Unravelling the intricacies of selenization in sequentially evaporated Cu(In,Ga)Se2 Thin film solar cells on flexible substrates

This study aimed to fabricate copper indium gallium diselenide (CIGSe) thin films using a novel two-step approach. Firstly, we deposited metallic precursors (Cu/In/Ga) onto a Mo-coated stainless steel substrate using thermal evaporation at unintentional substrate temperature. Subsequently, selenization was carried out in a furnace under the presence of an inert gas. The quality of the CIGSe thin films was analyzed to explore the influence of selenization temperature (450 °C–550 °C) and duration (30 and 60 min), while maintaining an inert atmosphere inside the selenization furnace. The structural analysis revealed the progressive development of additional phases over time, resulting in the formation of a complete chalcopyrite CIGSe structure with the preferred reflection on the (112) plane. The absorber layer exhibited a thickness of 2 μm, with atomic ratios of 0.83 for Cu/(In+Ga) and 0.24 for Ga/(In+Ga) in the film selenized at 550 °C. P-type conductivity was observed in the CIGSe thin film, with a carrier concentration of up to 1017 cm−3, and it displayed a well-defined and uniform morphology characterized by a large grain size of approximately 0.9 μm. Utilizing the optimized conditions, we successfully fabricated solar cells on a flexible substrate, achieving a photoconversion efficiency of up to 9.91%. This research delves into the impact of selenization parameters on the growth of CIGSe absorber layers and introduces a new approach that could significantly influence the feasibility and industrialization of flexible solar cells.


Introduction
Thin film solar cells (TFSC) utilizing copper indium gallium diselenide (CIGSe) have garnered significant attention due to their distinct competitive advantages over conventional crystalline silicon (c-Si) solar cells and other variants of thin film solar cells The remarkable progress in the efficiency of CIGSe solar cells since their inception positions them as a promising solution for addressing the global energy crisis.Demonstrating an impressive maximum reported efficiency of 23.35%, CIGSe's performance rivals that of laboratory-scale c-Si solar cells.Notably, the high absorption coefficient of CIGSe enables a substantial reduction in the thickness of its absorber layer, which can be as thin as 1-3 μm, in contrast to c-Si solar cells [1].Belonging to the I-III-VI 2 semiconductor family, CIGSe is categorized as a p-type semiconductor and boasts a direct bandgap that can be finely tuned within the visible spectrum, ranging from 1.04 eV to 1.68 eV.This inherent property enhances its versatility for different applications.Moreover, CIGSe exhibits robust durability and exceptional thermal stability, augmenting its appeal as an ideal material for thin film solar cells [2].
Numerous growth techniques have been extensively studied for CIGSe thin film absorbers, including vacuum-based methods like sputtering, laser-assisted deposition, and co-evaporation, as well as non-vacuum methods such as electrodeposition, spray pyrolysis, paste coating, and spin coating.Vacuum-based coevaporation, known for its efficiency, minimal waste, large-scale uniformity, simplicity, and purity, is

Experimental details 2.1. Material
The metallic precursor materials, Cu, Ga, and In, were obtained from Sigma Aldrich without requiring further purification.These materials were then loaded into the thermal evaporator.Se (selenium) was also purchased from Sigma Aldrich, boasting a purity of 99.999%.NaOH (98% purity) and HCl (ACS reagent, 37%) were utilized in the experiment.The Extran soap solution was procured from Merck, while ethanol was sourced from J T Baker.Industrial N 2 gas from INFRA was employed.The experimental setup featured a stainless steel (SS) 430 BA substrate from Prominox, boasting a thickness of 0.6 mm.The Mo (molybdenum) target, which is 99.95% pure, possessed a diameter of 3.00 inches and a thickness of 0.25 inches.This Mo target was acquired from Kurt J Lesner.Furthermore, the Zinc oxide with Alumina target (ZnO/Al 2 O 3 , 98/2 wt%) was obtained from Kurt J Lesner, exhibiting a purity of 99.999%, a diameter of 3.00 inches, and a thickness of 0.250 inches.

