NREL is a national laboratory of the U.S. Department of Energy, Office of Energy 

Efficiency & Renewable Energy, operated by the Alliance for Sustainable Energy, LLC.

Contract No. DE-AC36-08GO28308

Kesterites and Chalcopyrites: 

A Comparison of Close Cousins 

Preprint

Ingrid Repins, Nirav Vora, Carolyn Beall, 

Su-Huai Wei, Yanfa Yan, Manuel Romero, 

Glenn Teeter, Hui Du, Bobby To, Matt Young, 

and Rommel Noufi

Presented at the 2011 Materials Research Society Spring Meeting

San Francisco, California

April 25–29, 2011

Conference Paper

NREL/CP-5200-51286

May 2011
NOTICE

The submitted manuscript has been offered by an employee of the Alliance for Sustainable Energy, LLC 

(Alliance), a contractor of the US Government under Contract No. DE-AC36-08GO28308. Accordingly, the US 

Government and Alliance retain a nonexclusive royalty-free license to publish or reproduce the published form of 

this contribution, or allow others to do so, for US Government purposes.

This report was prepared as an account of work sponsored by an agency of the United States government. 

Neither the United States government nor any agency thereof, nor any of their employees, makes any warranty, 

express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of 

any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately 

owned rights. Reference herein to any specific commercial product, process, or service by trade name, 

trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recommendation, 

or favoring by the United States government or any agency thereof. The views and opinions of authors 

expressed herein do not necessarily state or reflect those of the United States government or any agency thereof.

Available electronically at http://www.osti.gov/bridge

Available for a processing fee to U.S. Department of Energy

and its contractors, in paper, from:

U.S. Department of Energy

Office of Scientific and Technical Information

P.O. Box 62

Oak Ridge, TN 37831-0062

phone: 865.576.8401

fax: 865.576.5728

email: mailto:reports@adonis.osti.gov

Available for sale to the public, in paper, from:

U.S. Department of Commerce

National Technical Information Service

5285 Port Royal Road

Springfield, VA 22161

phone: 800.553.6847

fax: 703.605.6900

email: orders@ntis.fedworld.gov

online ordering: http://www.ntis.gov/help/ordermethods.aspx

Cover Photos: (left to right) PIX 16416, PIX 17423, PIX 16560, PIX 17613, PIX 17436, PIX 17721

Printed on paper containing at least 50% wastepaper, including 10% post consumer waste.
1 

Kesterites and Chalcopyrites: A Comparison of Close Cousins

Ingrid Repins, Nirav Vora, Carolyn Beall, Su-Huai Wei, Yanfa Yan, Manuel Romero, Glenn 

Teeter, Hui Du, Bobby To, Matt Young, Rommel Noufi 

National Renewable Energy Laboratory, 1617 Cole Blvd., Golden, CO 80401, U.S.A. 

ABSTRACT

Chalcopyrite solar cells based on CuInSe2 and associated alloys have demonstrated high 

efficiencies, with current annual shipments in the hundreds of megawatts (MW) range and 

increasing. Largely due to concern over possible indium (In) scarcity, a related set of materials, 

the kesterites, which comprise Cu2ZnSnS4 and associated alloys, has received increasing 

attention. Similarities and differences between kesterites and chalcopyrites are discussed as 

drawn from theory, depositions, and materials characterization. In particular, we discuss 

predictions from density functional theory, results from vacuum co-evaporation, and 

characterization via x-ray diffraction, scanning electron microscopy, electron beam-induced 

current, quantum efficiency, secondary ion mass spectroscopy, and luminescence.

INTRODUCTION

Chalcopyrite solar cells have progressed from first proof of concept on melt-grown crystals 

[1] to high-efficiency thin-film laboratory devices [2,3

As production volumes increase, concern has arisen as to whether limits in the world 

supply of In will restrict the amount of chalcopyrite photovoltaics that can be manufactured at 

low cost. While the current cost of In adds only 1 to 10 ¢/Watt to the price of module 

manufacturing [

] to manufacturing presently in the 

hundreds of MW range. Manufacturing volumes are expected to push into the gigawatt (GW) 

range in coming years due to the combination of relatively high efficiency (compared to CdTe or 

α-Si) and potentially low processing costs (compared to single-crystal Si).

