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

Technology Development for 

High-Efficiency Solar Cells and 

Modules Using Thin (<80 μm) 

Single-Crystal Silicon Wafers 

Produced by Epitaxy

June 11, 2011 — April 30, 2013

T.S. Ravi

Crystal Solar, Inc.

Santa Clara, California

NREL Technical Monitor: Harin Ullal

Subcontract Report

NREL/SR-5200-58593

May 2013
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

15013 Denver West Parkway

Golden, Colorado 80401

303-275-3000 • www.nrel.gov

Contract No. DE-AC36-08GO28308

Technology Development for 

High-Efficiency Solar Cells and 

Modules Using Thin (<80 μm) 

Single-Crystal Silicon Wafers 

Produced by Epitaxy 

June 11, 2011 — April 30, 2013

T.S. Ravi

Crystal Solar, Inc.

Santa Clara, California

NREL Technical Monitor: Harin Ullal

Prepared under Subcontract No. NEU-31-40054-01

Subcontract Report

NREL/SR-5200-58593

May 2013
This publication was reproduced from the best available copy

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

NOTICE

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iv

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Executive Summary

The objective of this 18-month program was to demonstrate the viability of high-efficiency thin 

(less than 80 μm) monocrystalline silicon (Si) solar cells and modules with a low-cost epitaxial 

growth process. 

The program was in two phases with goals that were progressively designed to show the 

commercial viability of the Crystal Solar approach. Phase I focused on showing the basic 

feasibility of in-house-developed equipment for the porous Si creation and the epitaxial growth, 

and on demonstrating full-size cells that exceeded 15% in efficiency. 

Phase II was focused on further improvement of the equipment productivity and the cell 

efficiency and demonstrating the pathway to industrialization of this process. Crystal Solar 

demonstrated a batch of 24 wafers and a growth rate of >4 microns/minute using the production 

version of our epitaxial reactor. Furthermore, working with Prof. Rohatgi of Georgia Tech, we 

were able to demonstrate close to 17% efficiency on cells made with this epitaxial reactor. One 

deliverable was to demonstrate substrate reuse >50 times, followed by making epitaxial cells 

from successive peels. In the past, Crystal Solar has demonstrated that substrates can be reused 

>50 times without loss in the epitaxial quality as evidenced by the lifetime. However, we were 

not able to demonstrate five subsequent cells before the program’s end due to constraints in the 

cycle times involved with substrate reclaim within the current operating capacity at Crystal Solar 

and with our processing partners. 

Another important objective of the program was to show a plan for a 100-megawatt (MW) 

factory using Crystal Solar’s gas-to-module approach using the best cost numbers as measured 

using our in-house epitaxial reactor and anodic etcher as well as cost estimates from various 

equipment and chemical and materials vendors. Crystal Solar demonstrated that a cost of 

<$0.50/watt is feasible for a 100-MW line with a capital cost of ~$0.70/watt. 

In summary, while one of the deliverables was not met due to the timing of the program, we 

believe that all of the factors required to enable industrialization of the epitaxial approach were 

demonstrated. 

All images in this report are property of Crystal Solar, Inc.
v 

This report is available at no cost from the National Renewable Energy Laboratory at www.nrel.gov/publications. 

Table of Contents

Executive Summary................................................................................................................................... iv

Introduction ................................................................................................................................................. 1

Phase I Deliverables Summary.................................................................................................................. 2

Phase II Deliverables Summary................................................................................................................. 5

Tasks 1 and 7: Porous Si Process and Tool Development..................................................................... 7

Tasks 2, 5, and 8: Epitaxial Reactor Hardware, Process Development, and Throughput................... 9

Tasks 3 and 5: Thin Si Solar Cell Fabrication ........................................................................................ 13

Tasks 4 and 11: Module Fabrication and Testing.................................................................................. 16

Task 10: Scale Up Simulation .................................................................................................................. 20

Tasks 6 and 12: Manufacturing Flow, Capex Analysis, Cost Modeling, and Factory Planning........ 21

Thin-Epitaxial-Wafer-Based Heterojunction Cell Development at NREL (NREL PI: Dr. Qi Wang).... 23

Summary.................................................................................................................................................... 25

Future Work ............................................................................................................................................... 25

Acknowledgements .................................................................................................................................. 25
1 

This report is available at no cost from the National Renewable Energy Laboratory at www.nrel.gov/publications. 

