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Enhanced Conversion Efficiency of Cu(In,Ga)Se2 Solar Cells via Electrochemical Passivation Treatment Hung-Wei Tsai, Stuart Thomas, Chia-Wei Chen, Yi-Chung Wang, Hsu-Sheng Tsai, Yu-Ting Yen, Cheng-Hung Hsu, Wen-Chi Tsai, Zhiming Wang, and Yu-Lun Chueh ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b11863 • Publication Date (Web): 27 Jan 2016 Downloaded from http://pubs.acs.org on February 16, 2016

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ACS Applied Materials & Interfaces

Enhanced Conversion Efficiency of Cu(In,Ga)Se2 Solar Cells via Electrochemical Passivation Treatment Hung-Wei Tsai1, Stuart R. Thomas1,2, Chia-Wei Chen1, Yi-Chung Wang1, Hsu-Sheng Tsai1, Yu-Ting Yen1, Cheng-Hung Hsu1, Wen-Chi Tsai1, Zhiming M. Wang2 and Yu-Lun Chueh1,* 1

Department of Materials Science and Engineering, National Tsing Hua University, Hsinchu 30013,

Taiwan 2

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

China, People’s Republic of China *

E-mail: [email protected]

Abstract Defect control in CIGS materials, no matter what the defect type or density is a significant issue, correlating directly to PV performance. These defects act as recombination centers and can be briefly categorized into interface recombination and Shockley-Read-Hall (SRH) recombination, both of which can lead reduced PV performance. Here, we introduce an electrochemical passivation treatment for CIGS films that can lower the oxygen concentration at the CIGS surface as observed by X-Ray photoelectron spectrometer analysis. Temperature-dependent J-V characteristics of CIGS solar cells reveal that interface recombination is suppressed and an improved rollover condition can be achieved following our electrochemical treatment. As a result, the surface defects are passivated and the power conversion efficiency performance of the solar cell devices can be enhanced from 4.73 to 7.75 %.

Keyword: Electrochemical passivation, CIGS solar cells, rollover condition, temperature-dependent JV characteristics, EA activation energy

1 ACS Paragon Plus Environment


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1. Introduction With global energy consumption increasing, renewable solar based energy production offers a potential alternative to our reliance on finite carbon based energy sources.1 Single p-n junction solar cells offer the highest theoretical energy conversion efficiency (ƞ) of ~30 %,2 and are therefore considered a practical and cost effective way to convert solar energy into electricity. To date, numerous solar cell technologies have been demonstrated with extraordinary energy conversion efficiencies reported. Inorganic solar cells such as crystalline Si, III-V compounds of GaAs and InP, Cu(In,Ga)Se2 (CIGS), and CdTe have all been shown to produce efficiencies over 20 %.3 CIGS with its record breaking efficiency of 21.7 % is an attractive material candidate for thin-film photovoltaics (PV) due to its high absorption coefficient of 105 cm-1,3 whilst its tunable band gap (1.0 eV to 1.7 eV), achieved by controlling the ratio of In/Ga, also enables optimization of charge carrier transport.4, 5 Typical CIGS absorber layers with thicknesses of ~2 µm are sufficient to absorb most incident sunlight, and remain suitably robust for fabrication on flexible substrates.6 CdS is a commonly used buffer layer in CIGS solar cells, used to form a p-n heterojunction,7-9 However, imperfections at the CdS/CIGS heterojunction can induce interface defects, leading to severe interface recombination. Defect control in CIGS, regardless of defect type or density, is a significant issue correlating directly to solar cell performance. Defects in CIGS act as recombination sites and can be briefly categorized into interface recombination and SRH recombination, all resulting in inferior device performance.10, 11 Surface defects in CIGS can arise from the formation of secondary phases during selenization of the non-stoichiometric Cu/In/Ga precursors12 during crystallinity control processing of the CIGS grains and also from oxidation at the CIGS film surface. The Cu2-xSe secondary phase can be removed by KCN wash13 and the oxidation of CIGS can be cleaned by dipping in either KCN- or NH3-based aqueous solutions.14 Here, we introduce an alternative surface modification method that electrolyzes the CIGS thin film in nitric acid solution to passivate its surface and thus enhance the performance of our CIGS solar cells from 4.73

