{"paper_id":"159ffa2d-38d7-478f-ab61-102d23f27d3b","body_text":"Interplay Between Strain and Charge in Cu(In,Ga)Se2 Flexible Photovoltaics | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Article Interplay Between Strain and Charge in Cu(In,Ga)Se2 Flexible Photovoltaics William Jo, Ha Kyung Park, Kanghoon Yim, Jiyoon Lee, Yunae Cho, and 5 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-4364503/v1 This work is licensed under a CC BY 4.0 License Status: Published Journal Publication published 16 Sep, 2024 Read the published version in npj Flexible Electronics → Version 1 posted You are reading this latest preprint version Abstract Flexible and lightweight Cu(In,Ga)Se 2 (CIGS) thin-film solar cells are promising for versatile applications, but there is limited understanding of stress-induced changes. In this study, the charge carrier generation and trapping behavior under mechanical stress was investigated using flexible CIGS thin-film solar cells with various alkali treatments. Surface current at the CIGS surface decreased by convex bending, which occurs less with the incorporation of alkali metals. The formation energy of the carrier generating defects increased in convex bending environments clarifying the degradation of the surface current. Moreover, alkali-related defects had lower formation energy than the intrinsic acceptors, mitigating current degradation in mechanical stress condition. The altered defect energy levels were attributed to the deformation of the crystal structure under bending states. This study provides insights into the mitigating of strain-induced charge degradation for enhancing the performance and robustness of flexible CIGS photovoltaic devices. Furthermore, direct atomic force microscopy-based probing techniques on bent material surfaces can contribute to the understanding of interplay between the strain and charge in other flexible optoelectronic devices. Physical sciences/Energy science and technology/Renewable energy/Solar energy/Photovoltaics/Solar cells Physical sciences/Materials science/Materials for energy and catalysis/Solar cells Flexible Cu(In Ga)Se2 solar cells alkali incorporation stress-induced property surface current Figures Figure 1 Figure 2 Figure 3 Figure 4 Introduction The recent expansion of the solar energy market has amplified the crucial role of flexible and lightweight thin-film solar cells in reducing manufacturing cost and broadening photovoltaic applications. Among the light-absorbing materials, Cu(In,Ga)Se 2 (CIGS) is a promising candidate for commercialized flexible thin-film solar cells due to its high efficiency and stability. The most recently reported record cells have achieved a high efficiency of over 22% with alkali fluoride (AlkF) post-deposition treatments (PDT), especially when Rb and Cs are used. 1, 2, 3 The incorporation of heavy-alkali (Rb, and Cs) PDT in addition to light-alkali (Na) has a synergistic effect to maximize the device performance increasing the carrier concentration and improving the grain growth. 4 Furthermore, the PDT of alkali elements boosts the performance of flexible CIGS solar cells with a substrate that does not contain alkali elements. 5, 6, 7 Notably, flexible CIGS solar cells with a polyimide (PI) substrate exhibited the efficiency of 20.82% when the doping of heavy-alkali (RbF) PDT was controlled. 8 Recently, Ishizuka et al. reported flexible minimodules with an efficiency of 18.65% by Rb-PDT in addition to Na-PDT and K-PDT. 9 For flexible solar cells, understanding the characteristics of CIGS materials under stress is crucial to developing mechanically robust devices. However, changes in the flexible CIGS solar cells under mechanical stress and stress-induced modifications in CIGS materials have not been extensively examined. Few reports on the degradation of device performance after mechanical bending tests are available. 10, 11 In light of this, we previously demonstrated that the incorporation of alkali in flexible kesterite solar cells improved the stress-induced loss in the local V OC by designing a measurement system based on scanning probe microscopy that can be applied to a bent surface. 12 In this study, changes in the electrical charge states of flexible CIGS thin-film solar cells doped with alkali (e.g., Na, Rb and Cs) were investigated under mechanical bending stress. Under mechanical bending, the lattice constants changed, leading to the rearrangement of elements. Herein, based on scanning probe microscopy, the surface current of a bent surface was measured by attaching flexible CIGS samples on metal holders with specific curvatures. The distribution of current on the surface of the flexible CIGS improved with alkali incorporation and was degraded under mechanical bending stress due to alterations in the charged states. However, CIGS with additional alkali incorporation exhibited significantly suppressed degradation even under the bending states. Density functional theory (DFT) study provided insights into the strain effect on CIGS films at the atomic level, potentially revealing the underlying mechanism for the reduced degradation observed in alkali-incorporated CIGS films under bending stress. We conclude that defect-level redistribution with alkali incorporation enhances the tolerance of CIGS thin films to mechanical stress. Results and discussions CIGS crystal structure and distribution of alkali elements at the grain boundaries (GBs) were determined from high-resolution scanning transmission electron microscopy (STEM) image, as shown in Fig. 1 a. The d-spacing obtained using the fast-Fourier transform (FFT) patterns was 0.329 nm, which corresponded to the CIGS grain with (1 1 2) orientation and assuming (1 1 2) facets facing GBs. The distribution of alkali elements was investigated by STEM-energy dispersive spectroscopy (EDS) measurements. In Figs. 1 b and c , the GBs of the heavy-alkali-doped CIGS showed a Cu-depleted composition. By contrast, the concentration of heavy alkali increased at the GBs compared to the intragrain regions, suggesting heavy alkali occupation of the Cu vacancy sites. Several studies have demonstrated empirically and theoretically the segregation of heavy alkalis toward the GBs, in which the heavy alkalis pushed light alkalis away from the GBs. 13, 14, 15 The segregation of heavy alkali toward GBs passivated the detrimental defects and modified the energy level of the Cu vacancy ( V Cu ) sites. 16, 17 However, the migration of expelled light alkalis is debating. On the one hand, several studies have indicated that relatively light alkali was pushed away toward the surface. On the other hand, some studies showed that the repelled light alkali accumulated in the grain interior. 18, 19, 20 The EDS images of other elements are presented in Figure S1 in the Supporting Information . Herein, the concentration of Na was under the detection limit of EDS, which hindered the identification of the presence of Na. In addition to the expulsion of the light alkali at the GBs, the in-depth intensity of Na decreased significantly after the heavy-alkali PDT compared to that of the CIGS sample with NaF PDT only. Figure 1 d shows that the in-depth distribution of Na was hardly detected in the heavy-alkali PDT CIGS, and this result was comparable to the TEM-EDS results. The significant decrease in Na in the entire film suggested that the light alkalis from the GBs repelled by the heavy alkalis apparently moved toward the surface and washed away during the chemical bath deposition of CdS. The intensity of heavy alkalis at the GBs was higher than that of Na although the nominal thickness of the alkali PDT was identical. The heavy alkalis slightly increased at the back of the CIGS absorber. A previous study has demonstrated that the concentration of Rb at the GBs increased toward the Mo back contact. 4 To investigate the carrier behavior under mechanical stress, the current at the surface was probed using conductive-atomic force microscopy (c-AFM). As shown in Fig. 2 a, a sample holder with curvature mold was prepared to apply artificial concave and convex bending to the flexible CIGS thin-film. 21, 22 Mechanical stress was continuously applied as the samples were attached to the holder. Therefore, formation of the surface current during mechanical bending was obtained directly as the bent CIGS absorber surface was probed. Severe cracks or breakage induced by the stress were not observed in the topographies, indicating that the CIGS mechanically tolerated the strain with bending radius of 20.47 mm (See Figure S2 in the Supporting Information ). A previous study has also presented that the CIGS thin-film tolerated tensile stress lower than 1.0%, and stress of more than 1.0% resulted in grain boundary (GB) cracking that affected the carrier transportation along the GBs. 23 The surface current flowed along the GB of CIGS thin film, as depicted in the surface current map in Fig. 2 b. A two-dimensional (2D) topography and surface current map of all CIGS can be found in Figure S3 in the Supporting Information . This illustration highlights the GBs as the main conduction channels, particularly, when an external bias is applied to the sample emitting electrons from the samples to the grounded tip. 24 The samples with alkali PDT showed enhanced formation of the surface current presumably due to the increased carrier concentrations. Additionally, Sharma et al. recently demonstrated that the CIGS thin film treated with Rb and Cs possessed higher carrier concentrations and enhanced homogeneity of carrier concentration. 25 The segregated alkalis at GBs changed the charged defect states, resulting in different electrostatic properties. 26, 27, 28 The quantitative distribution of the surface current from the three-dimensional current maps is shown in Fig. 2 c, in which the surface current values from 20 GBs were extracted. The distribution of the surface current significantly decreased under convex bending, indicating a decrease in conductivity. A comparison between samples with and without mechanical stress history is presented in Figure S4 . Under convex bending, the surface current decreased in the CIGS samples both with and without alkali PDT. However, CIGS with alkali PDT showed less degradation in surface current compared to the CIGS without PDT, which showed a remarkable decrease in current. Some GBs of the alkali PDT CIGS maintained original values even under convex bending, indicating that the presence of alkali elements at the GBs hindered the decrease in conductivity. The electrical properties at the GBs could be attributed to the distribution of defect states. Therefore, changes in the surface current under mechanical bending might have originated from the redistribution of defect states at the GBs. To investigate how the changes in defect states at CIGS GBs depend on strain, we conducted DFT calculations to determine the formation energies of various defect states, including intrinsic and alkali-incorporated defects, under varying strain conditions. Assuming that GBs perpendicular to the film surface are relevant to the region where the highest current peak is observed in c-AFM images, we consider the strain conditions in the concave and convex bending states as shown in Fig. 2 d. Under concave bending, the compressive axial stress within the film induces planar expansion of grain boundaries (GBs). Conversely, the opposite effect, GB contraction, is expected under convex bending. Given that (1 1 2) facets with copper deficiency are expected to be dominant GBs in CIGS as shown in Fig. 2 d, we adopt the Cu-deficient (1 1 2) surface structure obtained from our previous study to effectively mimic the defect chemistry at GBs. 15 Among all calculated defect configurations (see Figure S5 and Table S1 for the all