4

4

], a tenfold increase in In price would be problematic for the necessary sub-

$1/W manufacturing cost. Estimates of when In scarcity will become important to module prices 

range in CuIn1-x,GaxSe2 (CIGS) manufacturing volumes from 4 GW/yr to over 100 GW/yr 

[ ,5,6,7

To mitigate possible future effects of In scarcity, it has been proposed that CIGS can be 

replaced by Cu2ZnSn(S,Se)4 in the kesterite structure, in which every two group III (In or Ga) 

atoms in chalcopyrite structure are replaced by a Zn (group II) and Sn atom (group IV), thus 

maintaining the octet rule. To date, solar cells with up to 9.7% efficiency [

]. If estimates of 4 GW/yr are correct, In scarcity could impact the industry within 10 

years. If estimates of over 100 GW/yr are correct, and multiple technologies continue to be close 

in cost, In scarcity may not impact the photovoltaic industry. 

8] have been made 

using these In-free kesterite absorbers. In our work with the co-evaporation of chalcopyrites and 

kesterites, we have observed both similarities and differences between absorbers made using 

these closely related crystal structures. Several aspects of this comparison are discussed in the 

sections below. 
2 

PREDICTIONS FROM DENSITY FUNCTIONAL THEORY

Structural, electronic, and defect properties of both kesterites and chalcopyrites have been 

investigated using density functional theory [9

Positions of conduction and valence band edges have been calculated for both kesterites 

and chalcopyrites [

] (DFT). These examinations have revealed both 

similarities and differences between kesterites and chalcopyrites.

10,11,12], as shown in Table I for CuInSe2 (CISe), Cu2ZnSnSe4 (CZTSe), 

CuInS2 (CIS), and Cu2ZnSnS4 (CZTS). In the table, positions of conduction and valence band 

edges are shown relative to vacuum, where the relative band offsets have been calculated from 

DFT, and the experimental value of the electron affinity for CISe, 4.6 eV [13], is taken from

experiment. For the selenides and sulfides, the kesterite band gaps are very similar to (slightly 

less than) the corresponding chalcopyrite band gaps. Furthermore, each material maintains a 

reasonable alignment with the CdS conduction band at -4.3 eV, explaining the success of the 

CdS/i-ZnO/ZnO:Al buffer and window layers in finishing both chalcopyrite and kesterite 

devices. It should be noted that for both the chalcopyrite and the kesterite, other crystal 

structures with the same stoichiometry exist. For I-III-VI2 compounds, a chalcopyrite or CuAulike structure may form. For I2-II-IV-VI4 compounds, a kesterite, stannite, or primitive mixed 

CuAu structure may form. However, DFT calculation predicts that the chalcopyrite and kesterite 

structures are the lowest energy configuration for the materials considered here [14

Table I. Calculated band gaps, valence band edges, and conduction band edges (relative to 

vacuum) for selenide and sulfide chalcopyrites and kesterites.

].

Material Eg (eV) Ev (eV) Ec (eV)

CISe 

(chalcopyrite)

1.04 -5.64 -4.60

CZTSe 

(kesterite)

1.00 -5.56 -4.56

CIS

(chalcopyrite)

1.53 -5.92 -4.39

CZTS 

(kesterite)

1.50 -5.71 -4.21

The stability of single-phase stoichiometric material has been calculated as a function of 

the chemical potential of constituent atoms for both chalcopyrites [15] and kesterites [16

Chemical defects impose further limits on compositions or reaction paths that may yield 

high-efficiency solar cells. Because there is one more element in the kesterite structure than in 

the chalcopyrite structure, there are a larger number of possible intrinsic defects for CZTS and 

related compounds than for CIS. Formation energies and transition energy levels within the gap 

have been calculated for the 13 dominant defects that do not involve the unlikely situation of a 

]. For 

CuInSe2, single-phase material can occur for Cu chemical potentials -0.4 < μCu < 0 eV and the 

corresponding In chemical potential -0.4 < μCu < -1.8 eV. For CZTS, single-phase material 

occurs only at a much smaller chemical potential space around (μCu = -0.20 eV, μZn = -1.23 eV, 

and μSn = -0.50 eV). It is thus predicted that constraints on forming stoichiometric CZTS are 

more strict for kesterites than for chalcopyrites.
3 

cation on an anion lattice site or vice versa [11]. Of these 13 defects, five have relatively low 

formation energy for Cu-poor kesterite. These five low-formation-energy defects are, in order of 

increasing formation energy: copper on zinc site (denoted CuZn), copper vacancy (denoted VCu), 

