A national laboratory of the U.S. Department of Energy

Office of Energy Efficiency & Renewable Energy

National Renewable Energy Laboratory 

Innovation for Our Energy Future 

ZnO:Al Doping Level and 

Hydrogen Growth Ambient Effects 

on CIGS Solar Cell Performance 

Preprint 

J.N. Duenow, D.M. Wood, and B. Egaas, 

Colorado School of Mines

T.A. Gessert, R. Noufi, and T.J. Coutts 

National Renewable Energy Laboratory 

Presented at the 33rd IEEE Photovoltaic Specialists Conference 

San Diego, California 

May 11–16, 2008 

Conference Paper 

NREL/CP-520-42537 

May 2008 

NREL is operated by Midwest Research Institute ● Battelle Contract No. DE-AC36-99-GO10337 
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ZNO:AL DOPING LEVEL AND HYDROGEN GROWTH AMBIENT EFFECTS ON CIGS 

SOLAR CELL PERFORMANCE 

Joel N. Duenow,1

 Timothy A. Gessert,2 David M. Wood,1 Brian Egaas,1 Rommel Noufi,2 Timothy J. Coutts2 

1

Colorado School of Mines, 1500 Illinois St., Golden, CO 80401 2

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

ABSTRACT 

Cu(In,Ga)Se2 (CIGS) photovoltaic (PV) cells require a 

highly conducting and transparent electrode for optimum 

device performance. ZnO:Al films grown from targets 

containing 2.0 wt.% Al2O3 are commonly used for this 

purpose. Maximum carrier mobilities of these films grown 

at room temperature are ~20-25 cm2

V-1s-1. Therefore, 

relatively high carrier concentrations are required to 

achieve the desired conductivity, which leads to free 

carrier absorption in the near infrared (IR). Lightly doped 

films (0.05 - 0.2 wt.% Al2O3), which show less IR 

absorption, reach mobility values greater than 50 cm2

V-1s-1

when deposited in H2 partial pressure. We incorporate 

these lightly doped ZnO:Al layers into CIGS PV cells 

produced at the National Renewable Energy Laboratory 

(NREL). Preliminary results show quantum efficiency 

values of these cells rival those of a past world-record cell 

produced at NREL that used 2.0 wt.% Al-doped ZnO films. 

The highest cell efficiency obtained in this trial was 18.1%. 

INTRODUCTION 

CIGS PV cells incorporate a transparent conducting 

oxide (TCO) layer as their top electrode. ZnO:Al targets 

containing 2.0 wt.% Al2O3 are commonly used for this 

purpose because ZnO:Al offers acceptable electrical and 

optical properties when grown using the low substrate 

temperatures preferred for the final steps of CIGS device 

processing. Other common TCOs, such as In2O3:Sn, can 

offer higher electrical and optical performance, but only at 

higher deposition temperatures. Maximum mobilities of 

ZnO:Al films grown from targets containing 2.0 wt.% Al2O3

at room temperature are ~20-25 cm2

V-1s

-1. High carrier 

concentrations or thick films are therefore required to 

achieve the desired sheet resistance. However, high TCO 

conductivity is best achieved by increasing mobility rather 

than carrier concentration for PV device applications [1]. 

High carrier concentration produces high infrared (IR) free 

carrier absorption, while high mobility reduces free carrier 

absorption. In this study, we investigate the extent to 

which lighter Al doping coupled with H2 incorporation in the 

Ar sputtering ambient can improve ZnO:Al film electrical 

and optical properties for utilization in PV devices. We 

examine undoped ZnO and ZnO:Al films grown by RF 

magnetron sputtering from targets with Al2O3 contents 

from 0.05 to 2.0 wt.%. Mobility values exceed 50 

cm2

V-1s

-1 for films containing 0.05 – 0.2 wt.% Al2O3 when 

grown at substrate temperatures up to 200oC. IR 

transmittance is increased strongly at these lower Al levels 

compared to that of the standard 2.0 wt.% Al2O3 films. 

