.4 11.76 Y. Cui etal. (2022) [45] CZTSSe DMSO N2, glove box 550°C 529 33.7 72.9 13.0 Y. Gong etal. (2022) [12] CZTSSe MOE Stirring (60°C) 550°C 505.5 39.3 64.8 12.87 H. Geng etal. (2022) [46] CZTSSe DMF Stirring 550°C 520 39.08 63.32 12.86 M. Wang etal. (2023) [47] CZCTS DMSO Stirring 625°C 640 27.8 71.0 12.3 X. Pan etal. (2023) [48] CZTSSe MOE Stirring 550°C 526.1 36.37 66.52 13.02 Y. Qi etal. (2023) [49] CZTSSe MOE, DMSO, DMF Stirring (60°C) 535 °C 551.20 35.74 71.73 13.8 J. Zhou etal. (2023) [50] CZTSSe C2H4(SH)2. and C2H4(NH2)2 Stirring (55°C) 550°C 513.85 38.69 69.27 13.77 L. Cao etal. (2024) [51] CZTSSe DMF Not mentioned Not mentioned 471.7 34.98 73.61 12.15 J. Zhou etal. (2024) [52] CZTSSe MOE Stirring 520°C 576.0 36.9 70.0 14.9 Y. Li etal. (2024) [53] CZTSSe MOE Stirring 560°C 573.0 35.1 70.1 14.1 Y. Gong etal. (2024) [54] CZTSSe MOE Stirring (50°C) 540°C 555 36.7 71.2 14.5 J. Wang etal. (2024) [55] CZTSSe MOE Stirring 555°C 580 36.63 67.43 14.3 Y. Zhao etal. (2024) [56] CZTS MOE Stirring (60°C) 620°C 664 24.28 62.33 10.0 F. Ye etal. (2024) [57] TABLE 1 | (Continued) 1099159x, 0, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/pip.3899 by Readcube (Labtiva Inc.), Wiley Online Library on [21/03/2025]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License 5 of 16 of Cdbased CZTS thin films. By fabricating CZTS absorbers using a Cu+- Sn4+- DMSO solution (supporting information [SI], FigureS1), the present study demonstrates the transformative impact of precise temperature control on the properties of the CZTS and corresponding device. Photovoltaic cells based on Cd free CZTS absorber layers crystallized at different temperatures had a maximum PCE of 4.08% and a Voc of 576 mV (FigureS2 and TableS1). The analysis reveals that high sulfurization temperatures induce a transformation in crystallinity, driving the transition from an amorphous to a crystalline structure. This process is accompanied by significant grain growth, which effectively reduces band tailing and enhances the optoelectronic properties of the films. By introducing Cd in the CZTS, along with band gap decrease, the CdS/CZCTS interface change in positive manner the photovoltaic properties. As a result, the PCE increases, peaking at 10.4% at an optimal sulfurization temperature of 680°C, underscoring the critical role of grain boundary passivation and interface engineering. However, further increasing the sulfurization temperature to 700°C led to performance degradation, attributed to secondary phase formation and interface degradation, thus highlighting potential tradeoffs in optimizing processing conditions. Importantly, the incorporation of 20% Cd into CZTS not only stabilized the conduction band offset but also significantly reduced the Voc loss for CZCTS devices, even without postheat treatment of the CZCTS/CdS heterojunction. This work establishes a novel approach for tuning kesteritebased thin films through a combination of Cdalloying and sulfurization temperature optimization, providing valuable insights into achieving higher efficiencies and advancing the field of scalable, solutionprocessed photovoltaics. 2 | Materials and Methods 2.1 | Reagents and Materials Dimethyl sulfoxide (DMSO), Thiourea (99%), CuCl (99.99%), SnCl4 (99.998%), Zn (Ac)2 (99.99%), and CdCl2 (98%), all purchased from SigmaAldrich, have been used as chemicals. 2.2 | CZCTS Precursor Film Preparation The precursor solution was prepared under ambient air condition, with all chemicals used as received without further purification. The 6 mmol of CuCl, 3 mmol of SnCl4, 3.7 mmol of Zn (Ac)2, 0.74 mmol of CdCl2 (corresponding to 20% of the total Zn + Cd molar content), and 27 mmol of thiourea were dissolved in 10 mL of dimethyl sulfoxide (DMSO) and stirred until complete dissolution. The yellow and clear precursor solution was obtained after 2h stirring. The fabrication process described involves the spincoating of the aprotic molecular ink onto Mo/ SiO2/Glass substrate (Suzhou ShangYang Solar Technology Co.), followed by annealing (300°C) to produce CZCTS films. The twostep sequence (spincoating and annealing process), is repeated seven times to achieve a precursor film with a thickness of approximately 1 μm (FigureS3). The sulfurization of precursor films was conducted in a tubular furnace under precisely controlled conditions. The process began with a temperature rampup at a rate of 10°C/min, leading to the target temperature, which ranged between 620°C and 700°C (FigureS4). Once the desired temperature was reached, the films were held at this temperature for 15 min to allow thorough sulfur incorporation and complete the reaction. The sulfurization process was carried out under atmospheric pressure within a dedicated graphite box containing sulfur pellets (0.1 g) as the source material. The graphite box was specifically designed to establish an optimal sulfurrich environment, effectively reducing contamination risks while ensuring a uniform distribution of sulfur vapor. This setup facilitated consistent sulfur incorporation across the film, which is crucial for achieving homogeneity in the materials structural and compositional properties. 