Bifacial silicon solar cells - an overview

1. © 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

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Some Notes on the Application of Bifacial Modules

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Some Notes on the Application of Bifacial Modules
 Usage of the albedo effect
www bSolar
Kreinin et al., IEEE PVSC (2010)

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Some Notes on the Application of Bifacial Modules
 Usage of the albedo effect
 Vertical installation
Uematsu et al., Solmat (2003)

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Some Notes on the Application of Bifacial Modules
 Usage of the albedo effect
 Vertical installation
 Internal reflection in module
Sliver cells
Hitachi

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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-converterBifacial Solar Cell
After: Truppke und Würfe

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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

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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)

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The Zoo of Bifacial Cell Structure
So many different cell structures!
Need for professional help from biology!

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

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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

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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

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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).

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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.

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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)

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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

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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).

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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)

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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)

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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)

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J-H: Heterojunction
 Sanyo´s HIT cell structure
 11% more output over the year
Mishima et al., SolMat 95 (2011)

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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

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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.

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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)

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J-D: MWT-Cells
 ECN’s ASPIRe bifacial MWT cells
 Influence of rear passivation
I. Romijn et al., EU-PVSEC (2007)

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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

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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).

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J-J: Sliver solar cell
 18.5% efficiency achieved at ANU
 Edge contacts
 Two junctions
 Similar to Mori’s cell
Illumin
ation
Illumin
ation
Perfectly bifacial
MetalMetal
Boron
diffusion
Phosphorus
diffusion
Surface texturing
Phosphorus diffusion
AR coating
50µm 1-2mm

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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

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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)

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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)

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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

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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)


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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)

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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)

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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

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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)

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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
Oxide
Glunz et al., IEEE PVSEC (1997)

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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