Effect of growth conditions on microstructure of sputtered precursor for CuIn1-xGaxSe2 (CIGS) absorber layer deposited on stainless steel substrates

Accepted Manuscript Effect of growth conditions on microstructure of sputtered precursor for CuIn1-xGaxSe2 (CIGS) absorber layer deposited on stainless steel substrates

M. Behr, M. Sharma, S. Sprague, N. Shinkel, J. Kerbleski, C. Alvey, S. Rozeveld, T. Hasan, C. Wintland, M. Mushrush, A. Wall PII: DOI: Reference:

S0040-6090(18)30584-4 doi:10.1016/j.tsf.2018.08.045 TSF 36855

To appear in:

Thin Solid Films

Received date: Revised date: Accepted date:

21 May 2018 23 August 2018 31 August 2018

Please cite this article as: M. Behr, M. Sharma, S. Sprague, N. Shinkel, J. Kerbleski, C. Alvey, S. Rozeveld, T. Hasan, C. Wintland, M. Mushrush, A. Wall , Effect of growth conditions on microstructure of sputtered precursor for CuIn1-xGaxSe2 (CIGS) absorber layer deposited on stainless steel substrates. Tsf (2018), doi:10.1016/j.tsf.2018.08.045

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ACCEPTED MANUSCRIPT Effect of growth conditions on microstructure of sputtered precursor for CuIn1-xGaxSe 2 (CIGS) absorber layer deposited on stainless steel substrates M. Behr*, M. Sharma, S. Sprague, N. Shinkel, J. Kerbleski, C. Alvey, S. Rozeveld,

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T. Hasan, C. Wintland, M. Mushrush, A. Wall

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The Dow Chemical Company, Midland, MI, USA

ACCEPTED MANUSCRIPT Abstract In this paper, we report the effect of growth conditions on the properties of sputtered precursor thin films for CuIn1-xGaxSe2 (CIGS) absorber layers.

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Specifically, precursor films containing Cu, In, Ga, and Se were deposited via co-

The impact on precursor film phase,

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compositions and deposition conditions.

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sputtering on flexible Mo-coated stainless steel substrates over a wide range of

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morphology, and elemental distribution was investigated as a function of precursor

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Se content, substrate temperature, target type (CIG/CIG vs. In/Cu 3Ga), and Na content. Precursor films selenized at high temperature (> 500 °C) to form

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stoichiometric CIGS and completed using a CdS n-type buffer layer and

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transparent conducting oxide window layers exhibited full-cell efficiencies as high

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as 11.5 %.

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*Corresponding Author ([email protected])

ACCEPTED MANUSCRIPT 1. Introduction Copper indium gallium diselenide CuIn 1-xGaxSe2 (CIGS) based thin-film photovoltaic cells have seen significant improvement over the last decade with lab-

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scale and commercial size module efficiencies reaching 22.6% and 19.2%,

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respectively [1,2]. These record efficiencies utilize glass substrates, while the

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highest reported module efficiency on stainless steel is significantly lower at

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15.7% [1]. Presence of a direct band gap, its tunability via composition (i.e.

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Ga/(In+Ga) or Ga/III), and ease of manufacturability using industrial-scale roll-toroll processes make CIGS an attractive candidate for solar cells [3,4,5].

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Two methods have been widely used for CIGS absorber layer deposition: 3-stage

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co-evaporation and two-step precursor-selenization [3,4]. Although lab-scale 3-

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stage co-evaporation enables fine control of the Ga-gradient, precursor-selenization

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is more conducive to industrial-scale manufacture. In a typical precursor selenization process, metal precursor containing Cu, In, Ga (with or without Se) is

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first deposited on back electrode-coated glass or stainless steel substrates. The precursor then undergoes a high-temperature (>500 °C) selenization process using elemental Se or H2Se to form the CIGS phase (Fig.1). One of the inherent problems with this precursor-selenization process is the aggregation of Ga towards the back electrode due to the difference in reactivity of In and Ga with Se [3,5,6]. Deficiency of Ga at the top surface leads to a lower bandgap, which affects open

ACCEPTED MANUSCRIPT circuit voltage (Voc). Post-sulfidization with thermal annealing [7] and precursor modification [8] are two examples of strategies that have been explored to obtain a more uniform Ga-gradient. In this work, we have hypothesized that the precursor

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microstructure and composition will dictate the resulting post-selenized CIGS film

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structure and resulting electrical properties.

