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Crystalline Engineering Toward Large-Scale High-Efficiency Printable Cu(In,Ga)Se2 Thin Film Solar Cells on Flexible Substrate by Femtosecond Laser Annealing Process Shih-Chen Chen, Nian-Zu She, Kaung-Hsiung Wu, Yu-Ze Chen, Wei-Sheng Lin, Jia-Xing Li, Fang I Lai, Jenh-Yih Juang, Chih Wei Luo, Lung-Teng Cheng, Tung-Po Hsieh, Hao-Chung Kuo, and Yu-Lun Chueh ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b00082 • Publication Date (Web): 10 Mar 2017 Downloaded from http://pubs.acs.org on March 12, 2017

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ACS Applied Materials & Interfaces

Crystalline Engineering Toward Large-Scale High-Efficiency Printable Cu(In,Ga)Se2 Thin Film Solar Cells on Flexible Substrate by Femtosecond Laser Annealing Process Shih-Chen Chen†, Nian-Zu She†, Kaung-Hsiung Wu†,*, Yu-Ze Chen§, Wei-Sheng Linǁ, Jia-Xing Li†, Fang-I Lai⊥, Jenh-Yih Juang†, Chih Wei Luo†, Lung-Teng Chengǁ, Tung-Po Hsiehǁ, Hao-Chung Kuo‡, and Yu-Lun Chueh§,η, £, ζ*

†

Department of Electrophysics and ‡Department of Photonics and Institute of Electro-Optical

Engineering, National Chiao-Tung University, Hsinchu 30010, Taiwan, ROC §

Department of Materials Science and Engineering, National Tsing Hua University, Hsinchu

30013, Taiwan, ROC η

School of Materials Science and Engineering, State Key Laboratory of Advanced Processing

and Recycling of Non-Ferrous Metals in Gansu Province, Lanzhou University of Technology, Lanzhou City 730050, Gansu Province, P. R. China ζ

Institute of Fundamental and Frontier Sciences, University of Electronic Science and

Technology of China, Chengdu 611731, P. R. China £

Department of Physics, National Sun Yat-Sen University, Kaohsiung, 80424, Taiwan, ROC.

⊥

Department of Photonic Engineering, Yuan-Ze University, Taoyuan 32003, Taiwan, ROC

ǁ

Compound Semiconductor Solar Cell Department, Next Generation Solar Cell Division,

Green Energy and Environment Research Laboratories, Industrial Technology Research Institute, Hsinchu 31040, Taiwan, ROC

*

E-mail: [email protected] and [email protected].

1

ACS Paragon Plus Environment


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ABSTRACT- Ink-printing method emerges as a viable way for manufacturing large-scale flexible Cu(In,Ga)Se2 (CIGS) thin film photovoltaic (TFPV) devices owing to its potential for the rapid process, mass production and low-cost non-vacuum device fabrication. Here, we brought the femtosecond laser annealing (fs-LA) process into the ink-printing CIGS thin film preparation. The effects of fs-LA treatment on the structural and optoelectronic properties of the ink-printing CIGS thin films were systematically investigated. It was observed that, while the film surface morphology remained essentially unchanged under superheating, the quality of crystallinity was significantly enhanced after the fs-LA treatment. Moreover, a better stoichiometric composition was achieved with an optimized laser scanning rate of the laser beam, presumably due to the much reduced indium segregation phenomena, which is believed to be beneficial in decreasing the defect states of InSe, VSe, and InCu. Consequently, the shunt leakage current and recombination centers were both greatly decreased, resulting in a near 20 % enhancement in photovoltaic conversion efficiency. Keywords: solar cell, Cu(In,Ga)Se2, laser annealing, ink-printing, flexible

1.

