© Fraunhofer ISE

Bifacial Silicon Solar Cells – An Overview

S. W. Glunz1, A. Cuevas2

1 Fraunhofer Institute for Solar Energy Systems, Freiburg, Germany 2 Australian National University, Canberra Bifacial Workshop Konstanz, April 2012

© Fraunhofer ISE

2

Some Notes on the Application of Bifacial Modules

© Fraunhofer ISE

3

Some Notes on the Application of Bifacial Modules

Usage of the albedo effect

www bSolar

Kreinin et al., IEEE PVSC (2010)

© Fraunhofer ISE

4

Some Notes on the Application of Bifacial Modules

Usage of the albedo effect

Vertical installation

Uematsu et al., Solmat (2003)

© Fraunhofer ISE

5

Some Notes on the Application of Bifacial Modules

Usage of the albedo effect

Vertical installation

Internal reflection in module

Sliver cells

Hitachi

© Fraunhofer ISE

6

Some Notes on the Application of Bifacial Modules

Usage of the albedo effect

Vertical installation

Internal reflection in module

Up-conversion

See e.g. S. Fischer et al., JAP 108 (2010)

Mirror

Up-converter Bifacial Solar Cell

After: Truppke und Würfe

© Fraunhofer ISE

7

What Defines a Bifacial Solar Cell?

Accepts light from both surfaces

Converts it efficiently to electric power

Desired: Not only high efficiency and high bifaciality

Overview on this cell type hides two major challenges:

Long history

Great variety

© Fraunhofer ISE

8

Bifacial Solar Cells: History

Bifacial activities can be found even in the Roman Empire! Janus was originally the Roman God of Light and Sun and later the God of Beginning (January!) and Ending.

A. Cuevas, Early History of Bifacial Solar Cells, EU-PVSEC (2005)

© Fraunhofer ISE

9

The Zoo of Bifacial Cell Structure

So many different cell structures!

Need for professional help from biology!

© Fraunhofer ISE

10

Taxonomy of Biology

Classes Orders Families …. Swedish botanist Carl Linnaeus Systema Naturae, 1st Edition in 1735

© Fraunhofer ISE

11

Taxonomy of Bifacial Cells Class Two-side contact

devices Two-side junction devices

Back-junction devices

Type J-H J-D J-J J-IJ Jfloat -IJ Hfloat-IJ

p-Si └npp┘ └np┘ └npn┘ └np┘n┘ n└pn┘ p└pn┘

n-Si └pnn┘ └pn┘ └pnp┘ └pn┘p┘ p└np┘ n└np┘

Front Junction Junction Junction Junction Junction High-Low

Front Contact

yes yes no yes No No

Rear High-Low Dielectric passivation

Junction Interdigitated Junction

Interdigitated Junction

Interdigitated Junction

Rear contact

Base Base Edge contacts

Emitter and Base

Emitter and Base

Emitter and Base

© Fraunhofer ISE

12

Taxonomy of Bifacial Cells: J-H Class Two-side contact

devices Two-side junction devices

Back-junction devices

Type J-H J-D J-J J-IJ Jfloat -IJ Hfloat-IJ

p-Si └npp┘ └np┘ └npn┘ └np┘n┘ n└pn┘ p└pn┘

n-Si └pnn┘ └pn┘ └pnp┘ └pn┘p┘ p└np┘ n└np┘

Front Junction Junction Junction Junction Junction High-Low

Front Contact

yes yes no yes No No

Rear High-Low Dielectric passivation

Junction Interdigitated Junction

Interdigitated Junction

Interdigitated Junction

Rear contact

Base Base Edge contacts

Emitter and Base

Emitter and Base

Emitter and Base

© Fraunhofer ISE

13

J-H: BSF Silicon Solar Cells

Russia (All-Union Scientific Research Institute of Energy Sources, VNIIT, Moscow) Bordina et al. "Semiconductor Photoelectric

Generator". USSR Certificate of Authorship N 434872 (1970).

