Group III Elemental Composition Dependence of RbF Post-Deposition

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Group III Elemental Composition Dependence of RbF Post-Deposition Treatment Effects on Cu(In,Ga)Se Thin-Films and Solar Cells 2

Shogo Ishizuka, Noboru Taguchi, Jiro Nishinaga, Yukiko Kamikawa, Shingo Tanaka, and Hajime Shibata J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b00079 • Publication Date (Web): 31 Jan 2018 Downloaded from http://pubs.acs.org on February 2, 2018

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The Journal of Physical Chemistry

Group III Elemental Composition Dependence of RbF Post-Deposition Treatment Effects on Cu(In,Ga)Se2 Thin-Films and Solar Cells Shogo Ishizuka1*, Noboru Taguchi2, Jiro Nishinaga1, Yukiko Kamikawa1, Shingo Tanaka2, and Hajime Shibata1 1

Research Center for Photovoltaics, National Institute of Advanced Industrial Science and

Technology (AIST), 1-1-1 Umezono, Tsukuba, Ibaraki 305-8568, Japan 2

Research Institute of Electrochemical Energy, National Institute of Advanced Industrial Science

and Technology (AIST), 1-8-31 Midorigaoka, Ikeda, Osaka 563-8577, Japan

ABSTRACT The effects of RbF post-deposition treatment (RbF-PDT) on Cu(In,Ga)Se2, CuInSe2, and CuGaSe2 thin-films and solar cell devices are comparatively studied. Similar to the effect of KF post-deposition treatment (KF-PDT), Cu(In,Ga)Se2 and CuInSe2 film surfaces show significant pore formation resulting in a rough surface morphology with RbF-PDT, whereas this is not the case for In-free CuGaSe2. The device properties of In-containing and In-free Cu(In,Ga)Se2 solar cells also show contrasting results, namely, Cu(In,Ga)Se2 or CuInSe2 devices show an increase in open circuit voltage (Voc) and fill factor (FF) values and almost constant or a slight decrease in short-circuit current density (Jsc) values with RbF-PDT, whereas CuGaSe2 devices show no significant improvements in Voc and FF values but a substantial increase in Jsc values. These results suggest that the alkali effects on Cu(In,Ga)Se2 film and device properties strongly depend on the group III elemental composition in Cu(In,Ga)Se2 films as well as alkali-metal species.

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1. INTRODUCTION Cu(In,Ga)Se2 (CIGS) solar cells with energy conversion efficiencies over 20% have been reported from several groups to date.1 Today, most studies demonstrating such high efficiencies have employed alkali-metal (heavier than Na) fluoride post deposition treatments. Thus, post deposition treatments using KF, RbF, or CsF (KF-PDT, RbF-PDT, or CsF-PDT) are now recognized as promising techniques to boost CIGS device performance.2-5 In contrast to the success of these alkali-halide PDTs, the detailed mechanism behind the beneficial effects of alkali-metals on CIGS devices are still open to discussion. The role of alkali-metals and mechanisms behind the effects in CIGS are occasionally discussed based upon ternary CuInSe2 (CIS) to simplify the model.6,7 Current record efficiencies have been demonstrated using CIGS materials with relatively low [Ga]/([Ga]+[In]) (GGI) compositions of about 0.3, equivalent to the band-gap energy (Eg) of about 1.1 eV,3,4,8 and thus the material properties of CIGS are expected to be close to CIS rather than CuGaSe2 (CGS). Studies of alkali-metal effects based upon CIS may be, therefore, important and useful. On the other hand, wide-gap materials which can be applied to the top cell of tandem structure type solar cells have recently attracted attention9,10 to obtain high efficiencies beyond the Shockley-Queisser limits.11 Wide-gap CIGS materials, namely, with large Ga and/or S content CIGS are promising candidates for practical highefficiency and low-cost top cell materials. Note that the Eg values of CIGS materials system can be controlled using these elements in the range of 1.04 eV (CuInSe2), 1.53 eV (CuInS2), 1.68 eV (CuGaSe2), and 2.43 eV (CuGaS2). The effects of alkali-halide PDTs on wide-gap CIGS are, therefore, one of important topics to extend the application of chalcopyrite materials from single junction solar cells to a multiplicity of uses.