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Specifically, we have studied the effect of Se content, sputtering target source,

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deposition temperature, and Na content on the resulting microstructure of the

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precursor layer. A variety of analytical techniques including wide-angle x-ray diffraction, scanning electron microscopy (SEM), x-ray energy dispersive

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spectroscopy (XEDS), transmission electron microscopy (TEM), and inductively-

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coupled plasma mass spectrometry (ICP-MS) were used to characterize film

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properties. A representative set of precursor samples selenized and made into full

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cells using CdS/i-ZnO/ITO layers exhibited efficiencies > 10 %.

2. Experimental Details CIGS precursor films were deposited by confocal DC co-sputtering employing either (1) a combination of two CuInGa (CIG) targets with different Cu/(In+Ga) (or Cu/III) ratios or (2) using In and Cu3Ga targets as shown in Fig. 2. Argon was used as process gas. Se was provided using a graphite pot with temperature

ACCEPTED MANUSCRIPT controlled by resistive cartridge heaters. Hereafter, the two target sets as described

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in Fig. 2 will be referred to as CIG/CIG or CuGa/In.

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Steel substrates of size 124 mm x 124 mm were coated with a back electrode comprising Ti and Mo via sputtering. It has been reported in literature that such

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multilayered back contacts can act as a good diffusion barrier without loss in cell

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efficiency [9,10]. 20 nm NaF was then deposited at 4.5 mTorr, 50 W RF power,

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and ambient temperature. Substrates were then transferred in situ to the precursor

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deposition chamber. Prior to the start of film deposition, both targets in the chamber were co-sputtered at 90 W, and 3 mTorr pressure for 10 minutes as a

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conditioning/cleaning step. The substrate and Se pot were then brought up to the

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desired temperature over the course of ~20 minutes, during which the sample

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shutter was kept closed, and the targets remained on at 30 W power at 3 mTorr pressure. After this temperature ramp, the targets were adjusted to their desired power settings for one minute, and then the shutter was opened to enable deposition on the substrate for a specified deposition time. For samples made using CIG/CIG targets, the gun power was set at 75 and 78 W for Cu35In45.5Ga19.5 and Cu60In28Ga12, respectively. Whereas for samples prepared using Cu 3Ga and In

ACCEPTED MANUSCRIPT targets, the power settings were 95 and 30 W, respectively. It is worthwhile to point out that owing to its low melting point and the geometry of the chamber, the In target was susceptible to shorts and often failed after a few experiments. At the

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end of the deposition, the sample shutter was closed, and the targets, Se pot heater,

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and substrate heater were turned off, with the chamber left at 3 mTorr during cool

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down.

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Full cells were completed by deposition of CdS (~80 nm) via chemical bath

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comprising ammonium hydroxide, thiourea, and cadmium sulfate. Intrinsic ZnO (iZnO) and ITO were sputter deposited to complete the solar cells.

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The crystalline structure was characterized using a Bruker AXS D4 diffractometer

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operated at 40 kV and 30 mA using Co Kα radiation. A 1 cm x 1 cm square piece

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of sample was cut and mounted in a polycarbonate sample holder for the analysis.

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XRD patterns were obtained in the 2θ range of 10 – 90° with a collection time of

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45 minutes. Phase identification was performed using MDI Jade 10 software. Film composition was measured using a PerkinElmer Optima 7300DV (Shelton, CT, USA) Inductively Coupled Plasma (ICP) Optical Emission Spectrometer in axial viewing mode. Films were prepared for analysis by dissolving in 50 % nitric acid in a hot water bath (80 °C) or hot block at 80 °C. Briefly, film sections ~1 cm2 were cut from regions located near the center of the deposited film area which

ACCEPTED MANUSCRIPT were free of surface defects/scratches and transferred to a polypropylene tube. 2 mL of 50 % (v/v) trace metal-grade nitric acid were added to the sample tube and then capped. The contents were heated on a hot plate at 80 °C for 30 minutes. The

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dissolved solution was then diluted in deionized water and transferred to auto-

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sampling tubes for analysis. The sample introduction system consists of a

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peristaltic pump and a concentric nebulizer with a cyclonic spray chamber. Single

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element standards of copper (1000 mg l-1), indium (1000 mg l-1), gallium (1000 mg l-1), and selenium (1000 mg l-1) used for preparing calibration standard solutions

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were purchased from Spex (Metuchen, NJ, USA). Detection of iron in the CIGS

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film was not possible due to contributions from the stainless steel substrate.