Bordina et al., "Operation of a thin silicon photo converter under illumination on both sides", Applied Solar Energy, No. 6, (1975)

Germany H. Fisher and W. Pschunder, 8th IEEE

Photovoltaic Specialists Conf. (1970) America P. Iles, 8th IEEE Photovoltaic Specialists

Conf. (1970) J. Mandelkorn and J. H. Lamneck, Jr., 9th

IEEE Photov. Specialists Conf. (1972).

© Fraunhofer ISE

14

J-H: Bifacial BSF Solar Cell Patents

Bordina N.M., Zadde V.V., Zaitseva A.K., Landsman A.P., Strebkov D.S.,

Streltsova V.I., Unishkov V.A., Patent GFR N 2452263, 1971. U.S. Patent no. 3.948.682, Application 31 Oct. 1974, publication 6 April

1976. Y. Chevalier and I. Chambouleyron, “Capteur photovoltaique a retro-

illumination”, French Patent 77 24669, application August 1977.

A. Luque, “Procedimiento para obtener celulas solares bifaciales” Spanish patent 458514, application May 1977.

© Fraunhofer ISE

15

J-H: p-type Substrates

B3 cell of Hitachi

13.7% (front),13.2%(rear)

Voltage increase due to higher injection

Uematsu et al., Solmat (2003)

© Fraunhofer ISE

16

J-H: p-type Substrates

Riegel et al., EU-PVSEC (2009)

Gloger et al., EU-PVSEC (2009)

UKN: 18.2% (front),14.8%(rear)

Bifaciality increases with decreasing cell thickness

p-type Cz exhibits limited diffusion length

© Fraunhofer ISE

17

J-H: n-type Bifacial BSF Solar Cells

In 1978 Fossum and Burgess reported 16.8% p+nn+ BSF solar cells.

The use of boron diffusions and n-Si led to p+nn+ cells with efficiencies of 15.7% (front) and 12.7% (rear).

Designed for static concentrators, with heavy phosphorus and boron diffusions.

carrier diffusion length 2-3 times the wafer thickness → bifaciality factor ≈ 94%.

A. Cuevas, Solar Cells 3, pp. 337-340 (1981).

© Fraunhofer ISE

18

J-H: n-type Substrate

IPM, Madrid

FZ-Si: 18.1 (front) 19.1% (rear)

Cz-Si: 15.2% (front), 17.7% (rear))

Lower performance from emitter side imperfect boron profile or passivation (SiO2)

Moehlecke et al., 1st WCPEC (1994)

Canizo et al., IEEE TED (2001)

© Fraunhofer ISE

19

J-H: n-type Substrate

ISC Konstanz

Better passivation of boron emitter

18.3% (front), 16.4% (rear)

High bifaciality allows new module concepts

Mihailetchi et al., EU-PVSEC (2010)

© Fraunhofer ISE

20

J-H: Benefiting from n-type Cell Technology

Similar developments for industrial n-type cells at ECN, Yingli, INES, ISE, …

Production average at PVGS, Japan: 19% (front), 18.5% (rear)

Most structures are bifacial

Boost for bifacial technology

Burgers et al., EU-PVSEC (2010)

Veschetti et al., IEEE JPV (2011)

© Fraunhofer ISE

21

J-H: Heterojunction

Sanyo´s HIT cell structure

11% more output over the year

Mishima et al., SolMat 95 (2011)

© Fraunhofer ISE

22

Taxonomy of Bifacial Cells: J-D Class Two-side contact

devices Two-side junction devices

Back-junction devices

Type J-H J-D J-J J-IJ Jfloat -IJ Hfloat-IJ

p-Si └npp┘ └np┘ └npn┘ └np┘n┘ n└pn┘ p└pn┘

n-Si └pnn┘ └pn┘ └pnp┘ └pn┘p┘ p└np┘ n└np┘

Front Junction Junction Junction Junction Junction High-Low

Front Contact

yes yes no yes No No

Rear High-Low Dielectric passivation

Junction Interdigitated Junction

Interdigitated Junction

Interdigitated Junction

Rear contact

Base Base Edge contacts

Emitter and Base

Emitter and Base

Emitter and Base

© Fraunhofer ISE

23

J-D: Bifacial cells with dielectric passivation

In 1977 Chevalier and Chambouleyron used tin oxide (SnO2) to passivate the rear surface of this simple n+p device, and measured a bifaciality factor of 63%.

