Heterojunction

silicon based solar cells

Miro Zeman

Photovoltaic Materials and Devices Laboratory, Delft University of Technology

Outline

Introduction to Si PV technologies

Motivation for developing HTJ Si solar cells

Achievements

Challenges

HET-Si project

Summary

Introduction to Si PV technologies

Wafer-based crystalline silicon

½ century of manufacturing history, ~90% of 2008 markethighest performance of flat-plate technologiesgood track record and reliabilitycost reduction is main overall challengemodule efficiencies:

-

12 ~ 20% (now)-

18 ~ > 22% (long term)

Wim

Sinke

(ECN, Leader of WG 3 : Science, technology & applications of EU

PV Technology Platform)

Introduction to Si PV technologies

Thin-film silicon

Wim

Sinke

(ECN, Leader of WG 3 : Science, technology & applications of EU

PV Technology Platform)

low-cost potential and new application possibilitiesapplication of micro-crystalline siliconefficiency enhancement is major challengestable module efficiencies:

- 6 ~ 9% (now)- 10 ~ 15% (longer term)

http://us.sanyo.com/Dynamic/customPages/docs/solarPower_HIT_Solar_Power_10-15-07.pdf

Introduction to Si PV technologies

High performance Low-cost potentialHybrid technology HIT solar cell

Sanyo started R&D in 1990

HIT: Heterojunction

with Intrinsic Thin Layer

Most popular Si PV technologies:

Motivation for HTJ solar cells

Solar cell operating principles:

Thermodynamic approach:

Conversion of energy of solar radiation into electrical energy

Two-step process:

1.

Solar energy → Chemical energy

of electron-hole pairs

2.

Chemical energy

→ Electrical energy

χe

absorber

EF

EC

EV

-qψ

Solar cell operating principles

Χe

electron affinity

1.

Solar energy → Chemical energy

of electron-hole pairs

-qψ

Solar cell operating principles

EFV

-μeh

EFCEC

EV

absorber

1.

Solar energy → Chemical energy

of electron-hole pairs

2.

Chemical energy

→ Electrical energy

-qψ

Solar cell operating principles

EFV

-μeh

EFCEC

EV

absorber

2.

Chemical energy

→ Electrical energy

Solar cell operating principles

absorber

EV

-qψ

EFC -qVOCEFV

Semi-

permeable membrane

for electrons

EC

Semi-

permeable membrane for holes

2.

Chemical energy

→ Electrical energy

Solar cell operating principles

absorber

EV

-qψ

EFC -qVOCEFV

Semi-

permeable membrane

for electrons

EC

Semi-

permeable membrane for holes

n-typep-type

2.

Chemical energy

→ Electrical energy

Solar cell operating principles

absorber

EV

-qψ

EFC -qVOCEFV

Semi-

permeable membrane

for electrons

EC

Semi-

permeable membrane for holes

n-typep-type

2.

Chemical energy

→ Electrical energy

Solar cell operating principles

absorber

χeEC

EV

-qψ

EFC

χe

E

χe

EFV

Semi-

permeable membrane

for electrons

Semi-

permeable membrane for holes

-qVOC

2.

Chemical energy

→ Electrical energy

Solar cell operating principles

absorber

EC

EV

-qψ

EFCE

EFV

Semi-

permeable membrane

for electrons

Semi-

permeable membrane for holes

-qVOC

n-typep-type

2.

Chemical energy

→ Electrical energy

Solar cell operating principles

absorber

EC

EV

-qψ

EFCE

EFV

Semi-

permeable membrane

for electrons

Semi-

permeable membrane for holes

-qVOC

n-typep-type

EF

Eg1

N c-Si P c-Si

Eg1

Silicon based solar cells

Eg1

N c-SiP a-Si

Eg2

EF

1. Tunneling2. Thermionic emission3. Trap-assisted tunneling

Homojunction Heterojunction

(band off-set)

Real world:

• Between p and n-type materials there is an intrinsic a-Si:H layer.

