Conference Paper

NREL/CP-520-46235

July 2009

Required Materials Properties for 

High-Efficiency CIGS Modules

Preprint

I. Repins, S. Glynn, J. Duenow, T.J. Coutts, 

W. Metzger, and M.A. Contreras

To be presented at the Society of Photographic Instrumentation 

Engineers (SPIE) 2009 Solar Energy + Technology Conference

San Diego, California

August 2-6, 2009
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1

Required Material Properties for High-Efficiency CIGS Modules

Ingrid Repins, Stephen Glynn, Joel Duenow, Timothy J. Coutts, Wyatt K. Metzger, and 

Miguel A. Contreras

National Renewable Energy Laboratory

ABSTRACT

Relatively high proven efficiencies of CIGS devices are often cited regarding its choice as a semiconductor for 

photovoltaic manufacturing. Module efficiency is an important parameter, as a number of factors in the cost per watt are 

driven downward by increasing efficiency. Some of these factors include materials costs, throughput for a given capital 

investment, and installation costs. Thus, realizing high-efficiency (e.g. 15%) large-area CIGS modules is key in both 

reducing cost per watt and differentiating the technology from other thin films. This paper discusses the material 

properties required of each layer of the CIGS device such that large-area CIGS modules can achieve efficiencies 15%, 

which is substantially higher than the current industrial state-of-the-art. The sensitivity of module performance to the 

important material parameters is quantified based on both experimental data and modeling. Necessary performance 

differences between small-area devices and large-area modules imposed by geometry are also quantified. Potential 

technical breakthroughs that may relax the requirements for each layer are discussed.

Keywords: CIGS, module, material properties, photovoltaics

1. Overview of material properties

At NREL, small area CIGS device efficiencies of 20.0% have been demonstrated,1 as shown in Figure 1. Table 1 shows 

measured material properties for each electrically active layer of an NREL 20% efficient device. These values will be 

referred to in the discussion as “best proven” values. In the following sections, each layer of the device is discussed, 

starting from the light-incident side. For each layer, the sensitivities of the performance to deviations from the best 

proven values of material properties are discussed. For critical properties, a range of acceptable values for 15% module 

performance is established. 

2. Grids and transparent conductor

Grids and the transparent conducting oxide (TCO) must be considered together, as both are involved in lateral current 

collection and optimized properties are therefore interdependent. Power losses for the grid and TCO are calculated 

following Green.2

These power losses for an NREL 20.0% device are calculated and listed in the second column of 

 

Table 2. Power 

absorbed in the TCO, PTCO, is based on measured TCO absorption (Figure 2), and is calculated by

PTCO = Vmp A(λ) I AM1.5 ∫ (λ) QE(λ) dλ (1)

where Vmp= maximum power voltage

A(λ) = fractional absorption as a function of wavelength

I(λ) = AM1.5 photon current per wavelength

QE(λ) = 20% device quantum efficiency

The absorbed photon current listed in Table 1 as “ZnO bi-layer AM1.5 absorption” is simply PTCO/Vmp.
2

Figure 1: JV curve of 20.0% CIGS device, using the recently-adopted standard ASTM G173 global spectrum. Note that the 

same device measures 19.9%, as published, under the earlier standard spectrum.

Table 1: Relevant material properties of NREL 20% device.

Layer Property Value Units Measurement Technique

Mo Sheet Resistance < 0.24 Ω/ Four-point probe

CIGS Thickness 2.2 μm Mechanical profilometer

CIGS Ga ratio 0.28 / 0.20 / 0.40 notch unitless Auger profiling

CIGS Lifetime ~200 ns Time-resolved photoluminescence

CIGS Carrier density 2 x 1016 cm-3 Capacitance-voltage

CdS Thickness 50 nm Mechanical profilometer

ZnO bi-layer Sheet Resistance 69 Ω/ Four-point probe

ZnO bi-layer AM1.5 Absorption 1.0 mA/cm2 Transmission spectrophotometry

ZnO: Al Carrier density 5.6 x 1020 cm-3 Hall Effect

ZnO: Al Mobility 13 cm2/V-sec Hall Effect 

ZnO: Al Thickness 120 nm Mechanical profilometer

Grid finger Sheet resistance 0.009 Ω/ Resistance test structure

Grid finger Contact resistance 4.5 x 10-7 Ω-m2 Resistance test structure

Grid finger Minimum width 45 μm Mechanical profilometer
3

Reflection losses are calculated in an analogous fashion, with A(λ) in (1) replaced by the reflection, R(λ). Based on the 

reflection data of Figure 2, these losses integrate to 4.1 mA/cm2

. However, use of a MgF2 anti-reflective (AR) coating 

increases the short-circuit current by 5% (i.e. by ~1.8 mA/cm2

), implying reflection losses are reduced to 2.3 mA/cm2. 