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An FEI Helios G3 FIB dual beam system was used to perform FIB milling to

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prepare cross sections. Prior to ion milling, a 0.5 µm Pt protective layer was locally deposited on the sample surface using a Pt needle in the FIB system. A rough cut

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was then performed using Ga ions at a current of 9.3 nA, followed by a series of

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fine polishes at a current of 0.79 nA. A JEOL 2010F 200 kV (scanning) transmission electron microscope (S)TEM was used to image films in cross-section. The JEOL 2010F is also equipped with a Bruker AXS XFlash 5030 energy-dispersive silicon drift detector, with an energy resolution of 133 eV. High-angle annular dark-field (HAADF) images with corresponding XEDS maps of Cu, In, Ga, and Se were collected in STEM mode

ACCEPTED MANUSCRIPT with an electron probe ~2 nm in diameter. A 50 micron condenser aperture was used, and the sample was tilted 5 degrees towards the x-ray detector. Spectral data

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were processed using the Bruker Quantax Esprit 1.8.5 software package.

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3. Results and Discussion

Results in this section are organized into four distinct groups. The effect of Se

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content, sputtering target type, deposition temperature, and Na content will all be

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discussed separately. For all samples Ga/III was in the range of 0.24-0.26 as

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determined by ICP measurements. In the first group of samples, the effect of Se

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content in the deposited precursor film was explored. Specifically, samples were deposited via co-sputtering of two CIG targets at 280 °C with a wide range of Se

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levels, from 0 – 50 at. %. In the second group of samples, the effect of sputtering

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target type was explored. Samples were produced via co-sputtering either In and

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CuGa or two CIG targets at three different Se levels at a substrate temperature of 280 °C. The third group of samples were produced to explore the effect of precursor deposition temperature. Samples were deposited via co-sputtering of two CIG targets over a substrate temperature range of 150 – 380 °C. In addition, two different Se levels were used at each temperature level. It was hypothesized that higher deposition temperature combined with higher Se content would be

ACCEPTED MANUSCRIPT preferable; by comparison, CIGS made via co-evaporation typically utilizes temperatures above ~450 °C. Although higher temperature deposition was desired, the deposition chamber was limited to 380 °C. It should be noted that both samples

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produced at high Se levels were below their target level of 50 at. % (actual 35 and

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40 at. %). Finally, the effect of NaF content was studied in the fourth group of

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samples via co-sputtering from two CIG targets at 280 °C. NaF film thicknesses

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ranging from 10 – 30 nm were deposited prior to precursor deposition.

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3.1. Effect of Se content on precursor film morphology and phase

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The precursor film morphology, crystalline phase, and phase distribution were characterized using SEM, XRD and STEM-XEDS, respectively. Peak positions

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corresponding to Cu 9Ga4 and Cu9In4 are marked with dotted lines herein. It is

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Cu9(Ga,In)4 .

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inferred that peaks present between these two dotted lines are indicative of mixed

Fig. 3 shows an overlay of XRD patterns obtained from precursor films deposited using CIG targets at various Se levels. At the lowest Se content (10 at. %), diffraction peaks consistent with the crystalline phases of In, Cu9(Ga,In)4, and In4Se3 were present. An increase in Se content to 15 at. % results in the selenization of most of the In to In 4Se3. With further increase in Se to 25 at. %, the

ACCEPTED MANUSCRIPT remaining In is selenized to In 4Se3; in addition, peaks associated with the CIGS phase start to appear. At the highest Se level of 49 at. %, CIGS was in fact the primary phase with a small amount of Cu 2Se also present. It is important to note

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that even for such fully selenized precursor samples, no MoSe 2 formation was

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observed, indicating that MoSe2 formation is driven by high-temperature

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selenization.