ZnSn, VZn, and CuSn. The formation energy of CuZn is significantly lower than that of VCu and 

ZnSn, predicting that CuZn should be the dominant intrinsic defect for CZTS. The predicted 

abundance of CuZn is a different situation than for the chalcopyrite, where VCu is the lowest 

formation energy defect [15] and is responsible for the p-type doping. The CuZn acceptor level is 

predicted to lie about 0.1 eV above the valence band maximum (VBM), as opposed to the 

shallower acceptor VCu (0.02 or 0.03 eV above the VBM in the kesterite or chalcopyrite, 

respectively). Since kT ≈ 0.025 eV at room temperature, the fraction of the dominant acceptors 

ionized may be significantly different in the kesterite than in the chalcopyrite. For both the 

chalcopyrite [17] and the kesterite [11], n-type doping is predicted to be unlikely due to the high 

formation energy of the donor defects and low formation energy of acceptor defects. 

For both chalcopyrites and kesterites, low-formation-energy defects with energy levels 

deep within the gap exist, but may be largely benign due to the formation of defect complexes. 

For CISe, the most important defect for carrier recombination is predicted to be InCu due to both 

its low formation energy and its predicted level 0.34 eV below the CBM [15]. However, the 

electronic activity of this defect is to some extent passivated by the formation of [2VCu + InCu] 

defect complexes [15]. Of the five low-formation-energy defects previously mentioned for 

CZTS, CuSn is expected to be the most active recombination center, with an energy level close to 

mid-gap, ~0.6 eV above the VBM [11]. As is the case for CISe, DFT calculations predict that 

charge-compensated defect complexes such as [CuSn + SnCu] are likely to form and passivate 

deep levels in the kesterites[11]. The dominant defect complex in CZTS is [CuZn +ZnCu]. 

However, unlike in CISe, where the formation of [2VCu + InCu] defect complexes leads to 

charge-separated α and β phases [17], the formation of [CuZn +ZnCu] does not have this possibly 

beneficial effect observed in chalcopyrite compounds. 

The effects of alloying CZTS and CZTSe have also been investigated using DFT 

calculations [18

FILM DEPOSITION

]. It is shown that the enthalpy of formation for the CZT(S1-xSex) is very small, 

indicating that the mixed-anion alloys are highly miscible, and that the cations maintain the same 

ordering preferences as in pure kesterites. The band gaps of the random alloy decrease with Se 

content almost linearly. The conduction band down-shift contributes more to the gap decrease 

than the valence band up-shift. The shift of the band edges makes the CZT(S1-xSex) alloys with 

high Se concentration easier to be doped both n-type and p-type. The balance between the band 

gap size and the doping ability may therefore be important in determining the optimal alloy 

composition to achieve high-efficiency CZT(S1-xSex)-based solar cells.

Co-evaporation of CIGS films at the National Renewable Energy Laboratory (NREL) 

and their incorporation into device structure has been previously discussed in detail [19]. To 

make kesterite films and devices, identical procedures were followed, except that In and Ga shot 

were replaced in the evaporator by Zn and Sn shot, and appropriate optical filters (202.55 nm for 

Zn and 284.0 nm for Sn) were substituted for the Ga and In filters in the electron impact 

emission spectrometer rate monitor. For devices discussed in this study, anti-reflective coating 

was not applied. 
4 

The two types of CZTSe recipes used for films in this study are shown in Figure 1. In all 

depositions, rates and temperatures were stabilized, then a substrate shutter was opened at time = 

0. Substrate temperature was held constant in the range of 470-500°C throughout the 

deposition. Temperature variations within this range are noted in the text when discussing 

different samples. Deposition temperatures are relatively low, due to decomposition of the 

kesterite at temperatures typically used during CIGS formation [20]. Depositions lasted about 20 

minutes. In all cases, Se and Sn were supplied in overpressure throughout the deposition, similar 

to the conditions for Se flux during CIGS deposition. Cu and Zn fluxes were supplied in two 

variations. In the first variation, shown in Figure 1a, Cu and Zn fluxes are constant throughout 

the deposition period. Thus, the Cu/Zn and Cu/(Zn+Sn) atomic ratios in the film are constant 

with time. This type of deposition is analogous to “one-stage” depositions used for CIGS. In the 

second type of recipe, shown in Figure 1b, the Cu rate is increased so that Cu/Zn and 

Cu/(Zn+Sn) can be greater than one during most of the deposition, and yet the final film can be 

Cu-poor. We observe during such depositions that the Cu-rich or Cu-poor character of the film 

is indicated by emissivity signature [21], in much the same way as for CIGS films, presumably 

due to the effects of excess copper selenide.