• Thin-layer: optimum thickness of the intrinsic a-Si:H is about 4 to 5 nm.

n-doped c-Si

p-doped a-Si:H

intrinsic a-Si:H

Heterojunction

Si solar cells

Sanyo HIT (Heterojunction with Intrinsic Thin Layer) solar cell:

http://us.sanyo.com/Dynamic/customPages/docs/solarPower_HIT_Solar_Power_10-15-07.pdf

UNSW PERL c-Si solar cell Sanyo HIT solar cell

http://pvcdrom.pveducation.org/MANUFACT/LABCELLS.HTM

http://sanyo.com/news/2009/05/22-1.html

Efficiency record

25% 23%

Manufacturing

Complicated diffusion, oxidation Formation of pn junction, passivation, photomasking BSF are all completed by PECVD

Temperature High temperature processes Less than 200 ˚C requirement

(up to 1000˚C)

Heterojunction

Si solar cells

Comparison with homojunction

c-Si solar cell:

Jsc

, Voc

, FF, Area

42.7 mAcm-2, 0.705 V, 0.828, 4 cm2

39.5 mAcm-2, 0.729 V, 0.80, 100 cm2

Good stability under light [1] and thermal exposure [2]

High efficiency (capability of reaching efficiency up to 25%)

• Negligible SWE due to very thin a-Si:H layer

• Favorable temperature dependence of the conversion efficiency

[1] T. Sawada, et al, Photovoltaic Energy Conversion, 2

(1994) 1219--1226

[2] Maruyama, E. et al, Photovoltaic Energy Conversion, 2

(2006) 1455--1460

Heterojunction

Si solar cells

Potential:

1. Low thermal budget

2. Avoiding bowing of thin wafers. Route to use very thin wafers

3. Suppressing lifetime degradation of minority carriers; possible use low quality c-Si

Heterojunction

Si solar cells

Industrial benefits:

200

400

600

800

1000

Proc

ess

tem

pera

ture

[C°]

Time [min]

c-Si conventional technology

Junction diffusion

ARC

Contacts

Firing

30’

0,5’ 2’

0,3’

200

400

600

800

1000

Proc

ess

tem

pera

ture

[C°]

Plasma

3’

TCO

10’

Front/back contact

Firing

0,3’

a-Si/c-Si technology

Low Tem

perature

Rapid ProcessTime [min]

F. Roca, ENEA

FZ/CZ Area Jsc Voc FF Efficiency

(cm2) (mA/cm

2) (mV) (%) (%)

Sanyo n CZ 100 39.5 729 80 23,0

AIST n CZ 0.2 35.6 656 75 17.5

Helmholtz

centre Berlin

n FZ 1 39.3 639 79 19.8

p FZ 1 36.8 634 79 18.5

IMT EPFL n FZ 0.2 34 682 82 19.1

p FZ 0.2 32 690 74 16.3

NREL p FZ 0.9 35.9 678 78.6 19.1

n FZ 0.9 35.3 664 74.5 17.2

Achievements

Laboratory solar cells:

• The maximum efficiency was 12.3%

•

Low Voc and FF compared to c-Si homojunction

results from large interface state density.

n c-Si

p a-Si:H

TCO

metal

Achievements

Development of HIT solar cells at Sanyo:

M. Tanaka, et al, “Development of New a-Si/c-Si Heterojunction Solar Cells: ACJ-HIT (Artificially Constructed Junction-Heterojunction with Intrinsic Thin-Layer)”, Appl. Phys., 31 (1992) 3518-3522

• The maximum conversion efficiency is 14.8%

• Voc

is improved by 30 mV due toexcellent passivation

of a-Si:H

• FF is improved to 0.8

•

Thin intrinsic a-Si layer

introduced, better passivation

of silicon wafers

Achievements

Development of HIT solar cells at Sanyo:

ACJ-HIT

n c-Si

p a-Si:H

TCO

metal

i a-Si:H

M. Tanaka, et al, “Development of New a-Si/c-Si Heterojunction Solar Cells: ACJ-HIT (Artificially Constructed Junction-Heterojunction with Intrinsic Thin-Layer)”, Appl. Phys., 31 (1992) 3518-3522

•

Application of textured substrate

and back surface field

(BSF), the maximum conversion efficiencyincreases to 18.1% for 1cm2 area.

