Film growth mechanism for electrodeposited copper indium selenide compounds

Thin Solid Films 524 (2012) 20–25

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Film growth mechanism for electrodeposited copper indium selenide compounds Yan Li, Shahid S. Shaikh, Shalini Menezes ⁎ InterPhases Solar, Inc., 668 Flinn Ave # 23, Moorpark, CA 93021, USA

a r t i c l e

i n f o

Article history: Received 19 December 2011 Received in revised form 28 July 2012 Accepted 30 July 2012 Available online 10 August 2012 Keywords: Copper indium selenide Electrodeposition Ordered defects Chalcopyrite Reaction mechanism Thin films Solar cells

a b s t r a c t The Cu2Se–In2Se3 system comprises several copper indium selenide (CIS) compounds with solar-matched bandgaps along with the optimum properties of the CuInSe2 compound. This work investigates electrochemical growth of CIS films under various conditions, initially identified with cyclic voltammetry. The film growth, monitored with X-ray fluorescence analysis, shows excellent composition and thickness uniformity. The results agree with secondary ion mass spectroscopy profiles and X-ray diffraction data, indicating the conversion of initially formed binary phases to homogenous ternary compound. Deposition potential and substrate/electrolyte interface control the film formation mechanism and hence its composition. Electrolyte composition and agitation influence the film thickness. Judicious combination of process parameters is essential to obtain CIS films with optimum properties. © 2012 Elsevier B.V. All rights reserved.

1. Introduction Many inherent properties of the copper indium selenide (CIS) semiconductor make it an optimum absorber for photovoltaic (PV) energy conversion. Its potential in PV devices has long been demonstrated by achieving high efficiency and unusual stability for PV cells made from single crystal CIS, as well as thin film CIS-alloys with Ga and S to form CuInGa(SeS)2 (CIGS) [1–4]. The wider bandgap of CIGS overcomes the efficiency limitation of the narrow bandgap CuInSe2 absorber. The single junction conversion efficiency for laboratory scale CIGS PV cell now exceeds 20% [4]. Manufacturing issues associated with controlling the CIGS deposition process still hamper the scale-up and cost competitiveness of CIGS PV technology. The bandgap engineering of the CIS absorber through alloying with foreign elements (Ga, S) tends to perturb its thermodynamically favored defect structure [5], thus complicating the CIGS deposition. On the other hand, the CuInSe2 material is a compound that is amenable to low-cost, non-vacuum, large area coating methods such as electrodeposition, chemical spray or chemical bath deposition. Electrodeposition is particularly advantageous because of: inexpensive precursors and capital equipment; low raw material needs and low waste; high material utilization; easier, safer, efficient coating for large areas; conformal application to irregular surfaces; and amenability to industrial scale manufacturing.

⁎ Corresponding author. Tel./fax: +1 805 530 1193. E-mail addresses: [email protected] (Y. Li), [email protected] (S.S. Shaikh), [email protected] (S. Menezes). 0040-6090/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.tsf.2012.07.131