Figure 1. Schematic representation of recipes used for samples in this study. For CIGS, these 

types of recipes are commonly called a) “one-stage” and b) “two-stage.” 

a) 

b) 
5 

A summary of the composition measurements for the samples described in this paper is 

shown in Table II. Compositions are shown in terms of atomic ratio. Film compositions were 

measured by x-ray fluorescence (XRF) calibrated against inductively coupled plasma optical 

emission spectrometry (ICP-OES). The table also shows the recipe type for each sample (either 

one-stage or two-stage) and the figure in which data for each sample is presented. X-ray 

diffraction, Raman spectroscopy, energy dispersive spectroscopy, and resistance measurements 

were used to confirm that the films in this paper are kesterites, with the occasional ZnSe 

precipitate in Zn-rich samples. The results and application of these phase identification 

techniques to co-evaporated CZTSe will be discussed in a separate publication [22

Table II. Summary of compositions and recipe types for films used in this study. 

].

Sample Zn/Sn Cu/(Zn+Sn) Cu/Sn Recipe type Figure in which sample is 

characterized

M3214 1.2 1.0 2.15 One-stage Figure 3a

M3250 1.35 0.8 1.9 One-stage Figure 3b

M3253 1.0 0.85 1.7 Two-stage Figure 2a

M3251 1.2 0.95 2.0 Two-stage Figure 2b

M3272 1.1 0.95 2.0 Two-stage Figure 2c

M3268 1.2 0.85 1.8 Two-stage Figure 2d

M3257 1.05 0.9 1.9 Two-stage Figure 4

M3215 0.75 1.2 2.1 One-stage Figure 5

M3244 1.1 0.9 1.9 Two-stage Figure 6, Figure 8

MORPHOLOGY

Parallels are observed in the morphology versus vacuum growth conditions of 

chalcopyrites and kesterites. These similarities include increasing grain size with both 

temperature and occurrence of a Cu-rich growth period.

Grain size in CIGS films increases with temperature [23

23

]. CZTSe films in this study have 

shown a similar trend. Figure 2 shows scanning electron micrograph (SEM) images of CZTSe 

films grown under nearly identical conditions, except for increasing substrate temperature. The 

compositions for these films, as measured by x-ray fluorescence (XRF), are also listed in Table I. 

Feature size in the SEM images increases with substrate temperature. Note that the temperature 

range in Figure 2 is considerably narrower than that which has been examined for CIGS [ ]. 

The films of Figure 2b and Figure 2d, with the highest Zn/Sn ratios in the sample set, begin to 

show segregation of ZnSe, which is evident as the bright spots near the Mo in the SEM images. 

Energy dispersive spectroscopy (EDS), used to examine these bright spots while the films are 

mounted in the electron microscope, confirms that the bright spots are mostly Zn and Se. For the 

films in this study, the increase in grain size with temperature did not yield an increase in device 

performance.
6 

Figure 2. SEM cross-sections of CZTSe films grown under nearly identical conditions, except 

for varying substrate temperatures of a) 468°C, b) 486°C, c) 492°C, or d) 502°C.

Chalcopyrite films have been documented to exhibit increased grain sizes when a period of 

Cu-rich (i.e., Cu/(In+Ga) >1) growth is included in the deposition [24,25,26]. This improved 

grain growth has been associated with a flux recrystallization of chalcopyrite grains by a liquid 

CuxSe phase. In the present work, kesterite films have been observed to exhibit a similar 

dependence on Cu-rich vs. Cu-poor growth. Figure 3 shows two kesterite films made under 

nearly identical conditions. For both, the substrate temperature was about 490°C, and a onestage recipe was utilized. However, for the film in Figure 3a, Cu rate was chosen such that the 

Cu/(Zn+Sn) ratio just exceeds one. For the film in Figure 3b, Cu rate was chosen such that the 

Cu/(Zn+Sn) ratio was always below one. The film grown under Cu-rich conditions exhibits 

considerably larger grain size. Other researchers have also observed large grain sizes under Curich growth conditions [27,28

The relationship between Cu ratio and grain size for CZTSe films is likely multi-variate 

and should be investigated more fully. It has been shown for CIGS that the amount of benefit 

obtained via a Cu-rich growth period is a function of deposition temperature [

]. For the film in Figure 3a, a sodium cyanide etch was used to 

remove excess copper selenide prior to acquiring the SEM image. 