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Among the alkali-metals, elemental Rb has been known to have similar effects as K and Cs in enhancing CIGS device performance.3,12 For instance, we have also demonstrated over 20% solar cell efficiencies from 1 cm2 size CIGS devices with RbF-PDT using a conventional device structure, that is, SLG/Mo/CIGS/CdS/i-ZnO/AZO, where SLG, i-ZnO and AZO are sodalime glass, intrinsic ZnO and Al-doped ZnO, respectively (Figure S1). In this work, the effects of RbF-PDT on CIGS thin-films and devices with various GGI values are comparatively studied. In particular, the effects of Rb in CGS are focused upon towards developments in wide-gap CIGS materials. Here we present that the effect of RbF-PDT strongly depends on the GGI value in CIGS films and as a consequence ternary CGS devices show contrasting behaviors to Incontaining CIGS devices.

2. EXPERIMENTAL CIGS, CIS, and CGS thin-films were grown on Mo-coated soda-lime glass (SLG) substrates by the three-stage process using elemental Cu, In, Ga, and Se Knudsen cell sources in a vacuum chamber. Elemental In and/or Ga, and Se were supplied during the first and third stages at a substrate temperature of 350℃ and Cu and Se were supplied during the second stage at a substrate temperature of 550℃. The typical CIGS film composition ratios [Cu]/([Ga]+[In]) and [Ga]/([Ga]+[In]) were 0.9 and 0.2-0.3, respectively. The film thickness of the CIGS or CIS layers was 2.0-2.5 µm, whereas the thickness of the CGS films was about 1.7 µm. The reason for the use of thinner CGS films is due to the short carrier collection length of CGS films compared with CIS or CIGS13 and we have optimized the CGS device process using a thickness of 1.7 µm.14 RbF-PDT was carried out using a RbF Knudsen cell source with a Se-vapor supply after film growth in an identical growth chamber used for the film growth at a substrate temperature of


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350℃ for 10 min. The amount of RbF material supplied to films was adjusted by varying the RbF Knudsen cell temperature (TRbF) ranging from 560 to 620 ℃. Solar cell devices were fabricated using a chemical bath deposited CdS buffer-layer and successively sputter-deposited intrinsic ZnO (i-ZnO) and conductive ZnO:Al layers. The thicknesses of CdS buffer-layers used were about 30 nm for CIGS and CIS devices and 60 nm for CGS devices. The thicknesses of iZnO and ZnO:Al layers commonly used for all devices were about 50 nm and 300 nm, respectively. Grid contacts made of Al or Ni/Al metals were formed on the ZnO:Al transparent conductive oxide (TCO) layer and selected cells were coated with a MgF2 or a moth-eye antireflection film. The solar cell device area was 0.52 cm2. Heat-light soaking (HLS) treatments were performed on cell devices under 0.5 sun illumination at 90℃ in a N2 atmosphere or in vacuum for 100 hours with the expectance of activation of the metastable acceptors in CIGS and a consequent enhancement in device performance.15,16 Films and solar cell devices were characterized using scanning electron microscopy (SEM), Auger electron spectroscopy (AES), tunneling electron microscopy-energy dispersive x-ray spectroscopy (TEM-EDX), secondary ion mass spectrometry (SIMS) measurements, and current-voltage (I-V), external quantum efficiency (EQE), and capacitance-voltage (C-V) measurements. The nominal carrier density (NCV) values were calculated using the equation NCV (W) = 2/(q・εCIGS・ε0・S2・[d(1/C2)/dV])

(1)

Where q is the electric charge and W is the depletion width expressed as W = εCIGS・ε0・S/C.