Introduction

Crystalline Si has continued to lead the market in the worldwide adoption of solar energy with 

over 85% market share. Much of this growth has happened in the last few years since the 

photovoltaic (PV) modules have dropped in price to below $1/watt due to massive vertical 

integration driven largely by Chinese manufacturers. Although this growth has been impressive, 

the cost of making these panels has not scaled down, thus resulting in significantly reduced gross 

margins and in many cases a net loss for the manufacturers. It is also clear that even with 

massive vertical integration with Si cell and module manufacturing, within one company the cost 

of making a panel has only leveled off to about $0.60/watt, well short of the <$0.50/watt 

numbers suggested by the U.S. Department of Energy (DOE) for true grid parity. What is needed 

is a completely different approach to manufacturing the Si, which still constitutes the single 

largest cost factor in the solar module.

Although kerfless approaches like implant-based cleaving and epitaxial thin Si on porous Si have 

been suggested in the past as ways to reduce the consumption and cost of the Si in the panel, to 

date there exists no production-proven way to use these approaches cost-effectively to make a 

kerfless module. Furthermore, since many of these approaches also use thin Si (less than 70 

microns thick), a further barrier exists in terms of handling the thin Si all the way through cell 

creation and module formation. 

Crystal Solar has addressed both these issues, first by developing the world’s first productionworthy high-throughput solar epitaxial reactor and using this reactor to demonstrate thin cells 

and modules with high yield. The following pages document the results generated during the 18-

month NREL SunShot program, starting with the porous Si creation and all the way to module 

formation. A significant part of the work had to do with showing the true cost of the process and 

planning of the commercialization of this technology. 

We have demonstrated not only the manufacturing viability of the epi-based kerfless process 

using in-house-developed epitaxial reactors, but we have also shown that this approach can 

indeed bring the overall PV module costs down to <0.50/watt. 
2 

This report is available at no cost from the National Renewable Energy Laboratory at www.nrel.gov/publications. 

Phase I Deliverables Summary

Phase I

Deliverable Criteria and Deliverable Date Crystal Solar Notes

D1 3.1.2.1 – D1: NREL 

monitors Crystal Solar’s 

three epitaxial Si 

depositions and the 

observed samples will be 

shipped to NREL to verify 

that these three <80-μm 

epi-Si wafers were grown 

at a rate of >3 μm/minute 

with uniformity better than 

80%. 

D1: M2 Completed successfully: Crystal Solar 

demonstrated average growth rate of 

3.1 microns/min in their prototype 

epitaxial reactor. 

D2 3.1.2.3 – D2: Four thin 

(<80 μm) free-standing 

gettered epitaxial Si 

wafers (~100 cm2

) will be 

submitted to NREL to 

verify that they exhibit 

resistivity ~1 Ohm-cm and 

minority carrier lifetimes 

>10 μ-sec. In addition, 

four <80-μm-thick 2-cm x 

2-cm wafers will be 

submitted to NREL to 

verify BSRV of <1000 

cm/s.

D2: M4 Completed successfully: NREL 

measured >10 microseconds average 

on the shipped thin epitaxial wafers. 

D3 3.1.3.1 – D3: Deliver 

three 125 mm x 125 mm 

thin (<80 μm) epitaxial Si 

solar cells, bonded to 

glass, with 15% efficiency 

to NREL for I-V and DIV 

characterization and 

spectral response

measurements.

D3: M6 Completed successfully although 

delayed by 3 months: NREL measured 

>15.2% efficiency on each of the wafers 

submitted for this purpose. 
3 

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D4

Reclaim 

Demonstration

3.1.4.3 – D4: Deliver five 

125 mm x 125 mm (<80

μm) epitaxial Si cells with 

efficiency >15% with all 

cells fabricated from and 

lifted off of a single Si 

substrate. 