2.3 | Film Characterization XRD patterns were collected using an Xray diffractometer featuring Cu Kα as the radiation source (Empyrean, PANalytical). Raman spectra were obtained using a Raman spectrometer (LabRAM HR Evolution, HORIBA) with a 532 nm laser diode as the excitation source. Detailed scanning electron microscopy (SEM) images were captured on a Hitachi S4800 SEM, utilizing a 10 kV accelerating voltage. The microstructure and elemental distribution of the sulfurized films were analyzed using a JEOLF200CF STEM equipped with an energy dispersive Xray spectroscopy (EDS) system. 2.4 | Device Fabrication and Characterization For the device fabrication, initially, a 50 nmthick CdS buffer layer was deposited onto the sulfurized films (FigureS5) using the chemical bath deposition method. Following this, radio frequency sputtering was employed to deposit intrinsic ZnO (iZnO) and indium tin oxide (ITO) to form the essential window layer. Thermal evaporation of Silver (Ag) using mechanical masks completed the entire device. More details are presented in the SI. As mention in the introduction, the optimization of the device fabrication has been done on Cdfree CZTS solar cells, structural and photovoltaic properties being also revealed in the SI. The current density–voltage (J–V) curves were recorded using a Keithley 2400 Source Meter under simulated AM 1.5 sunlight at 100 mW cm−2, calibrated with a Si reference cell (12.8% efficiency over an aperture area of 0.23 cm2). EQE measurements were conducted using the Enlitech QER test system, employing calibrated Si and Ge diodes as references. 3 | Results and Discussion The XRD patterns of precursor films subjected to varying sulfurization temperatures are presented in Figure1a. Aside from the peak originating from Mo (110 reflection, 2θ = 40.5°, ICDD PDF no. 000040809), the films show peaks at approximately 28.4°, 32.92°, 47.13°, 56.05°, and 58.71°, which correspond to tetragonal CZTS 112, 220 and 312 diffraction lines (ICDD PDF no. 040157542). The absence of significant deviations in the diffraction pattern implies a high level of structural integrity and successful alignment with present fabrication objectives. Moreover, from the observed patterns, the crystallite size evolution as function of sulfurization temperature provide useful insights into the film production process (SI, FigureS6). The gradual increase of 1099159x, 0, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/pip.3899 by Readcube (Labtiva Inc.), Wiley Online Library on [21/03/2025]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License 6 of 16 Progress in Photovoltaics: Research and Applications, 2025 the mean crystallite size (the coherence length along the crystallographic directions) up to 700°C shows a link between higher sulfurization temperatures and advantageous microcrystallites formation. The subsequent decrease in crystallinity at 700°C implies the onset of adverse effects, potentially linked to decomposition processes. This nuanced relationship between sulfurization temperature and crystallinity points out the need for a well understanding of the thermal factors during the fabrication process. The progressive increase in grain size with higher sulfurization temperatures (Figure1b) indicates that elevated temperatures promote enhanced crystal growth and coalescence, leading to the formation of larger, welldefined grains within the CZCTS films, as shown in SEM images provided in the Figures1b and S7. The improvement in grain morphology is vital for reducing grain boundary density, which directly impacts the performance of CZCTSbased solar cells. Grain boundaries are significant recombination centers, and their density directly influences the recombination rate. Since the recombination rate is proportional to the total grain boundary area, reducing the area inherently decreases recombination losses. This implies that the carrier lifetime at the grain boundaries is inversely proportional to the grain boundary area. Therefore, by promoting