εCIGS, ε0, and S are the dielectric constants of CIGS and vacuum, device area, respectively. The values of εCIGS ~ 13.5 and εCGS ~ 11.5 were used in this study. Detailed characterization conditions can be found elsewhere.17


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3. RESULTS AND DISCUSSION Figure 1 compares the CIGS, CIS, and CGS film surface morphology variation with RbFPDT. Granular particles formed after RbF-PDTs (Figure 1a, d, g, j) were found to disappear with a washing treatment in a dilute ammonia solution regardless of the group III elemental composition in films (Figure 1b, e, h, k). When compared to a CIGS film grown without RbFPDT (Figure 1f), prominent pore formation can be observed on the surface of RbF-PDT CIGS films with a wide range of TRbF values from 560 to 620℃ (Figure 1b, e). A similar trend to the CIGS film behavior was observed in ternary CIS films as shown in Figure 1h and 1i. Although KF-PDTs are also known to cause a similar pore formation, this is not the case for NaF-PDT of CIGS,18 implying that the effects of surface modification depend on the alkali-metal species. From this result, the effect of elemental Rb on CIGS is expected to be close to elemental K rather than Na. On the other hand, In-free CGS films showed no significant variation (no pore formation) in the surface morphology upon washing treatment after a RbF-PDT (TRbF ~ 580℃) as shown in Figure 1k, l, though a larger amount of RbF supply (TRbF ~ 620℃) led to a rough surface morphology (Figure S2). This result suggests that the formation of pores with alkali-halide PDTs strongly depends on the group III elemental composition in CIGS films and the presence of elemental In rather than Ga enhances surface modifications. The granular particles formed on the CIGS film surface with RbF-PDT were found to be very unstable. These particles can be easily removed with washing treatment as mentioned above. These particles were also found to easily disappear and left pores behind upon the electron beam irradiation used for SEM-AES measurements as shown in Figure 2a. Once pores were formed upon washing treatment, no further variations with electron beam irradiation were observed


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(Figure 2b). Even after the removal of the particles with electron beam irradiation or washing treatment, pore modified CIGS surfaces showed the presence of elemental Rb as determined by SEM-AES (Figure S3) and TEM-EDX measurements (Figure 2c). Figure 2c and 2f show TEMEDX results focusing on elemental Rb in the CIGS/CdS interface region and in the CIGS layer of a solar cell device (Figure 2e), respectively. Note that a RbF-PDT (TRbF ~ 620℃) CIGS solar cell device demonstrated a relatively high cell efficiency of 21.1% with an open circuit voltage (Voc) of 0.737 V, a short circuit current density (Jsc) of 35.9 mA/cm2, and a fill factor (FF) of 0.798 (in-house measurements with estimated errors less than ±3%, designated area: 0.52 cm2) and a CIGS film grown in an identical growth batch with this was used for the measurements shown in Figure 2a, b, c, f. Figure 2c shows high-angle annular dark-field (HAADF) TEM images and a corresponding elemental Rb EDX mapping image obtained from the CIGS/CdS interface region. It is expected that granular particles formed on the CIGS surface after RbF-PDT were removed with a dilute ammonia solution used during the CdS buffer-layer deposition process. Nonetheless, the presence of elemental Rb can be clearly observed and a distinguishable long-periodic structure, which does not belong to CIGS or CdS, was observed at the interface as indicated with white arrows in Figure 2c. Note that typical CIGS films grown without RbF-PDT show a CIGS/CdS hetero-epitaxy structure as shown in Figure 2d and the presence of this longperiodic structure can not be observed. This result is attributable to the formation of Rb-related compounds, such as Rb(In,Ga)Se2, as predicted in the literature.7 Our preliminary analysis on the unique structure is supportive to the model of the formation of Rb-III-Se2 compounds, though detailed analyses and discussions will be made in a separate paper and we will not go into details in this study.


