National Renewable Energy Laboratory 

Innovation for Our Energy Future 

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

Office of Energy Efficiency & Renewable Energy

NREL is operated by Midwest Research Institute ● Battelle Contract No. DE-AC36-99-GO10337 

Uniform, High Efficiency, Hybrid 

CIGS Process with Application 

to Novel Device Structures 

Annual Technical Report 

15 March 2005 — 14 March 2006 

A.E. Delahoy, L. Chen, and B. Sang 

Energy Photovoltaics, Inc. 

Princeton, New Jersey 

Subcontract Report 

NREL/SR-520-40145 

June 2006 
Uniform, High Efficiency, Hybrid 

CIGS Process with Application 

to Novel Device Structures 

Annual Technical Report 

15 March 2005 — 14 March 2006 

A.E. Delahoy, L. Chen, and B. Sang 

Energy Photovoltaics, Inc. 

Princeton, New Jersey 

NREL Technical Monitor: Harin S. Ullal 

Prepared under Subcontract No. ZXL-5-44205-05 

Subcontract Report 

NREL/SR-520-40145 

June 2006 

National Renewable Energy Laboratory

1617 Cole Boulevard, Golden, Colorado 80401-3393 

303-275-3000 • www.nrel.gov 

Operated for the U.S. Department of Energy 

Office of Energy Efficiency and Renewable Energy 

by Midwest Research Institute • Battelle 

Contract No. DE-AC36-99-GO10337
This publication was reproduced from the best available copy 

submitted by the subcontractor and received no editorial review at NREL.

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Preface 

The leading commercial thin film technologies are, broadly, a-Si:H and related alloys 

(possibly combined with nc-Si:H), CdTe, and CIGS; also under development are thinfilm c-Si, dye-sensitized TiO2, and organics. All have advanced considerably in the last 

few years. A new consideration, resulting from the recent booming of the PV industry 

worldwide, especially in the dominant c-Si technologies, has been the onset and growth 

of a supply shortage of Si raw materials. This shortage will definitely persist for a few 

years, and some argue that because of the conservative nature of Si suppliers and the 

financial risk associated with expansion, the shortage may persist for an even longer 

period. This situation offers a window of opportunity for thin films, and there is much 

commercial activity (capacity expansion, venture capital infusion, acquisitions, and 

agreements) to capitalize on it. 

Most thin-film PV processes offer the following advantages over wafer-based PV: 

monolithic design and large substrates (leading to reduced parts handling), low 

consumption of both direct and indirect materials, low specific energy for production, and 

fewer process steps. 

Within each thin-film semiconductor technology category, various deposition or growth 

methods have been devised. The long term commercial success of the various 

approaches is not automatically assured, but is dependent on a combination of factors 

including module efficiency, manufacturing cost per watt, final cost of the electricity 

produced, market niche, and even competitive forces within a given company. CIGS 

continues to hold the efficiency record, but the technology, although having entered the 

realm of manufacturing, is arguably not yet sufficiently evolved to be cost-competitive 

for production of standard power modules. To be economically viable in the long run, a 

PV technology must overcome any deficiencies in all of the following areas: scale up, 

yield, equipment uptime, capital equipment cost, module longevity. Naturally, process 

complexity can exacerbate several of these challenges. While high module efficiency is 

desirable, module efficiency figures do not tell the whole story, and the market is 

becoming more sophisticated in analyzing the true cost of electricity resulting from a 

given product type. 

Energy Photovoltaics, Inc. (“EPV”) is a manufacturer of tandem junction, amorphous 

silicon PV modules and of Integrated Manufacturing Systems for a-Si module 

production. In addition, EPV is developing CIGS technology with a view towards costeffective manufacturing of higher efficiency CIGS modules. EPV has consistently 

pursued a vacuum-based approach to CIGS production using soda-lime glass substrates. 

It has also deliberately chosen not to use the poisonous gases H2Se or H2S. These 

strategically-important choices offer a low-cost substrate, control over layer homogeneity 

and purity, and production without significant hazards, all of which help to lower the 

processing costs. 

NREL has for a number of years operated the Thin-Film Photovoltaics Partnerships 

Program to facilitate the development of CIGS, CdTe, and Si-based thin-film 

 iii
technologies. The long-term objective of the TFPPP is to demonstrate commercial, lowcost, reproducible modules of 15% aperture-area efficiency [1]. 

As part of the TFPPP, EPV is currently performing as an R&D Partner (Solar Cell 

Process Developer (Optimizer)) in a three-phase, cost-shared subcontract entitled 

“Uniform, High Efficiency, Hybrid CIGS Process with Application to Novel Device 

Structure”. EPV also participates in the National CIS Team Meetings. This annual 

technical report describes the major results obtained during the first phase of the 

subcontract (March 15, 2005 - March 14, 2006). 

One of the main Phase I objectives of this subcontract was for EPV to demonstrate 14% 

efficient CIGS devices using a hybrid process. The processing was also required to have 

good controllability. These goals were successfully accomplished. 

 iv
Contents 

Preface............................................................................................................................. iii 

1.0 Introduction..............................................................................................................1

2.0 Development of a Simplified Hybrid Process Combined with Thin CIGS 

Absorber in the R&D Hercules System...................................................................2 

 2.1 Development of Simplified Hybrid Process .....................................................2 

 2.2 Optimization of Process and Device Performance ...........................................5 

 2.3 Improvement of Indium Delivery .....................................................................7

3.0 CIGS Development in the Large Area Zeus System ...............................................7 

 3.1 Maintenance and Repair ...................................................................................7 

 3.2 Transfer of the Simplified Hybrid Process .......................................................7 

 3.3 Large Area Modules with CIGS Absorber of Sub-micron Thickness..............9 

 

4.0 Exploration of Post-CIGS Treatment and Buffer Layer .......................................11 

 4.1 Post-CIGS Treatment and Curing Effect of Aged CIGS Samples .................11 

 4.2 Exploration of Improving CdS Buffer Layer..................................................12 

5.0 New Front and Back Contacts with RE-HCS........................................................13 

 5.1 Titanium-doped Indium Oxide TCO Films .....................................................13 

 5.2 ZnO:Al Front TCO produced by RE-HCS ......................................................14 

 5.3 New Back Contact – TiN.................................................................................14 

 5.4 Application of TiN to CIGS Devices...............................................................16 

6.0 Phase I Summary ...................................................................................................18 

