Subcontract Report

NREL/SR-520-46434

September 2009

Characterization of the Electronic 

and Chemical Structure at the 

Thin Film Solar Cell Interfaces 

June 2005 – June 2009

C. Heske

University of Nevada, Las Vegas 

Las Vegas, Nevada
National Renewable Energy Laboratory

1617 Cole Boulevard, Golden, Colorado 80401-3393

303-275-3000 • www.nrel.gov

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

Office of Energy Efficiency and Renewable Energy

Operated by the Alliance for Sustainable Energy, LLC

Contract No. DE-AC36-08-GO28308

Subcontract Report

NREL/SR-520-46434

September 2009

Characterization of the Electronic 

and Chemical Structure at the 

Thin Film Solar Cell Interfaces 

June 2005 – June 2009

C. Heske

University of Nevada, Las Vegas 

Las Vegas, Nevada

NREL Technical Monitor: Bolko von Roedern

Prepared under Subcontract No. XXL-5-44205-12
This publication was reproduced from the best available copy

Submitted by the subcontractor and received no editorial review at NREL

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1 

Summary

This project was devoted to deriving the electronic structure of interfaces in Cu(In,Ga)(S,Se)2 and CdTe

thin film solar cells. By using a unique combination of spectroscopic methods (photoelectron 

spectroscopy, inverse photoemission, and X-ray absorption and emission) a comprehensive picture of 

the electronic (i.e., band alignment in the valence and conduction band) as well as chemical structure can 

be painted. The work focused on (a) deriving the bench mark picture for world-record cells, (b) analyze 

state-of-the-art cells from industrial processes, and (c) aid in the troubleshooting of cells with 

substandard performance.

 

First funds for this project became available in the middle of July 2005. After that, the workforce of the 

group was expanded to the size required for this project. The necessary experimental instrumentation at 

UNLV was commissioned and contacts within the Thin Film PV Partnership Program (TFPPP) were 

established to secure a supply of adequate samples, even well beyond the official termination of the 

TFPPP on 9/30/2006. Samples were analyzed both in the lab at UNLV as well as during our 

experimental campaigns at the Advanced Light Source, Lawrence Berkeley National Laboratory. 

Most recently, we have investigated the chemical and electronic surface structure of the current world 

record (20.0%-efficient) Cu(In,Ga)Se2 (CIGSe) thin film solar cell absorber and the corresponding 

CdS/CIGSe interface of samples provided by the National Renewable Energy Laboratory (NREL). For 

the 20.0%-efficient CIGSe absorber, our measurements reveal that the surface composition is more Cudeficient compared to less efficient absorbers and that the respective position of the conduction band 

minimum is comparatively shifted further away from the Fermi energy. Furthermore, we find a valence 

band offset of -0.90 (±0.15) eV and (as expected for a high-efficiency solar cell device) a flat conduction 

band alignment. 

To investigate whether the composition of Cu(In,Ga)(S,Se)2 (CIGSSe) chalcopyrite absorber has an 

impact on the formation of the Cu-poor, Eg-widened region and its extension into the absorber bulk, 

different chalcopyrite thin film solar cell absorbers provided by the Institute of Energy Conversion

(IEC), University of Delaware were investigated. Our photon and electron spectroscopies were able to 

gain depth-dependent Eg information. For all investigated samples we find an increasing band gap 

energy with decreasing information depth and the formation of a surface region with significantly higher 

Eg. In addition, we find that the Eg-widened surface region extends further into the bulk of the absorber 

for the sulfur-free CIGSe absorber (i.e., the absorber with smaller bulk band gap) than for the CIGSSe 

absorber.

In addition to our investigations of the chemical and electronic surface structure of CIGSSe absorbers, 

we also studied the deeply buried CIGSSe/Mo interface. For the first time, we could draw a complete 

picture of the chemical and electronic properties of that interface. For these experiments CIGSe and 

CIGSSe/back contact samples were used, again prepared by the IEC group. We found a pronounced 

chemical interaction between absorber and back contact, namely the formation of MoSe2 (and 

Mo(S,Se)2) and a “diffusion” of Ga into the Mo layer. In addition, we could derive a flat valence band 

alignment at this interface. 
2

Besides our studies on chalcopyrite thin film solar cell absorbers, we also investigated the chemical and 

electronic properties of structures relevant for CdS/CdTe-based thin film solar cells. Our results suggest 

that the CdS/CdTe interface is heavily intermixed, in particular after the CdCl2 treatment. Efforts are still 

ongoing to reveal the electronic structure at that interface.

In order to shed light on the chemical and electronic structure in real-world industrial-grade samples, 

i.e., manufactured in large-scale, high-throughput equipment in an industrial environment, we have also 

studied selected samples (both, front and back sides) directly taken out of Global Solar Energy, Inc. 

(“GSE”) production process after each preparation step. GSE has pioneered a unique robust process to 

manufacture CIGSe solar cell devices which can hardly be simulated on laboratory scale. While other 

companies pursue the approach of in-line deposition on rigid glass substrates, GSE is the only company 

to date using a roll-to-roll coating of the complete solar cell thin film layer stack on flexible substrates. 

Upon CIGSe formation our data shows that the back side exhibits MoSe2 and absorber related XPS 

features. Compared to the front side, we find an increased amount of Ga on the back side which suggests 

a pronounced interaction with the front side upon roll-up. 

Detailed Description of the Activities:

1. Establishing the group at UNLV

First funds for this project became available in the middle of July 2005. The initial activity was devoted 

to the expansion of the leadership work force for this project. With the arrival of Dr. Marcus Bär, a postdoctoral fellow (recipient of the prestigious German Emmy Noether Scholarship of the Deutsche 

Forschungsgemeinschaft) in mid-August 2005, and of Dr. Lothar Weinhardt, in January 2006, this 

expansion was fortunately very fast and successful. 

Dr. Bär performed the research for his doctoral thesis at the Hahn-Meitner-Institut in Berlin, Germany, 

specializing in the optimization of interfaces between novel buffer layer materials and Cu(In,Ga)(S,Se)2

thin film solar cell absorbers by chemical surface pretreatments. At UNLV, his primary focus was on 

modifying interface properties for an optimization of thin film solar cells with wide-gap chalcopyrite 

absorbers. Dr. Weinhardt, who came from the University of Würzburg, Germany, has pioneered the 

combination of UV- and Inverse Photoemission for the routine study of band alignment at thin film solar 

cell interfaces, as well as the to-date least intruding cleaning method for air-exposed thin film 

chalcopyrite surfaces (50 eV Ar+

 ion treatment). Both post-docs brought significant expertise in 

optimization and analysis of surfaces and interfaces in thin film solar cells into this project and have 

jointly led the TFPPP activities of the UNLV group. After Dr. Weinhardt and Dr. Bär left UNLV (at the 

end of 2007 and 2008, respectively) to start their own groups in Germany, Sujitra Pookpanratana (a 

graduate student from UNLV) took over UNLV’s TFPPP activities. 
3

2. Establishing a spectroscopic “tool chest” at UNLV

For the proposed characterization of the chemical and electronic surface and interface structures in thin 

film solar cells the necessary soft x-ray and electron-based spectroscopes needed to be established at 

UNLV. 

During the course of this project, x-ray and UV photoelectron spectroscopy (XPS, UPS) was routinely 

used to investigate the chemical and electronic (occupied states) structure of the thin film solar cell 

samples. For the investigation of unoccupied states, inverse photoemission measurements (IPES) were 

conducted. The ultra-high vacuum (UHV) apparatus for these lab-based XPS/UPS experiments at 

UNLV was successfully commissioned and optimized for routine investigations after its relocation from 

the University of Würzburg, Germany during Spring/Summer 2005. During the last quarter of 2005 also 

the IPES setup was successfully included, such that first IPES spectra of reference samples could 

already be recorded by the end of 2005. Fig. 1 shows the first Fermi edge (of a Ag calibration sample) 

measured with our setup (open circles). The red line represents a fit of the spectrum which is used to 

derive the Fermi energy and in addition gives the total energy resolution of our setup (440 meV). 

With funds from a different project, we were able to replace the old ESCALab MkII electron analyzer of 

our surface/interface characterization system, which showed reappearing electronic shortage problems in 

the first quarter and electronic communication problems in the second and third quarter of the first

project year, by a high-performance state-of-the-art instrument (SPECS PHOIBOS150 MCD) in April 

Fig. 1 First inverse photoemission spectrum at UNLV (open circles). The red line 

represents a fit of the experimental data. The derived Fermi energy is given by the dashed 

vertical line.

6.5 7.0 7.5 8.0 8.5

 data

 fit

Intensity

Electron Energy Energy (eV)

EF = 7.58 eV

Experimental resolution:

FWHM = 440 meV

EF
4

2006. After a downtime of only two weeks the new electron analyzer was commissioned and put to 

normal operation. The increased spectral resolution and an improvement of the signal-to-noise ratio in 

XPS by about two orders of magnitude greatly benefited the XPS and UPS results of our project and 

significantly reduced the experiment times. During the course of this project, we also replaced the old xray source by a high-performance state-of-the-art new x-ray source (SPECS XR-50), which again 

decreased the downtime of our surface analysis system due to maintenance issues and hence improved 

our sample throughput. For routinely performed XPS (UPS) measurements, we used Mg Kα or Al Kα

(He I/He II) excitation and our PHOIBOS 150 electron analyzer with a multi-channeltron detector. The

IPES experiments were conducted with a low-energy electron gun (STAIB) and a Dose-type detector 

with a SrF2 window and Ar: I2 filling were used. The experiments were performed in UHV with a base 

pressure below 1⋅10-10 mbar. 

In 2006, we also added a N2-purged glovebox to our surface analysis system in order to minimize the 

exposure to ambient air of samples sent by our TFPPP collaborators to UNLV, which was critical for 

our surface-sensitive XPS, UPS, and IPES characterization. Usually the samples were individually 

packed in two plastic bags filled with dry nitrogen and some desiccant immediately after preparation in 

the labs of our collaborators. The sample exposure time to ambient air (before the first characterization) 

could thus be drastically minimized. Once at UNLV, the samples were unpacked in our glovebox under 

dry nitrogen atmosphere and directly introduced into our UHV surface characterization system. 

Due to funds available from different projects, we could add a high-resolution XPS/UPS surface analysis 

system to our existing high-dynamic range measurement setup in 2007. This enabled us to investigate 

the thin film solar cell samples with much higher XPS and UPS spectral resolution (when necessary). 

Fig. 2 shows the development of our surface analysis system at UNLV during the course of this project. 

In addition to the characterization of the thin film solar cell samples by these surface-sensitive, electronbased spectroscopies, we also used soft x-ray based techniques [x-ray emission (XES) and absorption 

spectroscopy (XAS)] for our studies. Due to the fact that these spectroscopies (XAS in fluorescence 

yield) are “photon-in-photon-out” techniques, their information depth is not limited by the inelastic 

mean free path of electrons but by the attenuation length of the soft x-rays in the probed material. Hence, 

the information depth is increased from a few nm (for XPS, UPS, and IPES) to several 10 nm up to a 

few 100 nm. These measurements were performed at the Advanced Light Source (ALS, Lawrence 

Berkeley National Laboratory). For the XES (XAS) measurements, we used the permanently installed 

SXF spectrometer of Beamline 8.0 (a channeltron mounted in front of the sample). Fig. 3 shows a 

picture of Beamline 8.0 with the SXF spectrometer.

In the last two project years, we (in close collaboration with Würzburg University) also designed and 

commissioned our own endstation at the ALS. The SALSA (Solid And Liquid Spectroscopic Analysis) 

endstation equipped with a high throughput and high transmission VLS (variable line spacing) soft x-ray 

spectrometer enabled us also to measure thin film solar cell samples more efficiently and with a higher 

signal-to-noise-ratio. A picture of the SALSA endstation is presented in Fig. 4. 
5

(a)

(b)

(c)

X-rays (in) Sample

Electrons (out)

Electrons (in)

UV (in)

E-beam evaporation

ECR Plasma

Etching

Fig. 2 Picture of the ultra-high vacuum 

surface analysis instrument (a) still at 

Würzburg University, Germany and (b) 

after relocation to UNLV. Note the 

already replaced electron analyzer (red 

circle), the IPES setup (blue circle) and 

a scanning probe microscope 

(purchased with funds from a different 

project; green circle). (c) Current 

status of the surface analysis system at 

UNLV.
6

Fig. 4 SALSA endstation at Beamline 8.0 of the ALS.

Fig. 3 Beamline 8.0 (with SXF spectrometer in the front) at the Advanced Light Source 

(Lawrence Berkeley National Laboratory).
7

3. Experimental Results

In the following, we will present some selected results of our work within the TFPPP program. First, we 

will report on our studies on laboratory-manufactured chalcopyrite-based thin film solar cell absorbers 

(A1-A5), then some results of our investigations on CdS/CdTe structures will be presented (B1, B2), and 

finally we will exemplary demonstrate how our spectroscopic methods can also help in the development 

of industrial-grade mass-produced Cu(In,Ga)Se2 absorbers (C1). 

 

A1. Chemical and electronic structure of 20.0% efficient Cu(In,Ga)(S,Se)2 thin film solar cell 

absorbers

[Collaboration with I. Repins, M.A. Contreras, and R. Noufi, National Renewable Energy Laboratory, 

NREL]

In February 2008, the NREL group announced a new world record efficiency (20.0%, [1,2]) for 

Cu(In,Ga)Se2 “CIGSe” – based thin-film solar cells. This recent efficiency gain (compared to the former 

world record - 19.5% [3]), is believed to be caused by a small – but apparently significant – change in 

the three-stage process [1]. In comparison to the deposition process used earlier, the difference resulting 

in the recent world record CIGSe absorber was a termination of the third (and thus last) stage without Ga 

and hence is considered to be “In-terminated” [1]. 

 Fig. 5 XPS survey spectra of the investigated CIGSe samples (as-received).

1000 800 600 400 200 0Ga 3d/ In 4d Se 3d Cu 3p

Na KLL

Ga 3p

Cu LMM

Cu 3s/ In 4s

Se 3s

Ga LMM

C 1s

Se 3p

In 3d

O 1s

In 3p

O KLL

In MNN

Cu 2p

C KVV

Na 1s

M2992-PR

Normalized Intensity

Binding Energy [eV]

Mg Kα

M2995-PR

Ga 2p
8

Fig. 6 XPS survey spectra of the investigated CIGSe samples (after NH3-dip). 