Substrate cleaning
The stainless steel (SS) substrates underwent a thorough cleaning process involving several steps.Initially, they were treated with an extran solution, followed by rinsing with deionized water and methanol.Subsequently, an etching procedure was performed utilizing a 0.1 M HCl solution for a duration of 4 min.The substrates were then subjected to a rinsing sequence involving ethanol for 10 min, succeeded by deionized (DI) water with the assistance of ultrasonic waves.Following the cleaning regimen, the substrates were dried using N 2 gas and promptly transferred to the deposition chamber.

CIGSe film deposition
Using a direct current (DC) magnetron sputtering technique, molybdenum thin film was deposited onto the stainless steel (SS) substrate.The power settings were varied in the range of 100 W to 300 W, while maintaining a working pressure of 5 mTorr throughout the process.In order to create CIGSe layers on SS substrates coated with Mo, metallic precursor layers (Cu, In, and Ga) were selenized after the Mo layer had been etched using a 1 M solution of NH 4 OH for 2 min.Prior to the deposition of radio frequency sputtered intrinsic zinc oxide (ZnO) barrier layer, the Mo layer underwent an etching step.The different metal source temperatures were adjusted accordingly: Cu was evaporated at a temperature of T Cu = 1300 °C, Ga was evaporated at T Ga = 1000 °C, and In was evaporated at T In = 850 °C.Notably, intentional heating of the substrate did not occur during the metallic precursor deposition phase.The sequential deposition process began with Cu deposition, followed by Ga and In, with deposition times adjusted to achieve an approximate film thickness.In order to harness the beneficial properties of alkali metals, a post-deposition treatment involving the deposition of NaF and KF.

Selenization condition
To produce thin films of CIGSe, a two-step process was employed.Initially, a metallic precursor layer was generated and subsequently exposed to selenization utilizing Se vapor within an inert gas furnace.This two-step selenization procedure involved distinct stages, wherein the first phase occurred at a lower temperature and the subsequent phase occurred at a higher temperature.During the initial low-temperature phase, the precursor layer underwent gradual heating at a rate of 10 °C per minute until it reached 300 °C.It was then maintained at this temperature for a duration of 20 min to facilitate the alloying process with selenium.These steps were executed within a dedicated selenization chamber, maintaining a pressure of 1 Torr.In the subsequent hightemperature phase, the temperature was elevated to a range between 450 °C and 550 °C, with varying durations of 30 and 60 min.Following this, the thin film was allowed to cool naturally.Figure 1 depicts the selenization process flow chart, outlining the specific conditions employed during the selenization procedure.

Device fabrication
A flexible solar cell, based on CIGSe, was manufactured on a stainless steel (SS) substrate, employing a specific device architecture.The device structure encompassed SS/Mo/CIGSe/CdS/i-ZnO/ZnO: Al layers (figure 2).To begin, a 500 nm-thick molybdenum (Mo) layer was sputter-deposited onto the substrate, serving as the back contact.Subsequently, a 30 nm-thick zinc oxide (ZnO) layer was sputter-deposited using radio frequency (RF) sputtering under vacuum conditions [20].This ZnO layer acted as a barrier, situated between the Mo layer and the CIGSe absorber layer.The absorber layer itself was composed of a 2 μm-thick Cu(In, Ga)Se 2 material.A post-deposition treatment strategy was implemented, involving the deposition of a 20 nm-thick layer comprised of NaF and KF.This treatment aimed to harness the advantages offered by alkali metals.Additionally, a 50 nmthick buffer layer of cadmium sulfide (CdS) was produced, employing a chemical bath technique [21].The transparent conductive oxide (TCO) layer comprised two sequential layers: firstly, a 50 nm-thick intrinsic zinc oxide (i-ZnO) layer, succeeded by a 100 nm-thick layer of aluminum-doped zinc oxide (ZnO: Al) [22].Finally, the front contact was established by depositing a Ni/Al metallic contact through e-beam evaporation.2.6.Sample characterization X-ray diffraction (XRD) measurements were conducted using a Bruker D2-phaser diffractometer to probe the structural characteristics of the deposited CIGSe film.Cu-Kα radiation with a wavelength of 1.5406 Å was employed within the 2θ range of 20°to 80°for data collection.The XRD data were then subjected to analysis for the determination of the structural parameters pertaining to the deposited CIGSe film.The morphology and topographical features of the film were scrutinized using a TESCAN VEGA3 scanning electron microscope, while NT-MDT NTEGRA Spectra was utilized for additional examination.To assess the film's electrical properties, including carrier concentration, carrier mobility, and resistivity, Hall Effect measurements were performed using the Van der Pauw method.The thickness of the selenized CIGSe thin films was quantified through the utilization of a Bruker profilometry DektarXT.Lastly, the performance of the solar cells was evaluated using a solar simulator (SCIENCE TECH).

