Surface Treatment of CuInGaSe2

Thin Films and Its Effect on the 

Photovoltaic Properties of Solar 

Cells 

Preprint 

October 2002 • NREL/CP-520-32950 

K. Ramanathan, F.S. Hasoon, S. Smith, D.L. Young, 

and M.A. Contreras 

National Renewable Energy Laboratory 

Golden, Colorado

P.K. Johnson, A.O. Pudov and J.R. Sites 

Colorado State University 

Fort Collins, Colorado 

To be presented at the 13th International Conference on 

Ternary and Multinary Compounds 

Paris, France 

October 14-18, 2002 

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Surface Treatment of CuInGaSe2 Thin Films and Its Effect on the Photovoltaic Properties 

of Solar Cells 

K. Ramanathan, F. S. Hasoon, S. Smith, D.L. Young, and M.A. Contreras 

National Renewable Energy Laboratory, 1617 Cole Boulevard, Golden CO 80401 

and 

P. K. Johnson, A. O. Pudov, and J. R. Sites 

Physics Department, Colorado State University, Fort Collins CO 80523 

Abstract 

Solar cells have been fabricated with 

partial electrolyte treatments of CuInGaSe2 

(CIGS) thin film absorbers in lieu of a CdS 

layer. Treatment of the absorbers in a Cd or 

Zn containing solution is shown to produce 

conditions under which efficient solar cells 

can be fabricated. A similar effect is also 

observed in CuInGaSSe2 (CIGSS) graded 

band gap absorbers. These observations can 

be explained by the ability of Cd and Zn to 

produce n-type doping or inversion in the 

surface region. We also provide a brief 

review of similar work done elsewhere and 

identify directions for future investigations. 

1. Introduction 

Thin film solar cells based on CuInSe2 

(CIS) have been developed as a competitive 

option for low cost power generation. 

Photovoltaic cells and modules made using 

alloys of CIS with Ga and or S have 

demonstrated the highest efficiencies and 

good stability in outdoor tests [1]. In research 

laboratories, CIGS thin films are generally 

fabricated by co-evaporation from elemental 

sources [2]. The processes used for large area 

module manufacture, however, involve 

deposition of metallic precursor stacks and 

the subsequent formation of the compound in 

a selenium and sulfur ambient [3]. The two 

examples represent a sharp contrast in 

preparation methods, and the commonality is 

that one attempts to fabricate a single-phase, 

chalcopyrite CIS alloy thin film with a 

specific Ga or S content and a Cu/(In+Ga) 

ratio. The photovoltaic junction is generally 

completed by the deposition of a thin CdS 

layer by chemical bath deposition. Though 

the most efficient CIGS solar cells are 

fabricated in this manner, there are a few 

problems associated with the CdS 

technology. First, the band gap of the CdS 

layer is still low enough to limit the short 

wavelength part of the solar spectrum that can 

reach the absorber, and this leads to a 

reduction in the current that can be collected. 

This reduction becomes proportionally more 

severe for higher band gap CIGS cells. From 

a practical standpoint, the generation of large 

quantities of hazardous waste and its disposal 

present challenges to the manufacturer. 

Finding an equally attractive alternative to 

the CBD CdS technology has been a 

recurring theme in many studies. Among the 

alternative layers that have been investigated 

are ZnS, ZnO, Zn(S,O), ZnSe, In2S3, 

In(OH)xSy, and perhaps others [4]. When 

these alternative “buffer layers” have been 

inserted in place of CdS, the deposition 

processes have often still involved wet 

chemistry, and not uncommonly, the devices 

fabricated in this manner require post 

deposition anneals or light soaking to become 

fully active. It has also been found useful in 

many cases to chemically modify the surface 

of the absorber before deposition of the 

alternative n-type layer [5]. In this paper, we 

1
report the results of surface treatments of effects of sample inhomogeneity, the microCIGS thin films in an aqueous solution Photoluminescence (micro-PL) technique was 

containing Cd or Zn ions followed by the used to excite and collect the PL within a 1 

completion of the solar cells with a ZnO micron diameter spot size (excitation 

layer. wavelength 632.8 nm, excitation power 120 

nW) and subsequently dispersed onto an 

2. Experimental Techniques InGaAs diode array. Linear scans were taken 

with up to 50 spectra, separated by 1 μm 

CIGS thin films were grown at NREL by steps. 

the three-stage process on Mo/soda lime glass 

substrates. The resulting absorber material 3. Results and Discussion 

was 2.5 μm thick, and it was p-type with a 

hole density of 1-3x1016 cm-3. The nominal Fig. 1 shows the current-voltage (J-V) 

composition was CuIn0.7Ga0.3Se2. The Cu characteristics of three types of solar cells 

concentration in the near-surface region was fabricated from NREL-deposited CIGS 

carefully controlled so that the overall absorbers. For the case of ZnO deposited 

