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

Efficiency & Renewable Energy, operated by the Alliance for Sustainable Energy, LLC.

Contract No. DE-AC36-08GO28308

ZnMgO by APCVD Enabling 

High-Performance Mid-Bandgap 

CIGS on Polyimide Modules 

October 2009 — October 2010

Lawrence Woods

Ascent Solar Technologies, Inc. 

Littleton, Colorado

Subcontract Report

NREL/SR-5200-51379

April 2011
NREL is a national laboratory of the U.S. Department of Energy, Office of Energy 

Efficiency & Renewable Energy, operated by the Alliance for Sustainable Energy, LLC.

National Renewable Energy Laboratory

1617 Cole Boulevard

Golden, Colorado 80401

303-275-3000 • www.nrel.gov

Contract No. DE-AC36-08GO28308

ZnMgO by APCVD Enabling 

High-Performance Mid-Bandgap 

CIGS on Polyimide Modules 

October 2009 — October 2010

Lawrence Woods

Ascent Solar Technologies, Inc. 

Littleton, Colorado

NREL Technical Monitor: Brian Keyes

Prepared under Subcontract No. NEU-0-99010-02

Subcontract Report

NREL/SR-5200-51379

April 2011
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 

Introduction 

Ascent Solar Technologies (AST) has recently begun producing lightweight and flexible 

photovoltaic modules to address emerging building-integrated photovoltaic (BIPV), electronicintegrated photovoltaic (EIPV), transportation and space related PV markets. AST’s baseline PV 

technology primarily consists of typical low-bandgap CuInGaSe2 (CIGS) with a CdS emitter 

(buffer) layer, and using roll-to-roll (R-2-R) processing on a polyimide substrate that enables

monolithic integration of modules. This Pre-Incubator project was designed to increase the “real 

world” CIGS based photovoltaic module performance and decrease the Levelized Cost of Energy 

(LCOE) of systems utilizing those modules compared to our traditional CIGS based photovoltaic 

modules. This will be enabled by a) increasing the CIGS bandgap and b) developing better 

matched device finishing layers to the mid-bandgap CIGS based photovoltaics; including 

window and buffer layers (and eventually the TCO). Our approach to replace the traditional 

device finishing layers is to use metal-oxide alloys such as ZnMgO deposited by atmospheric 

pressure chemical vapor deposition in a roll-to-roll configuration. Furthermore, elimination of 

the small amount of Cd in our product will reduce costs and enable markets where Cd is either 

currently restricted or is being considered for restricting Cd-bearing materials.

Background

Typical high-efficiency CIGS devices utilize a low-bandgap (approx. 1.15 eV) CIGS solar 

absorber layer and a CdS based buffer layer adjacent to the solar absorber. The low-bandgap of 

typical CIGS devices results in module power losses (lower efficiency) due to two important 

factors: (1) High current modules resulting in high joule heating or I2

R power losses, and (2) 

Large negative voltage and power temperature coefficients resulting in significantly reduced 

power at nominal operating temperatures of 40 to 80 oC. This latter factor is a market barrier for 

structurally integrated modules (ex. BIPV) and structurally applied (ex. BAPV, EIPV, etc.) 

modules based on CIGS solar absorbers as it impedes the realization of high-efficiency flexible 

modules. Higher bandgap CIGS solar absorbers would significantly reduce these loss factors and 

our internally module modeling indicates that module performance could be increased by as 

much as 10 to 30% over the lower bandgap module at nominal operating temperatures. However 

research to date shows that the CIGS device (small-area) efficiency decreases as the band-gap is 

increased to more desirable single-junction levels (1.35 to 1.45 eV). The cause of the diminished 

performance is attributed to two theories: (1) Deep defect levels becoming more mid-gap with 

increasing bandgap and thus becoming more effective recombination centers,1 and (2) traditional 

device layers window and buffer layers are not well matched to the wider-bandgap CIGS (higher 

Ga) solar absorber. 2,3,4

1

 The former has limited mid-bandgap CIGS efficiencies to about 13% 

(small-area) on high temperature glass substrates when using traditional window and buffer 

(CdS) layers. Regarding the latter, a better matched buffer layer may enable better mid-bandgap 

CIGS performance over CdS buffers due to better conduction band line-up and a reduction in 

interface recombination. Furthermore, the Cd in the CdS layer is a market barrier in some 

countries (ex. Japan), and as such results in an indirect cost increase. In addition, costs for proper 

hazardous material handling at the manufacturing facility and waste removal and treatment of 

CdS further increases the costs associated with this layer. Thus there is room for considerable 

improvements in traditional CIGS devices as they relate to module performance, especially in 

structurally integrated markets.
2 

Project Approach

The approach of this Pre-Incubator project was to produce mid-bandgap CIGS solar absorbers 

using multi-source co-evaporation on molybdenum coated polyimide substrates, and optimize 

the CIGS composition in the bandgap range of 1.35 to 1.45 eV. Anticipated variables include 

the final Cu/III ratio, Ga/III ratio, sodium content, and the substrate heater temperature. As 

planned, we initially used a small bell jar co-evaporation system with a stationary substrate 

configuration for the CIGS depositions, and later in the program migrated to a larger Roll-to-Roll 

(R-2-R) co-evaporation system from the pilot line. The R-2-R system provided more CIGS 

material and more uniform CIGS compositions as needed for the buffer layer development and 

good comparison with buffer layer controls (with CdS). The R-2-R system was also used for 

CIGS depositions that further incorporated sodium (Na) into the mid-bandgap solar absorber 

make-up. Finally, the R-2-R system was used in AST’s internal development that produced 

novel CIGS based mid-bandgap alloys and that were leveraged for use with the novel metaloxide buffer layers being developed under this project. 

