© Fraunhofer ISE Bifacial Silicon Solar Cells – An Overview S. W. Glunz 1 , A. Cuevas 2 1 Fraunhofer Institute for Solar Energy Systems, Freiburg, Germany 2 Australian National University, Canberra Bifacial Workshop Konstanz, April 2012
© 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