considered defects), defect complexes combining alkali interstitials (Na i , Rb i , and Cs i ) and Cu vacancy ( V Cu ) have the lowest defect formation energies. More specifically, larger alkali metals tend to have lower defect formation energies when incorporated next to the V Cu defect. This explains why rubidium (Rb) and cesium (Cs) dominate at grain boundaries (GBs) in NaF + RbF and NaF + CsF PDT CIGS, respectively. These defect complexes (Na i - V Cu , Rb i - V Cu , and Cs i - V Cu ) differ from Cu substitutional defects (Na Cu , Rb Cu , and Cs Cu ) as alkali metals does not occupy the Cu sites. Notably, Rb Cu and Cs Cu have much larger formation energy compared to Rb i - V Cu , and Cs i - V Cu suggesting that Rb and Cs cannot permeate into intra-grain. Interestingly, while these defect complexes (Na i - V Cu , Rb i - V Cu , Cs i - V Cu ,) are electrically neutral, the formation energies of additional V Cu near the Na i - V Cu complex is even lower than V Cu without PDT. This provides feasible explanation for the origin of the largest current increasement at GBs in NaF PDT CIGS. In Fig. 2 e, the calculated formation energies of major carrier-generating defects are compared under axial strains on CIGS (1 1 2) surface, ranging from − 2% to + 2% of the lattice parameters. Under compressive axial strain applied to the (1 1 2) surface of CIGS, which corresponds to the convex bending condition, the formation energy of V Cu − defects increases, leading to a decrease in carrier concentration. On the contrary, tensile axial stress boost the formation of V Cu + . Notably, the formation energies of V Cu near Na i - V Cu complex ( V Cu @Na i− V Cu ) is much lower than V Cu that explains the reason that NaF PDT CIGS shows lowest degradation compared to other conditions. Herein, it was assumed that grain boundaries possess a single type of alkali element as heavy alkalis tended to segregate into the GBs by pushing away Na from the GBs as presented in Fig. 1 . To investigate the structural changes in the CIGS with mechanical stress, angle-tilted X-ray diffraction (XRD) measurements were performed for the CIGS both in the planar and bent states. Herein, Ψ indicates the angle between the diffraction scattering vector and the normal direction of samples. In the planar state, the CIGS (3 1 2) orientation peak presented the peak shift toward the lower diffraction angle by tilting angle Ψ, indicating tensile stress. On the other hand, the Mo (1 1 0) peak presented the peak position shift toward a higher angle, indicating compressive stress (Figs. 3 a, b). Several previous studies have reported the stress analysis of CIGS thin-films with intrinsic compressive stress when they were deposited on the SLG. 29, 30 Tensile stress formed in the CIGS thin film was probably attributed to the relatively thicker Mo back contact layer with smaller thermal expansion coefficient than the CIGS thin film. 31 The angle-tilted XRD patterns of the CIGS samples without PDT and NaF PDT under mechanical bending both presented tensile stress, indicating that mechanical bending did not compensate the intrinsic tensile stress (See Figure S6 and S7 in the Supporting Information ). As shown in Fig. 3 c, NaF PDT CIGS showed a smaller slope than that of the sample without PDT when the tilting angle was increased, indicating that NaF PDT CIGS had lower residual tensile stress. The NaF PDT CIGS sample presented slightly larger d-spacing compared to that without PDT, because the size of the Na atom was larger than the other elements, thus widening the crystal structure. Therefore, incorporation of the Na atoms produced relative compressive strain through the lattice site by enlarging the d-spacing, thereby slightly compensating the intrinsic tensile stress in the film. Out-of-plane XRD patterns for the lattice of the thin film were recorded under planar and bent states to measure the crystal plane spacing. The crystal plane spacing perpendicular to the film presents a coherent direction with the crystal structure model of Fig. 2 d, with the vertical direction of (1 1 2) facets assumed to be facing GBs. Figures 3 d and 3 e demonstrate the shifts of the (1 1 2) and (3 1 2) orientation peaks under mechanical stress. The (1 1 2) and (3 1 2) orientation peaks shifted toward lower diffraction angle (i.e., larger d-spacing) under convex bending, while it shifted toward higher diffraction angle (i.e., smaller d-spacing) under concave bending. These d-spacing changes supported our model for the microdeformation of the crystal structure due to mechanical stress as shown in Fig. 2 d. The calculated lattice parameters of CIGS crystal under biaxial strain (see Table S2 ) are also consistent with the XRD analysis. The correlation between carrier distribution and crystal structure modification is depicted in Fig. 3 g. Under convex bending, the biaxial compressive stress in the plane direction led to an increase in the acceptor energy levels, hindering the generation of charge carriers. By contrast, biaxial tensile stress decreased the energy levels of the acceptor defects under concave bending, thereby maintaining high carrier concentrations. This modification in carrier concentrations according to the mechanical bending can be related to the piezophototronic or flexoelectric effect in a dielectric semiconductor or device. 32 The piezophototronic effect in heterojunction devices facilitates or impedes the transport of photo-generated carriers at the heterojunction interface by mechanical stress modulating the junction region, which in turn affects the photoresponsivity or photovoltaic parameters. The piezophototronic response of the flexible CIGS thin-film optoelectronic devices is increased by the application of artificial static compressive strain, thereby modifying the interface energy band. 33, 34 Thus, the enhancement in carrier concentration at the CIGS absorber surface under concave bending can be potentially attributed to the piezophototronic effect in flexible CIGS optoelectronic devices. Figure 4 demonstrated the performances and carrier dynamics of flexible CIGS thin film solar cells. Figure 4 a shows the current density-voltage ( J-V ) curve of the flexible CIGS thin-film solar cells, and the photovoltaic parameters are presented in Table 1 . CIGS devices with AlkF PDT had enhanced device performances compared to the device without PDT. Among the CIGS samples, the NaF PDT sample exhibited the highest power conversion efficiency, followed by NaF + RbF and NaF + CsF PDT CIGS. The V OC was enhanced with alkali PDT, leading to comparable values among the alkali PDT samples. The alkali PDT samples showed improved fill factor (FF) compared to the samples without PDT, but FF decreased with additional heavy-alkali PDT. The decrease in FF could be attributed to the formation of a thin Alk–In–Se compounds layer that acted as an electron barrier by inducing a downward valence band edge on the low-Ga CIGS surface. 18, 35, 36 A box plot of the solar cell parameters is presented in Figure S8 (Supporting Information) . Figure 4 b presents the temperature-dependent V OC of various alkali-treated CIGS solar cells. Temperature-dependent J-V curves for all temperature steps are presented in Figure S9 . Activation energy ( E A ) could be extracted at the point T = 0 K from the linear fitting of the V OC -T plot when the diode ideality factor was not dependent on temperature. 37 The CIGS devices with AlkF PDT provided the larger E A compared to the CIGS device without PDT, which exhibited a value of 0.989 eV. Thus, interface recombination was alleviated by AlkF PDT, regardless of the type of alkali used. Among the samples, the NaF PDT samples exhibited the highest E A value of 1.050 eV. Table 1 Photovoltaic device parameters of flexible CIGS solar cell devices with best efficiency in each treatment and N CV . Sample V oc (V) J sc (mA/cm 2 ) FF (%) PCE (%) N CV (cm − 3 ) Non-PDT 0.550 37.93 58.0 12.10 1.28 × 10 15 NaF PDT 0.642 36.2 72.8 16.9 2.87 × 10 16 NaF + RbF PDT 0.639 36.29 64.43 14.95 9.05 × 10 15 NaF + CsF PDT 0.642 36.4 63.38 14.81 3.59 × 10 15 Carrier concentrations and defect energy levels of various alkali-doped CIGS solar cell were investigated by measuring the capacitance-voltage ( C-V ) characteristics. The results from C-V measurements showed that the increase in carrier concentrations ( N CV ) of samples with alkali PDT samples (Fig. 4 c). All alkali PDT samples demonstrated improved N CV compared to the Non-PDT sample, as they enhanced the p-type conductivity. The NaF PDT CIGS had the highest N CV , as it increased the charge carrier density most effectively. 38 In addition, the NaF PDT CIGS sample showed the highest density of free carriers and the lowest density of the deep defects among the alkali PDT samples as presented in Figure S10, Table S3 and S4 in supporting information. The depletion width decreased in alkali PDT samples, which could lead to a reduction in J SC due to hindered photocarrier collection and separation, consistent with previous studies. 5, 39 Admittance spectroscopy (AS) measurements were performed to determine the defect energy levels. The energy levels of the defect states as shown in Fig. 4 d were calculated using the linear fitting of the Arrhenius plot derived from the derivatives of the AS spectra in Figs. 4 e–h. The AS results indicated that irrespective of the type of alkali used, the defect energy levels decreased after alkali treatment compared to the sample without PDT. 40 The alkali PDT samples possessed lower defect energy levels due to the defect passivation effect of the alkali elements. 41 The high density of the defects resulted in the low carrier concentration of the samples without PDT. By contrast, alkali PDT samples possessed lower trap densities and reduced defect energy levels compared to the samples without PDT. This characteristic could be attributed to higher carrier concentrations as evident in planar state surface current map presented in Fig. 2 b. In particular, improved free carrier concentrations with lowered activation energy presumably contribute to lower charge carrier degradation under mechanical bending conditions. Conclusion In conclusion, the results of the c-AFM measurement performed to probe the surface current of the CIGS thin-film under different mechanical bending states showed that the quantitative surface current was degraded under convex bending. Materials modelling demonstrated that convex bending increased the acceptor defect formation energy, resulting in lower carrier concentrations as observed using the surface current map. Nevertheless, alkali PDT CIGS showed less degradation in the surface current under convex bending, which was attributed to the lower energy level of the alkali-related acceptors. The redistribution of the alkali-related defects was enabled due to the change in crystal plane spacings under the mechanical bending states. These findings suggested that the incorporation of alkali elements improved the stress tolerance in terms of carrier generation. In addition, the mitigation of intrinsic tensile stress in the CIGS thin films deposited on the flexible PI substrates contributed to the reduction in acceptor defect levels and generation of carriers. Thus, the developed probing technique to map the current distribution on a bent surface provides practical information as it simulates the environment experienced by flexible electronic devices in real-world applications. Experimental Film/device fabrication A PI film was used as the flexible substrate with a DC-sputtered Mo coating (700 nm) as a back contact. Three-stage co-evaporation method was used to grow the CIGS thin films with additional in - situ alkali PDT. The nominal thickness of the alkali (Na, Rb, Cs) PDT layer was 10 nm. Additional annealing