Introduction Photovoltaic power has been touted as a promising candidate among renewable energies

to revolutionize the energy industry. The clean and more affordable solar power is the bright solution for human being. However, for a highly-developed metropolis where intensive electricity supply is needed, power grids are required to supply electricity drained from solar farms distributed at its surrounding satellite areas. As a more suitable and cost-efficient approach to energy collection via the use of solar power through sophisticated integration of high-efficiency solar cell into the metropolis, such as building-integrated installations and vehicle charging systems, is an effective alternative, for which infrastructures of power grids 2


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are not needed. Recently, thin-film photovoltaics (TFPV) are being extensively applied to these prosperous applications due to their superior properties and low-cost in raw materials as well as the flexibility of the devices that enable a wide range of thin film solar cells integrated into high population areas. Thus, the demand for thin film solar cells is prosperously growing.1-2 Cu(InGa)Se2 (CIGS) compound is one of the most favorable materials currently and hotly pursued in solar cell technologies for realizing cost-effective power generation. In addition to the inherent excellent absorption characteristics due to only a layer of few microns-thick being used, the advantages of low-cost (less material needed) and high-rate of deposition over a large area are also indispensable factors to be considered.3 Nowadays, the high-efficiency CIGS thin-film solar cells of approaching 25 % have been developed.4 The remarkable long-term stability of CIGS solar cells has also demonstrated in outdoor tests conducted by National Renewable Energy Laboratory (NREL).5 The exhibited high radiation tolerance further promises its great potential for space applications.6 Various approaches for preparing CIGS thin films have been developed, including vacuum processes (sputtering,7 co-evaporation,8 and pulsed laser deposition9) and nonvacuum processes (electrochemical deposition,10 spin-coating,11 and ink-printing12). Among these processes, the ink-printing method is the most desirable approach for manufacturing large-scale flexible CIGS TFPV devices because of its potential for the rapid process, mass production and low-cost non-vacuum device fabrication. Previously, we successfully demonstrated the advantage of plasmonic gold nanoparticles for efficient enhancement of flexible ink-printing CIGS TFPV.13 However, researchers are still looking for a reliable method to develop large-scale high-efficiency printable CIGS TFPV. In this study, a laser annealing (LA) treatment using a femtosecond (fs) laser was practiced immediately following the selenization process in the nonvacuum ink-printing technique to enhance the crystalline quality of CIGS thin film. The results evidently showed 3



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that the LA treatment could lead to significantly enhanced conversion efficiency. The selective annealing resulted from the LA treatment can effectively activate the absorber layer without heating other regions after properly choosing a wavelength of the laser beam. Thus, the process has been ubiquitously employed in small-area dopant-activation of semiconductors currently to replace the more conventional annealing methods, including furnace annealing (FA) and rapid thermal annealing (RTA).14-16 In addition, laser crystallization has been implemented in solar cell production for attaining higher device performances than conventional thermal annealing.17 Previous studies, using continuous-wave (CW) and ns-pulsed lasers, have indicated that LA treatment indeed is a viable approach for improving the crystallinity and mobility of CIGS thin films obtained by different processes.18-21 However, these laser sources turn out to be inappropriate for the LA treatment process applied to CIGS thin films from the current non-vacuum synthesis, because these non-vacuum CIGS films are very sensitive to residual heat.22 On the other hand, the fs-laser has been recognized as a novel tool for annealing amorphous silicon without inducing heating effect to vicinity regions.23 As a result, the crystalline structures of amorphous silicon has been significantly improved and the optoelectronic properties can substantially meliorated after the fs-laser treatment.23 In this regard, we brought the femtosecond laser annealing (fs-LA) treatment into the non-vacuum ink-printing CIGS thin film preparation. To the best of our knowledge, this perhaps is the first attempt of this kind, which turned out to result in the significant enhancement of the conversion efficiency. Extensive examinations on fs-LA CIGS films using X-ray diffraction (XRD) patterns, Raman and photoluminescence (PL) spectra indicated that the obtained enhancements are intimately related to the much improved film crystallinity and substantial defect reductions. The ultrafast optical pump-probe spectroscopy measurements revealed that the much prolonged carrier lifetime observed in the fs-LA CIGS thin film may be the primary reason, leading to the much enhanced conversion efficiency realized in the 4