23]. Furthermore, 

a) T = 468°C b) T = 486°C

c) T = 492°C d) T = 502°C
7 

a marked dependence of CZTSe grain size on temperature is demonstrated in Figure 3. 

Variations in composition other than Cu excess may also have an effect on morphology.

Figure 3. SEM cross-section of CZTSe films grown at the same temperature and constant 

deposition fluxes, except Cu rates were adjusted to produce only a) Cu-rich growth or b) Cu-poor 

growth.

COMPOSITION PROFILES

Figure 4 shows Auger emission spectroscopy (AES) profiling of a CZTSe film that yielded 

4.4% devices. In Figure 4a, the entire film thickness (1.5 μm) is shown. Composition is fairly 

constant throughout the bulk. Figure 4b expands the near-surface region of the same data, 

plotted this time in terms of atomic ratio, in order to examine trends in the constituent elements 

without seeing effects from several atomic percent oxygen or carbon at the surface. It is evident 

in Figure 4b that, while the amount of Zn and Sn in the film is constant, the amount of Cu drops 

considerably near the surface. The two minutes of sputtering time over which this drop occurs 

corresponds to roughly 70 nm. Cu-poor surfaces are also observed in CIGS [29].

Figure 4. AES profile of a) entire film and b) near-surface region.

a) b) 

a) b)
8 

Secondary ion mass spectrometry (SIMS) profiling was used to compare the amount of Na 

in standard CIGS with that in the CZTSe films. Na is known to be important in CIGS devices 

for increasing free carrier density and decreasing recombination [30,31]. Positive secondary 

data are shown compared for CIGS (thin dashed lines) and CZTSe (thick solid lines) in Figure 5. 

For both films, Se makes up ~50% of the lattice, and Se counts are normalized to 103

 to allow for 

comparison of amounts of other elements. The CZTSe film is only half the thickness of the 

CIGS film, so the SIMS data is horizontally offset so that the back contact (i.e., the area of Mo 

signal increase) lines up for both films. The CZTSe film contains much less Na than the CIGS, 

as might be expected from the necessarily lower deposition temperature, since diffusion through 

the Mo is the main barrier to Na inclusion in CIGS [32]. Determining from device results 

whether the optimum amount of Na in CZTSe is the same as that in CIGS should be an area of 

future examination. 

Figure 5. SIMS data comparing CIGS (thin dashed lines) and CZTSe (thick solid lines).

ELECTRO-OPTICAL CHARACTERISTICS

Both CIGS and CZTSe samples have been mapped using electron beam-induced current 

(EBIC) and cathodoluminescence (CL). The methodology and apparatus used for luminescence 

microscopy has been described in earlier publications [33

To examine the effect of morphology on carrier collection, cross-sectional SEM images 

and cross-sectional electron beam induced current (EBIC) maps were compared between CIGS 

and CZTSe. Figures 6a and b show typical SEM and EBIC images for a ~16%-efficient CIGS 

device. The dotted lines in Figures 6a and b are benchmarks to indicate the same physical 

location on the sample in each image. Also shown in Figure 6b is a linescan, i.e., an x-y 

representation of the image brightness at the location where the linescan is drawn. The salient

feature of Figures 6a and b is that the response is basically constant as the electron beam moves 

away from the junction, although grain boundaries and an occasional grain show reduced 

response. Figures 6b and c show the same type of characterization for a 4.1% CZTSe device. 

Carriers are collected only within tenths of a micron of the junction, and this response length is 

nonuniform over the sample. The data in Figure 6 imply that improving collection length is 

extremely important in early work in improving CZTSe devices. 

].
9 

This conclusion is consistent with quantum efficiency (QE) data taken on progressively 

improving devices in this study. The QE data are shown in Figure 7 and show increasing red 

collection with improving efficiency. A 20.0%-efficient CIGS device is also shown in Figure 7 

for comparison. The minimal red loss (i.e. flat QE from ~700-900 nm) is apparent in the CIGS 

device, in strong contrast with the kesterite devices shown. Also evident is the difference in 

band gap between the CIGS and CZTSe (wavelength at which QE approaches 0) and the lack of 

anti-reflective coating on the CZTSe devices (~5% reduction in maximum QE). 