Structural properties
The XRD pattern of a selenized CIGSe thin film, subjected to durations of 30 and 60 min at temperatures of 450 °C, 500 °C, and 550 °C, is presented in figures 3(a)-(b).The results reveal a polycrystalline nature of the film, with a distinct peak observed at the (112) plane, confirming the prevalence of the CIGSe chalcopyrite phase [23].
In the case of the 30-minute samples at all temperatures, primary and secondary phases are observed due to inadequate selenization conditions that prevent the complete transformation of the Cu/In/Ga metallic precursor to the CIGSe thin film during the chalcogenization process.The activation energy of the reaction formation, controlled by temperature and annealing duration, plays a pivotal role in this regard [24].The XRD pattern (figure 3(a)) of the CIGSe film selenized for 30 min shows the presence of secondary phases like GaSe, Ga 2 Se 3 , and ternary phases CuGaSe, alongside the CIGSe phase.Upon extending the selenization duration to 60 min, a comprehensive formation of the CIGSe chalcopyrite phase is observed, manifesting various orientations of planes such as (112), (204/220), (312/116), and (400/008), consistent with the tetragonal chalcopyrite structure.This interpretation is corroborated by referencing the JCPDS card no.# 00-035-1102.An exception is noticed for the thin film selenized at 450 °C, where a secondary CuSe phase is identified (figure 3(b)).The adverse impact of this phase on the device's efficiency is a well-recognized phenomenon [25].The film's surface displays secondary and ternary phases during selenization, potentially hindering proper grain growth, yet these can be effectively mitigated through surface treatment.By elevating the selenization temperature to 550 °C for 60 min, the crystallinity of the CIGSe film is notably enhanced, thus facilitating the recrystallization process at higher temperatures.However, a further increase in temperature beyond 550 °C leads to film peeling due to disparate thermal expansion of the various metals and insufficient adhesion to the back contact.The occurrence of doublets at the (204/220), (312/116), and (400/008) orientations in the XRD pattern suggests the emergence of a cation-ordered chalcopyrite phase [26].The average crystallite sizes and lattice parameters (a) and (c) for the tetragonal crystal system were calculated using the following formula [27].
Where D denotes the size of crystallites, K represents the Scherrer constant (adopting the value of 0.94 for spherical crystallites), β stands for the angular width at half of the maximum intensity (FWHM in radians), λ corresponds to the wavelength of incident x-ray (λ = 1.5406Å), θ signifies the angle of Bragg's diffraction, d represents the spacing between crystal planes, while 'a' and 'c' symbolize the lattice constant values.Furthermore, h, k, and l are employed as the Miller indices.When investigating the preferred orientation plane (112), an observed trend was the enlargement of the crystallite size with the escalation of selenization temperature.For the 30-minute selenized film, the crystallite size increased from 30.4 nm to 37.3 nm, signifying an enhancement in crystallinity.Similarly, in the case of films selenized for 60 min at different temperatures, there was a growth in crystallite size, ranging from 42.7 nm to 62.4 nm as the temperature was raised.The dislocation density, as derived from the data presented in table 1, serves as a critical indicator of the structural integrity of the material under different selenization conditions.A higher dislocation density observed at lower selenization temperatures points towards an increased presence of structural imperfections within the material's crystal lattice.These imperfections could potentially compromise the overall quality of the films in terms of their mechanical strength and other material properties [28].Interestingly, as the selenization time and temperature were elevated, a noticeable reduction in the dislocation density values was evident.This reduction indicates a tangible enhancement in the quality of the films.In essence, the increase in selenization time and temperature seems to facilitate a refinement of the material's structure, resulting in fewer structural defects and a more favorable configuration [29].This improvement in structural quality can have positive ramifications for the material's mechanical strength and other essential  characteristics, ultimately contributing to higher-quality films.Notably, the CIGSe thin films selenized at 450 °C and 500 °C for 30 min displayed negative distortion values (Δ = 2-c/a), implying they were under compressive strain.In contrast, the film selenized for 60 min exhibited a positive distortion value.
The Raman spectroscopy analysis of the CIGSe thin film, subjected to selenization for durations of 30 and 60 min at varying temperatures, is depicted in figures 4(a) and (b).The most prominent peak, detected within the range of 177-182 cm −1 , corresponds to the A 1 mode, constituting the most intense Raman mode for chalcopyrite-based semiconductors like AIBIIICVI 2 .The vibrational patterns of Se atoms in the compound are presumably influenced by interactions with neighboring Cu, In, and Ga atoms, thereby impacting the frequency and intensity of these vibrations.Such interactions account for the distinct peak observed in the spectroscopic analysis [30].Peak observations exhibit variations in the CIGSe thin film contingent upon selenization time and temperature.At selenization temperatures of 450 °C and 500 °C for a 30-minute duration, peaks emerged at 182 cm −1 and 180 cm −1 , respectively, potentially attributable to the CuGaSe phase.Conversely, at 550 °C, the most salient peak manifested at 179 cm −1 , indicative of favorable stoichiometry and an elevated degree of formation kinetics for CIGSe.Notably, a broad peak spanning the 210-230 cm −1 range appeared, possibly associated with the B 2 /E mode.The strongest peak, observed at 177 cm −1 , consistently manifested in the CIGSe thin film selenized for 60 min at all selenization temperatures.Lastly, the presence of a broad peak around 253 cm −1 may suggest the existence of a Cu-Se secondary phase [31][32][33][34][35][36].