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present flexible TFPV devices via J–V characteristics measurements. 2. Experimental Section Preparation of the CIGS Absorber Layer by the Ink-Printing Process CIGS thin films with an average thickness of 2 µm were deposited by the non-vacuum nanoparticle method. The copper oxide (CuO), indium oxide (In2O3) and gallium oxide (Ga2O3) were mixed with a Ga/(In+Ga) ratio of 0.3 and Cu/(In+Ga) ratios of 0.8 to 1, followed by dispersing them in a water-based solution using the agent where transforming particles with initial sizes of a few micrometers into particles with sizes of only 50–80 nm in a stable non-flocculated state were confirmed. The solid content of the stable ink with 20 g/100mL was coated on the Mo/Cr/stainless steel substrate using a scalpel. Then, the samples were annealed at 450 oC in the H2 gas for 30 min. Subsequently, the precursor was selenized at 400 oC in the H2Se gas for 30 min. Laser Annealing System CIGS thin films fabricated by the nonvacuum ink-printing process were placed on a three-axis translation stage for adjusting the location. For comparison purposes, a KrF excimer laser (wavelength = 248 nm, pulse width = 20 ns, pulse repetition rate = 10 Hz) and a Ti:sapphire mode-locked laser (wavelength =800 nm, pulse width = 100 fs, pulse repetition rate = 5,000 Hz) were used as the light sources for the LA treatment process. The laser beam was collimated and guided to a laser scanner to irradiate the samples. Characterizations Structures and phases of CIGS films were characterized by a Bruker D2 PHASER X-ray spectrometer (Ettlingen, Germany) under irradiation of monochromatic Cu-Kα (λ approximately 1.54 Å). The morphologies and elemental compositions of CIGS films were observed by field emission scanning electron microscope (SEM, JSE-7001, JEOL, Tokyo, Japan) with attached accessory of energy dispersive spectroscopy (EDS INCA analysis 5



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system, Oxford Instruments, Oxford shire, UK). For measurements of pump-probe spectra, a commercial Ti:sapphire fs-laser system outputting short pulses (~30 fs) with a repetition rate of 75 MHz and a wavelength of 800 nm was employed. The pump beam was focused at a diameter of ~50 µm with pump fluences in a range of 15.2 to 45.7 µJ/cm2 and the fluence of the probe beam was fixed at 1 µJ/cm2 with a spot diameter of 20 µm. The pumping pulses were modulated by a chopper at 2 KHz. The time delay between the pump and probe pulses was varied with a mechanical delay stage. The transient reflectivity change ∆R/R of the probe beam was acquired as a function of the pump-probe delay time. The small reflected signals were detected and fed into a lock-in amplifier. A 635 nm diode laser was employed to excite samples for PL measurements. Temperature-dependent PL measurements were conducted at temperatures ranging from 10 to 80 K under a constant excitation power of 5 mW. Power-dependent PL measurements were conducted at a constant temperature of 10 K with excitation fluences ranging from 0.5 to 5 mW. PL signals emitted from the CIGS samples were guided to a monochromator with a 600 groove/mm grating and detected by an infrared photomultiplier tube. J-V measurements were conducted following the procedure described in international standard CEI IEC 60904-1. Both the pristine and the fs-LA-treated CIGS solar cells were measured under a simulated Air Mass 1.5, Global (AM1.5G) illumination with a power of 1000 W/m2. The temperature was actively maintained at 25 oC during the measurements. 3. Results and Discussion Figures 1(a) and (b) show a schematic diagram and a photo image of the LA system used in this study, respectively. Figures 1(c) to 1(e) show the surface morphologies of the CIGS thin films before and after the LA treatment process with different laser pulses. It is apparent that the surface morphology of the CIGS film treated with the ns-pulsed laser (Figure 1d) exhibits the typical feature of melting, while that of the fs-LA-treated CIGS film 6