D4: M8 Completed successfully.

D5

Module 

Fabrication

3.1.4.5 – D5: Deliver four 

mini-modules (260 mm X 

260 mm in size) to NREL 

composed of four (125

mm X 125 mm) thin (<80

μm) epitaxial Si cells for IV characterization. The 

mini modules must exhibit 

a module efficiency of 

13%.

D5: M8 Completed with average efficiency 

>13%. 

D6

Reclaim 

Demonstration

3.1.4.6 – D6: Deliver 20 

successfully released thin 

(<80 μm) epitaxial Si 

samples (>1 cm2 in area) 

substrates from a single 

Si substrate that exhibit 

<15% degradation in 

lifetime (from 10 μ-sec) 

over twenty reuses. 

D6: M9 Completed successfully: NREL 

measured minority carrier lifetimes for 

20 epitaxial Si substrates.

D7

Epitaxial Tool 

Throughput 

Demonstration

3.1.5 – D7: Deliver a 

report that documents 

production operation of 

two epitaxial chambers 

and associated cool-down 

chamber and susceptor 

handling robotics. 

Demonstrate a perchamber tool throughput 

of 25 wafers per hour (3 

μm/min dep rate; 80-μmthick epi) and document 

throughput, yield, and 

TAKT time. 

D7: M9 Completed successfully: NREL 

observed tool productivity and 

measured thickness uniformity. 
4 

This report is available at no cost from the National Renewable Energy Laboratory at www.nrel.gov/publications. 

D8

Manufacturing 

Flow and 

Capex 

Analysis

3.1.6 – D8: Deliver a 

report that documents a 

detailed cost model of the 

manufacturing process 

flow that includes tool 

throughput, yield, 

consumable costs, 

optimal number of Si 

wafer reuses, optimal cell 

and module efficiency, 

and demonstrate factory 

capex costs of <$1/Wp for 

100 MW/year capacity. 

D8: M9 Completed successfully: A factory 

report was generated using 

commercially available cell and module 

equipment. 
5 

This report is available at no cost from the National Renewable Energy Laboratory at www.nrel.gov/publications. 

Phase II Deliverables Summary

D9

Epitaxial 

Reactor and 

Process 

Development

3.2.2.1 – D9: Report 

documenting three epitaxial Si 

depositions of 24 wafers/batch 

using the Rev 3.0 tool. 24 

samples from a single batch 

will be shipped to NREL and 

random sampling will be used 

to verify thickness uniformity of 

>90% based on FTIR mapping.

The report shall also include inhouse FTIR results for all three 

epi-Si depositions that confirm 

thickness uniformity of >90% 

and a deposition rate of >4

μm/min.

D9: M10 Completed successfully: NREL 

observed epitaxial equipment and 

deposition rates and with FTIR 

mapping verified thickness 

nonuniformity of <10%.

D10

Solar Cell 

Fabrication

3.2.3.1 – D10: Deliver three 

125 mm x 125 mm thin (<80

μm) epitaxial Si solar cells with 

17% efficiency to NREL for I-V 

and DIV characterization and 

spectral response 

measurements and report 

documentation for this task.

D10: M14 Completed successfully: Verified 

>16.7% at NREL (which is within the 

3% error margin). 

D11

Cell

Efficiency 

with Reclaim

3.2.4 – D11: Deliver to NREL 

five thin (<80 μm) epitaxial Si 

solar cells (125 mm x 125 mm) 

fabricated with epi-Si wafers 

grown on the same Si 

substrate after 50 reuses. The 

five deliverable cells will be 

fabricated from reuses 51-55 

(from a single Si substrate) 

with three of the five cells 

exhibiting at least 16% 

efficiency.

D11 M19 Did not complete; although we were 

able to demonstrate 50 reuses of the 

substrate, we couldn’t make the cells 

in time before the program ended. 
6 

This report is available at no cost from the National Renewable Energy Laboratory at www.nrel.gov/publications. 