CIGS thin-film solar cells are known to comprise a distinct Cudeficient ordered defect chalcopyrite (ODC) layer on the surface of the absorber [6]. The ODC layer, comprising a n-CuIn3Se5 compound plays an important role in determining the PV properties of the CIGS solar cell. The phase equilibria for the quasibinary Cu2Se–In2Se3 isopleth [7] include several stable off-stoichiometric ODC compounds from the Cu2Se(In2Se3)n series, where n>1. These In-rich ODC compounds offer consistent n-type conductivity and larger solar-matched bandgaps relative to the stoichiometric CuInSe2 phase, Table 1 [8,9]. Thus, the wide bandgap CIS ODC phases, such as CuIn5Se8, CuIn3Se5, Cu2In4Se7 and Cu3In5Se9, retain the optimal electro-optic properties and chemical stability of CuInSe2, without the limitations that arise from alloying it with Ga and S, as in the standard CIGS absorber. Further, the formation of these ODC compounds is attributed to exceptionally low energy for the formation − 2+ 2− 2+ − of defect pairs such as (2 VCu +InCu ) and (2 CuIn +InCu ). The VCu , 2− 2+ CuIn or InCu defects, forming the charge compensated defect pairs impart electrically benign character to the large defect population inherent in CIS semiconductors [8]. They lead to electronically passivated, self-stabilizing ODC compositions. This important attribute of the ODCs allows wide tolerance to processing variability [10,11]. This work exploits this ODC property to provide a single step electrodeposition method required to uniformly coat ODC films on large substrates. Literature research is generally directed to find ideal conditions to deposit CuInSe2-based absorbers for solar cells. This paper investigates process parameters and reaction mechanisms for electrodeposited CIS films through deliberate deviation from ideal conditions. It seeks to demonstrate that within a reasonably wide range of process conditions, one can avail of thermodynamic reaction control to produce surprisingly stoichiometric ODC compositions, which are off-stoichiometric relative to CuInSe2.

Y. Li et al. / Thin Solid Films 524 (2012) 20–25 Table 1 ODC composition and bandgaps from the (Cu2Se) (In2Se3)n series.

21

Table 2 Process parameters and properties of electrodeposited CIS films.

Phase

n

Cu2Se units

In2Se3 units

Eg

CuInSe2 CuIn2Se3.5 CuIn3Se5 CuIn4Se6.5 CuIn5Se8

1 2 3 4 5

1 1 1 1 1

1 2 3 4 5

1.04 1.18 1.26 1.30 1.34

1.1. Electrochemical formation of CIS films The electrodeposition approach takes advantage of the thermodynamically driven formation of CIS-ODC compounds. Previous investigations for CuInSe2 have shown that the Cu–In–Se system presents a relatively complex chemistry. The electrodeposition from an electrolyte containing dissolved Cu 2+, In 3+ and Se 4+ ions, can lead to many alternate reaction paths and multiple solid phases of Se, CuSe, Cu2Se and In2Se3 [12–18]. The schematic in Fig. 1 illustrates the mechanism for the formation of a Cu2Se(In2Se3)n compound. As in the case of CuInSe2, the deposition of the CIS ODC compounds takes place via two steps, at potentials E1 and E2, corresponding to the formation and reduction of CuSe. The second step E2 triggers chemical (C) reactions that can lead to the formation of at least 3 solid phases, as shown in Fig. 1. The composition of the solid phase formed in Cu 2+/In 3+/Se 4+ electrolyte depends on the applied ED and concentration ratio of [Cu 2+]:[In 3+]:[Se 4+]. Initially, the Cu 2+ and Se 4+ ions reduce simultaneously to CuSe at E1 and further to Cu2Se and Se 2− at E2. Cu2Se and Se 2− react with the In 3+ from the solution via a homogenous chemical reaction (C1) to form CuInSe2, and further reactions (Cn) to form the In-rich Cu2Se(In2Se3)n compounds. Note that the Se 2− may chemically react with Se 4+ to form Se 0 (C″), or with In 3+ to form In2Se3 (C′). Sufficient excess of [In 3+] is needed to induce the formation of higher order CIS ODC compounds from the Cu2Se(In2Se3)n series. It is thus essential to avoid the reaction C″ and direct the process parameters to reaction path C1. The process parameters such as temperature, electrolyte composition, pH, mass transport and deposition potential, ED can be manipulated to control the kinetics of competing reactions in order to produce single phase stoichiometric ternary compounds, listed in Table 2. 2. Experimental details The electrodeposition of CIS ODC compounds was examined with cyclic voltammetry using Pine Instruments or ADI potentiostats to

Sample Fig. no. no.