Cu/(In+Ga) ratio is less than unity. Samples with neither CdS nor the Cd PE treatment, 

provided by Shell Solar Industries, formerly 

Siemens Solar Industries, (SSI) were graded 40 

band-gap CIGSS absorbers. Reference cells 

were made with CdS thin films deposited 

from an aqueous solution containing 20 

0.0015M CdSO4, 1.5M NH4OH, and 

0.0075M thiourea. The temperature of the 

bath was gradually raised from room 0 

temperature to 60 ̊C, and the samples were 

held in the bath for 15 min, during which a 40 

to 60 nm CdS film was deposited. For the -20 

non-CdS cells, partial electrolyte treatments 

of the absorber surface, designated as Cd PE 

and Zn PE, were performed by immersing the -40 

samples in a bath similar to the CdS 

direct ZnO 

Cd PE 

CdS 

-0.4 -0.2 0.0 0.2 0.4 0.6 0.8 

deposition, but excluding thiourea. The Voltage (V) 

maximum temperature during the Cd PE 

treatment was 80 ̊C, and the total immersion Fig. 1. Current-voltage characteristics of CIGS 

time was 22 min. Solar cell fabrication was solar cells with no treatment, Cd PE treatment, 

completed by depositing a two-layer ZnO and CdS layer. All have ZnO window layers. 

film. The first layer of 80 to 100 nm was 

deposited from a pure ZnO target, and the inferior photovoltaic activity is observed.

second layer of 120 nm was deposited from The open circuit voltage and the fill factor are 

an Al2O3 doped target. Ni/Al contacts were low. A detailed analysis of the J-V curves

evaporated on the ZnO. Current-voltage reveals that the reverse saturation current

characteristics of the solar cells were density and the diode factor are higher

measured in dark and under light simulated to compared to the CdS case. The shape of the

correspond to AM1.5 Global spectrum. J-V curve under illumination indicates that

Photoluminescence (PL) spectra of CIGS thin the light generated current may be controlled

films were measured at 4K. To minimize the by voltage-dependent recombination. The

Current Density (m

A/cm2) 

2
treatment of the absorbers in a Cd PE solution 

prior to the deposition of ZnO helps remove 

these limitations. A comparison of the J-V 

curves of this diode and the one with the CdS 

window layer shows that the Cd treatment 

alone is sufficient to produce an efficient 

photovoltaic cell. We had shown previously 

that similar results are obtained with CIS thin 

films [6]. 

In Fig. 2, the spectral dependence of the 

internal quantum efficiency of the two types 

of devices are compared. The CdScontaining cell exhibits a lower collection in 

the short wavelength part of the spectral 

range, which is attributed to the absorptive 

and collection losses in the CdS layer. The 

absence of these losses in the Cd PE device is 

evident, and an increase of the short circuit 

current density of 1-2 mA/cm2 is obtained 

80 

60 

40 

20 

CdS 

Cd PE 

CdS Cd PE

400 600 800 1000 

Wavelength (nm) 

Fig. 2. Quantum efficiency vs. wavelength of 

CIGS solar cells with Cd PE treatment and CdS 

layer. 

when the CdS layer is eliminated or a 

comparable wide-gap window layer is 

substituted. This is the obvious attraction to 

removing the CdS window layer. However, 

one finds that the devices without CdS layers 

exhibit slightly lower open circuit voltages 

and fill factors. These can be attributed to the 

nature of the interface created by the 

dissimilar materials and the defect states 

introduced during the processing steps. The 

lattice constants of CdS and CIGS are closely 

matched, and the interface created between 

the two will have minimal lattice mismatch 

and lower defect density compared to other 

window layers. It has also been shown that 

there is some inter-diffusion across the 

CdS/CIGS interface [7], and the resulting 

compositional mixing will further reduce the 

deleterious effects of interface recombination. 

The best cell fabricated using the Cd PE 

treatment had a conversion efficiency of 

15.7%. The use of Zn PE treatments, in 

which the Cd compound is replaced by a Zn 

compound, lead to J-V and QE curves similar 

to those shown in Figs. 1 and 2. We also find 

that similar Cd PE results are observed with 

CIGSS alloy thin films fabricated by a 

manufacturing process. An example is shown 

in Fig. 3 where the cells were completed on 

absorbers provided by SSI. 

