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Contract No. DE-AC36-99-GO10337 

C.L. Perkins, F.S. Hasoon, H.A. Al-Thani, 

S.E. Asher, and P. Sheldon 

Prepared for the 31st IEEE Photovoltaics Specialists 

Conference and Exhibition 

Lake Buena Vista, Florida 

January 3–7, 2005

February 2005 • NREL/CP-520-37419 

XPS and UPS Investigation of 

NH4OH-Exposed Cu(In,Ga)Se2

Thin Films 
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XPS AND UPS INVESTIGATION OF NH4OH-EXPOSED Cu(In,Ga)Se2 THIN FILMS 

Craig L. Perkins, Falah S. Hasoon, Hamda A. Al-Thani, Sally E. Asher, and Pete Sheldon 

National Renewable Energy Laboratory, 1617 Cole Blvd., MS/3215, Golden, CO, USA 

ABSTRACT

Photoelectron spectroscopy was used to 

determine the compositional and electronic changes 

occurring in Cu(In,Ga)Se2 thin films as a result of 

immersion in aqueous ammonia solution. We find that 

NH4OH-treated CIGS surfaces are preferentially etched of 

indium and gallium, resulting in the formation of a thin 

layer of a degenerate Cu-Se compound that we tentatively

identify as Cu2Se. The work function of ammonia-treated 

samples is found to increase by 0.6 eV relative to asgrown CIGS thin films. The uniformity of chemical bath 

effects (etching & deposition) was found to be improved by

the addition to the bath of a non-ionic surfactant. Initial 

device results show that the new surfactant-based

chemical bath deposition (CBD) method may lead to better 

and thinner CdS buffer layers.

INTRODUCTION 

To date, the most efficient thin film Cu(In,Ga)Se2

(CIGS) based solar cells use an n-type “buffer layer” of a 

sulfide, usually CdS, which is deposited on the p-type 

absorber via an aqueous chemical bath. A typical CBD of 

CdS uses NH3(aq), (usually described as a cation 

complexing agent) CdSO4 as the source for Cd, and 

thiourea to provide S2-. Although NH3 is known to play an

important role in controlling the formation of CdO, other 

effects attributed to the presence of this molecule include 

removal of Cu from the CIGS lattice and an increase in the 

rate of thiourea hydrolysis. Generally, the complex 

chemical reactions occurring both on the surface of the 

CIGS absorber and in solution are not well understood 

even though they can have large effects on the 

performance of finished devices. Even the basic question

of whether CIGS/CdS devices are generally best 

represented as homojunctions or heterojunctions has not 

been settled. EBIC data indicate abrupt heterojunctions in 

these structures[1], yet SIMS and XPS and AES sputter 

profiling show diffusion of Cd deep into the CIGS 

absorber[2]. Recent scanning capacitance microscopy

data[3] indicates a buried homojunction 80-90 nm from the 

CdS-CIGS boundary, but XPS data from Rockett et al. 

indicate that Cd was constrained to the first few atomic 

layers of a CIS single crystal film that had been exposed 

to Cd electrolyte [4]. In this paper we specifically address 

how ammonia and water affect the composition and 

electronic structure of the CIGS surface, and report initial 

results on a modification to water-based processes on 

CIGS that may lead to improved sulfide buffer layers. 