CIS-A CIS-B CIS-C CIS-D CIS-E

4 7 8 9 10

CIS-F

11

Substrate Electrolyte Cu:In:Se

Mo foil Mo foil Mo foil Mo foil SnO/ glass SnO/ glass

Solution Potential vs. ref, V

Film properties at 50 min Cu: In:Se

Thickness, μm 1.4 2.1 0.93 0.92 1.6

1:4:2 1:2:1 1:4:2 1:4:2 1:6:2

Stir Stir Stir No Stir

−0.525 −0.525 −0.500 −0.525 −0.500

1:3:5 1:3:5 1:1:2 2:4:7 2:3:5

1:6:2

Stir

−0.475

2:3:8 2.4

identify deposition parameters. A standard 3-electrode cell was used, comprising a Pt counter electrode and Ag/AgCl reference. The substrate was vertically suspended in the electrolyte during film growth and removed for analysis at various stages of growth. The current was monitored during film deposition. A number of CIS films were electrodeposited on back insulated Mo foil and SnO coated glass with an exposed area of ~ 5 cm 2, by varying the deposition parameters that control the composition and conductivity type. The solution parameters: supporting electrolyte, pH range, and solution temperature were maintained constant. The variables investigated include: deposition potential (ED), deposition time, [Cu]:[In]:[Se] metal concentration, and agitation. Table 2 lists representative samples that were investigated to determine the dependence of composition uniformity by changing the process parameters as listed. Most films were deposited at constant ED from a single 0.1 M KCl electrolyte containing Cu 2+, In 3+ and Se 4+ ions in different ratios, as shown in Table 2. The composition range for the dissolved metal ions was 0.5–1 mM CuCl, 2–3 mM InCl3, and 1 mM SeO2. X-ray fluorescence (XRF) analysis was performed with Spectro Midex to obtain the CIS film composition and thickness at specific time intervals during electrodeposition. The sample was removed from the electrodeposition system for approximately 15 min for each XRF measurement. Each XRF data point is acquired over a 3 min period. After the measurement, the CIS film deposition was resumed without noticeable effect, either on the deposition current or film morphology during the deposition run. Fig. 2 shows the approximate location of the spots that were analyzed across two types of CIS samples, deposited on Mo and SnO/glass substrates. The data collected for areas near the bottom (Point 1), middle (Point 2) and top (Point 3) of the Mo sample in Fig. 2a is plotted as a function of deposition parameters. The data collected for the two spots, bottom (Point 1) and top (Point 2) on the SnO/glass sample of Fig. 2b are also plotted. Note that the top spots are closest to the electrical contact

(a)

3

(b)

2 2 1 Fig. 1. Chemical (C) and electrochemical (E) reaction sequence for the formation of solid compounds in a solution containing Cu2+/In3+/Se4+ species. Modeled after Ref. [17].

1

Fig. 2. Location of points 1, 2 and 3 analyzed with XRF during CIS film growth on (a) Mo foil, and (b) SnO2/glass substrates.

Y. Li et al. / Thin Solid Films 524 (2012) 20–25

and the liquid surface, and the bottom spots are most distant. Secondary ion mass spectrometry (SIMS) depth profiling was used for post deposition characterization of the films with a Phi 6650 Dynamic quadrupole SIMS system. X-ray diffraction (XRD) plots were scanned with a PANalytical X'Pert PRO X-ray diffraction system at Cu K-alpha wavelength from 10 to 60° 2 theta in steps of 0.017. SIMS and XRD were obtained for the electrodeposited CIS samples after rapid thermal annealing. The samples were radiated through a glass plate using fifteen 10 s pulses from a 500 W infrared lamp at a distance of about 4 cm during the annealing. 3. Results and discussion

a

80

Point 1 Point 2 Point 3

70

Atomic Percent (at %)

22

60

Se

50 40

In

30 20

Cu

10

3.1. CIS deposition on alternate substrates

0

C3 10

20

30

40

50

60

40

50

60

Time (min)

b Point 1 Point 2

Thickness (µm)