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(Figure 1e) remains nearly intact as compared to that of the pristine CIGS films. This suggests that the CIGS thin film fabricated by the non-vacuum ink-printing process is indeed very sensitive to residual heat acquired from the laser beams,22 which often results in serious phase segregation and leads to the degraded device performance. Interestingly, the melting phenomenon was not observed in co-evaporated CIGS thin films while using excimer laser for carrying out the LA treatment process18, presumably that the vacuum process-derived CIGS films are having a better crystalline quality to begin with and, hence, having a higher tolerance to the residual heat. On the other hand, since the pulse duration is much shorter than the heat-conduction time (in the order of picosecond) when using fs-laser to carry out the LA treatment, resulting in a non-melting annealing effect. (Figure S1 shows the thermal images of the laser-irradiated CIGS films). As a result, the surface morphology of the CIGS film after the fs-LA treatment remains essentially the same as that of the pristine sample (cf. Figures 1c and 1e). Figure 2(a) shows the full-width at half-maximum (FWHM) of the (112)-diffraction peak, which also appears to be the preferred orientation of the present CIGS thin films after the fs-LA treatments with different laser fluences. (The full XRD patterns are presented in Figure S2.) The results indicate that the smallest FWHM of the (112) peak is obtained with an optimized fs-LA fluence of 6.8 mJ/cm2. Closer inspection at the XRD patterns displayed in Figure 2(b) for the pristine and the optimal fs-LA-treated CIGS thin films reveals that the positons of all characteristic peaks at (112), (220)/(204) and (312)/(116) peaks, corresponding to chalcopyrite CIGS crystalline structure24, remained with the (112) being the preferred growth orientation. The results confirm that the fs-LA did not induce drastic melting and regrowth in CIGS films. Nevertheless, after the fs-LA treatment, the FWHM of the (112) diffraction peak evidently reduced from 0.164o to 0.125o, indicating that significant increase in grain size is realized with the fs-LA treatment.25 In particular, it is noticed that feature of seemingly “selective” significant enhancement in the (112)-preferred orientation is very 7



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different from the results obtained by using CW or ns-pulsed laser annealing processes on CIGS films, wherein all of the characteristic peaks appeared to be enhanced more evenly.18-19 This unique phenomenon can be attributed to the atomic rearrangement on the polar (112) planes of the CIGS film by cations or anions migration as illustrated schematically in Figure S3.26 This is believed to arise mainly from significant enhancement in the vibration modes of cations and anions driven by the intensive electric field of fs laser pulses.27-28 The migration of ions, in turn, facilitates the elimination of electronic defects and modify the crystalline structures. The Raman spectra and the mapping image shown in Figure 2(c) further provide a direct evidence of atomic rearrangement-induced crystallinity modification resulted from the fs-LA treatment where the signal intensity of the A1 mode in the chalcopyrite structure of the CIGS after the process is significantly enhanced.29 In addition to the laser fluence, appropriate conversion of the incoming photon energy into chemical energy plays the decisive role in determining the reaction for maintaining proper stoichiometry.30 Thus, the scanning rate of the laser beam during the fs-LA process plays an important role. In general, for CIGS film, the deviation from the stoichiometric composition usually arises from the selenization process.31 Especially, the lack of dissociated Se vapor often leads to the segregation of In and results in unwanted off-stoichiometry products.31 In the present study, the composition (in atomic percent, at%) of the pristine CIGS films were determined to be: Cu:In:Ga:Se = 21.64 : 36.64: 4.09: 37.63. Figure 3(a) plots the difference in at%, defined as:△at.% = at.%(fs-LA)–at.%(pristine), of each element after the fs-LA treatment with various laser scanning rates at the optimized laser fluence of 6.8 mJ/cm2. It is evident that the amount of In is significantly decreased with the decrease in the laser scanning rate, while the amount of copper exhibits a changing trend opposite to that of In. This is suggestive that there might have an intimate interaction between In and Cu in the stoichiometric reaction driven by the incoming laser energy. On the contrary, the changes in Se and Ga concentrations appeared to be relatively insignificant and insensitive to the change 8