EXPERIMENTAL DETAILS 

TCO films were deposited by radio frequency (RF) 

magnetron sputtering at 13.56 MHz using a Unifilm 

Technology PVD-300 system in a sputter-down 

configuration with a source-to-substrate distance of ~1.5 

cm. Targets, purchased from CERAC, Inc. (Milwaukee, 

WI), were hot-pressed to 3-inch planar form. The 

undoped ZnO target was stated as 99.999% pure by the 

vendor. The ZnO:Al targets, of 99.995% purity, contained 

0.05, 0.1, 0.2, 0.5, 1.0, and 2.0 wt.% Al2O3. Corning 7059 

and 1737 glass substrates were soaked for 3 hours in a 

mechanically stirred solution of 1% detergent (Liqui-Nox) 

in deionized water maintained at ~90oC, subsequently 

rinsed in two tanks of clean deionized water, and allowed 

to dry in air. A radiative boron nitride heating element 

permitted substrate temperatures up to ~360oC. A 

cryopump established chamber base pressures between 

4×10-8 and 2×10-7 Torr prior to deposition. Ar, H2, and O2

gas flows were controlled with needle valves and 

measured using an ion gauge. Chamber pressure was 

throttled to 15 mTorr during deposition as measured by a 

capacitive manometer. Sputtering was performed in 

constant power mode at 72 W (with corresponding voltage 

~100 V) to deposit films to a nominal thickness of 500 nm. 

Substrates were allowed to equilibrate on the heater for 

one hour prior to deposition and allowed to cool for 45 

minutes in the sputtering ambient after deposition. 

Thickness measurements were performed by stylus 

profilometry (Veeco Dektak) and spectroscopically (190–

1000 nm; n&k Technology model 1280 Analyzer). Carrier 

concentration, mobility, and resistivity were obtained at 

room temperature from Hall analysis using the van der 

Pauw method (BioRad HL5500). Optical reflectance and 

transmittance were obtained using a spectrophotometer 

(Cary 5G) with integrating sphere (Labsphere DRA-CA50). 

CIGS photovoltaic cells were grown on 2-mm-thick 

soda lime glass with a 1-μm-thick layer of Mo deposited by 

sputter deposition. The CIGS absorber layer was 

deposited to a nominal thickness of 2.5 μm using a 3-

stage process [2]. In place of the commonly used CdS 

layer, a bilayer of CdS and ZnS was formed by chemical 

bath deposition on one of the samples. ZnO:Al 

depositions on these devices are discussed below. 

1
RESULTS AND DISCUSSION 

ZnO:Al Studies 

Based on initial studies of highly-doped films 

deposited in 100% Ar [3], a substrate temperature of 

200oC was chosen for sputtering ambient studies. The 

effects of ambient were investigated by adding small 

amounts of O2 or H2 to the Ar sputtering gas. 

When O2 is added to the Ar ambient, carrier 

concentration decreases sharply for all Al2O3 levels (Fig. 

1). The most significant decrease occurs for the undoped 

ZnO because it contains no potentially ionized extrinsic 

dopant species to contribute carriers. Mobility values also 

decrease significantly when small amounts of O2 are 

added to the ambient regardless of Al doping level. 

60

50

40

30

20

10

0

Mobility (cm 2

V-1 s-1 )

2.01.51.00.50.0

H2 /Ar Ratio (%)

0.5 0.4 0.3 0.2 0.1 0.0

O2 /Ar Ratio (%)

10 16

10 17

10 18

10 19

10 20

10 21

Carrier Conc. (cm

-3 )

Undoped ZnO

 ZnO:Al (0.05 wt.%)

ZnO:Al (0.1 wt.%)

ZnO:Al (0.2 wt.%)

ZnO:Al (0.5 wt.%)

ZnO:Al (1.0 wt.%)

ZnO:Al (2.0 wt.%)

100% Ar

Fig. 1. Carrier concentration (upper panel) and mobility 

(lower panel) vs. O2/Ar ratio (left) and H2/Ar ratio (right) of 

the sputtering gas for depositions performed at 200oC. 