larger and more uniform grains, the total grain boundary area is minimized, leading to enhanced carrier lifetimes and better charge transport. This improvement translates into higher electrical conductivity and superior device efficiency. The SEM images reveal that at temperatures up to 680°C, the films exhibit compact and uniform grains, contributing to superior structural integrity and optoelectronic properties. However, at 700°C, pinholes become evident in the film, suggesting that excessive sulfurization temperatures may induce localized thermal decomposition or material defects, which compromise the film's quality. These observations align closely with the XRD results, where the increase in grain size observed in SEM correlates with the enhanced crystallite size and reduced lattice strain evident in the diffraction patterns. Thus, the complementary SEM and XRD analyses confirm that optimizing sulfurization temperature is critical to achieving the desired balance between improved crystallinity and minimal defect formation, thereby optimizing the CZCTS absorber's photovoltaic performance. The Raman spectroscopy analysis of CZCTS films provides crucial insights into the structural characteristics of the material (Figure2). The A1 vibration mode in the Raman spectra is FIGURE 1 | (a) XRD and (b) SEM images of CZCTS thin films sulfurized at different temperature. All bars correspond to 1 μm (Mag = 10.00 KX). [Color figure can be viewed at wileyonlinelibrary.com] FIGURE 2 | Raman spectroscopy of CZCTS thin films. [Color figure can be viewed at wileyonlinelibrary.com] 1099159x, 0, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/pip.3899 by Readcube (Labtiva Inc.), Wiley Online Library on [21/03/2025]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License 7 of 16 observed as a dominant and welldefined peak at 335.6 cm−1, which confirms the presence of the CZCTS kesterite crystal structure, validating the successful synthesis of the desired phase. The A1 mode is associated with Sonly vibrations, with CuSn and CuZn vibrations against the [110] and [1–10] crystallographic directions, respectively, with a net sulfur contribution. The presence of the A1 mode across varying sulfurization temperatures indicates the stability of the CZCTS phase throughout the temperature range investigated, with minimal structural changes. Notably, there are no significant shifts or broadening in the A1 mode, suggesting that sulfurization up to 700°C does not induce notable thermal degradation or phase transitions, which is further corroborated by the XRD data showing stable crystallite sizes up to this temperature. Furthermore, at the higher sulfurization temperature of 700°C, an additional distinct peak emerges around 407 cm−1, corresponding to the MoS2 phase, which indicates the presence of this phase on or under the surface of the CZCTS, which might alter also the photovoltaic properties of the final device. Table2 summarizes values for distinctive modes within the KS phase of CZCTS films across a temperature spectrum (620°C to 700°C). Notably, B (TO)269 characterizes CZTS cations aligning against S atoms||[001], with reported values spanning 250 to 252 cm−1. B (LO)285 signifies the alignment of all cations against S atoms||[001], exhibiting values from 285 cm−1 to 288 cm−1. In E (TO)305/A305, Cu + Sn aligns against [110], and Cu + Zn aligns against [11¯0], involving a net sulfur contribution in the (xy) plane, with reported values ranging from 302 to 306 cm−1. A335/B (LO)335 captures Sonly and cation vibrations against [110] and [11¯0], respectively, with values in the range of 334 to 337 cm−1. E (LO)341 reveals Zn aligning against [110] with Cu oppositely positioned, incorporating specific S atoms against Zn and Cu, and reports values between 345 cm−1 and 349 cm−1. B (TO)354 features CuZn alignment against [001] with S in the (xy) plane, presenting values from 355 to 367 cm−1. Across the temperature range, the Raman peak location of the A335/B (LO)335 mode is comparatively consistent, with a slight TABLE 2 | Raman shifts (cm−1) corresponding to phonon modes identified from the Lorentzian components (shown in Figure2) for the CZCTS films. Row CZCTS 620°C CZCTS 635°C CZCTS 650°C CZCTS 680°C CZCTS 700°C Modes in KS phase Atomic Movements Experimental reported values 1255.69 251.71 255.60 250.99 251.10 B (TO)269 CZTS cations (Cu, Zn. Sn)||[001], against S 250 [65] 252 [66] 2287.93 288.03 285.50 288.29 286.75 B (LO)285 All cations||[001], against S 285 [66] 287 [67, 68] 288 [58, 69] 3303.51 302.11 307.06 — — E (TO)305 /A305 Cu + Sn||[110], Cu + Zn||[1 