20 

10 

0 

-10 

-20 

-30 

CdS 

direct ZnO 

Cd PE 

-0.4 -0.2 0.0 0.2 0.4 0.6 

Voltage (V) 

Fig. 3. Current-voltage characteristics of CIGSS 

solar cells with no treatment, Cd PE treatment, 

and CdS layer. All have ZnO window layers. 

3 

Internal QE (%) 

Current Density (mA/cm2) 
Fig. 4. shows the PL spectra of a CIGS surface region. It is well known that the 

thin film treated in ammonium hydroxide electrical properties of the I-III-VI2 

only and one in Cd PE. The two pieces were compounds are determined by a critical 

cut from the same sample along the direction balance of electrically active native defects 

of uniform Ga distribution. The PL intensities and complexes arising from deviations from 

exhibited minor variation along the length stoichiometry [9]. The effect of extrinsic 

probed, but the spectral features remained impurities tends to be marginal because of 

constant. The spectral dependence shows two limitations to dopant solubility and the 

broad peaks centered around 1.05 eV and readiness with which compensating defects of 

1.12 for the ammonium hydroxide case. It is the opposite kind form to neutralize the donor 

well known that the two transitions are donor- or acceptor impurities introduced externally 

to-acceptor pair transitions between shallow [10]. However, the lower band gap Cu-Indonor and acceptor energy levels close to the VI2 compounds, CuInSe2 and CuInS2, can be 

band edge [8]. In the Cd PE sample, the made n- and p-type conducting by annealing 

higher energy emission is quenched by the the crystals in minimum or maximum 

action of Cd. This involves a transition chalcogen pressure [9]. Efficient n-type 

between a donor level and the shallowest conductivity is also obtained in CuInSe2 by 

acceptor, and its disappearance indicates that the use of extrinsic doping with Cd and Zn. 

the acceptor level is neutralized by Cd. This The donor level introduced by Cd doping was 

can cause a change in the compensation ratio, found to have an ionization energy of 5 meV, 

and can result in a change of conductivity and a rapid diffusion of Cd was observed at 

type in the surface region. low temperatures [11]. In a separate study of 

thermally evaporated CdS on CIS single 

6000 

6328Å 

4K 

Cd PE 

NH4OH 

crystals, it was found that homojunctions 

were formed by the deposition of CdS, and 

5000 the photovoltaic effect was still present after 

the CdS layer was removed [12]. Hence, we 

4000 find ample evidence in the foregoing studies 

for the ability of IIB impurities (Cd, Zn) to 

3000 produce n-type doping. 

2000 Our experiments with the Cd and Zn PE 

solutions and the resulting solar cell 

1000 properties indicate that a similar effect could 

be created in a CBD environment involving 

0 Cd or Zn compounds and ammonium 

0.95 1.00 1.05 1.10 1.15 hydroxide. The polycrystalline films are Photon Energy (eV) deliberately grown Cu-poor (III rich), which 

creates an abundance of Cu vacancies. The 

Fig. 4. Photoluminescence spectra of CIGS thin ionic radii of Cu and Cd are very close, and film treated in ammonium hydroxide and Cd PE. the substitution of Cd in Cu vacancy sites is

Excitation wavelength is 6328 Å, sample most likely. One can envisage out-diffusion

temperature 4 K. of Se and the formation of sulfo-selenide 

The above observations lead us to a compounds or solid solutions at the interface. 

discussion of the effect of the surface Compositional profiles of Cd PE treated 

treatments on the electrical properties of the

samples indicate that there is an appreciable 

concentration of Cd in the near surface 

Intensity (arb. units) 

4
region. We further assume that the Cd 

impurities are readily ionized, leading to an 

increase in the electron density in the surface 

region and resulting in band bending or 

inversion. Controlled incorporation of Zn 

impurities in CIGS thin films grown by the 

three-stage process has clearly demonstrated 

that high efficiency solar cells can be 

fabricated by Zn doping of the surface region 

of polycrystalline films [13]. Nakada et al 

[14] find that the ZnS window layers 

deposited by CBD do not directly yield the 

best solar cells, but that a post deposition 

annealing step is needed to optimize the Zn 

diffusion profile. It is quite possible that the 

ionization of Zn donors is also taking place 

during the anneal step. A similar 

combination of effects can occur in any 

junction formation step in which Zn or Cd 

compounds are deposited on a heated 

substrate, and where the precursors are 

reactive. 

Although the notion of n-type doping is not 

new to the ternary chalcopyrites, it is to the 

thin film solar cell field and is still a 

contentious issue. The experimentalist is 

challenged to develop measurement 

techniques that can probe the electrical 

activity and conduction type in a very thin, 

surface layer of polycrystalline film. This is a 

difficult task, but it is central to the 

development of alternative device structures 

in II-VI and I-III-VI2 thin films. 