EXPERIMENTAL

The CIGS thin films were grown by a three-stage 

process on Mo-coated soda-lime glass substrates and 

were stored in air for several days prior to the experiments 

described here. Surface analytical experiments were

conducted in the NREL Surface Analysis Team’s 

homemade cluster tool. The apparatus will be described

in detail elsewhere [5], but briefly, the in-line cluster tool 

consists of a nitrogen-purged glove box, deposition

chamber, photoemission system, and a field emission

scanning Auger spectrometer, all interfaced to each other 

via a 24’ long UHV transfer system. Photoemission data 

was obtained on a modified PHI 5600 XPS system. The 

binding energy scale of the photoemission system was 

calibrated according to the method of Powell[6] using the 

sharp Fermi edge observed in ultraviolet photoelectron 

spectra (UPS) of clean gold foil and with x-ray 

photoelectron spectroscopy (XPS) data covering the Au 

4f7/2 and Cu 2p3/2 core levels. Aqueous chemical 

treatments reported here were conducted within the 

system’s glove box, and were set up to emulate conditions 

that a CIGS surface would experience during growth of a 

typical CdS buffer layer. Aqueous reagents were purged 

with flowing N2 prior to their insertion in the glove-box in 

an attempt to minimize effects of dissolved gases. Room 

temperature DI water and aqueous ammonia were added 

(1.8 M) to a stirred, double-walled beaker heated by

flowing 65 °C water, followed immediately by the addition 

of the CIGS films. Films were treated as described, blown 

dry with N2, and moved without air exposure into the UHV 

transfer system. Carbon content of the films was not 

quantified due to spectral interference between the C 1s 

photoelectron line and Ga and Se Auger transitions. 

 

RESULTS AND DISCUSSION 

Figure 1 shows the evolution of surface 

composition as a function of film preparation. The shortest 

air exposure time is a conservative estimate of oxygen 

exposure within the cluster tool glove-box and 

corresponds to a film treated in the ammonia-only CBD 

bath and transferred directly into the UHV analysis 

system. The longest air exposure corresponds to the asgrown film which sat in the lab air for about five months. 

Air exposure times of 5, 60, and 1400 minutes were 

performed on ammonia-exposed CIGS. The data for this 

film show a trend observed in many different batches of 

CIGS of different composition: hot aqueous ammonia 

removes compounds of Na and O, as well as causes an 

increase in the relative concentration of Cu, similar to 

1
results on CuInSe2 reported previously [7, 8]. This trend is 

more clearly observed in Table 1 and on the basis of the 

main constituents of the film, Cu, In, Ga, and Se. The data 

in Table 1 show the compositional changes as a result of 

the ammonia-only CBD bath on CIGS film M2737 starting

from four different surface preparation conditions, asgrown, sputter-cleaned, Br2-methanol etched, and a wateronly chemical bath. It can be seen that the Group III 

elements, Ga in particular, are preferentially etched from 

the surface. In the case of the as-grown film, it is certain 

that some of the changes in stoichiometry are due to the 

removal of oxides that have Cu, In, Ga, and Se 

concentrations different from the bulk of the CIGS 

absorber. Of interest is that even in the case of the 

sputter-cleaned, oxide-free surface, gallium is 

preferentially removed from the surface. A similar result is

seen for the clean Br2-methanol etched surface. These 

two results demonstrate that removal of surface oxides 

cannot explain all of the chemistry seen to occur between 

Cu(In,Ga)Se2 and NH4OH. The implications for CIGS 

device finishing are that indium and gallium vacancies on

the surface are available for substitution by Cd or Zn, a 

doping situation that is at odds with the generally accepted 

picture of Cd substituting for Cu, but is consistent with 

XANES data unambiguously showing that at least some 

VIn and VGa are populated by Cd atoms[9]. Another 

important point can be seen in the last three rows of Table 

1. Water alone is seen to bring about the majority of the 

changes that have previously been attributed to NH3[7, 8]. 

Ammonia was found to effect important changes in the 

relative position of the valence band maximum and the 

Fermi energy, as seen in Figure 2. 