Acknowledgements.........................................................................................................19 

Publications.....................................................................................................................19 

References.......................................................................................................................20 

 v
List of Tables

Table I. Device performance with simplified hybrid process and thin CIGS ...........3 

Table II. Properties of CIGS films from Zeus and performance of processed 

 devices..........................................................................................................8 

Table III. Physical and performance parameters for three CIGS modules 

 (A, B, C) with different CIGS thicknesses ..................................................9 

Table IV. Device performance versus time elapsed before CIGS treatment and 

 and CdS deposition .....................................................................................12

Table V. Properties of two In2O3:Ti films produced by RE-HCS ............................14 

Table VI. Resistivity of In2O3:Ti films as a function of Ts........................................................... 14 

Table VII. Comparison of CIGS solar cells produced with RE-HCS and RF 

 magnetron sputtered ZnO:Al layers...........................................................14 

Table VIII. Performance of glass/RE/CIGS/CdS/ZnO/ITO solar cells with 

 different rear electrode (RE) structures......................................................17 

List of Figures 

Fig. 1. QE curve of the cell with a 0.74 μm CIGS absorber layer ................................4 

Fig. 2. SEM images of a 0.92 μm thick CIGS film (H052405-6 in Table I) ................4 

Fig. 3. External QE curve of H121405-2 #2 .................................................................5 

Fig. 4. I-V curve of H121405-2 #2 as measured by NREL ..........................................6 

Fig. 5. J-V and QE curves of the cell with a 1.3 μm CIGS absorber layer (no 

 AR coating)........................................................................................................8 

Fig. 6. SEM cross-section of a 1 μm CIGS cell showing the Mo, CIGS, and 

 ZnO:Al layers.....................................................................................................9 

Fig. 7. I-V curve (taken in sunlight) for a CIGS module with CIGS thickness 

 of 0.75 μm........................................................................................................10 

Fig. 8. Measured short-circuit current densities Jsc for modules A, B, C of 

 Table III compared to those calculated as a function of CIGS thickness; 

 values are normalized to unity at a thickness of 2.5μm...................................10 

Fig. 9. Auger depth profile of the CIGS region of a completed cell with CIGS 

 thickness of 1.0 μm ..........................................................................................11 

Fig. 10. Fill factor and efficiency as a function of post-CIGS treatment type ..............12 

Fig. 11. Scatter plot of transmittance and resistivity of In2O3:Ti films.........................13 

Fig. 12. Total reflectance of TiN/Mo and Mo films......................................................15 

Fig. 13. XRD spectrum of TiN/Mo films deposited with four different substrate 

 temperature and bias combinations (here LT denotes unheated, HT 

 denotes 250°C).................................................................................................16 

 vi
1.0 Introduction

As part of the Thin Film Photovoltaics Partnership Program, EPV has conducted research 

to help generate a technology base for development of high efficiency CIGS solar cells as 

well as for production of CIGS PV modules. The method of CIGS formation is based on 

vacuum deposition. This strategy is consistent with the observation that, despite there 

being several approaches to forming CIGS of varying degrees of quality, vacuum 

deposition has maintained the world record for the highest efficiency CIGS device. This 

record currently stands at 19.5% (692mV, 35.2 mA/cm2

, FF 79.9%) for a 0.41 cm2 device 

grown at NREL by the three-stage process [2]. 

For CIGS formation, EPV uses vacuum equipment incorporating novel linear evaporation 

sources and magnetron sputtering cathodes for uniform coating of large (0.43m2

), heated,

moving substrates [3]. During EPV’s last TFPPP subcontract (2002-2005), EPV 

successfully developed a hybrid (combined sputtering/evaporation) CIGS process [P1]. 

Using this process in our large area deposition system (Zeus), we achieved a device 

efficiency of 13% and a module efficiency of 7.5% (power > 26W). The main merits of 

the hybrid process include: 

• Controllable copper deposition by sputtering 

• Delivery of In, Ga, and Se by linear thermal sources 

• Ability to perform gallium profiling 

• Use of elemental Se (no hazardous H2Se or H2S) 

• Ability to scale up the coating width 

The hybrid process was developed in a small scale R&D system (Hercules), and was 

transferred to Zeus for module production. However, the original hybrid process had a 

processing sequence that limited throughput, and we came to doubt its suitability for costeffective manufacturing. We concluded that simplified processing was required. 

Recently, a common area of focus for the TFPPP emerged as a result of concern 

regarding both the rising price of indium and its availability, the focus being on CIGS 

thickness reduction. In order to reduce CIGS module cost and to increase the ultimate 

manufacturing capacity of CIGS, it was resolved at the National CIS Team Meeting of 

March 2005 that CIGS cells and modules having a reduced absorber layer thickness of 

1μm (and below) would be developed, in contrast to the more generally used R&D 

thicknesses of about 2.5 μm. This strategy promised to reduce In and Ga consumption, 

reduce direct materials cost, increase throughput and/or reduce capital equipment costs, 

and to reduce waste. 

Project Objectives for this Subcontract 

In light of the two concerns described above, the principal objective for this subcontract 

was agreed to be the development of high efficiency CIGS solar cells with an absorber in 

 1
the thickness range of approximately one micron to submicron by a simplified hybrid 

CIGS process. The envisaged Statement of Work encompasses: 

• Exploration of a new hybrid process to simplify the original hybrid process. 

• Systematic optimization of the simplified hybrid process. 

• Reduction of potential manufacturing cost by demonstration of competitive ultrathin cells. 

• Achievement of 14% and above cell efficiency. 

• Development of a more reflective back contact for ultra-thin cells. 

• Further investigation of post-CIGS treatment. 

• Improvement of the buffer layer. 

• Development and utilization of superior TCOs. 

• Demonstration of the viability of new technologies via module fabrication. 

• The uncovering and mitigation of potential cell and module degradation 

mechanisms. 

It will be seen that during Phase I, we successfully developed a new, simplified hybrid 

process. A highlight of intensive work was the achievement of a 14.0% NREL-verified 

device at a CIGS thickness of 1.13 μm. The simplified hybrid process considerably 

reduces CIGS film formation time and offers the promise of being a truly cost-effective 

and manufacturable one. It is considered to be one of the more attractive CIGS processes 

in the industry. 