1000 800 600 400 200 0

M2995-PR

Ga LMM

Ga 3p Cu 3s/ In 4s

Ga 3d/ In 4d

Cu 3p Se 3d

Se 3p

Se 3s

C 1s

Cu LMM

C KVV

Na 1s

O KLL

Ga 2p

Cu 2p

In MNN

In 3p

O 1s

Normalized Intensity

Binding Energy [eV]

Mg Kα

In 3d

M2992-PR

In order to shed light on the expected different chemical surface structure, we investigated a sample 

from the world record absorber batch (M2992) by XPS. As already mentioned this technique is very 

surface sensitive (information depth a few nm) and thus well suited to address questions of surface 

termination. In addition, we also characterized a CIGSe absorber (M2995) deliberately terminated with 

Ga for comparison. 

Fig. 5 shows the XPS survey scans of the investigated CIGSe samples. As indicated by the highintensity O-related XPS (O 1s) and XAES (O KLL) features, the surface of both samples is significantly 

oxidized. Corresponding detail spectra of photoemission and Auger features show that especially In and 

Ga are oxidized. The surfaces also show large amounts of Na and C. The C 1s photoemission line is 

composed of two pronounced contributions, of which the high-binding energy feature at approx. 289.6 

eV is ascribed to carbonate. This interpretation also agrees with the line position of the O1s XPS peak. 

The position of the Na 1s photoemission line together with the modified Auger parameter of Na (Na 1s 

+ Na KLL) furthermore suggests the formation of NaCO3 at the sample surfaces. This pronounced 

sample surface contamination/oxidation can be explained by the extended storage of the samples in a 

desiccator and occasional exposure to air during different preceding characterization campaigns. 

In the survey spectra in Fig. 5, we also observe a lower intensity of the Ga XPS and Auger features for 

the "In-terminated" sample (M2992) compared to the “Ga-terminated” sample (M2995). This could 

indeed be the first indication for the expected different Ga/In surface ratio for the two samples. Note 

that, however, the higher degree of surface contamination for the "In-terminated" sample (as evident 
9

from the larger Na 1s, O 1s, and C 1s peaks) also leads to lower intensities of the corresponding Inrelated peaks. Since the emission of the Ga and In lines are governed by different attenuation lengths, a 

reliable quantitative analysis would require a detailed model of the nature, thickness, and morphology of 

the contamination layer, which is not available.

Usually, our group uses a mild (50 eV Ar+) ion treatment to clean the samples we are investigating. This 

ion energy is chosen since it is below the sputter threshold for chalcopyrites, leading primarily to ionstimulated desorption of adsorbates and minimizing potential damage to the actual surface. In the 

present case, however, the surface contamination/oxidation was very pronounced, and thus we decided 

to clean the samples chemically by means of a short (2 min) dip in aqueous ammonia (at room 

temperature) performed in our N2-filled glovebox. The corresponding XPS survey spectra are shown in 

Fig. 6. 

Compared to the XPS survey spectra in Fig. 5, the spectra in Fig. 6 show that the intensity of the Na-, 

O-, and C-related features is significantly reduced. Correspondingly, the Cu and In photoemission and 

Auger feature detail spectra do not show any indications for the presence of oxides at the sample surface 

anymore. Hence, as expected, the NH3 dip cleaned the surface of the investigated CIGSe samples (note 

that the residual oxygen signal is ascribed to residue from the aqueous solution dip). 

To minimize the impact of the residual surface contamination for the quantitative analysis of the surface 

composition, we have analyzed only photoemission lines in the low binding energy (EB) region. Low EB

photoelectrons correspond to high kinetic energy photoelectrons, which exhibit lower surface sensitivity 

(and susceptibility to surface contamination). Furthermore, the analyzed photoemission lines have 

kinetic energies within range of each other, which allows us to neglect the energy-dependence of 

inelastic mean free path of electrons and the spectrometer transmission function.

Fig. 7 shows the region of the low binding energy photoemission lines of the investigated In- (M2992, 

red) and Ga- (M2995, black) terminated CIGSe absorbers, respectively. For normalization of the two 

spectra, we derived the area intensity of the shown Cu, In, Ga, and Se core levels, weighted them with 

the respective photoionization cross sections [4], and summed them to give the overall spectral weight. 

Then, a normalization factor was chosen such that the normalized overall spectral weight for both 

spectra was equal. 

At first sight, the spectra look very similar to each other -- they both show all expected Cu, In, Ga, and 

Se photoemission lines. However, close inspection of the Se 3d and Ga 3d/In 4d regions reveals small, 

but significant differences in the respective line intensities. As confirmed by the difference spectrum 

([(a) – (b) = M2992 – M2995]; bottom spectrum in Fig. 7), the In-terminated absorber shows more 

intensity in the spectral region of the Se 3d and Ga 3d/In 4d XPS peaks. Due to the overlap of the Ga 3d 

and In 4d photoemission lines, it is not trivial to ascribe the higher overall intensity of the superimposed 

XPS peaks to a higher/lower Ga or In surface content. In order to address this situation, the Ga 3d and In 

4d contribution to the Ga 3d/In 4d line was determined by a simultaneous fit of the corresponding 

spectral region for both samples. The inset in Fig. 7 shows the magnified region of the Ga 3d/In 4d XPS 

peaks along with the fitted Ga 3d and In 4d contributions for the spectra at 18 eV. The fit agrees well 

with the energetic position of the In 4d contribution. Close inspection of the difference spectrum also 
10

shows that the position of the minimum (around 20 eV) is in accordance with that of the Ga 3d 

contribution. Hence, the In-terminated sample has a higher In content (by 7 ± 1 %) and the Gaterminated sample a higher Ga content (by 9 ± 1 %) at the surface (as expected). 

Taking the photoionization cross sections [4] into account, one can determine that the (In+Ga) surface 

content of sample M2992 is (2 ± 1) % higher than that of M2995. In addition, one can compute the 

Ga/(In+Ga) ratio of sample M2992 and M2995 are 0.29 and 0.32 (±0.02), respectively (note that these 

values are in good agreement with the surface composition of the pristine samples reported in Ref. 1). 

Contrary to the intensity increase of the spectral Ga 3d/In 4d region observed for the In-terminated 

sample, a decrease in intensity is observed for the Cu 3p/In 4p region (as best seen in the difference 

spectrum in Fig. 7). This suggests that the Cu surface content for the In-terminated sample is lower. In 

order to estimate the impact on the signal intensity in the Cu 3p/In 4p region caused by the observed In 

4d variations, the In 4p photoemission line of an In metal reference (scaled according to the In 4d signal 

intensity of sample M2995) is shown in Fig. 7 for comparison. Apparently, the small change observed in 

the In surface content would have only a minor (i.e., negligible) influence on the overall intensity in the 

spectral region of the Cu 3p/In 4p photoemission lines. 

As additional features in the difference spectrum in Fig. 7 one can observe a local minimum and 

maximum at around 55 eV and a small dip at 64 eV. The latter feature is in the spectral region of the Na 

2s photoemission line. This can be interpreted as a slightly higher Na surface content of sample M2995 

sample compared to sample M2992. Since no pronounced Na 2s photoemission line can be observed, the 

Na concentration is close to the XPS the detection limit. Note that the photoionization cross section [4]

of the accompanying Na 2p line (which one would expect at 31 eV) is less than that of the Na 2s line at 

80 60 40 20 0 Na 2s

In Ref.

Cu 3p/In 4p

Ga 3d/In 4d

Intensity [a.u.]

Binding Energy [eV]

(a) M2992

(b) M2995

Mg Kα

[(a) - (b)] x 5

Se 3d

22 20 18 16

In 4d

(a) - (b) x 3

Ga 3d

Fig. 7 Region of the low binding energy photoemission lines of the investigated In- (M2992, red 

spectrum) and Ga- (M2995, black spectrum) “terminated” CIGSe absorbers. The magnified difference 

spectrum ([(a) – (b) = M2992 – M2995; blue spectrum] as well as the (scaled) In 4p spectrum of an In 

metal reference (gray spectrum) are also shown. The inset shows the magnified region of the Ga 3d/In 4d 

XPS peaks for both samples, together with the fitted Ga 3d and In 4d contribution for sample M2995. 

The computed difference spectrum is again shown for comparison.
11

this excitation energy, and hence the related Na 2p difference feature is not visible due to an insufficient 

signal-to-noise ratio of the measured XPS spectra. The feature in the vicinity of the Se 3d XPS peak 

(around 55 eV), can be explained by a slight shift of the M2992 Se 3d photoemission line to lower 

binding energies. 

The increase of the (Ga+In) and decrease in Cu surface content (at a similar Se surface content) for the 

In-terminated sample can also be interpreted as a more pronounced Cu deficiency for the In-terminated 

sample (note that both samples are Cu deficient compared to the stoichiometric Cu:(In+Ga):Se = 1:1:2 

bulk composition). This is in good agreement with earlier publications, which show a Cu-poor surface 

composition for high-efficiency chalcopyrite absorbers [5-7]. 

In order to investigate the influence of different absorber terminations on the electronic surface structure, 

corresponding UPS and IPES measurements were performed. The recorded UPS (IPES) spectra of 

sample M2992 (in red) and M2995 (in black) are shown on the left (right) side of Fig. 8 on a common 

energy scale relative to the Fermi energy (EF). A close look at the valence band (VB) region reveals that 

the UPS spectrum of the Ga-terminated sample M2995 has more intensity in the spectral region with 

contributions from Cu 3d-derived states (approx. 3 eV below EF). This observation agrees with the 

earlier interpretation that the surface of the In-terminated sample is more Cu deficient. However, the 

position of the valence band maximum (VBM; defined by the leading edge of the UPS spectra) does not 

seem to be influenced by the changes in the surface stoichiometry, as best seen in the corresponding 

region shown in the inset of Fig. 8. The respective linear approximation [shown for the UPS spectrum of 

sample M2992; Fig. 8 (inset)] determines the VBM for both samples to be – 0.75 ± 0.10 eV. The 

spectral conduction band (CB) region of both samples appears very similar, but the IPES spectrum (and 

hence the respective conduction band maximum [CBM]) of sample M2992 is clearly shifted upward

when compared to M2995 [∆CBM = (0.20 ± 0.05) eV]. As apparent from the inset of Fig. 8, the leading 

edge of the IPES spectra (of as-introduced samples) is not as clearly defined (linear) as the respective 

UPS spectra or as IPES spectra of samples cleaned by mild Ar+ sputtering [6], leading to an increased 

uncertainty for the determination of the absolute CBM position. The linear approximation shown for the 

IPES spectrum of the In-terminated sample (Fig. 8, inset) results in a CBM estimate of (0.8 ± 0.3) eV 

above EF, and the CBM is determined as (0.6 ± 0.3) eV for the Ga-terminated sample. Together with the 

determined VBM positions, the corresponding electronic surface band gaps (Eg

Surf) are [1.55 ± 0.3] eV 

and [1.35 ± 0.3] eV for the In- and Ga-terminated samples, respectively. These values are in good 

agreement with previously published surface band gaps for CIGSe absorbers [6,7]. However, the 

estimated Eg

Surf values are higher than the expected bulk band gap (∼1.17 eV according to the quantum 

efficiency measurement of M2992 shown in Ref. 1) and the expected “theoretical” band gap [8] based 

on the here-determined surface Ga/(In+Ga) ratio (1.17 and 1.19 eV for sample M2992 and M2995, 

respectively). Our Eg

Surf finding is consistent with a Cu-deficient surface (found for both samples). Note 

that the observed higher Ga surface content of sample M2995 does not result in a larger 

Eg

Surf value (compared to M2992). Apparently, the less pronounced Cu deficiency (leading to a decrease 

in the band gap) counteracts the (expected) band gap increase with Ga content. 
12

The different solar cell performance of devices made from M2992 or M2995 absorbers is inferred from 

the observed different chemical and electronic surface structure. The larger Eg

Surf of the In-terminated 

absorber will have an impact on the interface formation with the CdS buffer layer. In order to address 

this question, a detailed analysis of the interface formation between the buffer layer and its respective 

absorber using XPS, UPS, and IPES is currently under way. 

In summary, we have investigated the chemical and electronic surface structure of (world-record) CIGSe 

absorbers stemming from deposition processes with different growth termination. As expected, the In 

(Ga) “terminated” sample has a higher In (Ga) surface content. Furthermore, the surface of the Interminated absorber is also (comparatively) Cu poor, which was interpreted as an increased Cu 

deficiency for the In-terminated sample. The surface band gap of the In terminated sample was found to 

be larger than that of the Ga terminated sample, indicating that the degree of Cu deficiency has a more 

pronounced impact on the surface band gap than the Ga/(In+Ga) ratio at the surface. Future experiments 

will clarify if these findings provide an explanation to the origin of higher solar cell device efficiencies 

in In-terminated CIGSe (as opposed to Ga-terminated) absorbers. 

A2. The chemical and electronic structure of the CdS/Cu(In,Ga)Se2 interface

[Collaboration with M.A. Contreras and R. Noufi, National Renewable Energy Laboratory, NREL] 

In order to investigate the chemical and electronic structure of the CdS/CIGSe interface (note that CdS is 

the conventional junction partner of the CIGSe absorber in chalcopyrite-based thin film solar cells), we 

investigated a set of CdS/CIGSe samples from NREL. For those samples the CdS layer thickness was 

varied by means of taking the samples out of the chemical deposition bath after different times (0 - 16 

min). See Table I for a complete list. All samples were characterized by XPS, UPS, IPES, and XES. 

Fig. 8 UPS (IPES) spectra of sample M2992 (red) and M2995 (black) are shown on the left (right) side 

on a common energy scale relative to the Fermi energy EF. The inset shows the magnified region of the 

edge onsets. The linear approximation of the leading edges to derive the VBM and CBM is shown for 

sample M2992. 