The approach to produce non-CdS based window and buffer layers for the mid-bandgap solar 

absorbers was to utilize atmospheric pressure chemical vapor deposition (APCVD) as a ‘gentle’ 

technique (not damaging to CIGS surface) in a belted (amenable to R-2-R) deposition system to 

develop metal-oxides such as ZnMgO for use as the window and buffer layer with mid-bandgap 

CIGS. The proposed mid-bandgap CIGS device layers in cross-section versus the baseline lowbandgap CIGS device layers are illustrated in Figure 1 below.

Figure 1 - The proposed mid-bandgap CIGS device layers in cross-section (right) versus the 

baseline low-bandgap CIGS device layers (left).

Several process variables related to the fabrication of the metal-oxide buffer layer and device 

junction were identified and planned for testing during the program. The variables have been 

categorized below, and their anticipated effect on the buffer layer material is noted.

• CIGS surface preparation and/or pre- treatment (Pre-CVD deposition) 

o Clean, passivate, create buried homojunction 

(TCO top contact)

Polyimide (substrate)

1.4 eV CuInGaSe2 (p-type absorber)

ZnMgO Window layer (n-type)

Mo 

Back 

contact

ZnMgO buffer layer (n-type)

(TCO top contact)

Polyimide (substrate)

1.15 eV CuInGaSe2 (p-type absorber)

CdS Window layer (n-type)

ZnO buffer layer (n-type)

Mo 

Back 

contact
3 

• APCVD Buffer Layer deposition parameters: example ZnMgO: Zn conc., Mg conc., 

Zn/Mg (alloy) ratio, metals/oxygen ratio, substrate temperature, film thickness, 

dopant level. 

o Resistivity, stoichiometry, transmission, and composition. 

• Post APCVD Buffer Layer Annealing 

o Junction Cross-diffusion, layer crystallinity, resistivity and 

transmission, passivation of CIGS.

• Post Device Annealing 

o Same as above, but this now includes i-ZnO and TCO and their interfaces.

Summary of Program Goals and Objectives

The overall goal of the program was to develop the technology for higher performance midbandgap (1.35 eV to 1.45 eV) CIGS based photovoltaics on polyimide substrates that utilize 

metal-oxide buffer/window layers (no CdS) as deposited by non-surface damaging APCVD. 

The Technical Improvement Opportunities (TIO’s) addressed in the project are: (1) Absorber 

(CIGS), and (2) Cells and Contacts (window and buffer layers). 

The small area (1 cm2

) device efficiency goals for the novel mid-bandgap devices on polyimide

substrates were as follows:

• 6% eff. by month 6 

• 8% eff. by month 9 

• 9% eff. by month 12 

Key objectives of the development effort enabling progress toward these performance goals were 

as follows:

• Produce mid-bandgap CIGS solar absorbers of about 1 micron thickness using multisource roll-to-roll (R-2-R) co-evaporation on molybdenum coated polyimide 

substrates, and optimize the CIGS composition (Eg = 1.35 to 1.45 eV) for higher 

Open Circuit Voltage (Voc) and lower temp. coefficients. 

o Critical success factor related to KPPs: Bandgap > 1.35 eV (reduced Indium), 

Thickness of about 1 micron (reduced materials). 

• Replace CdS and utilize R-2-R Atmospheric Pressure Chemical Vapor Deposition 

(APCVD) to develop metal-oxides such as ZnMgO for use as the window and buffer 

layer with mid-bandgap CIGS.

o Critical success factor related to KPPs: Higher performance device at nominal 

operating temperatures (vs. low bandgap), Cost of metal-oxide buffer < cost 

of CdS 
4 

Accomplishments of Performance Milestones and/or Deliverables

Several milestones were planned as a check on the projects progress. These milestones are listed 

in Table 1 below along with their due dates and final status.

Table 1 – Planned milestones, due dates, and final status.

Milestone Month Summary Status

Task 3.1 Fabricate Hardware Baseline 1 On time. Accepted at NREL

Task 3.2 Mid-bandgap (Eg >1.35eV) 

CIGS samples fabricated

1 On time. Mid-bandgap development was

ongoing throughout the remainder of program.

Subtask 3.2.1 Mid-bandgap 

(Eg>1.35eV) CIGS with Na samples 

fabricated

3 On time. Mid-bandgap development with Na 

was ongoing throughout the remainder of the 

program.

Subtask 3.2.2 Mid-bandgap 

(Eg>1.35eV) thin < 2 μm thick) CIGS 

samples fabricated

5 On time. Mid-bandgap development with 

thinner absorbers was ongoing throughout the 

remainder of the program.

Task 3.3 APCVD Metal-oxide films 

fabricated with targeted composition, 

and optical properties.

2 On time. Metal-oxide depositions were 

ongoing throughout the remainder of the 

program.

Subtask 3.3.1 Surface pretreatment 

effectiveness determined

4 On time. Surface pretreatments were ongoing 

throughout the remainder of the program.