The substrate type plays a crucial role in the mechanism of the compound formation. The reduction of Cu 2+/In3+/Se 4+ species at Mo and SnO/glass substrates was investigated using cyclic voltammetry. The current (I)–voltage (V) scans in Fig. 3 were obtained in ~0.5 mM Cu+ 3 mM In+ 1 mM Se+ 0.1 M KCl solution with Mo foil and SnO2/ glass electrodes. Comparison of the first and second scans, obtained at the Mo electrode shows a distinct peak at −0.5 V for scan 1 that is absent in the subsequent scan 2. This may be related to the initial reaction of substrate with Se. The next wave for CuSe formation and conversion to CIS is limited by the Cu concentration as previously observed [12–19]. The substrate-Se reaction peak is absent in curve 3, obtained with SnO2 coated glass. The CuSe formation wave occurs at potentials positive of −0.5 V. The lower conductivity of SnO2 slows the kinetics and contributes to the higher overvoltage for CIS formation.

C1 0

1

Point 3

0.5

0 0

3.2. Effect of process parameters on composition and thickness

10

20

30

Time (min) Fig. 4a shows a plot of film composition for sample CIS-A, Table 2, during growth, measured along the three spots in Fig. 2a. The plot shows that the initially Cu-rich film converts sharply to an In-rich film within the first 10 min. At that point the film is only 0.3 μm thick. This is consistent with the mechanism for CIS electrodeposition via electrochemical formation of CuSe and its subsequent chemical conversion to CuInSe2 as shown in Fig. 1 and in previous literature [12–19]. However, the In assimilation reactions start very early during the film formation. The profile indicates at least 10% of In in the film within the first minute of electrodeposition. The In and Cu atomic % profiles cross within the first 2 min, showing the formation of stoichiometric (C1) CuInSe2 with large excess of Se. Subsequently, the film composition changes rapidly to form nearly perfect (C3) CuIn3Se5 ODC stoichiometry. Thus, under the conditions of this experiment, the

Current (mA)

0

-1

-2

-3 -1

1st scan at Mo 2nd scan at Mo 1st scan at SnO2/glass

-0.8

-0.6

-0.4

-0.2

0

Voltage (V) Fig. 3. I–V curves in a solution containing Cu2+, In3+ and Se4+ show (1) first, and (2) second scans for Mo foil; and (3) first scan for SnO2/glass.

Fig. 4. (a) Composition, and (b) thickness changes in three areas shown in Fig. 2 for film CIS-A, deposited with agitation at −0.525 V vs. AgCl/KC under conditions listed in Table 2.

growth profile most likely changes via formation of (E1) CuSe → (C1) CuInSe2 → (C2) CuIn2Se3.5 → (C3) CuIn3Se5 compositions as the film grows. The film composition reaches an equilibrium stage at a Cu: In:Se ratio of 1:3:5. Thus two or more phases that form during the initial 10 min appear to eventually coalesce into a single CuIn3Se5 phase. Note that the composition obtained at each point in the plot of Fig. 4a represents the cumulative composition of the film grown up to that point in time. Also the values measured by the XRF probe represent an average composition, measured through the depth of the film thickness. Notably, the composition profile for the different spots, 1–3 on this film is identical, indicating excellent uniformity without optimizing the current distribution in the deposition cell. With further adjustments in deposition parameters it was possible to electrodeposit most of CIS-ODC compounds from the In-rich Cu2Se(In2Se3)n series with the compositions listed in Table 1. The film thickness was also monitored at different time intervals during growth, for the same 3 points near the top, center and bottom areas of the sample CIS-A as in Fig. 4a. The data is plotted as a function of the film thickness vs. time, Fig. 4b. No variation in film thickness was seen during the initial phase of film growth. The plots show less than 10% deviation towards the later phase of film growth, where the thickness exceeds 1 μm. Substantially thicker films of up to 5 μm could be deposited under these conditions. The thickness profile plot corroborates with composition results in Fig. 4a. Identical thickness for the top, center and bottom regions of the film indicates excellent uniformity also, without attempting to optimize the deposition cell geometry. The electrodeposited CIS films were subsequently annealed and profiled using SIMS, Fig. 5. The concentration profile after full growth

