Impact of KF Post-Deposition Treatment on Aging of the Cu(In,Ga)Se2

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Impact of KF Post-Deposition Treatment on Aging of the Cu(In,Ga)Se Surface and its Interface with CdS 2

Nicoleta Nicoara, Sylvie Harel, Thomas Lepetit, Ludovic Arzel, Nicolas Barreau, and Sascha Sadewasser ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b00365 • Publication Date (Web): 10 May 2018 Downloaded from http://pubs.acs.org on May 16, 2018

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ACS Applied Energy Materials

Impact of KF Post-Deposition Treatment on Aging of the Cu(In,Ga)Se2 Surface and its Interface with CdS Nicoleta Nicoara,1,* Sylvie Harel,2 Thomas Lepetit,2 Ludovic Arzel,2 Nicolas Barreau,2 and Sascha Sadewasser1 1

International Iberian Nanotechnology Laboratory (INL), 4715-330 Braga, Portugal

2

Institut des Matériaux Jean Rouxel (IMN) - UMR6502, Université de Nantes, CNRS, 2 rue de

la Houssinière, BP 32229, 44322 Nantes Cedex 3, France KEYWORDS: chalcopyrite, thin-film solar cells, alkali fluoride post-deposition treatments Kelvin probe force microscopy, photoelectron spectroscopy

ABSTRACT

Recent world record efficiencies for thin film solar cells based on Cu(In,Ga)Se2 (CIGS) have been realized with absorbers subject to alkali fluoride post deposition treatments (PDT). We investigated the effect of ambient air exposure on the electronic properties of CIGS with KFPDT in a combined time-dependent Kelvin probe force microscopy and X-ray photoelectron

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spectroscopy study. We also studied the early stage formation of the absorber/buffer interface after the initial deposition of CdS in the chemical bath. Our study shows that the KF-PDT, as well as the CdS deposition process induce an increase in the overall surface work function, as compared to bare CIGS. A K-In-Se compound forms after the KF-PDT, accompanied by a stable In oxide which explains the remarkable stability of the contact potential difference to air exposure, confirming phenomenological observations in many laboratories. In clear contrast to the untreated CIGS, the KF-treated CIGS/CdS interface shows a significant variation in the surface potential (∼ 360 mV) over approximately 7 hours air-exposure. We attribute this variation to a Cd-In intermixing at interface, whose chemical stability is susceptible to airexposure. Our results contribute to the understanding of the electronic properties of the KFtreated and untreated CIGS/CdS junction during the early stages of the interface formation, which impact the overall device properties. INTRODUCTION The recent record efficiencies for Cu(In,Ga)Se2 (CIGS) thin-film solar cells have been realized using alkali fluoride post-deposition treatments (PDT) after the deposition of the CIGS absorber layer, reaching now 22.6% power conversion efficiency for co-evaporated CIGSe1 and 22.9 % for a sequential Cu(In,Ga)(S,Se)2 process, with an enhanced surface treatment of the absorber layer.2 A potassium fluoride (KF) PDT was introduced initially,3 which is now widely used in numerous labs and is started to be explored by industry. The PDT modifies the optoelectronic properties of CIGS bulk (e.g. a higher charge carrier concentration) and surface (e.g. formation of KInSe2 surface phase).4,5,6,7 Additionally, it is generally observed that the CdS buffer layer deposited by chemical bath (CBD) can be reduced in thickness (by decreasing deposition time), maintaining high efficiencies3,8,9 and increasing current collection in the 500 nm wavelength