Figure 1. Effects of air and NH4OH exposure on 

Cu(In,Ga)Se2 composition 

Figure 2 shows UPS data taken with 21.2 eV He I 

excitation in the vicinity of the Fermi level of films that 

have undergone various treatments. Films having a native 

oxide were exposed to the NH4OH bath for 15’, a typical 

time for completion of a CdS buffer layer, and were then 

studied with UPS. After ammonia etching, some films 

were removed from the analysis chamber, exposed to air, 

and again studied with UPS and XPS. UPS spectra of the 

NH4OH-treated CIGS films (0’ air) show distinct features at 

1.0 and 2.9 eV below the Fermi level. These spectral 

features were observed in all ammonia-treated films 

studied, including CuGaSe2, and most importantly, CIGS 

that had been initially heavily etched with Br2/methanol 

and subsequently exposed to NH4OH. The fact that the 

spectral features were not observed for films etched only

Br2/methanol, sputtered, or mechanically cleaned, yet 

appear in CIGS films that have undergone both 

Br2/methanol etching and NH4OH-exposure strongly 

implies that the spectral features are the signature of a 

Figure 2. UPS data near EF on Cu(In,Ga)Se2

films with different surface preparations

Table 1. NH4OH effects on CIGS m2737 

Surface Atomic Concentration, (%)

Condition Cu In Ga Se

as-grown 15.1 2.0 8.2 74.7 

NH4OH 39.0 1.9 3.1 56.0 

% change +160 -5 -62 -25 

sputter-cleaned 18.4 22.1 4.1 55.4 

NH4OH 18.0 20.3 0.7 61.1 

% change -2 -8 -83 +10 

Br2/methanol 8.0 19.5 5.0 67.5 

NH4OH 23.2 12.0 1.1 63.6 

% change +190 -38 -78 -6

water 25.7 2.9 1.6 69.8 

NH4OH 27.7 3.2 2.5 66.5 

% change +8 +9 +60 -5

2
specific surface structure that results from NH4OHtreatment. Theoretical [10] and experimental [11] 

investigations of chalcopyrite valence band density of 

state structure have attributed these two spectral features 

to Cu 3d and Se 4p hybridized bonding. Photoemission 

data taken on well-characterized Cu2Se thin films showed 

similar features[12], including the degenerate p-type

surface that is implied by the coincidence of EF and the 

valence band maximum (VBM) of the NH4OH-etched 

samples. The position of the VBM, the presence of the 

1.0 and 2.9 eV features, and the large increase in the Cu 

concentration all point towards the generation of a thin 

layer of a Cu2Se compound on the surface as a result of 

hot NH4OH treatment of Cu(In,Ga)Se2. 

Figure 3 shows how the work function of 

Cu(In,Ga)Se2 changes as a result of exposure to the 

NH4OH bath. The general trend for this and other films is 

that the work function was increased by the bath, as one 

would expect with the removal of alkali metals and other 

contaminants. Curiously, exposure to air of ammoniaetched samples for different lengths of time did not result 

in linear changes to the work function values. It should be

noted here that work function values determined via 

photoemission generally are determined by the region 

within the analysis area having the lowest work function, 

unlike a Kelvin probe which spatially averages a 

heterogeneous surface, and that the experiments detailed 

in this paper necessarily involved sample movements that 

may have resulted in an analysis being conducted in a 

slightly different area than desired. During the course of 

experiments with aqueous reagents on Cu(In,Ga)Se2, it 

was observed that the heated solutions evolved dissolved 

gases. It was postulated that bubbles adhering to the 

absorber surface inhibited both the effects of the ammonia 

as well as the deposition of CdS, that this was contributing 

to the non-uniformity of work function values, and that a 

wetting agent , or surfactant might increase the uniformity

of both ammonia-etching as well as the deposition of CdS. 

Figure 4. Formation of adherent bubbles during 

standard CdS CBD 

Figure 3. Secondary electron cutoffs of UPS data

from CIGS m2737 after different chemical 

treatments 

Figures 4-5 are photos showing the effects of the 

addition (0.7 mM) of Triton X-100, a common non-ionic 

surfactant, to the standard NREL CdS chemical bath. It 

can be seen that the number of large bubbles adhering to 

the absorber surface is greatly reduced by the surfactant. 