Y. Li et al. / Thin Solid Films 524 (2012) 20–25

a

50000

In

40000 Cu

30000

Se

20000 10000

Substrate O

0

0

200

400

600

800

1000

Atomic Percent (at %)

Secondary Ion (Counts)

60000

23

70 60

Se

50 40

In

30

Point 1 Point 2 Point 3

20

Cu

10

Time (s) 0 Fig. 5. SIMS profile for CIS film electrodeposited as in Fig. 4 and annealed.

3.2.1. Metal concentration The film CIS-B was deposited by doubling the Cu concentration in the electrolyte. The profiles in Fig. 7a are essentially identical to those in Fig. 4a. Except for some minor initial variation in the Cu, In and Se atomic % profiles for Point 1, the composition profile appears reproducible as long as the other deposition parameters remain constant, Table 2. The initial crossover of Cu and In lines that occurs for the 2000

Intensity (Count)

1800 1600

112

1400 1200 1000

220/204

800 312/116

600 400 200 0 10

20

30

40

50

2 Theta (degree) Fig. 6. XRD plot for electrodeposited and annealed CIS film.

60

10

20

30

40

50

60

40

50

60

Time (min)

b 2.5

Point 1 Point 2 Point 3

2

Thickness (µm)

and anneal is relatively uniform and quite similar to that obtained with XRF after the first 10 min of film growth. Surface roughness tends to broaden the SIMS peaks at the front surface and also at the CIS/substrate interface, and is quite likely to have caused the higher In and Se content. Also, annealing is known to promote the formation of In-rich ODC compound at the film surface. The homogenous depth profile attests to the absence of CuSe phase near the CIS/substrate interface. The XRD plot in Fig. 6 shows a single sharp peak and 2 small peaks that are representative of single phase CIS compounds. The result agrees with previous XRD data for CuIn2Se3.5 and CuIn3Se5 polycrystals [9]. The peak positions are similar but intensities vary slightly for the electrodeposited nanocrystalline films. The SIMS and XRD scans indicate the absence of CuSe in the electrodeposited films. Thus, the CuSe film formed within the first few minutes, Fig. 4a, must convert completely to a ternary CIS composition during subsequent growth. The high uniformity of composition and thickness achieved for sample CIS-A required substantial adjustment of deposition parameters. Below we examine the effects of deviating from the optimum parameters used for sample CIS-A, on the film composition and thickness. The electrodeposition of samples CIS-B through CIS-F in Table 2 was carried out by changing the variables, such as [Cu]:[In]: [Se] concentration ratio, ED, agitation and substrate type, relative to those used for sample CIS-A. The deliberate deviation of process parameters enables investigation of their effects on the reaction mechanism.

0

1.5

1

0.5

0

0

10

20

30

Time (min) Fig. 7. Experiment of Fig. 4 showing (a) composition, and (b) thickness plots for sample CIS-B, deposited by increasing the Cu:In concentration ratio in the electrolyte.