with Se supply (15 Å/s) was performed after the alkali PDT. The temperature for the whole process was maintained under 450 ℃. To complete the device fabrication, 40–60 nm of the CdS buffer layer was deposited on the CIGS thin film by using chemical bath deposition. Then, 50 nm of intrinsic zinc oxide (i-ZnO) and 150 nm of indium tin oxide (ITO) layer were deposited by radio frequency (RF) sputtering. An aluminum (Al) top electrode was deposited, and 110 nm of magnesium fluoride (MgF 2 ) was deposited for anti-reflect coating by using electron beam evaporation. Characterization The J-V characteristics of the finished devices were measured with a solar simulator (WACOM, WXS-155S-L2), under a standard test condition of AM 1.5 G spectrum and intensity of 1 sun (100mW·cm –2 ) at 25°C. The temperature-dependent V OC ( V OC -T ) was performed using a semiconductor characterization system (Keithley 4200, Tektronix, Inc.). A vacuum probe stage with a temperature controller (HFS600E-PB4, Linkam) was used to vary the temperature from 300 K to 80 K with an interval of 10 K at a pressure of 10 –3 Torr. For the temperature-dependent C-V , DLCP, and AS measurements, a LCR meter (Agilent 4284A) was used with a probe station (LTS 350, Linkam) applying the same temperature interval as the V OC -T measurement. The microstructures and element distribution were characterized by transmission electron microscopy (TEM; JEM-ARM200F “NEO ARM”, JEOL) and energy-dispersive X-ray spectrometry (EDS, JED-2300T(Dual), JEOL), respectively. The accelerating voltage for the STEM observations was 200 kV. A cross-section lamella of the CIGS thin film for the TEM measurement was prepared using a focus-ion beam system. An O 2 sputtering source was used for the time-of-flight secondary ion mass spectrometry (ToF-SIMS) measurements instead of the Cs source to avoid the detection of a potential Cs source. Commercial atomic force microscopy (Nanofocus, n-tracer) was used to perform c-AFM measurement. Contact mode was used to scan the surface, and the set point value was 1 nN. A Pt/Ir-coated tip (Nanocensors Inc.) was used for the measurement. An external bias of 0.5 V (DC) was applied to the sample stage with the tip grounded, and the detection limit of the surface current was 10 nA. The scan area was 2 × 2 µm 2 , and the scan resolution was 256 × 256 pixels (256-line scan). During the scanning, the scan rate was maintained below 0.3 Hz to minimize the tip damages. Detail method for c-AFM measurement on the bent surface is explained in the supporting information. For the XRD measurements, the height of the samples loaded were adjusted to be identical to avoid the influence of the difference of the distance between the sample and the detector. DFT calculations Total energy and electronic structure calculations were performed using density functional theory (DFT). The Vienna Ab initio Simulation Package 42 with projector augmented-wave pseudo-potentials was employed. The generalized gradient approximation was applied for the exchange-correlation function, and the DFT + U method was used for Cu with U eff = 4 eV for the d states. 43 A CuIn 0.5 Ga 0.5 Se 2 unit cell was used to represent the complex configurational space of CIGS (See Figure S5 in the Supporting Information ). The energy cut-off of the plane-wave basis was set to 300 eV, and k -points were sampled as 3 × 3 × 2 Monkhorst-pack mesh that satisfy the convergence criteria of energy/atom < 0.01 eV and pressure < 10 kBar. To calculate the defect chemistry at the CIGS GBs, a 148-atom slab structure with (112) surface was used. The defect formation energy ( E f ) was calculated as follows $${E}_{\\text{f}}\\left(defect\\right)={E}_{\\text{t}\\text{o}\\text{t}}\\left(defect\\right)-{E}_{\\text{t}\\text{o}\\text{t}}\\left(perfect\\right)+ \\sum {\\text{N}}_{\\text{i}}{\\mu }_{\\text{i}}+q\\left({\\epsilon }_{\\text{F}}+{\\epsilon }_{\\text{V}\\text{B}\\text{M}}\\right)+{E}_{\\text{c}\\text{o}\\text{r}\\text{r}}$$ , where E tot is the total free energy of the supercell; and N i and µ i are the quantities and chemical potentials of the exchanging chemical species, respectively. For a charged defect, q is the charge state of the defect, ε F is the Fermi level from the valence band maximum ( ε VBM ), and E corr is the charge correction energy determined using the Makov-Payne method. 44 To calculate the changes in lattice parameters under biaxial strain, we modify the VASP code to allow the lattice relaxion only along the z-axis. For all calculations, atomic configurations were optimized, until the atomic force converged within 0.01 eV/Å. Declarations Author Contributions H.K.P. and K.Y. contributed equally to this work and the manuscript was mainly written by the first authors, H.K.P. and K.Y. H.K.P. designed the research and performed the T-V OC , DLCP, AS, c-AFM, SEM measurements. K.Y. conceived the computational research and performed the DFT simulations. J.L. contributed to the c-AFM measurement. Y.C. assisted the experiments and characterizations. I.J., D.S. and K.K. designed the fabrication of the flexible CIGS solar cells and conducted the IV measurements. J.G. supervised the research program. A.W. contributed the validation of computational study and revised the manuscript. K.K. and W.J. supervised the overall work. All authors discussed the results and commented on the manuscript. Competing interests The authors declare no competing interests. Acknowledgements This research was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (NRF-2018R1A6A1A03025340) and Ministry of Science, Technology, ICT, and Future Planning (NRF-2021R1A2B5B02001961). This work was supported by a grant (NRF-2022M3J1A1064229) of the Technology Development Program for the solving of climate change, and the grant was provided through the NRF funded by the Ministry of Science and ICT. The first author, H.K.P., gratefully acknowledges the scholarship support received from the Hyundai Motor Chung Mong-Koo Foundation. References Nakamura M, Yamaguchi K, Kimoto Y, Yasaki Y, Kato T, Sugimoto H. Cd-Free Cu(In,Ga)(Se,S) 2 Thin-Film Solar Cell With Record Efficiency of 23.35%. IEEE J. Photovolt. 9, 1863–1867 (2019). Jackson P, Wuerz R, Hariskos D, Lotter E, Witte W, Powalla M. Effects of heavy alkali elements in Cu(In,Ga)Se 2 solar cells with efficiencies up to 22.6%. Phys. 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C 123, 17757–17764 (2019). Cheng S, Zhang K, Lin S, Zhang Y, Sun Y, Liu W. Analysis of the Heavy Alkali Element Postdeposition Treatment: Which Factors Determine the Electronic Structure and Transport Properties of the Heterojunction in CIGS Thin Film Solar Cells. ACS Appl. Energy Mater. 4, 3279–3287 (2021). Cho Y, et al. Photon-induced defects and dynamics of photogenerated carriers in Cu(In,Ga)Se 2 thin film solar cells. Sol. Energy Mater. Sol. Cells 220, 110860 (2021). Pianezzi F, et al. Unveiling the effects of post-deposition treatment with different alkaline elements on the electronic properties of CIGS thin film solar cells. Phys. Chem. Chem. Phys. 16, 8843–8851 (2014). Khatri I, Matsuura J, Sugiyama M, Nakada T. Effect of heat bias soaking on cesium fluoride treated CIGS thin film solar cells. Prog. Photovolt.: Res. Appl. 27, 22–29 (2018). Werner F, et al. Alkali treatments of Cu(In,Ga)Se 2 thin-film absorbers and their impact on transport barriers. Prog. Photovolt.: Res. Appl. 26, 911–923 (2018). Colombara D, Conley K, Malitckaya M, Komsa H-P, Puska MJ. The fox and the hound: in-depth and in-grain Na doping and Ga grading in Cu(In,Ga)Se 2 solar cells. J. Mater. Chem. A 8, 6471–6479 (2020). Kresse G, Hafner J. Ab initio molecular dynamics for liquid metals. Phys. Rev. B Condens Matter 47, 558–561 (1993). Wang L, Maxisch T, Ceder G. Oxidation energies of transition metal oxides within theGGA + Uframework. Phys. Rev. B 73, 195107 (2006). Makov G, Payne MC. Periodic boundary conditions in ab initio calculations. Phys. Rev. B Condens Matter 51, 4014–4022 (1995). Additional Declarations There is NO Competing Interest. Supplementary Files NatCommunSIAlkaliCIGSHKP240503.docx Supplementary Information Cite Share Download PDF Status: Published Journal Publication published 16 Sep, 2024 Read the published version in npj Flexible Electronics → Version 1 posted You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. We do this by developing innovative software and high quality services for the global research community. Our growing team is made up of researchers and industry professionals working together to solve the most critical problems facing scientific publishing. Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {\"props\":{\"pageProps\":{\"initialData\":{\"identity\":\"rs-4364503\",\"acceptedTermsAndConditions\":true,\"allowDirectSubmit\":false,\"archivedVersions\":[],\"articleType\":\"Article\",\"associatedPublications\":[],\"authors\":[{\"id\":300759754,\"identity\":\"0b6b5f6b-0afe-44fe-8e05-53ade3738a45\",\"order_by\":0,\"name\":\"William 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University\",\"correspondingAuthor\":false,\"prefix\":\"\",\"firstName\":\"Ha\",\"middleName\":\"Kyung\",\"lastName\":\"Park\",\"suffix\":\"\"},{\"id\":300759756,\"identity\":\"d54cb1cb-e954-4c54-becd-69183e1188ed\",\"order_by\":2,\"name\":\"Kanghoon Yim\",\"email\":\"\",\"orcid\":\"https://orcid.org/0000-0002-3945-0215\",\"institution\":\"Korea Institute of Energy Research\",\"correspondingAuthor\":false,\"prefix\":\"\",\"firstName\":\"Kanghoon\",\"middleName\":\"\",\"lastName\":\"Yim\",\"suffix\":\"\"},{\"id\":300759758,\"identity\":\"fd5aba7b-a896-4615-af2c-f7de0f13a671\",\"order_by\":3,\"name\":\"Jiyoon Lee\",\"email\":\"\",\"orcid\":\"\",\"institution\":\"Ewha Womans University\",\"correspondingAuthor\":false,\"prefix\":\"\",\"firstName\":\"Jiyoon\",\"middleName\":\"\",\"lastName\":\"Lee\",\"suffix\":\"\"},{\"id\":300759760,\"identity\":\"282be9ad-18c8-4ae5-88b9-676b65193397\",\"order_by\":4,\"name\":\"Yunae Cho\",\"email\":\"\",\"orcid\":\"\",\"institution\":\"Korea Institute of 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Research\",\"correspondingAuthor\":false,\"prefix\":\"\",\"firstName\":\"Jihye\",\"middleName\":\"\",\"lastName\":\"Gwak\",\"suffix\":\"\"},{\"id\":300759765,\"identity\":\"05f5084b-a6f2-427a-922f-760dee2e7158\",\"order_by\":8,\"name\":\"Aron Walsh\",\"email\":\"\",\"orcid\":\"https://orcid.org/0000-0001-5460-7033\",\"institution\":\"Imperial College London\",\"correspondingAuthor\":false,\"prefix\":\"\",\"firstName\":\"Aron\",\"middleName\":\"\",\"lastName\":\"Walsh\",\"suffix\":\"\"},{\"id\":300759766,\"identity\":\"bd0ae924-dd15-4041-a06e-b170b41660eb\",\"order_by\":9,\"name\":\"Kihwan Kim\",\"email\":\"\",\"orcid\":\"\",\"institution\":\"Korea Institute of Energy Research\",\"correspondingAuthor\":false,\"prefix\":\"\",\"firstName\":\"Kihwan\",\"middleName\":\"\",\"lastName\":\"Kim\",\"suffix\":\"\"}],\"badges\":[],\"createdAt\":\"2024-05-03 13:55:54\",\"currentVersionCode\":1,\"declarations\":\"\",\"doi\":\"10.21203/rs.3.rs-4364503/v1\",\"doiUrl\":\"https://doi.org/10.21203/rs.3.rs-4364503/v1\",\"draftVersion\":[],\"editorialEvents\":[{\"content\":\"https://doi.org/10.1038/s41528-024-00347-7\",\"type\":\"published\",\"date\":\"2024-09-16T04:00:00+00:00\"}],\"editorialNote\":\"\",\"failedWorkflow\":false,\"files\":[{\"id\":60170282,\"identity\":\"dc93a5b0-fee7-4868-97a2-3f9779a95d21\",\"added_by\":\"auto\",\"created_at\":\"2024-07-12 15:05:36\",\"extension\":\"png\",\"order_by\":1,\"title\":\"Figure 1\",\"display\":\"\",\"copyAsset\":false,\"role\":\"figure\",\"size\":234131,\"visible\":true,\"origin\":\"\",\"legend\":\"\\u003cp\\u003e\\u003cstrong\\u003eDevice properties of various alkali-doped flexible CIGS thin-film solar cells. (a) \\u003c/strong\\u003eCurrent-voltage (\\u003cem\\u003eJ-V\\u003c/em\\u003e) curves of four different samples. The NaF+RbF PDT and NaF+CsF PDT samples exhibited comparable curves resulting in an overlap. \\u003cstrong\\u003e(b)\\u003c/strong\\u003e Temperature-dependent \\u003cem\\u003eV\\u003c/em\\u003e\\u003csub\\u003eOC\\u003c/sub\\u003e plot for the flexible CIGS thin-film solar cells under illumination. \\u003cstrong\\u003e(c)\\u003c/strong\\u003e Drive-level carrier concentrations (\\u003cem\\u003eN\\u003c/em\\u003e\\u003csub\\u003eDL\\u003c/sub\\u003e) of CIGS thin-film solar cells obtained from drive-level capacitance profiling (DLCP) measurements. \\u003cstrong\\u003e(d)\\u003c/strong\\u003e Defect energy levels calculated from the Arrhenius plot. (\\u003cstrong\\u003ee–h\\u003c/strong\\u003e) Admittance spectroscopy of the flexible CIGS thin-film solar cells obtained from 80 K to 300 K.