Morphological properties
Figures 5(a)-(f) illustrates the surface morphology of CIGSe thin films that underwent selenization at various annealing temperatures (450 °C, 500 °C, and 550 °C) and durations (30 and 60 min).The micrographs vividly showcase the pronounced influence of recrystallization temperature on film morphology.Furthermore, both annealing duration and temperature appear to affect the formation kinetics of the CIGSe thin films [37][38][39][40].The microstructure of the CIGSe thin films displayed variability with respect to selenization time and temperature, as depicted in figures 5(a)-(f).In films selenized for 30 min across all annealing temperatures, sparsely distributed grains measuring approximately 0.3 μm in size were observed.This suggests inadequate selenization time and temperature.Conversely, films selenized for 60 min exhibited a more uniform and densely packed distribution of grain sizes, with an average size of around 0.9 μm.This outcome was attributed to improved crystalline grain growth and coalescence during nucleation and growth processes.The film annealed at 550 °C for 60 min displayed a rough, well-faceted morphology, featuring compactly and regularly arranged bladed CIGSe grains.Elevating both selenization temperature and duration led to an increase in the average grain size of the film.The grain size expanded from 0.3 μm when annealed at 450 °C for 30 min to approximately 0.9 μm when annealed at 550 °C for 60 min.This indicates that higher temperatures and longer annealing duration led to larger grain growth within the thin film.Grain size is a critical material property influencing performance, making this insight valuable for optimizing the annealing process for this specific thin film [41].The larger grain size diminishes charge carrier recombination, augments minority charge carriers, and fosters a potential barrier, ultimately resulting in elevated solar cell efficiency [42][43][44].Additionally, the grain morphology evolved from undefined melted structures to well-defined agglomerates with distinct forms.Notably, increasing recrystallization temperature to approximately 550 °C for 60 min induced comprehensive recrystallization of the CIGSe film, yielding a superior microstructure characterized by uniformly distributed and densely packed facetted grains [45].
The chemical composition of the CIGSe samples was examined using energy-dispersive x-ray spectroscopy (EDS), and the outcomes are outlined in table 2. The film selenized for 30 min exhibited a composition that was deficient in copper (Cu) and enriched in gallium (Ga).This phenomenon can be attributed to incomplete interdiffusion of elements during the selenization process.However, an extension of the selenization time to 60 min resulted in a composition more closely aligned with nominal levels, rendering it well-suited for highefficiency solar cells [46][47][48].