• Jsc

is improved by 20% to 37.9 mA/cm2

Achievements

Development of HIT solar cells at Sanyo:

TCO

p a-Si:H

i a-Si:H

n c-Si

metal

n a-Si:H

M. Tanaka, et al, “Development of New a-Si/c-Si Heterojunction Solar Cells: ACJ-HIT (Artificially Constructed Junction-Heterojunction with Intrinsic Thin-Layer)”, Appl. Phys., 31 (1992) 3518-3522

•

The symmetrical structure

can suppress both thermal and mechanical stress.

• The maximum conversion efficiency is 21.3% for 100 cm2.

TCO

p a-Si:H

i a-Si:H

n c-Si

n a-Si:H

i a-Si:H

metal

TCO

Achievements

Development of HIT solar cells at Sanyo:

M. Tanaka, et al, “Development of hit solar cells with more than 21% conversion efficiency and commercialization of highest performance hit modules”, Photovoltaic Energy Conversion, 1 (2003) 955--958

Achievements

Development of HIT solar cells at Sanyo:

Y. Tsunomura, et al, “Twenty-two percent efficiency HIT solar cell”, Solar Energy Materials and Solar Cells, 93 (2009) 670--673

1. Improving the a-Si:H/c-Si heterojunction

Conversion efficiency 22.3% has been achieved in 2008 by further optimization:

2. Improving the grid electrode

3. Reducing the absorption in the a-Si:H and TCO

Achievements

Sanyo HIT modules:

Achievements

Sanyo HIT Double Bifacial modules:

Achievements

Development of HIT solar cells at Sanyo:

Conversion efficiency 23,0% has been achieved in May 2009:

http://us.sanyo.com/News/SANYO-Develops-HIT-Solar-Cells-with-World-s-Highest-Energy-Conversion-Efficiency-of-23-0-

Voc(V) 0.729

Jsc(mA/cm2) 39.5

FF 0.8

Efficiency 23%

c-Si Thickness (µm) >200

Achievements

Development of HIT solar cells at Sanyo:

Conversion efficiency 22.8% with 98 µm thick c-Si (EU-PVSEC Hamburg 2009):

http://techon.nikkeibp.co.jp/english/NEWS_EN/20090923/175532/

Highest Voc

for c-Si type solar cell, Voc

= 0.743V

Achievements

Production development of HIT solar cells at Sanyo:

http://www.pv-tech.org/news/_a/sanyo_targets_600mw_hit_solar_cell_production_with_new_plant/

Achievements

National Institute of

Advanced Industrial Science and Technology:

H. Fujiwara, et al, “Crystalline Si Heterojunction Solar Cells with the Double Heterostructure of Hydrogenated Amorphous Silicon Oxide”, Jpn. J. Appl. Phys., 48 (2009) 064506

Al

n c-Si

p a-SiO:HITO

i a-SiO:H

i a-SiO:Hn a-SiO:H

ITO

Ag

• a-SiO:H i layer can suppress epitaxial growth completely

• Efficiency decreases with decreasing thickness of c-Si

Achievements

Institute of Microtechnology

(IMT) Neuchatel (EPFL):

Al or Ag

n c-Si

p a-Si:H/µc-Si:HITO

i a-Si:H

i a-Si:Hn a-Si:H/µc-Si:H

ITO

S.Olibet, PhD thesis, 2008

• a-Si:H/uc-Si:H layers fabricated by VHF-CVD

• Small area (0.2 cm2) cells without front metal contact

• no intrinsic a-Si:H layer results in low Voc

Achievements

Helmholtz Center Berlin for Materials and Energy:

AZO

p a-Si:H

n c-Si

n a-Si:H

Al

M.Schmidt, et al, “Physical aspects of a-Si:H/c-Si hetero-junction solar cells”, Thin Solid Films, 515 (2007) 7475--7480

• reduction of optical loss due to thinner a-Si layer

• a-Si:H layers fabricated by HW CVD

Achievements

National Renewable Energy laboratory (NREL):

n a-Si:H

p c-Si

p a-Si:H

i a-Si:H

metal

ITO

i a-Si:H

metal

Q. Wang, et al, “Crystal Silicon Heterojunction Solar cell by Hot-Wire CVD”, The 33rd IEEE Photovoltaic Specialists Conference, 2008.