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Fig. 4 shows secondary electron images obtained at 10 kX magnification of the top

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surfaces of each precursor film in plan-view. The corresponding STEM-HAADF and STEM-XEDS images of the films in cross-section are shown in Fig. 5. At low

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Se content (10 at. %) the film consists of relatively large particles, ~1 micron in

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size, sparsely distributed among a collection of smaller particles, which results in

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high film roughness. STEM-XEDS reveals that the largest particles are composed of Cu, In, and Ga, with observed Cu/III atomic ratios of ~1.2 – 2.1. In direct

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contact with these large particles are intermediate-size particles, ~100 nm in

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diameter that are composed primarily of In, Ga, and Se, with observed III/Se atomic ratios ~1.2 – 1.4. Lastly, located near the bottom of the precursor film are domains rich in In and Se, with typical III/Se atomic ratios ~1.6 – 2. Small In-rich pockets are also observed within these domains. As the Se content in the film is increased to 15 at. %, the average particle size decreases and becomes more uniform in diameter; however, significant film

ACCEPTED MANUSCRIPT porosity remains. Cross-section STEM-HAADF images show particles ~100 – 400 nm in diameter dispersed on a continuous layer, ~200 nm thick on average. Particles contain Cu, In, and Ga, with typical Cu/III ~1.75; however, the relative

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ratio between In and Ga was found to vary significantly among particles. The

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majority of particles observed exhibited Ga/III ~0.3; however some exhibited

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much higher Ga content, with Ga/III ~0.7. In addition, small Cu-rich pockets were

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observed in some regions. The continuous bottom layer is primarily composed of In and Se, with average III/Se ~1.5. It should be noted however that the In and Se

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distribution within this layer is not uniform.

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As Se content is further increased to 25 at. %, significant changes in film

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morphology are observed. These changes can be most clearly observed in STEM-

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HAADF images obtained in cross-section, which show a dense, relatively smooth film composed of both particle and layered domains. STEM-XEDS reveals two

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main types of phases within the film, one containing primarily Cu and In (Cu/III

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~1.8 – 2), and the second containing primarily In, Ga, and Se (III/Se ~0.9 – 1, ~50 at. % Se). The largest domains (>200 nm) of Cu and In are observed near the bottom of the film, while those containing In, Ga, and Se are just above this layer. The remaining volume above and between these larger particles are filled with thinner layers of these two phases. Finally, a small number of domains containing

ACCEPTED MANUSCRIPT primarily In and Se are observed at the bottom of the film. These domains exhibit III/Se ~1.2 – 1.4. As Se content is further increased to 40 at. %, multiple distinct phases are still

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observed; however, their compositions and morphology have changed. Large

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domains are present in this sample, as well as the appearance of some porosity.

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Three distinct phases are observed distributed throughout the film. The most

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prevalent phase observed contains Cu, In, Ga, and Se (III/Se ~0.8, 50 at. % Se).

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This phase contains more copper than the corresponding phase observed at 25 at. % Se, and is present as relatively large domains throughout the sample. The second

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phase contains primarily Cu and In (Cu/III ~1.6 – 1.8), and exists both as relatively

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large particles near the bottom of the film and as smaller particles distributed

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throughout the first phase. Finally, a small number of domains rich in In and Se

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(III/Se ~1.3) were observed near the bottom of the film. Precursor films deposited at the highest Se content, 49 at. %, exhibit significantly

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different morphology and distribution of elements than those films deposited at lower Se content. STEM-HAADF images of the film cross-section show a relatively compact film, with slightly layered structure, and a small level of porosity throughout the film. Except for a few nanometer-size regions that appear bright in these images, the majority of the film exhibits uniform intensity, indicating that the distribution of elements throughout the film is uniform. Indeed,

ACCEPTED MANUSCRIPT STEM-XEDS maps show a uniform distribution of elements, with average composition consistent with the presence of a stoichiometric CIGS phase (Cu/III ~0.75, Ga/III ~0.24, 49 at. % Se). The few bright nanometer-size domains were

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determined to be a copper-rich phase. Finally, the grain size of this film is quite

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3.2. CIG/CIG vs. In/CuGa sputtering targets

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small, as determined by selected-area electron diffraction analysis (not shown).

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The impact of precursor sputtering target type was also studied in this work. To

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elucidate the contribution to film characteristics of each target type, separate

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precursor films were deposited using both In/CuGa and CIG/CIG target combinations at multiple Se levels. Specifically, films were deposited using each

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target type combination at a substrate temperature of 280 °C with Cu/III ratio in

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the range of 0.85 – 0.89. Three different Se levels were targeted (0, ~15, and ~40

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at. %) though measured Se content differed slightly from these values (see caption Figure 6). Fig. 6 shows XRD characterization of the crystalline phases present for each sample. Regardless of the target combination (i.e. CIG vs. CuGa/In), at a given Se level, similar phases were observed. Also for each target combination the evolution of phases with increasing Se level was similar to that discussed previously.