to 7.75 % (max) with

enhancement of 63 %. The efficacy of our electrochemical passivation treatment (EP herein) was confirmed by a lower oxygen compositions near the CIGS surface, confirmed by XPS analysis. Temperature-dependent J-V characteristics of CIGS solar cells reveal that the interface recombination


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mechanism can be suppressed and an improved rollover condition can be achieved after the EP treatment. As a result, this treatment is able to passivate the defects with hydrogen and generate n-type doping at the surface of CIGS.15 2. Experimental Section Electrochemical passivation treatment: The EP treatment was processed by a potentiostatic method in a two-electrode system. Diluted nitric acid with a pH value of 1.2 was used as the electrolyte solution, and a bias of -3 V was applied for durations between 30 and 180 seconds at a current density of approximately 16 mA/cm2. After the EP treatment the CIGS films were cleaned using only deionized water. Synthesis of CIGS films: On top of soda lime glass (SLG), we deposited a molybdenum (400 nm) electrode using a two-stage sputtering process, followed by our 10 metal precursor layers of Cu/In/Ga sputtered individually. This was followed by a selenization process using Se pellets in a graphite box loaded into a furnace at 570 °C leading to a final CIGS film thickness of ~1.2 µm. Fabrication of PV devices: The CIGS thin film was dipped into a 10 wt. % KCN solution for KCN etching treatment, and then a 50 nm-buffer layer of CdS was capped by chemical bath deposition. Finally, i-ZnO and ITO with thicknesses of 70 nm and 550 nm respectively were deposited by sputtering system to complete the whole CIGS devices. Characterization: The crystallization and composition of the CIGS films were conﬁrmed by X-ray diffraction using a Rigaku TTRAX Ⅲ diffractometer with a Cu Kα radiation source at a wavelength of 0.154 nm, and using a micro-Raman spectrometer with a 632.8 nm He-Ne laser with a laser intensity of 2 mW. The surface morphology and composition of the films were examined by scanning electron microscopy using JOEL JSM 6500-F system operated at 15 kV with resolution of 3.0 nm, by X-ray photoelectron spectroscopy (XPS, Thermo Fisher Scientific Theta Probe), and by energy-dispersive xray spectrometry using an Oxford X-MAX SDD at 80mm2. Time-resolved photoluminescence (TRPL) was used for determining the carrier life time by recording when the electrons were excited and then emitted fluorescence caused by the recombination. The current–voltage characteristics were measured



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using a Keithley 4200-SCS parameter analyzer system under class AAA AM 1.5 G solar simulator with a power density of 100 mW cm2, at a constant temperature of 25 °C. 3. Results and Discussion The EP treatment was carried out in a two-electrode cell, consisting of a CIGS thin-film sample as the working electrode (WE) and a graphite bar as both counter electrode (CE) as well as reference electrode (RE) as shown in Figure 1(a). Diluted nitric acid with a pH value of 1.2 was used as the electrolyte solution, and a bias of -3 V was applied with durations of 30, 60 and 180 sec for the EP treatment. Following the treatment, a clear change in contrast over the reactive area was observed as shown in Figures S1(a) to S1(c) whilst scanning electron microscope (SEM) images (Figure 1b to 1e) of the CIGS thin films show similar surface morphologies before and after the treatment, indicating that the EP treatment will not change the CIGS grain size and the film morphologies. In addition, no significant thickness change can be observed before and after the EP treatment (Figure S2). Photoluminescence (PL) spectroscopy was performed in order to investigate the quality of CIGS films before and after the EP treatment as shown in Figure 2(a). Clearly, the enhanced PL peak intensity can be observed after the EP treatment was applied for 30 seconds with an unchanged PL peak position centered at 1215 nm (Figure S1d), whilst no clear intensity enhancement was found following the EP treatment for more than 30 sec. However, the peeling observed in CIGS thin films (Figure S3a) following the longer duration EP treatment is likely the cause of the slight degradation in PL peak intensity. From the PL results and SEM images, we concluded that the 30 second EP treatment duration is the most suitable treatment time for further solar cell fabrication. Furthermore, the elemental compositions of copper (Cu), indium (In), gallium (Ga) and selenium (Se) in CIGS thin films were measured by energy dispersive spectrometer (EDS) equipped in our SEM as shown in Figure 2(b), showing the consistent elemental compositions of CIGS films after the EP treatment. The corresponding Raman spectra as shown in Figure 2(c) reveals the two intrinsic peaks at 175 and 214 cm-1, attributed to Raman-active vibration A1 and B2, E-mixed modes respectively.16 Following initial device performance assessments, we found that when compared to untreated films, the solar cell performance of devices utilising films subjected to 30 seconds of EP show the highest values for VOC and JSC as shown in Figure S3(b). The extracted device parameters are detailed in Table S1.