These decreasing values could have a number of 

causes. Because these films are polycrystalline, with an 

average grain size of ~30 nm [3], the grain boundary 

density is high. O may adsorb on these grain boundaries 

[4-7]. Adsorbed O could remove free carriers from the 

exterior of the grains (and thus from the conduction band) 

to become O-

 [4], reducing the measured carrier 

concentration. Trapped charge due to the adsorbed Owould establish electrostatic potential energy barriers that 

could inhibit carrier transport between grains, lowering the 

measured Hall mobility [6]. Other mechanisms may also 

describe this behavior. O vacancies (VO) and Zn 

interstitials (Zni) have been suggested as providers of 

carriers in ZnO [8-11]. Addition of O2 to the ambient, 

therefore, could fill VO or decrease the number of Zni

formed, decreasing the number of free carriers. Other 

workers have suggested, however, based on density 

functional theory calculations [12-15], that neither of these 

latter mechanisms likely provide carriers in ZnO. 

Calculations indicated that VO are deep donors, while Zni

have a low activation energy for diffusion from their 

interstitial sites [13]. In recent years, H has been 

suggested as a source of carriers in ZnO [16-22]. The 

addition of O2 to the growth ambient could remove 

beneficial effects (detailed below) of any residual H in the 

growth chamber. 

Controlled amounts of H2 increase both carrier 

concentration and mobility (Fig. 1). Carrier concentration 

values for films grown in a H2-rich ambient increase 

systematically from 1 to 8×1020 cm-3 as the Al2O3 content 

of the target increases from 0.05 to 2.0 wt.%, respectively. 

The highest mobilities, particularly for undoped and lightly 

Al doped films, were obtained when a small amount of H2

was added to the Ar sputtering ambient (optimal H2/Ar 

~0.3%). Remarkably, films containing 0.05 - 0.2 wt.% 

Al2O3 exceed the mobility of undoped ZnO, reaching 

values greater than 50 cm2V-1s-1 near 0.3% H2/Ar. 

H2 added to the sputtering ambient could form 

complexes with adsorbed O-

 on ZnO grain boundaries, 

causing the O-

 to be removed or passivated. This could 

return carriers to the conduction band, increasing the 

carrier concentration. The size of electrostatic barriers 

between grains would also be reduced, increasing the 

measured Hall mobility. Alternatively, the use of H2

enables a reducing environment that could be more 

conducive to the formation of VO and Zni. The decrease in 

mobility with larger amounts of Al2O3 and with excessive 

H2 content (>0.3% H2/Ar) is likely due to increased ionized 

or neutral impurity scattering because both Al and H 

constitute scattering centers. 

Differences in the substrate temperature dependence 

of electrical properties were observed between films 

grown in the 100% Ar and 0.3% H2/Ar ambients when 

using the 0.2 wt.% Al2O3 target (Fig. 2). Films grown in 

100% Ar display a broad maximum in both carrier 

concentration and mobility at 150-200°C, whereas films 

Fig. 2. Carrier concentration (upper panel) and mobility 

(lower panel) vs. substrate temperature for films grown in 

100% Ar and 0.3% H2/Ar from the 0.2 wt.% Al2O3 target. 

2
grown in 0.3% H2/Ar show a slight monotonic decrease in 

both values with increasing substrate temperature. Thus, 

lightly doped ZnO:Al films with the best electrical 

properties can be obtained by room temperature 

deposition in a 0.3% H2/Ar partial pressure ambient, 

without the need for substrate heating. This low 

temperature deposition is ideal for utilization in CIGS PV 

devices that require processing temperatures below 

~200°C to prevent degradation. Note that the addition of 

H2 increases both mobility and carrier concentration over a 

wide range of deposition temperatures, which improves 

manufacturability and process robustness. 