ACCEPTED MANUSCRIPT Fig.7 shows secondary electron images obtained at 20 kx magnification of the top surfaces of each of the precursor films in plan-view and STEM-HAADF images. The STEM-XEDS images of certain films in cross-section are shown in Fig. 8.

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Films deposited in the absence of Se exhibit a morphology and elemental

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distribution that are significantly different compared to films deposited with Se.

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The plan-view SEM images of each film (Fig. 7(a,b)) and the associated cross-

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section STEM-HAADF/XEDS images (Figs. 8(a), 7(g)) show relatively large

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micron-size In particles (See Fig. 8(a) inset) located on top of and separated by a collection of smaller, ~100-200 nm size Cu/Ga nanoparticles, in various ratios. It is

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interesting to note that sputtering target type does in fact appear to affect the film

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morphology under these process conditions.

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Films deposited in the presence of Se, under conditions that yielded ~15 at. % Se,

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exhibited a more uniform particle film. The top surfaces of both films consist of nanoparticles with sizes of ~100 – 400 nm. Cross-section STEM-HAADF images

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and associated STEM-XEDS maps reveal that precursor films deposited under these conditions yielded a rough continuous bottom layer rich in In and Se, though some Cu was present in small domains. The distribution of In and Se within this layer is not homogeneous. In fact, In-rich domains, ~10 – 20 nm in size, were observed distributed throughout this In- and Se-containing layer. Located on top of this bottom layer are larger nanoparticles, up to ~400 nm in size that consist

ACCEPTED MANUSCRIPT primarily of Cu, Ga, and In, though the distribution of these elements is not homogenous. Finally, each particle is covered with a layer rich in Se, but also containing Cu, In, and Ga. Whereas in the film produced using the CIG/CIG target

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combination this top layer is relatively dense, and ~150 nm thick, the CuGa/In

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target combination yields a more porous layer that is more intermixed with the

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underlying particles.

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With increasing Se, films grow thicker and exhibit less porosity. The top surfaces of both films produced under these conditions consist of particles, relatively

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uniform in size, ~400 – 500 nm in diameter. Cross-sections of both films show

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distinct columnar growth, with a fair amount of porosity present near the bottom of

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the film, and less at the top film surface. The distribution of elements within the

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film is heterogeneous, though the degree of heterogeneity is less than that observed in films produced with lower Se content. Domains rich in In and Se are observed

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near the bottom of each film, along with nanoparticles, ~100 nm in size, composed

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primarily of Cu, Ga, and In. The bulk of the film, which exhibits a columnar growth with distinct layers, is composed of Cu, Ga, In, and Se in relative amounts approaching stoichiometric CIGS. Within the bulk of the film additional smaller Cu- and Ga-rich domains are present, either as distinct nanoparticles or intergrown throughout the bulk. It is interesting to note that the film deposited using CuGa/In

ACCEPTED MANUSCRIPT targets produced Cu- and Ga-rich domains that were on average smaller in size and more uniformly distributed throughout the bulk of the film.

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3.3. Effect of substrate temperature

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Precursor films were deposited from CIG/CIG sputtering targets at substrate temperature of 150, 280, and 380 °C, using two different Se levels, ~25, and ~40

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at. %. For all these samples except those that were deposited at the highest

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substrate temperature, the Cu/III ratio was in the range of 0.86 – 0.89.

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Interestingly, samples prepared at 380 °C exhibited a low Cu/III ratio of 0.75 at

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both Se levels.

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Within this substrate temperature range and for a given Se level, no differences were observed in the crystal phases as measured by XRD (Fig. 9). At 25 at. % Se,

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all samples exhibited peaks consistent with the presence of In 4Se3, Cu9(In,Ga)4 and

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a small amount of CIGS. Note that peaks corresponding to In 4Se3 (031)/(330) planes and CIGS (112) are in close proximity to each other, and therefore the broad peak around 2θ ~32o for samples (a) , (b), (c) in Fig. 9 could be a result of convolution from all these phases. At 40 at. % Se, CIGS became the dominant phase while peaks associated with In 4Se3 were significantly reduced in intensity. Cu9(In,Ga)4 phase was still present at 40 at. % Se.