Subtask 3.3.2 Doping effectiveness 

determined

6 On time. Doping and alloying were ongoing 

throughout the remainder of the program.

The baseline device deliverable was sent in before the end of the first month of the program, and 

was been accepted by NREL. Toward the end of the 2nd quarter (6 months), the 6 month 6% eff. 

device deliverable was sent in to NREL for performance verification and accepted. Toward the 

end of the 3rd quarter (9 months), the 9 month 8% eff. and the 12 month 9% eff. device 

deliverables were sent in to NREL for performance verification and accepted. By the 11th month 

of the program, a 12% efficient device (as measured by AST) on polyimide substrates was 

produced using a mid-bandgap CIGS based alloy and metal-oxide buffer and finishing layers (no 

CdS). The bandgap of this device was determined to be about 1.40 eV as determined by the 

examination of the absorber band edge from quantum efficiency measurements. 

Detailed Progress on Key Performance Parameters (Project Results)

The mid-bandgap CIGS deposition process on the temperature restricting polyimide substrates 

was developed and optimized through-out the program, and overall there have been 23 large 

multi-variable experiments performed using the mid-bandgap CIGS with APCVD buffer layers. 

A summary of the all the experimental variables tested during the program are summarized in 

Table 2. 
5 

Table 2 – Summary of all the experimental variables for the metal-oxide buffer layer development 

on mid-bandgap CIGS and organized according to category. 

CdS buffer layer controls devices were fabricated and tested for all experiments and midbandgap solar absorbers. In all experiments each experimental buffer layer sample and 

CdS/CIGS control were finished into approximately twelve individual 1 cm2 small area devices, 

then prepped (back contact access) and tested using simulated AM1.5 light IV testing. In many 

cases, quantum efficiency (QE) measurements were performed to assess the device spectral 

response and effective bandgap from the long wavelength cut-off. The composition of the midbandgap CIGS solar absorber was determined using x-ray fluorescence measurements, and was 

also used as a guide to achieve targeted bandgaps when used in conjunction with published data 

on CIGS bandgap versus Ga/(In+Ga) ratio. 

During the first quarter of the project all CIGS depositions were accomplished in a small bell jar 

with a stationary substrate configuration. Prior to the CIGS sample fabrication, the maximum 

substrate heater temperature was approx. determined by visual and mechanical inspection of the 

substrate after several calibration runs. The CIGS produced for these early experiments had 

Ga/III ratios in the range of 0.5 (Eg > 1.3 eV) to 0.6 (Eg > 1.35 eV). Some of the primary 

variables tested during the first quarter were the Cu/III ratio of the mid-bandgap CIGS, the effect 

of pre-treatments to the CIGS surface, and the substrate temperature during the APCVD metaloxide deposition. No sodium was incorporated into the solar absorber during these experiments 

and the primary metal-oxide under consideration was ZnMgO. The Mg/(Zn+Mg) flow ratio was 

CIGS

Solar Absorber

CIGS

Surface Treatments

APCVD Alloyed 

ZnO Deposition

Post-Anneal

Low Cu/(In+Ga) ratio None Substrate Temp. None

Mid Cu/(In+Ga) ratio #1 Metals/O2 ratio Anneal #1

High Cu/(In+Ga) ratio #2 ( 1X and 2X conc.) Metals conc.

CIGS Surface composition 

variations.

#3 Metals Alloy %

With Sodium #1 or #2, and #3 Metals type

R-2-R CIGS #1 and/or #3

R-2-R CIGS with Sodium #4, #5, #6, #7 or #8

R-2-R CIGS alloy With 

Sodium

Pre Anneal #1

R-2-R CIGS alloy Without 

Sodium

Pre Anneal #2
6 

fixed at a relatively high value during these initial experiments. The results from one experiment 

is given below, and represents the best performance during the first quarter.

Figure 2 – Example of device efficiency results from ZnMgO/CIGS based devices (and CdS 

controls) with CIGS bandgap > 1.4 eV (no NaF). Best devices eff. (Top).

Device performance results from these initial experiments yielded a best experimental (ZnMgO 

buffer) small-area device efficiency of 4.5%, (at STC) and utilized a CIGS pretreatment, while 

the best CdS/CIGS control tested at 6% eff. Voc’s follow eff. (not shown), higher than lowbandgap CIGS devices. The best Voc’s were near 700 mV on controls, and near 600 mV on 

experimental devices. These best experimental and control samples also had the lower Ga/III 

ratio (Eg > 1.3), and higher Cu/III ratio, which is not indicated in the Figure. 