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Nevertheless, without the supplementary KCN wash treatment for the removal of the Cu2-xSe secondary phases, the energy conversion efficiency of CIGS solar cell remains below optimum. We expect that the KCN wash step is still necessary and not fully replaceable by electrochemical passivation treatment directly. As a result of our initial findings, we focused our investigation on how our EP treatment can enhance solar cell performance when used in addition to the KCN wash process. Here, we fixed the duration of the EP treatment at 30 seconds according to our previous results, and CIGS thin films were prepared under four conditions, including pristine CIGS film, CIGS film with KCN wash applied only, CIGS film with the EP treatment applied only and CIGS film with both the KCN wash and EP treatment applied. The optical properties of each CIGS thin film, measured by UV-Vis spectroscopy, reveal approximately similar reflectance in the visible and near-infrared region (over the range of 500 – 1200 nm) as shown in Figure 3(a). By plotting (αhυ)2 versus incident photon energy, approximate band gap values of 1.08 eV were extracted, indicating that any difference observed in solar cell performance should not be due to modification of the optical properties of the films after the EP treatment. The surfaces of CIGS thin films were analyzed by grazing incidence X-ray diffraction spectrometry. Figure 3(c) shows the approximately identical series of XRD peaks among all of the CIGS thin films and no peak shift was observed in any of the films as confirmed by inset in Figure 3(c), verifying that the surface elemental compositions remained unaffected following the EP treatment. Time-resolved photoluminescence spectrometry was also utilized to measure the minority carrier lifetime and to distinguish the degree of carrier recombination for each of the films. By utilizing the fitting equation of = − + − , we are able to calculate the minority carrier lifetimes, τ1 and τ2 as shown in Figure 3(d) for four kinds of sample conditions. As a result, CIGS thin films utilising the combined KCN wash and EP treatment show superior minority carrier lifetimes for τ1 and τ2 of 1.178 ns and 2.342 ns respectively. For the CIGS film with only the KCN etching treatment, we observed minority carrier lifetimes for τ1 and τ2 of 0.635 ns and 1.315 ns respectively. For the CIGS film with only EP, we observe minority carrier lifetimes for τ1 and τ2 of 0.273 ns and 1.136 ns, respectively. However, the CIGS film without any treatment shows the inferior minority carrier lifetimes of τ1 and τ2 of 0.237 ns and 1.098 ns, respectively.