ACCEPTED MANUSCRIPT Film morphology as well as the distribution of elements within the film changed significantly with changes in substrate temperature during deposition. Crosssection STEM-HAADF images and STEM-XEDS elemental maps are shown in

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Fig. 10 and Fig. 11. It should be noted that not all films from this set were

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examined; however general trends can still be identified.

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At low temperature (150 °C), the film deposited with 25 at. % Se exhibited very

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little porosity, and showed relatively uniform distribution of the elements Cu, In,

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Ga, and Se. Phase segregation of domains rich in Cu and Ga from those containing all four elements occurred over much shorter length scales than that observed in

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films deposited at higher temperatures. In addition, at this low deposition

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temperature, phases rich in In and Se were not observed.

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At 280 °C, deposited films exhibited phase segregation over longer length scales

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than that observed in films deposited at low temperature. Separate phases observed

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in these films were discussed previously. At the highest temperature examined, 380 °C, films exhibited phase segregation over the longest length scales. Domains close to ~1 micron in diameter are present. It is interesting to note that at this deposition temperature domains rich in In and Se were distributed throughout the film, not located preferentially at the bottom of the

ACCEPTED MANUSCRIPT film. In addition, films deposited at this high temperature exhibited the highest level of porosity.

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3.4. Effect of NaF content

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Finally, the effect of NaF film thickness was studied in this work. The beneficial role of Na in literature has been ascribed to passivation of grain boundaries,

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moderation of cation diffusion, or to facilitation of the formation of the MoSe2

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layer [11-13]. To examine its effect at the precursor stage, precursor films were

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deposited from CIG/CIG targets at 280 °C onto Ti/Mo coated stainless steel

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substrates with three different NaF film thicknesses (10, 20, and 30 nm). NaF was deposited prior to deposition of the precursor layer. Cu/III ratio and Se content

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were kept constant at 0.86 and 25 at. %, respectively.

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Fig. 12 shows XRD patterns obtained from each film. As expected within the

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explored Na range, at a given Se level all samples exhibit a similar XRD pattern, consistent with the presence of In 4Se3, Cu9(In,Ga)4, and CIGS phases. For these three samples, ICP measurements showed Na content of 3.58 at. %, 4.32 at. %, and 5.75 at. %, respectively. It should be noted that microscopy characterization of these samples was not conducted.

ACCEPTED MANUSCRIPT 3.5. Selenized films A representative set of precursor samples was selenized at high temperature (>500 °C) and cells were built with deposition of CdS, ZnO and ITO layers. Fig. 13

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shows the x-ray diffraction data of three such cells with different Se content at the

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precursor stage. All three samples have bulk Ga/III ratio of 0.24, as measured by

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ICP. No evidence of partially selenized phases was found in these samples, and

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CIGS films exhibited preferred orientation in the (112) direction. The CIGS peak

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(Fig. 13b) shows bifurcation, which is typical of the formation of In and Ga-rich regions with In migrating to the surface. Interestingly, the relative intensity of the

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In-rich (low 2θ) and Ga-rich (high 2θ) peaks changed as the Se content in the

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precursor was increased. The sample with lowest Se (~0.28 at. %) in the precursor

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exhibits a more intense In-rich peak post-selenization, while the film with highest

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Se content (~17.35 at. %) at the precursor stage has a higher-intensity, Ga-rich peak. This suggests that increasing Se content in the precursor helps to better

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homogenize Ga towards the surface, corroborating some of the reported findings in the literature and highlighting the effect of precursor microstructure on resulting CIGS structure and elemental distribution. The figures of merit for all cells are listed in Table 1. These values are averaged over a minimum of eight cells, each with an area of 0.46 cm2. Among these three samples, the highest cell efficiency of 11.48% was obtained for the film with

ACCEPTED MANUSCRIPT highest Se content in the precursor, primarily owing to larger FF, due to its lower series resistance (Rs). Open circuit voltage (Voc) seems to be independent of the Se

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content in the precursor layer.

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4. Conclusions

This study has explored the effect of precursor deposition conditions including Se

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content, substrate temperature, target type (CIG/CIG vs. CuGa/In), and Na content

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on the phase, morphology, and elemental distribution of the resulting precursor

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film. We have found that Se content has a strong effect on the relative fraction of

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phases (Cu9(In,Ga)4, InxSey, CIGS) and morphology of the precursor. The length scale over which phase segregation occurs decreases with increasing Se content.