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region. For the fabrication of solar cell devices, in the laboratory as well as at industrial level, the CIGS absorber layer is generally transferred through air after the PDT from the vacuum growth system to the CBD process. The oxidation process occurring during this transfer time modifies the CIGS surface and thus the pn-junction at the CIGS/buffer interface. It is common experience that the deposition of the CdS should be carried out as soon as possible after the exposure of the CIGS to air. The oxidation behavior of CIGS has been addressed in several studies, finding that the formation of some Ga oxides occurs within 3 min.10,11,12,13 Nevertheless, it is generally accepted that the exposure of the CIGS absorber layer to ambient conditions for short times has no dramatic consequences on the solar cell device properties, since the subsequent CBD process acts as a surface cleaning and conditioning that etches the oxides. However, most of the work studying the effect of air exposure on CIGS focused on absorbers without any alkali-fluoride PDT,13 where typically Na is the dominant alkali element through diffusion from the soda-lime glass. Since the alkali-fluoride PDT leads to a strong modification of the surface chemistry, with a Cu and Ga depletion,14,15 the presence of alkali elements,16 and the potential formation of additional surface phases (e.g. In2Se3 or K-In-Se 6,14,17 or CdIn2S4 after the chemical bath deposition18) the surface properties of CIGS subjected to an alkali-fluoride PDT should be re-evaluated in order to understand the interface formation with the CdS buffer layer. Here, we present a study of the effect of air exposure on the surface of KF-treated CIGS and its interface with the CdS buffer layer. We study the evolution of the surface work function measured by Kelvin probe force microscopy (KPFM) with exposure time to ambient air and characterize the detailed surface chemical composition using X-ray photoelectron spectroscopy (XPS). We show that the KF-treated CIGS surface is stabilized due to the formation of a


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passivating indium oxide, confirming phenomenological observations in many laboratories. Additionally, an intermixing occurs between the KF-PDT CIGS surface and CdS after a short deposition time in the CBD process leading to a Cd-In-(S,O/OH)4 interface composition. EXPERIMENTAL SECTION We studied a set of four different samples, consisting of CIGS and KF-PDT CIGS, with and without a 1 minute CBD CdS, denominated: CIGS, CIGS/CdS, KF-CIGS, and KF-CIGS/CdS, respectively. CIGS absorber layers were co-evaporated on Mo-coated soda-lime glass (SLG) following the 3-stage process.19 A standard KF-PDT was applied to the KF-CIGS samples: 150 Å of KF were evaporated at 0.1 Å/s under Se atmosphere directly onto the CIGS absorber heated at 350°C. The CdS layer was grown by CBD in an open reactor heated at 60°C, containing ammonia (1 mol/l), cadmium acetate dehydrate ( 2.6×10-3 mol/l) and thiourea (9.5 10-2 mol/l). CIGS/CdS and KF-CIGS/CdS were dipped for 1 minute in the bath and then rinsed with deionized water and dried with N2 gas. CIGS and KF-CIGS were also used to complete solar cell devices to verify the beneficial effect of the treatment. Details of this process were reported previously.20 The conversion efficiency values obtained for complete solar cell devices without and with KF-PDT were 16.6 ± 0.3% and 18.0 ± 0.3%, respectively. The samples were sealed under N2 atmosphere after the growth to reduce any uncontrolled modification of the surfaces. They were taken from the N2 atmosphere immediately prior to measurements. Amplitude-modulated KPFM in dual pass, using a lift height of 5 nm, was performed in a Bruker Dimension Icon atomic force microscope (AFM) operated in ambient environment. We used Pt/Ir-coated cantilevers (PPP-EFM NanosensorsTM) with a nominal tip radius of 25 nm, spring constant of 2.8 N/m, and 75 kHz resonance frequency. To ensure comparability of the results the


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tips were calibrated using an Au-coated Si sample. The contact potential difference (CPD) is defined as VCPD = -VDC = e(Φsample - Φtip), where Φ is the work function.