A description of the equipment and analytical facilities available for this research can be 

found in earlier reports, e.g. [P1]. 

2.0 Development of a Simplified Hybrid Process Combined with Thin 

CIGS Absorber in the R&D Hercules System 

2.1 Development of a Simplified Hybrid Process 

The development of a more manufacturing-friendly variant of the hybrid process, i.e. a 

hybrid process with high efficiency and high throughput, has been in our mind since day 

one when we first developed the hybrid process. As one of the sub-tasks in this new 

TFPPP contract, we proposed to simplify our hybrid process, thereby leading EPV to a 

truly cost-effective and manufacturable process. Another sub-task targeted development 

of device structures with sub-micron thick CIGS in order to reduce materials costs 

(especially that of indium) and to increase machine productivity. At the start of the 

contract, we decided to embark on these tasks simultaneously by making films of about 

one micron thickness using simplified processes. 

We started development of the simplified hybrid process in the R&D Hercules system. 

The first obstacle we had to overcome was the problem of peeling of the film from the 

Mo/glass substrate. After trying different approaches, targets and processes, we found a 

reliable method of making adherent CIGS films in a controllable manner. 

 2
The next step was to make the simplified hybrid CIGS device with the right copper ratio. 

The devices in the first couple of runs exhibited CIS-like behavior with high current and 

low voltage. However, devices with 10% plus in efficiency were soon reached after a few 

optimization runs. 

Following the third step, viz. the addition of more gallium with proper profiling, we are 

now in the position of being able to consistently achieve 12% plus device efficiency in a 

CIGS absorber thickness around 1 μm and below. Table I lists the performance of some 

devices from the Hercules system made with the simplified hybrid process. (Some of the 

earlier results have already been published [P4].) For comparison, a device with a normal 

CIGS thickness using the original hybrid process is listed in the last row. Using the EPV 

simplified hybrid process we are able to fabricate devices of efficiencies of 12-13% with 

a CIGS absorber thickness in the range of 0.75-1.30 μm. The CIGS film thickness was 

determined using a Dektak III, and was also confirmed by SEM measurement. 

Table I. Device performance with simplified hybrid process and thin CIGS. 

ID Process Voc1

(mV)

FF 

(%) 

Jsc

2

(mA/cm2)

Efficiency 

(%)

Thickness

(μm)

H071205-4(8) s hybrid 590 70.5 29.1 12.1 0.74 

H052005-1 s hybrid 605 67.9 29.4 12.1 0.82 

H052305-5 s hybrid 608 70.5 27.9 12.0 0.85 

H052405-6 s hybrid 638 70.0 28.7 12.8 0.92 

H121405-2 s hybrid 622 72.7 29.4 13.3 1.13 

H080205-4 s hybrid 587 68.5 33.2 13.4 1.30 

H146-5 hybrid 569 73.5 32.3 13.5 2.51 

1 T>25C 

2 QE current (active area) without AR coating; all contacts are mechanically pressed indium

Figure 1 shows the QE curve of the cell with a 0.74 μm CIGS absorber layer. The device 

has a respectable collection efficiency even with a very thin absorber layer. However, a 

fall off in QE is evident in the long wavelength region between 700 and 1000 nm. We do 

not yet understand the precise origin of this, whether it is only due to insufficient optical 

absorption or whether other effects such as back contact recombination play a role. 

Comparing the 0.74 μm (simplified hybrid) with the 2.51 μm (hybrid) device, we find a 

drop in current density of only about 10% upon thickness reduction of 70%. But a drop 

in current density of more than 10% can be observed if the 0.74 μm device is compared 

to the 1.30 μm device (both simplified hybrid). This might be related to Ga profiling 

although the Ga/(In+Ga) ratios of these films are very close. A systematic investigation 

(using the simplified hybrid process) is planned by changing film thickness while trying 

to keep the Ga ratio and Ga profile unchanged in the films. 

 3
Wavelength (nm)

400 500 600 700 800 900 1000 1100 1200

External QE

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

H071205-4 (29.1 mA/ cm2, 0.74 μ m)

Fig. 1. QE curve of the cell with a 0.74 μm CIGS absorber layer. 

In Fig. 2, cross-sectional and top view SEM images for run H052405-6 (see Table I) are 

given. The images show a large columnar grain structure with an apparent grain size of 

~ 0.5 – 0.7 μm (some possibly larger), and a film thickness consistent with the profiler 

measurement (0.92 μm). 

 

Fig. 2. SEM images of a 0.92 μm thick CIGS film (H052405-6 in Table I). 

Some crevices in the film are evident. However, it is interesting to note that there is no 

sign of the horizontal interface in the middle of the CIGS film which was a common 

feature of our original hybrid process. In other words, the bi-layer structure has totally 

disappeared in the simplified process. The higher Voc that we observe with the new 

process may be indirectly related to this structural/growth difference. 

The crevices in the film are cause for concern. The IV curves for cells with sub-micron 

thick CIGS not infrequently exhibit a noticeable dark leakage current. It was not clear 

 4
whether this was caused by an intrinsic compactness problem with the film or whether it 

resulted from a quick contacting process involving indium pressure contacts to the ZnO. 

By evaporating Al grids onto the ZnO we found the dark shunting substantially 

improved, though some dark leakage still remained. We therefore believe that the 

crevices are, at least in part, responsible for shunting paths in the device and hence for 

increased dark leakage. The leakage tends to depress Voc and FF and may partially 

explain why the FF is good but not exceptional in these CIGS devices with sub-micron 

thickness. 

 

2.2 Optimization of Process and Device Performance 

Much effort was devoted to optimizing the simplified hybrid process. The effort included 

investigations of substrate temperature, selenium flux, pre and post selenization time and 

flux, Ga profiling, substrate temperature profiling at the very last stage of evaporation 

and during the post selenization period. As a result, some excellent devices were 

produced. From Run H121405, we fabricated a CIGS device having a total area 

efficiency of 14% with an absorber layer thickness of 1.13 μm. The efficiency of this cell 

was 13.3% before depositing the metal grid collection electrode and AR coating, and 

14% (644 mV, 30.4 mA/cm2, 71.6% (EPV data); 654 mV, 29.3 mA/cm2, 72.9% (NREL 

data)) after depositing the metal grid and AR coating. The small difference in Voc

between the EPV and NREL measurement is likely due to higher measurement 

temperature at EPV, while the higher current density at EPV reflects the use of active 

area current density as deduced from integrated QE data. Plotted in Fig. 3 is the external 

quantum efficiency of this cell measured at EPV; it corresponds to the active area Jsc of 

30.4 mA/cm2. Plotted in Fig. 4 is the I-V curve of this cell as measured by NREL. It is 

gratifying that the simplified hybrid process has yielded peak efficiencies somewhat 

higher than those achieved with the original hybrid process [P6, P7]. 