D12

Module 

Fabrication

3.2.5.4 – D12: Deliver to NREL 

four mini-modules (260 mm x 

260 mm in size) composed of 

four thin (<80 μm) epitaxial Si 

cells for I-V characterization 

and reliability tests. The minimodules must exhibit a module 

efficiency of 14%.

D12: M8 Completed successfully: NREL 

measured 14% efficiency on Crystal 

Solar sub modules. 

D13

Module 

Testing

3.2.5.5 – D13: Deliver modules 

for testing. NREL shall conduct 

1,000 hours of damp heat 

testing on the modules in 

accordance with IEC 61215.

Two of the modules must 

exhibit less than a 10% drop in 

maximum power (Pmax) 

(compared to initial LIV results) 

after 500 hours of damp heat 

testing (85C/85RH). 

D13: M6 Partially successful: one of the 

modules passed with 10% 

degradation after 500 hours of damp 

heat testing.

D14

Cost 

Modeling and 

Factory 

Planning

3.2.6 – D14: Provide a D14 

deliverable report that 

documents the development of 

a detailed cost model with 

individual demonstrated 

parameters (e.g., epi 

deposition rate, yield, 

consumables costs, optimal 

number of Si wafer reuses, 

capex, cell and module 

efficiency, TAKT time) coupled 

with capex model developed in 

Phase 1 to demonstrate 

<$1/Wp manufacturing cost 

when scaled to volume. A 

report will be provided in the

D14 deliverable report.

D14: M18 Completed successfully: A 

comprehensive cost analysis and 

factory modeling was done for Crystal 

Solar’s “gas to module” process. The 

results were discussed in terms of 

total capital expenditures, as well as 

all-in module costs. A baseline 

scenario demonstrated a production 

capital cost of $77M (including onetime expenditures related to gas 

handling and facilitization), thereby 

demonstrating significantly less than 

$1/W (as originally targeted in the 

original proposal to DOE by Crystal 

Solar). A total cost of $0.45/W was 

also modeled. Both of the above 

metrics allow Crystal Solar to achieve 

DOE module cost targets of $0.50/W.
7 

This report is available at no cost from the National Renewable Energy Laboratory at www.nrel.gov/publications. 

Tasks 1 and 7: Porous Si Process and Tool 

Development 

The objective was to further develop anodic etching technology for the fabrication of porous Si 

layers on large-area single-crystal Si substrates for subsequent epitaxial deposition. 

Figure 1 shows a cross-section of a representative porous Si layer. The key challenge here was to 

get a uniform thickness. We achieved this by carefully engineering the field around the substrate 

wafer in an anodic etch cell. The thickness uniformity from the center to the edge as measured by 

SEM cross-section is shown in Figure 2. 

Figure 1. Representative SEM cross-section of anodically etched wafers. 

Figure 2. Thickness variation from center to edge showing an average nonuniformity of 13.7%. 

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

0123456789

Porous layer thickness -

μ

m

Distance across wafer - 125 mm

Average thickness nonuniformity - 13.7%
8 

This report is available at no cost from the National Renewable Energy Laboratory at www.nrel.gov/publications. 

 

Figure 3. Production anodic etch reactor and 10-wafer batch loading system. 

By the end of Phase II we had developed a multi-wafer batch anodic etcher. Figure 3 shows the 

system in the Crystal Solar lab along with a 10-wafer vacuum chuck for holding the substrates.

We have successfully tested this, and Figure 4 shows the visual appearance uniformity within a 

batch of 10 wafers. Having a batch of at least 10 wafers enables throughputs of greater than 200 

wph, thus qualifying for our production use. 

Figure 4. Visual appearance of anodic etched wafers in a batch showing uniformity from wafer to 

wafer.

+ Front Mid Back -
9 

This report is available at no cost from the National Renewable Energy Laboratory at www.nrel.gov/publications. 