Innovation: the rear metal made direct contact with the p-type substrate on a restricted area, while most of it remained passivated.

PrEcuRsor Cell of the well known PERC structure.

Y. Chevalier and I. Chambouleyron, “Getting more power out of silicon", Proc. 1st. European Com. Conf. on Photovoltaic Solar Energy, Luxembourg, (1977), pp. 967-976.

© Fraunhofer ISE

24

J-D: Bifacial cells with SiN passivation

Jaeger and Hezel used PECVD silicon nitride passivation in 1987 to make 15% (front), 13.2% (rear) bifacial solar cells.

These devices had a Metal-Insulator Semiconductor-Inversion Layer front junction.

Ten years later, the MIS-IL junction was replaced with a diffused pn junction to produce at ISFH bifacial cells with 20.1% front and 17.2% rear efficiencies.

Jaeger, Hezel, IEEE PVSC (1987)

Hübner et al., EU PVSEC (1997)

© Fraunhofer ISE

25

J-D: MWT-Cells

ECN’s ASPIRe bifacial MWT cells

Influence of rear passivation

I. Romijn et al., EU-PVSEC (2007)

© Fraunhofer ISE

26

Taxonomy of Bifacial Cells: J-J Class Two-side contact

devices Two-side junction devices

Back-junction devices

Type J-H J-D J-J J-IJ Jfloat -IJ Hfloat-IJ

p-Si └npp┘ └np┘ └npn┘ └np┘n┘ n└pn┘ p└pn┘

n-Si └pnn┘ └pn┘ └pnp┘ └pn┘p┘ p└np┘ n└np┘

Front Junction Junction Junction Junction Junction High-Low

Front Contact

yes yes no yes No No

Rear High-Low Dielectric passivation

Junction Interdigitated Junction

Interdigitated Junction

Interdigitated Junction

Rear contact

Base Base Edge contacts

Emitter and Base

Emitter and Base

Emitter and Base

© Fraunhofer ISE

27

J-J: First proposals of Bifacial Silicon Solar Cells

p+np+ Double junction cell. 1: n-type silicon, 2 and 2’: p-type emitter regions.

In Russia: A.K. Zaitseva and O.P. Fedoseeva, “Study of possibility of bifacial silicon solar cell applications”, Teploenergetika, 1961.

In Japan: H. Mori, "Radiation energy transducing device", U.S. Patent 3.278.811, Oct. 1966 (priority Oct. 1960).

© Fraunhofer ISE

28

J-J: Sliver solar cell

18.5% efficiency achieved at ANU Edge contacts Two junctions Similar to Mori’s cell

Illum

inatio

n

Illu

min

atio

n

Perfectly bifacial

Metal Metal

Boron diffusion

Phosphorus diffusion

Surface texturing Phosphorus diffusion AR coating

50µm 1-2mm

© Fraunhofer ISE

29

Taxonomy of Bifacial Cells: J-J Class Two-side contact

devices Two-side junction devices

Back-junction devices

Type J-H J-D J-J J-IJ Jfloat -IJ D/Hfloat-IJ

p-Si └npp┘ └np┘ └npn┘ └np┘n┘ n└pn┘ p└pn┘

n-Si └pnn┘ └pn┘ └pnp┘ └pn┘p┘ p└np┘ n└np┘

Front Junction Junction Junction Junction Junction High-Low or Dielectric paasivation

Front Contact

yes yes no yes No No

Rear High-Low Dielectric passivation

Junction Interdigitated Junction

Interdigitated Junction

Interdigitated Junction

Rear contact

Base Base Edge contacts

Emitter and Base

Emitter and Base

Emitter and Base

© Fraunhofer ISE

30

J-IJ: Transistor-like Solar Cell

Advantages:

tolerant of low quality material.

good IR response

Disadvantage:

fabrication complexity, 3 terminals.