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

400 500 600 700 800 900 1000 1100 1200

Wavelength (nm)

QE 

Fig. 3. External QE curve of H121405-2 #2. 

 5
Fig. 4. I-V curve of H121405-2 #2 as measured by NREL. 

 6
2.3 Improvement of Indium Delivery 

The parameter that is most important to control in the R&D Hercules system in order to 

secure a predictable CIGS composition is the amount of indium evaporated in the hybrid 

process. Indium delivery is from a heated crucible. (The delivery of copper can be well 

controlled by sputtering power and time.) Previously, the indium evaporation rate was 

controlled by the voltage applied to the crucible heater from the secondary winding of a 

transformer. Recently, in order to obtain better control over the crucible current, a 

current regulator was designed in house and was installed. It has worked very well thus 

far and reproducibility runs are now being conducted. The results of these reproducibility 

tests will be described in the next report. 

3.0 CIGS Development in the Large Area Zeus System 

3.1 Maintenance and Repair 

During the period of proving the concept and establishing the recipe for the simplified 

hybrid process in the R&D Hercules system, we focused on maintenance and repair of 

our large area Zeus system. 

The large deposition chamber was opened in order to replace all the lamps used for 

substrate heating. This represented arduous work as the entire innards had to be pulled 

out and cleaned. The lamps were replaced taking care to protect the connections from 

attack by Se. We also took the opportunity to replace some worn transportation parts. 

The electrical circuit of the power for the sources was modified to give increased 

protection against overloading. A temperamental stop curtain between the loading 

chamber and the deposition chamber was also fixed. 

Some exhaust pipes that were deteriorating and showing leakage were replaced. The 

roughing pump showing potential leakage was also repaired by replacing all sealing 

gaskets. 

3.2 Transfer of the Simplified Hybrid Process 

Following the success of the simplified hybrid process in the R&D scale Hercules 

system, the simplified hybrid process was transferred to the large area deposition system, 

Zeus. Large-area CIGS plates (17” x 38”) were made and device fabrication was 

undertaken. Our efforts demonstrated that the simplified hybrid process is transferable, 

just as was found for our original hybrid process. In order to indicate the processing 

progress, the composition and thickness of CIGS films resulting from some Zeus runs as 

well as performance of the processed devices are listed in Table II. 

 7
 Table II. Properties of CIGS films from Zeus and performance of processed devices. 

Sample Thickness 

(μm) 

Cu/(In+Ga) Ga/(In+Ga) Voc

(mV) 

FF 

(%) 

Jsc(QE) 

(mA/cm2) 

Eff 

(%) 

Z1779 2.38 0.39 0.40 - - - - 

Z1780 1.78 0.54 0.40 - - - - 

Z1781 1.59 0.69 0.46 645 63.4 21.5 8.8 

Z1782 1.32 0.90 0.45 576 67.0 27.3 10.5 

Z1783 1.38 0.80 0.35 565 65.7 28.1 10.4 

Z1784 1.26 0.90 0.35 534 68.4 32.0 11.7 

Z1785 1.36 0.87 0.34 538 68.5 33.1 12.2 

Z1787 1.30 0.93 0.35 558 70.3 30.7 12.0 

Z1794 1.30 1.00 0.39 602 72.7 30.0 13.2 

The major problems we met and solved were: 1) Peeling. In the first couple of runs 

before Z1784, we found that the CIGS films had poor adhesion to the Mo and peeled off 

either after deposition or later during chemical bath deposition of CdS. After optimizing 

the selenization time, all films later than run Z1784 showed good adhesion to Mo; 2) 

Composition control. Film composition was gradually adjusted to the suitable range for 

devices by adjusting the In and Ga source temperatures and deposition time (see 

composition changes from Z1779 to Z1783). Right now, we are able to consistently 

maintain our average film composition in a good range (Cu/(In+Ga): ~ 0.90; Ga/(In+Ga): 

~ 0.35), and to fabricate devices of 11-12%. The CIGS absorber layer thickness is 1.3 - 

1.4 microns. 

 The best device (without AR coating or grid electrode) so far is 13.2%. J-V and QE 

curves are shown in Fig 5. This level of performance almost matches the highest 

efficiency achieved in the Hercules system (13.4%, as shown in Table I). The successful 

achievement of high performance devices will be followed in Phase II by fabrication of 

large area modules using the simplified hybrid process. 

Z1794-9-B2

Voltage (V)

-0.2 -0.1 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7

Current Density (mA/ cm2

)

-35

-30

-25

-20

-15

-10

-5

0

5

10

15

20

Voc: 602 mV

Jsc: 30.0 mA/ cm2

 (From QE)

FF: 72.7 %

Eff: 13.2 %

Z1794-9-B2

Wavelength (nm)

400 500 600 700 800 900 1000 1100 1200

External QE

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

Fig.5. J-V and QE curves of the cell with a 1.3 μm CIGS absorber layer (no AR coating). 

 8
3.3 Large Area Modules with CIGS Absorber of Sub-micron Thickness 

Use of a sub-micron thick CIGS absorber for module fabrication raises, among others, 

the following concerns: 

• Device shunting might be more severe in large-area, thinner CIGS. 

• An additional shunting path might develop between isolated Mo segments if the 

CIGS thickness is smaller than that of the Mo. 

To test these concerns, micron and sub-micron thick CIGS modules were produced at 

EPV. The main purpose was to find out if there are any additional problems in module 

processing for ultra-thin CIGS. Modules with two CIGS thicknesses, namely 1.0 μm and 

0.75 μm, were fabricated using the original hybrid process. We were gratified to learn 

that there appear to be no shunting problems in our (previously standard) module process 

at those thickness levels. Indeed, the performance of modules with CIGS thickness 1.0 

μm and 0.75 μm were found to be not that much inferior to that of modules with a normal 

thickness (2.5 μm). Results are listed in Table III. From Table III, we see that the FF and 

Voc per cell for thin CIGS are essentially unchanged from those obtained at 2.5μm, while, 

as expected, the Jsc is somewhat reduced [P2]. 