Optical transmittance data (Fig. 3) show that films of 

all Al doping levels have visible transmittance values of 

~85% on average. Higher Al doping reveals a systematic 

increase in bandgap with increasing carrier concentration 

from its intrinsic value of ~3.3 to ~3.8 eV. The lightly 

doped films show significantly less IR absorption than the 

films with higher Al content. Although this is due primarily 

to the lower carrier concentration of these films, the higher 

mobility of films with lower Al content also reduces IR 

absorption [1]. Because high transparency up to ~1100 

nm is required for CIGS PV devices, we believe lowering 

the Al content is a promising avenue toward improved 

device performance. 

100%

90

80

70

60

50

40

30

20

10

0

Transmittance (%)

750500250 1000 1250 1500 1750 2000

Wavelength (nm)

0.5%

 Al

1.0%

 Al 

2.0%

 Al

Undoped

 ZnO

0.1%

 Al 0.2%

 Al wt.% Thick.

 Al 2O3 (nm)

 0 390

 0.1 370

 0.2 415

 0.5 410

 1.0 495

 2.0 470

1.1 eV

(CIGS,

nc-Si:H)

Fig. 3. Transmittance vs. wavelength for the highest 

mobility film at each Al2O3 level, deposited at 200oC in 

0.3% H2/Ar (except for the 2.0 wt.% Al2O3 film, deposited 

in 100% Ar). The CIGS bandgap appears for comparison. 

Implementation on CIGS Devices 

To test whether ZnO:Al films with lower Al content 

might be used successfully in CIGS PV devices, two 

samples from the same CIGS deposition were selected. 

CIGS samples were grown to a nominal thickness of 2.5 

μm using a 3-stage process [2] on 2-mm-thick soda lime 

glass with a 1-μm-thick layer of Mo deposited by sputter 

deposition. One sample was processed with a bilayer of 

CdS and ZnS, while the other received the standard CdS 

treatment by chemical bath deposition. The bilayer was 

used to preserve the beneficial qualities of the CdS/CIGS 

interface while improving the transparency of this layer at 

lower wavelengths. The CdS/ZnS sample was coated 

with 50 nm of intrinsic ZnO (i-ZnO) deposited at room 

temperature in 100% Ar and 190 nm of ZnO:Al deposited 

from a target containing 0.1 wt.% Al2O3 at room 

temperature in 0.3% H2/Ar. The CdS-treated sample, 

receiving the standard processing, was coated with 100 

nm of i-ZnO and 120 nm of ZnO:Al deposited from a target 

containing 2.0 wt.% Al2O3 at room temperature in 100% 

Ar. Each sample received a MgF2 anti-reflection coating 

and was patterned using photolithography into 0.42 cm2

devices. The performance of the device utilizing ZnO:Al 

containing 0.1 wt.% Al2O3 compares favorably with the 

highest-efficiency device produced from the CdS-treated 

control sample using the standard ZnO:Al deposition 

(Table 1). The slightly lower fill factor for the 0.1 wt.% 

Al2O3 device (with the CdS/ZnS bilayer) is likely due to the 

higher series resistance of ZnS than CdS. The opencircuit voltage values for the two devices are comparable 

within the measurement uncertainty. The slightly higher 

short-circuit current for the 0.1 wt.% Al2O3 device (with the 

CdS/ZnS bilayer) is attributed to improved transparency of 

this layer compared to the typical CdS layer. 

The absolute external quantum efficiency (QE) of the 

device utilizing ZnO:Al containing 0.1 wt.% Al2O3 was 

compared to that of the 19.5% NREL CIGS device that 

held the world record (WR) until recently (Fig. 4). 

100

80

60

40

20

0

Absolute External QE (%)

200 400 600 800 1000 1200

Wavelength (nm)

18.1% eff.