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The CIGS thin films utilising combined KCN wash and EP treatment with the best two minority carrier lifetimes were used for solar cell fabrication, which required the additional deposition of CdS, i-ZnO, tin-doped indium oxide (ITO) and aluminum electrode to complete the device structure (full details of the device fabrication were discussed in the experimental section). Figure 4(a) shows the J-V characteristics of CIGS solar cells under air mass 1.5 illumination. Clearly, the cells with the combined KCN wash and EP treatment exhibit higher VOC, JSC, FF and ƞ than cells after the KCN wash treatment only. We observe a significant enhancement in conversion efficiency from 4.73 % to 7.75 % (max), following the EP treatment. A complete set of extracted parameters are listed in table 1 with statistics results of VOC, JSC, FF and ƞ for each set of CIGS solar cells (six devices/set) shown in Figure S4. Note that the defects inside the CIGS solar cells will also contribute defect levels in band diagram and hence limit the open circuit voltage of the device.17 This is why an increased Voc could be achieved after passivating the defects on the CIGS surface by electrochemical passivation treatment. The EQE spectra shown in Figure 4(b) indicate that the enhanced photon-to-electron conversion efficiency region is over the range of 550-1150 nm. However, within this range, each of the CIGS thin films show approximately the same reflectance, negating the possibility that the enhanced conversion efficiency is a result of higher absorbance in the EP treatment processed CIGS film. To determine the source of enhancement in solar cells with the EP treatment, temperaturedependent J-V measurements were measured between 140 K and 320 K, using a measurement interval of 10 K (Figure S5). The temperature-dependent J-V characteristics of CIGS solar cells can be used to distinguish whether the dominant recombination mechanism is interface recombination due to Fermilevel pinning at the heterojunction or Shockley-Read-Hall (SRH) recombination in bulk.18 The temperature-dependent VOC can be defined as Eq. (1), =

−

!

(1)

where q is the electron charge, n is the diode ideality factor, k is the Boltzmann constant, T is the temperature in Kelvin, J00 is photon-generated current density and JL is saturation current density prefactor.18 Therefore, EA, the activation energy of the dominant recombination mechanism, can be determined by extrapolating the VOC(T) from the high temperature linear regime to T = 0 K. Figure 5(a) shows the extrapolated values of EA at ~1.00 eV and ~0.975 eV for CIGS solar cells with both KCN


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wash and EP treatment and with KCN wash only, respectively. Solar cells utilizing the combined KCN wash and EP treatment observe a comparatively higher EA, closer to Eg (1.08 eV, Figure 3a), indicating the SRH recombination as the more dominant recombination mechanism,10 confirming that the interface defects at CIGS surface have been passivated after the EP treatment. Typically, CIGS surface defects can induce an increased barrier height at the CdS/CIGS interface, the effect of which can be observed by J-V characteristics under partial saturation of forward current (rollover),19 becoming more obvious and severe with an increase in the barrier height at a lower measurement temperature. Figure 5(b) exhibits the illuminated J-V characteristics of CIGS solar cells measured at 140, 220 and 300 K, respectively. We can observe that at lower temperatures, the current saturates in the forward bias region (beyond VOC), and the device utilising the KCN wash only, suffers a more severe rollover condition, implying more defects at the surface of the CIGS film. To investigate the defect types and concentrations at the surface of CIGS films, we performed Xray photoelectron spectrometer on surface and also at different depths (via surface sputtering with durations of 3 and 33 sec, corresponding to depths (d) of 0.3 and 3.3 nm) for CIGS films after both KCN wash and EP treatment or KCN wash only, as shown in Figures 6(a) and 6(b), respectively. The O1s XPS spectra region show that the surface oxygen composition of the CIGS film after both KCN wash and EP treatment (Figure 6a) is generally lower than that of the CIGS film after the KCN wash only (Figure 6b). The XPS spectra of the remaining constituent elements are shown in Figure S6(a)S6(d) and we observed that the peaks for the surface of CIGS show a small shift compared with those observed at depths of 0.3 and 3.3 nm possibly due to the higher oxidation phase. Note that negatively charged oxygen atoms could occupy vacant Se sites as chemically given by Eq. (2), in turn reducing donor defects and hence retard the band-bending at the CIGS surface and grain boundaries due to the decreased number of vacant Se sites, which then degrade the performance of CIGS solar cells.20 ∙∙ & 0 , "#$ '()* + "+ &-.' ↔ "+#$ &'()*