Table III. Physical and performance parameters for three CIGS modules (A, B, C) 

with different CIGS thicknesses. 

Module 

( #cells) 

CIGS 

th.(μm) 

Area 

(m2) 

Voc 

(V) 

Isc 

 (A) 

Pmax 

(W) 

FF 

 (%) 

Voc

/cell 

Jsc

(mA/cm2)

Eff 

 (%) 

A (71) 2.5 0.345 38.5 1.20 26.0 56.4 0.542 24.7 7.52 

B (71) 1.0 0.330 37.7 1.01 21.1 55.7 0.531 21.8 6.41 

C (34) 0.75 0.134 18.6 0.80 8.48 57.0 0.547 20.3 6.32 

Figure 6 shows a SEM cross-section for a completed device (Mo/CIGS/ZnO) with 1 μm 

thick CIGS. The CIGS appears to have a compact grain structure. The I-V curve for the 

module with CIGS thickness 0.75 μm is shown in Fig 7. 

Fig. 6. SEM cross-section of a 1 μm CIGS cell showing the Mo, CIGS, & ZnO:Al layers. 

 9
Voltage (V)

0 5 10 15 20

Current (A)

-1.0

-0.8

-0.6

-0.4

-0.2

0.0

CIGS module ID 1762, 34 cells, 0.134m2

CIGS thickness 0.75μm

Irrad. 1110W/m2

Module temp. 49o

C

Voc 17.0V

I

sc 0.893A

FF 57.0%

Pm 8.6 W

Fig. 7. I-V curve (taken in sunlight) for a CIGS module with CIGS thickness of 0.75μm. 

In Fig. 8, the calculated Jsc (based on optical absorption in CIGS with Eg = 1.1 eV, 

integrated over an AM1.5 spectrum) is compared with the measured module Jsc. Reasons 

for the faster fall-off in the observed currents could be inferior CIGS quality or severe 

back diffusion and recombination in the thin CIGS film. It might also be due to 

insufficient bandgap profiling, although we had deliberately attempted to grade the Ga 

ratio (see Fig. 9). 

CIGS thickness [μm]

0.0 0.5 1.0 1.5 2.0 2.5 3.0

Normalized Jsc

0.75

0.80

0.85

0.90

0.95

1.00

1.05

Calculation

Module measurement

Fig. 8. Measured short-circuit current densities Jsc for modules A, B, C of Table III 

compared to those calculated as a function of CIGS thickness; 

 values are normalized to unity at a thickness of 2.5μm. 

 10
Sputter time (min)

50 60 70 80 90

Element ratio

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

Profile through completed ZnO/CdS/CIGS (1.0 μm) cell; CIGS region

Cu/(In+Ga)

Ga/(In+Ga)

Fig. 9. Auger depth profile of the CIGS region of a completed cell with 

 CIGS thickness of 1.0 μm. 

4.0 Exploration of post-CIGS treatment and buffer layer 

4.1 Post-CIGS Treatment and Curing Effect of Aged CIGS Samples

We continued to investigate post-CIGS treatment during Phase I of this subcontract. As 

we have mentioned in our earlier reports [3], EPV’s standard treatment and the new 

treatment have shown good consistency in improving device performance on EPV 

absorbers. 

It would, of course, be interesting to see if EPV post-CIGS treatments also improve 

devices on CIGS produced by other organizations. A set of NREL CIGS samples from 

Run C1896 was kindly provided by Miguel Contreras. These samples were vacuum 

sealed for shipping and were stored in a N2 box at EPV before using them. We divided 

the six samples into two groups of three. Two separate post-CIGS treatment experiments 

were conducted. In each experiment, one sample was treated with the standard EPV 

process, the second sample with the new EPV process, while the third sample was left 

untreated as a reference. Plotted in Fig. 10 is a comparison of device efficiency and FF 

of cells with the two types of treatment and of the untreated reference cells. The results 

from both experiments consistently confirmed that EPV post-CIGS treatments do 

improve device performance on NREL absorber layers when devices are finished at EPV. 

The data clearly shows improvement of cell performance with treatment is statistically 

significant. Increase of device efficiency results mostly from substantial improvement in 

fill factor. 

 

 11
50

55

60

65

70

75

80

85

90

95

100

-1 0 1 2 3

Treatment Type

FF (%)

6.0

7.0

8.0

9.0

10.0

11.0

12.0

13.0

14.0

15.0

16.0

Efficiency (%)

Nontreated New Treatment Old Treatment 

Hollow for 

efficiency

Solid for 

FF

Fig. 10. Fill factor and efficiency as a function of post-CIGS treatment type. 

CIGS aging behavior is another important issue to be addressed in module production 

since, for batch mode deposition of CdS, the CIGS films cannot always be kept fresh 

before CBD CdS deposition. We have found that the EPV treatment also has the merit of 

refreshing aged CIGS, thereby curing the aging problem. We list one set of results in 

Table IV. The data unarguably demonstrates that CIGS device performance need not 

suffer aging deterioration even if the CIGS film has been exposed to ambient air for as 

long as 97 days before treatment and CdS deposition. The post-CIGS treatment cures the 

CIGS aging effect [P4]. 

Table IV. Device performance versus time elapsed before CIGS treatment and CdS deposition. 

Sample ID Treatment & CdS 

(Days) after CIGS 

Voc 

(mV) 

FF 

 (%) 

Jsc

(mA/cm2) 

Eff 

(%) 

Z1730-5 0 543 70.1 33.4 12.7 

Z1730-19 84 573 72.3 30.3 12.5 

Z1730-27 91 557 70.0 33.0 12.9 

Z1730-36 92 540 71.5 32.4 12.5 

Z1730-41 97 542 70.3 32.7 12.4 

 

4.2 Exploration of Improving CdS Buffer Layer

Two directions for improving the currently used CBD CdS process were pursued in 

Phase I. 