One particular area of concern was identified during this first quarter and as a result of x-ray 

photoemission spectroscopy (XPS) analysis of a ZnMgO film deposited by the belt APCVD 

process. It was found that the Mg/(Zn+Mg) ratio varied considerably as a function of film depth, 

presumable due to different Zn and Mg Precursor reaction rates as the substrate traverses under 

the injector head. The XPS profile through the film is given in Figure 3 below. The XPS 

analysis was performed by Dr. Glenn Teeter at the National Renewable Energy Laboratory 

(NREL). The profile shows that the Zn and Mg concentrations are oppositely correlated and 

with Zn peaking at the beginning and ending of the film, and Mg peaking in the middle of the 

film. The oxygen profile also seems to peak in the middle of the film, though this variation is 

much more subtle. The repercussions of this type of profile were not immediately clear, but the 

ZnMgO design strategy was to match or slightly exceed the conduction band energy of the midbandgap CIGS solar absorber layer at the ZnMgO/CIGS heterointerface (similar to the CdS/low0

1

2

3

4

5

6

7

No PreCVD,

Tsub =T0, 

No PreCVD,

Tsub =T0+40, 

No PreCVD,

Tsub =T0+60, 

No PreCVD,

Tsub =T0+80, 

No PreCVD,

Tsub =T0+125, 

Wet

PreTreatment

#1, Tsub=T0, 

Wet

PreTreatment

#1,

Tsub=T0+60, 

Wet

PreTreatment

#2,

Tsub=T0+40 

Efficiency (%)

Mid Cu/In+Ga

CdS Controls

High Cu/In+Ga

CdS Controls

Mid Cu/In+Ga

ZnMgO

High Cu/In+Ga

ZnMgO
7 

bandgap CIGS band line-up). Earlier absorption spectroscopy (not shown - prior to this 

program) showed that the ZnMgO did not have a sharp band edge, potentially now attributable to 

the non-uniform compositional profile, but with an ill-defined bandgap in the range of 3.3 to 3.6 

eV. Thus, indication is that we will have a mostly ZnO composition adjacent to the mid-bandgap 

CIGS absorber. In an effort to determine if non-uniform buffer layer bandgap profiling would be 

problematic for the mid-bandgap CIGS devices, then device modeling was performed by Dr. 

Kanevce at NREL using commercial device physics based simulation software. This modeling is 

described in a separate section below and was ongoing throughout the majority of the program. 

Nonetheless, it was also decided to utilize other metals for alloying with the ZnO, in addition to 

the ZnMgO. Hereafter the alloyed metal-oxide buffer is collectively referred to as ‘alloyed ZnO’.

Figure 3 – XPS profile of ZnMgO film on a glass slide. 

During the 2nd quarter of the project, the mid-bandgap CIGS depositions shifted to the larger rollto-roll (R-2-R) depositions toward the latter part of the quarter. This enabled better substrate 

temperature control and more plentiful mid-bandgap CIGS solar absorber material for the buffer 

layer device experiments. As a part of the 2nd quarter experiments several mid-bandgap CIGS 

solar absorber ‘types’ were tested, including three levels of Cu/III ratio (low, medium, and high), 

sodium additions, CIGS surface composition variations, and combinations thereof. In addition, 

mid-bandgap CIGS and CIGS alloys were produced from a roll-to-roll deposition system and 

tested with the APCVD buffer layer variations. Three different CIGS surface treatments were 

also tested. Regarding the APCVD deposition itself, variations in the alloyed ZnO content and 

Sample 2a - ZnMgO/glass (91203B-5B)

0

10

20

30

40

50

60

70

80

600 640 680 720 760 800 840 880 920 960 1000

Etch Time (s)

Atomic Percent (%)

O 1s

Zn 2p

Mg 1s

Si 2p

C 1s
8 

substrate temperature were tested. A summary of the results grouped according to mid-bandgap 

absorber layer type are as follows:

• Small Bell Jar Stationary Substrate Mid-Bandgap CIGS (no sodium addition) 

o Alloyed metal oxide buffer layer were better than some of the CdS buffer 

layer controls. 

o Best devices tended to be samples with a CIGS pre-treatment (#2)

o Samples with a medium level of Cu/III performed best.

o Overall efficiencies were still less than 5% eff.

• R-2-R Mid-Bandgap CIGS with sodium. 

o The best CdS/mid-bandgap CIGS was over 9% eff. (Eg. = 1.35 eV) 

o Alloyed ZnO buffer layer samples all tested out with much lower efficiencies 

(best 3% eff.) 

o The best alloyed ZnO buffer sample utilized a CIGS pre-treatment (#3-2 or #2 

+ #3-0) and a higher level of alloying with the ZnO. 

• R-2-R Mid-Bandgap CIGS alloy without sodium (from Internal Development) 

o The best CdS/mid-bandgap CIGS was measured to be over 8% eff. and Voc = 

0.57 V (Eg = 1.35 eV) 

o Alloyed ZnO buffer layer samples tested out with higher efficiencies than 

controls, best over 9% eff., Voc = 0.57 V (Eg = 1.35 eV). Best Voc’s near 600 

mV

o The best experimental sample did not utilize a CIGS pre-treatment, but some 

of the pre-treated samples did better than the controls as well.

o A higher level of ZnO alloying was also found to have a significant benefit

o Alloyed ZnO Buffer layer samples deposited at intermediate substrate 

temperatures performed best.

o Devices require post-fabrication annealing to maximize performance (FF).

The device results from one experiment during the second quarter are given in Figure 4 below as 

an example, and some of these samples represent the best performance during the 2nd quarter. 
9 

Figure 4 - Example of device efficiency results from an experiment with alloyed ZnO buffer layers 

(and CdS controls) and mid-bandgap CIGS alloyed based absorbers from R-2-R deposition 

system, no NaF. The CIGS bandgap is approximately 1.35 eV. 