-8 -6 -4 -2 0 2 4 6 8

IPES

Normalized Intensity

Energy rel. EF [eV]

M2992

M2995

UPS - He I

∆ CBM

[+/- 0.05 eV]

0.20 

-1 0 1(-/+

1.5(5)

1.3(5)

[+/- 0.3 eV]

ESurf

g

-11) 0 1
13

First, we will focus on our XES results. Fig. 9 shows the respective XES spectra of the Cd M4,5 and In 

M4,5 emission region on a linear (left) and logarithmic scale (right). Already after a deposition time of 1 

min (and above), a Cd M4,5 emission can be clearly identified (in particular on the logarithmic scale), 

which steadily increases with increasing deposition time. Consequently, the In M4,5 emission intensity 

from the CIGSe substrate decreases due to the attenuation by the increasingly thick CdS layer. Close 

inspection of the data shows that the In M4,5 emission is small, but still visible after a deposition time of 

16 min. In order to quantify the thickness of the CdS layer using the XES data, we compared the 

CdS/CIGSe data with (reference) spectra of a thick CdS layer (not shown) and of the uncovered CIGSe 

substrate (bottom spectrum in Fig. 9). All measured spectra were described (in terms of a χ2 fit) as a sum 

of the (weighted) reference CdS and substrate spectra:

sample CdSa b substrate )1( reference ⋅= ⋅+

To derive the thickness of the CBD-CdS layer deposited on CIGSe, we can use both, the attenuation of 

the In M4,5 signal as well as the increase of the Cd M4,5 intensity independently. If a homogeneous cover 

layer of thickness x attenuates the emission from the substrate, then the attenuated substrate emission 

intensity I

sub(x) can be written as 

 

)( .)2( λ*

x

sub

ref

sub eIxI

−

⋅=

Fig. 9 Cd M4,5 and In M4,5 X-ray emission spectroscopy of the investigated set of 

CdS/CIGSe samples on a linear (left) and logarithmic scale (right).
14

Similarly, the intensity of the emission from the cover layer I

cov(x) can be written as

.)3(1)( cov cov *















 −= −

λ

x

ref eIxI

sub

ref I and cov

ref I denote the reference emission intensity of an uncovered substrate and of a cover layer of 

sufficient thickness, respectively (“sufficient” corresponds to a material thickness that results in a 

saturated emission intensity). Furthermore, 

),4( sin

1

sin

1

*

1

















⋅ +















⋅ =

exc em βλαλλ

where λexc and λem are the attenuation lengths in the cover layer for the excitation and emission energy, 

respectively. α and β are the angles of excitation and emission relative to the sample surface, 

respectively (in our case α = β = 45°). In order to obtain the cover layer thickness x, we used the abovedetermined weighting factors a = )( cov xI : cov

ref I and b = xI )( sub : sub

ref I (see Eq. 1). The attenuation lengths 

associated with the Cd M4,5 and In M4,5 emission energies, the excitation energy, and the specific 

overlayer material (here: CdS) are listed in Table II. Assuming that the CdS layer homogeneously covers 

the substrate (as it can be expected if prepared by a wet-chemical deposition method such as CBD), the 

layer thicknesses on CIGSe were determined and are shown in Fig. 10 as a function of the deposition 

time. The given error is assumed to be dominated by the uncertainty in comparing absolute XES 

intensities due to sample (mis)alignment and is estimated to be 10% for the above-mentioned intensity 

ratios. For deposition times of 2 min and above, the values determined using the attenuation of the In 

M4.5 CIS emission are (within the error bars) quite similar to those calculated from the increasing Cd 

Table I

List of the investigated samples with corresponding CdS buffer 

deposition time. 

Sample CdS Deposition Time 

C2106-11 0 min, bare absorber

C2106-21 1 min 

C2106-12 2 min

C2106-22 4 min

C2106-18 8 min

C2106-23 (2 × 8) 16 min 
15

M4.5 cover layer emission intensity. Thus, both approaches (Eq. (2) and (3)) give consistent numbers. 

For thin cover layers the thickness determination based on the Cd M4,5 emission intensity is more 

reliable as indicated by the smaller error bars. The thickness of the CBD-CdS layer after a deposition 

time of 16 min (which corresponds to the standard CdS buffer) on CIGSe is determined to be (70 ± 11) 

nm.

S L2,3 spectra of the investigated samples and of a CdS reference were also recorded. They are shown in 

Fig. 11 again on a linear (left) and a logarithmic scale (right). The main peak of the CdS reference 

spectrum at 147.3 eV (which is actually a doublet indicated by the clearly visible shoulder at 149 eV) is 

due to S 3s electrons decaying into S 2p1/2 and S 2p3/2 core holes. In addition, the two peaks at 150.5 eV 

and 151.8 eV correspond to Cd 4d electrons decaying into the S 2p1/2 and S 2p3/2 core holes, 

respectively. They thus directly indicate sulfur atoms bound to Cd. Furthermore, we observe the upper 

valence band of CdS at about 156 eV. Comparing the spectra of the CdS/CIGSe samples with that of the 

CdS reference, it is obvious that they also show the typical features of a CdS S L2,3 spectrum, especially 

when compared on logarithmic scale. As expected, this becomes more distinct with increasing 

deposition time. 

Table II

X-ray attenuation lengths in CdS (taken from [9]). 

λCdS

Cd M4,5 

emission

(341.3 eV)

λCdS

In M4,5 

emission

(370.4 eV)

λCdS

excitation

(470.0 eV)

189 nm 216 nm 128 nm

0 2 4 6 8 10 12 14 16 18

1

10

100

Thickness [nm]

Deposition time [min]

CBD-CdS/CIGSe

 In M4,5

 Cd M4,5

Fig. 10 CdS layer thickness determined from the attenuation of the In M4,5 emission of 

the substrate or from the increase of the Cd M4,5 emission from the cover layer, 

respectively.
16

Fig. 12 Normalized S L2,3 emission of the investigated set of CdS/CIGSe samples (left). 

Magnified and smoothed presentation of the spectrum of the bare CIGSe sample (right): 

red dots: original data, black solid line: smoothed spectrum.

Fig. 11 S L2,3 emission of the investigated set of CdS/CIGSe samples on a linear (left) and 

logarithmic scale (right). For comparison also the spectrum of a CdS reference is shown.
17

In an earlier paper [10], the absence of the features indicating S-Cd bonds was indicative of intermixing 

processes at the CdS/CIGSe interface. For a detailed evaluation of whether such effects also play a role 

here, the new spectra are shown in Fig. 12, left, with normalization to their maximum. It can be observed 

that the spectrum of the bare (S-free!) CIGSe substrate shows different spectral features compared to the 

spectra of the S-containing samples. A magnified (smoothed) presentation of the CIGSe XES spectrum 

shown in Fig. 12 (right) reveals two spectral features, which are separated by 5.7 eV. Since the latter 

agrees well with the doublet separation of Se 3p1/2 and Se 3p3/2, the features can most likely be attributed 

to Se 4s electrons decaying into Se 3p1/2 and Se 3p3/2 core holes. Note that our group has (for the last ten 

years) repeatedly searched for such Se 3p emission peaks; only recently, a significant improvement of 

the XES spectrometer has made it possible to observe such very weak structures.

A similar analysis approach as used above (i.e., describing the CdS/CIGSe spectra as a sum of the 

weighted reference CdS- and CIGSe-spectra) was used to clarify whether the intensity ratio between the 

features directly indicating S-Cd bonds and the main S 3s peak changes with deposition time or whether 

the spectra of the CdS/CIGSe samples can be explained by a (weighted) superposition of CdS and bare 

CIGSe reference spectra. The exemplary comparison of the experimental data of the “1 min” sample 

with a respective fit is shown in Fig. 13. The fit agrees quite well with the experimental data except 

between 150 -153 eV (the spectral range of the features directly indicating S-Cd bonds). We thus 

conclude that the spectra taken for thin CdS films cannot be explained by a mere superposition of the Se 

substrate signal and the CdS reference. Nevertheless, in contrast to our earlier work, we find a clear Cd 

4d signature (albeit smaller than for the CdS reference) even for the thinnest CdS film. Surface-sensitive 

XPS experiments in Fig. 16 additionally show that after 16 min deposition the Se 3d XPS signal 

vanishes, which is again in contrast to earlier experiments on CdS/CIGSe samples from a different 

source [10].

Fig. 13 Comparison of the ‘S L2,3 spectrum’ of the “1 min” CdS/CIGSe sample with a 

weighted superposition fit. In addition, also the residuum (difference between data and 

fit) is shown.
18

-8 -6 -4 -2 0 2 4 6 8

Eg

[eV]

(+/- 0.15)

1.62

1.84

3.12

CBM

[eV]

(+/- 0.10)

0.78

0.90

1.42

Intensity

Energy rel. EF [eV]

VBM

[eV]

(+/- 0.10)

-0.84

-0.94

-1.70

UPS

He I

IPES

as-deposited

Ar+

-treatment

(50 eV)

40 min

20 min

-8 -6 -4 -2 0 2 4 6 8

Ar+-treatment

(50eV)

40 min

20 min

Eg

[eV]

(+/- 0.15)

2.50

2.58

2.94

CBM

[eV]

(+/- 0.10)

0.57

0.65

0.80

Intensity

Energy rel. EF [eV]

VBM

[eV]

(+/- 0.10)

-1.93

-1.93

-2.14

UPS

He I

IPES

as-deposited

Fig. 15 UPS and IPES spectra of absorber film (left) and the thick (16 min) CdS layer on CIGSe 

(right). To remove adsorbates from the surface both samples have been exposed to a mild Ar+ ion 

treatment. The positions of the valence band maximum and conduction band minimum as derived by a 

linear extrapolation of the leading edges are given next to the spectra. The distance between valence 

band maximum and conduction band minimum gives the surface band gap, which is also listed.

58 56 54 52

Intensity

Binding Energy [eV]

XPS

Mg Kα Se 3d

bare CIGSe

16 (2 x 8)

CBD-CdS

[min]

1

2

4

8

Fig. 14 Se 3d XPS detail spectra of the CdS/CIGSe sample set.
19

This can be interpreted in the following way: We do not find any evidence for a significant S/Se 

intermixing process. Nevertheless, the environment of the sulfur atoms at the growth start of the 

interface clearly deviates from a perfect CdS environment. Whether this is due to a less perfect 

crystalline structure (i.e., the formation of very small nm-scale nanoparticles [11]) or some sulfur 

diffusing into the CIGSe absorber cannot unambiguously be differentiated. 

In the following our measurements with respect to the electronic interface structure are presented and 

discussed. For determining the band alignment at the interface, we have measured the valence and 

conduction band edges of the absorber film and the thick CdS film using UPS and IPES, as shown in Fig. 

15. While samples were packed carefully, surface contaminations with C and O cannot be entirely 

prevented. These surface contaminations obscure the determination of the band edges, clearly seen by 

the attenuated valence band signal related to Cu 3d electrons for the CIGSe absorber film (bottom left 

spectrum in the left panel of Fig. 15). Only after mild Ar+ ion treatment of the surface (50 eV ions at a 

sample current of approx. 100 nA), we are able to remove these contaminations without structural 

damage to the absorber surface. After about 40 min of ion treatment, no further changes in the spectra 

are observed, and we can derive a surface band gap for the CIGSe absorber film of 1.63 (±0.15) eV. This 

band gap is increased with respect to the optical bulk value (1.23 eV), which can be explained by the 

copper-poor stoichiometry of the sample that we derive from our XPS measurements.

1 2 4

0.00

0.05

0.10

0.15

0.20

0.25

0.30

 In 3d/S 2s In 3d/Cd 3d

 Cu 2p/S 2s Cu 2p/Cd 3d

 Ga 2p/S 2s Ga 2p/Cd 3d

 Se 3d/S 2s Se 3d/Cd 3d

Viibb [eV]

Deposition Time [min]

Avrg.: (0.18 +/- 0.07) eV

Fig. 16 Values of the interface-induced band bending (iibb) as derived from monitoring 

the line shifts of different core levels.
20

Similarly, the investigation of the CdS surface shows an increased surface band gap for the as-prepared 

surface. After cleaning with Ar+ ions for 20 minutes the influence of the surface contaminations is 

largely removed, giving us a surface band gap of 2.50 (±0.15) eV, which agrees well with the (optically 

measured) bulk band gap of CdS. 

The band alignment at the CdS/CIGSe interface is now determined in two steps. In the first 

(approximate) step, we compare the band edge positions of the CIGSe absorber with those of the thick 

CdS/CIGSe film which are given in Fig. 15. This first approximation indicates a small cliff of 0.21 eV. 

This value has now to be refined in the second step to account for changes in band bending of the 

absorber occurring when the interface to the CdS buffer layer is formed. This can be achieved by 

following the shifts of XPS core level positions of the absorber and the CdS buffer layer using the thin 

CdS/CIGSe samples (1, 2, and 4 min) where both lines from the CIGSe absorber and from the CdS 

buffer layer are visible. To minimize effects due to chemical shifts caused by bonding at the interface as 

well as possible intermixing of the layers, we have used different combinations of core levels of the 

CIGSe absorber and the CdS buffer layer as shown in Fig. 16. In total we have derived 24 different 

values and get a mean value of 0.18 (±0.07) eV. Compared to earlier investigations of this interface with 

different CIGSSe absorbers [6,12,13], this is the largest interface-induced band bending we have found, 

indicating the presence of an interface dipole. Furthermore, the overall spread of the various Viibb values 

is smaller than that of other CdS/CIGSe interfaces for which significant S/Se intermixing at the interface 

could be shown [10] (note that, as reported above we did not find an indication for S/Se intermixing in 

the case of the investigated NREL samples). These findings indicate a somewhat different chemical and 

electronic character of the CdS/CIGSe interface formation process at NREL compared to our previous 

work. 

Fig. 17 Schematic diagram of the band alignment at the CdS/CIGSe interface (all values 

are given in eV). The band extrema of the CIGSe and CdS surfaces (as determined by 

UPS and IPES) are shown on the left and right, respectively. The center shows the band 

alignment at the interface after taking the interface-induced band bending into account. 

CIGSe

Surface

CdS

Surface

CIGSe/CdS

Interface

EF

VBM

CBM

CBO = -0.03

VBO = -0.90

0.57 1.93

0.78 0.85

Eg = 1.63

Eg = 2.50

CIGSe

Surface

CdS

Surface

CIGSe/CdS

Interface

EF

VBM

CBM

CBO = -0.03

VBO = -0.90

0.57 1.93

0.78 0.85

Eg = 1.63

Eg = 2.50
21

We can now apply the correction for the interface-induced band bending to the conduction band 

minimum and valence band maximum values derived above to get the band alignment at the CdS/CIGSe 

interface. The results are shown in Fig. 17. We find a valence band offset of -0.90 (±0.15) eV and a flat 

conduction band alignment with an offset of -0.03 (±0.15) eV. 