\\u003c/p\\u003e\",\"description\":\"\",\"filename\":\"Picture1.png\",\"url\":\"https://assets-eu.researchsquare.com/files/rs-4364503/v1/ffe73913adbc3dd163f1493a.png\"},{\"id\":60170277,\"identity\":\"8bb47633-71a6-485f-a421-8f33cf44870f\",\"added_by\":\"auto\",\"created_at\":\"2024-07-12 15:05:36\",\"extension\":\"png\",\"order_by\":2,\"title\":\"Figure 2\",\"display\":\"\",\"copyAsset\":false,\"role\":\"figure\",\"size\":1176790,\"visible\":true,\"origin\":\"\",\"legend\":\"\\u003cp\\u003e\\u003cstrong\\u003eDistribution of alkali elements in the bulk and grain boundaries (GBs).\\u003c/strong\\u003e \\u003cstrong\\u003e(a, b)\\u003c/strong\\u003e Scanning transmission electron microscopy (STEM)-energy dispersive spectroscopy (EDS) images of \\u003cstrong\\u003e(a) \\u003c/strong\\u003eNaF+RbF PDT and \\u003cstrong\\u003e(b)\\u003c/strong\\u003e NaF+CsF PDT samples.\\u003cstrong\\u003e \\u003c/strong\\u003eHigh-angle annular dark field (HAADF) images and EDS maps of Cu, Rb, and Cs at the GBs. White dashed line in the HAADF images indicate the random GBs. \\u003cstrong\\u003e(c)\\u003c/strong\\u003eHigh-resolution STEM image of the grain with (1 1 2) orientation. Inset presents the fast-Fourier transform (FFT) image of the white dashed box area in Figure 2c. \\u003cstrong\\u003e(d)\\u003c/strong\\u003e The depth profile of alkali elements in the alkali PDT CIGS samples was obtained by time-of-flight secondary ion mass spectrometry.\\u003c/p\\u003e\",\"description\":\"\",\"filename\":\"Picture2.png\",\"url\":\"https://assets-eu.researchsquare.com/files/rs-4364503/v1/4a170fa4b58b149806f9a630.png\"},{\"id\":60170278,\"identity\":\"8f960cef-dd35-4d54-955f-2d9df72324ea\",\"added_by\":\"auto\",\"created_at\":\"2024-07-12 15:05:36\",\"extension\":\"png\",\"order_by\":3,\"title\":\"Figure 3\",\"display\":\"\",\"copyAsset\":false,\"role\":\"figure\",\"size\":879789,\"visible\":true,\"origin\":\"\",\"legend\":\"\\u003cp\\u003e\\u003cstrong\\u003eCharge carrier generation and modification in the defect formation energy under the different bending states.\\u003c/strong\\u003e \\u003cstrong\\u003e(a)\\u003c/strong\\u003e Schematic diagram of the scanning bent surface by atomic force microscopy. \\u003cstrong\\u003e(b)\\u003c/strong\\u003eSurface topography and surface current images of the CIGS devices with the concave, planar, and convex bending settings. External bias of 0.5 V was applied to the samples, and the bending radii were 20.47 mm and 14 mm for curvature angles of 20° and 30°, respectively. \\u003cstrong\\u003e(c)\\u003c/strong\\u003e Box plot of the surface currents extracted from 20 GBs at each sample under planar and mechanical bending states. \\u003cstrong\\u003e(d) \\u003c/strong\\u003eSchematic illustration of strain relations of concave and convex bending states. Zoomed crystal structure depicts a model atomic structure of CIGS near the GB composed of two (1 1 2) facets under mechanical bending states. \\u003cstrong\\u003e(e)\\u003c/strong\\u003e Calculated defect formation energies of primary carrier-generating defects, with or without alkali defect complexes, under different biaxial strains.\\u003c/p\\u003e\",\"description\":\"\",\"filename\":\"Picture3.png\",\"url\":\"https://assets-eu.researchsquare.com/files/rs-4364503/v1/4d353db2c5bbfd43e823a003.png\"},{\"id\":60170280,\"identity\":\"e31287be-87ca-4feb-b24c-1b1d3083c76e\",\"added_by\":\"auto\",\"created_at\":\"2024-07-12 15:05:36\",\"extension\":\"png\",\"order_by\":4,\"title\":\"Figure 4\",\"display\":\"\",\"copyAsset\":false,\"role\":\"figure\",\"size\":745204,\"visible\":true,\"origin\":\"\",\"legend\":\"\\u003cp\\u003e\\u003cstrong\\u003eStress analysis of CIGS thin-film in the planar and bending states.\\u003c/strong\\u003e \\u003cstrong\\u003e(a, b)\\u003c/strong\\u003e Angle-tilted X-ray diffraction pattern shifting of the\\u003cstrong\\u003e (a)\\u003c/strong\\u003e CIGS (3 1 2) and \\u003cstrong\\u003e(b)\\u003c/strong\\u003e Mo (1 1 0) peaks. By tilting the angle Ψ, the value of sin\\u003csup\\u003e2\\u003c/sup\\u003eΨ value varied from 0 to 0.9 with a step of 0.1. \\u003cstrong\\u003e(c)\\u003c/strong\\u003e d-spacing change in the (3 1 2) diffraction peak by angle tilting in the samples without PDT and NaF PDT CIGS. Inset diagram represents the intrinsic stress in the CIGS thin-film deposited on the Mo back contact layer. \\u003cstrong\\u003e(d)\\u003c/strong\\u003e CIGS (1 1 2) and \\u003cstrong\\u003e(e)\\u003c/strong\\u003e (3 1 2) orientation peak shift with concave and convex mechanical bending. (\\u003cstrong\\u003ef)\\u003c/strong\\u003e Quantitative d-spacing change in the CIGS (1 1 2) and (3 1 2) orientation.\\u003cstrong\\u003e (g)\\u003c/strong\\u003e Schematic diagram of modifications in the GB carrier distribution according to the d-spacing changes under the bending states compared with those of the planar states.\\u003c/p\\u003e\",\"description\":\"\",\"filename\":\"Picture4.png\",\"url\":\"https://assets-eu.researchsquare.com/files/rs-4364503/v1/22ebc89ec52440483bd1f54e.png\"},{\"id\":65432029,\"identity\":\"e75fc65f-e057-45ac-b27c-d59d0e7cb1c1\",\"added_by\":\"auto\",\"created_at\":\"2024-09-27 12:01:36\",\"extension\":\"pdf\",\"order_by\":0,\"title\":\"\",\"display\":\"\",\"copyAsset\":false,\"role\":\"manuscript-pdf\",\"size\":3902026,\"visible\":true,\"origin\":\"\",\"legend\":\"\",\"description\":\"\",\"filename\":\"manuscript.pdf\",\"url\":\"https://assets-eu.researchsquare.com/files/rs-4364503/v1/d10165d7-0748-4937-a228-baddbb474159.pdf\"},{\"id\":60170279,\"identity\":\"8b72ebd1-467d-4c8a-bd17-de7c5668122b\",\"added_by\":\"auto\",\"created_at\":\"2024-07-12 15:05:36\",\"extension\":\"docx\",\"order_by\":1,\"title\":\"\",\"display\":\"\",\"copyAsset\":false,\"role\":\"supplement\",\"size\":6362415,\"visible\":true,\"origin\":\"\",\"legend\":\"Supplementary Information\",\"description\":\"\",\"filename\":\"NatCommunSIAlkaliCIGSHKP240503.docx\",\"url\":\"https://assets-eu.researchsquare.com/files/rs-4364503/v1/a18121c33e4e255d54cfed9b.docx\"}],\"financialInterests\":\"There is \\u003cb\\u003eNO\\u003c/b\\u003e Competing Interest.\",\"formattedTitle\":\"Interplay Between Strain and Charge in Cu(In,Ga)Se2 Flexible Photovoltaics\",\"fulltext\":[{\"header\":\"Introduction\",\"content\":\"\\u003cp\\u003eThe recent expansion of the solar energy market has amplified the crucial role of flexible and lightweight thin-film solar cells in reducing manufacturing cost and broadening photovoltaic applications. Among the light-absorbing materials, Cu(In,Ga)Se\\u003csub\\u003e2\\u003c/sub\\u003e (CIGS) is a promising candidate for commercialized flexible thin-film solar cells due to its high efficiency and stability.\\u003c/p\\u003e \\u003cp\\u003eThe most recently reported record cells have achieved a high efficiency of over 22% with alkali fluoride (AlkF) post-deposition treatments (PDT), especially when Rb and Cs are used. \\u003csup\\u003e1, 2, 3\\u003c/sup\\u003e The incorporation of heavy-alkali (Rb, and Cs) PDT in addition to light-alkali (Na) has a synergistic effect to maximize the device performance increasing the carrier concentration and improving the grain growth. \\u003csup\\u003e4\\u003c/sup\\u003e Furthermore, the PDT of alkali elements boosts the performance of flexible CIGS solar cells with a substrate that does not contain alkali elements. \\u003csup\\u003e5, 6, 7\\u003c/sup\\u003e Notably, flexible CIGS solar cells with a polyimide (PI) substrate exhibited the efficiency of 20.82% when the doping of heavy-alkali (RbF) PDT was controlled. \\u003csup\\u003e8\\u003c/sup\\u003e Recently, Ishizuka et al. reported flexible minimodules with an efficiency of 18.65% by Rb-PDT in addition to Na-PDT and K-PDT. \\u003csup\\u003e9\\u003c/sup\\u003e\\u003c/p\\u003e \\u003cp\\u003eFor flexible solar cells, understanding the characteristics of CIGS materials under stress is crucial to developing mechanically robust devices. However, changes in the flexible CIGS solar cells under mechanical stress and stress-induced modifications in CIGS materials have not been extensively examined. Few reports on the degradation of device performance after mechanical bending tests are available. \\u003csup\\u003e10, 11\\u003c/sup\\u003e In light of this, we previously demonstrated that the incorporation of alkali in flexible kesterite solar cells improved the stress-induced loss in the local \\u003cem\\u003eV\\u003c/em\\u003e\\u003csub\\u003eOC\\u003c/sub\\u003e by designing a measurement system based on scanning probe microscopy that can be applied to a bent surface. \\u003csup\\u003e12\\u003c/sup\\u003e\\u003c/p\\u003e \\u003cp\\u003eIn this study, changes in the electrical charge states of flexible CIGS thin-film solar cells doped with alkali (e.g., Na, Rb and Cs) were investigated under mechanical bending stress. Under mechanical bending, the lattice constants changed, leading to the rearrangement of elements. Herein, based on scanning probe microscopy, the surface current of a bent surface was measured by attaching flexible CIGS samples on metal holders with specific curvatures. The distribution of current on the surface of the flexible CIGS improved with alkali incorporation and was degraded under mechanical bending stress due to alterations in the charged states. However, CIGS with additional alkali incorporation exhibited significantly suppressed degradation even under the bending states. Density functional theory (DFT) study provided insights into the strain effect on CIGS films at the atomic level, potentially revealing the underlying mechanism for the reduced degradation observed in alkali-incorporated CIGS films under bending stress. We conclude that defect-level redistribution with alkali incorporation enhances the tolerance of CIGS thin films to mechanical stress.\\u003c/p\\u003e\"},{\"header\":\"Results and discussions\",\"content\":\"\\u003cp\\u003e \\u003c/p\\u003e \\u003cp\\u003eCIGS crystal structure and distribution of alkali elements at the grain boundaries (GBs) were determined from high-resolution scanning transmission electron microscopy (STEM) image, as shown in Fig.\\u0026nbsp;\\u003cspan refid=\\\"Fig1\\\" class=\\\"InternalRef\\\"\\u003e1\\u003c/span\\u003ea. The d-spacing obtained using the fast-Fourier transform (FFT) patterns was 0.329 nm, which corresponded to the CIGS grain with (1 1 2) orientation and assuming (1 1 2) facets facing GBs. The distribution of alkali elements was investigated by STEM-energy dispersive spectroscopy (EDS) measurements. In Figs.