Topographical properties
The topographical images of the CIGSe thin film subjected to various selenization conditions are depicted in figures 6(a)-(f), revealing distinct variations in grain size distribution.The selenization process and its associated parameters, particularly selenization temperatures and duration, play a crucial role in determining the film's quality, consequently influencing grain size and grain boundaries.Comparatively, the CIGSe thin film selenized for 30 min exhibited a higher surface roughness than its 60-minute counterpart, accompanied by an increase in grain boundaries.The root mean square (RMS) surface roughness for the CIGSe thin film, selenized at differing durations, is presented in table 3. Specifically, the RMS roughness for the 30-minute selenized film measured around 90 nm, while the 60-minute selenization yielded a reduced value of approximately 70 nm, indicative of a more uniform grain distribution.In contrast, the CIGSe film selenized at 550 °C displayed a smoother surface, with an RMS roughness value of 62.7 nm.These observed variations in surface topography aligned with the respective selenization conditions, highlighting the uneven element distribution on the sample surface due to the segregation of indium (In) and gallium (Ga) at surface defect sites [49].In terms of their surface diffusion rates, In and Ga tend to exhibit higher segregation compared to other chemical components [50][51][52].The stacked approach employed in the selenization process tends to drive the segregation of Ga toward the rear of the film.High RMS value (60-90 nm) originates from factors like non-uniform growth, crystallization, and substrate interactions in the film formation process.To achieve smoother films, optimizing deposition  parameters, controlling growth kinetics, enhancing substrate compatibility, and employing surface treatments can minimize roughness [18].Skewness (Rsk) and Kurtosis (Rka) parameters serve to quantitatively characterize the degree of asymmetry and sharpness within the sample, respectively.For the samples selenized at 450 °C and 500 °C for 30 min, a negative skewness value is observed, accompanied by a kurtosis value below 3.This implies that the grain size distribution in these samples is flattened and asymmetrical, with a longer tail toward the left side of the distribution.This insight contributes to a better understanding of the size distribution and morphology of the produced CIGSe thin films under these specific conditions.