In summary, we have derived the electronic structure of the CdS/CIGSe interface in thin film solar cells 

prepared by NREL. The conduction band alignment at the CdS/CIGSe interface is flat, as expected for a 

high efficiency CIGSe solar cell. Furthermore, we find direct evidence for a strong interface dipole. 

Together with the previously reported lack of significant intermixing at the CdS/CIGSe interface, we 

thus find a modified electronic and chemical interface structure compared to previously studied (less 

efficient) devices from other manufacturers.
22

A3. Depth-dependent band gap in chalcopyrite thin-film solar cell absorbers

[Collaboration with the S. Nishiwaki and W. Shafarman, Institute of Energy Conversion, IEC, U 

Delaware] 

This investigation is initially based on two kinds of samples: S-free CIGSe/Mo/glass and S-containing 

CIGSSe/Mo/glass test structures. The chalcopyrite absorber films (approx. 2 μm thick) were prepared 

using multi-source thermal co-evaporation without intentional composition gradients. The Mo-coated 

soda-lime glass substrate was held at 550°C during absorber formation (see Ref. 14 for more details). 

The samples were investigated by different spectroscopic techniques in order of greatest surface 

sensitivity. The samples were first characterized by UPS and IPES, since these (electron-based) 

techniques are most surface sensitive among the applied spectroscopies and thus would be most 

influenced by any surface contamination. Next, the electronic structure of the surface-near bulk of the 

samples was investigated by XES/XAS at the ALS. For the transport to the ALS, the samples were again 

carefully packed and sealed in an inert atmosphere. Once at the ALS, the samples were mounted in and 

transferred through ambient air into the UHV analysis chamber of the SXF endstation (base pressure 

below 5⋅10-8 mbar). Finally, the samples were transferred back to UNLV (in air) and optical reflection 

spectra of the CIGSe and CIGSSe samples were measured using a conventional UV-Vis-NIR 

spectrophotometer (Varian, Cary 5000). 

The spectra recorded for the CIGSe and CIGSSe samples by the different spectroscopic techniques are 

shown in Fig. 18. The different spectra (optical reflection, left; XES/XAS, center; UPS/IPES, right) 

allow a determination/estimation of the Eg of the investigated samples with three different information 

depths, as will be described in the following. As shown in Fig. 18 (left), the optical reflection spectra 

exhibit pronounced interference patterns. These (Fabry-Perot) oscillations are caused by interference of 

light reflected at the absorber/air and absorber/substrate interfaces and thus depend on the thickness d of 

the absorber and its absorption coefficient α. For photon energies higher than Eg, no oscillations are 

observed since the photons are completely absorbed in the CIGSe (CIGSSe) layer. In the spectral region 

with pronounced interferences the CIGSe (CIGSSe) absorber must consequently be transparent. Hence, 

in first approximation, the photon energy at which the oscillations in the reflection spectrum disappear 

corresponds to the Eg. To further refine this approach, we note that the Eg is actually shifted to lower 

energies since the absorption coefficient only gradually increases above Eg and since the thickness d of 

the absorber is sufficiently small that some light reflected at the absorber/substrate interface (although 

attenuated) will still be able to interfere with light reflected at the absorber/air interface. Thus, our 

approach to determine Eg from the “oscillation-free” reflectance R is as follows. We assume that the 

interferences oscillate around the “true” reflectance value and thus approximate R by the mean of the 

interference maxima and minima. Based on the respective envelopes of the extrema (fitted by 2nd order 

( )

( ) 2 ln )5(

min

max min 



 





−

− = RRRR

αd
23

polynomials; see Fig. 18, left) the mean spectral characteristic of R can be constructed. The absorption 

coefficient was estimated by applying Eq. (5) [15], where Rmax (Rmin) is the maximal (minimal) 

reflectance in the considered spectral range (note that this approach gives reasonable results since the 

absolute value of α is not relevant for the estimation of Eg). Since it is well known that Cu chalcopyrites 

are direct semiconductors, the approximated absorption coefficient is plotted as (αhν)

2 vs. the photon 

energy hν to determine Eg (see insets in Fig. 18, left). We find a band gap energy of [1.14 (1.38) ± 0.05] 

eV for the CIGSe (CIGSSe) absorber. 

Fig. 18 (center) shows the Cu L3 XES and XAS spectra of the investigated CIGSe and CIGSSe samples. 

For comparison also the spectra of metallic Cu are shown. Since XES and XAS probe occupied and 

unoccupied states, respectively, the combination of both spectra gives information about Eg. However, 

the band gap values based on these measurements are approximate and represent a lower limit due to the 

potential existence of core exciton features in the XAS spectra. Eg was determined as the intersection of 

the baseline with a linear extrapolation of the leading XES and XAS edges. The Eg for the CIGSe 

(CIGSSe) absorber is found to be [1.52 (1.64) ± 0.20] eV. 

Fig. 18 Left: Optical reflection spectra of the investigated CIGSe (top) and CIGSSe (bottom) sample. 

The respective envelopes of the interference extrema and the constructed mean values (red dotted 

lines) are also shown. Insets: Approximated absorption coefficient for the CIGSe (top) and CIGSSe 

(bottom) sample plotted as (αhν)

2 vs. the photon energy hν. The given band gap energies have an 

error of ± 0.05 eV.

Center: Cu L3 x-ray emission (XES, left) and absorption (XAS, right) spectra of the investigated 

CIGSe (middle spectra) and CIGSSe (bottom spectra) samples. For comparison, the spectra of 

metallic Cu (top) are also shown. The linear approximation of the leading edges for the determination 

of “Eg” (a lower bound for the true band gap value – see text) is indicated by the solid green and red 

lines. The given band gap energies “Eg” have an error of ± 0.20 eV. 

Right: UV photoelectron spectroscopy (UPS, left) and inverse photoemission (IPES, right) 

measurements of the investigated CIGSe (top spectra) and CIGSSe (bottom spectra) samples. For each 

sample, two sets of spectra are shown, one for the as-introduced (thin solid lines) and one for the 

cleaned samples (dots). The linear approximation of the leading edges for the determination of Eg is 

indicated by the solid green and red lines. The given band gap energies Eg have an error of ± 0.15 eV.

0.75 1.00 1.25 1.50 1.75

0.8 1.0 1.2

1.0 1.2 1.4

Normalized Reflection

Photon Energy [eV]

CIGSSe

CIGSe

(αhν)

2

1.14 eV

(αhν)

2

1.38 eV

-8 -6 -4 -2 0 2 4 6 8

UPS - He I

CIGSSe

Eg

[eV]

1.92

Cu 3d

CIGSe

Eg

[eV]

1.62

Normalized Intensity

Energy rel. EF [eV]

IPES

Cu 3d

925 930 935

CIGSSe

"Eg

" = 1.64 eV

Normalized Intensity

Photon Energy [eV]

CIGSe

"Eg

" = 1.52 eV

XES XAS

Cu

metal

Cu L3
24

Fig. 19 Schematic presentation of the determined band gap energies of the investigated CIGSe (top) 

and CIGSSe (bottom) samples using the three different spectroscopic approaches (optical reflection, 

left; x-ray emission and absorption, center; UV photoelectron spectroscopy and inverse photoemission, 

right). The respective information depths of the applied spectroscopies are also shown. The gray area 

represents the measurement error.

Finally, the corresponding surface-sensitive UPS and IPES spectra of the investigated CIGSe and 

CIGSSe samples are shown in Fig. 18, right. For each sample, two sets of spectra are shown: One for the 

as-introduced and thus surface-contaminated sample and one after cleaning the sample surface by an 

ion-stimulated desorption process with 50 eV Ar+ ions. Similar to the XES and XAS measurements, 

UPS and IPES give information about the occupied and unoccupied states, respectively. Hence, by 

combination of both techniques it is possible to measure the band gap at the surface. Again, linear 

extrapolation of the leading edges of the UPS and IPES spectra is used to determine the valence band 

maximum (VBM) and conduction band minimum (CBM), respectively, resulting in the surface band gap 

Eg = CBM - VBM. For the clean surface of the CIGSe (CIGSSe) absorber we find a band gap energy of 

[1.62 (1.92) ± 0.15] eV. 

In order to understand the observed variations in Eg, the information depth of the different spectroscopic 

techniques has to be considered. Under the assumption that α ≈ 104

cm

-1 for photon energies slightly 

above Eg (as reported for CuInSe2 [16]) the information depth of the optical reflection measurement is ≈

2000 nm, which is in the range of the thickness of the investigated CIGSe (CIGSSe) layers (note that for 

this and the following considerations the information depth is defined as the thickness from which 90% 

of the overall signal is collected). XAS (in total fluorescence yield mode) and XES have similar 
25

information depths ( ≈ 200 nm) (based on [9]). According to the “universal curve” [17], the inelastic 

mean free path (IMFP) of the detected photoelectrons for the UPS (He I) measurements of the 

investigated CIGSe (CIGSSe) samples is ≈ 10 Å, which results in an information depth of ≈ 2 nm. The 

information depth of our IPES measurements is approx. twice that of the UPS technique. 

Fig. 19 shows a summary of our results. It shows a schematic presentation of the determined band gap 

energies of the investigated CIGSe (CIGSSe) sample using the three different spectroscopic approaches 

(optical reflection, left; XES/XAS, center; UPS/IPES, right) as a function of the information depth. For 

the investigated CIGSe (CIGSSe) samples we find an increasing band gap energy with decreasing 

information depth and, in particular, the formation of a surface region with significantly higher Eg. The 

smaller difference of the band gap energies determined by XES/XAS and UPS/IPES for the CIGSe 

sample compared to that for the CIGSSe sample suggests that the Eg-widened (i.e., Cu-poor) region is 

more pronounced for the CIGSe absorber. 

Currently we are extending our experiments also to wide-gap chalcopyrites such as CuInS2 and CuGaSe2

in order to test the hypothesis that the Eg-widened surface region is less pronounced for absorbers with 

larger bulk band gaps. This might explain the lower performance of devices based on wide-gap 

chalcopyrite absorbers compared to the world record efficiencies reported for their low-gap counterparts.

In summary, we presented a detailed study that demonstrates how a combination of various 

spectroscopic techniques can be used to derive depth-dependent band gap information for compound 

semiconductors. We find that the band gap of Cu(In,Ga)(S,Se)2 thin film solar cell absorbers varies 

greatly as a function of depth and that this variation is dependent on the presence or absence of S at the 

absorber surface. This approach is expected to be very useful in determining the electronic structure of 

real-world thin film surfaces from a variety of materials systems for energy conversion devices.
26

A4. Chemical structure of different interfaces in Cu(In,Ga)Se2 thin film solar cell absorber 

structures

[Collaboration with K. Ramanathan, M. Contreras, and R. Noufi, National Renewable Energy 

Laboratory, NREL] 

These investigations were based on two different samples, namely Cu(In,Ga)Se2 “CIGSe”/Mo/glass and 

CdS/CIGSe/Mo/glass. To investigate also the interfaces buried beneath the absorber, namely the 

CIGSe/Mo interface and the Mo/glass interface, we prepared additional samples by cleaving the samples 

at those interfaces. For doing so, we have glued the front side of both samples to stainless steel plates 

and divided the stack into two parts. For the CIGSe/Mo/glass sample this cleavage takes place at the 

CIGSe/Mo-interface, as our measurements show. In contrast, the adhesion between Mo back contact and 

glass substrate was very weak for the investigated CdS/CIGSe/Mo/glass sample, such that this sample 

was cleaved at the Mo/glass interface. In total we thus had six different samples, which are 

schematically shown in Table III (the arrows show the direction of measurement):

Table III

Schematic presentation of the investigated sample surfaces/interfaces. Note that the arrows indicate the 

direction of measurement.

Sample Name in the text sketch

CIGSe/Mo/glass CIGSe front

CIGSe

Mo

glass

CIGSe/Mo/glass cleaved, 

top part 

CIGSe back

CIGSe

Mo

glass

CIGSe/Mo/glass cleaved, 

bottom part Mo front 

CIGSe

Mo

glass

CdS/CIGSe/Mo/glass CdS

CdS

CIGSe

Mo

glass

CdS/CIGSe/Mo/glass 

cleaved, top part Mo back

CdS

CIGSe

Mo

glass

CdS/CIGSe/Mo/glass 

cleaved, bottom part Glass front

CdS

CIGSe

Mo

glass

All samples were investigated by XES and the first three in the list also by XPS. Both techniques 

provide detailed information about the chemical properties of the investigated samples and complement 

each other with respect to their information depth (XES: bulk sensitive with an information depth of a 
27

few 100 nm, depending on the investigated line; XPS: surface sensitive with an information depth of a 

few nm).

Fig. 20 shows the XPS survey spectra of the CIGSe front, the CIGSe back and the Mo front. The names 

chosen for those samples are confirmed by those very surface sensitive spectra, since Mo is only found 

on the “Mo front“ sample and not on the “CIGSe back” sample.

Since the samples were inevitably exposed to air prior to the measurements, a contamination layer 

consisting of C and O compounds is formed on their surface, complicating an exact quantitative analysis 

of the peak intensities. However, quite some qualitative information can be gathered from the XPS 

survey spectra shown in Fig. 20. 

We find that the In 3d signal is stronger on the CIGSe front than on the CIGSe back side. This is 

because a higher amount of In is replaced by Ga at the absorber back side, which can be seen from the 

stronger Ga 2p signal at the absorber back side. 

The Na amounts on the three samples differ strongly. The highest Na content is found on the absorber 

front side, whereas much less Na is located around the CIGSe/Mo interface represented by the two other 

samples.