To link the KPFM results with the evolution of surface chemistry during air exposure we performed X-ray photoelectron spectroscopy (XPS) on “twin” samples (i.e. samples having undergone identical processes as those measured in AFM). The XPS measurements were performed with a Kratos AXIS Ultra spectrometer, using a monochromatic Al Kα x-ray source at 150 W for core levels and valence band. The analyzed area was 700 × 300 µm2. The energy scale was calibrated using Au 4f7/2 at 83.97 eV and Cu 2p3/2 peaks at 932.63 eV21, measured from sputter-cleaned Au and Cu films. The overall energy resolution, as determined from the Fermi edge of an Ag reference sample was (0.47 ± 0.03) eV at 20 eV pass energy. The Kratos charge neutralizer system was used during all experiments. The core levels were recorded using a 20 eV pass energy. Since the XPS measurements were performed in ultra-high vacuum, the samples were exposed to air for specified intervals to ensure comparability to the KPFM data. Immediately after being produced, all samples were stored in individually vacuum-sealed storage bags. In a short time range (less than 1 h), the sealed bags were opened and the samples were quickly transferred to the XPS system. This set corresponds to the 0 hours samples. After a first XPS analysis, all samples were taken out from UHV and exposed to air for 2 hours, then reintroduced into the XPS system. This set corresponds to the 2 h samples. Finally, the same samples were again taken out from UHV and exposed to air for 5 hours, corresponding to a total of 7 hours exposure to air. Before being transferred to the XPS system, part of the bare CIGS absorber was etched in a KCN solution (10 wt%, 1 min) to remove oxides, and KF-treated samples were rinsed for 1 min with diluted ammonia (1 mol/l, 1 min) to remove fluoride compounds. The XPS study was focused on the CIGS core levels, Cu 2p3/2, In 3d5/2 Ga 2p3/2 and


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In M45N45N45, Cd M45N45N45 Auger lines. Survey spectra were recorded for all samples. In addition, Cd 3d core levels were recorded for CIGS/CdS and KF-CIGS/CdS. Using combined measurements of core levels and corresponding Auger peaks, the modified Auger parameters22 α´ for In and Cd were determined:

α´(In) = Ebin(In 3d5/2) + Ekin(In M4N45N45)

(1)

and α´(Cd) = Ebin(Cd 3d5/2) + Ekin(Cd M4N45N45)

(2)

As reported previously,23,24 these parameters are very sensitive to the In or Cd chemical environment and are independent of sample charging; the latter is important since surface composition and sample charging may evolve with the different treatments. RESULTS Repeated KPFM measurements, consisting of the simultaneous acquisition of topography and CPD maps over the same sample area (8 µm × 8 µm), were performed over a period of ∼ 7 hours. To quantify the changes in the potential distribution, we performed a histogram analysis of the measured CPD maps (Figure 1). The values corresponding to the maximum CPD value and the variation of the potential distribution at 1/e of the counts at the maximum (the latter plotted as error bar) are extracted from the Gaussian fitting of the CPD histograms and are represented in Figure 2, which shows the development of the CPD with the exposure time to air for the CIGS, CIGS/CdS, KF-CIGS and KF-CIGS/CdS samples.


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Figure 1. Analysis of the KPFM measurements. (a) Topography image of the first measurement of the KF-CIGS sample and (b) corresponding CPD image. (c) From the CPD histogram the CPD value at the maximum is extracted and the variation of the CPD at 1/e of the counts at the maximum is used as a measure of the CPD distribution width, represented as error bars in Figure 2.

Figure 2. CPD determined for CIGS, CIGS/CdS, KF-CIGS and KF-CIGS/CdS as a function of exposure time to ambient conditions. Samples were removed from a protective N2 atmosphere immediately prior to the KPFM measurement. To ensure comparability, the AFM tip was


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calibrated repeatedly on an Au/Si reference sample (green symbols), which was assumed to have a constant work function of ΦAu = 5.1 eV. The first data point for each sample series corresponds to the CPD measured in the first 15 to 20 minutes of exposure to air, immediately after removing the sample from the N2 protective atmosphere. The following data correspond to consecutive images, acquired at interval times of ∼ 15 minutes, the minimum time required for a CPD scan. The error bars, with values in the range of ∼ 50 mV, indicate a fairly homogeneous surface potential for the selected 8 µm × 8 µm scanned areas. To ensure comparability of data, intermediate calibrations of the AFM tip are performed on a reference Au/Si sample, for which a constant work function of ∼ 5.1 eV is assumed.25 Comparison of the first CPD data point for each sample shows that both processes, the KF-PDT as well as the CdS CBD, increase the work function of the CIGS layers.20 In relation to the CPD evolution in time, the results show a variation of ∼ 100 mV for CIGS layers, an almost flat tendency (less than ∼ 50 mV) for the CIGS/CdS and KF-CIGS and a larger variation of ∼ 360 mV for the KF-CIGS/CdS sample. In the XPS measurements of the bare CIGS absorber (CIGS-0 h), in addition to the CIGS-related element core levels, the presence of Na diffused from the SLG and a significant amount of oxygen were detected, as shown in the survey spectra (see Supporting Information Figure S1). The evolution of the O 1s intensity with the time of air exposure indicates a gradual oxidation of the CIGS surface as shown in Figure S2 a. To obtain reference spectra of In and Ga in CIGS without any oxide contribution, In M45N45N45 Auger lines, In 3d5/2, and Ga 2p3/2 core levels were recorded for a KCN-etched CIGS, as shown in Figure 3. In 3d5/2 core levels and Auger lines (In M45N45N45) have been fitted with Gaussian-