\\u0026nbsp;\\u003cspan refid=\\\"Fig1\\\" class=\\\"InternalRef\\\"\\u003e1\\u003c/span\\u003eb \\u003cb\\u003eand c\\u003c/b\\u003e, the GBs of the heavy-alkali-doped CIGS showed a Cu-depleted composition. By contrast, the concentration of heavy alkali increased at the GBs compared to the intragrain regions, suggesting heavy alkali occupation of the Cu vacancy sites. Several studies have demonstrated empirically and theoretically the segregation of heavy alkalis toward the GBs, in which the heavy alkalis pushed light alkalis away from the GBs. \\u003csup\\u003e13, 14, 15\\u003c/sup\\u003e The segregation of heavy alkali toward GBs passivated the detrimental defects and modified the energy level of the Cu vacancy (\\u003cem\\u003eV\\u003c/em\\u003e\\u003csub\\u003eCu\\u003c/sub\\u003e) sites. \\u003csup\\u003e16, 17\\u003c/sup\\u003e However, the migration of expelled light alkalis is debating. On the one hand, several studies have indicated that relatively light alkali was pushed away toward the surface. On the other hand, some studies showed that the repelled light alkali accumulated in the grain interior. \\u003csup\\u003e18, 19, 20\\u003c/sup\\u003e The EDS images of other elements are presented in \\u003cb\\u003eFigure \\u003cspan refid=\\\"MOESM1\\\" class=\\\"InternalRef\\\"\\u003eS1\\u003c/span\\u003e\\u003c/b\\u003e in the \\u003cb\\u003eSupporting Information\\u003c/b\\u003e. Herein, the concentration of Na was under the detection limit of EDS, which hindered the identification of the presence of Na.\\u003c/p\\u003e \\u003cp\\u003eIn addition to the expulsion of the light alkali at the GBs, the in-depth intensity of Na decreased significantly after the heavy-alkali PDT compared to that of the CIGS sample with NaF PDT only. Figure\\u0026nbsp;\\u003cspan refid=\\\"Fig1\\\" class=\\\"InternalRef\\\"\\u003e1\\u003c/span\\u003ed shows that the in-depth distribution of Na was hardly detected in the heavy-alkali PDT CIGS, and this result was comparable to the TEM-EDS results. The significant decrease in Na in the entire film suggested that the light alkalis from the GBs repelled by the heavy alkalis apparently moved toward the surface and washed away during the chemical bath deposition of CdS. The intensity of heavy alkalis at the GBs was higher than that of Na although the nominal thickness of the alkali PDT was identical. The heavy alkalis slightly increased at the back of the CIGS absorber. A previous study has demonstrated that the concentration of Rb at the GBs increased toward the Mo back contact. \\u003csup\\u003e4\\u003c/sup\\u003e\\u003c/p\\u003e \\u003cp\\u003e \\u003c/p\\u003e \\u003cp\\u003eTo investigate the carrier behavior under mechanical stress, the current at the surface was probed using conductive-atomic force microscopy (c-AFM). As shown in Fig.\\u0026nbsp;\\u003cspan refid=\\\"Fig2\\\" class=\\\"InternalRef\\\"\\u003e2\\u003c/span\\u003ea, a sample holder with curvature mold was prepared to apply artificial concave and convex bending to the flexible CIGS thin-film. \\u003csup\\u003e21, 22\\u003c/sup\\u003e Mechanical stress was continuously applied as the samples were attached to the holder. Therefore, formation of the surface current during mechanical bending was obtained directly as the bent CIGS absorber surface was probed. Severe cracks or breakage induced by the stress were not observed in the topographies, indicating that the CIGS mechanically tolerated the strain with bending radius of 20.47 mm (See \\u003cb\\u003eFigure S2\\u003c/b\\u003e in the \\u003cb\\u003eSupporting Information\\u003c/b\\u003e). A previous study has also presented that the CIGS thin-film tolerated tensile stress lower than 1.0%, and stress of more than 1.0% resulted in grain boundary (GB) cracking that affected the carrier transportation along the GBs. \\u003csup\\u003e23\\u003c/sup\\u003e\\u003c/p\\u003e \\u003cp\\u003eThe surface current flowed along the GB of CIGS thin film, as depicted in the surface current map in Fig.\\u0026nbsp;\\u003cspan refid=\\\"Fig2\\\" class=\\\"InternalRef\\\"\\u003e2\\u003c/span\\u003eb. A two-dimensional (2D) topography and surface current map of all CIGS can be found in \\u003cb\\u003eFigure S3\\u003c/b\\u003e in the \\u003cb\\u003eSupporting Information\\u003c/b\\u003e. This illustration highlights the GBs as the main conduction channels, particularly, when an external bias is applied to the sample emitting electrons from the samples to the grounded tip. \\u003csup\\u003e24\\u003c/sup\\u003e The samples with alkali PDT showed enhanced formation of the surface current presumably due to the increased carrier concentrations. Additionally, Sharma et al. recently demonstrated that the CIGS thin film treated with Rb and Cs possessed higher carrier concentrations and enhanced homogeneity of carrier concentration. \\u003csup\\u003e25\\u003c/sup\\u003e The segregated alkalis at GBs changed the charged defect states, resulting in different electrostatic properties. \\u003csup\\u003e26, 27, 28\\u003c/sup\\u003e\\u003c/p\\u003e \\u003cp\\u003eThe quantitative distribution of the surface current from the three-dimensional current maps is shown in Fig.\\u0026nbsp;\\u003cspan refid=\\\"Fig2\\\" class=\\\"InternalRef\\\"\\u003e2\\u003c/span\\u003ec, in which the surface current values from 20 GBs were extracted. The distribution of the surface current significantly decreased under convex bending, indicating a decrease in conductivity. A comparison between samples with and without mechanical stress history is presented in \\u003cb\\u003eFigure S4\\u003c/b\\u003e. Under convex bending, the surface current decreased in the CIGS samples both with and without alkali PDT. However, CIGS with alkali PDT showed less degradation in surface current compared to the CIGS without PDT, which showed a remarkable decrease in current. Some GBs of the alkali PDT CIGS maintained original values even under convex bending, indicating that the presence of alkali elements at the GBs hindered the decrease in conductivity. The electrical properties at the GBs could be attributed to the distribution of defect states. Therefore, changes in the surface current under mechanical bending might have originated from the redistribution of defect states at the GBs.\\u003c/p\\u003e \\u003cp\\u003eTo investigate how the changes in defect states at CIGS GBs depend on strain, we conducted DFT calculations to determine the formation energies of various defect states, including intrinsic and alkali-incorporated defects, under varying strain conditions. Assuming that GBs perpendicular to the film surface are relevant to the region where the highest current peak is observed in c-AFM images, we consider the strain conditions in the concave and convex bending states as shown in Fig.\\u0026nbsp;\\u003cspan refid=\\\"Fig2\\\" class=\\\"InternalRef\\\"\\u003e2\\u003c/span\\u003ed. Under concave bending, the compressive axial stress within the film induces planar expansion of grain boundaries (GBs). Conversely, the opposite effect, GB contraction, is expected under convex bending. Given that (1 1 2) facets with copper deficiency are expected to be dominant GBs in CIGS as shown in Fig.\\u0026nbsp;\\u003cspan refid=\\\"Fig2\\\" class=\\\"InternalRef\\\"\\u003e2\\u003c/span\\u003ed, we adopt the Cu-deficient (1 1 2) surface structure obtained from our previous study to effectively mimic the defect chemistry at GBs. \\u003csup\\u003e15\\u003c/sup\\u003e Among all calculated defect configurations (see \\u003cb\\u003eFigure S5\\u003c/b\\u003e and \\u003cb\\u003eTable \\u003cspan refid=\\\"MOESM1\\\" class=\\\"InternalRef\\\"\\u003eS1\\u003c/span\\u003e\\u003c/b\\u003e for the all considered defects), defect complexes combining alkali interstitials (Na\\u003csub\\u003e\\u003cem\\u003ei\\u003c/em\\u003e\\u003c/sub\\u003e, Rb\\u003csub\\u003e\\u003cem\\u003ei\\u003c/em\\u003e\\u003c/sub\\u003e, and Cs\\u003csub\\u003e\\u003cem\\u003ei\\u003c/em\\u003e\\u003c/sub\\u003e) and Cu vacancy (\\u003cem\\u003eV\\u003c/em\\u003e\\u003csub\\u003eCu\\u003c/sub\\u003e) have the lowest defect formation energies. More specifically, larger alkali metals tend to have lower defect formation energies when incorporated next to the \\u003cem\\u003eV\\u003c/em\\u003e\\u003csub\\u003eCu\\u003c/sub\\u003e defect. This explains why rubidium (Rb) and cesium (Cs) dominate at grain boundaries (GBs) in NaF\\u0026thinsp;+\\u0026thinsp;RbF and NaF\\u0026thinsp;+\\u0026thinsp;CsF PDT CIGS, respectively. These defect complexes (Na\\u003csub\\u003e\\u003cem\\u003ei\\u003c/em\\u003e\\u003c/sub\\u003e-\\u003cem\\u003eV\\u003c/em\\u003e\\u003csub\\u003eCu\\u003c/sub\\u003e, Rb\\u003csub\\u003e\\u003cem\\u003ei\\u003c/em\\u003e\\u003c/sub\\u003e-\\u003cem\\u003eV\\u003c/em\\u003e\\u003csub\\u003eCu\\u003c/sub\\u003e, and Cs\\u003csub\\u003e\\u003cem\\u003ei\\u003c/em\\u003e\\u003c/sub\\u003e-\\u003cem\\u003eV\\u003c/em\\u003e\\u003csub\\u003eCu\\u003c/sub\\u003e) differ from Cu substitutional defects (Na\\u003csub\\u003eCu\\u003c/sub\\u003e, Rb\\u003csub\\u003eCu\\u003c/sub\\u003e, and Cs\\u003csub\\u003eCu\\u003c/sub\\u003e) as alkali metals does not occupy the Cu sites. Notably, Rb\\u003csub\\u003eCu\\u003c/sub\\u003e and Cs\\u003csub\\u003eCu\\u003c/sub\\u003e have much larger formation energy compared to Rb\\u003csub\\u003e\\u003cem\\u003ei\\u003c/em\\u003e\\u003c/sub\\u003e-\\u003cem\\u003eV\\u003c/em\\u003e\\u003csub\\u003eCu\\u003c/sub\\u003e, and Cs\\u003csub\\u003e\\u003cem\\u003ei\\u003c/em\\u003e\\u003c/sub\\u003e-\\u003cem\\u003eV\\u003c/em\\u003e\\u003csub\\u003eCu\\u003c/sub\\u003e suggesting that Rb and Cs cannot permeate into intra-grain. Interestingly, while these defect complexes (Na\\u003csub\\u003e\\u003cem\\u003ei\\u003c/em\\u003e\\u003c/sub\\u003e-\\u003cem\\u003eV\\u003c/em\\u003e\\u003csub\\u003eCu\\u003c/sub\\u003e, Rb\\u003csub\\u003e\\u003cem\\u003ei\\u003c/em\\u003e\\u003c/sub\\u003e-\\u003cem\\u003eV\\u003c/em\\u003e\\u003csub\\u003eCu\\u003c/sub\\u003e, Cs\\u003csub\\u003e\\u003cem\\u003ei\\u003c/em\\u003e\\u003c/sub\\u003e-\\u003cem\\u003eV\\u003c/em\\u003e\\u003csub\\u003eCu\\u003c/sub\\u003e,) are electrically neutral, the formation energies of additional \\u003cem\\u003eV\\u003c/em\\u003e\\u003csub\\u003eCu\\u003c/sub\\u003e near the Na\\u003csub\\u003e\\u003cem\\u003ei\\u003c/em\\u003e\\u003c/sub\\u003e-\\u003cem\\u003eV\\u003c/em\\u003e\\u003csub\\u003eCu\\u003c/sub\\u003e complex is even lower than \\u003cem\\u003eV\\u003c/em\\u003e\\u003csub\\u003eCu\\u003c/sub\\u003e without PDT. This provides feasible explanation for the origin of the largest current increasement at GBs in NaF PDT CIGS. In Fig.\\u0026nbsp;\\u003cspan refid=\\\"Fig2\\\" class=\\\"InternalRef\\\"\\u003e2\\u003c/span\\u003ee, the calculated formation energies of major carrier-generating defects are compared under axial strains on CIGS (1 1 2) surface, ranging from \\u0026minus;\\u0026thinsp;2% to +\\u0026thinsp;2% of the lattice parameters. Under compressive axial strain applied to the (1 1 2) surface of CIGS, which corresponds to the convex bending condition, the formation energy of \\u003cem\\u003eV\\u003c/em\\u003e\\u003csub\\u003eCu\\u003c/sub\\u003e\\u003csup\\u003e\\u0026minus;\\u003c/sup\\u003e defects increases, leading to a decrease in carrier concentration. On the contrary, tensile axial stress boost the formation of \\u003cem\\u003eV\\u003c/em\\u003e\\u003csub\\u003eCu\\u003c/sub\\u003e\\u003csup\\u003e+\\u003c/sup\\u003e. Notably, the formation energies of \\u003cem\\u003eV\\u003c/em\\u003e\\u003csub\\u003eCu\\u003c/sub\\u003e near Na\\u003csub\\u003e\\u003cem\\u003ei\\u003c/em\\u003e\\u003c/sub\\u003e-\\u003cspan type=\\\"ItalicUnderline\\\" class=\\\"ItalicUnderline\\\" name=\\\"Emphasis\\\"\\u003eV\\u003c/span\\u003e\\u003csub\\u003eCu\\u003c/sub\\u003e complex (\\u003cem\\u003eV\\u003c/em\\u003e\\u003csub\\u003eCu\\u003c/sub\\u003e@Na\\u003csub\\u003e\\u003cem\\u003ei\\u0026minus;\\u003c/em\\u003e\\u003c/sub\\u003e\\u003cem\\u003eV\\u003c/em\\u003e\\u003csub\\u003eCu\\u003c/sub\\u003e) is much lower than \\u003cem\\u003eV\\u003c/em\\u003e\\u003csub\\u003eCu\\u003c/sub\\u003e that explains the reason that NaF PDT CIGS shows lowest degradation compared to other conditions. Herein, it was assumed that grain boundaries possess a single type of alkali element as heavy alkalis tended to segregate into the GBs by pushing away Na from the GBs as presented in Fig.