Thus, the remaining power loss due to reflection in a high-efficiency device (coated with MgF2) is approximately 1.4 

mW/cm2

.

Figure 2: Measured transmission, reflection, and absorption as a function of wavelength for ZnO bilayer on glass 

companion piece to device M2992 of Figure 1.

When moving to a larger device size, power loss related to lateral current collection necessarily increases. However, 

these power losses are minimized by adjusting grid spacing, cell dimensions, and TCO conductivity and absorption (via 

TCO free carrier density). The necessary performance differences imposed by device size should be distinguished from 

processing choices made for throughput and cost advantages. For example, a manufacturer may utilize a substrate or 

deposition rate that has a known negative impact on absorber lifetime, but is advantageous in terms of throughput. In 

this paper, we only discuss material properties needed to achieve a given performance goal, independent of 

manufacturing trade-offs.

The geometry utilized to examine power losses for a large-area device is shown in Figure 3. A gridded, rather than 

monolithically-integrated, configuration is analyzed, as grids have been shown both experimentally and by simulation to 

maximize efficiency.3

 The cell length, A, is fixed at 10 cm, taken to be the smallest size reasonable for integration into 

a large-area module. The bus bar width, d, is fixed at 1.5 mm, a minimum reasonable size for automated stringing. The 

maximum bus bar sheet resistance is taken to be 0.14 mΩ/, consistent with 13 μm (0.5 mil) Cu ribbon. B, one half the 

cell width, and S, the distance between grid fingers, are optimized to minimize power losses.
4

A

d

S

B

Figure 3: Cell geometry used for analysis of large-area device.

The TCO properties, not just the cell dimensions, must also be optimized for the larger geometry to achieve the highest 

efficiency. It is assumed for this optimization that the mobility of the TCO is approximately fixed, since the mobility is 

a function of scattering mechanisms that may not be mutable for a given process or material. The free carrier density, 

however, is easily varied via oxygen vacancies or dopants in the target. For example, in ZnO films at NREL, changing 

the Al-doping from 0 to 2% by weight caused a 20x increase in carrier density, but only a 2x decrease in mobility.4

Effects of changing the free carrier density on power loss in the device can be calculated using classical harmonic 

oscillator theory,5,6

α = q

3

(377 ohms)

ω2m2 ε∞

n

μ

dictating that the absorption in the TCO is related to conduction following

(2)

σ = qμn (3)

A=1− e

−α t (4)

where α = absorption coefficient

σ = conductivity

A = absorption

q = electron charge

μ = mobility

n = carrier concentration

ω = angular frequency of light

m = carrier mass

ε∞ = high frequency dielectric constant

t = film thickness

This theory predicts a linear relationship between ln(1-A) and tn/μ, as is demonstrated in Figure 4 for ZnO films made at

NREL. In Figure 4, absorption is measured at 1.1 eV, and Al doping of the films ranges from 0 to 2% by weight. The 
5

comparison between experiment and theory should be considered approximate, as no attempt to account for reflection 

from the back surface of the film (ZnO/glass interface) has been made.

Figure 4: Absorption at 1.1 eV as a function of carrier density, thickness, and mobility measured on NREL ZnO films. The 

gray line is a linear regression to the data.

Thus, using the material properties of Table 1, the values for B, S, and nt product were adjusted to minimize power loss 

in the hypothetical large-area device of Figure 3. This minimization was accomplished numerically using a spreadsheet. 

Resulting parameters are B= 2.0 cm, S=1.1 mm, and nt=2.3 x 1015 cm-2

. Calculated power losses for this configuration 

are shown in column 3 of Table 2. Power losses not related to the grids and TCO are assumed to be identical to the 20% 

small cell. Using the best proven material properties, the highest possible large-area efficiency is thus 19.1%. 

Alternatively, if the cell width is fixed at 10 cm (i.e. B=5 cm), optimized values are S=0.61 mm and nt=1.3 x 1015 cm-2

, 

with a resulting efficiency of 18.1%. The power losses for this configuration are listed in column 4 of Table 2. The 1 to 

2% absolute efficiency difference between the small cell and large cell is consistent with published data and modeling.3

The 18 to 19% efficient large-area prediction is based upon a minimum finger width of grids deposited by e-beam 

evaporation. Other deposition techniques may necessitate substantially thicker grids. The maximum achievable largearea efficiency is strongly dependent on minimum grid finger width. As grid fingers are made thicker, they must be 

spaced further apart, resistive loss in the TCO increases, and the optimized TCO is less transmissive. This effect is 

illustrated in Figure 5, where optimized efficiency and grid spacing are calculated for a 10 cm wide device. To achieve a 

module efficiency of 15%, the maximum grid finger width is about 1 mm.