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Lorentzian functions and a linear background was subtracted. A modified Doniach-Sunjic function was used to fit the Ga 2p3/2 peak. In this case the In M45N45N45 Auger line exhibits a well-defined In M4N45N45 peak fitted by one contribution (A) and a single contribution fits the In 3d5/2 core level peak. These contributions are named In-A and In-1 in the following. The deduced Auger parameter α'(In-A), calculated using Eq. (1), is characteristic of CIGS, in agreement with previous works.17,24,26 All In 3d5/2 binding energies, In M4N45N45 kinetic energies, and corresponding α´(In) parameters are displayed in the Wagner plot shown at the end of the Results section (and summarized in Supporting Information Table S1). For the CIGS-0 hours (without the KCN etching) we observe a modification of the In M45N45N45 Auger peak shape (Figure 3a). An additional contribution In-B is included to obtain a reasonable peak fit. Also the In 3d5/2 exhibits an additional contribution In-2, shifted to higher binding energy (BE) with respect to the previously identified In-1 (Figure 3b). Both signals show increasing intensities with the air-exposure time, e.g., a significant increase from 16 % to 34 % of the total signal intensity is observed for the In-2 contribution after 7 hours. These new contributions In-2 and In-B can be attributed to an indium oxide formation and allow the calculation of the corresponding modified Auger parameter α'(In-B), also displayed in the Wagner plot shown at the end of this section For the bare absorber, the Ga 2p3/2 signal exhibits also a supplementary contribution, shifted to higher BE compared with KCN-etched CIGS, whose intensity also increases with the time of ambient exposure. This new contribution corresponds to a Ga oxide.27 SeOx is also detected on the Se 3d peak after 7 h of air exposure (Figure S2 b).


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At the surface of CIGS after 1 min CdS CBD, the XPS measurements show a strong Cd 3d peak and Cd M45N45N45 Auger peak. An attenuation of all CIGS-related signals (Cu 2p3/2, Ga 2p3/2, In M45N45N45, and In 3d5/2) is observed after the CdS deposition (see Figure 4 and Figure S1). No shift is observed for the Cu 2p3/2 and Ga 2p3/2 peaks after the CdS CBD. As for CIGS, In-A and In-B structures are detected in the In M45N45N45 spectrum displayed in Figure 4a. The In 3d5/2 core levels in Figure 4b exhibit also two components In-1 and In-2. The calculated α'(In-A) and α'(In-B) Auger parameters remain unchanged after the CdS CBD compared to bare CIGS. The Cd M45N45N45 Auger signal (Figure 4c) and Cd 3d5/2 line were also recorded to provide information on the nature of the Cd compound. The α'(Cd) parameter calculated according to Eq. (2) is 786.5 ± 0.2 eV. All Auger parameters, α'(In-A), α'(In-B), and α'(Cd) show no detectable evolution with time of air-exposure.

Figure 3. Evolution of the (a) In M45N45N45, (b) In 3d5/2, and (c) Ga 2p3/2 peaks as a function of exposure to air. The spectra correspond to CIGS-KCN etched, CIGS “as received“ (CIGS-0 h), after 2 h (CIGS-2 h) and 7 h (CIGS-7 h) air exposure. Offsets are added for clarity.


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Figure 4. Evolution of the (a) In M45N45N45, (b) In 3d5/2, and (c) Cd M45N45N45 peaks as a function of exposure time to air. The spectra correspond to CIGS-0 h and CIGS/CdS “as received“ (CIGS/CdS-0 h) and after 2 h (CIGS/CdS-2 h) and 7 h (CIGS/CdS-7 h) air exposure. Offsets are added for clarity.