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in laser scanning rates. It has been pointed out that the increase in the composition ratio of Cu/(Ga+In) will result in a substantial decrease in the Raman frequency of the A1 mode and can drastically modify the defect states.32 This is largely consistent with the results obtained in the Raman spectrum shown in Figure 2(c). Since the defect states existing in the CIGS thin films play a pivot role in the ultimate device performance by manifested themselves primary in the carrier dynamics,33-37 it is not only scientifically interesting but also necessary from a practical application point of view to further verify the above mentioned assertions on the defect reduction obtained by the fs-LA treatment. To this respect, carrier dynamics in the fs-LA CIGS thin films treated with different laser scanning rates were investigated by fs pump-probe spectroscopy, which has been proved to be a powerful technique to delineate non-equilibrium carrier dynamics in various materials, including CIGS films.38-39 Figure 3(b) shows the room-temperature reflectivity transient for CIGS films treated by fs-LA with different laser scanning rates. It is apparent that the longest defect-related carrier lifetime (τslow) is obtained in the fs-LA CIGS thin film with a scanning rate of 30 mm/s, suggesting an optimized condition with minimum defects (See Figure S4).38 The result appears to be associated with the turning point in atomic percent changes of Se as shown in Figure 3(a). In order to further investigate the defect states in the present CIGS films, the temperature-dependent PL spectra measurements were conducted. Figures 4(a) and 4(b) show the PL spectra obtained in the temperature range of 10~80 K for the pristine and fs-LA treated CIGS films, respectively. To delineate the corresponding physical origins related to individual specific defect giving rise to the photon emission40-41, the experimental data obtained at 30 K was chosen as the representative data for Gaussian fitting and the results for the pristine and the fs-LA treated CIGS films are shown in Figures 4(c) and 4(d), respectively. To further shed light on the physical mechanism associated with each individual photon emission peak, the exciting power-dependent PL spectra were measured and the results are displayed in Figure 9



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S5. According to the correlations between the peak shifts, temperatures and exciting powers, the four primary peaks used in the fitting process can be assigned as follows. Normally, peak 2 (P2) is unambiguously related to band-to-band transition, while P1, P3 and P4 are all related to donor–acceptor pair (DAP) transitions.42-43 The slight photon energy difference associated with P2, namely 1.08 and 1.09 eV for the pristine and fs-LA CIGS films, respectively, is believed to be resulted from the increased Ga/(Ga+In) ratio, as depicted in Figure 3(a).44 Values of the activation energy45, Ea, derived from an Arrhenius plot of I(T)= I(0)/ [1+ Cexp (-Ea/kBT)], where I(0), I(T), kB and C are the PL intensity at 0 and T K, Boltzmann constant and a constant, respectively, related to P1, P3 and P4 obtained from the fitting results were listed in Table 1. The identified defects, corresponding to InSe, VSe, and InCu for P1, P3 and P4, were also specifically indicated in Table 1.46 It is also interesting to note that, in comparison to the PL spectra obtained for the pristine CIGS film. P1 and P4 in the fs-LA-treated CIGS film are both “quenched” with vanishingly small contribution to the emission peak (cf. Figures 4c and 4d), suggesting that the InSe and InCu defects have been almost completely eliminated by the fs-LA treatment. The results are also in good agreement with the variation in atomic concentrations as shown in Figure 3(a), where the stoichiometric reaction and structural changes were simultaneously driven by the fs laser to result in substantial reduction of In atoms segregation on the surface of CIGS thin films. The excessive In atoms originally existing in the vicinity of film surface may have been driven by the fs-LA treatment and diffuse all way inward to promote more completed stoichiometric reaction (Figure S6).47 As a result, the interstitial defect states (InSe and InCu), mainly resulted from the existence of excessive In atoms, are effectively eliminated, which, in turn, retrieved the Cu and Se atoms to the correct sites. This scenario also explains the opposite trend observed in the elemental changes of Cu and In (Figure 3a). Moreover, the fact that no monotonic increase in the amount of Se was observed. It can be attributed to the Se-vacancies (VSe), which are filled by Se atoms with the optimized scanning rate (30 mm/s) that are in line with the substantially 10