CL images of the surface of CZTSe films were also compared with those from CIGS. 

Several aspects of this comparison are consistent with the high recombination center density 

indicated by the EBIC and the QE. First, the intensity of the luminescence is about two orders of 

magnitude lower for the CZTSe than for the CIGS. Second, the materials show a different 

behavior in emitted photon energy with increasing CL current. This comparison is shown in 

Figure 8. CL as a function of excitation current for CIGS of 1.15 eV band gap is shown in figure 

8a, and that for CZTSe is shown in Figure 8b. Figure 8b was taken on the same material used to 

make the 4.1% device of Figure 6. For both CZTS and CIGSe, the spectrum consists of a 

broadband luminescence associated with transitions between donor and acceptor defect bands. 

The electronic states participating in the formation of these bands are deeper for CZTSe (~0.2 eV 

below the band gap) than CIGS (~0.1 eV below the band gap). The spectrum shows a significant 

blueshift with the external excitation (measured by the electron beam current) for both 

chalcopyrites and kesterites. This blueshift is due to the filling of the bands while increasing the 

excitation. We observe that (i) the blueshift is much more pronounced for CIGS than CZTSe, 

and (ii) there is a saturation of the bands at high excitation in CIGS (confirmed by the change in 

the shape of the peak with the e-beam current for CIGS), which is not observed in CZTSe. Thus, 

not only are the dominant intrinsic defects leading to the formation of these bands deeper in 

energy for CZTSe when compared to CIGS but, based on these measurements, the density of 

these defects is much higher in CZTSe.

Figure 6. a) SEM and b) EBIC cross-sections for CIGS; and c) SEM and d) EBIC cross-sections 

for CZTSe.

a) b) c) d)
10 

Figure 7. QE for CZTSe devices of various efficiencies.

Figure 8. CL spectrum as a function of CL current for a) CIGS and b) CZTSe. 

CONCLUSIONS

A comparison of chalcopyrites and kesterites yields both significant similarities and 

differences. Theory predicts similar band edge energies, p-type doping, formation of benign 

defect complexes, and miscibility of alloys. However, theory also predicts differences: a 

smaller single-phase composition space and a different dominant intrinsic defect. During 

vacuum co-evaporation, the chalcopyrites and kesterites can be made using nearly identical 

equipment, recipes, and endpoint detection. However, co-evaporation of the kesterites requires 

a) b)
11 

Sn (not just Se) overpressure and lower temperatures. Kesterites and chalcopyrites both show an 

increase in grain size with temperature and with a Cu-rich growth period. A Cu-poor surface is 

observed in both types of film, although the universality of this observation has not been tested 

for the kesterites. The kesterite deposition recipes used in this study result in much less Na in the 

absorber than in optimized CIGS films, as might be expected from the necessarily lower 

deposition temperature. The kesterite films in this study show evidence of short diffusion 

lengths, in sharp contrast with good CIGS devices. This evidence of increased activity of 

recombination centers is evident in cross-sectional EBIC, QE, and CL. 

ACKNOWLEDGMENTS

This work was supported by the U.S. Department of Energy under Contract No. DE-AC36-

08-GO28308 with the National Renewable Energy Laboratory. The authors thank Jeffrey 

Blackburn and Jian V. Li for helpful discussions. 

REFERENCES

 

1. S. Wagner, J. L. Shay, and P. Migliorato, Appl. Phys. Lett. 25(8), 434-435 (1974). 

2. P. Jackson, D. Hariskos, E. Lotter, S. Paetel, R. Wuerz, R. Menner, W. Wischmann, M. 

Powalla, Prog. Photovoltaics, DOI: 10.1002 (2011). 

3. I. Repins, M.A. Contreras, B. Egaas, C. DeHart, J. Scharf, C.L. Perkins, B. To, and R. Noufi, 

Prog. Photovoltaics 16, 235–239 (2008).

4. V. Fthenakis, Renew. Sust. Energ. Rev. 13, 2746–2750 (2009). 

5. T. Sullivan, W.H. Kuo, P. Karayan, M. LoCascio, M. Holman, S. Udupa, B. Hilman, A. Stuk, 

“Solar State of the Market Q3 2008: The Rocky Road to $100 Billion,” LRSI-SMR-08-02, (Lux 

Research, 2008). 