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The location (occupying sites) of alkali-metals in CIGS films and its local environments have long been discussed to date.19-23 It was found that elemental Rb added in CIGS films with RbF-PDT concentrates not only in the CIGS grain boundaries but also in dislocation defects in the CIGS grain interior as shown in Figure 2f. The band-gap energy (Eg) of NaF- or KF-PDT CIGS is suggested to have widened values.24-26 RbF-PDT CIGS films are, therefore, also expected to have a similarly modified (widened) energy band structure. The TEM-EDX results shown here suggest that such a modification of structural and electrical properties of CIGS is especially expected to occur at the CIGS/CdS interface, CIGS grain boundaries, and also defects present in the CIGS grain interior where alkali-metals can be incorporated. Figure 3 compares the alkali-metal distribution profiles in CIS, CIGS, and CGS films grown with RbF-PDT (TRbF ~ 580℃, equivalent to a Rb concentration of about 3×1019 cm-3 in films as shown in Figure 3g, h, i). Elemental Na present in these films originates from diffusion from SLG/Mo substrates during film growth and RbF-PDT processes, whereas elemental Rb mostly originates from RbF-PDT. Although the CGS film surface showed no significant variation (no pore formation) with RbF-PDT as shown in Figure 1k, the presence of a substantial amount of elemental Rb in the film can be observed (Figure 3i), as well as in CIS and CIGS films (Figure 3g, h). The relatively light alkali-metal, Na, was found to decrease in concentration near the surface region in all the films with RbF-PDT (Figure 3d, e, f). This result is consistent with an ion exchange model, that is, heavy alkali elements substitute for light alkali elements, as reported in the literature.2 The substitution of elemental Na and K in CIGS with Cs has also been reported.3 It should be noted here that CIS films grown by the three-stage process consist of large-size grains (Figure 3a), whereas an incorporation of elemental Ga leads to a reduction of the grain size (Figure 3b, c). The Na concentration in CIGS or CGS films near the Mo layer was


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found to increase with RbF-PDT. This result is attributable to enhanced Na diffusion from the substrate into the CIGS or CGS film during the RbF-PDT process.27 On the other hand, the CIS film consisting of large-size grains showed no significant increase in Na concentration near the Mo layer (Figure 1d), implying that grain boundaries can be an important alkali-metal diffusion pathway. Next, variations in solar cell device properties are discussed. Figure 4a shows variations in CIGS solar cell parameters obtained with RbF-PDT (TRbF ~ 580℃) and HLS treatments. As indicated with black arrows, RbF-PDTs significantly enhanced the open circuit voltage (Voc) and fill factor (FF) values, though short-circuit current density (Jsc) values decreased slightly. As a consequence, CIGS cell efficiencies improved. This result is consistent with the effect of alkalimetal K as reported by other groups.3,16 A slight decrease in Jsc values with RbF-PDT is likely related to a decrease in the collection of carriers generated by long wavelength light as will be mentioned later. Solar cell parameter variations of CIS devices showed similar trends with CIGS with RbF-PDT as can be seen in Table 1 and thus hereafter a comparative study of CIGS and Infree CGS devices is focused upon. The red arrows indicate the trend of cell parameter variations with HLS treatments. RbF-PDT CIGS cells showed further enhancements in Voc and FF values and concomitant improvements in cell efficiencies with HLS treatments, though Jsc values slightly decreased (Figure 4a and 5b). One of the mechanisms behind the enhanced device performance is attributable to an increase in the space charge density NCV (nominal hole carrier density) in CIGS, as shown in Figure 5a. Although Figure 5a shows one example for an increased NCV value with RbF-PDT, the NCV value in CIGS sometimes decreases with RbF-PDT, even though the device performance improves. In this case, other effects such as CIGS/CdS interface modifications with RbF-PDT which lead to a reduction in carrier recombination are