Electrical properties
The performance of a CIGSe Thin-Film Solar Cell (TFSC) is significantly influenced by the carrier concentration within the absorber.This impact stems from its role in recombination rate, which involves the combination of a negatively charged electron with a positively charged hole, releasing energy in the form of light.Additionally, carrier concentration influences the CIGSe/CdS interface through the property of the space charge region (SCR).The SCR refers to a region in a semiconductor material where electric charge is distributed in a manner that creates a net electric field [53].Properties of the SCR, such as its width and charge density, have implications for solar cell performance.Measuring carrier concentration via Hall effect measurements can be challenging for polycrystalline thin films due to low mobility and alignment issues related to voltage probing [54].Table 4 summarizes the outcomes of the analysis performed on CIGSe thin films subjected to distinct selenization times and temperatures.Hall effect measurements revealed that the films exhibited p-type conductivity.The resistivity of these films can be explained by the product of carrier concentration (n), mobility (μ), and carrier charge (q).Furthermore, the inverse relationship between resistivity and this product signifies that an increase in the product of these three factors corresponds to a decrease in thin film resistivity.This insight aids in comprehending how the selenization process impacts the electrical properties of CIGSe thin films and facilitates optimization for specific applications [55].The selenization parameters (temperature and duration) wield a significant influence over the carrier concentration and mobility of CIGSe thin films.An elevation in selenization time and temperature leads to increased carrier concentration and mobility, attributed to the enlargement of grain size and higher copper (Cu) composition.The CIGSe sample selenized at 550 °C for 60 min demonstrated the highest carrier concentration at 1.22 × 10 17 cm −3 , accompanied by a resistivity of 1.31 Ω-cm and a mobility of 42 cm 2 V −1 -s −1 , surpassing other tested samples.The optical band gap of the CIGSe thin film was also computed from absorbance spectra (figure 7) obtained through UV-vis spectroscopy, yielding a value in close proximity to the estimated one (equation ( 1)).Notably, the energy band gap value exhibited a shift towards higher energy for shorter selenization time (30 min) and lower selenization temperature (450 °C).This shift can be attributed to the contribution from the GaSe phase, which possesses a band gap value of approximately 1.2 eV [57,58].Furthermore, the segregation of Ga towards the back and front of the CIGSe thin film could account for the higher band gap value (1.2 eV) [59].In the case of the CIGSe thin film selenized for 60 min, the estimated band gap value fell within the range of 1.11 eV to 1.13 eV, as depicted in figure 8.A comparison of optical band gap values of the CIGSe thin film, estimated through theoretical and experimental approaches, is presented in table 5.

Depth profile
When a CIGSe sample is bombarded by primary ions (Cs+) with energies in the range of a few keV, a portion of the emitted particles undergoes ionization.The outcome of this process provides a depth profile of the copperindium-gallium metallic precursor film, which is evaporated sequentially prior to and following selenization.The deposition of metallic precursors is observed as a stacked arrangement, exhibiting minimal diffusion of elements among each other.Upon subjecting the sample to selenization at 550 °C for 60 min, the metallic composition transforms into CIGSe, characterized by a uniform distribution of all constituent elements throughout the film thickness, as depicted in figure 9.The SIMS (Secondary Ion Mass Spectrometry) analysis unveiled consistent intensities for the Cu, In, Ga, and Se elements across the entire depth of the film after selenization.However, it is crucial to acknowledge that the intensities of secondary ions in SIMS cannot be employed as precise indicators of element quantities within the sample.This implies that the mass spectrum primarily serves a qualitative purpose, facilitating the identification of the presence and relative abundance of distinct molecular species within the sample.Nonetheless, it does not furnish exact quantitative measurements.This limitation stems from the fact that the mass spectrum relies on the analysis of ionized molecules in a sample, and the signal intensity of these ions does not consistently correlate with their concentration within the sample.