1000 800 600 400 200 0

Ga 3p

In 4s In 4p

In 4d/

Ga 3d

Ga LMM

Mo 3d

Se 3s Se 3p Se 3d

Na KVV

Cu LMM

In 3d

In 3p

In MNN

O KVV

C KVV

Cu 2p

C 1s

O 1s

Na 1s

Mo front

CIGSe back

Intensity

Binding Energy (eV)

CIGSe front

Mg Kα

Ga 2p

Fig. 20 XPS survey spectra of the CIGSe front, the CIGSe back, and the Mo front of a Cu(In,Ga)Se2

NREL absorber.
28

186 188 190 192 194 196 198 200

glass front

Mo back

CIGSe back

x10

Intensity

Emission Energy (eV)

x100

Mo M4,5

hv=250 eV

Mo front

900 950 1000 1050 1100

glass front

Mo back

Mo front

CdS

CIGSe front

CIGSSe back

hv=1200 eV

Na K

Ga L2,3

Intensity

Emission Energy (eV)

Cu L2,3

Ga 3s -> Ga 2p1/2

/20

 Fig. 21 Mo M4,5 XES spectra of Mo back, Fig. 22 Cu L2,3, Na K, and Ga L2,3 spectra 

 Mo front, CIGSe back, and glass front. of all investigated samples. 

We find strong indications for different intermixing processes at the CIGSe/Mo interface, as will be 

discussed in the following. While only trace amounts of In and (within the detection limit of the 

experiment) no Cu is found, the Se signal increases at the Mo front side, pointing towards the formation 

of a MoSe2 compound, as was found before for Cu(In,Ga)(S,Se)2 absorbers [18]. This finding is 

corroborated by the Mo M4,5 XES spectra shown in Fig. 21. Here the M4,5 emission of the Mo front side 

is compared with that of the Mo back side (note that the spectra of the CIGSe back and the glass front 

only show some small Mo remainders). In accordance with the assignment to MoSe2 (with a smaller Mo 

density than in metal Mo) the Mo signal is much weaker at the Mo front side.

Besides the Se diffusion, also a Ga diffusion into the back contact can be observed, which is manifested 

in the Ga 2p signal seen in the XPS survey spectrum of the Mo front side in Fig. 20. The more bulk 

sensitive XES measurements (mean free path of around 200 nm for energies around 1000 eV) in Fig. 22, 

where the Cu L2,3, Na K, and Ga L2,3 emission was recorded in one energy window, show that this Ga 

diffusion is very strong. While only small amounts of Cu can be found on the Mo front, the Ga L2,3

intensity is more than half of that on the CIGSe back. The high Ga L2,3 intensity found in the spectrum 

of the CIGSe back reveals that the Ga content at the CIGSe back compared to the CIGSe front is not 

only higher at those surfaces but in the whole surface near region. 

From the Na K emission lines in Fig. 22, additional information about the Na distribution can be derived. 

The strongest Na signal is found on the soda lime glass substrate, as expected. In contrast to the surface 

sensitive measurements above, the Na signal at the CIGSe back is stronger than that at the CIGSe front, 

which can be explained as follows. It is known that Na at the CIGSe front is mainly localized at its 
29

surface and only small amounts are found in the bulk or at grain boundaries near the front surface [19]. 

This localized Na gives a strong signal in the surface sensitive XPS measurements, whereas the Na 

content in the bulk and at grain boundaries next to the surface plays a more important role for the XES 

spectra. Therefore, the higher Na signal in the Na K XES spectra can be attributed to a higher Na content 

in the bulk region next to the back contact and/or at grain boundaries next to the CIGSe back. 

In summary, we have investigated the CIGSe/Mo and Mo/glass interfaces by cleaving suitable samples. 

We suggest the formation of a MoSe2 layer at the surface of the Mo back contact film. Furthermore, a 

strong Ga diffusion into the back contact can be observed. The actual compound formed during this 

diffusion and its role for the electronic structure and performance of the solar cell remains to be 

investigated in future experiments. Finally, we were able to draw a detailed picture of the Na distribution 

throughout the entire cell structure, showing Na mainly at the back side of the Mo back contact, at grain 

boundaries near the back contact, and, in particular, at the CIGSe front surface.
30

A5. The chemical and electronic structure of the deeply buried chalcopyrite/Mo interface

[Collaboration with S. Nishiwaki and W. Shafarman, Institute of Energy Conversion, IEC, U Delaware] 

This work focuses on the deeply buried interface between absorber and Mo back contact in chalcopyrite 

thin film solar cells. These investigations were based on two different types of samples, namely 

CIGSe/Mo/glass and CIGSSe/Mo/glass. In order to make the interface between absorber and Mo 

accessible for characterization by photoelectron spectroscopy, we developed a suitable lift-off (cleavage) 

technique, which allowed us to cleave the absorber/Mo/glass samples at the desired interface. It was 

found that gluing the front side of the absorber/Mo/glass thin film stack to a stainless steel plate using a 

conductive (Ag-containing) UHV compatible epoxy allows a subsequent division of the stack in two 

parts and provides the necessary conductivity for the XPS, UPS, and IPES measurements. Note that the 

“lift-off” process itself took place in a N2-filled glovebox or glovebag, which was directly connected to 

the load lock chamber of the UHV surface analysis system in order to minimize contamination of the 

freshly prepared cleavage planes. The scheme in Fig. 23 visualizes the different investigated surfaces for 

each lift-off process: 

(a) Surface

(b) Back Surface

(c) Mo-Side

Fig. 23 Scheme of the lift-off process and visualization of the different prepared and investigated 

surfaces.

(a) Surface

“Lift-off”

(in N2-filled glovebox)

(b) Back Surface

(c) Mo-Side

(a) Surface

“Lift-off”

(in N2-filled glovebox)

(b) Back Surface

(c) Mo-Side “Lift-off”

(in N2-filled glovebox)

(b) Back Surface

(c) Mo-Side

 CIG(S)Se

 glass

 Mo
31

First, we will focus on the results gained from the investigation of the samples by XPS. Fig. 24 shows 

the XPS survey spectra of the Surface, Back Surface, and Mo-Side of the CIGSSe (top panel) and 

CIGSe (bottom panel) samples. Although the samples were handled and shipped under inert gas 

atmosphere and stored in UHV, one can observe distinct peaks which can be ascribed to C and O on the 

Surface (a), stemming from a contamination layer formed on the absorber surface. In contrast, we find 

only minor amounts of oxygen on the Back Surface (b). This shows that the applied cleavage process in 

an N2 filled glovebag/glovebox and the immediate transfer of the cleaved samples into the attached 

UHV characterization system provides surfaces with minimized contamination (oxidation), which is of 

1000 800 600 400 200 0

(c)

(b)

Mo MNV

Mo MVV

Mo 4s

Mo 4p

Mo 3d

Mo 3p

Mo 3s

Ga 3d/In 4d

Ga LMM

Ga 3p Cu 3s

S 2s/Se 3s

S LMM Na 1s

In 3s

O KLL

C KVV

Cu LMM

Cu 3p

B 

Normalized Intensity

 

Mg Kα

S

In 3d

O 1s

C 1s

In 3p

Cu 2p

In MNN

Ga 2p

S 2p/Se 3p

Se 3d

(a)

1000 800 600 400 200 0 Mo 3d (O Kα

)

Se 3d (O Kα

)

C KVV

Na 1s

In 3s

In 3p

In 4p

Cu LMM

Ga LMM

In 4s/Ga 3s Ga 3p

Mo MNV

Mo 3s

Mo MVV

Mo 3p

Mo 3d

Mo 4p

In MNN

C 1s

Se 3s

In4d/Ga 3d

Se 3d

O 1s

Se 3p

Ga 2p

Normalized Intensity

Binding Energy [eV]

Mg Kα

(a)

(b)

(c)

In 3d

Cu 2p

Fig. 24 XPS survey spectra of the different accessible “surfaces” before and after lifting-off the

chalcopyrite absorber from the Mo/glass substrate (top: CIGSSe, bottom: CIGSe): (a) Surface, (b) Back 

Surface, and (c)Mo-side.
32

large importance for a subsequent determination of the electronic surface (and interface) structure. Note 

that the residual C 1s signal observed on the Back Surface points to a carbon incorporation into the 

absorber layer.

The intensity difference of all absorber features (e.g., Ga 2p, Cu 2p, and In 3d) between Surface and 

Back Surface can be explained by the different attenuation of the differently thick contamination layers. 

At first sight (see also discussion below) no Mo emission can be found on the Back Surface and only 

minor amounts of the absorber components (as indicated by the small In 3d peak - the most prominent 

absorber feature) can be observed on the Mo-Side. This confirms that the cleavage occurs at the 

absorber/Mo interface with only some chalcopyrite grains remaining on the back contact (this 

characteristic of the lift-off mechanism was already described in A4). In consequence, the comparatively 

large intensities of the photoemission and Auger lines of S and Se observed on the Mo Front point to the 

formation of a Mo(S,Se)2 and MoSe2 layer at the back contact for the CIGSSe and CIGSe sample, 

respectively. This was similarly reported/suggested in the past [20-25]. However, as shown in Fig. 25 

(which shows the S 2p/Se 3p lines of the different CIGSSe-based samples), the S/Se ratio in the 

Mo(S,Se)2 film does not mirror the S/Se ratio of the absorber. In this case, the formation of MoS2 is 

clearly preferred over the formation of MoSe2. 

170 168 166 164 162 160 158

Se 3p3/2

Normalized Intensity

Binding Energy [eV]

Mg Kα

S 2p

(a)

(b)

(c) x 0.5

Se 3p1/2

Fig. 25 S 2p/ Se 3p photoemission of the different “surfaces” before and after lifting-off the CIGSSe 

absorber from the Mo/glass substrate: (a) Surface, (b) Back Surface, and (c)Mo-side.
33

A more detailed analysis of our data indicates that (besides the formation of the Mo(S,Se)2) additional 

chemical interactions at the absorber/back contact interface take place. A detailed comparison of the S 

2s/Se 3s and Mo 3d energy range for the different samples (Surface, Back Surface, Mo-Side, Fig. 26) 

reveals that a (minor) Mo signal at the Back Surface can be identified at both absorber/back contact 

structures. This agrees with our earlier X-ray emission (XES) measurements of different 

chalcopyrite/back contact structures (see A4), which also showed Mo at the absorber back side. It is at 

present unknown whether this is due to Mo diffusion into the Back Surface or the presence of some 

residual Mo(S,Se)2 from the cleavage process.

A further important result from the spectra in Fig. 26 is the finding that the Mo(S,Se)2 layer (in the 

CIGSSe case) is apparently thinner than the MoSe2 layer (in the CIGSe case), as evidenced by the 

residual metallic Mo 3d doublet (filled black peaks in Fig. 26, bottom left) in the CIGSSe case.

Our previous XES data also showed an accumulation/diffusion of Ga at/into the back contact (see A4). 

Comparing the intensity of the most prominent photoemission lines of the absorber constituents (Ga 

2p3/2, Cu 2p3/2, and In 3d3/2) of the Back Surface and the absorber remainders at the Mo-Side, our present 

XPS data confirms the accumulation of Ga. As shown in Fig. 27, we find that the Ga 2p intensity from 

the Mo-side is significantly larger than the intensity of the other absorber elements; note that the peaks 

in Fig. 27 were normalized to the absolute intensity of the respective peaks observed for the Back 

Surface, and hence a larger Ga peak directly indicates the presence of additional Ga on/in the Mo-side 

surface. 

Fig. 26 (Overlapping) S 2s/Se 3s and Mo 3d photoemission lines of the different “surfaces” before and 

after removing the chalcopyrite absorbers from the Mo/glass substrate (left: CIGSSe, right: CIGSe): (a) 

Surface, (b) Back Surface, and (c)Mo-side. 

234 232 230 228 226 224

Mo 3d

Se 3s

Normalized Intensity

Binding Energy [eV]

Mg Kα

(a)

(b)

(c)

x 0.1

S 2s

Mo [II]

234 232 230 228 226

Normalized Intensity

Binding Energy [eV]

(a)

(b)

(c) x 0.1

Se 3s

Mo 3d

Mg Kα
34

170 168 166 164 162 160 158 156 Ga 3s Se 3p3/2 Se 3p1/2 S 2p3/2

Norm. Intensity

Binding Energy [eV]

Mg Kα

S 2p1/2

CIGSSe

front/ back

Fig. 28 Region of the S 2p, Se3p, and Ga 3s photoemission lines of the CIGSSe Surface (black dots) and 

CIGSSe Back Surface (red line). For the CIGSSe Surface spectrum also the corresponding fits (black 

lines) are shown. 

In the following we will focus on the comparison of the CIGSSe Surface and CIGSSe Back Surface in 

terms of their composition. In order to determine the Ga/(Ga+In) = X and the S/(S=Se) = Y composition 

of the front and back side of the Cu(In1-XGaX)(SYSe1-Y)2 absorber, the S 2p/Se 3p (Fig. 28) and the Ga 

3d/In 4d detail spectra (Fig. 29) are evaluated. For direct comparison of CIGSSe Surface and CIGSSe

Back Surface, the spectra are normalized to their maximum. In addition, the spectra of the latter have 

been shifted to lower binding energies by 0.1 eV for maximal overlap. The observed higher binding 

Fig. 27 Comparison of the Ga 2p3/2, Cu 2p3/2, and In 3d3/2 photoemission lines of the Back Surface 

(top) and absorber remainders at the Mo-Side (bottom). The intensities are normalized to the 

respective maximum of the Back Surface. The corresponding spectra of the CIGSSe/CIGSe samples 

are shown in the left and right three panels, respectively.

1120 1116

Normalized to max. Intens. of BackSurf signalAl Kα

Ga 2p3/2

935 930

Binding Energy [eV]

Mg Kα

Cu 2p3/2

454 452 450

In 3d3/2

Mg Kα

1120 1116 935 930 454 452 450

Binding Energy [eV]

In 3d3/2

Mg Kα

x 3

Cu 2p3/2

Mg Kα

x 3

Normalized Intensity

Ga 2p3/2

Mg Kα
35

energies for both, the S 2p/Se 3p and the Ga 3d/In4d spectra for the CIGSSe Back Surface point to an 

increased surface band bending compared to the CIGSSe Surface. Fig. 28 shows the region of the S 2p, 

Se 3p, and Ga 3s photoemission lines of the cleaved samples CIGSSe Surface (black dots) and CIGSSe 

Back Surface (red line). For the CIGSSe Surface spectrum also the corresponding fits (black lines) are 

shown. The comparison of the spectra clearly shows that the S/(S+Se) ratio at the CIGSSe Surface is 

higher than that of the CIGSSe Back Surface. For quantification of the S/(S+Se) ratio we have used the 

intensity of the S 2p3/2 and the Se 3p3/2 photoemission lines, which were determined by fitting the 

corresponding contributions of the spectra with Voigt area functions (exemplarily shown in Fig. 28). 