6. A. Feltrin, A. Freundlich, Renew. Energ. 33, 180–185 (2008). 

7. B.A. Andersson, Prog. Photovoltaics 8, 61-76 (2000). 

8. T. K. Todorov, K. B. Reuter, D. B. Mitzi, Adv. Mater. 22, 1-4 (2010). 

9. G. Kresse, J. Furthmuller, Comp. Mater. Sci. 6(1), 15-50 (1996).

10. S. Chen, X.G. Gong, A. Walsh, S.H. Wei, Appl. Phys. Lett. 94, 041903 (2009). 

11. S. Chen, J.H. Yang, X.G. Gong, A. Walsh, S.H. Wei, Phys. Rev. B 81, 245204 (2010). 

12. S.H. Wei, A. Zunger, J. Appl. Phys. (6), 3846-3856 (1995). 

13. N. Romeo, Jpn. J. Appl. Phys. 19-3, 5-13, (1980). 

14. D.S. Su, S.H. Wei, Appl. Phys. Lett. 74(17), 2483-2485 (1999). 

15. S. B. Zhang, S.H. Wei, A. Zunger, H. K. Yoshida, Phys. Rev. B 57(16), 9642-9656 (1998). 

16. S. Chen, X.G. Gong, A. Walsh, S.H. Wei, Appl. Phys. Lett. 96, 021902 (2010). 

17. S.H. Wei, B. Zhang , J. Phys. Chem. Solids 66, 1994 (2005). 

18. S. Chen, A. Walsh, J.-H. Yang, X.-G. Gong, L. Sun, P.-X. Yang, J.-H. Chu, and S.-H. Wei, 

Phys. Rev. B 83, 125201 (2011). 

19. A.M. Gabor, “The conversion of (In,Ga)2Se3 thin films to Cu(In,Ga)Se2 for application to 

photovoltaic solar cells,” PhD thesis, (University of Colorado, 1995), pp. 42-49. 

20. A. Weber, R. Mainz, H.W. Schock, J. Appl. Phys. 107, 013516 (2010). 

21. J. Kessler, J. Scholdstrom, L. Stolt, IEEE Phot. Spec. Conf. 28, 509-512(2000). 
12 

 

22. N. Vora et al, in progress. 

23. W.N. Shafarman, J. Zhu, Mater. Res. Soc. Symp. P. 668, H2.3.2-H2.3.6 (2001). 

24. A.M. Gabor, J.R. Tuttle, D.S. Albin, M.A. Contreras, R. Noufi, Appl. Phys. Lett. 65 (2), 198-

200 (1994). 

25. R.A. Mickelsen, W.S. Chen, Y.R. Hsiao, V.E. Lowe, IEEE Transactions on Electron 

Devices ED-31 (5), 542-546 (1984). 

26. J.R. Tuttle, M.A. Contreras, M.H. Bode, D. Niles, D.S. Albin, R. Matson, A.M. Gabor, A. 

Tennant, A. Duda, R. Noufi, J. Appl. Phys. 77 (1), 153-161 (1995). 

27. B.A. Schubert, B. Marsen, S. Cinque, T. Unold, R. Klenk, S. Schorr, H.W. Schock, Prog. 

Photovoltaics 19 (1), 93-96 (2011). 

28. T. Tanaka, A. Yoshida, D. Saiki, K. Saito, Q. Guo, M. Nishio, T. Yamaguchi, Thin Solid 

Films 518, S29-S33 (2010). 

29. S.H. Han, F.S. Hasoon, A.M. Hermann, D.H. Levi, Appl. Phys. Lett. 91, 021904 (2007). 

30. J.E. Granata, J.R. Sites, World Conf. Photo. Sol. Energ. Conv. 2, 604-607 (1998). 

31. D. Rudmann, “Effects of sodium on growth and properties of Cu(In,Ga)Se2 thin films and 

solar cells,” PhD thesis, (Swiss Federal Institute of Technology, 2004). 

32. M.B. Zellner, R.W. Birkmire, E. Eser, W.N. Shafarman, J.G. Chen, Prog. Photovoltaics 11, 

543-548 (2003). 

33. M.J. Romero, M.A. Contreras, I. Repins, C.S. Jiang, M.M Al-Jassim, Mater. Res. Soc. Symp. 

P. 1165, 419-424 (2010).