Figure 5: Calculated maximum large-area efficiency as a function of grid finger width. Also shown are the effects of grid 

finger width on optimum grid spacing.
6

Table 2: Optical and resistive losses in optimized devices of varying areas, calculated assuming best proven material 

properties.

3. CdS

The function of the bath-deposited CdS layer has been described not just as a heterojunction partner, but also in terms of 

band alignment,7 lattice matching,8 surface cleaning,9,10

10

and formation of a buried heterojunction via Cd doping of the 

CIGS. ,11,12,13,14

Device performance as a function of CdS deposition time (i.e. thickness) has been examined at NREL.

It is assumed in this analysis that the CdS deposition conditions allow enough time for adequate surface 

cleaning and any CIGS doping that might occur. Therefore, the remaining CdS property available for optimization is the 

CdS thickness. 

15

Figure 6

 From this 

study, shows open-circuit voltage (Voc), fill factor (ff), and current collection below 520 nm (from integrated 

quantum efficiency measurements) as a function of deposition time. Open circuit voltage and fill factor first increase 

with CdS deposition time then saturate. Current below 520 nm, on the other hand, decreases steadily as absorption in the 

CdS increases. 

The decrease in current with increasing CdS thickness is well-described by integrating the AM1.5 spectrum times the 

CdS absorption calculated using published optical constants. To perform this calculation of the CdS absorption, a CdS 

thickness is assigned to each data point from Figure 6 based on the measured quantum efficiency (QE). Assuming that 

the ratio of the QE at 520 nm (QE520) to that at 450 nm (QE450)is controlled by the transmission through the CdS, the 

thickness t of the CdS is given by

t = ln(QE 450 /QE 520 )

(α 520 −α 450) (5)

where α450 and α520 are the absorption coefficients at 450 and 520 nm, respectively, as available in published data.16

Using CdS thickness extracted by this method, the data of Figure 6 are used to graph device efficiency as a function of 

CdS thickness, and are shown in Figure 7. Points are measured data, while the dashed line shows the expected 

dependency at higher thicknesses due to absorption in the CdS. To keep losses from the CdS below 0.5 mW/cm2

, CdS 

thickness must be 40 nm +40/-10 nm. The fairly gentle decrease in efficiency with increased CdS thickness shown in 

Figure 7 accounts only for increased absorption in the CdS. In practice, it can be expected that for the largest 

thicknesses shown in the graph, fill factor decreases will occur due to reduced electric field in the CIGS and the 

Mechanism NREL 20% small cell 

losses (mW/cm2)

Optimized large 

area cell (mW/cm2)

Cell width fixed at 

10 cm (mW/cm2)

Grid shading 0.71 0.85 1.49

Grid lateral resistance 0.08 0.32 1.17

Grid contact resistance 0.14 0.12 0.07

TCO resistance 0.24 0.21 0.12

TCO absorption 0.25 0.21 0.12

Bus bar resistance 0.13 0.14 0.36

Bus bar shading 0.13 0.75 0.30

Reflection 1.4 1.4 1.4

Total: 3.1 4.0 5.0

Efficiency (%): 20.0 19.1 18.1
7

associated energy position of the CdS/CIGS conduction band offset.17

 

 The onset of this fill factor reduction is a function 

of the CdS and i-ZnO thicknesses and carrier densities.

0.55

0.6

0.65

0.7

0.75

0.8

10 11 12 13 14 15

Time (min)

4

4.2

4.4

4.6

4.8

5

5.2

5.4

5.6

5.8

Current below 520 nm (mA/cm2

)

Voc

fill factor

Current

Figure 6: Performance parameters as a function of CdS deposition time (i.e. thickness).

Figure 7: Device efficiency as a function of CdS thickness. Points are measured data, while the dashed line shows the 

expected dependency at higher thicknesses due to absorption in the CdS.

4. CIGS

CIGS, as the absorber and the depleted semiconductor, is the primary layer in determining junction properties and device 

performance. Its formation tends to be the chief challenge facing manufacturers, as the highest performing CIGS 

requires the difficult combination of elevated temperatures, extended deposition times, controlled composition and 

reaction of four elements, Na doping, and large grain size.

Properties of the CIGS that may change from one film to the next and are expected to affect device performance are 

band gap (Eg), lifetime (τ), carrier density (p), mobility (μ), and front and rear surface recombination velocities (SF and 

SR). Profiles (gradients) in these properties may occur through the film. 