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waned P3 intensity manifested in the PL spectra of the fs-LA CIGS film as shown in Figure 4(d). And the following increased amount of Se at low scanning rate (5 mm/s) is due to the formation of SeO2 nearby the surface of the CIGS film. Having established the effects of the fs-LA treatment on the quality of the CIGS films printed on flexible stainless steel substrates, a cadmium sulfide (CdS) thin film (buffer layer) was then deposited on the obtained CIGS films to form pn-junctions for investigating the effects on the performance of real devices. For fabricating the devices, a window layer and transparent conductive oxides, namely, i-ZnO and Al:ZnO (AZO), were capped on the pn-junction. Finally, the silver grid pattern was coated on the window layer by the printing process. The J-V characteristics were measured and the typical results and corresponding parameters for the pristine and fs-LA CIGS devices are shown in Figure 5(a) and listed in Table 2, respectively. The inset of Figure 5(a) further shows a photo image of a typical CIGS TFPV device fabricated in this study. It is evident from the displayed results that the conversion efficiency increases from 9.24 % to 11.05 % after the fs-LA treatment owing to the enhancement in filling factor (FF), corresponding to near ~20 % enhancement. Moreover, the open-circuit voltage increases because of an enlarged bandgap arisen from the increase in Ga/(Ga+In) ratio (Figure 3a). In addition, the dark J-V measurements were conducted, as shown in Figure 5(b), by injecting carriers into the circuit with electrical method rather than using light generated carriers because small fluctuation in light intensity can trigger considerable amount of noise to the system, making it difficult to precisely diagnose the diode properties under illumination. Clearly, the significant decrease in the shunt leakage current after the fs-LA treatment was obtained. The ideality factor (n) of the diode is an important indicator to elucidate the state of carrier recombination as well as the improvement in modified cells. It can be independently determined through the slope of the semi-logarithmic plot of dark current vs. voltage characteristics according to Shockley diode equation.48 An extracted ideality factor (n) decreases from 3.69 to 2.37 after the fs-LA treatment, which is 11



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indicative of a drastic decrease in the number of recombination centers. This result corresponds perfectly to the extended carrier lifetime observed in pump-probe spectroscopy (Figure 3b) due to the modified crystalline structures after the fs-LA treatment. Both the decrease in the shunt leakage current and ideality factor (n) can, thus, be attributed to the enhanced quality of crystalline structures and decreased defect states (InSe, VSe, and InCu) driven by the fs laser pulses. 4. Conclusions We have successfully demonstrated that the femtosecond laser annealing treatment can give rise to significant improvements in both crystalline structure and defects reduction for the nonvacuum ink-printing CIGS thin films without introducing melting effect. The results unambiguously indicate that while the film surface morphology remains essentially unchanged under superheating, the fs-LA treatment has evidently led to much enhanced intensity both in the preferred-(112) diffraction peak and A1 mode Raman signal of the chalcopyrite structure, indicating that much better film crystallinity has been achieved. Furthermore, the pump-probe spectroscopy revealed that the carrier lifetime can be substantially prolonged by optimizing the laser-scanning rate, presumably due to the better stoichiometry in CIGS thin films resulted from migrating the excessive In atoms at the surface inward and reacting with other ions. As a consequence, the defect states of InSe, VSe, and InCu were all reduced to vanishing level, which has been elaborately verified by extensively analyzing the temperature and power dependence of the PL spectra. The modification in chalcopyrite structure and stoichiometric composition via the fs-LA treatment also greatly decreases the shunt leakage current and recombination centers, resulting in a near 20% enhancement in photovoltaic conversion efficiency, from 9.24 % to 11.05 %. These results have convincingly manifested that the fs-LA treatment is a reliable method for boosting the conversion efficiency in flexible ink-printing CIGS TFPV. 12


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ASSOCIATED CONTENT Supporting Information Thermal images of laser-irradiated CIGS films; The XRD spectra of femtosecond laser treatment on CIGS films with different laser fluences; 3D sketches of (112) planes of CIGS crystalline structures; The curve fitting method for fs pump-probe spectroscopy; The PL spectra with various exciting fluences obtained from the pristine and fs-LA annealed CIGS films. The depth profile of elemental analysis detected by XPS measurements. This material is available free of charge via the Internet http://pubs.acs.org/.