Due to the similar binding energies for the S 2p and the Se 3p peaks, it was legitimately assumed that the 

inelastic mean free paths and the analyzer characteristics are the same for the corresponding 

photoelectrons. Thus, for the calculation of the S/(S+Se) ratio the corresponding peak intensities were 

only corrected by the respective cross-sections (from Ref. [4]). In consequence, Y (the S/(S+Se) ratio) of 

the CIGSSe Surface and CIGSSe Back Surface was determined as 0.79 and 0.65, respectively, as shown 

in Table II. For the determination of X (the Ga/(Ga+In) ratio) of the CIGSSe Surface and CIGSSe Back 

Surface, we again have used adjacent photoemission lines (as shown in Fig. 29). The direct comparison 

of the Ga 3d/In 4d spectra of the cleaved samples CIGSSe Surface (black line) and CIGSSe Back 

Surface (red dots) reveals that X of both sample surfaces is quite similar. Indeed, the quantification of 

the Ga 3d5/2 and the In 4d5/2 photoemission lines determines X values (0.36 and 0.33, see Table IV) 

which are, within the error margins, identical for the CIGSSe Surface and CIGSSe Back Surface.

Table IV

Surface composition as determined by our XPS analysis. 

sample X 

Ga/(Ga+In)

Y 

S/(S+Se)

Eg 

[eV]

CIGSSe 

Surface 0.36 0.79 1.68 

CIGSSe 

Back Surface 0.33 0.65 1.58 

Assuming a stoichiometric absorber composition (in particular no Cu deficiency towards the absorber 

surface) the X and Y compositions should allow a direct (“theoretical”) estimate of the absorber band 

gap (Eg). Using equation (6) (Ref. [8]) we have determined Eg for the CIGSSe Surface to 1.68 eV and 

for the CIGSSe Back Surface to 1.58 eV (see Table IV). 

)6(54.055.013.008.013.01 2 2 Eg ++= YXX XY +++ YX
36

In order to directly measure the band gap at the CIGSSe Surface and CIGSSe Back Surface, we 

additionally characterized a set of cleaved samples with UV photoelectron spectroscopy (UPS) and 

inverse photoemission (IPES). The corresponding UPS and IPES spectra for the CIGSSe Surface and 

Back Surface are shown in Fig. 30 left and right, respectively. The linear extrapolation of the leading 

edge of the UPS and IPES spectra results in the position of the valence band maximum (VBM) and 

conduction band minimum (CBM), respectively. Thus, the sum of the absolute values of VBM and 

CBM reveals the electronic surface band gap Eg

Surf. In addition to the UPS/IPES spectra and the 

respective VBM, CBM, and Eg

Surf values of the as-prepared CIGSSe Surface (left) and CIGSSe Back

Surface (right) samples, Fig. 30 also shows the electronic properties after several cleaning cycles with a 

mild Ar+ beam (Eion = 50 eV, Isample < 1μA). As can be observed in Fig. 30, the VBM, CBM, and Eg

Surf

values remain constant after 30 min (CIGSSe Back Surface) and 60 min (CIGSSe Surface) of Ar+

treatment. The difference in required treatment time is most likely due to the more pronounced 

contamination layer on the CIGSSe Surface, as discussed above. The respective XPS survey spectra 

after Ar+ treatment (not shown) also indicate clean sample surfaces without C- or O-containing 

contaminants. The experimentally determined electronic surface band gaps of (1.94 ± 0.15) eV for the 

CIGSSe Surface and (2.09 ± 0.15) eV for the CIGSSe Back Surface are larger than the band gaps 

calculated from the surface stoichiometry determined by XPS (see Table IV). Enlarged electronic

surface band gaps have in the past [6,12,13] been explained by the formation of a Cu-poor “Ordered 

Vacancy Compound” [26] or “Ordered Defect Compound” [27] surface phase, which deviates from the 

Cu : In+Ga : S+Se ratio of 1 : 1 : 2 forming on top of the chalcopyrite absorber. Our XPS results indeed 

show a Cu-poor surface stoichiometry for both, the CIGSSe Surface as well as the CIGSSe Back Surface. 

However, it is not clear whether the Cu-poor surface phase at the CIGSSe Back Surface is present from 

the beginning or whether its formation is induced by the lift-off process and subsequent exposure to N2

atmosphere in the glovebox/glovebag or UHV, respetively. 

24 21 18 15 In 4d5/2 Ga 3d5/2 Ga 3d3/2

Norm. Intensity

Binding Energy [eV]

Mg Kα CIGSSe

front/back

In 4d3/2

Fig. 29 Ga 3d and In 4d spectra of the cleaved samples CIGSSe Surface (black line) and CIGSSe Back

Surface (red dots). For the CIGSSe Back Surface spectrum also the corresponding fits (red lines) are 

shown.
37

In the past it was discussed whether the chalcopyrite/Mo interface results in an Ohmic contact [23,28,29] 

or whether a Schottky contact [30,31] is formed. Kohara et al. even suspected the observed MoSe2 layer 

to be responsible for an Ohmic contact at the chalcopyrite/Mo interface [23]. In a conventional CIGSSebased solar cell device the charge carriers are generated in the low-gap, p-type chalcopyrite absorber and 

are separated in the electric field caused by the pn-junction formed by depositing the wide-gap, n-type 

window material onto the absorber. Thus, the photogenerated electrons (holes) have to travel to the front 

(back) contact to be able to contribute to the photocurrent. In consequence, the bandoffset of the valence 

band at the CIGSSe/Mo interface has to be known in order to judge the quality of the interface in terms 

of unhindered current transport. Hence, in order to shed more light on the electronic properties of the 

chalcopyrite/Mo interface, also the CIGSSe Mo-side was additionally characterized by UPS and IPES. 

The corresponding spectra of a freshly cleaved sample are shown in Fig. 31. Again the VBM and CBM 

are determined by linear extrapolation of the leading edges of the UPS and IPES spectrum, respectively. 

The resulting surface band gap of (1.30 ± 0.15) eV agrees surprisingly well with that of MoS2 (1.20 – 

1.35 eV [32]), which confirms the preferred formation of MoS2 over that of MoSe2 at the CIGSSe/Mo 

interface as discussed above. 

 

Fig. 30 UPS and IPES spectra (black dots) and derived VBM, CBM, and surface band gap values of 

as-prepared CIGSSe Surface (left) and CIGSSe Back Surface (right) samples. In addition, the 

corresponding spectra are also shown after different cleaning steps by a mild (50 eV Ar+) sputter 

cleaning. The solid red lines represent the linear extrapolation of the leading edge of the respective 

spectra. Note that “0”on the energy scale indicates the position of the Fermi edge EF. 

-8 -6 -4 -2 0 2 4 6 8

Egap

[eV]

(+/- 0.15)

1.94

1.96

2.23

3.29

CBM

[eV]

(+/- 0.1)

1.10

1.14

1.14

1.43

Normalized Intensity

Energy rel. EF

VBM

[eV]

(+/- 0.1)

-0.84

-0.82

-0.99

-1.86

Cu 3d

approx. 

90 min

approx.

60 min

approx.

30 min

as-prepared50 eV Ar=-ions

(Isample < 1 μA)

UPS

He I IPES

-8 -6 -4 -2 0 2 4 6 8

Egap

[eV]

(+/- 0.15)

2.09

2.11

2.28

CBM

[eV]

(+/- 0.1)

1.22

1.22

1.26

Normalized Intensity

Energy rel. EF [eV]

VBM

[eV]

(+/- 0.1)

-0.87

-0.89

-1.02

UPS IPES

He I

Cu 3d

approx.

60 min

approx.

30 min

as-prepared50 eV Ar+-ions

(ISample < 1μA)
38

The scheme in Fig. 32 shows a summary of the positions of the valence band maxima, the conduction 

band minima and the surface band gaps for all investigated samples. Neglecting any potential interfaceinduced band bending, Fig. 34 shows that the valence band at the CIGSSe/Mo(S2) interface is (within 

the error bars) aligned and thus no barrier for hole transport across that interface is present. Furthermore, 

the conduction band at the CIGSSe Back is slightly higher than at the CIGSSe Surface, possibly 

suggesting the presence of a “back surface field” that repels minority charge carriers (electrons) from the 

CIGSSe/Mo interface.

Fig. 32 Schematic summary of the valence band maxima (VBM), conduction band minima (CBM), 

electronic surface band gaps, and Fermi energy position. All numbers are given in eV and have an 

error of 0.10 eV (VBM and CBM) and 0.15 eV (band gaps). 

Surface Back Surf. Mo-Side

0.40

0.90 0.84

1.10

0.87

1.22

CBM

[eV]

VBM

[eV]

EF

[eV]

1.94 2.09

1.30

Fig. 31 UPS and IPES spectra (black dots) and VBM, CBM, and surface band gap values of a freshly 

cleaved CIGSSe Mo-Side sample. The solid red lines represent the linear approximation of the leading 

edge of the respective spectra. Note that “0” on the energy scale indicates the position of the Fermi 

edge EF. 

-8 -6 -4 -2 0 2 4 6 8

Egap

[eV]

(+/- 0.15)

1.30 CBM

[eV]

(+/- 0.1)

0.40

Normalized Intensity

Energy rel. EF [eV]

VBM

[eV]

(+/- 0.1)

-0.90

IPES

UPS

He I

-8 -6 -4 -2 0 2 4 6 8

Egap

[eV]

(+/- 0.15)

1.30 CBM

[eV]

(+/- 0.1)

0.40

Normalized Intensity

Energy rel. EF [eV]

VBM

[eV]

(+/- 0.1)

-0.90

IPES

UPS

He I
39

Based on our investigations it is – for the first time – possible to draw a complete picture of the chemical 

and electronic properties of the deeply buried chalcopyrite/back contact interface. We have found a 

pronounced chemical interaction between absorber and back contact, namely the formation of MoSe2

(and Mo(S,Se)2) and a “diffusion” of Ga into the Mo layer. In addition, we could (tentatively) derive a 

flat valence band alignment at this interface. 
40

B1. The chemical structure of the CdTe/CdS interface investigated by XES 

[Collaboration with the group of A. Compaan, U Toledo] 

These investigations were based on two sets of samples, namely differently treated CdS thin films and 

CdTe/CdS thin film stacks, respectively (see Table V for a complete list). For the latter set of samples, 

the impact of CdCl2-treatment on the CdTe/CdS thin film stacks was investigated, while for the CdS thin 

films also the influence of Cu-diffusion was analyzed. In addition, some powder samples (CdS, CdSO4, 

CdCl2) were characterized for comparison.

Table V

In total, we thus investigated eight different samples.

Sample Treatment Name in the text 

CdS 

(thin film on glass) none as-grown CdS 

CdS 

(thin film on glass) CdCl2-treated CdCl2-treated CdS 

CdS 

(thin film on glass) Cu-diffused Cu-diffused CdS 

CdTe/CdS 

(thin film stack on glass) none 

as-grown CdTe/CdS 

CdTe/CdS

(thin film stack on glass) CdCl2-treated CdCl2-treated CdTe/CdS 

CdSO4, CdS, CdCl2

(powders) N/A CdSO4, CdS, CdCl2 references

All samples were investigated by X-ray emission spectroscopy (XES). As mentioned above, this 

technique provides detailed information about the chemical properties of the investigated samples, and, 

as a photon-in photon-out technique, probes the “near-surface” bulk. In our case, where we have focused 

on the S L2,3 and Cl L2,3 emission, XES has an information depth of about 100 nm. 

Fig. 33 shows the S L2,3 XES spectra of the set of CdS samples. At first sight, all spectra look identical. 

The main feature (1) at 147.3 eV (which is actually a doublet indicated by the clearly visible shoulder at 

149 eV) can be ascribed to S 3s electrons decaying into S 2p1/2 and S 2p3/2 core holes. In addition, the 

two peaks at 150.5 eV and 151.8 eV (2) correspond to Cd 4d electrons decaying into the S 2p1/2 and S 

2p3/2 core holes, respectively, and thus indicate sulfur atoms bound to Cd. Furthermore, we observe the 

upper valence band of CdS at about 156 eV. Altogether, all spectra show the typical features of a S L2,3 

spectrum of CdS, which is also confirmed by the respective spectrum of the CdS reference. However, a 

close inspection of the data shows small but significant differences for, e.g., the S L2,3 XES spectrum of 
41

145 150 155 160

(3)

(2)

CdS reference

Cu-diffused CdS

CdCl2

-treated CdS

Normalized Intensity

Emission Energy (eV)

S L2,3 XES

hv=200 eV

as-grown CdS

(1)

Fig. 33 S L2,3 XES spectra of a set of differently treated CdS thin films. In addition, a 

corresponding spectrum of a CdS reference is also shown (top spectrum). The main 

features are labeled (1) – (3).

the as-grown CdS (a) compared to that of the Cu-diffused CdS sample (b), as shown in Fig. 34. In Fig. 

34, the raw spectra and the corresponding difference spectrum are shown. The comparison of the 

(enlarged) difference (a)-(b) with a CdS and a CdSO4 reference spectrum reveals that the features in the 

difference spectrum can be ascribed to the formation of S-O bonds and a localization of the Cd 4dderived band. 

The spectra of the differently treated CdTe/CdS thin film stacks are shown in Fig. 35. Since the 

thickness of the CdTe layer, which covers the CdS, is significantly beyond the information depth of XES, 

one would not expect to observe a S L2,3 signal. This is indeed the case for the as-grown CdTe/CdS thin 

film stack. However, the S L2,3 spectrum of the CdCl2-treated CdTe/CdS sample clearly shows some 

small (note the magnification factor) spectral features, which are similar to the CdS spectra shown in Fig. 