Smaller, non-adherent bubbles were seen to sheet up the 

absorber surface during CdS deposition, a process that 

should minimize spots of leftover Na compounds and 

pinholes in the CdS layer. Initial scanning Auger and XPS 

compositional results indicate that the CdS purity and 

deposition rate are not affected by the surfactant. To test 

the idea that the surfactant could improve the quality of 

Figure 5. Elimination of adherent bubbles upon 

addition of surfactant to bath

3
thin CdS buffer layers, CIGS/CdS devices were References

constructed using CdS thin enough such that the effects of 

pinholes and poor CdS coverage (i.e. a low Voc) could be 

observed. Another set of cells was made with surfactant 

added to the bath. Table 2 shows that the surfactantmodified CBD method led to a large (23%) increase in the 

overall efficiency of the devices, almost all of that coming 

from the increased Voc. 

[1] M. J. Romero, M. M. Al-Jassim, R. G. Dhere, F. S.

Hasoon, M. A. Contreras, T. A. Gessert, and H. R. 

Moutinho, "Beam injection methods for characterizing thinfilm solar cells," Progress in Photovoltaics, vol. 10, pp. 

445-455, 2002.

[2] C. Heske, D. Eich, R. Fink, E. Umbach, T. van Buuren,

C. Bostedt, L. J. Terminello, S. Kakar, M. M. Grush, T. A. 

Callcott, F. J. Himpsel, D. L. Ederer, R. C. C. Perera, W. 

Riedl, and F. Karg, "Observation of intermixing at the 

buried CdS/Cu(In,Ga)Se2 thin film solar cell 

heterojunction," Applied Physics Letters, vol. 74, pp. 1451-

1453, 1999. 

Table 2. Comparison of device performance using 

standard CBD (cells 1-5) and using surfactantmodified CBD (cells 6-10) 

Cell Voc

(V) 

Jsc 

(mA/cm-2) 

Fill 

Factor

Efficiency

1 0.460 -29.9 63.69 8.75

2 0.450 -30.73 63.35 8.767 

3 0.461 -32.26 63.79 9.479 

4 0.469 -32.56 64.32 9.816 

5 0.480 -32.41 67.02 10.416 

Avg. 0.464 -31.57 64.43 9.446 

6 0.187 -27.66 24.97 1.292 

7 0.612 -29.78 66.16 12.054 

8 0.577 -31.46 67.82 12.301 

9 0.533 -31.67 68.05 11.482 

10 0.505 -31.79 67.38 10.825 

Avg.* 0.557 -31.18 67.35 11.666 

*Average without bad cell #6

[3] C. S. Jiang, F. S. Hasoon, H. R. Moutinho, H. A. AlThani, M. J. Romero, and M. M. Al-Jassim, "Direct 

evidence of a buried homojunction in Cu(In,Ga)Se2 solar 

cells," Applied Physics Letters, vol. 82, pp. 127-129, 2003. 

[4] D. X. Liao and A. Rockett, "Cd doping at the 

CuInSe2/CdS heterojunction," Journal of Applied Physics, 

vol. 93, pp. 9380-9382, 2003.

[5] G. Teeter, C. L. Perkins, S. E. Asher, M. Young, and 

P. Sheldon, To be published. 

[6] C. J. Powell, "Energy Calibration of X-Ray 

Photoelectron Spectrometers - Results of an 

Interlaboratory Comparison to Evaluate a Proposed

Calibration Procedure," Surface and Interface Analysis, 

vol. 23, pp. 121-132, 1995. 

[7] R. S. Hunger, T.; Lebedev, M.; Klein, A.; Jaegermann, 

W.; Kniese, R.; Powalla, M.; Sakurai, K.; Niki, S., 

"Removal of the surface inversion of CuInSe2 absorbers 

by NH3(aq) etching," presented at Proceedings of 3rd World 

Conference on Photovoltaic Energy Conversion, Osaka, 

Japan, 2003. 

