Subcontract Report

NREL/SR-520-48591

August 2010

New Metallization Technique 

Suitable for 6-MW Pilot Production 

of Efficient Multicrystalline Solar 

Cells Using Upgraded 

Metallurgical Silicon

Final Technical Progress Report

December 17, 2007 – June 16, 2009

K. Ounadjela and A. Blosse

CaliSolar, Inc.

Sunnyvale, California

A Project Funded under the U.S. DOE Solar Energy 

Technologies Program’s Photovoltaic Technology Incubator 
National Renewable Energy Laboratory

1617 Cole Boulevard, Golden, Colorado 80401-3393

303-275-3000 • www.nrel.gov

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

Office of Energy Efficiency and Renewable Energy

Operated by the Alliance for Sustainable Energy, LLC

Contract No. DE-AC36-08-GO28308

Subcontract Report

NREL/SR-520-48591

August 2010

New Metallization Technique 

Suitable for 6-MW Pilot Production 

of Efficient Multicrystalline Solar 

Cells Using Upgraded 

Metallurgical Silicon

Final Technical Progress Report

December 17, 2007 – June 16, 2009

K. Ounadjela and A. Blosse

CaliSolar, Inc.

Sunnyvale, California

A Project Funded under the U.S. DOE Solar Energy 

Technologies Program’s Photovoltaic Technology Incubator 

NREL Technical Monitor: Richard L. Mitchell

Prepared under Subcontract No. NAT-7-77015-04
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iii

Table of Contents

1. Summary................................................................................................................................... 1

2. Research and Development Activities.................................................................................... 2

2.1. Optimization of Laser-Grooved Front Contacts................................................................. 2

2.2. Electroplating Development ............................................................................................... 4

2.3. Surface Passivation............................................................................................................. 7

2.4. Solar Cells........................................................................................................................... 7

2.5. Mini-Modules ..................................................................................................................... 8

2.6. Commercial-Size Modules and Module Reliability ........................................................... 9

2.7. Pilot Production ................................................................................................................ 11

2.8. Building a 60-MW Manufacturing Line and Future Projections...................................... 12

3. Conclusions............................................................................................................................. 13
1

1. Summary

During the last 18 months, CaliSolar has enjoyed major success as a Photovoltaic Technology 

Incubator awardee within the U.S. Department of Energy’s (DOE’s) Solar Energy Technologies 

Program. Over this time span, the company evolved from a handful of employees to more than

100 scientists, engineers, technicians, and operators. On the technical side, the company 

transitioned itself from a proof-of-concept through pilot-scale to large-scale industrial production 

in just over two years. A fully automated 60-megawatt (MW) manufacturing line has been 

developed and commissioned in Sunnyvale, California. The facility is vertically integrated, 

converting upgraded metallurgical-grade (UMG) silicon (Si) feedstock to ingots, wafers, and 

high-efficiency, multicrystalline solar cells.

CaliSolar has pioneered the development and commercialization of high-efficiency Si solar cells 

based on 100% UMG Si feedstock. The technology we developed covers the whole value chain 

from Si feedstock to solar cells. We have developed an advanced casting process that results in 

UMG wafers with lifetimes exceeding 30 μs. Further, various parts of the cell were optimized to 

achieve conversion efficiencies competitive with those based on electronic-grade Si, but at 

significantly lower cost. During the period of this project, CaliSolar greatly improved its casting 

process, increased the size of its solar cells from 125 mm to 156 mm, optimized the front and 

back metal contacts, and improved the hydrogen passivation process. Collectively, these 

improvements have resulted in a champion cell efficiency of 16.73%, and the mean cell 

efficiency increased from 13.5% to 15.8% over a period of two years.

CaliSolar has partnered with major module manufacturers to test and qualify its cells. Modules 

of quality and reliability comparable to those based on electronic-grade Si cells were 

demonstrated. Additionally, various accelerated lifetime testing schemes showed little or no 

degradation in module efficiency. 
2

2. Research and Development Activities

The tasks carried out to reach the ultimate objectives of the program included work on the 

development of laser grooving and electroplating for front contacts, optimization of the surface 

passivation and back contact, the optimization of impurity gettering and bulk passivation, and the

development of a robust cell processing technology based on UMG Si. Much effort during 

Phase II was spent on the improvement of the cell and module efficiencies and the establishment 

of the manufacturing line. 