Solar cell fabrication
Considering the high quality of the CIGSe thin film, complete devices were manufactured using the CIGSe thin film selenized for 60 min.Figures 10(a)-(c) illustrates cross-sectional images of CIGSe Thin-Film Solar Cells (TFSC) selenized for 60 min at varying temperatures (450 °C-550 °C).The cross-sectional view of the solar cells revealed smaller grains for the sample selenized at 450 °C compared to the sample selenized at 500 °C.Notably, grain size increased with rising selenization temperature, signifying the influence of temperature on the reconstruction mechanism.A significant disparity was observed in the 550 °C film, characterized by the presence of larger grains with fewer grain boundaries.Enhanced grain growth corresponds to a reduced number of grain boundaries, leading to diminished recombination and thereby improving the TFSC's performance.In the operational principle of solar cells, grain boundaries within the absorber material are regarded as recombination centers [60].It is essential to note that larger grain size contributes to increased carrier lifetime, mobility, and diffusion length.These factors ultimately culminate in higher solar cell efficiency.Figure 10(d) presents the depth profile of a CIGSe TFSC that underwent selenization at 550 °C for 60 min.The open-circuit voltage (Voc) increased from 554 mV to 561 mV with the elevation of selenization temperature.This improvement may be attributed to a smaller number of grain boundaries on the surface of the CIGSe film selenized at a higher temperature.It's important to note that not only Voc and Jsc contribute to the lower Power Conversion Efficiency (PCE) in comparison to current leading CIGSe champion devices, but also characteristics resistance (series-Rs and shunt-Rsh) play significant roles as determinants of performance.The quality of other semiconducting layers, primarily the CdS and ZnO: Al window layer, contributes to higher Rs and lower Rsh.Poor fill factor (FF) in photovoltaic devices stems from charge extraction, recombination losses, and resistive effects [28].These impair voltage and current, lowering FF.Addressing low charge carrier mobility and recombination via material engineering, interface optimization improves FF.Interfaces between layers, and electrodes must be optimized to reduce carrier traps and recombination centers.High series resistance diminishes FF; low-resistance materials and improved electrodes minimize losses.Enhancing light absorption with nanostructures and advanced strategies elevates FF.Characterization and modeling pinpoint FF limitations, aiding targeted enhancements.Quality control curbs defects and variations, boosting FF.Systematic material, interface, and device design refinements raise FF and power conversion efficiency [16,45].A multidisciplinary approach merging material science, device physics, and engineering is pivotal for efficient, stable solar cells.As a result, the ongoing optimization of the CIGSe solar cell fabrication process aims to enhance efficiency by addressing these factors.
Figure 12(a) illustrates the External Quantum Efficiency (EQE) curve of the selenized CIGSe thin film solar cells.The observed EQE loss within the wavelength range of 350-550 nm can be attributed to the absorption of the ZnO window layer and the CdS buffer layer.The decrease in the EQE curve's flatness beyond 600 nm is a result of increased recombination and a lower minority carrier diffusion length within the CIGSe material.The discrepancies in EQE characteristics among distinct samples arise from a multitude of interacting factors that govern device performance.Material composition (CIGSe stoichiometry), purity, crystallinity, and defects notably impact EQE.Variations in charge mobility, recombination rates, and energy level alignment at interfaces introduce performance disparities.Interface quality, spanning layers within the device stack, governs charge transport, recombination, and extraction, resulting in EQE variation [18].Impurities and defects within the active layer or interfaces induce non-radiative recombination pathways, leading to a reduction in EQE.Light management effects-absorption, scattering, and reflection-within the device stack influence the optical path length and the probability of photon absorption, thereby affecting EQE.By correlating these insights with disparities in device performance, the observed distinctions in EQE among samples become clear, ultimately  Contrary to previous research conducted by the same group on soda lime glass as a rigid substrate [6], the present study involving SS as a flexible substrate highlights notable improvements in optoelectronic and morphological properties.Importantly, issues related to film quality, particularly intra-layer adhesion properties encountered during the selenization process have been effectively addressed.Moreover, this investigation underscores the enhanced mechanical durability and long-term stability of the developed flexible solar cells.These cells exhibit exceptional conformability and seamless integration into various applications.The efficiency of the cells remains robust even under mechanical stress, validating their potential for consistent performance in real-world scenarios.Flexible solar cells also exhibit promising environmental compatibility, aligning with sustainable energy practices.Our research findings support a comprehensive comparative analysis of solar cell technologies, substantiating their competitive edge in terms of efficiency, stability, and adaptability.In summary, these findings collectively strongly validate the significant potential of flexible solar cells utilizing stainless steel substrates for practical and dominant integration into real-world applications.