\\u0026nbsp;\\u003cspan refid=\\\"Fig1\\\" class=\\\"InternalRef\\\"\\u003e1\\u003c/span\\u003e.\\u003c/p\\u003e \\u003cp\\u003e \\u003c/p\\u003e \\u003cp\\u003eTo investigate the structural changes in the CIGS with mechanical stress, angle-tilted X-ray diffraction (XRD) measurements were performed for the CIGS both in the planar and bent states. Herein, Ψ indicates the angle between the diffraction scattering vector and the normal direction of samples. In the planar state, the CIGS (3 1 2) orientation peak presented the peak shift toward the lower diffraction angle by tilting angle Ψ, indicating tensile stress. On the other hand, the Mo (1 1 0) peak presented the peak position shift toward a higher angle, indicating compressive stress (Figs.\\u0026nbsp;\\u003cspan refid=\\\"Fig3\\\" class=\\\"InternalRef\\\"\\u003e3\\u003c/span\\u003ea, b). Several previous studies have reported the stress analysis of CIGS thin-films with intrinsic compressive stress when they were deposited on the SLG. \\u003csup\\u003e29, 30\\u003c/sup\\u003e Tensile stress formed in the CIGS thin film was probably attributed to the relatively thicker Mo back contact layer with smaller thermal expansion coefficient than the CIGS thin film. \\u003csup\\u003e31\\u003c/sup\\u003e\\u003c/p\\u003e \\u003cp\\u003eThe angle-tilted XRD patterns of the CIGS samples without PDT and NaF PDT under mechanical bending both presented tensile stress, indicating that mechanical bending did not compensate the intrinsic tensile stress (See \\u003cb\\u003eFigure S6 and S7\\u003c/b\\u003e in the \\u003cb\\u003eSupporting Information\\u003c/b\\u003e). As shown in Fig.\\u0026nbsp;\\u003cspan refid=\\\"Fig3\\\" class=\\\"InternalRef\\\"\\u003e3\\u003c/span\\u003ec, NaF PDT CIGS showed a smaller slope than that of the sample without PDT when the tilting angle was increased, indicating that NaF PDT CIGS had lower residual tensile stress. The NaF PDT CIGS sample presented slightly larger d-spacing compared to that without PDT, because the size of the Na atom was larger than the other elements, thus widening the crystal structure. Therefore, incorporation of the Na atoms produced relative compressive strain through the lattice site by enlarging the d-spacing, thereby slightly compensating the intrinsic tensile stress in the film.\\u003c/p\\u003e \\u003cp\\u003eOut-of-plane XRD patterns for the lattice of the thin film were recorded under planar and bent states to measure the crystal plane spacing. The crystal plane spacing perpendicular to the film presents a coherent direction with the crystal structure model of Fig.\\u0026nbsp;\\u003cspan refid=\\\"Fig2\\\" class=\\\"InternalRef\\\"\\u003e2\\u003c/span\\u003ed, with the vertical direction of (1 1 2) facets assumed to be facing GBs. Figures\\u0026nbsp;\\u003cspan refid=\\\"Fig3\\\" class=\\\"InternalRef\\\"\\u003e3\\u003c/span\\u003ed and \\u003cspan refid=\\\"Fig3\\\" class=\\\"InternalRef\\\"\\u003e3\\u003c/span\\u003ee demonstrate the shifts of the (1 1 2) and (3 1 2) orientation peaks under mechanical stress. The (1 1 2) and (3 1 2) orientation peaks shifted toward lower diffraction angle (i.e., larger d-spacing) under convex bending, while it shifted toward higher diffraction angle (i.e., smaller d-spacing) under concave bending. These d-spacing changes supported our model for the microdeformation of the crystal structure due to mechanical stress as shown in Fig.\\u0026nbsp;\\u003cspan refid=\\\"Fig2\\\" class=\\\"InternalRef\\\"\\u003e2\\u003c/span\\u003ed. The calculated lattice parameters of CIGS crystal under biaxial strain (see \\u003cb\\u003eTable S2\\u003c/b\\u003e) are also consistent with the XRD analysis.\\u003c/p\\u003e \\u003cp\\u003eThe correlation between carrier distribution and crystal structure modification is depicted in Fig.\\u0026nbsp;\\u003cspan refid=\\\"Fig3\\\" class=\\\"InternalRef\\\"\\u003e3\\u003c/span\\u003eg. Under convex bending, the biaxial compressive stress in the plane direction led to an increase in the acceptor energy levels, hindering the generation of charge carriers. By contrast, biaxial tensile stress decreased the energy levels of the acceptor defects under concave bending, thereby maintaining high carrier concentrations. This modification in carrier concentrations according to the mechanical bending can be related to the piezophototronic or flexoelectric effect in a dielectric semiconductor or device. \\u003csup\\u003e32\\u003c/sup\\u003e The piezophototronic effect in heterojunction devices facilitates or impedes the transport of photo-generated carriers at the heterojunction interface by mechanical stress modulating the junction region, which in turn affects the photoresponsivity or photovoltaic parameters. The piezophototronic response of the flexible CIGS thin-film optoelectronic devices is increased by the application of artificial static compressive strain, thereby modifying the interface energy band. \\u003csup\\u003e33, 34\\u003c/sup\\u003e Thus, the enhancement in carrier concentration at the CIGS absorber surface under concave bending can be potentially attributed to the piezophototronic effect in flexible CIGS optoelectronic devices.\\u003c/p\\u003e \\u003cp\\u003e \\u003c/p\\u003e \\u003cp\\u003eFigure \\u003cspan refid=\\\"Fig4\\\" class=\\\"InternalRef\\\"\\u003e4\\u003c/span\\u003e demonstrated the performances and carrier dynamics of flexible CIGS thin film solar cells. Figure\\u0026nbsp;\\u003cspan refid=\\\"Fig4\\\" class=\\\"InternalRef\\\"\\u003e4\\u003c/span\\u003ea shows the current density-voltage (\\u003cem\\u003eJ-V\\u003c/em\\u003e) curve of the flexible CIGS thin-film solar cells, and the photovoltaic parameters are presented in Table\\u0026nbsp;\\u003cspan refid=\\\"Tab1\\\" class=\\\"InternalRef\\\"\\u003e1\\u003c/span\\u003e. CIGS devices with AlkF PDT had enhanced device performances compared to the device without PDT. Among the CIGS samples, the NaF PDT sample exhibited the highest power conversion efficiency, followed by NaF\\u0026thinsp;+\\u0026thinsp;RbF and NaF\\u0026thinsp;+\\u0026thinsp;CsF PDT CIGS. The \\u003cem\\u003eV\\u003c/em\\u003e\\u003csub\\u003eOC\\u003c/sub\\u003e was enhanced with alkali PDT, leading to comparable values among the alkali PDT samples. The alkali PDT samples showed improved fill factor (FF) compared to the samples without PDT, but FF decreased with additional heavy-alkali PDT. The decrease in FF could be attributed to the formation of a thin Alk\\u0026ndash;In\\u0026ndash;Se compounds layer that acted as an electron barrier by inducing a downward valence band edge on the low-Ga CIGS surface. \\u003csup\\u003e18, 35, 36\\u003c/sup\\u003e A box plot of the solar cell parameters is presented in \\u003cb\\u003eFigure S8 (Supporting Information)\\u003c/b\\u003e. Figure\\u0026nbsp;\\u003cspan refid=\\\"Fig4\\\" class=\\\"InternalRef\\\"\\u003e4\\u003c/span\\u003eb presents the temperature-dependent \\u003cem\\u003eV\\u003c/em\\u003e\\u003csub\\u003eOC\\u003c/sub\\u003e of various alkali-treated CIGS solar cells. Temperature-dependent \\u003cem\\u003eJ-V\\u003c/em\\u003e curves for all temperature steps are presented in \\u003cb\\u003eFigure S9\\u003c/b\\u003e. Activation energy (\\u003cem\\u003eE\\u003c/em\\u003e\\u003csub\\u003e\\u003cem\\u003eA\\u003c/em\\u003e\\u003c/sub\\u003e) could be extracted at the point T\\u0026thinsp;=\\u0026thinsp;0 K from the linear fitting of the \\u003cem\\u003eV\\u003c/em\\u003e\\u003csub\\u003e\\u003cem\\u003eOC\\u003c/em\\u003e\\u003c/sub\\u003e\\u003cem\\u003e-T\\u003c/em\\u003e plot when the diode ideality factor was not dependent on temperature. \\u003csup\\u003e37\\u003c/sup\\u003e The CIGS devices with AlkF PDT provided the larger \\u003cem\\u003eE\\u003c/em\\u003e\\u003csub\\u003eA\\u003c/sub\\u003e compared to the CIGS device without PDT, which exhibited a value of 0.989 eV. Thus, interface recombination was alleviated by AlkF PDT, regardless of the type of alkali used. Among the samples, the NaF PDT samples exhibited the highest \\u003cem\\u003eE\\u003c/em\\u003e\\u003csub\\u003eA\\u003c/sub\\u003e value of 1.050 eV.\\u003c/p\\u003e \\u003cp\\u003e \\u003cdiv class=\\\"gridtable\\\"\\u003e\\u003ctable float=\\\"Yes\\\" id=\\\"Tab1\\\" border=\\\"1\\\"\\u003e \\u003ccaption language=\\\"En\\\"\\u003e \\u003cdiv class=\\\"CaptionNumber\\\"\\u003eTable 1\\u003c/div\\u003e \\u003cdiv class=\\\"CaptionContent\\\"\\u003e \\u003cp\\u003ePhotovoltaic device parameters of flexible CIGS solar cell devices with best efficiency in each treatment and \\u003cem\\u003eN\\u003c/em\\u003e\\u003csub\\u003eCV\\u003c/sub\\u003e.\\u003c/p\\u003e \\u003c/div\\u003e \\u003c/caption\\u003e \\u003ccolgroup cols=\\\"6\\\"\\u003e \\u003cdiv align=\\\"left\\\" class=\\\"colspec\\\" colname=\\\"c1\\\" colnum=\\\"1\\\"\\u003e\\u003c/div\\u003e \\u003cdiv align=\\\"char\\\" char=\\\".\\\" class=\\\"colspec\\\" colname=\\\"c2\\\" colnum=\\\"2\\\"\\u003e\\u003c/div\\u003e \\u003cdiv align=\\\"char\\\" char=\\\".\\\" class=\\\"colspec\\\" colname=\\\"c3\\\" colnum=\\\"3\\\"\\u003e\\u003c/div\\u003e \\u003cdiv align=\\\"char\\\" char=\\\".\\\" class=\\\"colspec\\\" colname=\\\"c4\\\" colnum=\\\"4\\\"\\u003e\\u003c/div\\u003e \\u003cdiv align=\\\"char\\\" char=\\\".\\\" class=\\\"colspec\\\" colname=\\\"c5\\\" colnum=\\\"5\\\"\\u003e\\u003c/div\\u003e \\u003cdiv align=\\\"char\\\" char=\\\"\\u0026times;\\\" class=\\\"colspec\\\" colname=\\\"c6\\\" colnum=\\\"6\\\"\\u003e\\u003c/div\\u003e \\u003cthead\\u003e \\u003ctr\\u003e \\u003cth align=\\\"left\\\" colname=\\\"c1\\\"\\u003e \\u003cp\\u003eSample\\u003c/p\\u003e \\u003c/th\\u003e \\u003cth align=\\\"left\\\" colname=\\\"c2\\\"\\u003e \\u003cp\\u003e\\u003cem\\u003eV\\u003c/em\\u003e\\u003csub\\u003eoc\\u003c/sub\\u003e (V)\\u003c/p\\u003e \\u003c/th\\u003e \\u003cth align=\\\"left\\\" colname=\\\"c3\\\"\\u003e \\u003cp\\u003e\\u003cem\\u003eJ\\u003c/em\\u003e\\u003csub\\u003esc\\u003c/sub\\u003e (mA/cm\\u003csup\\u003e2\\u003c/sup\\u003e)\\u003c/p\\u003e \\u003c/th\\u003e \\u003cth align=\\\"left\\\" colname=\\\"c4\\\"\\u003e \\u003cp\\u003eFF (%)\\u003c/p\\u003e \\u003c/th\\u003e \\u003cth align=\\\"left\\\" colname=\\\"c5\\\"\\u003e \\u003cp\\u003ePCE (%)\\u003c/p\\u003e \\u003c/th\\u003e \\u003cth align=\\\"left\\\" colname=\\\"c6\\\"\\u003e \\u003cp\\u003e\\u003cem\\u003eN\\u003c/em\\u003e\\u003csub\\u003eCV\\u003c/sub\\u003e (cm\\u003csup\\u003e\\u0026minus;\\u0026thinsp;3\\u003c/sup\\u003e)\\u003c/p\\u003e \\u003c/th\\u003e \\u003c/tr\\u003e \\u003c/thead\\u003e \\u003ctbody\\u003e \\u003ctr\\u003e \\u003ctd align=\\\"left\\\" colname=\\\"c1\\\"\\u003e \\u003cp\\u003eNon-PDT\\u003c/p\\u003e \\u003c/td\\u003e \\u003ctd align=\\\"char\\\" char=\\\".\\\" colname=\\\"c2\\\"\\u003e \\u003cp\\u003e0.550\\u003c/p\\u003e \\u003c/td\\u003e \\u003ctd align=\\\"char\\\" char=\\\".\\\" colname=\\\"c3\\\"\\u003e \\u003cp\\u003e37.93\\u003c/p\\u003e \\u003c/td\\u003e \\u003ctd align=\\\"char\\\" char=\\\".\\\" colname=\\\"c4\\\"\\u003e \\u003cp\\u003e58.0\\u003c/p\\u003e \\u003c/td\\u003e \\u003ctd align=\\\"char\\\" char=\\\".\\\" colname=\\\"c5\\\"\\u003e \\u003cp\\u003e12.10\\u003c/p\\u003e \\u003c/td\\u003e \\u003ctd align=\\\"char\\\" char=\\\"\\u0026times;\\\" colname=\\\"c6\\\"\\u003e \\u003cp\\u003e1.28 \\u0026times; 10\\u003csup\\u003e15\\u003c/sup\\u003e\\u003c/p\\u003e \\u003c/td\\u003e \\u003c/tr\\u003e \\u003ctr\\u003e \\u003ctd align=\\\"left\\\" colname=\\\"c1\\\"\\u003e \\u003cp\\u003eNaF PDT\\u003c/p\\u003e \\u003c/td\\u003e \\u003ctd align=\\\"char\\\" char=\\\".\\\" colname=\\\"c2\\\"\\u003e \\u003cp\\u003e0.642\\u003c/p\\u003e \\u003c/td\\u003e \\u003ctd align=\\\"char\\\" char=\\\".