First, we studied the effect of a surfactant following the report by Craig Perkins and Falah 

Hasoon [4]. An amount equal to 0.44 gm of a surfactant was added in 700 cc DI water in 

our normal CBD CdS dip. An increase in efficiency of about 1% on average in devices 

with added surfactant was observed. However, the improvement in efficiency 

 12
disappeared if these samples were treated with our post-CIGS treatment before CBD CdS 

process. This suggested that the EPV surface treatment may have a similar function to 

better wetting of the CIGS surface in addition to its other benefits. 

Second, we realized that in module production it may be desirable to slow down the onset 

of colloidal growth during the CdS process to suppress the size and formation of colloidal 

particles in the liquid phase so that the growth of the CdS film would be more uniform 

[5]. Various molar concentrations of sulfate anions were added into both our normal and 

raised temperature CBD CdS process. We did confirm the effect of (NH4)2SO4 additive in 

reducing the formation of large particle size colloids but have not been able to confirm 

that this additive could be beneficial to device performance as our normal process yielded 

higher efficiencies. Further investigation and optimization in this direction is under 

consideration. 

5.0 New Front and Back Contacts with RE-HCS 

 

5.1 Titanium-Doped Indium Oxide TCO Films 

Previous work at EPV (not conducted under the Thin Film Partnerships Program) had 

resulted in the production of excellent TCO films (e.g. Ti-doped In2O3 or ZnO:B) by 

reactive-environment hollow cathode sputtering (RE-HCS) [6]. Under the current 

contract we investigated whether it may be possible to retain and take advantage of the 

high mobility of these films in CIGS devices. 

 

As a first step, we prepared some In2O3:Ti films at 300°C. Compared to previous work, 

the oxygen was introduced in a different manner, and some system deficiencies were 

corrected. We also varied the oxygen flow and details of how the Ti was introduced. An 

overview of the results is shown in Fig. 11. The three films on the right-hand side of the 

plot are deficient in Ti or oxygen or both. Film quality does not appear to be influenced 

by substrate type at this deposition temperature. The properties of two films are shown in 

more detail in Table V. 

Resistivity (10-4 ohm-cm)

01234567

Transmittance (%)

86

88

90

92

94

96

borosilicate

7059

soda-lime 

Fig. 11. Scatter plot of transmittance and resistivity of In2O3:Ti films. 

 13
Table V. Properties of two In2O3:Ti films produced by RE-HCS. 

Sample 

number 

Substrate Film 

thickness 

(A) 

Sheet R 

(ohms/square)

Resistivity 

(10-4 Ω.cm) 

Optical 

transmission

(%) 

X443 Borosilicate 3800 6.6 2.5 93 

X444 Soda lime 3800 4.2 1.6 89 

We also investigated deposition at lower substrate temperatures Ts in order to try to apply 

the TCO with high electron mobility to CIGS devices. Film properties were optimized 

by changing the principal variables of pressure and oxygen flow rate. Table VI shows 

some of the film properties that were achieved on 3mm soda-lime glass substrates. A 

typical film obtained at 300°C is included for comparison, although in this particular case 

transmission was sacrificed somewhat for slightly lower resistivity. The film deposited at 

Ts = 170°C will be evaluated as a TCO on CIGS solar cells in the next quarter. 

Table VI. Resistivity of In2O3:Ti films as a function of Ts. 

Sample 

number 

Temperature

(°C) 

Resistivity 

(10-4Ω.cm)

Optical 

transmission (%) 

X433 300 1.8 88 

X451 170 3.4 90 

X462 150 5.5 90 

5.2 ZnO:Al Front TCO Produced by RE-HCS 

We have used RE-HCS ZnO:Al as a front TCO in CIGS devices, with control devices 

having ZnO:Al prepared by conventional RF magnetron sputtering. Both the i-ZnO and 

the n+-ZnO layers were prepared by RE-HCS. The RE-HCS process for ZnO:Al yielded 

devices that were similar in efficiency (see Table VII) and therefore could be adopted for 

routine device fabrication if so desired. 

 

Table VII. Comparison of CIGS solar cells produced with RE-HCS and RF magnetron 

sputtered ZnO:Al layers. 

ZnO 

function 

ZnO 

method 

Voc

(mV) 

Jsc

(mA cm-2) 

FF 

(%) 

Eff. 

(%) 

TCO on CIGS cella RE-HCS 577 29.4 66.1 11.2 

TCO on CIGS cella RF magnetron 533 29.7 70.3 11.1 a Mo/CIGS/CdS/i-ZnO/n+

-ZnO 

5.3 New Back Contact - TiN 

We mentioned earlier in section 2.1 that the QE of a device with a CIGS thickness of 

0.74μm dropped significantly in the long wavelength region. It is thought that at least 

part of this drop results from incomplete optical absorption and is unavoidable for a submicron thick CIGS active layer in a cell of otherwise identical construction. 

 14
One of the solutions to help restore absorption in the CIGS at long wavelengths is to 

substitute a more reflective back contact for the standard Mo. This is easier said than 

done, since Mo has firmly stood the test of time as being the most suitable back contact 

metal. We realized many, many years ago that TiN would be interesting to evaluate as a 

possible back contact as it possesses metallic conductivity and is chemically inert. 

However, we had no process in place to produce this material. Preliminary work by 

another group has since investigated TiN as a back contact for CIGS solar cells, 

apparently confirming a higher QE than Mo in the range 850 – 1050nm [7]. 

Internal R&D at EPV has now produced TiN by RE-HCS and recently both the quality of 

the films and the reproducibility of the process have been improved [P3]. The deposition 

process is stable and the deposition rate is high (50 nm/min static and 1.7 nm m/min 

dynamic). These attributes are a consequence of the Ti targets being sputtered in the 

metal mode. The films are gold colored and have a resistivity around 50 - 60 μΩ cm. A 

collaboration has been established with Dr. Bill Shafarman of IEC, Delaware to further 

characterize the films and their performance in devices of standard thickness. Devices of 

sub-micron thickness on TiN as back contact will be fabricated by EPV under this 

subcontract. The reflectivity of Mo as a control and Mo overcoated with different 

thicknesses of TiN is shown in Fig. 12. It can be seen that the IR reflectivity (>700nm) 

of TiN is considerably higher than that of Mo. Even a very thin TiN layer (25 nm) is 

sufficient to significantly improve the reflectivity. Therefore, it has the potential to 

contribute to a more reflective back contact system for CIGS devices, which would be 

especially useful in ultra-thin (submicron thickness) cells. 