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The amount of stoichiometric CIGS phase increases with increasing Se content,

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resulting in phase-pure CIGS with small grain size at the highest Se loading (~49

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at. %). Increasing substrate temperature over the range of 150 °C – 380 °C resulted in an increase in both surface roughness and length scale of phase segregation, but did not change the type of crystalline phases. Target type and Na content were not found to significantly affect precursor characteristics. Thus, we hypothesize that precursors containing high Se which results in an abundance of uniformly-

ACCEPTED MANUSCRIPT distributed CIGS phase may selenize to produce CIGS absorber films with uniform

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Ga distribution, thereby resulting in improved open circuit voltage.

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solar cells, Thin Solid Films 480-481 (2005) 520-525. 6. W. Witte, D. Abou-Ras, K. Albe, G. H. Bauer, F. Bertram, C. Boit, R. Brüggemann, J. Christen, J. Dietrich, A. Eicke, D. Hariskos, M. Maiberg, R. Mainz, M. Meessen, M. Müller, O. Neumann, T. Orgis, S. Paetel, J. Pohl, H. Rodriguez-Alvarez, R. Scheer, H.W. Schock, T. A. Unold, Gallium

ACCEPTED MANUSCRIPT gradients in Cu(In,Ga)Se2 thin-film solar cells, Prog. Photovolt: Res. Appl. 23 (2015) 717-733. 7. C. Y. Huang, W. C. Lee, and A. Lin, A flatter gallium profile for high-

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efficiency Cu(In,Ga)(Se,S) 2 solar cell and improved robustness against

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sulfur-gradient variation, J. Appl. Phys. 120 (2016) 094502-(1-15).

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8. S. Ishizuka, L. M. Mansfield, C. DeHart, M. Scott, B. To, M. R. Young, B.

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Egass, and R. Noufi, Rapid fabrication of Cu(In,Ga)Se2 thin films by the two-step selenization process. IEEE Journal of Photovoltaics 3 (2013) 476-

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482.

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9. R. Wuerz, A. Eicke, M. Frankenfeld, F. Kessler, M. Powalla, P. Rogin, O.

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Yazdani-Assl, CIGS thin-film solar cells on steel substrates, Thin Solid Films 517 (2009) 2415-2418.

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10. P. Blösch, S. Nishiwaki, T. Jaeger, L. Kranz, F. Pianezzi, A. Chirilă, P.

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Reinhard, S. Buecheler, A. N. Tiwari, Alternative back contact designs for

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Cu(In,Ga)Se2 solar cells on polyimide foils, Thin Solid Films 535 (2013) 220–223.

11. T. P. Erslev, J. W. Lee, W. N. Shafarman, J. D. Cohen, The influence of Na on metastable defect kinetics in CIGS materials, Thin Solid Films 517 (2009) 2277–2281.

ACCEPTED MANUSCRIPT 12. A. Rockett, The effect of Na in polycrystalline and epitaxial single-crystal CuIn1-xGaxSe2, Thin Solid Films 480-481 (2005) 2-7. 13. D. Rudmann, D. Bre´maud, A.F. da Cunha, G. Bilger, A. Strohm, M.

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Kaelin, H. Zogg, A.N. Tiwari, Sodium incorporation strategies for CIGS

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growth at different temperatures. Thin Solid Films 480-481 (2005) 55-60.

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Fig. 1. Schematic illustrating a typical two-step precursor-selenization process for

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CIGS absorber deposition

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Fig. 2. Picture of the (a) Se pot and (b) interior of the deposition chamber showing location of the sputtering guns and Se pot. Chemical composition of the targets

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used in this study are listed in the table.

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Fig. 3. X-ray diffraction patterns of precursor films prepared at a substrate temperature of 280 °C with (a) 10 at. % Se, (b) 15 at. % Se, (c) 25 at. % Se, (d) 40 at. % Se, and (e) 49 at. % Se. Measured Cu/III ratio for these samples was in the range of 0.86 – 0.92.

ACCEPTED MANUSCRIPT Fig. 4. Top-down SEM images and cross-section STEM-HAADF image montages of precursor films deposited at 280 °C with (a, f) 10 at. % Se, (b, g) 15 at. % Se, (c, h) 25 at. % Se, (d, i) 40 at. % Se, and (e, j) 49 at. % Se. The back electrode

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appears at the bottom of STEM-HAADF images.