While some of these properties are measured routinely, others are not fully quantified for CIGS. Band gap profile and 

carrier density profile fall in the more routine category. Band gap profiles are typically deduced from Auger or x-ray 

photoelectron spectroscopy profiling, and subsequent conversion of the atomic Ga/(In+Ga) ratio to band gap. Carrier 

densities can be deduced from capacitance-voltage or drive-level capacitance profiling, although the separation of free 

carrier, interface, and deep-level response can be non-trivial.18 Lifetime measurements by time-resolved 

photoluminescence (TRPL) can yield information relevant to device performance,19,20,21 although only a single value, 
8

rather than a profile, is generally extracted. Some (single-value) measurements of hole mobility in CIGS are available in 

the literature,22,23,24

4.1 BAND GAP PROFILE

while estimates of the electron mobility are based largely on assumptions regarding the ratio 

between electron and hole mobilities, or from measurements on single-crystal materials. Little data exists characterizing 

surface recombination velocities in the CIGS device.

Band gap profile is one of the most thoroughly characterized and understood properties of the CIGS layer. Numerous 

publications detail how the band gap profile can be manipulated to match the solar spectrum, decouple optical absorption 

and voltage, and reduce of the effects of back surface recombination and short diffusion length.25

The overall band gap to match the solar spectrum is ideally around 1.4 eV.

 In general, the salient 

features are 1) overall band gap magnitude, 2) single-grading, i.e. increasing the band gap toward the back contact of 

the device, and 3) double-grading, i.e. creating a “notch” profile that both increases the band gap toward the back and 

front of the device.

26

Figure 1

 In practicality, for CIGS, it is presently 

limited to about 1.15 eV, due to the impact of midgap defects at higher band gaps. The measured band gap profile for the 

20.0% device of is shown in Figure 8. In this data, the atomic ratio Ga/(In+Ga) was measured by Auger 

spectroscopy profiling and converted to band gap (Eg) using constants provided by Alonso.27 The average band gap is 

around 1.15 eV.

Figure 8: Measured band gap profile of 20.0% device.

A conspicuous feature of Figure 8 is the “notch” in the band gap profile. The measured profile is similar to the optimum 

identified by modeling efforts,25,28

25

although the measured band gap minimum is slightly farther from the sample surface 

and is slightly more shallow than recommended by the models. Based on modeling, the expected benefits of this profile 

compared to an ungraded absorber are ~0.5% absolute. Most of this expected benefit is attributed to the grading 

toward the back of the device, which reduces back surface recombination and assists in collection. The grading toward 

the front of the device, which to some extent decouples electrical and optical response, makes a smaller contribution to 

the improvement.

4.2 LIFETIME, CARRIER DENSITY, AND MOBILITY

Lifetime, carrier density, and mobility are logically examined together, as they all occur in a similar fashion in the 

relationship between forward current and voltage. The forward current density, JF, in a p-n junction is given by 29

J F = q μ kT

τ q

ni

2

p

e

qV

kT + qWn i

2τ

e

qV

2kT (6)

where q = electron charge

k = Boltzmann constant
9

T = temperature

μ = electron mobility

τ = electron lifetime

ni = intrinsic density = NCNVe

−Eg / kT

NC = conduction-band density of states

NV = valence-band density of states

p = hole density

W = depletion width≅ 2εVbi /q p

ε = dielectric constant

Vbi = built-in potential = E g − kTln(NV / p)

V = voltage applied to the junction

The first term in the right hand side of (6) is forward current due to diffusion (JD), while the second term is forward 

current due to depletion region recombination (JR). The equation should only be considered a first-order approximation 

for transport in CIGS devices, as the calculation neglects the effects of finite absorber thickness and assumes uniform 

absorber properties. Nevertheless, order of magnitude agreement between voltages and lifetimes using this relationship 

has been demonstrated,21 and (6) thus should be a valid vehicle for examining the general trends in expected sensitivity 

of CIGS device performance to transport parameters. 

General trends in the expected impact of μ, τ, and p on device performance can be illustrated by examining the forward 

current near the maximum power point for several parameter values. For such calculations, μ, τ, and p were varied, 

while other material parameters were set to baseline values recommended by Gloeckler,30 apart from the electron and 

hole effective masses (me/m0 and mh/m0, respectively): CIGS me/m0 = 0.09, and CIGS mh/m0 = 0.72.31

Figure 9

 The effective 

masses affect values for band edge densities of states. The lines in a show the resulting ratio of the diffusion to 

recombination current at V= the maximum power voltage Vmp. For this exercise, Vmp is taken to be 0.591 mV, as in 

Figure 1. The lines in Figure 9b show the total forward current. Calculations are performed for three values of p and two 

values of μ, as listed in the figure. The lowest curve (dashed light gray) is calculated using the baseline values for high 

efficiency devices, p = 2 x 1016 cm-3 and μ = 100 cm2

/V-sec.30

Figure 9: Calculated a) ratio of diffusion current to recombination current, and b) total forward current, as a function of 

lifetime, for several values of the mobility and carrier density. In the legend, carrier density is listed in cm-3

, and 

mobility as cm2

/V-sec.