The primary finding during the 2nd quarter was that the Na containing mid-bandgap solar 

absorbers with traditional CdS buffer layers performed well and much better the alloyed ZnO 

based buffer layers, but the alloyed ZnO based buffer layers performed as well or better than the 

CdS buffer layers when no Na was incorporated into the CIGS, but overall the performance 

seemed to be limited to 9% eff. One other significant finding was noted during this quarter, and 

that is that some of the devices required proper storage to stabilize the performance. The initial 

indications were that that the storage ambient depended on whether or not there was sodium in 

the CIGS alloys. 

During the 3rd quarter, the large scale experiments focused on the alloyed ZnO buffer layer 

development with a mid-bandgap CIGS alloy solar absorber (from internal development). The 

mid-bandgap CIGS alloys were obtained from a roll-to-roll (R2R) deposition system and during 

the 3rd quarter the program utilized material from four different runs; three of these had sodium 

added. In addition, several different CIGS surface treatments were tested, including several new 

ones (#4 - #8) in an effort to improve the device performance with the Na treated CIGS alloy. 

Some of the different pre-treatments were sometimes done in combination with other 

pretreatments. Regarding the APCVD deposition itself, variations in the ZnO alloy content, 

metals/oxidant ratio, and substrate temperature were tested. 

In the first few experiments, the CIGS and CIGS alloy compositions with sodium (Na) were 

utilized to improve the device performance when using the Na containing solar absorbers.

Several new pre-treatments were tested to help remove excess Na, and were tested with several 

Experiments 100301, 100303, and 100324A

R-2-R CIGS alloy

0

1

2

3

4

5

6

7

8

9

10

none Pre-treatment #1 Pre-treatment #2 

Efficiency (%)

CdS Controls

ZnO 5%alloy

Tsub=T0

with Na

ZnO 5%alloy

Tsub=T0

No PreTreat

ZnO 50%alloy

Tsub=T0

Pre-treat. #3-0

ZnO 50%alloy

Tsub=T0

Pre-treat.#3-2

ZnO 50%alloy

Tsub=T0

Pre-treat.#3-1

ZnO 50%alloy

Tsub=T0

ZnO 5%alloy

Tsub=T0+60

with Na

ZnO 5%alloy

Tsub=T0+60

ZnO 5%alloy

Tsub=T0+120
10

different variations in the APCVD process. An example of the device efficiency results from one 

of the experiments is shown in Figure 5. Some of the new pre-treatments were found to be 

beneficial and the ZnO alloy process variations were influential as well. One of the experimental 

ZnO alloy buffer layer devices was tested at close to 10% eff., the best ZnO alloy/mid-bandgap 

CIGS device to date, but the mid-bandgap CIGS with the CdS buffer layer tested at about 12% 

eff. (Eg ≅ 1.35 eV). 

Figure 5 – Summary of Best Device Performance from ZnO alloy/mid-gap CIGS alloy based 

devices (with Na) and CdS buffer controls as a function of APCVD pre-treatment from exp. 100521. 

Later in the 3rd quarter, another advance in the alloyed ZnO/mid-bandgap CIGS alloy 

performance was obtained, and the best device was measured (after light soaking) at over 11% 

eff. (Voc = 0.615), now performing much closer to the CdS buffer layer control which again 

tested at 12% eff. (Voc = 0.687). The best experimental sample utilized a pre-anneal, pretreatment #5, and a higher ZnO alloying %. The device light IV curves from this best alloyed

ZnO buffer layer and corresponding CdS buffer layer devices with the mid-bandgap CIGS alloy 

are shown in Figure 6. The best alloyed ZnO/mid-bandgap CIGS alloy device was submitted as 

a deliverable and also tested at NREL at over 11% eff. and Voc of 637 mV (after light soaking). 

The QE from the best device indicated a bandgap close to 1.32 eV (not shown). 

ZnO Alloy Exp. 100521 (IP-172B)

0

2

4

6

8

10

12

14

none #5 only 2 min [Lo]

#6, then #5

5 min [Lo] 

#6, then #5

2 min [Hi] 

#6, then #5

5 min [Hi] 

#6, then #5

10 sec [Lo]

#7, then #5

20 sec [Lo]

#7, then #5

10 sec [Hi]

#7, then #5

20 sec [Hi]

#7, then #5

Efficiency (%)

CdS Controls

ZnO alloy (50%), Tsub=T0, O2/metals=R2

ZnO alloy (50%), Tsub=T0, O2/metals=R1
11

 

Figure 6 – Light IV curves from an alloyed ZnO buffer with a mid-gap (1.32 eV) CIGS alloy (R-2-R) 

and CdS buffer controls. 

Eventually, another experiment was run using the CIGS alloy without Na to determine if these 

solar absorbers would also benefit from the latest APCVD ZnO alloy deposition conditions and 

tested in combination with different anneals (pre- and post-sputtered i-ZnO/TCO). The best 

device performance from this experiment was about 9% eff., and thus equaling the performance 

that which was obtained in earlier experiments with the non-Na containing solar absorbers. The

best alloyed ZnO/mid-bandgap CIGS alloy device from this experiment was submitted as a 

program deliverable and tested at NREL at over 8.5% efficient (after light soaking). The QE 

from the best device indicated a bandgap of about 1.35 eV (not shown). 

Despite the very good mid-bandgap performance on polyimide substrates during the 3rd quarter, 

it can be seen from Figure 6 that the Voc and FF of the alloyed ZnO buffer were still in need of 

improvement and perhaps suffering from low free carrier density (buffer layer or CIGS alloy)

and/or some interface recombination. 