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Fig. 5. Cross-section STEM-XEDS elemental maps of Cu (green), In (blue), Ga

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(yellow), and Se (red) of precursor films deposited at 280 °C with (a) 10 at. % Se,

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(b) 15 at. % Se, (c) 25 at. % Se, (d) 40 at. % Se, and (e) 49 at. % Se.

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Fig. 6. X-ray diffraction patterns of CIGS precursor films with (a) 1 at. % Se, (b) 15 at. % Se, (c) 38 at. % Se, (d) 0.3 at. % Se, (e) 14 at. % Se, (f) 40 at. % Se. Films

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(a), (b), (c) were deposited from CIG/CIG targets with different Cu/III ratios,

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while (d), (e), (f) were deposited from co-sputtering of CuGa/In.

Fig. 7. Top-down SEM images and cross-section STEM-HAADF image montages of precursor films deposited at 280 °C with various Se levels from CIG/CIG and CuGa/In sputtering targets: (a) 0.3 at. % Se, CIG/CIG; (b, g) 1 at. % Se, CuGa/In; (c, i) 14 at. % Se, CIG/CIG; (d, h) 15 at. % Se, CuGa/In; (e, j) 40 at. % Se,

ACCEPTED MANUSCRIPT CIG/CIG; and (f, k) 38 at. % Se, CuGa/In. A protective Pt layer used for FIB milling is visible covering the precursor films in (g), (h), and (k).

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Fig. 8. Cross-section STEM-XEDS elemental maps of Cu (green), In (blue), Ga

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(yellow), and Se (red) of precursor films deposited at 280 °C with various Se levels

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from CIG/CIG and CuGa/In sputtering targets: (a) 1 at. % Se, CIG/CIG (inset

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SEM-XEDS elemental map of large In domain); (b) 15 at. % Se, CuGa/In; (c) 14

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at. % Se, CIG/CIG; (d) 38 at. % Se, CuGa/In; (e) 40 at. % Se, CIG/CIG.

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Fig. 9. X-ray diffraction patterns of precursor films prepared at various substrate temperatures and Se levels: (a) 150 °C, 25 at. % Se, (b) 280 °C, 25 at. % Se, (c)

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380 °C, 25 at. % Se, (d) 150 °C, 35 at. % Se, (e) 280 °C, 40 at. % Se, (f) 380 °C,

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40 at. % Se.

Fig. 10. Cross-section STEM-HAADF image montages of precursor films deposited at various substrate temperatures and Se levels from CIG/CIG sputtering targets: (a) 150 °C, 25 at. % Se; (b) 280 °C, 25 at. % Se; (c) 280 °C, 40 at. % Se. A protective Pt layer used in FIB milling is visible covering the precursor film in (a).

ACCEPTED MANUSCRIPT Fig. 11. Cross-section STEM-XEDS elemental maps of Cu (green), In (blue), Ga (yellow), and Se (red) of precursor films deposited at various substrate temperatures and Se levels from CIG/CIG sputtering targets: (a) 150 °C, 25 at. %

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Se; (b) 280 °C, 25 at. % Se; (c) 280 °C, 40 at. % Se; (d) 380 °C, 25 at. % Se.

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Fig. 12. X-ray diffraction patterns of precursor films deposited with: (a) 10 nm

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NaF, (b) 20 nm NaF, (c) 30 nm NaF.

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Fig. 13. Top: X-ray diffraction patterns of full cells made from precursor films

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CIGS (312) XRD peak.

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with: (a) 0 at. % Se, (b) 13 at. % Se, (c) 17 at. % Se. Bottom: Magnified view of

Fig. 14. Top-down SEM and cross-section STEM-HAADF images of full cells

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made from precursor films with: (a, b) 17 at. % Se (c, d) 13 at. % Se Table 1. Figures of merit of full cells.

Precursor Se

Eff.

Voc

Jsc

(at.%)

(%)

(mV)

(mA/cm )

FF 2

(%)

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Rsh (Ω2

(Ω-cm )

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cm )

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Highlights

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 Microstructure of sputtered precursor for CuIn1-xGaxSe2 (CIGS) absorber is studied

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 Polycrystalline CIGS thin films are obtained on Mo-coated steel substrates

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 Full cells with efficiencies upto 11.48% have been fabricated

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