2.1. Optimization of Laser-Grooved Front Contacts

The first phase of this contract was mainly concerned with technology optimization. We focused 

our development activity primarily on laser grooving for front-side contact and on the 

electroplating processes for the front-side metallization. We were also able to demonstrate the 

maturity of our novel technology by establishing a baseline of 14% cell efficiency on 100% 

UMG Si wafers early in this contract.

During the first quarter, we validated the process flow to form laser-grooved front contacts on 

100% UMG Si multicrystalline wafers (125 mm x 125 mm). The laser used was a 1,064-nm 

Nd:YAG laser with a power of 100/20 W. During the experiments, different laser parameters 

such as speed, frequency, and current were varied to optimize the process and to create grooves 

with 30-μm width and 20-μm depth before KOH etching. A SEM cross section of a trench 

created by laser ablation is shown in Figure 1. We can clearly observe a lot of residues on the 

side of the trench either on top of the antireflection coating (ARC) layer on the flat surface of the

wafer, or within the trench.

These residues were subsequently removed in a KOH bath. Approximately 5 μm of the damaged 

Si region in the trench and all the residues on the top of the ARC were removed. A clean trench 

and the top ARC surface are shown in Figure 2. The optimization of the laser ablation process 

allowed us to develop contact trenches with 50 μm width after damage removal, hence meeting 

the requirements of the deliverable.
3

Figure 1. SEM image of a laser-

 grooved wafer 

The above deliverable was an intermediate milestone for our development work to demonstrate 

trenches with a width of <30 μm. The laser used for the latter was a UV laser at 354-nm 

wavelength. During the experiments, the laser parameters such as speed, frequency, and current 

were varied to optimize the process. The ultimate goal of this trial was to get trench width 

< 20 μm after laser etching, while keeping the width less than 30 μm after laser damage removal

(Figure 3). The main difficulty was to keep the width of the trench independent of the grain 

orientation in our polycrystalline wafers.

Figure 3. SEM plan view of a <20-μm-wide trench after laser damage removal

Figure 2. SEM image of lasergrooved wafer after damage removal
4

2.2. Electroplating Development

The goal of this task is to develop a metal deposition process using metal-plating techniques 

instead of the conventional screen-printing metallization. The main advantage of this technique 

is to form metallic lines at low temperatures with a high aspect ratio, good contact to the emitter, 

and low electrical resistance. The process temperature required is less than 500°C in order to 

retain the benefits of the gettering of the metallic impurities in the bulk UMG Si. The first step of 

the development was to demonstrate a metal-plating process capable of filling a 70-μm-wide 

trench with a first electrolessly deposited metal layer with low contact resistance to n+ type Si 

and good barrier properties to Cu. The latter is subsequently deposited with an electroplating

process. This metal-plating process was demonstrated on laser-grooved front side contacts with

an n+/n++ emitter with a resistivity of 100 and 10 Ω/, respectively (Figure 4).

Our initial efforts were to develop the capability of filling 70-μm-wide trenches, first with an 

electrolessly deposited metal layer with low contact resistance to n+ type Si and good barrier 

properties to copper, which is subsequently deposited with an electroplating process. After the 

optimization of the groove cleaning, prior to metal plating and the process conditions for metal 

plating, we demonstrated a continuous filling of ~30-μm-wide trenches with electroplated metal

(Fig. 5). This metal plating process resulted in laser-grooved front-side contacts with an n+/n++ 

emitter with a resistivity of 100 and 10 Ω/, respectively.

Figure 4a. Plan view of a 56-μmwide laser-grooved trench filled 

with electroplated copper

Figure 4b. Plan view of an 

interdigitated bus bar on the 

same wafer in Figure 4a
5

In the second phase of this project, we carried out the metallization development activities with 

some changes that were introduced to make the process more compatible with large-scale 

industrial production. This decision was driven by a careful study of the cost of producing the 

cells with this new metallization, and by risk mitigation to ramp up the factory using a wellstabilized process on a manufacturing proven tool. With this new approach, the new 

metallization procedure was based on front-side screen printing with optimized printing of 

narrower fingers. After co-firing and before laser edge isolation, the plating process takes place. 