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expected.28 Note that alkali-metals incorporated in CIGS are suggested to play multiple roles to be both acceptors IIICu → ACu (A: a heavy alkali-metal such as K, Rb, or Cs) and donors NaCu → (A-A)Cu (dumbbell interstitial donor formation model).6 On the other hand, the NCV value in CIGS increases with HLS treatments in almost cases. These variation trends observed with RbFPDT are quite similar to the effect of KF-PDT as reported in the literature.15,16 The slight decrease in Jsc values observed with RbF-PDT and HLS treatments is attributable to an excessively large value of NCV, namely the hole carrier density, which is expected to reduce the carrier diffusion length resulting in enhanced recombination in CIGS films as manifested by a decrease in EQE values in the long wavelength region (Figure 4c). Also, the decrease in the depletion width with increasing NCV values may lead to a decrease in the collection of carriers generated by long wavelength photons due to the decreased electric field near the Mo back contact region in CIGS films. On the other hand, In-free CGS devices showed far different behavior from CIGS devices. Figure 4b shows variations in the CGS solar cell parameters with RbF-PDT (TRbF ~ 580℃) and with HLS treatments. Unlike the case for CIGS devices, RbF-PDT was not significantly effective in improving CGS device performance. RbF-PDT CGS devices showed a slight increase in Voc values and a decrease in FF values. Although Figure 4b shows improved Voc values with RbFPDT, degradation in Voc values can be sometimes observed with RbF-PDT from CGS devices. Consequently, not so remarkable improvements in CGS cell efficiencies were demonstrated with RbF-PDT. It should be noted here, however, that RbF-PDT CGS devices showed a significant increase in Jsc values, in contrast to the result observed from CIGS devices. HLS treatments performed on completed (after mechanical scribing to separate eight cell devices on a substrate as shown in Figure 5e) CGS cell devices led to a significant degradation in Voc and FF values as


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indicated with red arrows in Figure 4b and 5d. The Jsc values, however, clearly increased with HLS treatments. The variations observed in Jsc values are consistent with the variation in EQE curves (Figure 4d). It was found that the NCV value obtained from CGS devices showed no significant variation with RbF-PDT (Figure 5c). HLS treatments performed on the CGS devices, however, led to a significant decrease in NCV values, that is, the completely opposite trend to the result observed in CIGS devices. In our previous study, we observed that In-free CGS devices are very sensitive to exposure to heat and oxygen in comparison to CIGS, and post CGS/CdS interface formation processes performed using heat and oxygen, such as TCO layer depositions, can cause critical degradation of CGS device performance.29,30 In this study, HLS treatments were carried out in a N2 atmosphere or in vacuum as mentioned above. To avoid the effect of residual humidity and oxygen in the chamber used for HLS treatments, we examined the effect of HLS treatments carried out before the mechanical scribing of cells (Figure 5e) to prevent the device cross section from being exposed to the atmosphere during HLS treatments and the result is shown in Figure 5f. An increase in Jsc values was reproducibly observed. A decrease in FF values, attributable to a dent in the current density-voltage (J-V) curve shape around the Pmax region implying degradation in the p-n junction properties, was observed. An increase in Voc values was, however, reproducibly observed in CGS devices with HLS treatments performed before cell separation, though the NCV values decreased (not shown). From this result, it can be said that RbF-PDT and HLS treatments are basically effective in improving Voc values regardless of CIGS or CGS. It is, however, difficult to explain the mechanism behind the increase in Voc values only with the relevant NCV value variations because the trend observed from CIGS and CGS devices is completely opposite to each other. Although more detailed investigations are necessary to clarify


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the mechanism behind the increased Jsc values in the CGS devices, elemental Ga-related metastable states (including the effects of the complex vacancy defect VSe-VCu on neighboring Ga atoms),31 which may be different from In-related ones, present in CGS grain boundaries and/or grain interior modified with elemental Rb are expected to affect carrier transport in CGS films resulting in Jsc value variations with RbF-PDT and HLS treatments.