For the KF-CIGS surface (the survey spectra is shown in Figure S3) we found a similar intensity of the O 1s line and a strongly reduced Na 1s signal intensity compared with the CIGS surface (Figure S4). As a result of the KF-PDT, In enrichment and Ga depletion (Figure 5b and 5c) are observed for the KF-CIGS sample compared to the bare CIGS in agreement with a recent study.7 Other authors have identified mostly Ga depletion and no significant modification of the In content.3,16 The KF-CIGS In 3d5/2 peak in Figure 5b exhibits an intense higher BE contribution (In-2) linked to an intense In-B structure in the In M4N45N45 Auger peak (Figure 5a). A noticeable intensity stability with time of air exposure is observed for both In-2 and In-B structures. This behavior is distinct from what is observed for CIGS. Additionally, the calculated values for the α'(In-A) and α'(In-B) parameters decrease compared to CIGS, indicating that In has a different chemical environment for KF-treated CIGS samples. Similarly, the Ga 2p3/2 peak


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exhibits a contribution at higher BE, presumably related to a Ga oxide (Figure 5c). Contrary to what is observed for CIGS, with time of air-exposure the intensity of the In- oxide contribution does not increase. We cannot exclude instead, a slight increase of the Ga–oxide contribution, however some difficulty in fitting the Ga 2p3/2 peak (i.e. reduced signal intensity due to Ga depletion after the KF-PDT) has to be noted. The α' (In-B) parameter varies by less than 0.2 eV for KF-CIGS and no significant variation is detected for the α'(In-A) parameter with time of air exposure. For the KF-CIGS/CdS sample, In M45N45N45, Cd M45N45N45 Auger peaks, and In 3d5/2 core level measurements were performed (Figure 6). As for CIGS/CdS, we observe a decrease of Cu 2p3/2 and Ga 2p3/2 signal intensity in the survey spectra (Figure S3) but no intensity decrease of the In M45N45N45 Auger peak and In 3d5/2 photoelectron peak as seen in Figure 6a and 6b. The In 3d5/2 core level spectrum in Figure 6b shows an overall peak modification after the buffer deposition. The intensity of the In 3d5/2 component at higher BE (In-2), which represents 84 % of the total signal intensity, is considerably attenuated after the CdS CBD and the opposite effect is seen for the In-1 component.


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Figure 5. Evolution of the (a) In M45N45N45, (b) In 3d5/2, and (c) Ga 2p3/2 peaks with the time of air exposure. The spectra correspond to CIGS as received (CIGS-0 h), KF-CIGS “as received“ (KF-CIGS-0 h) and after 2 h (KF-CIGS-2 h) and 7 h (KF-CIGS-7 h) air exposure. Offsets are added for clarity.

Figure 6. Evolution of the (a) In M45N45N45, (b) In 3d5/2, and (c) Cd M45N45N45 peaks as a function of time of air exposure. The spectra correspond to KF-CIGS-0 h and KF-CIGS/CdS “as received“ (KF-CIGS/CdS-0 h) and after 2 h (KF-CIGS/CdS-2 h) and 7 h (KF-CIGS/CdS-7 h) air exposure. Offsets are added for clarity.


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A moderate increase of ∼ 0.15 eV for the α'(In-A) Auger parameter value and a significantly larger increase of ∼ 0.6 eV for the α'(In-B) is seen for the KF-CIGS after the CdS CBD (Figure 7) and all Auger parameter, α'(In-A), α'(In-B), and α'(Cd), undergo an increase of 0.2 eV after 7 h air-exposure indicating an aging effect. Note that potassium is clearly detected (Figure S5) after the CdS CBD.