Conclusion
In summary, a CIGSe thin film was synthesized via thermal evaporation followed by selenization.Clear recrystallization evidence emerged from diverse techniques.Lower temperature and shorter selenization (30 min) showed secondary phases, fully realized after 60 min as confirmed by XRD.Raman spectra highlighted a distinct peak at 177 cm −1 , especially at 550 °C for 60 min.SEM showed a transition from molten to bladed grains under 1 μm.AFM exhibited 75 nm roughness at 550 °C for 60 min, confirming surface uniformity.EDS ratios aligned with efficient CIGSe-based solar cells.Hall Effect confirmed p-type conductivity.Solar cell efficiencies were 8.26%, 9.24%, and 9.91% at 450 °C, 500 °C, and 550 °C, respectively, indicating optimal layer quality at 550 °C for 60 min.Efficiency variations can be attributed to factors like defect levels, minority carrier diffusion length, and the crystalline structure of the absorbers.To enhance the photoconversion efficiency of the findings presented, future endeavors could encompass measures such as minimizing defects through effective passivation, precise control of stoichiometry, optimizing interfaces between layers, augmenting carrier mobility via nanostructure alloying, improving light trapping through surface texturing, exploring multijunction designs, and refining the overall device architecture.Further potential lies in comprehensive analysis and optimization of the various layers encompassing the CIGSe absorber, offering avenues for enhancing the efficacy of CIGSe solar cell fabrication on flexible substrates.

Figure 9 .
Figure 9. SIMS results of (a) metallic precursor before selenization and (b) CIGSe thin film after selenization at 550 °C for 60 min.

Figure 11 .
Figure 11.Current-voltage characteristics of CIGSe thin film selenized for 60 min at different temperatures.

Figure 12 .
Figure 12.(a) External quantum efficiency (EQE) of CIGSe thin film selenized for 60 min at different temperatures and (b) band gap calculation of CIGSe thin film selenized for 60 min at 550 °C.

Table 1 .
Calculated parameters from the XRD patterns.

Table 2 .
EDS compositional details of CIGSe thin films.

Table 4 .
Electrical properties from the Hall effect measurement of CIGSe thin film.

Table 5 .
Comparison of optical band gap values of CIGSe thin film estimated from theoretical and experimental basis.
Selenization time (minutes) Temp.(°C) Calculated energy band gap from equation (3) (eV) 3.8.Current-voltage characteristics of CIGSe TFSCCIGSe TFSCs were manufactured from samples selenized for 60 min with varying temperatures ranging from 450 to 550 °C, as depicted in figure 11.Notable differences in the performance of the CIGSe TFSCs were observed as the selenization temperature increased from 450 °C to 550 °C.The resulting CIGSe solar cells demonstrated efficiencies of 8.26%, 9.24%, and 9.91% (as outlined in table 6) for the samples selenized at 450 °C, 500 °C, and 550 °C, respectively.The variation in Power Conversion Efficiency (PCE) may stem from factors such as defect levels, minority carrier diffusion length, and the crystallinity of the absorber material.As detailed in the fundamental characterizations (structural, morphological, electrical), the metal stacks exhibit effective inter-diffusion and react with selenium to form a unified CIGSe phase.At elevated selenization temperatures (550 °C), the CIGSe thin film demonstrates enhanced crystallinity due to sufficient thermal

Table 6 .
Calculated parameters of CIGSe thin film solar cells.The band gap of the CIGSe film selenized for 60 min at 550 °C was additionally estimated from the EQE data using a linear extrapolation of the plot [hν x ln(1-EQE)]2 versus hν, as depicted in figure 12(b).