0.0

0.2

0.4

0.6

0.8

1.0

300 500 700 900 1100 1300

Wavelength (nm)

Reflection

Glass/Mo control

250A TiN

700A TiN

2000A TiN

Fig. 12. Total reflectance of TiN/Mo and Mo films. 

To use a TiN layer as a back contact, it has to have good conductivity. We found that a 

substrate electrical bias during deposition is essential to secure high TiN conductivity. 

After optimization, with proper substrate bias and temperature we were able to reach a 

resistivity of 53 μΩ cm. This lies in the range of resistivity values typically reported for 

polycrystalline TiN thin films, namely 30 - 150 μΩ cm. X-ray diffraction was performed 

to determine the crystal structure of the TiN films. The result is shown in Fig. 13. 

 15
2 theta (degree)

30 40 50 60 70 80

Intensity (a.u.)

0

200

400

600

800

1000

1200

1400

TiN (111) Mo (110)

Mo (211)

TiN (222)

From top to bottom:

LT -220 V bias

HT -220 V bias

LT -60 V bias

HT -60 V bias

Fig. 13. XRD spectrum of TiN/Mo films deposited with four different substrate 

temperature and bias combinations (here LT denotes unheated, HT denotes 250°C). 

The (111) diffraction peak near 36° for cubic TiN is seen indicating a (111) preferred 

orientation. 

5.4 Application of TiN to CIGS Devices 

An initial set of experiments was designed to test the ability of TiN to serve as a rear 

electrode material for CIGS solar cells. Three pairs of substrates were prepared, each 

pair having co-deposited TiN layers, the three TiN thicknesses being 25 nm, 70 nm, and 

200 nm, and the TiN being deposited on Mo-precoated glass. Since a good ohmic contact 

is known to result when CIGS is deposited on Mo, one TiN substrate from each pair was 

over-coated with a very thin layer (2 nm) of Mo by electron beam evaporation. Such a 

thin layer probably becomes selenized during CIGS deposition and is expected to be 

optically insignificant. The six substrates plus one normal Mo substrate as reference 

were then coated at IEC, Delaware with 2.5 μm CIGS and completed as solar cells in the 

configuration glass/RE/CIGS/CdS/i-ZnO/ITO where RE denotes the rear electrode. 

The performance of the best cell on each of these seven different rear electrode structures 

is listed in Table VIII. The cells on TiN are functional, and perform quite well; however, 

there is a steady decline in efficiency relative to the control cell on thick Mo as the 

thickness of the TiN increases. It is also clear that at any thickness of TiN the cell 

performance is improved by the presence of the 2 nm thick Mo layer. A major 

unresolved issue is whether the Na concentration in the CIGS film has been reduced by 

the presence of the TiN, thereby contributing to the generally lower values of Voc and FF. 

 16
Table VIII. Performance of glass/RE/CIGS/CdS/ZnO/ITO solar cells 

with different rear electrode (RE) structures. 

Rear electrode Eff. 

(%) 

FF 

(%) 

Voc 

(mV) 

Jsc 

(mA cm-2)

Roc 

(Ω cm2) 

Gsc 

(S cm-2) 

Mo 13.65 70.8 599 32.17 2.1 1.3 

Mo/TiN (25nm)/Mo(2nm) 12.27 66.7 577 31.90 2.4 0.9 

Mo/TiN (25nm) 11.70 63.2 579 31.98 2.4 2.6 

Mo/TiN (70nm)/Mo(2nm) 12.30 67.8 588 30.85 2.3 0.1 

Mo/TiN (70nm) 10.80 62.6 569 30.34 2.7 3.2 

Mo/TiN (200nm)/Mo(2nm) 9.43 59.0 518 30.82 2.8 2.9 

Mo/TiN (200nm) 9.62 54.9 557 31.47 3.1 10.1 

 17
6.0 Phase I Summary

Energy Photovoltaics, Inc. 

Uniform, High Efficiency, Hybrid CIGS Process 

 with Application to Novel Device Structures 

Subcontract No. ZXL-5-44205-05 

• In Phase I, a new CIGS process, the simplified hybrid process, was successfully 

developed. It offers promise as a truly cost-effective and manufacturable one. The 

process speeds up the production of CIGS modules. In our view, it is one of the more 

attractive CIGS processes in the industry. 

• The simplified process was optimized in the R&D scale Hercules system. Devices 

with CIGS of sub-micron thickness were intensively developed. We were able to 

achieve 11%-13% device efficiency with a CIGS absorber thickness in the range of 

0.75-0.9 μm, in particular obtaining 12.1% at a thickness of 0.74 μm. 

• An efficiency of 14.0% was verified by NREL for a device with a CIGS thickness of 

only 1.1 μm. The main Phase I goal of this subcontract (demonstrate a 14% efficient 

CIGS device with a hybrid process) was successfully accomplished. 

• The simplified hybrid process was successfully transferred to the large area system 

Zeus. An efficiency of 13.2% was achieved for a device with a CIGS thickness of 1.3 

μm and without any AR coating or grid electrode. This is close to the highest 

efficiency achieved in the R&D (Hercules) system for similar devices (13.4%). The 

successful achievement of high performance devices is being followed by fabrication 

of large area modules. 

• Test modules with micron and sub-micron (down to 0.75 μm) CIGS thickness were 

produced using the original hybrid process. They showed 85% of the power output of 

our best 2.5 μm module. We did not observe significant shunting I-V behavior even 

at a CIGS thickness of 0.75 μm. 

• Post-CIGS surface treatment with EPV’s standard process and with a newer process 

have shown good consistency in improving device performance on EPV CIGS 

absorbers. 

• A new type of TCO (In2O3:Ti), produced by reactive-environment hollow cathode 

sputtering (RE-HCS), has been produced at temperatures compatible with device 

fabrication (170°C) while maintaining adequately low resistivity. It will be applied 

to CIGS devices. 

• IR-reflective TiN (resistivity 50 - 60 μΩ cm) has been successfully produced by REHCS. The IR reflectivity of TiN is considerably higher than that of Mo. The intial 

application of TiN to 2.5μm CIGS devices showed that the cells on TiN are 

functional, and perform quite well. Experimental devices of sub-micron thickness on 

TiN as a back contact are being fabricated. 