Figure 7. (a) Wagner plot of the modified Auger parameters α´(In), α´(In) = Ebin(In 3d5/2) + Ekin(In M4N45N45) and (b) corresponding legend. Solid symbols indicate values obtained in our measurements. The different shapes correspond to CIGS (square), CIGS/CdS (circle), KF-CIGS (triangle) and KF-CIGS/CdS (rhomb) samples. The colors indicate the two contributions In-A and In-B of the In M4N45N45 Auger peak (as indicated in (c)) and In-1 and In-2 of the In 3d5/2 core levels. The values of the modified Auger parameters α´(In) are indicated on the diagonal grid. Open symbols refer to different chemical states of In, reported in literature.


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DISCUSSION For the bare CIGS absorber (CIGS-0h), the In Auger parameter α'(In-B), calculated from the In2 contribution of the In 3d5/2 photoemission peak and the In-B contribution of the In M4N45N45 is attributed to an indium oxide according to a previous work by Cahen et al.28 The evolution of the In M45N45N45 Auger signal as a function of air-exposure time is also in good agreement with previous reports.11,12 The increase of the intensity of these In oxide contributions, the Ga-oxide component, and the O 1s photoemission peak with time of air exposure indicates the progressive oxidation of the CIGS surface. Degradation of CIGS and CIG(S,Se)2 absorber properties under different environmental conditions was previously studied.11,12,29,30 Metzger et al.

29,30

showed

that the carrier lifetime measured by time-resolved photoluminescence decreases by two orders of magnitude when high quality CIGS samples are exposed to air for 1 day, as compared with samples for which immediate CdS deposition is done after the CIGS growth. Clear indications for CIG(S,Se)2 oxidation were reported by Hauschild et al.12 in a XPS and Auger electron spectroscopy (AES) study, where CIG(S,Se)2 was systematically exposed to 3 different environmental conditions for a total period of 14 days. For exposure to ambient air, the authors observed a continuous increase in the O 1s core level intensity reaching a 6-times higher value as compared to bare CIG(S,Se)2. At the same time, they observed oxide formation with the elements In, Cu, and Se. In our case, the O 1s intensity increases by a factor of 1.5 after 7 h exposure to air and the ratio between oxidized and non-oxidized In is 35:65, due to a reduced exposure time, in comparison to the 50:50 ratio reported for 14 days of exposure to air.12 In oxide was also reported by Lehmann et al. after 100 h exposure to air11 and by other authors who observed In oxide removal after the etching of CIGS by KCN or H2O2/H2SO4 treatments.31 Previous KPFM studies have shown that the correlation between grain topography and work


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function distribution vanished with the exposure time (i.e., exposure to damp heat up to 5 hours) and was interpreted as a degradation of the CIGS surface properties.32 Changes in the CPD (i.e. surface work function) can result from several independent processes or combinations thereof, i.e., the formation of an interface dipole, a surface induced band bending, or a change in the electron affinity or work function. The increase in the CPD (increase of the surface work function) in the present KPFM results could be tentatively attributed to the CIGS oxidation, as determined from XPS, though other contributing processes cannot be excluded. For the CIGS/CdS sample, the presence of an intense Cd 3d photoemission peak and Cd M45N45N45 Auger signal indicates that Cd-related species are already formed during the first minute of the CBD. The similarity of the calculated α'(Cd) Auger parameter with values reported in the literature33 suggests that the observed Cd lines correspond to a Cd and S containing compound. The calculated α'(In-A) and α'(In-B) Auger parameters corresponding to In in CIGS and to In in an oxidized CIGS chemical environment, respectively, are unchanged after CdS deposition (see Figure 7). These results indicate that the chemical environment of In in CIGS is barely modified during the first step of the CdS bath. The slight increase in the work function for the CIGS/CdS sample can be attributed to an incomplete or inhomogeneous CdS buffer layer resulting from the short CBD time,8 thus leading to a poor CIGS passivation. This can also be related to previous reports showing that specific CIGS grain orientations inhibit at initial growth stage a perfect CdS layer coverage.34 For KF-CIGS a negligible variation of the surface potential of less than ∼ 50 mV is observed, without a clear trend of an increase or decrease of the work function with time of air exposure. In this case, the α'(In-A) parameter value differs significantly from that calculated for CIGS-KCN