During the 4th quarter, the large scale experiments again focused on the alloyed ZnO buffer layer 

development with a mid-bandgap CIGS alloy solar absorber (from internal development). The 

mid-bandgap CIGS alloys were obtained from a roll-to-roll (R2R) deposition system and during 

the 4th quarter the program utilized material from a couple of different runs; all with sodium 

added. During this quarter, we targeted higher bandgap solar absorbers (> 1.4 eV) to be tested 

along with the latest pre-treatments/pre-anneals and buffer layer conditions that had produced the 

best results to date. Regarding the APCVD deposition itself, variations in the alloyed ZnO 

composition, metals/oxidant ratio, and substrate temperature were tested. 

-40

-20

0

20

40

60

80

-0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0

Voltage (V)

Current Density (mA/cm2

)

ZnO alloy/mid-bandgap CIGS

alloy (IP-174 C1-A3)

CdS/mid-bandgap CIGS alloy

(IP-174 C1-A4)

Device ID Eff (%) Voc (V) Jsc (mA/cm^2) Fill Factor

Alloyed ZnO/Mid-Bandgap CIGS Alloy (IP-174 C1-A3 - 10 min LS ) 11.16 0.615 29.74 0.61

CdS/Mid-Bandgap CIGS Alloy (IP-174 C3 NO LS RETEST-A4) 12.15 0.687 27.29 0.65
12

An example of the device efficiency results from one of the mid-bandgap CIGS alloy 

experiments (Eg ≅ 1.40 eV) is shown in Figure 7 below, which also further includes a lowbandgap (Eg ≅ 1.16 eV) baseline device for comparison. One of the experimental alloyed ZnO 

buffer layer devices was tested at over 12% eff., representing the best alloyed ZnO alloy/midbandgap CIGS device to date, and the highest bandgap. However, again the mid-bandgap CIGS 

with the CdS buffer layer tested out slight higher at about 13% eff. (Eg ≅ 1.40 eV). The power 

temperature coefficient in the temperature range of 20 to 65 °C is indicated in Figure 9.

Figure 7 – AM1.5 light IV curves of alloyed ZnO buffer layers on mid-bandgap CIGS alloy solar 

absorber device (1.4 eV), and low-bandgap (1.16 eV) CdS/CIGS control. 

-40

-30

-20

-10

0

10

20

30

40

-0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0

Voltage (V)

Jsc (mA/cm2)

Low-Bandgap (1.16 eV) CdS/CIGS (CRC

0292 C27-A5)

Mid-Bandgap (1.40 eV) Alloyed ZnO/CIGS alloy

(IP-196 B3-B4)

Device ID Eff (%) Voc (V) Jsc (mA/cm^2) Fill Factor

Mid- Bandgap (1.40 eV) Alloyed ZnO/CIGS alloy (IP-196 B3-B4) 12.1 0.676 30.0 0.60

Low Bandgap (1.16 eV) CdS/CIGS (CRC 0292 C27-A5) 11.6 0.598 29.1 0.66
13

Figure 8 –QE curves of alloyed ZnO buffer layers on mid-bandgap CIGS alloy solar absorber 

device (1.4 eV) and Low Bandgap (1.16 eV) CdS/CIGS control. 

Figure 9 – % Power Loss vs. temperature for alloyed ZnO and CdS buffer layers on mid-bandgap 

CIGS alloy solar absorber devices, and standard CdS/CIGS low bandgap device.

0

10

20

30

40

50

60

70

80

90

100

400 500 600 700 800 900 1000 1100

wavelength (nm)

QE (%)

Low-Bandgap (1.16 eV) CdS/CIGS 

(CRC 0292 C27-A5)

Mid-Bandgap (1.40 eV) alloyed

ZnO/CIGS alloy (IP-196 B3-B4)

% Power Loss vs. Temperature

-25%

-20%

-15%

-10%

-5%

0%

5%

20 25 30 35 40 45 50 55 60 65

Temperature (degrees Celcius)

% Power Loss

Low-Bandgap (1.16 eV) CdS/CIGS

Mid-Bandgap (1.40 eV) CdS/CIGS alloy

Mid-Bandgap (1.40 eV) Alloyed ZnO/CIGS alloy
14

Device Stability

Early in the program, the unencapsulated mid-bandgap devices with the alloyed ZnO based 

buffer layers showed a higher level of performance instability and light soaking sensitivity than 

the CdS buffer layer counterparts. The inconsistent performance initially interfered with our 

ability to achieve good correlation between measurements here at AST, and later measurements 

at NREL. It was eventually determined that the inconsistencies were largely due to our lack of 

experience with these types of devices, especially the effects of light soaking, and post 

fabrication annealing and storage conditions. Through additional testing it was first determined 

that the devices required light soaking at Voc (or Pmax) of at least a few minutes to maximize 

the device performance, and this timing sometimes varied with the ambient storage conditions. 