CdS/ZnS, 0.1% Al 2O3

19.5% eff. (WR)

CdS, 2.0 wt.% Al 2O3

Fig. 4. External quantum efficiency (QE) vs. wavelength 

for CIGS device utilizing ZnO:Al containing 0.1 wt.% Al2O3

(18.1% efficiency) compared to QE of the past worldrecord (WR) 19.5% NREL CIGS device using 2.0 wt.% 

Al2O3.

Table 1. CIGS PV devices incorporating lightly doped (0.1 wt.% Al2O3) and standard (2.0 wt.% Al2O3) ZnO:Al. 

Al2O3 Content 

(wt.%) Treatment Efficiency (%) Fill Factor (%) Open-curcuit 

voltage (mV) 

Short-circuit 

current (mA/cm2

) 

0.1 CdS/ZnS 18.1 76.2 671 35.4 

2.0 CdS 18.1 79.1 666 34.4 

3
Differences in QE at the lower wavelengths are primarily 

due to the transparency difference between the CdS/ZnS 

bilayer (used on the 0.1 wt.% Al2O3 device) and CdS layer 

(on the 2.0 wt.% Al2O3 WR device). At the higher 

wavelengths, the device produced with the ZnO:Al 

containing 0.1 wt.% Al2O3—in one trial, without 

optimization—demonstrates QE meeting or exceeding that 

of the record device through much of the higher 

wavelength range. These findings suggest that, with 

optimization of the device processing to take advantage of 

the higher ZnO:Al mobility, utilization of the lightly-doped 

ZnO:Al may result in higher device performance. 

CONCLUSIONS 

Lighter Al doping, when combined with controlled 

incorporation of H2 in the sputtering ambient, produces 

ZnO:Al films with higher near-IR transmittance and carrier 

mobility values compared to those grown using the more 

typical 2.0 wt.% Al2O3. In addition, H2 incorporation 

increases both mobility and carrier concentration over a 

wide range of deposition temperatures, including room 

temperature, improving manufacturability and process 

robustness. For these reasons, lightly doped ZnO:Al films 

grown in a controlled H2 partial pressure ambient at room 

temperature may offer advantages over films with higher 

Al concentrations deposited in 100% Ar, particularly where 

low-temperature deposition is a requirement. 

ACKNOWLEDGMENTS 

This work was supported by the U.S. Department of 

Energy under Contract No. DE-AC36-99G010337 and 

NREL subcontracts KXEA-3-33607-11 and -24. 

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5
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ZnO:Al Doping Level and Hydrogen Growth Ambient Effects on 

CIGS Solar Cell Performance: Preprint 

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14. ABSTRACT (Maximum 200 Words)

Cu(In,Ga)Se2 (CIGS) photovoltaic (PV) cells require a highly conducting and transparent electrode for optimum 

device performance. ZnO:Al films grown from targets containing 2.0 wt.% Al2O3 are commonly used for this 

purpose. Maximum carrier mobilities of these films grown at room temperature are ~20-25 cm2V-1s-1. Therefore, 

relatively high carrier concentrations are required to achieve the desired conductivity, which leads to free carrier 

absorption in the near infrared (IR). Lightly doped films (0.05 - 0.2 wt.% Al2O3), which show less IR absorption, 

reach mobility values greater than 50 cm2V-1s-1 when deposited in H2 partial pressure. We incorporate these lightly 

doped ZnO:Al layers into CIGS PV cells produced at the National Renewable Energy Laboratory (NREL). 

Preliminary results show quantum efficiency values of these cells rival those of a past world-record cell produced at 

NREL that used 2.0 wt.% Al-doped ZnO films. The highest cell efficiency obtained in this trial was 18.1%. 

15. SUBJECT TERMS

PV; transparent electrode; device; optimum performance; hydrogen; quantum efficiency; carrier concentration; 

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