For low values of τ, the forward current is dominated by recombination. (i.e. There is a very low value of JD/JR in Figure 

9a). Even above 10 ns, JR remains the dominant contribution to JF, although JD is non-negligible. The critical task of 

reducing recombination current to increase efficiency is consistent with recent record efficiencies achieved with 
10

decreasing diode quality factor.1 Devices measured at NREL whose material properties are consistent with fabrication 

of >15% modules (i.e. small-area efficiencies ≥17% after application of AR coating) exhibit lifetimes almost exclusively 

greater than 50 ns. The highest efficiency devices exhibit even longer lifetimes.20 It should be noted that meaningful 

comparison of lifetime across studies requires careful control of measurement conditions as described in the 

references.19,20

In Figure 9b, below ~10 ns, the forward current is insensitive to μ, as μ appears only in the prefactor for JD. , and JR

dominates in this lifetime range. Above ~10 ns, the forward current exhibits some modest dependence on μ (for constant 

p), as evidenced by the difference between the solid black and dashed gray lines, or between the dashed black and solid 

gray lines. Measurements for hole mobilities in CIGS range from 0.1 to 20 cm2

/V-sec.22,23,24 However, little is known 

about the typical range of electron mobilities in CIGS samples and values required for high performance devices. 

Because the mobility appears only in the diffusion term of the forward current, it is likely that mobility is of secondary 

importance compared to lifetime or carrier density. For very high efficiencies, further study of mobility is necessary to 

comment further on its role in device performance.

For all values of τ, the forward current depends significantly on p, consistent with data in the literature. For small-area 

devices in the efficiency range of 15 to 18%, a relatively wide range of carrier densities are observed, from around 5 x 

1015 to 2 x 1016 cm-3

.

21,24,32,33,34,35

4.3 SURFACE RECOMBINATION VELOCITIES

 For the devices with efficiencies greater than 19%, however, carrier densities measure 

over 1016 cm-3. Devices with very low carrier density (<1015 cm-3) typically exhibit dramatically reduced efficiency 

(<10%).

Although recombination at the back surface is recognized as a mechanism for efficiency loss in the CIGS device,36

For high-quality CIGS devices, an upper limit of 103 cm/s has been deduced for the bare CIGS surface and the 

CIGS/CdS interface.

little 

data exists characterizing this quantity. Thus, we are unable to comment here on acceptable ranges or typical values for 

high efficiency devices. 

19 Device models predict that a surface recombination velocity of this order of magnitude should 

have virtually no effect on device performance.

5. Mo

Primary functions of the Mo layer are to provide a low-resistance back contact, and – in the case of Na-containing 

substrates- an adequate Na supply for the growing absorber. How well the latter function is performed depends on the 

Mo thickness and microstructure, and is evident in the absorber lifetime and carrier concentration. 

The conductive requirements for the Mo are fairly easily satisfied, since, unlike the case of the TCO, no transmission is 

needed. Resistive loss in the Mo is calculated as a function of sheet resistance and cell size by standard methods37

Figure 10

and 

shown in . The four lines in Figure 10 correspond to four different geometries for connecting to the back 

contact of the device in Figure 3. For line A (dashed gray), calculations are performed assuming both long edges are 

contacted with B set at 2.0 cm, as optimized for the TCO. In this geometry, the largest distance any carrier must travel 

through the Mo is 2.0 cm. For line A, all values of sheet resistance considered – even those substantially above the

demonstrated 0.2 Ω/ (marked with an arrow) – yield a low power loss. Line B is calculated assuming the same 

geometry as line A, except only one long edge of the device is contacted. Carriers may travel up to 4 cm in the Mo. 

Once again power loss is low. Line C is calculated assuming the two short edges of the device are contacted, implying a 

maximum travel distance of 5 cm. Line D is calculated assuming just one short edge of the device is contacted, implying 

a maximum travel distance of 10 cm. Only for the largest sheet resistances and maximum travel distances shown does 

power loss in the Mo exceed 1 mW/cm2

.

The requirements on Mo sheet resistance imposed by Figure 10 apply only to devices on insulating substrates. 