\\\" colname=\\\"c3\\\"\\u003e \\u003cp\\u003e36.2\\u003c/p\\u003e \\u003c/td\\u003e \\u003ctd align=\\\"char\\\" char=\\\".\\\" colname=\\\"c4\\\"\\u003e \\u003cp\\u003e72.8\\u003c/p\\u003e \\u003c/td\\u003e \\u003ctd align=\\\"char\\\" char=\\\".\\\" colname=\\\"c5\\\"\\u003e \\u003cp\\u003e16.9\\u003c/p\\u003e \\u003c/td\\u003e \\u003ctd align=\\\"char\\\" char=\\\"\\u0026times;\\\" colname=\\\"c6\\\"\\u003e \\u003cp\\u003e2.87 \\u0026times; 10\\u003csup\\u003e16\\u003c/sup\\u003e\\u003c/p\\u003e \\u003c/td\\u003e \\u003c/tr\\u003e \\u003ctr\\u003e \\u003ctd align=\\\"left\\\" colname=\\\"c1\\\"\\u003e \\u003cp\\u003eNaF\\u0026thinsp;+\\u0026thinsp;RbF PDT\\u003c/p\\u003e \\u003c/td\\u003e \\u003ctd align=\\\"char\\\" char=\\\".\\\" colname=\\\"c2\\\"\\u003e \\u003cp\\u003e0.639\\u003c/p\\u003e \\u003c/td\\u003e \\u003ctd align=\\\"char\\\" char=\\\".\\\" colname=\\\"c3\\\"\\u003e \\u003cp\\u003e36.29\\u003c/p\\u003e \\u003c/td\\u003e \\u003ctd align=\\\"char\\\" char=\\\".\\\" colname=\\\"c4\\\"\\u003e \\u003cp\\u003e64.43\\u003c/p\\u003e \\u003c/td\\u003e \\u003ctd align=\\\"char\\\" char=\\\".\\\" colname=\\\"c5\\\"\\u003e \\u003cp\\u003e14.95\\u003c/p\\u003e \\u003c/td\\u003e \\u003ctd align=\\\"char\\\" char=\\\"\\u0026times;\\\" colname=\\\"c6\\\"\\u003e \\u003cp\\u003e9.05 \\u0026times; 10\\u003csup\\u003e15\\u003c/sup\\u003e\\u003c/p\\u003e \\u003c/td\\u003e \\u003c/tr\\u003e \\u003ctr\\u003e \\u003ctd align=\\\"left\\\" colname=\\\"c1\\\"\\u003e \\u003cp\\u003eNaF\\u0026thinsp;+\\u0026thinsp;CsF PDT\\u003c/p\\u003e \\u003c/td\\u003e \\u003ctd align=\\\"char\\\" char=\\\".\\\" colname=\\\"c2\\\"\\u003e \\u003cp\\u003e0.642\\u003c/p\\u003e \\u003c/td\\u003e \\u003ctd align=\\\"char\\\" char=\\\".\\\" colname=\\\"c3\\\"\\u003e \\u003cp\\u003e36.4\\u003c/p\\u003e \\u003c/td\\u003e \\u003ctd align=\\\"char\\\" char=\\\".\\\" colname=\\\"c4\\\"\\u003e \\u003cp\\u003e63.38\\u003c/p\\u003e \\u003c/td\\u003e \\u003ctd align=\\\"char\\\" char=\\\".\\\" colname=\\\"c5\\\"\\u003e \\u003cp\\u003e14.81\\u003c/p\\u003e \\u003c/td\\u003e \\u003ctd align=\\\"char\\\" char=\\\"\\u0026times;\\\" colname=\\\"c6\\\"\\u003e \\u003cp\\u003e3.59 \\u0026times; 10\\u003csup\\u003e15\\u003c/sup\\u003e\\u003c/p\\u003e \\u003c/td\\u003e \\u003c/tr\\u003e \\u003c/tbody\\u003e \\u003c/colgroup\\u003e \\u003c/table\\u003e\\u003c/div\\u003e \\u003c/p\\u003e \\u003cp\\u003eCarrier concentrations and defect energy levels of various alkali-doped CIGS solar cell were investigated by measuring the capacitance-voltage (\\u003cem\\u003eC-V\\u003c/em\\u003e) characteristics. The results from \\u003cem\\u003eC-V\\u003c/em\\u003e measurements showed that the increase in carrier concentrations (\\u003cem\\u003eN\\u003c/em\\u003e\\u003csub\\u003eCV\\u003c/sub\\u003e) of samples with alkali PDT samples (Fig.\\u0026nbsp;\\u003cspan refid=\\\"Fig4\\\" class=\\\"InternalRef\\\"\\u003e4\\u003c/span\\u003ec). All alkali PDT samples demonstrated improved \\u003cem\\u003eN\\u003c/em\\u003e\\u003csub\\u003eCV\\u003c/sub\\u003e compared to the Non-PDT sample, as they enhanced the p-type conductivity. The NaF PDT CIGS had the highest \\u003cem\\u003eN\\u003c/em\\u003e\\u003csub\\u003eCV\\u003c/sub\\u003e, as it increased the charge carrier density most effectively. \\u003csup\\u003e38\\u003c/sup\\u003e In addition, the NaF PDT CIGS sample showed the highest density of free carriers and the lowest density of the deep defects among the alkali PDT samples as presented in \\u003cb\\u003eFigure S10, Table S3 and S4\\u003c/b\\u003e in supporting information. The depletion width decreased in alkali PDT samples, which could lead to a reduction in \\u003cem\\u003eJ\\u003c/em\\u003e\\u003csub\\u003eSC\\u003c/sub\\u003e due to hindered photocarrier collection and separation, consistent with previous studies. \\u003csup\\u003e5, 39\\u003c/sup\\u003e\\u003c/p\\u003e \\u003cp\\u003eAdmittance spectroscopy (AS) measurements were performed to determine the defect energy levels. The energy levels of the defect states as shown in Fig.\\u0026nbsp;\\u003cspan refid=\\\"Fig4\\\" class=\\\"InternalRef\\\"\\u003e4\\u003c/span\\u003ed were calculated using the linear fitting of the Arrhenius plot derived from the derivatives of the AS spectra in Figs.\\u0026nbsp;\\u003cspan refid=\\\"Fig4\\\" class=\\\"InternalRef\\\"\\u003e4\\u003c/span\\u003ee\\u0026ndash;h. The AS results indicated that irrespective of the type of alkali used, the defect energy levels decreased after alkali treatment compared to the sample without PDT. \\u003csup\\u003e40\\u003c/sup\\u003e The alkali PDT samples possessed lower defect energy levels due to the defect passivation effect of the alkali elements. \\u003csup\\u003e41\\u003c/sup\\u003e The high density of the defects resulted in the low carrier concentration of the samples without PDT. By contrast, alkali PDT samples possessed lower trap densities and reduced defect energy levels compared to the samples without PDT. This characteristic could be attributed to higher carrier concentrations as evident in planar state surface current map presented in Fig.\\u0026nbsp;\\u003cspan refid=\\\"Fig2\\\" class=\\\"InternalRef\\\"\\u003e2\\u003c/span\\u003eb. In particular, improved free carrier concentrations with lowered activation energy presumably contribute to lower charge carrier degradation under mechanical bending conditions.\\u003c/p\\u003e\"},{\"header\":\"Conclusion\",\"content\":\"\\u003cp\\u003eIn conclusion, the results of the c-AFM measurement performed to probe the surface current of the CIGS thin-film under different mechanical bending states showed that the quantitative surface current was degraded under convex bending. Materials modelling demonstrated that convex bending increased the acceptor defect formation energy, resulting in lower carrier concentrations as observed using the surface current map. Nevertheless, alkali PDT CIGS showed less degradation in the surface current under convex bending, which was attributed to the lower energy level of the alkali-related acceptors. The redistribution of the alkali-related defects was enabled due to the change in crystal plane spacings under the mechanical bending states. These findings suggested that the incorporation of alkali elements improved the stress tolerance in terms of carrier generation. In addition, the mitigation of intrinsic tensile stress in the CIGS thin films deposited on the flexible PI substrates contributed to the reduction in acceptor defect levels and generation of carriers. Thus, the developed probing technique to map the current distribution on a bent surface provides practical information as it simulates the environment experienced by flexible electronic devices in real-world applications.\\u003c/p\\u003e \"},{\"header\":\"Experimental\",\"content\":\"\\u003cp\\u003e \\u003cstrong\\u003eFilm/device fabrication\\u003c/strong\\u003e \\u003cp\\u003eA PI film was used as the flexible substrate with a DC-sputtered Mo coating (700 nm) as a back contact. Three-stage co-evaporation method was used to grow the CIGS thin films with additional \\u003cem\\u003ein\\u003c/em\\u003e-\\u003cem\\u003esitu\\u003c/em\\u003e alkali PDT. The nominal thickness of the alkali (Na, Rb, Cs) PDT layer was 10 nm. Additional annealing with Se supply (15 \\u0026Aring;/s) was performed after the alkali PDT. The temperature for the whole process was maintained under 450 ℃. To complete the device fabrication, 40\\u0026ndash;60 nm of the CdS buffer layer was deposited on the CIGS thin film by using chemical bath deposition. Then, 50 nm of intrinsic zinc oxide (i-ZnO) and 150 nm of indium tin oxide (ITO) layer were deposited by radio frequency (RF) sputtering. An aluminum (Al) top electrode was deposited, and 110 nm of magnesium fluoride (MgF\\u003csub\\u003e2\\u003c/sub\\u003e) was deposited for anti-reflect coating by using electron beam evaporation.\\u003c/p\\u003e \\u003c/p\\u003e \\u003cp\\u003e \\u003cstrong\\u003eCharacterization\\u003c/strong\\u003e \\u003cp\\u003eThe \\u003cem\\u003eJ-V\\u003c/em\\u003e characteristics of the finished devices were measured with a solar simulator (WACOM, WXS-155S-L2), under a standard test condition of AM 1.5 G spectrum and intensity of 1 sun (100mW\\u0026middot;cm\\u003csup\\u003e\\u0026ndash;2\\u003c/sup\\u003e) at 25\\u0026deg;C. The temperature-dependent \\u003cem\\u003eV\\u003c/em\\u003e\\u003csub\\u003eOC\\u003c/sub\\u003e (\\u003cem\\u003eV\\u003c/em\\u003e\\u003csub\\u003e\\u003cem\\u003eOC\\u003c/em\\u003e\\u003c/sub\\u003e\\u003cem\\u003e-T\\u003c/em\\u003e) was performed using a semiconductor characterization system (Keithley 4200, Tektronix, Inc.). A vacuum probe stage with a temperature controller (HFS600E-PB4, Linkam) was used to vary the temperature from 300 K to 80 K with an interval of 10 K at a pressure of 10\\u003csup\\u003e\\u0026ndash;3\\u003c/sup\\u003e Torr. For the temperature-dependent \\u003cem\\u003eC-V\\u003c/em\\u003e, DLCP, and AS measurements, a LCR meter (Agilent 4284A) was used with a probe station (LTS 350, Linkam) applying the same temperature interval as the \\u003cem\\u003eV\\u003c/em\\u003e\\u003csub\\u003e\\u003cem\\u003eOC\\u003c/em\\u003e\\u003c/sub\\u003e\\u003cem\\u003e-T\\u003c/em\\u003e measurement. The microstructures and element distribution were characterized by transmission electron microscopy (TEM; JEM-ARM200F \\u0026ldquo;NEO ARM\\u0026rdquo;, JEOL) and energy-dispersive X-ray spectrometry (EDS, JED-2300T(Dual), JEOL), respectively. The accelerating voltage for the STEM observations was 200 kV. A cross-section lamella of the CIGS thin film for the TEM measurement was prepared using a focus-ion beam system. An O\\u003csub\\u003e2\\u003c/sub\\u003e sputtering source was used for the time-of-flight secondary ion mass spectrometry (ToF-SIMS) measurements instead of the Cs source to avoid the detection of a potential Cs source. Commercial atomic force microscopy (Nanofocus, n-tracer) was used to perform c-AFM measurement. Contact mode was used to scan the surface, and the set point value was 1 nN. A Pt/Ir-coated tip (Nanocensors Inc.) was used for the measurement. An external bias of 0.5 V (DC) was applied to the sample stage with the tip grounded, and the detection limit of the surface current was 10 nA. The scan area was 2 \\u0026times; 2 \\u0026micro;m\\u003csup\\u003e2\\u003c/sup\\u003e, and the scan resolution was 256 \\u0026times; 256 pixels (256-line scan). During the scanning, the scan rate was maintained below 0.3 Hz to minimize the tip damages. Detail method for c-AFM measurement on the bent surface is explained in the supporting information. For the XRD measurements, the height of the samples loaded were adjusted to be identical to avoid the influence of the difference of the distance between the sample and the detector.