Challenges

Losses in HIT solar cell:

Optical losses:1. Textured surface2. Low absorption of TCO and a-Si3. High aspect ratio of grid electrode

Recombination losses:1. cleaning2. Hydrogen termination of wafer surface3. High quality a-Si:H

Resistance losses:1. High conductivity TCO2. Good ohmic

contact between different layers

n c-Si

a-Si:H (i/n)

TCOa-Si:H (p/i)

TCO

Grid electrode

reflection absorption shading

Optical losses (Jsc)

+-

Recombination losses (Voc)

Res

ista

nce

loss

es (

FF)

Challenges

1. Wafer cleaning

Partial passivation by H2 or HF solution to saturate dangling bonds

Remove particles and metallic contaminants from the surface

SC1 + SC2 (RCA Cleaning) NaOH : H2OHNO3 : HFHF : H2OHCl:HFCH3OH:HFCH3CH(OH)CH3:HF (or HI)HF:H2O2:H2OCF4/O2 (8% Mix)NF3H2N2O2Ar

wet

Chemicals

dry

PVMD/DIMES

results:

F. Roca, ENEA

Challenges

2. Epitaxial growth at the heterojunction

interface

H. Fujiwara, et al, “Impact of epitaxial growth at the heterointerface of a-Si:H/c-Si solar cell”, Appl. Phys. Lett., 90 (2007) 013503--3

Optimum growth temperature and rf power density

Suppression of the epitaxial growth

Challenges

3. Controlling layer thickness

Efficiency is highly related to the thickness of the intrinsic and doped layers

T. Sawada, et al, “High efficiency a-Si/c-Si heterojuction solar cell”, IEEE Photovoltaic Specialists Conference, Vol. 2 (1994) 1219—1226

•

Thicker intrinsic a-Si:H

layers lead to rapid reduction in Jsc

and FF

•

Jsc

is sensitive to thickness of p-type a-Si:H

layer.

Optical loss in short wavelength region is caused by the absorption of a-Si.

Optical loss in long wavelength region is caused by the free carrier absorption of TCO.

Challenges

4. Reducing absorption loss in a-Si and TCO

E.Maruyama, et al, “Sanyo's Challenges to the Development of High-efficiency HIT Solar Cells and the Expansion of HIT Business”, Photovoltaic Energy Conversion, 2 (2006) 1455--1460

Solutions:1. High-quality wide gap alloys such as a-SiC:H2. High-quality TCO with high carrier mobility and

relatively low carrier density.

Surface-textured substrates are used due to optical confinement effect

Challenges

5. Surface-textured wafer surface

M. Tucci, et al, “CF4/O2 dry etching of textured crystalline silicon surface in a-Si:H/c-Si heterojunction for photovoltaic applications”, Solar energy materials and solar cells, 69 (2001) 175-185

Problems:

1. Fabrication of an uniform a-Si layer on the textured c-Si

2. Insufficient cleaning of c-Si surfaces before a-Si film growth

Solutions:1. Optimization of deposition condition

2. Clean c-Si surface with hydrogen plasma treatment

Finer width (W) and no spreading area of grid electrode reduce shade losses

Challenges

6. Improvement of grid electrode

Solutions:

1. Optimize viscosity and rheology of silver paste2. Optimize process parameters in screen printing

Y.Tsunomura, et al, “Twenty-two percent efficiency HIT solar cell”, Solar Energy Materials and Solar Cells, 93 (2009) 670--673

00/00/200800/00/200800/00/2008

Project concept and objectivesHetorojunction

concepts for high

efficiency

solar

cells

Short-term target:

demonstrate the industrial feasibility

of heterojunction

solar cells

in EuropeMedium term target:

demonstrate the concept of ultra-

high efficiency rear-contact cells

based on a-Si/c-Si heterojunction

00/00/200800/00/200800/00/2008

Project partnershipHETSI partnership

1.

HTJ Si solar cells offer promising potential to conventional c-Si solar cells-

lower production cost-

better thermal stability-

higher electrical yield

Summary

2. HIT Si solar cells contain a-Si/c-Si heterojunction

and use intrinsic a-Si:H

for high-quality passivation

3. The efficiency record of HIT solar cells is 23.0%

4.

Challenges to fabricate high-efficiency HTJ Si solar cells-

clean and textured c-Si surfaces-

abrupt heterojunctions

with low interface-defect densities-

optimum a-Si :H deposition conditions and layer thickness- TCO

Acknowledgements