Conducting substrates, e.g. stainless steel foil, are much thicker than the typical Mo coating, and therefore can carry the 

necessary current without imposing conductivity requirements on the Mo.
11

 

A B

C D

Figure 10: Power loss in the Mo back contact as a function of Mo sheet resistance and geometry. Arrow shows location on 

graph of demonstrated 0.2 Ω/ sheet resistance. Schematics on right show geometry used to calculate each line in 

graph.

6. Impact of Potential Breakthroughs

A number of opportunities exist for technological breakthroughs that may relax the material properties requirements 

delineated in the sections above. These include larger-bandgap absorbers, non-absorbing heterojunction partners that 

maintain Voc, improved TCOs, and use of optics.

Improved current collection has been the aim of investigation of both larger band gap heterojunction partners and 

improved TCO’s. In the AM1.5 spectrum, 8 mA/cm2 of photon current is available below 520 nm, and the optimized 

CdS layer (Figure 7) absorbs 3 mA/cm2 of it. Thus, a larger band gap heterojunction partner could allow conversion of 3 

mA/cm2 more current, or approximately 1.8 mW/cm2 more power. Likewise, a perfect TCO (no absorption, resistance, 

or reflection) would eliminate the grid and resistive layer losses that are shown, for an optimized module, in column 3 of 

Table 2. These losses total 4 mW/cm2 to be gained with the development of a perfect TCO.

Larger bandgap absorbers also provide avenues for potential improvement. In theory, larger band gap materials should 

yield both a better match of the band gap to the solar spectrum, and lower resistive losses. The efficiency gain due to a 

better match to the solar spectrum, changing the present ~1.15 eV to the optimum 1.4 eV is estimated at 2.5% absolute.26 

Reduced resistive losses, on the other hand, can be expected to yield about 0.4% absolute efficiency increase. This

estimate is produced by repeating the TCO/grid optimization of section 2, using a hypothetical 20.0% efficient wideband gap device with the same outer dimensions as utilized for the optimized large-area device of Table 2 (4 cm x 10 

cm), with Vmp = 0.800 V, and Jmp = 23.6 mA/cm2

. Presently, however, wide band gap chalcopyrites have not yet 

achieved the absorber and/or interface quality necessary to realize these benefits.38

Use of optics produces some efficiency advantages for CIGS solar cells. It should be noted that optics are a slightly 

different class of improvements than those already discussed, as they require adding components to the device stack, 

rather than improving or modifying an existing layer. The use of low concentration (14x) has been demonstrated to 

improve small-area CIGS device efficiency by 3.6% absolute.39 Maintaining low resistances is key in achieving this 

efficiency increase under concentration. Luminescent frequency conversion has been shown to increase short-circuit 

current.40,41

40

 In this technique, luminescent dyes absorb blue photons that might otherwise be lost in the CdS or ZnO. 

The dyes then emit at wavelengths for which the device exhibits a high QE. This technique has been shown to increase 

short circuit current up to 1.5 mA/cm2 for CIGS devices (i.e approximately 0.9 mW/cm2 for a very high-efficiency 

device). 1.0 mA/cm2 of this current is due to enhanced blue response, and 0.5 mA/cm2 due to decreased reflection of a 

mini-module without antireflective coating.
12

7. Conclusions

Material properties of each layer in an NREL 20.0%-efficient, small-area, CIGS device have been documented. Based 

on best proven material properties, 18%-efficient, 100 cm2 devices suitable for integration into large-area modules are 

possible. Successful transfer of high efficiencies from small to large areas requires careful attention to minimum grid 

finger width, optimized cell design, high-quality transparent conductors, and AR coating. For some layers (e.g. CdS, 

Mo), windows of acceptable material properties for 15% devices are fairly wide. The key challenge in creating 15% 

large-area modules is likely maintaining long lifetimes (> ~50 ns) and high carrier densities (> ~ 1016 cm-3

) in the CIGS, 

particularly in high-throughput processes over large areas. A better understanding of electron mobility in CIGS devices 

is needed to put firm limits on the required values for lifetime and carrier density. New developments related to highquality large-bandgap absorbers, non-absorbing heterojunction partners that maintain Voc, improved TCO’s, and use of 

optics have the potential to add a few percent to module efficiency, thus relaxing requirements on the CIGS.

ACKNOWLEDGMENTS

This work was supported by the U.S. Department of Energy under Contract No. DOE-AC36-08GO28308 with the 

National Renewable Energy Laboratory.

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pp. 235-239, 2008. DOI: 10.1002/pip.822

[2] M.A. Green, Solar Cells: Operating Principles, Technology, and System Applications, Prentice-Hall, 1982, pp. 153 

ff.

[3] J. Kessler, J. Wennerberg, M. Bodegard, L. Stolt, “Highly efficient Cu(In,Ga)Se 2 mini-modules,” Solar Energy 

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[4] J.N. Duenow, “Dopants and Transport Properties of Transparent Conducting Zinc Oxide Thin Films,” Doctoral 

thesis: Colorado School of Mines, 2008, pg 90.