 18
ACKNOWLEDGMENTS 

This research was carried out by EPV’s CIGS group: Dr. M. Akhtar, Dr. L. Chen (Group 

Leader), Dr. A. Delahoy (Principal Investigator), R. Govindarajan, Dr. B. Sang, and R. 

Saramak. The Company wishes to thank all members for their sterling work. Dr. S. Guo 

and J. Cambridge participated in the project in the area of hollow cathode sputtering. 

Notable technical support was also provided by A. Foustotchenko, R. Zhang, R. Lyndall, 

and J. Allen (consultant), as well as by manufacturing, equipment development, facilities, 

and the machine shop. 

The following individuals (and institutions) are also acknowledged for their analyses, 

support, advice, or interest: J. Pankow, S. Asher, R. To, M. Contreras, K. Ramanathan, R. 

Noufi, H. Ullal, K. Zweibel (National Renewable Energy Laboratory); J. Sites (Colorado 

State University); W. Shafarman (Institute of Energy Conversion/University of 

Delaware); Neelkanth Dhere (University of Central Florida). 

 

Thanks are due to all members of the National CIS Team for their collaborations and 

discussions. 

Finally, EPV thanks the US DOE for support under subcontract ZXL-5-344205-05 with 

NREL. 

PUBLICATIONS (2005 - 2006) 

P1. A.E. Delahoy and L. Chen, “Advanced CIGS Photovoltaic Technology”, Final 

Technical Report, NREL/SR-520-38356, Aug. 2005. 

P2. B. Sang, L. Chen, M. Akhtar, R. Govindarajan, S. Guo, A. Delahoy, “Simplified 

Hybrid Process: Application to Normal, Submicron, and Light-trapping CIGS 

Devices”, DOE Solar Energy Technologies Program Review Meeting, Denver, Nov. 

2005. 

P3. S. Guo, W. Shafarman, A. Delahoy, “TiN and TiO2:Nb Thin Film Prepared Using 

Hollow Cathode Sputtering with Application to Solar Cells”, Proc. AVS 52nd 

International Symposium and Exhibition, Boston (2005) in press. 

P4. L. Chen, B. Sang, M. Akhtar, R. Govindarajan, S. Guo, J. Cambridge, A. Delahoy, 

M. Contreras, “Development of Large-Area, High Efficiency Thin Film CIGS 

Modules”, Technical Digest of the 15th PVSEC, Shanghai, China (2005), p. 85. (This 

paper received an award: Best Oral Presentation.) 

P5. A. Delahoy, S. Guo, “Transparent and Semi-Transparent Conducting Film Deposition 

by Reactive-Environment, Hollow Cathode Sputtering”, J. Vac. Sci Technol. A23, 

1215 (2005). 

 19
P6. A. Delahoy, L. Chen, B. Sang, S. Guo, J. Cambridge, R. Govindarajan, M. Akhtar 

“CIGS PV Modules: Progress Regarding the Hybrid Process and TCO Deposition” 

Proc. 20th European PVSEC, Barcelona, Spain (2005) p. 1843. 

P7. B. Sang, L. Chen, M. Akhtar, R. Govindarajan, A. Delahoy, J. Pankow, 

“Cu(InGa)Se2 Thin Films by Elemental Co-Evaporation/Sputtering and Large Area 

Module Results”, Proc. of the 31st IEEE PVSC, FL, 2005, p. 215. 

REFERENCES 

[1] U.S. DOE Solar Energy Technologies Program: Multi-Year Technical Plan 2003-

2007 and beyond. 

[2] M.A. Contreras, K. Ramanathan, J. Abushama, F. Hasoon, D.L. Young, B. Egaas, and 

R. Noufi, Prog. Photovoltaics 13 (2005) p. 209. 

[3] A.E. Delahoy, L. Chen, M. Akhtar, B. Sang, and S. Guo, Solar Energy 77 (2004) p. 

785. 

[4] C. Perkins and F. Hasoon, Proc. AVS 52nd International Symposium and Exhibition, 

Boston (2005) in press. 

[5] J. Raul Ortega-Borges and Daniel Lincot, J. Electrochem. Soc. 140, (1993) p. 3464. 

[6] A. Delahoy and S. Guo, J. Vac. Sci Technol. A23, (2005) p. 1215. 

[7] J. Malmström, O. Lundberg, and L. Stolt, Proc. 3rd WCPEC, Osaka, Japan (2003), p. 

344.

 20
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PLEASE DO NOT RETURN YOUR FORM TO THE ABOVE ORGANIZATION.

1. REPORT DATE (DD-MM-YYYY)

June 2006 

2. REPORT TYPE

Subcontract Report 

3. DATES COVERED (From - To)

15 March 2005–14 March 2006 

5a. CONTRACT NUMBER

DE-AC36-99-GO10337 

5b. GRANT NUMBER

4. TITLE AND SUBTITLE

Uniform, High Efficiency, Hybrid CIGS Process with Application to 

Novel Device Structures: Annual Technical Report, 15 March 2005 - 

14 March 2006 

5c. PROGRAM ELEMENT NUMBER

5d. PROJECT NUMBER

NREL/SR-520-40145 

5e. TASK NUMBER

PVB65101 

6. AUTHOR(S)

 A.E. Delahoy, L. Chen, and B. Sang 

5f. WORK UNIT NUMBER

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

 Energy Photovoltaics, Inc. 

P.O. Box 7456 

 Princeton, NJ 08543 

8. PERFORMING ORGANIZATION 

REPORT NUMBER

ZXL-5-44205-05 

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NREL 

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National Renewable Energy Laboratory 

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13. SUPPLEMENTARY NOTES

NREL Technical Monitor: Harin S. Ullal 

14. ABSTRACT (Maximum 200 Words)

One of the main Phase I objectives of this subcontract was for EPV to demonstrate 14%-efficient CIGS devices using 

a hybrid process. The processing was also required to have good controllability. These goals were successfully 

accomplished. It will be seen that during Phase I, we successfully developed a new, simplified hybrid process. A 

highlight of intensive work was the achievement of a 14.0% NREL-verified device at a CIGS thickness of 1.13 μm. 

The simplified hybrid process considerably reduces CIGS film formation time and offers the promise of being a truly 

cost-effective and manufacturable one. It is considered to be one of the more attractive CIGS processes in the 

industry. 

15. SUBJECT TERMS

PV; thin film; solar cells; manufacturer; modules; high efficiency; hybrid process; CIGS; controllability 

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