Tasks 2, 5, and 8: Epitaxial Reactor Hardware, Process 

Development, and Throughput 

Crystal Solar was founded on the basis of developing a high-throughput epitaxial reactor for 

solar applications. Our approach is based on high-temperature CVD of trichlorosilane to Si in a 

mini batch reactor. Figure 5 shows our prototype reactor. It is a manually loaded reactor with an 

eight-wafer batch. During Phase I we had a target growth rate of 3 microns/minute and a target 

nonunformity of 20%. We exceeded these requirements by a wide margin. Figure 6 shows the 

average growth rate and thickness nonuniformity from this prototype reactor. 

Figure 5. Crystal Solar experimental prototype epitaxial reactor. 

Figure 6. Growth rate for three consequent runs in Crystal Solar prototype epi reactor with <10% 

nonuniformity. 
10 

This report is available at no cost from the National Renewable Energy Laboratory at www.nrel.gov/publications. 

In Phase II, we not only had to demonstrate >4 microns/min deposition rate but also a thickness 

nonuniformity of better than 10%. In addition we had to show that the epi wafer was of high 

quality. 

Figure 7 shows the Crystal Solar pilot epi reactor with automatic loading and unloading of the 

substrates.

Figure 7. Crystal Solar production epi reactor with automatic loading and unloading. 

We were able to exceed both the growth rate and uniformity targets. Using the 24-wafer batch 

we were able to demonstrate a throughput of >30 wph/chamber, exceeding the target set by the 

program. 

Figures 8 and 9 show the thickness and the growth rate of a 24-wafer batch.
11 

This report is available at no cost from the National Renewable Energy Laboratory at www.nrel.gov/publications. 

Figures 8 and 9. Thickness and growth rate variations of Crystal Solar’s 24-wafer batch epi 

reactor. 

Epi Quality: The production system has shown consistently good-quality epi for P-type and Ntype wafers. Figure 10 shows the average lifetime of P-type and N-type wafers during several 

epitaxial runs, showing the consistent quality of our epitaxial films.

0

20

40

60

80

100

120

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24

 24-wafer batch epitaxy (69 μm Avg) 

Non-Unif = 5.3 %, 1 Sigma

0.00

1.00

2.00

3.00

4.00

5.00

6.00

7.00

8.00

9.00

10.00

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24

 24-wafer batch epitaxy growth rate 

Average 4.8 μm/min
12 

This report is available at no cost from the National Renewable Energy Laboratory at www.nrel.gov/publications. 

Figure 10. Epi quality from run to run with P-type (top chart) and N-type (bottom chart) epi. 

Epi Reactor Throughput: With the production reactor we were able to verify the target 

throughput. Figure 11 shows the the timing diagram of our epi reactor with a 24-wafer batch. 

This was done in the presence of the NREL program monitor. The total cycle time for a batch of 

24 wafers is about 55 minutes, which trnaslates to a throughput of >300 wph. 

Figure 11. Timing diagram for a 24-wafer epi batch, total cycle time <55 m (verified by NREL). 
13 

This report is available at no cost from the National Renewable Energy Laboratory at www.nrel.gov/publications. 

Tasks 3 and 5: Thin Si Solar Cell Fabrication 

Crystal Solar has developed a simple screen-printed Ag process for thin Si solar cell fabrication 

in collaboration with Prof. Rohatgi’s group at Georgia Tech. By optimizing the various steps of 

texture etching, diffusion, nitride antireflective coating, and screen printing, we were able to 

demonstrate with the prototype epi reactor efficiencies under glass of around 15.5%. One of the 

limitations was that the prototype epi reactor produced lower lifetime due to lack of automation 

in loading and unloading samples, which resulted in contamination of the reactor. 

During Phase II of the program we were able to increase the wafer size to 125 mm x 125 mm and 

along with significantly better material (see previous section) from the production epi reactor 

(with automatic loading) we were able to demonstrate the efficiency close to 17%, the original 

target. Figure 12 shows an NREL-measured I-V curve for a 16.7% thin epi cell measured under 

glass. The cell itself is shown in Figure 13 with the measurement tabs sticking out. We believe 

that taking into account transmission losses (of around 9%) this is equivalent to ~18.3% absolute 

cell efficiency. 