(2)

However, the elemental compositions taken from XPS results suggest the Se-rich surface for all CIGS films as shown in Figure 6(c), where solid columns represent CIGS films with both KCN wash and EP treatment, while shaded columns represent CIGS films with KCN wash only. Thus, the excess oxygen atoms likely come from the formation of metal oxides such as copper oxide, indium oxide or gallium



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oxide. These oxides can act as recombination centers at the heterojunction and are detrimental to the performance of CIGS solar cells. As such, we believe that the enhanced performance of our devices is most likely due to the elimination or reduction of the metal oxides confirmed by XPS after the EP treatment as the nitrate ions in the electrolyte can be reduced to nitrous ions under the applied bias of 3 V by Eq. (3).21 1+2, + 345 + 2 , → 41+ + 4 + , : ; = +0.94 @ A4:

(3)

These nitrous ions may then further react with the metal oxides, forming metal nitrates such as copper nitrate, indium nitrate and gallium nitrate as chemically represented by Eq. (4) where M indicates “metal”. B+0 + 1+, → B1+2 + +0

(4)

Interestingly, these metal nitrates are soluble in aqueous solution22 and can then be removed from the CIGS film surface, spontaneously. An additional mechanism that may play a contributing factor could be n-type doping on the surface and passivation by hydrogen ions after the EP treatment as expressed by Eq. (5). 5 5 ; , 4C → 4.D-E , 4.D-E , 4.D-E

(5)

The hydrogen may interact with the defect complex (2VCu+InCu)0, yielding shallow donors and passivating the acceptor-like copper vacancies,

15

which is consistent with the Cu-poor condition

observed at the surface of CIGS films by XPS results. 4. Conclusions An electrochemical treatment has been introduced to passivate the surface of CIGS thin films as a method to significantly enhance the performance of CIGS solar cells, increasing power conversion efficiency from 4.73 to 7.75 %. Temperature-dependent J-V characteristics reveal that using this method has led to the suppression of the interface recombination and an improved rollover condition. The surface composition of the CIGS films analyzed by XPS shows that the oxygen content is reduced due to the elimination of metal oxides. Our electrochemical surface treatment offers a potentially large-scale, low-cost, room temperature and industry-compatible method for significantly enhancing the performance of CIGS based solar cells.


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ASSOCIATED CONTENT Supporting Information Reactive areas of CIGS thin films; CIGS thin film peeling following excessive EP processing; Photovoltaic characterisation statistics; Illuminated J-V characteristics of CIGS solar cells measured between 140 K and 320 K with an interval of 10 K; XPS spectra for copper, indium, gallium, and selenium concentrations measured on as-deposited films and at different surface depths. This material is available free of charge via the Internet at http://pubs.acs.org.

Acknowledgements The research is supported by Ministry of Science and Technology through Grant through grants no, 104-2628-M-007-004-MY3, 104-2221-E-007-048-MY3, 104-2633-M-007-001, 104-2622-M-007002-CC2 and the National Tsing Hua University through Grant no. 104N2022E1. Y.L. Chueh greatly appreciates the use of facility at CNMM, National Tsing Hua University through Grant No. 104N2744E1.