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and CIGS samples (Figure 7), indicating an In chemical environment different from that in CIGS. This In chemical environment corresponds to a K-In2Se3 or K-In-Se material (amorphous In2Se3 containing K) formed at the CIGS surface during the KF-PDT, as previously reported.14,17 The α'(In-B) parameter (850.5 ± 0.2) eV was already reported in a previous work.6,24 This value deviates from the α'(In-B) value calculated for the In-O bonds of the untreated CIGS samples (CIGS and CIGS/CdS), demonstrating that the In oxide induced by the presence of K (or by the KF treatment) is distinct from the In oxide formed during CIGS oxidation. The α'(In-B) value is rather close to that reported for In2O3,33 indicating that the KF-PDT treatment could lead to the fast formation of a stable InxOy, not sensitive to the duration of air exposure. It should be emphasized that for KF-CIGS the In 3d5/2 (In-2) and In M4N45N45 (In-B) contributions exhibit a noticeable intensity stability. This behavior completely differs from what is observed for CIGS and confirms that the In oxide formed at KF-CIGS surfaces is different. This also suggests that the KF-PDT of CIGS has a significantly different effect on the CIGS stability against oxidation as compared to CIGS where the alkali provenience is only from the diffusion from the SLG. The presence of a stable In oxide at the KF-CIGS surface is a key point in explaining the almost constant CPD evolution with air exposure. The KF-CIGS/CdS sample exhibits a very different behavior compared to CIGS/CdS with a significantly larger CPD variation observed with the air-exposure time. The unchanged intensities of In signals (In M45N45N45 and In 3d5/2) in combination with the decrease of the Cu and Ga signals after the CdS bath indicate that the CdS growth process on KF-CIGS is considerably modified compared to that on CIGS. According to the In M4N45N45 Auger electron inelastic-mean free path value in CdS calculated using the IMFPWIN code,

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the information

depth in the In M45N45N45 energy range is around 3 nm ± 20%. This means that In is present at


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the outermost surface layer. The slight variation by 0.15 eV of the α'(In-A) Auger parameter for KF-CIGS can be interpreted by the intermixing or chemical reaction of the K-In-Se compound and the CdS bath species leading to a Cd-In-(S,O/OH)4 interface composition , as already proposed.6 The calculation of the α'(In-B) Auger parameter for the KF-CIGS/CdS sample allows to conclude that a partial reduction of InxOy present after the KF treatment occurs after 1 min CBD. The calculated value corresponds rather well to oxidized CIGS. The decrease of In-2 component intensity confirms this partial reduction. The increase of the α'(In-A), α'(In-B), and α'(Cd) Auger parameters by 0.2 eV indicates that this compound is less chemically stable than CdS. The significantly larger CPD variation observed with exposure time for KF-CIGS/CdS might be explained by an incomplete formation of a Cd-In-(S,O/OH)4-based compound with poor chemical stability against air-exposure. It must be highlighted that K is detected at the surface after CdS CBD bath indicating diffusion of K, possibly located at grain boundaries.

36,37

Finally

the different surface conditions are schematically illustrated in Figure 8.


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Figure 8. Schematic illustration of the surface processes occurring during air-exposure.

CONCLUSIONS We presented a combined KPFM and XPS study on CIGS, CIGS/CdS, KF-CIGS and KFCIGS/CdS as a function of exposure time to air, reflecting what happens in solar cell fabrication in the laboratory and industry environment. We find that the KF-PDT strongly affects the processes occurring upon air-exposure, leading finally to a more stabilized CIGS surface through the formation of a passivating In-oxide, explaining also the negligible variation of the work function of this sample with air exposure time. The KF-PDT has a strong effect also on the interface formation with the CdS buffer layer, where a Cd-In-(S,O/OH)4 composition is observed. The present results provide new insights on the CIGS surface properties and formation of CIGS/CdS interface and unravel the physical origin of hitherto phenomenological observations. The knowledge gained on CIGS surface oxidation is expected to positively impact the optimization of the KF-PDT process by research laboratories and facilitate the industrial uptake. ASSOCIATED CONTENT Supporting Information XPS survey spectra for CIGS, CIGS/CdS, KF-CIGS and KF-CIGS/CdS as received (0-hour air exposure), O 1s and Se 3d core levels for CIGS as received (0 h) and CIGS after 7 hours exposure to air, O 1s and Na 1s peaks for CIGS-0 h and KF-CIGS-0 h, K 2p signal for KFCIGS-0 h and KF-CIGS/CdS-0 h and peak positions for calculating the modified Auger parameter of indium α´(In).