Figure 12. Epi thin Si cell measured under glass. 
14 

This report is available at no cost from the National Renewable Energy Laboratory at www.nrel.gov/publications. 

Figure 13. Epi thin silicon cell attached to glass.

The IQE analysis of the cell is shown in Figure 14. Based on this, it is expected that further 

improvements can be made with better emitter passivation, backside passivation, and light 

trapping. The backside passivation is also critical for high Voc and different passivation materials 

are currently being investigated at Crystal Solar.

Figure 14. IQE analysis of the cell shown in Figure 13. 

0

20

40

60

80

100

300 400 500 600 700 800 900 1000 1100 1200

Internal Quantum Efficiency IQE (%)

Wavelength λ (nm)

IQE-R8945

Reflectance-R8945
15 

This report is available at no cost from the National Renewable Energy Laboratory at www.nrel.gov/publications. 

Cell efficiency as a function of bulk lifetime was further modeled using PC-1D to demonstrate 

the pathway to achieve efficiency >19% for a 50-μm-thick Si solar cell. Crystal Solar has 

consistently demonstrated high lifetime, as shown in Figure 10. According to the modeling 

results, the lifetime achieved at Crystal Solar is no longer the limiting factor of the efficiency. 

Further increases in efficiency will depend on improvement of the front surface passivation, back 

surface passivation, light trapping, and tabbing techniques, as indicated in Figure 15. 

Figure 15. PC-1D modeling: efficiency as a function of bulk lifetime. 
16 

This report is available at no cost from the National Renewable Energy Laboratory at www.nrel.gov/publications. 

Tasks 4 and 11: Module Fabrication and Testing 

Crystal Solar has made multiple 2 x 2 and larger-size modules using 125 mm x 125 mm cells. 

Our module-making process involves transferring our thin epi to glass using the encapsulant and 

then metallizing the back side of the solar cells to make contacts. Figure 16 shows the NREL I-V 

test for one of the thin Si modules showing a 14% module efficiency. Figure 17 shows a 24-cell 

(105 mm x 105 mm) module with a 35W output and an aperture area efficiency of ~ 14.4%. 

Figure 16. I-V curve of a 2 x 2 module made showing 14% efficiency. 
17 

This report is available at no cost from the National Renewable Energy Laboratory at www.nrel.gov/publications. 

Figure 17. A larger module (24 wafers, 38W) using Crystal Solar epi thin Si cells. 

A Crystal Solar Technology Concept Module (>15% efficiency) was displayed at the Solar 

Power International Conference and at the EUPVSEC in 2012. Figure 18 shows the module. This 

is the first example of commercial-scale modules fabricated with our epitaxial Si technology. 

Figure 18. Epi Thin-Silicon concept module. 
18 

This report is available at no cost from the National Renewable Energy Laboratory at www.nrel.gov/publications. 

Module testing: We have obtained preliminary data from NREL on the HAST testing and were 

able to show that we can pass the 500-hour damp heat test with a maximum of 10% degradation 

in power. Figure 19 shows the dual-cell module that was tested at NREL, and Figure 20 shows 

the before and after I-V curves. As can be seen the degradation is ~10%. We are continuing these 

tests internally with larger modules. 

Figure 19. Two-cell submodule tested at NREL. 
19 

This report is available at no cost from the National Renewable Energy Laboratory at www.nrel.gov/publications. 

Figure 20. I-V curves before and after 500 hours of damp heat test at NREL showing ~10% 

degradation in power. 

 
20 

This report is available at no cost from the National Renewable Energy Laboratory at www.nrel.gov/publications. 

Task 10: Scale Up Simulation 

Substrate reclaim is key to achieving low cost. Our cost models indicate that a minimum of 50 

reuses are needed to achieve cost goals with the Epi Thin-Silicon lift-off approach. Crystal Solar 

has developed a simple reclaim process that involves using low-cost chemicals to reclaim the 

surface. Using this approach we have shown over 50 reclaims with a baseline lifetime of >10 

microseconds using the prototype epitaxial reactor. 