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Lewis, N. S.; Nocera, D. G., Powering the planet: Chemical Challenges in Solar Energy Utilization. Proc. Natl. Acad. Sci. U. S. A. 2006, 103, (43), 15729-15735. Shockley, W.; Queisser, H. J., Detailed Balance Limit of Efficiency of p‐n Junction Solar Cells. J. Appl. Phys. 1961, 32, (3), 510-519. Green, M. A.; Emery, K.; Hishikawa, Y.; Warta, W.; Dunlop, E. D., Solar cell Efficiency Tables (Version 45). Prog. Photovoltaics 2015, 23, (1), 1-9. Dullweber, T.; anna, G. H.; Rau, U.; Schock, H. W., A New Approach to High-efficiency Solar Cells by Band Gap Grading in Cu(In,Ga)Se2 Chalcopyrite Semiconductors. Sol. Energy Mater. Sol. Cells 2001, 67, (1–4), 145-150. Jaffe, J. E.; Zunger, A., Anion Displacements and the Band-Gap Anomaly in Ternary ABC2 Chalcopyrite Semiconductors. Phys. Rev. B 1983, 27, (8), 5176-5179. Chirilă, A.; Buecheler, S.; Pianezzi, F.; Bloesch, P.; Gretener, C.; Uhl, A. R.; Fella, C.; Kranz, L.; Perrenoud, J.; Seyrling, S.; Verma, R.; Nishiwaki, S.; Romanyuk, Y. E.; Bilger, G.; Tiwari, A. N., Highly Efficient Cu(In,Ga)Se2 Solar Cells Grown on Flexible Polymer Films. Nat. Mater. 2011, 10, (11), 857-861. Rau, U.; Schmidt, M., Electronic Properties of ZnO/CdS/Cu(In,Ga)Se2 Solar Cells — Aspects of Heterojunction Formation. Thin Solid Films 2001, 387, (1–2), 141-146. Kronik, L.; Burstein, L.; Leibovitch, M.; Shapira, Y.; Gal, D.; Moons, E.; Beier, J.; Hodes, G.; Cahen, D.; Hariskos, D.; Klenk, R.; Schock, H. W., Band Diagram of the Polycrystalline



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CdS/Cu(In,Ga)Se2 Heterojunction. Appl. Phys. Lett. 1995, 67, (10), 1405-1407. Repins, I.; Contreras, M. A.; Egaas, B.; DeHart, C.; Scharf, J.; Perkins, C. L.; To, B.; Noufi, R., 19.9%-efficient ZnO/CdS/CuInGaSe2 Solar Cell with 81.2% Fill Factor. Prog. Photovoltaics 2008, 16, (3), 235-239. Turcu, M.; Pakma, O.; Rau, U., Interdependence of Absorber Composition and Recombination Mechanism in Cu(In,Ga)(Se,S)2 Heterojunction Solar Cells. Appl. Phys. Lett. 2002, 80, (14), 2598-2600. Dullweber, T.; Rau, U.; Contreras, M. A.; Noufi, R.; Schock, H., Photogeneration and Carrier Recombination in Graded Gap Cu(In, Ga)Se2 Solar Cells. IEEE Trans. Electron. Dev. 2000, 47, (12), 2249-2254. Park, J. S.; Dong, Z.; Kim, S.; Perepezko, J. H., CuInSe2 Phase Formation During Cu2Se/In2Se3 Interdiffusion Reaction. J. Appl. Phys. 2000, 87, (8), 3683-3690. Bär, M.; Klaer, J.; Weinhardt, L.; Wilks, R. G.; Krause, S.; Blum, M.; Yang, W.; Heske, C.; Schock, H.-W., Cu2-xS Surface Phases and Their Impact on the Electronic Structure of CuInS2 Thin Films – A Hidden Parameter in Solar Cell Optimization. Adv. Energy Mater. 2013, 3, (6), 777-781. Lehmann, J.; Lehmann, S.; Lauermann, I.; Rissom, T.; Kaufmann, C. A.; Lux-Steiner, M. C.; Bär, M.; Sadewasser, S., Reliable Wet-chemical Cleaning of Natively Oxidized High-efficiency Cu(In,Ga)Se2 Thin-film Solar Cell Absorbers. J. Appl. Phys. 2014, 116, (23), 233502. Kilic, C.; Zunger, A., N-type Doping and Passivation of CuInSe2 and CuGaSe2 by Hydrogen. Phys. Rev. B 2003, 68, (7). Rincón, C.; Ramírez, F. J., Lattice Vibrations of CuInSe2 and CuGaSe2 by Raman Microspectrometry. J. Appl. Phys. 1992, 72, (9), 4321-4324. Lany S; Zunger A. Limitation of the open-circuit voltage due to metastable intrinsic defects in Cu(In,Ga)Se2 and strategies to avoid these defects. 33rd IEEE Photovolatic Specialists Conference, 2008, May 11–16, 1-3. Cao, Q.; Gunawan, O.; Copel, M.; Reuter, K. B.; Chey, S. J.; Deline, V. R.; Mitzi, D. B., Defects in Cu(In,Ga)Se2 Chalcopyrite Semiconductors: A Comparative Study of Material Properties, Defect States, and Photovoltaic Performance. Adv. Energy Mater. 2011, 1, (5), 845-853. Topič, M.; Smole, F.; Furlan, J., Examination of Blocking Current-voltage Behaviour Through Defect Chalcopyrite Layer in ZnO/CdS/Cu(In,Ga)Se2/Mo Solar Cell. Sol. Energ. Mat. Sol. Cells 1997, 49, (1–4), 311-317. Kronik, L.; Rau, U.; Guillemoles, J.-F.; Braunger, D.; Schock, H.-W.; Cahen, D., Interface Redox Engineering of Cu(In,Ga)Se2 – Based Solar Cells: Oxygen, Sodium, and Chemical Bath Effects. Thin Solid Films 2000, 361–362, (0), 353-359. Kim, K.-W.; Lee, E.-H.; Choi, I.-K.; Yoo, J.-H.; Park, H.-S., Electrolysis of Nitric Acid by Using a Glassy Carbon Fiber Column Electrode System. J. Radioanal. Nucl. Chem. 2000, 245, (2), 301308. Solubilities of Inorganic and Organic Compounds. A Compilation of Quantitative Solubility Data from the Periodical Literature, by Atherton Seidell, Ph. D., Washington. Supplement to the second edition containing data published during the years 1917–1926 inclusive. D. van Nostrand Company, Inc., New York/Gauthier-Villars et Cie., Paris, 1928. 569 Seiten. Arch Pharm 1928, 266, (7), 544c-544.