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AUTHOR INFORMATION Corresponding Author * Nicoleta Nicoara. [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Funding Sources This project has received funding from the FCT-Pessoa/France project. ACKNOWLEDGMENT We thank Mark C. Biesinger for his expertise in Auger parameter and fruitful discussions.

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Figure 1. Analysis of the KPFM measurements. (a) Topography image of the first measurement of the KFCIGS sample and (b) corresponding CPD image. (c) From the CPD histogram the CPD value at the maximum is extracted and the variation of the CPD at 1/e of the counts at the maximum is used as a measure of the CPD distribution width, represented as error bars in Figure 2. 48x12mm (300 x 300 DPI)


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Figure 2. CPD determined for CIGS, CIGS/CdS, KF-CIGS and KF-CIGS/CdS as a function of exposure time to ambient conditions. Samples were removed from a protective N2 atmosphere immediately prior to the KPFM measurement. To ensure comparability, the AFM tip was calibrated repeatedly on an Au/Si reference sample (green symbols), which was assumed to have a constant work function of ΦAu = 5.1 eV. 84x56mm (300 x 300 DPI)



Figure 3. Evolution of the (a) In M45N45N45, (b) In 3d5/2, and (c) Ga 2p3/2 peaks as a function of exposure to air. The spectra correspond to CIGS-KCN etched, CIGS “as received“ (CIGS-0 h), after 2 h (CIGS-2 h) and 7 h (CIGS-7 h) air exposure. Offsets are added for clarity. 87x43mm (300 x 300 DPI)


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Figure 4. Evolution of the (a) In M45N45N45, (b) In 3d5/2, and (c) Cd M45N45N45 peaks as a function of exposure time to air. The spectra correspond to CIGS-0 h and CIGS/CdS “as received“ (CIGS/CdS-0 h) and after 2 h (CIGS/CdS-2 h) and 7 h (CIGS/CdS-7 h) air exposure. Offsets are added for clarity. 88x43mm (300 x 300 DPI)



Figure 5. Evolution of the (a) In M45N45N45, (b) In 3d5/2, and (c) Ga 2p3/2 peaks with the time of air exposure. The spectra correspond to CIGS as received (CIGS-0 h), KF-CIGS “as received“ (KF-CIGS-0 h) and after 2 h (KF-CIGS-2 h) and 7 h (KF-CIGS-7 h) air exposure. Offsets are added for clarity. 87x43mm (300 x 300 DPI)


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Figure 6. Evolution of the (a) In M45N45N45, (b) In 3d5/2, and (c) Cd M45N45N45 peaks as a function of time of air exposure. The spectra correspond to KF-CIGS-0 h and KF-CIGS/CdS “as received“ (KF-CIGS/CdS-0 h) and after 2 h (KF-CIGS/CdS-2 h) and 7 h (KF-CIGS/CdS-7 h) air exposure. Offsets are added for clarity. 87x43mm (300 x 300 DPI)



(a) Wagner plot of the modified Auger parameters α´(In), α´(In) = Ebin(In 3d5/2) + Ekin(In M45N45N45) and (b) corresponding legend. Solid symbols indicate values obtained in our measurements. The different shapes correspond to CIGS (square), CIGS/CdS (circle), KF-CIGS (triangle) and KF-CIGS/CdS (rhomb) samples. The colors indicate the two contributions In-A and In-B of the In M45N45N45 Auger peak (as indicated in (c)) and In-1 and In-2 of the In 3d5/2 core levels. The values of the modified Auger parameters α´(In) are indicated on the diagonal grid. Open symbols refer to different chemical states of In, reported in literature. 96x52mm (300 x 300 DPI)


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Figure 8. Schematic illustration of the surface processes occurring during air-exposure. 70x29mm (300 x 300 DPI)



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Impact of KF Post-Deposition Treatment on Aging of the Cu(In,Ga)Se2

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