33. A comparison with the spectrum of the as-growm CdS thin film as well as with the CdSO4 reference 

reveals that the S L2,3 spectrum of the CdCl2-treated CdTe/CdS thin film stack can be described as a 

superposition of spectral features of both reference samples. Most prominently, the two peaks at 150.5 

eV and 151.8 eV directly indicate S-Cd bonds, and the peaks at 153.9 eV, 155.1 eV, and 161.0 eV can 

be directly ascribed to S-O bonds. In consequence, this points to a CdCl2-treatment-induced crack or 

void formation of the CdTe layer or, more likely, to a strong intermixing (the latter is commonly 

accepted in the community). For the CdTe/CdS thin film stacks we also investigated the Cl L2,3 XES 

spectra, as shown in Fig. 36 (multiplied by the given magnification factors). The two major features of 

the observed spectra at 182.3 eV and 183.8 eV can again be ascribed to 3s electrons decaying into the 

2p1/2 and 2p3/2 core holes, this time for electrons in Cl. As expected, we find a Cl L2,3 XES spectrum for 

the CdCl2-treated CdTe/CdS sample (middle spectrum in Fig. 36), the main features of which are quite 

similar to those of the CdCl2 reference. The fact that the structures between 185 and 190 eV and between 
42

145 150 155 160

difference spectrum (a)-(b)

 (a) Cu-diffused CdS

 (b) as--grown CdS

Intensity

Emission Energy (eV)

S L2,3 XES

hv=200 eV

CdSO4 reference

Fig. 34 S L2,3 XES spectra of a Cu-diffused CdS thin film (a) plotted upon that of an asgrown CdS sample (b). In addition, the corresponding (enlarged) difference spectrum (ab) and a CdSO4 reference spectrum is also shown. 

Fig. 35 S L2,3 XES spectra of a set of differently treated CdTe/CdS thin film stacks. In 

addition, the corresponding spectrum of an as-grown CdS thin film and a CdSO4

reference spectrum are also shown. Note the different magnification factors.

145 150 155 160

as-grown CdS

CdCl2

-treated CdTe/CdS

x500

S L2,3 XES

hv=200 eV

Intensity

Emission Energy (eV)

x140

as-grown CdTe/CdS

CdSO4 reference
43

190 eV and 194 eV are less pronounced than in the reference sample, indicates the presence of Cl atoms 

that are not directly bound to Cd. Surprisingly, one can also identify a weak (note the magnification 

factor) Cl L2,3 XES spectrum for the as-grown CdTe/CdS thin film stack, which shows all the 

characteristics of CdCl2. Whether this is due to extrinsic contamination or a result of the used sample 

preparation (and thus significant) is the topic of future experiments. 

 

Based on these results, we will continue the investigation of these CdTe/CdS samples and extend them 

by investigating customized sample series with UPS and IPES to get insight into the band alignments at 

the various interfaces of the device structure of the CdTe-based solar cell. 

175 180 185 190 195

as-grown CdTe/CdS 

CdCl2-treated CdTe/CdS x15

Normalized Intensity

Emission Energy (eV)

x140

Cl L2,3 XES

hv=220 eV

CdCl2 reference

Fig. 36 Cl L2,3 XES spectra of a set of differently treated CdTe/CdS thin film stacks. In 

addition, the corresponding spectrum of CdCl2 reference spectrum is also shown. Note 

the different magnification factors.
44

B2. The chemical structure of the CdTe/CdS interface investigated by XPS

[Collaboration with the group of X. Wu, National Renewable Energy Laboratory, NREL] 

In order to become familiar with the special characteristics of CdTe/CdS samples with respect to their 

investigation in photoemission measurements, we conducted first XPS measurements of CdTe/CdS 

samples prepared by the group of X. Wu (NREL). The first set of test structures consisted of the 

following samples: 

Table VI

Complete list of investigated samples.

Sample Treatment Name in the text 

CdTe/CdS 

(thin film stack on glass) none #1 

CdTe/CdS 

(thin film stack on glass) CdCl2-treated and etched #3 

CdTe/CdS 

(thin film stack on glass) CdCl2-treated #4 

Cu (5nm)/CdTe/CdS 

(thin film stack on glass) 

Before Cu deposition: CdCl2-

treated and etched

After Cu deposition: None 

#5 

Cu (150nm)/CdTe/CdS 

(thin film stack on glass)

Before Cu deposition: CdCl2-

treated and etched

After Cu deposition: None 

#6 

Cu (5nm)/CdTe/CdS 

(thin film stack on glass) 

Before Cu deposition: CdCl2-

treated and etched

After Cu deposition: 

Annealing for 30s @ 250oC 

#7 

Cu (150nm)/CdTe/CdS 

(thin film stack on glass) 

Before Cu deposition: CdCl2-

treated and etched

After Cu deposition: 

Annealing for 30s @ 250oC 

#8 

In view of the high surface sensitivity of XPS, all samples showed significant surface contamination (see 

discussion below), prohibiting a meaningful subsequent characterization by UPS or IPES. However, 

there is evidence that the contamination (oxidation) is concentrated at the surface, most likely caused by 

exposing the samples to ambient air. This indicates that with proper sample handling and, possibly, a 

low-energy (50 eV) ion desorption step, sufficiently clean surfaces can be obtained. The fact that Te is 

indeed only oxidized at the sample surface is confirmed by the Te 3d3/2 spectra shown in Fig. 37 (left). 

All spectra show a significant high binding energy feature at approx. 587 eV, indicative for oxidized 

tellurium, except the corresponding spectrum of sample #6. Here, the tellurium is covered by a thick 
45

[nominal 150 nm] Cu layer preventing its oxidation. (Note that a tellurium XPS-signal is visible 

“through” the thick Cu layer, which is a strong indication that the Cu does not cover the underlying 

CdTe completely.) The “buried” non-oxidized tellurium indicates that the oxidation takes place after the 

actual production process. Furthermore, the graph shows the Te 3d3/2 photoelectrons not only excited 

with Mg Kα (hν = 1253.6 eV) but also with Al Kα excitation (hν = 1486.6 eV). Using Al Kα excitation, 

the electrons have a higher kinetic energy and thus according to the “universal curve” of the inelastic 

mean free path (IMFP) [17] a larger IMFP. Therefore, the corresponding spectra (Fig. 39 (left), blue) are 

less surface sensitive. It can be observed that the intensity of the TeOx component (high binding energy 

feature) in those spectra is smaller as compared to that of the respective spectra conducted with Mg Kα

excitation, also confirming that the tellurium is only oxidized at the sample surface. 

An additional interesting result is shown in Fig. 37, right. While sample #6 (thick [nominal 150 nm] Cu 

layer on the CdTe/CdS stack) shows metallic Cu, the very thin [nominal 5nm] Cu film of sample #5 is 

partly oxidized, as indicated by a second peak at higher binding energies around 943 eV. The spectra 

clearly show the presence of CuO, since, for Cu2O, the peak would only be shifted by ~1 eV to higher 

binding energies with respect to the Cu 2p3/2 photoemission line of metallic Cu. Furthermore, for CuO 

two peaks as observable in the present case are typical. In addition, it can be observed that after 

annealing of the Cu/CdTe/CdS thin film stacks the thin Cu layer is vanished (compare spectra of 

samples #5 and #7 Fig. 37, right) and the thick Cu layer is also partly oxidized (compare spectra of 

samples #6 and #8 Fig. 37, right), respectively. 

590 588 586 584 582 580

Al Kα

#6

#5

#4

#3

Intensity

Binding Energy (eV)

#1

XPS

Te 3d3/2

TeOX

Mg Kα

945 940 935 930

#8

Intensity

Binding Energy (eV)

#7

#6

#5

CuO

Cu

XPS

Mg Kα

Cu 2p3/2

 before and

 after annealing

Fig. 37 Detail spectra of the Te 3d3/2 (left) and Cu 2p3/2 (right) photoemission line of the 

different investigated CdTe/CdS thin film stacks.
46

C1. Chemical surface structure of CIGSe absorbers on stainless steel 

[Collaboration with Global Solar Energy Inc.] 

Surprisingly, very little is known about the chemical and electronic structure in real-world industrialgrade samples, i.e., manufactured in large-scale, high-throughput equipment in an industrial 

environment. Global Solar Energy, Inc. (“GSE”), e.g., has pioneered a unique robust process to 

manufacture CIGSe solar cell devices which can hardly be simulated on laboratory scale. While other 

companies pursue the approach of in-line deposition on rigid glass substrates, GSE is the only company 

to date using a roll-to-roll coating of the complete solar cell thin film layer stack on flexible substrates. 

Central questions of the latter approach are how the chemical structure of the deposited materials differs 

from that of conventionally prepared materials and how the process-specific parameters influence the 

material properties. The roll-to-roll process itself presents a set of process-specific challenges as well, 

such as chemical interactions between the deposited material (front side) and the back side of the 

flexible substrate during roll-up after each preparation step.

In order to investigate the material properties in CIGSe-based thin film solar cells induced by roll-to-roll 

deposition on flexible substrates a set of samples directly taken out of the production process of Global 

Solar Energy, Inc. (“GSE”) was investigated. For the different samples which represent the status after 

individual deposition steps in the manufacturing process, both, the front and back side was investigated 

by XPS. 

Fig. 38 shows respective schemes of the investigated test structures. Shown is a typical structure used by 

GSE for their commercial PV process. Different materials are being utilized at GSE for the top 

electrode. In this specific case ZnO and ITO was used.

Fig. 38 Schemes of the investigated samples.
47

Fig. 39 XPS survey spectra of the front (top panel) and back side (bottom panel) of the 

investigated test structures.

Si 2p

Si 2s

Mo 3d*

Ga 2p

In 4s

In 4p Mo 4s

Mo 4p

Fe 2s

Cr 2p3/2

Fe LMM

Fe 2p

Fe 3s

Fe 3p

In 4d

S 2s

S LMM

S 2p

Mo MNV

Mo 3s

Mo 3p

Mo 3d

Cd 4d In 4d

Ga LMM

Zn 2p

Cu 3s

Cu 3p

Cu LMM

Se 3s

Se 3d

Se 3p

C KVV

C 1s

Cu 2p

Na 1s

Cd 3p

Sn MNN

O KLL

In 3p

In MNN

Cd MNN

Cd 3d

Sn 3d

In 3d

Normalized Intensity

O 1s

Mg Kα

* excited with O Kα

5

4

3

2

1sample

1200 1000 800 600 400 200 0 Cu 2p Ga 2p In 3p Si 2s Fe 3s Se 3d* Zn 3s Zn 3d Zn 3p Se MMN Mo 3d* Se 3p Se 3d In 3d Cr 2p3/2 Si 2p Fe 3p Fe 2s Fe 2p C KVV

Mo MNV

Mo 4s

Mo 4p

C 1s

Mo 3s

Mo 3p

Mo 3d

O KLL

O 1s

Normalized Intensity

Binding Energy [eV]

Mg Kα

Zn 2p

Zn LMM

sample

5

4

3

2

1
48

238 236 234 232 230 228 226

5

4

3

Normalized Intensity

Binding Energy [eV]

Mg Kα

Mo 3d

Mo

Mo-O

Mo-Se

sample

2

B k Sid

Fig. 41 XPS Mo 3d detail spectra of the 

back side samples #2-5 (experimental 

data shown as dots, fits indicated by 

solid lines). 

Fig. 39 shows the survey spectra of the front (top panel) and back side (bottom panel) of the investigated 

test structures. For the front side samples we find the elements of the respective deposited material. As 

indicated by the small C 1s (and O 1s) XPS peak the surface contamination could successfully be 

1122 1120 1118 1116 938 936 934 932 930 928 456 454 452 450

Normalized Intensity

Mg Kα Al Kα

Ga 2p3/2

Sample 4

Back Side

Sample 3 x 20

 Binding Energy [eV]

Al Kα

Cu 2p3/2

x 20

Front Side

Sample 3

x 20

In 3d3/2

x 20

Fig. 40 XPS Ga 2p3/2 (left), Cu 2p3/2 (center), and In 3d3/2 (right) detail spectra of the front 

side sample #3 (top spectra) and the back side samples #3 and #4 (bottom two spectra). 
49

minimized due to a suitable packaging and shipping procedure. We find a relatively high amount of 

oxygen on sample #2, which points to the formation of MoOx. The back side samples (except #1) are 

dominated by Mo-related XPS signals. Surprisingly, we find a relatively high amount of selenium for 

samples #3 - #5 which is indicative for the formation of MoSe2 upon absorber formation (see also detail 

spectra in Fig. 41). Furthermore, we find CIGSe-related XPS peaks on back side sample #3 and #4 (see 

also the detail spectra in Fig. 40), which are explained by an interaction of front side absorber material 

with the back side during roll-up of the stainless steel tape after CIGSe deposition. 

The comparison of the Ga 2p3/2, Cu 2p3/2, and In 3d3/2 detail spectra of the front side sample #3 and the 

back side samples #3 and #4 in Fig. 40 shows an abundance of Ga at the back side sample surfaces (note 

the magnification factors indicated for the Cu 2p3/2 and In 3d3/2 XPS peaks of the back side samples). 

This can be interpreted not only as a simple deposition of front side material on the back side, but 

indicates a significant chemical interaction leading to the observed Ga accumulation. 

Fig. 41 shows the XPS Mo 3d detail spectra of the back side samples #2-5. For all spectra, we find 

pronounced spectral features which deviate from a Mo 3d spectrum of a single Mo species. By peak 

fitting the spectra of sample #1, we were able to identify two Mo 3d doublets which we ascribe to 

metallic Mo and Mo-O bonds, respectively. As indicated by the pronounced change in the spectral shape 

of the Mo 3d spectra after CIGSe deposition (sample #3) and confirmed by our peak fit analysis, the 

(oxidized) Mo is converted into MoSe2 upon absorber formation. The fact that the metallic Mo 3d 

doublet is still visible indicates that the MoSe2 layer might not be closed or too thin (i.e., thinner than the 

XPS information depth) to completely attenuate the Mo 3d photoelectrons stemming from the 

underlying metallic Mo.
50

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52

Presentations (contributed oral presentations, unless marked as “poster” or “invited oral”)

T1. M. Bär, “Formation of the buffer/CuInS2 interface”, WCPEC-4, Hawaii, May 2006. 

T2. L. Weinhardt, “Unfavorable band alignment at the CdS/Cu(In,Ga)S2 interface in thin film solar 

cells”, WCPEC-4, Hawaii, May 2006. 