4. SUMMARY To summarize, RbF-PDT performed on CIGS or CIS films significantly modified the surface morphology and was effective in enhancing device performance. These effects are similar to other heavy alkali-metal (K or Cs) fluoride PDT reported to date.3,12,18 TEM measurements showed the formation of Rb-related compounds between the CIGS/CdS interface with RbF-PDT, suggesting that a modification of the energy band diagram occurs at the interface and contributes to improvements in Voc values as in the case of elemental K. On the other hand, In-free CGS showed no prominent surface morphology variations in comparison with CIGS or CIS, and no significant improvements in device performance with RbF-PDT were demonstrated, though a substantial increase in Jsc values was observed. At present, it has been found that the same PDT conditions which are effective in improving CIGS or CIS devices are not useful for CGS devices, though there is room for further discussion if RbF-PDTs are really not so effective in improving CGS device performance or if it is only an issue of process optimization. As a consequence, the effect of an alkali-metal Rb on CIGS device performance was found to strongly depend on the group III elemental composition.


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ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.xxxxxxx. The original data sheet of the independently certified efficiency, SEM images of CGS films, SEM-AES images of CIGS films (PDF)

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. ORCID Shogo Ishizuka: 0000-0002-4404-5257 Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS We thank H. Higuchi, M. Iioka, Y. Ueno, and H. Takahashi for their help with the experiments and technical support. We also thank P. J. Fons for fruitful discussions. This work was supported by JSPS KAKENHI Grant Number 16K04969, also supported in part by AIST internal funds, the Department of Energy and Environment Innovation Program.

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Page 18 of 25

Figure Captions Figure 1. Surface SEM images of CIGS films grown with RbF-PDT (a, d) before and (b, e) after washing treatment, (c) a schematic of the sample structure used for SEM measurements, (f) a surface SEM image of a CIGS film grown without RbF-PDT for reference. ‘RbF-PDT 560’ and ‘RbF-PDT 620’ mean that RbF-PDTs were carried out using TRbF values of 560℃ and 620℃, respectively. (g) CIS and (j) CGS film surfaces grown with RbF-PDT (TRbF ~ 580℃) and (h, k) after a washing treatment. (i) CIS and (l) CGS film surfaces grown without RbF-PDT treatment for reference. The magnification of these images is identical. Figure 2. RbF-PDT CIGS film surface variations with electron-beam (e-beam) irradiation observed from (a) non-washed and (b) washed surfaces. (c) Cross sectional TEM-EDX images obtained from near the CIGS/CdS interface region in a RbF-PDT CIGS cell device. (d) A TEM image obtained from near the CIGS/CdS interface region in a CIGS cell device grown without RbF-PDT. (e) A schematic of the device structure used for TEM measurements. (f) TEM-EDX images obtained from the CIGS layer in an identical device. Figure 3. Cross sectional SEM images observed from (a) CIS, (b) CIGS, and (c) CGS cell devices. SIMS elemental distribution profiles of (d, e, f) Na and (g, h, i) Rb in the CIS, CIGS, and CGS layers. SIMS measurements were carried out using Cs+ as the primary ion with an acceleration voltage of 5 kV. The Na and Rb concentrations in all the films were calculated using standard CIGS samples prepared by ion implantation, and thus the concentration values in CIS and CGS may deviate from accurate values and should be used only for relative comparisons. Figure 4. Solar cell parameter variations with RbF-PDT and HLS treatments obtained from (a) CIGS and (b) CGS cell devices. (c, d) Typical EQE curves obtained from these cell devices. Black arrows indicate variation trends observed with RbF-PDT, while red arrows indicate variation trends observed with HLS treatments. Figure 5. (a, c) NCV-depletion width profiles calculated from C-V measurements for CIGS and CGS cell devices grown with (solid lines) and without (dotted lines) RbF-PDT, and before (black lines) and after (red lines) HLS treatments for 100 h. (b, d) J-V curves obtained from CIGS and CGS cell devices grown with RbF-PDT before (black lines) and after (red lines) HLS treatments performed after cell separation. (e) A picture of cell devices fabricated on a 3×3 cm2 substrate and a schematic of the cell separation. (f) J-V curves obtained from a CGS cell device grown with RbF-PDT before and after HLS treatments performed before cell separations. All RbFPDTs were carried out with the TRbF value of 580℃. Red arrows indicate variation trends observed with HLS treatments.