Figure 21 shows the deviation from the baseline lifetime based on this approach showing no 

trend in lifetime after 50 reuses. 

Figure 21. Lifetime as a function of number of reuses; baseline lifetime is 10 μs. 

We had launched a similar reclaim experiment using the production epi reactor with the goal that 

after 50 reclaims five cells would be made in succession. Unfortunately, by the end of the 

program we couldn’t complete this milestone. We had however shown some data in Phase I that 

showed no degradation (or correlation) in the efficiency numbers with subsequent reclaims. This 

is shown in Figure 22. 

Figure 22. Data on efficiency versus substrate reuse. 

Substrate use Voc-mV Isc-mA/cm2 FF-% Efficiency%

New wafer (Use 1) 605 31.0 73.7 13.8

Use 2 605 31.6 72.8 13.9

Use 3 621 34.1 75.9 16.1

% of Baseline Lifetime
21 

This report is available at no cost from the National Renewable Energy Laboratory at www.nrel.gov/publications. 

Tasks 6 and 12: Manufacturing Flow, Capex Analysis, 

Cost Modeling, and Factory Planning

We developed an integrated process flow for manufacturing thin Si modules starting with 

trichlorosilane gas. The major benefit of this approach comes with the avoidance of expensive 

poly-Si, ingoting, and wafering of Si, and it enables vertical integration on a much smaller scale. 

Crystal Solar’s high-throughput epi reactor has enabled this. 

The factory layout is quite flexible and can be adapted to fit into an existing structure. For this 

design, a circular flow was utilized. The cell-fab area occupies the largest percentage of the 

factory, and the flow of this section generally proceeds from left to right down the length of the 

building. The wafer-fab area is located below on the left side of the cell-fab, and the module-fab 

area is below on the right side. 

The footprint of the 100-MW integrated plant is compact and highly suitable for modular 

expansion. The total fab area is 170 x 55 m2

, which is approximately 100,000 ft2. Allowing for 

additional non-fab space for logistics, gas/chemical handling, and storage, the entire facility can 

be housed in a space of ~250,000 ft2. Figure 23 shows the layout of a 100-MW factory that was 

developed as part of this deliverable. 

Figure 23. Layout of 100-MW fab for epi reactors with porous Si. 

Note that only eight epi reactors are needed for a 100-MW line.

 
22 

This report is available at no cost from the National Renewable Energy Laboratory at www.nrel.gov/publications. 

Equip 

Capex

($)

Wafer $40,929,536

Cell $24,619,265

Module $4,751,400

Sum $70,300,201

All-in Module Cost Summary

Depreciation Consumables Labor Rent All-in-cost

($/W) ($/W) ($/W) ($/W) ($/W)

Wafer $ 0.044 $ 0.105 $ 0.006 $ 0.004 $ 0.159 

Cell $ 0.028 $ 0.113 $ 0.007 $ 0.007 $ 0.155 

Module $ 0.006 $ 0.118 $ 0.005 $ 0.004 $ 0.133 

Sum $ 0.077 $ 0.337 $ 0.018 $ 0.015 $ 0.446 

Figure 24. Capex and cost/watt for a 100-MW, 50-micron-thick epi integrated module line. 

Figure 24 shows the capex and the total cost for a 100-MW line based on the detailed analysis of 

Crystal Solar’s process flow and all the equipment necessary for cell and module manufacturing.

The total cost of the module is <$0.45/watt with a capex of $0.77/watt. 

In comparison, a typical Crystalline Si line including the Poly has a capex of $1.50/watt with the 

best cost numbers for the module around $0.60/watt. This shows the potential of the epitaxial 

kerfless approach.
23 

This report is available at no cost from the National Renewable Energy Laboratory at www.nrel.gov/publications. 

Thin-Epitaxial-Wafer-Based Heterojunction Cell 

Development at NREL (NREL PI: Dr. Qi Wang) 

This work was done jointly with Dr. Qi Wang and is based on work done by Dr. Wang on P-type 

HIT cells. For standard HIT cell fabrication, the thin epi wafer was extracted from the substrate 

and processed as a thin wafer. While this proved difficult, Dr. Wang was able to make a few 

cells, and Figure 25 shows the best results to date. 