T3. C. Heske, “Chemical and electronic properties of the CdS/Cu(In,Ga)(S,Se)2/Mo junctions in thin 

film solar cells”, EMRS2006, Nice, France, May 2006. 

T4. C. Heske, “Surface and interface analysis in Las Vegas”, Seminar Experimentelle Physik II, 

University of Würzburg, Germany, July 20, 2006 (invited oral). 

T5. C. Heske, “Characterizing surfaces and interfaces in thin film solar cells by soft X-ray 

spectroscopy", Symposium: Photovoltaic, Solar Energy Materials and Thin Films, XV 

International Materials Research Congress, Cancun, Mexico, August 2006 (invited oral). 

T6. C. Heske, “How to reveal the chemical and electronic properties of interfaces, buried layers, and 

liquids with soft x-ray spectroscopy”, Materials Science and Engineering Department, Stanford 

University, August 31, 2006 (invited oral). 

T7. C. Heske, “How to reveal the chemical and electronic properties of interfaces, buried layers, and 

liquids with soft x-ray spectroscopy”, Physics Department, Boston University, December 8, 2006

(invited oral). 

T8. M. Bär, “Chemical and Electronic Properties of the Front and Back Surfaces of Chalcopyrite Thin 

Film Solar Cell Absorbers”, 2007 MRS Spring Meeting, San Francisco, April 2007. 

T9. L. Weinhardt, “X-ray and electron spectroscopy investigation of interfaces and surfaces in CdTe

thin film solar cells”, 2007 MRS Spring Meeting, San Francisco, April 2007. 

T10. J. Zhou (NREL), “The mechanism of J-V “roll-over” in CdS/CdTe devices”, 2007 MRS Spring 

Meeting, San Francisco, April 2007. 

T11. C. Heske, “Introduction” at the Young Scientist Tutorial, 2007 MRS Spring Meeting, San 

Francisco, April 2007 (invited oral). 

T12. M. Bär, L. Weinhardt, “Surface/interface characterization and modification” at the Young 

Scientist Tutorial, 2007 MRS Spring Meeting, San Francisco, April 2007 (invited oral). 

T13. C. Heske, “Characterization of the electronic and chemical structure at Cu(In,Ga)(S,Se)2 and CdTe 

thin film solar cell interfaces”, 2007 Solar Review Meeting, Denver, April 2007 (poster). 

T14. C. Heske, “Surface and Interface Analysis of Real-World Cu(In,Ga)(S,Se)2 Thin Film Solar 

Cells”, Global Solar, Tucson, AZ, July 2007. 
53

T15. M. Bär, “Bandoffset Tailoring – A Tool To Improve Thin Film Solar Cells“, Hahn-MeitnerInstitute, Berlin, July 2007 (invited oral). 

T16. M. Bär, “Surface, bulk, and interface properties of Cu(In,Ga)Se2-based thin film solar cell 

structures, 15th International Conference on Vacuum Ultraviolet Radiation Physics, Berlin 

(Germany), July 29 – August 3, 2007 (poster). 

T17. M. Bär, “The chemical structure of the CdS/Cu(In,Ga)Se2 interface in high-efficiency thin film 

solar cells”, 22nd European Photovoltaic Solar Energy Conference, Milan (Italy), September 3-7, 

2007. 

T18. M. Bär, “Improving Thin Film Solar Cells by Deliberate Interface Tailoring”, Auswahlsitzung 

Helmholtz-Gemeinschaft Berlin, November 2007 (invited oral). 

T19. M. Bär, “Bandoffset Tailoring – A Tool To Improve Thin Film Solar Cells”, Physikalisches 

Kolloquium, Brandenburgische Technische Universität Cottbus, April 2008 (invited oral). 

T20. M. Bär, “Depth-dependent band gap energies in chalcopyrite thin film solar cell absorbers”, 33rd

IEEE Photovoltaic Specialists Conference, San Diego (CA), USA, May 11-16, 2008. 

T21. C. Heske, “Soft x-ray spectroscopy of the Cu(In,Ga)(S,Se)2/Mo back contact interface”, European 

Materials Research Society Spring Meeting, Strasbourg, France, May 26-30, 2008 (invited oral).

T22. M. Bär, “Spectroscopic Characterization of Thin Films Used in Energy Conversion Devices”, 

American Chemical Society 42nd Western Regional Meeting, Las Vegas, USA, September 22-26, 

2008 (invited oral). 

T23. M. Bär, “How soft x-ray spectrosocpies can help to improve thin film solar cells”, 

Berufungsvortrag, Brandenburgische Technische Universität Cottbus, February 2009 (invited 

oral). 

T24. M. Bär, “How soft x-ray spectroscopies can help to reveal the secrets of chalcopyrite-based thin 

film solar cells”, Spring Meeting of the Materials Research Society, San Francisco (CA), USA, 

April 13-17, 2009 (invited oral). 

T25. C. Heske, “Introduction” at the Young Scientist Tutorial, 2009 MRS Spring Meeting, San 

Francisco, April 13, 2009 (invited oral). 

T26. S. Pookpanratana, “Spectroscopic Analysis of the Chemical Structure at the CdS/Cu(In,Ga)Se2

Interface in High-Efficiency Solar Cell Devices”, 34th IEEE Photovoltaic Specialists Conference, 

Philadelphia (PA), USA, June 7–12, 2009. 

T27. Xiangxin Liu (U of Toledo), “Migration and Oxidation of Sulfur at the Back Contact in CdTe 

Cells”, 34th IEEE PVSC, Philadelphia, PA, June 7-12, 2009. 
54

T28. M. Bär, “Chemical structure of the CdS/CuGaSe2 interface – pushing the limit of depth-resolved 

analysis by soft x-ray emission spectroscopy”, E-MRS Spring Meeting 2009, Strasbourg, France, 

June 8-12, 2009 (poster). 

Publications

Deliverables:

1st year – one or more submitted conference articles

2nd year – a submitted journal and one or more submitted conference articles

3rd year – a submitted journal and >1 submitted conference articles 

P1. Conference: "Chemical bath deposition of CdS thin films on CuInS2 and Si substrates – a 

comparative x-ray emission study", M. Bär, L. Weinhardt, O. Fuchs, J. Klaer, J. Peiser, H.-W. 

Schock, E. Umbach, and C. Heske, Proceedings 4th World Conference on Photovoltaic Energy 

Conversion (WCPEC), Waikoloa, Hawaii, May 2006, p. 416-419.

P2. Conference: "Comparison of band alignments at various CdS/Cu(In,Ga)(S,Se)2 interfaces in thin 

film solar cells", L. Weinhardt, O. Fuchs, D. Groß, G. Storch, N.G. Dhere, A.A. Kadam, S.S. 

Kulkarni, C. Heske, and E. Umbach, Proceedings 4th World Conference on Photovoltaic Energy 

Conversion (WCPEC), Waikoloa, Hawaii, May 2006, p. 412-415.

P3. Journal: "Surface modifications of Cu(In,Ga)S2 thin film solar cell absorbers by KCN and 

H2O2/H2SO4 treatments", L. Weinhardt, O. Fuchs, D. Groß, E. Umbach, C. Heske, N.G. Dhere, 

A.A. Kadam, and S.S. Kulkarni, J. Appl. Phys. 100, 024907 (2006). 

P4. Conference/Journal: "Chemical properties of the Cu(In,Ga)Se2/Mo/glass interfaces in thin film 

solar cells", L. Weinhardt, M. Blum, M. Bär, C. Heske, O. Fuchs, E. Umbach, J.D. Denlinger, K. 

Ramanathan, and R. Noufi, Thin Solid Films 515, 6119-6122 (2007) (Proceedings EMRS2006, 

Nice, France, May 2006). 

P5. Conference: “The mechanism of J-V “roll-over” in CdS/CdTe devices”, J. Zhou, X. Wu, Y. Yan, S. 

Asher, J. L. F. Da Silva, Su-Huai Wei, L. Weinhardt, M. Bär, and C. Heske, ”Thin-Film 

Compound Semiconductor Photovoltaics – 2007”, Materials Research Society Symposium 

Proceedings, San Francisco, April 2007, Vol. 1012, p. 491-496. 

P6. Conference: “Band Gap Energy of Chalcopyrite Thin Film Solar Cell Absorbers Determined by 

Soft X-Ray Emission and Absorption Spectroscopy”, M. Bär, L. Weinhardt, S. Pookpanratana, C. 

Heske, S. Nishiwaki, W. Shafarman, O. Fuchs, M. Blum, W. Yang, and J.D. Denlinger, Proc. 33rd

IEEE Photovoltaic Specialists Conference, San Diego (CA), USA, May 11-16, 2008. 

P7. Journal: “Depth-resolved band gap in Cu(In,Ga)(S,Se)2 thin films”, M. Bär, S. Nishiwaki, L. 

Weinhardt, S. Pookpanratana, O. Fuchs, M. Blum, W. Yang, J.D. Denlinger, W.N. Shafarman, and 

C. Heske, Appl. Phys. Lett. 93, 244103 (2008). 
55

P8. Journal: “Chemical structures of the Cu(In,Ga)Se2/Mo and Cu(In,Ga)(S,Se)2/Mo interfaces”, M. 

Bär, L. Weinhardt, C. Heske, S. Nishiwaki, and W. N. Shafarman, Phys. Rev. B 78, 075404 

(2008).

P9. Journal: “Electronic level alignment at the deeply buried absorber/Mo interface in chalcopyritebased thin film solar cells”, M. Bär, S. Nishiwaki, L. Weinhardt, S. Pookpanratana, W. N. 

Shafarman, and C. Heske, Appl. Phys. Lett. 93, 042110 (2008). 

P10. Journal: “Resonant inelastic soft x-ray scattering map of CdS”, L. Weinhardt, O. Fuchs, A. 

Fleszar, M. Bär, M. Blum, M. Weigand, J.D. Denlinger, W. Yang, W. Hanke, E. Umbach, and C. 

Heske, Phys. Rev. B 79, 165305 (2009). 

P11. Journal: “Chemical and electronic surface structure of 20%-efficient Cu(In,Ga)Se2 thin film solar 

cell absorbers”, M. Bär, I. Repins, M.A. Contreras, L. Weinhardt, R. Noufi, and C. Heske, Appl. 

Phys. Lett., in print. 

P12. Conference (submitted to Proc. 34th IEEE PVSC): “Spectroscopic Analysis of the Chemical 

Structure at the CdS/Cu(In,Ga)Se2 Interface in High-Efficiency Solar Cell Devices”, S. 

Pookpanratana, I. Repins, M. Bär, R. Félix, M. Blum, L. Weinhardt, W. Yang, J.D. Denlinger, 

M.A. Contreras, and C. Heske 

P13. Conference (submitted to Proc. 34th IEEE PVSC): “Migration and Oxidation of Sulfur at the Back 

Contact in CdTe Cells”, Xiangxin Liu (U Toledo), A.D. Compaan, Kai Sun, L. Weinhardt, M. Bär, 

S. Pookpanratana, C. Heske, O. Fuchs, W. Yang, and J.D. Denlinger 

Other deliverable: “A solar cell (manufactured by one of the partners within the TFPPP) with novel 

attributes where the junction were engineered according to Subcontractor’s recommendation” 

The fulfilment of this deliverable was pursued with several partners in parallel: K. Ramanathan (NREL), 

X. Wu (NREL), M. Beck (Global Solar), K. Ramanathan (Miasolé), M. Beck (Solyndra), and U. Schoop 

(Global Solar). Due to the unexpected termination of the TFPPP program on 9/30/2006 (i.e., after 16 

months after the receipt of first funds in our project) and significant changes in personnel and goals of 

the various partners, this deliverable could not be achieved. Nevertheless, substantial insights into 

various interfaces of CuInGaSe2- and CdTe-based solar cells could be obtained and were (are) published 

in the openly accessible literature and presented at pertinent conferences, such that the results can be 

easily incorporated into the processes at former TFPPP and other partners. 
F1146-E(10/2008) 

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PLEASE DO NOT RETURN YOUR FORM TO THE ABOVE ORGANIZATION.

1. REPORT DATE (DD-MM-YYYY)

September 2009

2. REPORT TYPE

Subcontractor Report

3. DATES COVERED (From - To)

June 2005 - June 2009

4. TITLE AND SUBTITLE

Characterization of the Electronic and Chemical Structure at the 

Thin Film Solar Cell Interfaces

5a. CONTRACT NUMBER

DE-AC36-08-GO28308

5b. GRANT NUMBER

5c. PROGRAM ELEMENT NUMBER

6. AUTHOR(S)

C. Heske

5d. PROJECT NUMBER

NREL/SR-520-46434

5e. TASK NUMBER

PVB91110

5f. WORK UNIT NUMBER

7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES)

University of Nevada, Las Vegas

Las Vegas, Nevada 89154-4003

8. PERFORMING ORGANIZATION

REPORT NUMBER

XXL-5-44205-12

9. SPONSORING/MONITORING AGENCY NAME(S) AND ADDRESS(ES)

National Renewable Energy Laboratory

1617 Cole Blvd.

Golden, CO 80401-3393

10. SPONSOR/MONITOR'S ACRONYM(S)

NREL

11. SPONSORING/MONITORING 

AGENCY REPORT NUMBER

NREL/SR-520-46434

12. DISTRIBUTION AVAILABILITY STATEMENT

National Technical Information Service

U.S. Department of Commerce

5285 Port Royal Road

Springfield, VA 22161

13. SUPPLEMENTARY NOTES

NREL Technical Monitor: Bolko von Roedern

14. ABSTRACT (Maximum 200 Words)

This project was devoted to deriving the electronic structure of interfaces in Cu(In,Ga)(S,Se)2 and CdTe thin-film 

solar cells. By using a unique combination of spectroscopic methods (photoelectron spectroscopy, inverse 

photoemission, and X-ray absorption and emission), a comprehensive picture of the electronic (i.e., band alignment 

in the valence and conduction band) as well as chemical structure can be painted. The work focused on the 

following: (1) deriving the benchmark picture for world-record cells, (2) analyzing state-of-the-art cells from industrial 

processes, and (3) troubleshooting of cells with substandard performance. 

15. SUBJECT TERMS

PV; CIGSS; CdTe; electronic structure; interfaces; thin film; solar cells; photoelectron spectroscopy; inverse 

photoemission; X-ray absorption and emission; 

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UL

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