[5] P. Drude, Wied. Ann. 46, 353 (1882); ibid. 48, 122 (1883); ibid. 49, 690 (1883). 

[6] See, for example, K. Seeger, Semiconductor Physics: An Introduction, Springer-Verlag, 1991, pg. 348.

[7] D. Schmid, M. Ruckh, H.W. Schock, “A comprehensive characterization of the interfaces in Mo/CIS/CdS/ZnO 

solar cell structures”, Solar Energy Materials and Solar Cells 41(2), pp.281-29.

[8] K.A. Jones, “The lattice mismatch between (112) chalcopyrite films and (0001) CdS substrates” Journal of Crystal 

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[9] A. Kylner, “The Role of CdS Buffer Layer in the Cu(In,Ga)Se2 Thin Film Solar Cell,” Doctoral thesis: Uppsala 

University, 1998.

[10] J. Kessler, M. Ruckh, D. Hariskos, U. Ruhle, R. Menner. H.W. Schock, “Interface engineering between CuInSe2 and 

ZnO ,” Proceedings of the IEEE Photovoltaics Specialists Conference 23, 1993, pg. 447.

[11] D. Lincot, R. Ortega-Borges, J. Vedel, M. Ruckh, J. Kessler, K.O. Velthaus, D. Hariskos, H.W. Schock, “Chemical 

Bath Deposition of CdS on CuInSe2: Combining Dry and Wet Processes for High Efficiency Thin Film Solar Cells,” 

Proceedings of the EC PVSEC 11, 1992, pg. 870.
13

 [12] K. Ramanathan, R.N. Bhattacharya, J. Granata, J. Webb, D. Niles, M.A. Contreras, H. Wiesner, F.S. Hasoon, R. 

Noufi, “Advances in the CIS Research at NREL,” IEEE Photovoltaics Specialists Conference, 26, 1997, pp. 319-

322.

[13] K. Ramanathan, H. Wiesner, S. Asher, D. Niles, R.N. Bhattacharya, J. Keane, M.A. Contreras, R. Noufi, “High 

Efficiency Cu(In,Ga)Se2 Thin Film Solar Cells Without Intermediate Buffer Layers”, Proceedings of the World 

Conference on Photovoltaic Energy Conversion 2, 1998, pp. 477-481.

[14] T. Nakada, “Nano-structural investigations on Cd-doping into Cu(In,Ga)Se2 thin films by chemical bath deposition 

process,” Thin Solid Films 361-362 (21), 2000, pp. 346-352.

[15] M.A. Contreras, M.J. Romero, B. To, F. Hasoon, R. Noufi, S. Ward, K. Ramanathan, “Optimization of CBD CdS 

process in high-efficiency Cu(In,Ga)Se2-based solar cells”, Thin Solid Films 403-404, 2002, pp. 204-211.

[16] S. Ninomiya, S. Adachi, “Optical Properties of Wurtzite CdS,” Journal of Applied Physics 78(2), 1995, pp. 1183-

1190. 

[17] A.O. Pudov, “Impact of Secondary Barriers on CuIn1-xGaxSe2 Solar-Cell Operation,” Doctoral thesis: Colorado 

State University, 2005, pp. 64 ff.

[18] M. Cwil, M. Igalson, P. Zabierowski, S. Siebentritt, “Charge and doping distributions by capacitance profiling in 

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[19] W.K. Metzger, I.L. Repins, M. Romero, P. Dippo, M. Contreras, R. Noufi, D. Levi, “Recombination kinetics in 

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[20] W.K. Metzger, I.L. Repins, M. Contreras, “Long recombination lifetimes in high-efficiency Cu(In,Ga)Se2 solar 

cells,” Applied Physics Letters 93, 2008, DOI 022110.

[21] I.L. Repins, W.K. Metzger, C.L. Perkins, J.V. Li, and M.A. Contreras, “Measured Minority-Carrier Lifetime and 

CIGS Device Performance,” IEEE Photovoltaics Specialists Conference, 34, 2009.

[22] J.W. Lee, J. D. Cohen, W.N. Shafarman, “The determination of carrier mobilities in CIGS photovoltaic devices 

using high-frequency admittance measurements,” Thin Solid Films 480-481, 2005, pp. 336-340.