1 0]; net S contribution/Sonly in the (xy) plane 302 [70] 303 [58] 305 [71] 306 [66, 72] 4335.16 335.10 335.06 335.15 335.37 A335 /B (LO)335 Sonly/CuSn||[110], Cu + Zn||[1 1 0]; net S contribution 334 [66] 335 [73] 336 [74] 337 [67, 75] 5349.24 348.79 346.03 —345.78 E (LO)341 Zn||[110], Cu roughly opposite; two S against Zn and two other S against Cu 347 [66] 6363.70 365.60 363.19 362.66 B (TO)354 CuZn||[001]; S in (xy) plane 355 [76] 364 [77] 366 [65, 71] 367 [58] FIGURE 3 | FWHM and peak area of A335/B (LO)335 vibration mode of the CZCTS samples. [Color figure can be viewed at wileyonlinelibrary.com] 1099159x, 0, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/pip.3899 by Readcube (Labtiva Inc.), Wiley Online Library on [21/03/2025]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License 8 of 16 Progress in Photovoltaics: Research and Applications, 2025 larger Raman shift observed at 700°C. This trend is similar to what has been reported in other works investigating the temperature dependence of vibrational modes in CZTS and CZCTS films [58]. The number of active vibrational modes is represented by the area of the Raman peak, which increases typically with temperature. It reaches its maximum at 680°C and then starts to decrease slightly at 700°C. Up to 680°C, this trend suggests an improvement in crystalline quality or a decrease in defect density, consistent with observations from recent studies on the impact of temperature on CZTS crystallinity [78]. At higher temperatures, structural degradation or defect formation is likely, as indicated by the decrease in peak intensity and the broadening of the full width at half maximum (FWHM). The FWHM, an indication of crystalline quality that is inversely correlated, drops from 8.00 at 620°C to 6.15 at 650°C (Figure3). Nonetheless, a minor rise in FWHM at 680°C and 700°C suggests that lattice strain or defect development may be beginning at these high temperatures. The intensity of the vibrational mode is represented by the height of the Raman peaks, which normally rise with temperature (the ratio A335/B285 follows the same trend). The peak is reached around 650°C, and as temperatures rise, they gradually decrease. Up to 650°C, improvements in crystallinity are consistent with this pattern; at higher temperatures, there may be structural alterations or the introduction of defects. Figure4 shows the EDS coupled with SEM mapping, illustrating the uniform distribution of elements across the sample surface. The elemental composition (Table3) confirms a desirable Cupoor and Znrich profile in all films, which is known to favor enhanced photovoltaic performance by reducing deeplevel defect states [79]. The Cd concentration remains consistent across all samples, while Zn content is similarly stable, supporting the FIGURE 4 | EDS spectra, and the surface EDS mapping of the components elements of the sample sulfurized at different temperature. [Color figure can be viewed at wileyonlinelibrary.com] TABLE 3 | Chemical elemental composition (at %) of the CZCTS films. Sample Cu (at.) Zn (at.) Cd (at.) Sn (at.) S (at) Cu/(Zn + Cd + Sn) (Zn + Cd)/Sn S/cations CZCTS 700°C 15.86 12.09 3.22 13.63 50.72 0.54 1.12 1.13 CZCTS 680°C 19.68 11.73 3.99 10.65 53.92 0.75 1.47 1.17 CZCTS 650°C 18.51 11.79 3.70 9.16 56.81 0.75 1.69 1.31 CZCTS 635°C 19.77 11.45 2.64 10.62 55.50 0.80 1.32 1.24 CZCTS 620°C 21.35 13.23 4.72 11.27 49.41 0.73 1.59 0.97 1099159x, 0, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/pip.3899 by Readcube (Labtiva Inc.), Wiley Online Library on [21/03/2025]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License 9 of 16 reproducibility of the alloying strategy. However, the tin concentration is relatively lower at lower sulfurization temperatures (620°C), while the sulfur content is also less at these temperatures, reaching a minimum at 635°C. Interestingly, at the highest sulfurization temperature (700°C), the Cu/(Zn + Cd + Sn) ratio decreases significantly to approximately 0.5, reflecting pronounced Cu depletion and unexpectedly high Sn concentrations. This anomalous composition at 700°C, which is typified by an excess of Sn and a Cu deficiency, may contribute to the lower performance in these samples. At higher temperatures, one might normally predict Sn loss through diffusion toward the surface following the evaporation, while the evaporated Sn is reabsorbed on the CZCTS surface considering the closed space feature of the graphite box. Alternatively, Cu may migrate to the MoS₂/Mo interface, as suggested by evidence from similar systems, driven by thermodynamic