18

Page 19 of 25 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


Table 1. Typical variations in solar cell parameters with RbF-PDT (TRbF ~ 580℃) for CIS, CIGS, and CGS cell devices measured without HLS treatments (in-house measurements with estimated errors less than ±3%, designated area: 0.52 cm2). Sample

RbF-PDT

Efficiency (%)

Voc (V)

Jsc (mA/cm2)

FF (%)

CIS

Yes

14.35

0.466

41.87

73.6

CIS

No

10.41

0.396

41.86

62.7

CIGS

Yes

20.14

0.717

36.35

77.3

CIGS

No

18.43

0.682

36.75

73.5

CGS

Yes

9.02

0.818

15.80

69.9

CGS

No

8.59

0.816

14.80

71.1


19


Figure 1. Surface SEM images of CIGS films grown with RbF-PDT (a, d) before and (b, e) after washing treatment, (c) a schematic of the sample structure used for SEM measurements, (f) a surface SEM image of a CIGS film grown without RbF-PDT for reference. ‘RbF-PDT 560’ and ‘RbF-PDT 620’ mean that RbF-PDTs were carried out using TRbF values of 560℃ and 620℃, respectively. (g) CIS and (j) CGS film surfaces grown with RbF-PDT (TRbF ~ 580℃) and (h, k) after a washing treatment. (i) CIS and (l) CGS film surfaces grown without RbF-PDT treatment for reference. The magnification of these images is identical. 170x162mm (300 x 300 DPI)


Page 20 of 25

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Figure 2. RbF-PDT CIGS film surface variations with electron-beam (e-beam) irradiation observed from (a) non-washed and (b) washed surfaces. (c) Cross sectional TEM-EDX images obtained from near the CIGS/CdS interface region in a RbF-PDT CIGS cell device. (d) A TEM image obtained from near the CIGS/CdS interface region in a CIGS cell device grown without RbF-PDT. (e) A schematic of the device structure used for TEM measurements. (f) TEM-EDX images obtained from the CIGS layer in an identical device. 170x168mm (300 x 300 DPI)



Figure 3. Cross sectional SEM images observed from (a) CIS, (b) CIGS, and (c) CGS cell devices. SIMS elemental distribution profiles of (d, e, f) Na and (g, h, i) Rb in the CIS, CIGS, and CGS layers. SIMS measurements were carried out using Cs+ as the primary ion with an acceleration voltage of 5 kV. Na and Rb concentrations in all the films were calculated using standard CIGS samples prepared by ion implantation, and thus the concentration values in CIS and CGS may deviate from accurate values and should be used only for relative comparisons. 170x139mm (300 x 300 DPI)


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Figure 4. Solar cell parameter variations with RbF-PDT and HLS treatments obtained from (a) CIGS and (b) CGS cell devices. (c, d) Typical EQE curves obtained from these cell devices. Black arrows indicate variation trends observed with RbF-PDT, while red arrows indicate variation trends observed with HLS treatments. 170x171mm (300 x 300 DPI)



Figure 5. (a, c) NCV-depletion width profiles calculated from C-V measurements for CIGS and CGS cell devices grown with (solid lines) and without (dotted lines) RbF-PDT, and before (black lines) and after (red lines) HLS treatments for 100 h. (b, d) J-V curves obtained from CIGS and CGS cell devices grown with RbFPDT before (black lines) and after (red lines) HLS treatments performed after cell separation. (e) A picture of cell devices fabricated on a 3×3 cm2 substrate and a schematic of the cell separation. (f) J-V curves obtained from a CGS cell device grown with RbF-PDT before and after HLS treatments performed before cell separations. All RbF-PDTs were carried out with the TRbF value of 580℃. Red arrows indicate variation trends observed with HLS treatments. 170x198mm (300 x 300 DPI)


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TOC Graphic 66x44mm (300 x 300 DPI)


Group III Elemental Composition Dependence of RbF Post-Deposition

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