SUMMARY 

XPS data show that Group III elements are 

preferentially removed from CIGS surfaces as a result of 

NH4OH and that water is responsible for many but not all 

of the compositional changes. UPS shows a VBM within a 

few kT of the Fermi level, indicating that freshly NH4OHetched CIGS surfaces are degenerate and p-type. These 

data are consistent with the formation of a Cu-Se 

compound that we tentatively identify as Cu2Se. The 

thickness of this layer is < 30 Å based on the fact that In 

and Ga core-level signals from the underlying substrate 

are still observable. The Cu-Se layer produced by

NH4OH-etching has a distinctive feature in He I ultraviolet 

photoelectron spectroscopy (UPS) data at 1.0 and 2.9 eV 

that disappears with brief air exposure. Work functions 

determined by extrapolating the secondary electron cutoff 

in UPS spectra to the energy axis show that the work 

function of ammonia-treated samples increases by 0.6 eV 

relative to as-received CIGS thin films covered with a 

native oxide. Lateral uniformity of chemical bath effects 

was found to be affected by the formation of bubbles 

adhering to the absorber surface. Addition of a common 

non-ionic surfactant, Triton X-100, was found to aid the 

wetting of CIGS surfaces during aqueous chemical 

treatments and eliminate adhering bubbles. Initial results 

obtained using the new surfactant-modified CBD method 

show that a surfactant can greatly improve the 

performance of devices made with thin CdS buffer layers. 

[8] K. O. V. J. Kessler, M. Ruckh, R. Laichinger, H.W. 

Schock, D. Lincot, R. Ortega, J. Vedel, "Chemical bath 

deposition of CdS on CuInSe2, etching effects, and growth 

kinetics," presented at 6th International Photovoltaics 

Science and Engineering Conference, New Delhi, India, 

1992. 

[9] P. Fons, K. Sakurai, A. Yamada, K. Matsubara, K. 

Iwata, T. Baba, Y. Kimura, H. Nakanishi, and S. Niki, "The 

chemical environment about Cd atoms in Cd chemical 

bath treated CuInSe2 and CuGaSe2," Journal of Physics 

and Chemistry of Solids, vol. 64, pp. 1733-1735, 2003. 

[10] J. E. Jaffe and A. Zunger, "Electronic-Structure of the 

Ternary Chalcopyrite Semiconductors CuA1S2, CuGaS2, 

CuInS2, CuA1Se2, CuGaSe2, and CuInSe2," Physical 

Review B, vol. 28, pp. 5822-5847, 1983. 

[11] H. Sommer, A. Weiss, H. Neumann, and R. D. 

Tomlinson, "Comparative Photoemission-Study of the 

CuInC2-Vi Chalcopyrite Compounds," Crystal Research

and Technology, vol. 25, pp. 1183-1187, 1990. 

[12] K. S. Art J. Nelson, John Moreland, "Growth and 

characterization of the binary defect alloy Cu2-xSe and the 

relation to the II-VI/I-III-VI heterojunction formation," 

presented at Materials Research Society symposium 

proceedings, 1995. 

4
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5a. CONTRACT NUMBER

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4. TITLE AND SUBTITLE

XPS and UPS Investigation of NH4OH-Exposed Cu(In,Ga)Se2 Thin 

Films 

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NREL/CP-520-37419 

5e. TASK NUMBER

PVA53301 

6. AUTHOR(S) 

C.L. Perkins, F.S. Hasoon, H.A. Al-Thani, S.E. Asher, and 

P. Sheldon 

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14. ABSTRACT (Maximum 200 Words)

Photoelectron spectroscopy was used to determine the compositional and electronic changes occurring in Cu(In,Ga)Se2

thin films as a result of immersion in aqueous ammonia solution. We find that NH4OH-treated CIGS surfaces are 

preferentially etched of indium and gallium, resulting in the formation of a thin layer of a degenerate Cu-Se compound 

that we tentatively identify as Cu2Se. The work function of ammonia-treated samples is found to increase by 0.6 eV 

relative to as-grown CIGS thin films. The uniformity of chemical bath effects (etching & deposition) was found to be 

improved by the addition to the bath of a non-ionic surfactant. Initial device results show that the new surfactant-based 

chemical bath deposition (CBD) method may lead to better and thinner CdS buffer layers. 

15. SUBJECT TERMS

PV; chemical bath deposition (CBD); thin film; etching; deposition; non-ionic surfactant; buffer layers; indium and gallium; 

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