Subsequently, the plating, the edge isolation, and the sorting are carried out. The goal is to 

improve the front contact by electrodepositing dense layers on printed fingers. Therefore, the 

improvement in cell performance is due to better series resistance. 

The plating is an electrochemical, potential bias-enhanced deposition process. First a Ni layer is 

plated on the screen-printed Ag fingers. The Ni layer is only 1.5 μm thick and the goal is to 

prevent the Cu from diffusing into the Si. After rinsing the wafer, the Cu layer is deposited on 

the Ni. The thickness of the Cu layer determines the conductivity of the fingers. In this work, ~ 

8-μm-thick layers were deposited. In the last step, performed after rising, a 3-μm-thick Sn layer 

is deposited. The Sn layer is needed for the soldering of the cells. An SEM cross section of a 

plated finger is shown in Figure 6.

Figure 5. SEM plan view of <50-μm-wide trenches 

in Si filled with an electroless metal-plating 

deposited film
6

Figure 6. SEM cross section of copper electroplated on a screen-printed finger

The plating process is sequentially illustrated in Figure 7 below.

After 2.5 minutes (1.5 μm) of Ni plating

After 6 minutes (8 μm) of Cu plating
7

After 2 minutes (3 μm) of Sn plating 

Figure 7. Optical micrographs illustrating the electroplating sequence

2.3. Surface Passivation

As in conventional Si solar cell technology, good surface passivation is a critical step for 

achieving good cell performance in UMG-based cells. Our process consists of hydrogenation of 

Si with microwave induced remote hydrogen plasma (MIRHP). The conditions for optimal 

hydrogenation were studied in relation to the sample conditions, such as the surface texturing 

and the active carrier concentration in the bulk. This hydrogenation process is key to obtaining

good efficiency when using a low thermal budget with electroplated metallization.

2.4. Solar Cells

Obtaining cell efficiencies on 100% UMG cells that are comparable to those obtained from 

electronic-grade cells, but with a significant cost advantage, was a key aspect of this work. At 

the beginning of this project our mean efficiency was around 13.5%. One of the most important 

milestones in this project was reaching cell efficiencies of 16% based on 100% UMG Si. This 

was reached in December 2008 with the demonstration of energy conversion efficiency greater 

than 16% on two 6-in. cells using 100% UMG Si feedstock. The conversion efficiency measured 

was 16.24% and 16.23%, respectively. These values were confirmed by NREL’s Cell 

Measurement Group. The process used to demonstrate these high efficiencies was basically the 

same as our baseline process, but with several process improvements enabling this major 

enhancement in cell efficiency. The main contributors were:

i. Optimization of the groove etching and cleaning process, enabling a more uniform 

growth of the electroplated film

ii. Improvement of the passivation of bulk defects by better control of the release of 

hydrogen from the ARC layers and during the last hydrogenation step

iii. Optimization of the Si casting process.

On January 9, 2009, eight additional cells with the electrical characteristics reported in Table 1 

below were sent to NREL for the main cell efficiency deliverable (deliverable D-14).
8

Table 1. Electrical Characteristics of the Cells Sent to NREL for Deliverable D-14

Nr Cell-ID Area Temp FF jsc Voc Eta

1 H102C_098 151.29 25.1 77.95 33.04 625.8 16.11

2 H102C_066 151.29 25 78.22 32.74 627.5 16.07

3 H102C_063 151.29 25 78.54 32.62 627.2 16.07

4 H102C_080 151.29 25.1 78.32 32.61 628.2 16.05

5 H102C_092 151.29 25 77.92 32.84 626.2 16.03

6 H102C_086 151.29 25 78.09 32.74 627.3 16.04

7 H102C_076 151.29 25 78.45 32.53 628 16.03

8 H102C_056 151.29 25 78.35 32.59 626.8 16.01

9 H102A_054 141.61 25 79.74 32.44 627.6 16.24

10 H102A_055 141.61 25 79.59 32.5 627.6 16.23

2.5. Mini-Modules

One of the peculiarities of solar cells built with 100% UMG Si is that they exhibit a significantly 

lower diode-breakdown voltage compared to cells processed with electronic-grade Si. This 

parameter is important in case some cells are shaded on a fully operational module. In a worstcase scenario, where just one cell is shaded whereas all other cells in the string are under full 

illumination, the shaded cell functions as a load that resists the total voltage of the string and the 

voltage of the bypass diode. One solution could be to reduce the length of the string, but at a 

higher cost for the module manufacturer. A very elegant alternative solution was proposed by 

Day4Energy, whose module concept will not be disclosed in this report due to confidentiality 

issues.