formation of CuInSe2 phase is also absent. After the initial changes, the film composition remains constant at nearly perfect CuIn3Se5 stoichiometry, similar to the profile of CIS-A. Good composition uniformity was also obtained for other areas of the sample, confirming that the film composition is most likely controlled by the compound formation reactions rather than by the current distribution. Interestingly, the higher Cu concentration in the electrolyte has a strong effect on the thickness of film CIS-B, Fig. 7b. The film thickness almost doubles to 3 μm with the higher Cu concentration. This film attains the thickness of film CIS-A in about half the time. The deviation for the 3 points increases with the thickness. Thus the variation at 30 min for CIS-B is about the same as that found after 60 min for film CIS-A in Fig. 4b. As the film grows thicker, the deviation between the top and bottom edges of sample CIS-B spreads to about 20%. 3.2.2. Applied ED Fig. 8a shows similar composition plots at 3 different points during growth for the film CIS-C deposited at a potential 25 mV more positive than that used for the films CIS-A and CIS-B. As in Figs. 4a and 7a, the variation between the three points is minimal. However, the composition profiles are very different, particularly for Cu and In. The profiles change substantially with only a small change of potential. The atomic % of In is initially higher than in Fig. 4a, but does not increase to attain ODC composition. Notably, the film attains precise CuInSe2 stoichiometry via reaction C1 at 60 min. If the deposition time is extended beyond 60 min, the In and Cu atomic % profiles cross over, indicating a change in reaction mechanism. The Cu content keeps increasing beyond 30%. The film thickness at 60 min is less than 1 μm and the spread for the 3 different points is ~ 40%. These results suggest that a different reaction mechanism is operative at

24

Y. Li et al. / Thin Solid Films 524 (2012) 20–25

a

60

Atomic Percent (at %)

Se 50 40 30

In

20

Cu

Point 1 Point 2

10

Point 3

0 0

20

40

60

80

100

120

140

Time (min)

b Point 1

Thickness (µ µm)

1

Point 2 Point 3

0.5

0

0

20

40

60

80

100

120

experimental conditions for CIS-E are the same as those for CIS-C. The interface chemistry and lower conductivity of SnO/glass substrate appear to affect the reaction kinetics and hence the growth profile for sample CIS-E. The substrate dependency of the composition profiles is attributed to the different rates of chemical and electrochemical reactions in Fig. 1, on the metallic and glass substrates. The growth profile for film CIS-F, shown in Fig. 11 was obtained at 25 mV more positive than for CIS-E. Comparison of the results for CIS-E and CIS-F samples indicates identical Se profiles. The Cu and In profiles are also similar but the Cu/In ratio is lower for the CIS-E film, deposited at a lower (negative) ED relative to the ED for film CIS-F. Overall, the composition of CIS films deposited on SnO/glass substrates is more sensitive to the deposition parameters, i.e. the range for parameter variability is narrower than for metal substrates. Very uniform and shiny films with ODC composition were more difficult to obtain due to the slow electrochemical reactions on SnO coated glass. Changing to SnO/glass substrates required re-formulating the deposition process with a new set of parameters. With substantial fine-tuning, narrower ranges for the ED, pH and electrolyte composition were identified to electrodeposit better quality CIS on SnO/glass. The pH affects the rate of the first reaction, i.e. the Se reduction, which then transmits to all subsequent reactions. Higher Cu concentrations of about 0.25–0.5 mM, and vigorous stirring were required. These conditions produce smooth, uniform, shiny and thick CIS films with composition ratios close to 1:2:3 or between 1:1:2 and 1:2:3. The deposition of higher order ODC phases (n > 1) on SnO/glass substrates was somewhat difficult.

140

Time (min)

3.2.3. Agitation Composition variation for the different areas of the sample is even less in an unstirred solution. The profiles for CIS-D in Fig. 9a are similar to those of Fig. 4a except for initial variation in the atomic % of Se and In. As the film grows thicker over a period of two hours, the Se content decreases, with a corresponding rise in Cu content. The resulting film composition approaches the CuIn2Se3.5 stoichiometry and remains nearly constant during the two hour deposition period. Thus the film CIS-D appears to be vertically homogenous. As expected from the extremely low metal concentrations used in these experiments, there is substantial variation in thickness of film CIS-D in the unstirred solutions. At 60 min, the thickness for point 3 is more than twice that for points 1 and 2. The growth rate is 50% slower and the morphology is rougher in unstirred solutions. 3.2.4. Effect of substrate The plots in Figs. 4 and 7–9 were obtained for CIS film deposition on Mo foil. Initial chemical interaction with the substrate, curve 1 in Fig. 3 may have an effect on the mechanism of electrochemical formation of the CIS compound and hence its electronic properties. As suggested by the data of curve 3, Fig. 3, CIS deposits on SnO/glass substrates at a more positive ED than on Mo foil. The growth profile for film CIS-E shown in Fig. 10 was obtained during CIS deposition on SnO/glass substrate. Except for the difference in substrates, the