favorability or interfacial stabilization. This migration could also alter the stoichiometry and account for the Cu deficiency. The performance of CZCTS based photovoltaic devices (Figure5), as detailed in Table4, reveals a welldefined trend with respect to sulfurization temperature. Starting at 620°C, the achieved PCE was 5.14%, accompanied by an opencircuit voltage (Voc) of 0.480 V, a shortcircuit current density (Jsc) of 18.97 mA cm−2, and a fill factor (FF) of 56.75%. Subsequently, increasing the sulfurization temperature to 635°C resulted in an improved PCE of 6.16%, driven by a Voc of 0.608 V and a Jsc of 22.03 mA cm−2, albeit a slightly reduced FF of 46.32%. As the temperature further rose to 650°C, the PCE continued increasing, reaching 7.12% with a Voc of 0.608 V, Jsc of 19.79 mA cm−2, and an improved FF of 59.32%. At 680°C, the device achieved the maximum performance, with a PCE of 10.4%, attributed to a combination of the highest Voc of 0.701 V, a Jsc of 24.31 mA cm−2, and an FF of 60.81%. However, at 700°C, the PCE declined to 8.51%, corresponding to a slight reduction in Voc of 0.624 V, Jsc of 24.07 mA cm−2, while maintaining a relatively high FF of 59.13%. This trend aligns with observations reported in recent studies, where optimal sulfurization temperatures improve grain growth, crystallinity, and defect passivation, contributing to enhanced device performance. For example, Pan etal. [48] observed a similar increase in PCE with higher sulfurization temperatures due to reduced grain boundary recombination, with diminishing returns at excessively high temperatures attributed to interface degradation and secondary phase formation. The obtained results corroborate these findings, particularly the strong correlation between larger grain size at 680°C and reduced recombination losses. The decline in PCE at 700°C can be ascribed to a delicate equilibrium between the beneficial and detrimental effects of the sulfurization process on the CZCTS absorber film. While increasing sulfurization temperatures generally lead to improved crystallinity, grain size, and charge carrier mobility, pushing the limits to 700°C might induce adverse effects, such as interfacial decomposition, phase segregation, secondary phase formation, or defect generation, which compromise electronic properties and overall photovoltaic performance as similarly reported in previous studies [21, 80, 81]. Additionally, at higher temperatures, there is a likelihood of enhanced grain boundary recombination and increased surface roughness, negatively impacting the charge transport properties within the film. Therefore, the decline in PCE at 700°C shows the critical importance of carefully optimizing sulfurization temperatures, recognizing that exceeding a certain limit can introduce detrimental factors that offset the gains achieved at lower temperatures, emphasizing the nuanced balance required for maximizing the efficiency of CZCTS absorber films [21, 80, 81]. The crosssectional SEM images depicted in Figure 6 offer insights into both the absorber and the devices engineered from the highperformance CZCTS films, prepared at 680°C. Notably, the images showcase a typical largegrain structure, with certain grains extending seamlessly throughout the entire thickness of the CZCTS layer. The strategic presence of such large grains holds intrinsic benefits for device performance, effectively minimizing the likelihood of recombination of photogenerated carriers at grain boundaries. This inherent characteristic demonstrates the potential of CZCTS films synthesized FIGURE 5 | J–V curves of CZCTS based devices. [Color figure can be viewed at wileyonlinelibrary.com] TABLE 4 | Detailed parameters of photovoltaic devices based on the CZCTS films. Sample Jsc (mA/ cm2) Voc (mV) FF (%) PCE (%) CZCTS 700°C 23.1 624 59.1 8.5 CZCTS 680°C 24.3 701 60.8 10.4 CZCTS 650°C 19.8 608 59.3 7.1 CZCTS 635°C 22.0 608 46.3 6.2 CZCTS 620°C 18.8 480 56.7 5.1 1099159x, 0, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/pip.3899 by Readcube (Labtiva Inc.), Wiley Online Library on [21/03/2025]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License 16 of 16 Progress in Photovoltaics: Research and Applications, 2025 74. J. He, L. Sun, S. Chen, Y. Chen, P. Yang, and J. Chu, “Composition Dependence of Structure and Optical Properties of Cu2ZnSn(S,Se)4 Solid Solutions: An Experimental Study,” Journal of Alloys and Compounds 511, no. 1 (2012): 129–132, https:// doi. org/ 10. 1016/j. jallc om. 2011. 08. 099. 75. X. Fontané, V. IzquierdoRoca, E. 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