We decided to use Day4Energy for building the modules to be delivered to NREL. One of the 

main advantages of this assembly technique is that no busbar is required for the front side, hence

reducing the light shadowing. The cells’ characteristics used for this deliverable are reported in

Table 2. Compared to the cells of Table 1, processed with a front side with a busbar, we can 

observe, as expected, a significant increase in both the short-circuit current and the energy 

conversion efficiency with several cells at a 16.4% efficiency record for CaliSolar. The cells 

were sorted by Impp; the 12 best cells were assembled into two 6-cell mini-modules. 
9

Table 2. Cells Used in the Fabrication of Mini-Modules

2.6. Commercial-Size Modules and Module Reliability

Beginning in April 2008, CaliSolar started to make contact with different major module 

manufacturers interested in evaluating solar cells based on UMG Si. Our selection was based on 

the capability of the engineering department of the company we partnered with to support our 

request, in case some problem would appear because of the specificity of our material. 

To make our cells more compatible with mainstream module manufacturers, considerable efforts 

were put into investigating the main causes of lower breakdown voltage in UMG-based cells. 

Steady improvement in the quality of our feedstock and the casting conditions have yielded 

breakdown voltages >12 V, as evidenced in Figure 8.
10

Figure 8. Steady improvement in our UMG technology has yielded breakdown voltages >12 V.

Particular emphasis was placed on cell and module reliability. One important degradation 

mechanism that we investigated was light-induced degradation (LID). With a non-optimized 

UMG cell process, a marked degradation in short-circuit current and efficiency was observed 

(Figure 9). However, with the improvement of the quality of our UMG feedstocks and the 

casting conditions, CaliSolar’s UMG and electronic-grade Si devices can have equally low LID.

Figure 9. CaliSolar UMG and electronic-grade Si can have equally low light-induced degradation.
11

2.7. Pilot Production

In keeping with the PV Technology Incubator concept of starting with a prototype and ending 

with a pilot line, we invested considerable resources in setting up a 6-MW pilot line in 

Sunnyvale, California. This was accomplished by the end of Phase I of this project. Operating 

this pilot line proved to be critical in testing and further improving various steps in our 

fabrication process. Further, it was a necessary prerequisite to setting up our manufacturing line. 

The experience we gained in pilot operation proved invaluable in tool selection for the 

production line and also in the development of our cell efficiencies.

Cell efficiency improvement has been a main focus area throughout this project. Figure 10 shows 

the evolution of cell efficiency with time from the beginning of this project to date. Clearly, the 

optimization of the casting conditions and various cell parameters has resulted in a dramatic 

increase in cell efficiency. Currently, our champion cell has an efficiency of 16.73%. Some of 

these improvements were integral parts of this PV Incubator project. However, many other 

process developments were not directly related to this project, but nonetheless were well-aligned 

with the DOE Solar Program’s goals of reducing the cost of solar electricity. 

Figure 10. Cell efficiency evolution within the duration of this project
12

2.8. Building a 60-MW Manufacturing Line and Future Projections

CaliSolar was able to close the second round of funding to build a 60-MW commercial plant. 

Different locations were investigated worldwide. This selection process was quickly narrowed 

down to California and more precisely to the south of the Bay Area. Among more than 10 

potential industrial sites, a 130,000-ft2 site in Sunnyvale was selected at the end of October 2008,

and the lease was signed on December 17th. The design of the production floor layout started as 

soon as the site was selected. All facilities work was defined and the demolition work started on 

December 18. 