Atomic Percent (at %)

ED = − 0.5 V. The In atomic % decreases with time during growth. The CIS-C film appears to become Cu-rich, probably due to the formation of a Cu-rich ternary phase or CuSe inclusions into the CuInSe2 compound. Interestingly, the profile changes occur simultaneously for all the 3 spots on the sample, again confirming that the reactions taking place at the applied ED primarily control the film composition.

a

60

Se

50 40

In

30

Point 1 Point 2

20

Point 3

Cu 10 0

0

10

20

30

40

50

60

Time (min)

b Point 1 1 Point 2

Thickness (µm)

Fig. 8. Experiment of Fig. 4 showing (a) composition, and (b) thickness plots for sample CIS-C, deposited at −0.50 V vs. AgCl/KCl.

Point 3

0.5

0

0

20

40

60

Time (min) Fig. 9. Experiment of Fig. 4 showing (a) composition, and (b) thickness plots for sample CIS-D, deposited without stirring.

Y. Li et al. / Thin Solid Films 524 (2012) 20–25

a

a

60

Atomic Percent (at %)

Atomic Percent (at %)

Se 50 40

In

30

Cu

20

0

10

20

30

40

50

50

Point 1 Point 2

40 30

In

20

Cu

10 10

20

30

40

50

60

Time (min)

b

b Point 1

1.5

0

60

Time (min)

2.5

2

Thickness (µm)

Point 2

1

1.5

1

Point 1 Point 2

0.5

0.5

0 0

Se

60

0

Point 2

Thickness (µm)

70

Point 1

10 0

25

0

10

20

30

40

50

0

Fig. 10. Experiment of Fig. 8 showing (a) composition, and (b) thickness for sample CIS-E deposited on SnO2/glass substrate.

4. Conclusions The stoichiometry of electrodeposited CIS ODC films is controlled by the thermodynamic driving force associated with stable compound formation. Judicious control of electrolyte composition and deposition parameters enables the electrodeposition of In-rich CIS ODC compounds on metal foil. The film growth profiles concur with previous research results for CuInSe2 [12–19], indicating 2-step mechanism for the electrochemical formation of CIS compounds. Assimilation of In into the Cu–Se binary occurs very early during the deposition. Two or more phases that form during the first 5 min appear to eventually coalesce into a single phase CIS ODC compound such as CuIn3Se5. The results show surprisingly stoichiometric composition as well as film thickness uniformity even under less than optimum configuration for deposition cell. The results enable extending the benefits of electrodeposition to produce wide bandgap CIS compounds that could serve as solar absorbers. The deposition potential has a predominant effect on the film composition. A 25 mV potential shift can alter the deposition mechanism and hence the film composition from In-rich to Cu-rich phases. The film growth mechanism is also sensitive to the chemistry at the substrate/electrolyte interface, and changes considerably for substrates with different conductivities. The composition profile is less sensitive to small variations in the electrolyte composition and agitation. These parameters have a significant effect on the film thickness. CIS deposition on non-metallic glass-based substrates is less tolerant to variation in deposition parameters than that on metal substrates. Nevertheless, a judicious combination of concentration, deposition potential and stirring rate is essential to obtain CIS films with optimum electronic properties for a given substrate within a reasonably short deposition period. The deposition conditions can be controlled to synthesize most of the self-stabilizing CIS ODC compositions from the entire Cu2Se(In2Se3)n series.

20

30

40

50

60

Time (min)

60

Time (min)

10

Fig. 11. Experiment of Fig. 10 showing (a) composition, and (b) thickness plots for sample CIS-F deposited at −0.475 V vs. AgCl/KCl.

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