Acknowledgments The authors show their great appreciation to Prof. Pavlos Lagoudakis from the School of Physics and Astronomy, University of Southampton, UK for the great support in data analysis and fruitful discussion. The research was supported by Ministry of Science and Technology through grants:

105-2112-M-009-011, 106-2917-I-564-024, 104-2628-M-007-004-MY3,

104-2221-E-007-048-MY3,

105-2633-M-007-003,

104-2622-M-007-002-CC2,

and

by

National Tsing Hua University through Grant no. 104N2022E1. Y.L. Chueh greatly appreciates the use of facilities at CNMM, National Tsing Hua University through Grant No. 105A0088J4. Funding for this work was also provided by the Department of Industrial Technology, Ministry of Economic Affairs, Taiwan. References (1) Yu, W.; Li, F.; Wang, H.; Alarousu, E.; Chen, Y.; Lin, B.; Wang, L.; Hedhili, M. N.; Li, Y.; Wu, K.; Wang, X.; Mohammed, O. F.; Wu, T. Ultrathin Cu2O as An Efficient Inorganic Hole Transporting Material for Perovskite Solar Cells. Nanoscale 2016, 8, 6173–6179. (2) Lee, C. H.; Kim, D. R.; Zheng, X. Transfer Printing Methods for Flexible Thin Film Solar 13



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Table Captions Table 1 Activation energies and the assigned defects for individual PL peak in CIGS thin films. peak 1

activation energies

4 meV

related defects

InSe

peak 2

peak 3

peak 4

band to band 16 meV 18 meV transition VSe

InCu

Table 2 J-V characteristics obtained from the pristine and fs-LA CIGS TFPV devices.

Voc(V)

Jsc(mA/cm2)

FF(%)

efficiency(%)

pristine

0.50

33.07

55.24

9.24

fs-LA

0.53

34.08

61.16

11.05

flexible CIGS TFPV

20


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Figure captions Figure 1 (a) Schematic diagrams and (b) the photo of laser annealing system. Surface morphologies of CIGS thin films before (c) and after different pulsed laser treatment using (d) KrF excimer laser (ns-LA) and (e) Ti:sapphire mode-locked laser (fs-LA), respectively. (Laser fluence ~10 mJ/cm2)

Figure 2 (a) Full-width at half-maximum (FWHM) as a function of laser fluence used in fs-LA treatment for the diffraction peak of the (112)-preferred orientation for CIGS films with the fs-LA treatment at different laser fluences. (b) XRD spectra and (c) Raman spectra and the mapping image for CIGS thin film before and after the fs-LA treatment (laser fluence = 6.8 mJ/cm2, irradiation time: 10 seconds).

Figure 3 (a) Different atomic ratios of each element after the fs-LA treatment with various laser scanning rates. (b) Reflectivity transient in CIGS films treated with different scanning rates at room temperature (The laser fluences of the pump beam and probe beam are 45.7 µJ/cm2 and 1 µJ/cm2, respectively).

Figure 4 PL spectra of CIGS films (a) before and (b) after the fs-LA treatment in the temperature ranges between 10~80 K, respectively. The experimental data and the Gaussian fitting at 30K for (c) pristine and (d) fs-LA CIGS films.

Figure 5 (a) J–V characteristics and (b) dark current of CIGS TFPVs with and without the fs-LA treatment.

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Figure 1

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Figure 2

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Figure 3

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Figure 4

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Figure 5

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TOC

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Thin Film Solar Cells o - ACS Publications - American Chemical Society

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