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Table Captions Table 1. Extracted photovoltaic properties of CIGS solar cells VOC (mV)

JSC (mA/cm2)

FF (%)

η (%)

RS (Ω·cm2)

RSH (Ω·cm2)

KCN+EP

449.5

33.70

51.1

7.75

3.75

131.72

KCN wash only

420.0

27.40

41.1

4.73

9.56

83.39

Table 1



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Figure Captions Figure 1.

a) Illustration of electrochemical treatment for CIGS surface passivation. Surface morphology SEM images of CIGS thin film after electrochemical treatment for b) 30 sec, c) 60 sec, d) 180 sec and e) pristine CIGS thin film.

Figure 2.

a) Statistical values for photoluminescence peak intensities of CIGS films after different durations of the EP treatment, b) Elemental compositions measured by energy-dispersive X-ray spectrometry (EDS), and c) Raman spectra of pristine CIGS thin film and following the EP treatment on 30 sec, 60 sec, 180 sec.

Figure 3.

a) UV-vis spectra of CIGS thin films showing approximate reflectance in visible and near-infrared region (over the range of 500 – 1200 nm). b) Plots as the function of (αhυ)2 at different incident photon energies and extracted band gaps for four different conditions of CIGS thin films. c) Grazing incidence X-ray diffraction spectra of CIGS thin films confirm that no change in surface compositions following electrochemical treatment (black line : CIGS KCN + EP, red line : CIGS KCN only, green line : CIGS EP only, blue line : CIGS as deposited), and d) Time-resolved photoluminescence spectra of CIGS thin films and their corresponding fitted lifetimes τ1 and τ2.

Figure 4.

a) Current density-Voltage (J-V) characteristics measured under air mass 1.5 illumination and b) external quantum efficiencies (EQE) for CIGS solar cells after the combine KCN wash and EP treatment, exhibiting superior cell performance and enhanced photon-toelectron conversion efficiency over the range of 550-1150 nm.