12.7% efficient, 4 cm2 “Transcell” devices fabricated at the Polytechnical University of Madrid between 1977 and 1980.

A. Cuevas, A. Luque, and J. M. Ruiz, "A n+pn+ double-sided solar cell for optimal static concentration", 14th IEEE PVSC, (1980)

© Fraunhofer ISE

31

J-IJ: Influence of Transistor Structure

Hitachi’s transistor-structure cells 21.3% (front) and 19.8% (rear) Triode vs. floating junction (RFE)

Ohtsuka et al., PiP 8 (2000)

© Fraunhofer ISE

32

Taxonomy of Bifacial Cells: J-J Class Two-side contact

devices Two-side junction devices

Back-junction devices

Type J-H J-D J-J J-IJ Jfloat -IJ D/Hfloat-IJ

p-Si └npp┘ └np┘ └npn┘ └np┘n┘ n└pn┘ p└pn┘

n-Si └pnn┘ └pn┘ └pnp┘ └pn┘p┘ p└np┘ n└np┘

Front Junction Junction Junction Junction Junction High-Low or Dielectric paasivation

Front Contact

yes yes no yes No No

Rear High-Low Dielectric passivation

Junction Interdigitated Junction

Interdigitated Junction

Interdigitated Junction

Rear contact

Base Base Edge contacts

Emitter and Base

Emitter and Base

Emitter and Base

© Fraunhofer ISE

33

D-IJ (or Hfloat-IJ): Bifacial Pegasus Cells

A variant of SunPower’s back contact cell

21.9% (front), 13.9% (rear) Infrared transmission

Zhou, Verlinden, Crane, Swanson, Sinton, IEEE PVSC (1997)

© Fraunhofer ISE

34

Hfloat-IJ: Laser-Grooved Buried Contacts

IBC variant of LGBC solar cells 17% (front), 15.7% (rear) Positive effect of thinner wafer

Guo, Cotter, IEEE-TED 51 (2004)

© Fraunhofer ISE

35

D-IJ: SiNx on p-type Silicon

ISFH’s Back-Oeco cell: 21.5% (front) and 17.7% (rear) No photolithograpy

Müller, Merkle and Hezel, 20th EU-PVSEC (2005)

© Fraunhofer ISE

36

Taxonomy of Bifacial Cells: J-J Class Two-side contact

devices Two-side junction devices

Back-junction devices

Type J-H J-D J-J J-IJ Jfloat -IJ D/Hfloat-IJ

p-Si └npp┘ └np┘ └npn┘ └np┘n┘ n└pn┘ p└pn┘

n-Si └pnn┘ └pn┘ └pnp┘ └pn┘p┘ p└np┘ n└np┘

Front Junction Junction Junction Junction Junction High-Low or Dielectric paasivation

Front Contact

yes yes no yes No No

Rear High-Low Dielectric passivation

Junction Interdigitated Junction

Interdigitated Junction

Interdigitated Junction

Rear contact

Base Base Edge contacts

Emitter and Base

Emitter and Base

Emitter and Base

© Fraunhofer ISE

37

Jfloat-IJ: First Cell Structures

A double-junction IBC solar cell was proposed by Texas Instruments researchers:

S.Y. Chiang, B.G. Carbajal and G.F. Wakefield, “High performance thin solar cell”, EU-PVSEC (1977)

© Fraunhofer ISE

38

Jfloat-IJ

Fraunhofer ISE’s double-junction cell 20.6% (front) and 20.2% (rear)

n++p+

Base Grid

Emitter Grid

Oxide

Emitter

Floating Emitter

OxideGlunz et al., IEEE PVSEC (1997)

© Fraunhofer ISE

39

Conclusion

Many different cell structures are suited for bifacial applications

Substrate quality plays a dominant role

Due to the strong industrial development JH-structures on n-type silicon could boost the bifacial market

Powerful support by the patron of bifacial solar cells, Janus