The tool selection process started as early as June of 2008. The methodology used for the tool 

selection was as scientific as it could be to identify the best equipment available on the market 

that was compatible with our technology. Purchase orders were issued between August and 

November for various pieces of equipment, such as casting furnaces, wet-benches, diffusion 

furnaces, and devices for antireflective coating and metallization. The most critical point during 

the negotiation with the vendors was the delivery time, as the backlog of orders for all vendors 

was varying between 18 weeks and 16 months! The strong interest of the PV community in 

UMG Si helped CaliSolar draw the attention of the tool suppliers and shorten the delivery time.

In addition to tool selection and purchasing, massive site preparation, and equipment installation, 

CaliSolar experienced a rapid increase in staffing. Currently, it employs nearly 100 engineers 

and technicians. At the time of writing this report, both production lines are up and running with 

various test runs being completed. It is expected that within three months the plant will be 

running at full one-shift capacity (Figure 11).

Figure 11. CaliSolar’s 130,000-ft2 site housing the 60-MW plant. Photo credit: CaliSolar
13

3. Conclusions

Most of the goals of this PV Incubator project were reached. CaliSolar was successful in 

transferring its novel UMG Si-based cell technology from the concept and feasibility phase, 

through a batch-pilot phase, and into large-scale manufacturing. The company drove technology 

in three areas: crystallization, wafer treatment, and cell processing. The cell size was increased 

from 125 mm to the industry standard of 156 mm, while the mean cell efficiency increased from 

13.5% at the beginning of this project to 15.8% in pilot production in 2009 with peak efficiency 

at 16.73%. The company transitioned from a handful of employees in 2007 to nearly 100 

engineers and technicians at the present time. 

At the time of writing this report, the company is operating a 60-MW, full manufacturing line 

complete with ingot casting, wafering, and advanced cell processing in a 130,000-ft2 facility in 

Sunnyvale, California.
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1. REPORT DATE (DD-MM-YYYY)

August 2010

2. REPORT TYPE

Subcontractor Final Report

3. DATES COVERED (From - To)

12/17/2007 – 6/16/2009

4. TITLE AND SUBTITLE

New Metallization Technique Suitable for 6-MW Pilot Production of 

Efficient Multicrystalline Solar Cells Using Upgraded Metallurgical 

Silicon: Final Technical Progress Report, December 17, 2007 – 

June 16, 2009

5a. CONTRACT NUMBER

DE-AC36-08-GO28308

5b. GRANT NUMBER

5c. PROGRAM ELEMENT NUMBER

6. AUTHOR(S)

Dr. Kamel Ounadjela

Dr. Alain Blosse

5d. PROJECT NUMBER

NREL/SR-520-48591

5e. TASK NUMBER

PV10.2010

5f. WORK UNIT NUMBER

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

CaliSolar Inc.

985 Almanor Avenue

Sunnyvale, CA 94085

8. PERFORMING ORGANIZATION

REPORT NUMBER

NAT-7-77015-04

9. SPONSORING/MONITORING AGENCY NAME(S) AND ADDRESS(ES)

National Renewable Energy Laboratory

1617 Cole Blvd.

Golden, CO 80401-3393

10. SPONSOR/MONITOR'S ACRONYM(S)

NREL

11. SPONSORING/MONITORING 

AGENCY REPORT NUMBER

NREL/SR-520-48591 

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National Technical Information Service

U.S. Department of Commerce

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13. SUPPLEMENTARY NOTES

NREL Technical Monitor: Richard L. Mitchell

14. ABSTRACT (Maximum 200 Words)

This report describes CaliSolar’s work as a Photovoltaic Technology Incubator awardee within the U.S. Department 

of Energy’s Solar Energy Technologies Program. The term of this subcontract with the National Renewable Energy 

Laboratory was two years. During this time, CaliSolar evolved from a handful of employees to over 100 scientists, 

engineers, technicians, and operators. On the technical side, the company transitioned from a proof-of-concept 

through pilot-scale to large-scale industrial production. A fully automated 60-megawatt manufacturing line was 

commissioned in Sunnyvale, California. The facility converts upgraded metallurgical-grade silicon feedstock to ingots, 

wafers, and high-efficiency multicrystalline solar cells.

15. SUBJECT TERMS

photovoltaics; PV; solar cells; upgraded metallurgical-grade silicon; UMG; industry; manufacturing

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