[23] E.A. Schiff, “Transport, Interfaces, and Modeling in Amorphous Silicon Based Solar Cells,” Subcontract report 

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[24] I.L. Repins, B.J. Stanbery, D.L. Young, S.S. Li, W.K. Metzger, C. Perkins, W.N. Shafarman, M.E. Beck, L. Chen, 

V.K. Kapur, D. Tarrant, M.D. Gonzalez, D.G. Jensen, T.J. Anderson, X. Wang, L.L. Kerr, B. Keyes, S. Asher, A. 

Delahoy, B. Von Roedern, “Comparison of Device Performance and Measured Transport Parameters in WidelyVarying Cu(In,Ga)(Se,S) Solar Cells” Progress in Photovoltaics 14, 2006, pp. 25-43.

[25] M. Gloeckler, J. R. Sites, “Band-gap grading in Cu(In,Ga)Se2 solar cells”, Journal of Physics & Chemistry of Solids

66(11), 2005, pp.1891-1894. 

[26] M.A. Green, Solar Cells: Operating Principles, Technology, and System Applications, Prentice-Hall, 1982, pg. 89.

[27] M.I. Alonso, M. Garriga, C.A. Durante Rincon, E. Hernandez, M. Leon, “Optical functions of chalcopyrite 

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[29] See, for example, S.M. Sze, Physics of Semiconductor Devices, John Wiley and Sons, 1981, pg. 92.

[30] M. Gloeckler, A.L. Fahrenbruch, J.R. Sites, “Numerical modeling of CIGS and CdTe solar cells: setting the 

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14

 [32] R.N. Bhattacharya, M.A. Contreras, B. Egaas, and R.N. Noufi, High efficiency thin-film CuIn1-xGaxSe2 photovoltaic 

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based photovoltaic cells from solution-based precursor films, Thin Solid Films 361-362, 2000, pp. 396–399.

[34] P.K. Johnson, J.T. Heath, J.D. Cohen, K. Ramanathan, and J.R. Sites, A comparative study of defect states in 

evaporated and selenized CIGS(S) solar cells, Progress in Photovoltaics 13, 2005, pp. 579–586.

[35] B. Egaas, C. DeHart, J. Scharf, B.E. McCandless, R. Noufi , “Characterization of 19.9%-Efficient CIGS 

Absorbers,” IEEE Photovoltaics Specialists Conference 33, 2008.

[36] M. Gloeckler, J.R. Sites, “Potential of Submicrometer thickness Cu(In,Ga)Se2 solar cells”, Journal of Applied 

Physics 98, 2005, DOI 103703.

[37]

 M.A. Green, Solar Cells: Operating Principles, Technology, and System Applications, Prentice-Hall, 1982, pg. 146.

[38] S. Siebentritt, “Wide gap chalcopyrites: material properties and solar cells,” Thin Solid Films 403-404, 2002, pp. 1-

8.

[39] J.S. Ward, K. Ramanathan, F.S. Hasoon, T.J. Coutts, J. Keane, M.A. Contreras, T. Moriarty, R. Noufi, “A 21.5% 

Efficient Cu(In,Ga)Se2 Thin-Film Concentrator, Solar Cell,” Progress in Photovoltaics 10, 2002, pp. 41-46. 

[40] G.C. Glaeser, U. Rau, “Improvement of photon collection in Cu(In,Ga)Se2 solar cells and modules by fluorescent 

frequency conversion,” Thin Solid Films 515, 2007, pp. 5964-5967.

[41] K.R. McIntosh, G. Lau, J.N. Cotsell, K. Hanton, D.L. Batzner, F. Bettiol, B.S. Richards, “Increase in External 

Quantum Efficiency of Encapsulated Silicon Solar Cells from a Luminescent Down-Shifting Layer,” Progress in 

Photovoltaics 17, 2009, pp. 191-197.
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Required Materials Properties for High-Efficiency CIGS Modules: 

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14. ABSTRACT (Maximum 200 Words)

Relatively high proven efficiencies of CIGS devices are often cited regarding its choice as a semiconductor for 

photovoltaic manufacturing. Module efficiency is an important parameter, as a number of factors in the cost per watt 

are driven downward by increasing efficiency. Some of these factors include materials costs, throughput for a given 

capital investment, and installation costs. Thus, realizing high-efficiency (e.g. 15%) large-area CIGS modules is key 

in both reducing cost per watt and differentiating the technology from other thin films. This paper discusses the 

material properties required for each layer of the CIGS device such that large-area CIGS modules can achieve 

efficiencies of >15%, substantially higher than the current industrial state of the art. The sensitivity of module 

performance to the important material parameters is quantified based on both experimental data and modeling. 

Necessary performance differences between small-area devices and large-area modules imposed by geometry are 

also quantified. Potential technical breakthroughs that may relax the requirements for each layer are discussed.

15. SUBJECT TERMS

PV; CIGS; module; material properties; high-efficiency; devices;

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