Figure 25. HIT cells fabricated by Dr. Qi Wang (NREL) on Crystal Solar 50-micron epi. 

As can be seen, while the Voc and FF are reasonable, the Jsc is quite low, and we attribute that to 

the poor lifetime of the epi (this was from our prototype epi reactor). In addition, these were freestanding samples, and as such, the yields were quite low and the actual area of the samples was 

less than 10 cm2. 

In the second phase of this program, to be able to handle large-area cells, Crystal Solar 

developed an approach where the front side of the epi wafer is processed like a typical HIT cell 

and the back side processed using Crystal Solar’s baseline approach of P++ Epitaxial BSF and 

backside aluminum deposition for metal contacts with the thin cell attached to a glass handle. 

As shown schematically in Figure 26, this structure represents a hybrid heterojunctionhomojunction device (patented by Crystal Solar). 

Figure 26. Concept for a heterojunction-homojunction hybrid cell. 

The HIT part of the cell was completed at NREL under Dr. Qi Wang’s supervision whereas the 

back side was completed at Crystal Solar. 
24 

This report is available at no cost from the National Renewable Energy Laboratory at www.nrel.gov/publications. 

Results and Prospects: Figure 27 shows the best effort to date on this hybrid cell. The results 

look quite promising, and although we couldn’t reach the very high efficiencies we achieved 

with the homojunction cell, we believe that the basic process flow works. 

In analyzing this we concluded that the main issue was a lack of effective passivation of the front 

side, followed by a lower fill factor due to front contact issues, and in general, the low lifetime of 

the epi that contributed to the low current. 

Figure 27. I-V curve of the first heterojunction/homojunction thin epi cell. 

We believe that this is an attractive approach since it eliminates all of the high-temperature steps 

that can potentially affect the yield of the thin Si processing. We therefore plan to continue this 

work internally at Crystal Solar. 

 
25 

This report is available at no cost from the National Renewable Energy Laboratory at www.nrel.gov/publications. 

Summary

Crystal Solar’s direct epi-wafer technology has the ability to radically change the Si PV 

landscape by allowing smaller-scale wafer (or integrated wafer+cell+module) plants that can be 

operated with sustained technical and cost advantages—rather than the huge scale and 

monolithic integration required by conventional Si PV.

We have not only demonstrated a high-throughput, low-COO epitaxial reactor but also highefficiency cells and modules with less than 80-micron-thick Si. In addition, we have shown a 

path to commercialization with a projected cost less than $0.50/watt by using standard cell and 

module equipment for the processing of the thin Si cells and modules.

Future Work

Based on the work done within this program we expect over the next 12 months to focus on the 

following towards a commercialization path for epi-based thin Si cells and modules.

1. Demonstrate >19% cell efficiency using further innovations in the cell-making process 

2. Build several KWs of large-area thin Si modules to test for various standards 

certifications

3. Build a pilot factory to demonstrate 25 MW for gas-to-module with an industrial partner 

4. Further develop advanced cell concepts such as HIT, IBC, and PERC cells with thin 

epitaxial Si. 

Acknowledgements

We thank DOE and NREL for supporting this project through the DOE SunShot Initiative. From 

NREL, we would like to specifically thank Paul Ciszek and Keith Emery for doing the cell 

measurements, Kent Terwilliger and Peter Hacke for doing the reliability testing of the Si minimodules, Alan Goodrich for reviewing the cost model, Qi Wang for helping in the processing of 

the HIT cells, and Harin Ullal for being the Technical Monitor of this subcontract. We further 

acknowledge the support of Prof. Ajeet Rohatgi and his team at Georgia Tech UCEP for the thin 

Si cell fabrication and testing and Duncan Harwood from D2 Solar for the module fabrication 

and testing. Finally, we want to thank the Crystal Solar team, specifically Ruiying Hao and K.V. 

Ravi (the original PI of this program), for enabling the equipment and the thin Si technology.