5. Conclusions 

We have compared the properties of solar 

cells fabricated in CIGS thin films by Cd PE 

treatment with those containing standard CdS 

window layers, and we show that the 

presence of Cd alone is sufficient to produce 

efficient solar cells. Similar results are 

obtained by using Zn PE baths. These results 

suggest that the column IIB impurities help 

change the electrical properties of the surface 

region. Hence, a model for the operation of 

the CdS/CIGS solar cell must consider the 

role of the absorber and window layer 

properties and the electrical and 

compositional changes to the intermediate 

region influenced by the processing 

conditions. 

Acknowledgments 

This work was performed for the Department 

of Energy under Contract No. DE-AC36-

99GO10337 to NREL. We thank D.E. 

Tarrant of SSI for the supply of samples and 

his support of this work; J. Dolan and J. 

Keane for expert technical assistance; T. 

Moriarty and D. Dunlavy for standardized 

measurements; and, R. Bhattacharya, and R. 

Noufi for helpful suggestions. 

References 

[1] H.S. Ullal et al, Proc. 28th IEEE 

Photovoltaic Specialists’ Conference, 

Anchorage, 2000, pp 301-305. 

[2] M. A. Contreras, B. Egaas, K. 

Ramanathan, J. Hiltner, A. Swartzlander, F. 

Hasoon, and R. Noufi, Prog. Photovol: Res. 

Appl. 7 (1999) 311. 

[3] D.E. Tarrant and J. Ermer, Proc. 23rd 

IEEE Photovoltaic Specialists’ Conference, 

Louisville KY, 1993, p 372. 

[4] J. Kessler, M. Ruckh, D. Hariskos, U. 

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[5] T. Wada, Y. Hashimoto, S. Nishiwaki, T. 

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(2001) 305. 

[6] K. Ramanathan, et. al., Second World 

Conference on Photovoltaic Solar Energy 

Conversion, Wien, Austria, 1998, pp 477-

481. 

[7] C. Heske et al., Appl. Phys. Lett., 74 

(1999) 1481. 

[8] S. Zott, K.Leo, M.Ruckh, and H.W. 

Schock, Appl. Phys. Lett., 68 (1996) 1144. 

5
[9] J. L. Shay and J. H. Wernick, “Ternary 

Chalcopyrite Semiconductors: Growth, 

Electronic Properties and Applications”, 

Pergamon Press, Oxford, 1975. 

[10] A. Zunger, S.B. Zhang, and Su-Huai 

Wei, Proc. 26th IEEE Photovoltaic 

Specialists’ Conference, Anaheim, 1997, p 

313 

[11] P.W. Yu, S.P. Faile, and Y.S. Park, 

Appl. Phys. Lett. 26 (1975) 384. 

[12] R. J. Matson, R. Noufi, K. J. Bachmann, 

and D. Cahen, Appl. Phys. Lett. 50 (1987) 

158. 

[13] T. Sugiyama, S. Chaisitsak, A. Yamada, 

M. Konagai, Y. Kudriavtsev, A. Godines, A. 

Villegas, and R. Asomoza, Jpn. J. Appl. 

Phys. 39 (2000) 4816. 

[14] T. Nakada and M. Mizutani, Jpn. J. 

Appl. Phys. 41 (2002) L165. 

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4. TITLE AND SUBTITLE

 Surface Treatment of CuInGaSe2 Thin Films and Its Effect on the Photovoltaic 

Properties of Solar Cells: Preprint

6. AUTHOR(S) 

 K. Ramanathan, F.S. Hasoon, S. Smith, D.L. Young, M.A. Contreras, P.K. Johnson*, 

A.O. Pudov*, and J.R. Sites* 

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13. ABSTRACT (Maximum 200 words: Solar cells have been fabricated with partial electrolyte treatments of CuInGaSe2 (CIGS) thinfilm absorbers in lieu of a CdS layer. Treatment of the absorbers in a containing Cd or Zn solution is shown to produce 

conditions under which efficient solar cells can be fabricated. A similar effect is also observed in CuInGaSSe2 (CIGSS) 

graded-bandgap absorbers. These observations can be explained by the ability of Cd and Zn to produce n-type doping or 

inversion in the surface region. We also provide a brief review of similar work done elsewhere and identify directions for 

future investigations. 

15. NUMBER OF PAGES 14. SUBJECT TERMS: PV; CuInGaSe2; thin-film solar cells; partial electrolyte treatment; 

electrical properties; window layer property; n-type doping; chalcopyrites; current 

voltage; stoichiometry; Cd; Zn 16. PRICE CODE

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