Figure 5.

a) Temperature dependent open-circuit voltage (VOC) for CIGS solar cells including

extrapolated activation energy (EA) as T → 0K, and b) illuminated J-V characteristics of CIGS solar cells measured at 140, 220 and 300 K. Figure 6.

Surface X-ray photoelectron spectra (XPS) of oxygen concentration at surface and different depths from surface (0.3 nm and 3.3 nm) for a) KCN wash and EP processed


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and b) KCN wash only processed CIGS thin films and c) elemental composition of CIGS thin films at different surface depths, measured by XPS.

Figure 1



PL intensity ( a. u.)

180 170

(a)

160 150 140 130 120 110 100 90 Pristine 50

(b)

40

-3 V -3 V -3 V 30 sec 60 sec 180 sec

-3 V, 30 sec -3 V, 60 sec -3 V, 180 sec Pristine

30 20 10 0

Cu K

Raman Intensity (a. u.)

Atomic (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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(c)

150

In L Ga K Elements

A1

B2,E

Se L

-3V, 30sec -3V, 60sec -3V, 180sec Pristine CIGS

200 250 Raman Shift(cm-1)

Figure 2


300

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(b) 2.0 2 8

20

2

15 10 5

1.5 1.0 0.5 0.0

400

600

800 1000 1200 1400

30

40

50

60

0.8

(d)

27

28

70

1.0 1.08 1.2

1.4

Energy (eV)

KCN + EP KCN EP Pristine

Intensity (counts)

26

CIGS(400)

25

0.6

CIGS(332)

CIGS(211)

CIGS(112)

CIGS(312)

Wavelength (nm)

(c)

20


-2

25

(α hυ) (10 cm eV )


CIGS(213) CIGS(220) CIGS(301)

Reflectance (%)

(a)30

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


τ1 = 1.178 ns, τ2 = 2.342 ns τ1 = 0.635 ns, τ2 = 1.315 ns τ1 = 0.273 ns, τ2 = 1.136 ns τ1 = 0.237 ns, τ2 = 1.098 ns

F G = HI JKL −G/NI + HO PQR −G/NO

80

4

2 theta (degree)

6

8

10

Time (ns)

Figure 3


12

14

2

(a)

KCN + EP KCN

40 30 20 10 0

0

100

200

300

400

500

Voltage (mV) Energy (eV) (b)

EQE (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Current density (mA/cm )


90 80 70 60 50 40 30 20 10 0

3.53 2.5

2

1.5

1

ZnO CdS

CIGSe2

KCN + EP KCN 400

600

800

1000

1200

Wavelength (nm)

1400

Figure 4


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(a) 1100

KCN + EP KCN

1000 VOC (mV)

900 800 700 600 500 400 300

0

40 80 120 160 200 240 280 320 Temperature (K)

(b)

80

Current density (mA/cm2)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


60 40

KCN + EP - 140K KCN + EP - 220K KCN + EP - 300K KCN - 140K KCN - 220K KCN - 300K

20 0 -20

-400 -200

0

200 400 600 800 1000

Voltage (mV)

Figure 5



(a) Intensity (a.u.)

1600 1400 1200

Surface 0.3 nm from surface 3.3 nm from surface

KCN+EP

Surface 0.3 nm from surface 3.3 nm from surface

KCN

1000 800 600 400 200 0

1600

(b) Intensity (a.u.)

1400 1200 1000 800 600 400 200 0 540

535

530

525

Binding energy (eV)

(c)

60 50

Atomic (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

40 30 20

KCN + EP - surface KCN - surface KCN + EP - 0.3 nm from surface KCN - [d] 0.3 nm from surface KCN + EP - 3.3 nm from surface KCN - 3.3 nm from surface

10 0 Cu

In

Ga

Se

O

Figure 6


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TOC


Enhanced Conversion Efficiency of Cu(In,Ga)Se2 ... - ACS Publications

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