The testing also revealed that the effects of post-device annealing were highly dependent upon 

the processing conditions. Almost every sample generated under this program had a different 

processing condition, making it very difficult to determine the best annealing and storage 

conditions for every sample. However in general, the annealing effects seemed to depend most 

on whether or not the CIGS had sodium added or not and the ambient gas being used. This was 

also true with regard to the storage ambient. The knowledge of annealing and storage conditions 

effects eventually enabled very good correlation between the device performance at AST and 

NREL. Furthermore, the continued retesting of unencapsulated novel devices kept in the proper 

ambient and in the dark and at Voc, did not show any obvious degradation when considering all 

the potential sources for device and testing variability. Figure 10 below shows the average 

sample efficiency and std. deviation (12 devices each sample) over a couple of months of this 

storage condition. The indication is that there is a systematic variation in the average 

performance of all the novel buffer layer devices that also affected one of the CdS buffer layer 

controls. This is seen as a performance increase after a couple of weeks then a performance 

decrease after 10 weeks. The exception is the CdS control for the “no Na” absorber, which 

showed only decreased performance that is likely due to being stored in the wrong ambient (air) 

as the other CdS control was stored under nitrogen. The system and test variations that may 

affect the unencapsulated device performance include the following factors: ambient humidity 

during testing (not controlled), air infiltration into nitrogen cabinet (no desiccant used in sample 

boxes), contacts during testing, variations in light level from solar simulator (uniformity of 

illumination, variation after calibration), and light soaking time. Finally, almost every sample 

generated under this program had a different processing condition, making it very difficult to 

determine the best storage conditions for every sample. Thus there is some likelihood that the 

unencapsulated samples shown in Figure 10 are not being optimally stored. In conclusion, the 

data to date on the unencapsulated novel devices does not indicate any obvious degradation 

trends. 
15

Figure 10 – Average device efficiency and std. deviation versus time in storage (dark, at Voc) from 

several alloyed ZnO/mid-bandgap CIGS alloy samples (approx. 12 devices each sample). Samples 

with CdS buffer layers instead of the alloyed ZnO are as indicated, as is the presence of sodium 

(Na) in the absorber (different storage ambient).

Mid-Bandgap CIGS Device Simulation Results

Device modeling and simulation was performed by Dr. Ana Kanevce at NREL (Electro-optical 

characterization group) using commercial device physics based simulation software in an effort 

to determine if non-uniform ZnMgO buffer layer bandgap profiling would be problematic for the 

mid-bandgap CIGS devices. The impetus for the modeling was earlier compositional profile 

measurements of the ZnMgO buffer layers (discussed above) that showed non-uniform Zn and 

Mg content through the film thickness, with Mg peaking in the middle of the film, and most 

likely a result of different Zn and Mg precursor reaction rates as the samples pass under the 

APCVD injector. The conduction band diagram for the ZnMgO/mid-bandgap CIGS material 

system is simulated and shown in Figure 11. 

Several scenarios were modeled, and included the following variations: 

• ZnMgO bandgap profile (constant vs. Gaussian), 

• ZnMgO/CIGS conduction band offsets, 

• ZnMgO/CIGS interface defect density, 

• ZnMgO and CIGS free carrier concentrations. 

0

2

4

6

8

10

12

14

0 2 4 6 8 10 12 14

Time (weeks)

Efficiency (%)

IP-145 A14 CdS IP-172B B2

IP-145 A15 IP-172B B5 CdS

IP-145 A17 IP-174 C1

IP-145 C23

 No Na in Absorber Na in Absorber
16

In the first simulation case, the peak conduction band energy of the Gaussian profile was made to 

match that of the conduction band energy of the constant band profile case, but at the 

heterointerface there is a conduction band cliff of about 0.3 eV with the Gaussian profile, and no 

cliff or slight barrier with the constant band profile, similar to that of Figure 11. 

Figure 11 – Simulated Conduction band diagram at V=0 for ZnO/ZnMgO/mid-bandgap CIGS (1.45 

eV) for two cases: small barrier at heterointerface and constant Mg in ZnMgO (black), and 0.3 eV 

cliff at heterointerface and Gaussian Mg profile in ZnMgO (red). 

The effect of a ZnMgO Gaussian composition profile on the light IV curve is shown in Figure 12

below, and is plotted as a function of the defect state density at the heterointerface. 

Conduction Band 

(CB) Offset
17

Figure 12 – Light IV device simulation results of ZnMgO (n=1018 cm-3) with Gaussian composition 

profile (top) and constant profile (bottom) as a function of defect density at the heterointerface

with mid-bandgap CIGS showing. (graphs have different y-scale) 

In general, the simulation shows that the low Mg at CIGS interface with the Gaussian profile has 

a more pronounced Voc loss with increasing interface defect density up to 1011

cm

-2. This 

confirmed the concern with the Gaussian ZnMgO composition profile. However, additional 

simulations showed that the Gaussian profile appears to be beneficial when the ZnMgO freecarrier concentration is lower. This is shown in Figure 13 below as the ZnMgO free-carrier 

concentration is reduced to n = 1013 cm-3 for both cases. With the lower free-carrier 

concentration, the Gaussian profile case shows much lower Voc losses with increasing interface 

defect density, similar to that of the high free-carrier concentration with constant bandgap (Mg) 

profile. Conversely, the constant profile case has voltage dependent current collection and Voc 

losses that become very pronounced as the interface state defect density is increased (compare 

with Figure 12, constant profile). The constant Mg profile in ZnMgO introduces secondary 

barrier when free carrier density is low under forward voltage (not shown, band diagram only 

shows V=0 case). Note that there is also a small barrier at the ZnO/ZnMgO interface when the 

ZnMgO free carrier density is high (see band diagram in Figure 13), that would not be present 

Constant Mg profile,

∆Ec = 0 eV 

ZMO free-carrier conc.