\\u003c/p\\u003e \\u003c/p\\u003e \\u003cp\\u003e \\u003cstrong\\u003eDFT calculations\\u003c/strong\\u003e \\u003cp\\u003eTotal energy and electronic structure calculations were performed using density functional theory (DFT). The Vienna Ab initio Simulation Package \\u003csup\\u003e42\\u003c/sup\\u003e with projector augmented-wave pseudo-potentials was employed. The generalized gradient approximation was applied for the exchange-correlation function, and the DFT\\u0026thinsp;+\\u0026thinsp;\\u003cem\\u003eU\\u003c/em\\u003e method was used for Cu with \\u003cem\\u003eU\\u003c/em\\u003e\\u003csub\\u003eeff\\u003c/sub\\u003e = 4 eV for the d states. \\u003csup\\u003e43\\u003c/sup\\u003e A CuIn\\u003csub\\u003e0.5\\u003c/sub\\u003eGa\\u003csub\\u003e0.5\\u003c/sub\\u003eSe\\u003csub\\u003e2\\u003c/sub\\u003e unit cell was used to represent the complex configurational space of CIGS (See \\u003cb\\u003eFigure S5\\u003c/b\\u003e in the \\u003cb\\u003eSupporting Information\\u003c/b\\u003e). The energy cut-off of the plane-wave basis was set to 300 eV, and \\u003cem\\u003ek\\u003c/em\\u003e-points were sampled as 3 \\u0026times; 3 \\u0026times; 2 Monkhorst-pack mesh that satisfy the convergence criteria of energy/atom\\u0026thinsp;\\u0026lt;\\u0026thinsp;0.01 eV and pressure\\u0026thinsp;\\u0026lt;\\u0026thinsp;10 kBar. To calculate the defect chemistry at the CIGS GBs, a 148-atom slab structure with (112) surface was used. The defect formation energy (\\u003cem\\u003eE\\u003c/em\\u003e\\u003csub\\u003ef\\u003c/sub\\u003e) was calculated as follows\\u003c/p\\u003e \\u003c/p\\u003e \\u003cp\\u003e \\u003cdiv id=\\\"Equa\\\" class=\\\"Equation\\\"\\u003e \\u003cdiv format=\\\"TEX\\\" class=\\\"mathdisplay\\\" id=\\\"FileID_Equa\\\" name=\\\"EquationSource\\\"\\u003e\\n$${E}_{\\\\text{f}}\\\\left(defect\\\\right)={E}_{\\\\text{t}\\\\text{o}\\\\text{t}}\\\\left(defect\\\\right)-{E}_{\\\\text{t}\\\\text{o}\\\\text{t}}\\\\left(perfect\\\\right)+ \\\\sum {\\\\text{N}}_{\\\\text{i}}{\\\\mu }_{\\\\text{i}}+q\\\\left({\\\\epsilon }_{\\\\text{F}}+{\\\\epsilon }_{\\\\text{V}\\\\text{B}\\\\text{M}}\\\\right)+{E}_{\\\\text{c}\\\\text{o}\\\\text{r}\\\\text{r}}$$\\u003c/div\\u003e \\u003c/div\\u003e,\\u003c/p\\u003e \\u003cp\\u003ewhere \\u003cem\\u003eE\\u003c/em\\u003e\\u003csub\\u003etot\\u003c/sub\\u003e is the total free energy of the supercell; and N\\u003csub\\u003ei\\u003c/sub\\u003e and \\u003cem\\u003e\\u0026micro;\\u003c/em\\u003e\\u003csub\\u003ei\\u003c/sub\\u003e are the quantities and chemical potentials of the exchanging chemical species, respectively. For a charged defect, \\u003cem\\u003eq\\u003c/em\\u003e is the charge state of the defect, \\u003cem\\u003eε\\u003c/em\\u003e\\u003csub\\u003eF\\u003c/sub\\u003e is the Fermi level from the valence band maximum (\\u003cem\\u003eε\\u003c/em\\u003e\\u003csub\\u003eVBM\\u003c/sub\\u003e), and \\u003cem\\u003eE\\u003c/em\\u003e\\u003csub\\u003ecorr\\u003c/sub\\u003e is the charge correction energy determined using the Makov-Payne method. \\u003csup\\u003e44\\u003c/sup\\u003e To calculate the changes in lattice parameters under biaxial strain, we modify the VASP code to allow the lattice relaxion only along the z-axis. For all calculations, atomic configurations were optimized, until the atomic force converged within 0.01 eV/\\u0026Aring;.\\u003c/p\\u003e \\u003c/div\\u003e\"},{\"header\":\"Declarations\",\"content\":\"\\u003cp\\u003e\\u003cstrong\\u003eAuthor Contributions\\u003c/strong\\u003e\\u003c/p\\u003e\\n\\u003cp\\u003eH.K.P. and K.Y. contributed equally to this work and the manuscript was mainly written by the first authors, H.K.P. and K.Y. H.K.P. designed the research and performed the \\u003cem\\u003eT-V\\u003csub\\u003eOC\\u003c/sub\\u003e\\u003c/em\\u003e, DLCP, AS, c-AFM, SEM measurements. K.Y. conceived the computational research and performed the DFT simulations. J.L. contributed to the c-AFM measurement. Y.C. assisted the experiments and characterizations. I.J., D.S. and K.K. designed the fabrication of the flexible CIGS solar cells and conducted the \\u003cem\\u003eIV\\u003c/em\\u003e measurements. J.G. supervised the research program. A.W. contributed the validation of computational study and revised the manuscript. K.K. and W.J. supervised the overall work. All authors discussed the results and commented on the manuscript.\\u003c/p\\u003e\\n\\u003cp\\u003e\\u003cstrong\\u003eCompeting interests\\u003c/strong\\u003e\\u003c/p\\u003e\\n\\u003cp\\u003eThe authors declare no competing interests.\\u003c/p\\u003e\\n\\u003cp\\u003e\\u003cstrong\\u003eAcknowledgements\\u003c/strong\\u003e\\u003c/p\\u003e\\n\\u003cp\\u003eThis research was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (NRF-2018R1A6A1A03025340) and Ministry of Science, Technology, ICT, and Future Planning (NRF-2021R1A2B5B02001961). This work was supported by a grant (NRF-2022M3J1A1064229) of the Technology Development Program for the solving of climate change, and the grant was provided through the NRF funded by the Ministry of Science and ICT. The first author, H.K.P., gratefully acknowledges the scholarship support received from the Hyundai Motor Chung Mong-Koo Foundation.\\u003c/p\\u003e\"},{\"header\":\"References\",\"content\":\"\\u003col\\u003e\\u003cli\\u003e\\u003cspan\\u003eNakamura M, Yamaguchi K, Kimoto Y, Yasaki Y, Kato T, Sugimoto H. Cd-Free Cu(In,Ga)(Se,S)\\u003csub\\u003e2\\u003c/sub\\u003e Thin-Film Solar Cell With Record Efficiency of 23.35%. IEEE J. Photovolt. 9, 1863\\u0026ndash;1867 (2019).\\u003c/span\\u003e\\u003c/li\\u003e \\u003cli\\u003e\\u003cspan\\u003eJackson P, Wuerz R, Hariskos D, Lotter E, Witte W, Powalla M. Effects of heavy alkali elements in Cu(In,Ga)Se\\u003csub\\u003e2\\u003c/sub\\u003e solar cells with efficiencies up to 22.6%. Phys. Status Solidi (RRL) \\u0026ndash; Rap. Res. Lett. 10, 583\\u0026ndash;586 (2016).\\u003c/span\\u003e\\u003c/li\\u003e \\u003cli\\u003e\\u003cspan\\u003eKato T, Wu J-L, Hirai Y, Sugimoto H, Bermudez V. Record Efficiency for Thin-Film Polycrystalline Solar Cells Up to 22.9% Achieved by Cs-Treated Cu(In,Ga)(Se,S)\\u003csub\\u003e2\\u003c/sub\\u003e. IEEE J. Photovolt. 9, 325\\u0026ndash;330 (2019).\\u003c/span\\u003e\\u003c/li\\u003e \\u003cli\\u003e\\u003cspan\\u003eRaghuwanshi M, \\u003cem\\u003eet al.\\u003c/em\\u003e Influence of RbF post deposition treatment on heterojunction and grain boundaries in high efficient (21.1%) Cu(In,Ga)Se\\u003csub\\u003e2\\u003c/sub\\u003e solar cells. Nano Energy 60, 103\\u0026ndash;110 (2019).\\u003c/span\\u003e\\u003c/li\\u003e \\u003cli\\u003e\\u003cspan\\u003eKim K, \\u003cem\\u003eet al.\\u003c/em\\u003e Mechanisms of extrinsic alkali incorporation in CIGS solar cells on flexible polyimide elucidated by nanoscale and quantitative analyses. 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A 8, 6471\\u0026ndash;6479 (2020).\\u003c/span\\u003e\\u003c/li\\u003e \\u003cli\\u003e\\u003cspan\\u003eKresse G, Hafner J. Ab initio molecular dynamics for liquid metals. Phys. Rev. B Condens Matter 47, 558\\u0026ndash;561 (1993).\\u003c/span\\u003e\\u003c/li\\u003e \\u003cli\\u003e\\u003cspan\\u003eWang L, Maxisch T, Ceder G. Oxidation energies of transition metal oxides within theGGA\\u0026thinsp;+\\u0026thinsp;Uframework. Phys. Rev. B 73, 195107 (2006).\\u003c/span\\u003e\\u003c/li\\u003e \\u003cli\\u003e\\u003cspan\\u003eMakov G, Payne MC. Periodic boundary conditions in ab initio calculations. Phys. Rev. B Condens Matter 51, 4014\\u0026ndash;4022 (1995).\\u003c/span\\u003e\\u003c/li\\u003e\\u003c/ol\\u003e\"}],\"fulltextSource\":\"\",\"fullText\":\"\",\"funders\":[],\"hasAdminPriorityOnWorkflow\":false,\"hasManuscriptDocX\":true,\"hasOptedInToPreprint\":true,\"hasPassedJournalQc\":\"\",\"hasAnyPriority\":false,\"hideJournal\":false,\"highlight\":\"\",\"institution\":\"\",\"isAcceptedByJournal\":true,\"isAuthorSuppliedPdf\":false,\"isDeskRejected\":\"\",\"isHiddenFromSearch\":false,\"isInQc\":false,\"isInWorkflow\":false,\"isPdf\":false,\"isPdfUpToDate\":true,\"isWithdrawnOrRetracted\":false,\"journal\":{\"display\":true,\"email\":\"info@researchsquare.com\",\"identity\":\"npj-flexible-electronics\",\"isNatureJournal\":false,\"hasQc\":true,\"allowDirectSubmit\":false,\"externalIdentity\":\"npjflexelectron\",\"sideBox\":\"Learn more about [npj Flexible Electronics](http://www.nature.com/npjflexelectron/)\",\"snPcode\":\"41528\",\"submissionUrl\":\"https://submission.springernature.com/new-submission/41528/3\",\"title\":\"npj Flexible Electronics\",\"twitterHandle\":\"\",\"acdcEnabled\":true,\"dfaEnabled\":true,\"editorialSystem\":\"stoa\",\"reportingPortfolio\":\"NPJ\",\"inReviewEnabled\":true,\"inReviewRevisionsEnabled\":true},\"keywords\":\"Flexible Cu(In, Ga)Se2 solar cells, alkali incorporation, stress-induced property, surface current\",\"lastPublishedDoi\":\"10.21203/rs.3.rs-4364503/v1\",\"lastPublishedDoiUrl\":\"https://doi.org/10.21203/rs.3.rs-4364503/v1\",\"license\":{\"name\":\"CC BY 4.0\",\"url\":\"https://creativecommons.org/licenses/by/4.0/\"},\"manuscriptAbstract\":\"\\u003cp\\u003eFlexible and lightweight Cu(In,Ga)Se\\u003csub\\u003e2\\u003c/sub\\u003e (CIGS) thin-film solar cells are promising for versatile applications, but there is limited understanding of stress-induced changes. In this study, the charge carrier generation and trapping behavior under mechanical stress was investigated using flexible CIGS thin-film solar cells with various alkali treatments. Surface current at the CIGS surface decreased by convex bending, which occurs less with the incorporation of alkali metals. The formation energy of the carrier generating defects increased in convex bending environments clarifying the degradation of the surface current. Moreover, alkali-related defects had lower formation energy than the intrinsic acceptors, mitigating current degradation in mechanical stress condition. The altered defect energy levels were attributed to the deformation of the crystal structure under bending states. This study provides insights into the mitigating of strain-induced charge degradation for enhancing the performance and robustness of flexible CIGS photovoltaic devices. Furthermore, direct atomic force microscopy-based probing techniques on bent material surfaces can contribute to the understanding of interplay between the strain and charge in other flexible optoelectronic devices.\\u003c/p\\u003e\",\"manuscriptTitle\":\"Interplay Between Strain and Charge in Cu(In,Ga)Se2 Flexible Photovoltaics\",\"msid\":\"\",\"msnumber\":\"\",\"nonDraftVersions\":[{\"code\":1,\"date\":\"2024-07-12 15:05:31\",\"doi\":\"10.21203/rs.3.rs-4364503/v1\",\"editorialEvents\":[{\"type\":\"communityComments\",\"content\":0}],\"status\":\"published\",\"journal\":{\"display\":true,\"email\":\"info@researchsquare.com\",\"identity\":\"npj-flexible-electronics\",\"isNatureJournal\":false,\"hasQc\":true,\"allowDirectSubmit\":false,\"externalIdentity\":\"npjflexelectron\",\"sideBox\":\"Learn more about [npj Flexible Electronics](http://www.nature.com/npjflexelectron/)\",\"snPcode\":\"41528\",\"submissionUrl\":\"https://submission.springernature.com/new-submission/41528/3\",\"title\":\"npj Flexible Electronics\",\"twitterHandle\":\"\",\"acdcEnabled\":true,\"dfaEnabled\":true,\"editorialSystem\":\"stoa\",\"reportingPortfolio\":\"NPJ\",\"inReviewEnabled\":true,\"inReviewRevisionsEnabled\":true}}],\"origin\":\"\",\"ownerIdentity\":\"ea7c368d-4b35-4773-a440-9fc37067e57a\",\"owner\":[],\"postedDate\":\"July 12th, 2024\",\"published\":true,\"recentEditorialEvents\":[],\"rejectedJournal\":[],\"revision\":\"\",\"amendment\":\"\",\"status\":\"published-in-journal\",\"subjectAreas\":[{\"id\":31735886,\"name\":\"Physical sciences/Energy science and technology/Renewable energy/Solar energy/Photovoltaics/Solar cells\"},{\"id\":31735887,\"name\":\"Physical sciences/Materials science/Materials for energy and catalysis/Solar cells\"}],\"tags\":[],\"updatedAt\":\"2024-09-27T10:16:55+00:00\",\"versionOfRecord\":{\"articleIdentity\":\"rs-4364503\",\"link\":\"https://doi.org/10.1038/s41528-024-00347-7\",\"journal\":{\"identity\":\"npj-flexible-electronics\",\"isVorOnly\":false,\"title\":\"npj Flexible Electronics\"},\"publishedOn\":\"2024-09-16 04:00:00\",\"publishedOnDateReadable\":\"September 16th, 2024\"},\"versionCreatedAt\":\"2024-07-12 15:05:31\",\"video\":\"\",\"vorDoi\":\"10.1038/s41528-024-00347-7\",\"vorDoiUrl\":\"https://doi.org/10.1038/s41528-024-00347-7\",\"workflowStages\":[]},\"version\":\"v1\",\"identity\":\"rs-4364503\",\"journalConfig\":\"researchsquare\"},\"__N_SSP\":true},\"page\":\"/article/[identity]/[[...version]]\",\"query\":{\"redirect\":\"/article/rs-4364503\",\"identity\":\"rs-4364503\",\"version\":[\"v1\"]},\"buildId\":\"qtupq5eGEP_6zYnWcrvyt\",\"isFallback\":false,\"isExperimentalCompile\":false,\"dynamicIds\":[84888],\"gssp\":true,\"scriptLoader\":[]}","source_license":"CC-BY-4.0","license_restricted":false}