Voltage (V)

Defect Density

Defect Density

Gaussian Mg profile,

∆Ec = - 0.3 eV (i.e. cliff)

ZnMgO free-carrier 

conc

 
18

with a Gaussian profile. However, voltage dependent current collection, without Voc loss is 

observed if we increase the peak energy in the Gaussian profile case (not shown). 

Figure 13 - Simulated device light IV curves (left) and band diagram (right) for constant ZnMgO 

bandgap, and 0 eV conduction band offset with mid-bandgap CIGS (bottom), and Gaussian 

ZnMgO profile. 

In summary the modeling and simulation results indicate that the Gaussian profile with 

conduction band cliff at the interface is not detrimental as long as the free-carrier concentration 

is keep lower, and the Gaussian peak energy is not too high. 

Conclusions

Incremental progress in the novel device performance was demonstrated throughout the program, 

and ultimately achieved performance results that exceeded the milestones ahead of schedule and 

set the stage for a follow-on incubator program. AST has successfully developed mid-bandgap 

CIGS based devices with metal-oxide buffer layers (no Cd) that match the average performance 

of the baseline low bandgap CIGS with CdS buffer layer small-area devices. Metal-oxide buffer 

layer devices with mid-bandgap CIGS alloys on polyimide substrates were produced with 

efficiencies of over 12%. Corresponding mid-bandgap devices with CdS buffers produced over 

13% efficient devices. Furthermore, no obvious degradation in the device performance has been 

observed to date, after proper storage ambient of the different types of unencapsulated devices 

ZMO free-carrier conc.

ZMO free-carrier conc.

n = 1013 cm-3 

Defect Density

Band-Diagram at V=0

Light IV curves:

Gaussian Mg profile,

 Defect Density

ZMO free-carrier conc. 

 1013 -3

Constant Mg profile,

∆Ec = 0 eV 
19

were identified. It was also determined from a device emulation program that the non-uniform 

Mg profile in the APCVD grown ZnMgO was not necessarily detrimental to the ZnMgO/midbandgap CIGS performance as long as the Mg non-uniformity was not excessive and the ZnMgO 

free-carrier concentration was lower.

 

1 J.D. Cohen, J. T. Heath, and W.N. Shafarman,”New Junction Capacitance Methods fo the Study of Defect 

Distributions and Carrier Properties in the Copper Indium Diselenide Alloys,” Table 1, MRS Symposium 

Proceeding, Vol. 763, pp429-440, (2003). 2 M. Gloeckler and J. Sites, “Efficiency limitations for wide-bandgap chalcopyrite solar cells,” Thin Solid Films, 

v480-481, pp 241-245, (2005) 3 T. Minemoto, et al., “Theoretical analysis of the effect of conduction band offset of window/CIS layers on 

performance of CIS solar cells using device simulation,” Sol. Energy Mat. & Sol. Cells, 67, pp83-88, (2001). 4 G. B. Turner, R.J. Schwartz, and J.L. Gray, “Band Discontinuity and Bulk vs. Interface Recombination in 

CdS/CuInSe2 Solar Cells,” 20th IEEE PVSC, pp 1457 – 1460, (1988).
F1146-E(10/2008) 

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April 2011

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3. DATES COVERED (From - To)

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4. TITLE AND SUBTITLE

ZnMgO by APCVD Enabling High-Performance Mid-bandgap CIGS 

on Polyimide Modules

5a. CONTRACT NUMBER

DE-AC36-08GO28308

5b. GRANT NUMBER

5c. PROGRAM ELEMENT NUMBER

6. AUTHOR(S)

Lawrence Woods

5d. PROJECT NUMBER

NREL/SR-5200-51379

5e. TASK NUMBER

PV10.1199

5f. WORK UNIT NUMBER

7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES)

Ascent Solar Technologies, Inc.

8120 Shaffer Pkwy. 

Littleton, CO 80127

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REPORT NUMBER

NEU-0-99010-02

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National Renewable Energy Laboratory

1617 Cole Blvd.

Golden, CO 80401-3393

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NREL

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National Technical Information Service

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13. SUPPLEMENTARY NOTES

NREL Technical Monitor: Brian Keyes

14. ABSTRACT (Maximum 200 Words)

This Pre-Incubator project was designed to increase the “real world” CIGS based photovoltaic module performance 

and decrease the Levelized Cost of Energy (LCOE) of systems utilizing those modules compared to our traditional 

CIGS based photovoltaic modules. This was enabled by a) increasing the CIGS bandgap and b) developing better 

matched device finishing layers to the mid-bandgap CIGS based photovoltaics; including window and buffer layers 

(and eventually the TCO). Incremental progress in the novel device performance was demonstrated throughout the 

program, and ultimately achieved performance results that exceeded the milestones ahead of schedule. Metal-oxide 

buffer layer devices with mid-bandgap CIGS alloys on polyimide substrates were produced with efficiencies of over 

12%. Corresponding mid-bandgap devices with CdS buffers produced over 13% efficient devices. Furthermore, no 

obvious degradation in the device performance has been observed to date, after proper storage ambient of the 

different types of unencapsulated devices were identified.

15. SUBJECT TERMS

Photovoltaic; CIGS; polyimide

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