Transcript Title

Photovoltaics
F.-J. Haug
Ecole Polytechnique Fédérale de Lausanne (EPFL), Institute of Microengineering (IMT),
Photovoltaics and Thin Film Electronics Laboratory (PV-Lab),
2000 Neuchâtel, Switzerland.
Outline
• Why Solar Cells?
• How do they work?
• Why thin film silicon solar cells?
• What do we do at PV-Lab?
–
–
–
–
–
thin film solar cells on glass and plastic substrates
thin films for wafer-based cells (hetero-junction cells)
transparent conducting oxides
(sensor applications)
(module design and reliability)
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The power source: Our Sun
• A low to medium size star
• Heated by fusion
(essentially 4H → He + 2e− , processes for Li, Be, B, etc.)
• Power output: 4x1026 W (convert 4 Mio t/s to energy)
• Delivers 5x1024 J/y to the earth
• 10’000 the global energy consumption
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Solar irradiation atlas
Central Europe: ca 1 kW/m2 under clear sky at noon
Yearly average: 1000 kWh/m2, ca 3 hours every day
SolarGIS
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The solar spectrum
The solar spectrum resembles white-hot body glowing at T=5700 K
• Usually measured with respect to wavelength (use a grating or a prism)
• In PV: often necessary to convert to photon energy: ℎ𝜈 𝑒𝑉 = 1241/𝜆(𝑛𝑚)
See www.pvlighthouse.com.au
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Filtering by atmosphere: air mass (AM)
AM0 (data)
AM1
AM1.5 (data)
AM2
AM3
2
Spectral irradiaton density (W/m nm)
2.0
1.5
AM0: space
integrated: 1366 W/m2
AM1: equator
AM1.5: central Europe
1000 W/m2
1.0
AM2: 30° above horizon
0.5
AM3: 20° above horizon
0.0
400
600
800
1000
Wavelength (nm)
Ozone
Water
Air mass number: 𝐴𝑀 = 1/ cos 𝜃, 𝜃 latitude
PV standard: AM = 1.5 => 𝜃 = 48°, central Europe, Canada/US border
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Is 1 kW/m2 a low energy density?
Example Germany: 30% of electricity supply by lignite (brown coal)
exploitation in open strip mines
some data for Hambach (Germany):
- lower ground water table by 400 m
- dislocate 3 villages (2 more planned)
- 85 km2 open mine and waste-deposit
- 40 Mio tons of coal per year
~7GW electricity
Formation of lignite: 25 Mio y
exploitation: 1984-2050
Perspective:
solar illumination on 80 km2: 80 GW
cover with solar cells of 10% efficiency => 8 GWp (alas, not continuously)
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Other “conventional” energy sources
MTR (mountain top removal) for
Appalachian coal (USA)
Strip mining of tar sands in
Alberta (Canada)
Less obvious, but still impact on large area:
dropping ground over coal shafts, shale-gas fracking, deep-sea oil drilling, etc.
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Solar electricity (photovoltaics)
Teplin Airstrip, Germany: 130 MW on 2 km2
installed in 4 months
c.f. Centrale de Gd Dixence:
2 GW, 8 km2 lakes + catchment area
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Roof top installation (Kaneka a-Si modules)
Suitable roof top area in Switzerland:
138 km2 (residential, commercial, etc.)
(IEA)
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What is inside a solar cell?
silicon wafer solar cell
p-n junction
thin film silicon solar cell
p-i-n junction
glass
glass
encapsulation
transparent front contact
p-layer
n-contact
p-type wafer
e
h
semiconductor
absorber layer
h
e
metallic back contact
n-layer
reflecting back electrode
encapsulation
encapsulation
glass
glass
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What is inside a solar cell?
active part: semiconducting
silicon (crystalline)
active part: semiconducting
silicon (thin film)
(also used: CdTe, CIGS etc.)
glass
glass
encapsulation
n-contact
200-300 μm
2-3 μm
encapsulation
encapsulation
glass
glass
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What is a semiconductor?
• It’s conducting, but not as good as metals
• Electric conductivity is normally associated with the flow of
electrons in metals
• Conductivity in semiconductors is different
• The flow of water can serve as analogue
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Conductivity in metals
Analogue:
a pond of
water
Flow of electrons similar to flow of water
Horizontal surface, no (very small) potential difference
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Electronic conductivity in a semiconductor
Winter:
the pond
is covered
with ice
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Electronic conductivity in a semiconductor
Winter:
the pond
is covered
with ice
Flow of electrons similar to flow of water (like metals)
Difference: Slower movement, needs potential difference
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Hole conductivity in a semiconductor
Imagine:
blow air
bubbles
under the
ice sheet
A new phenomenon, exclusive to semiconductors !
Bubbles under the barrier move upwards
In semiconductors: positive charge carriers called holes
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Why semiconductors for solar cells?
• Metals normally reflect light; semiconductors can absorb it
• Absorbed light creates pairs of electrons and holes
(water droplets above and bubbles below the ice sheet)
• An electric field between doped regions separates
electron-hole pairs (imagine you inclined the ice sheet)
e-
h+
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Absorption in semiconductors: bandgap
-
-
+
+
Photons with energy less than the band gap are not absorbed
Photon energy in excess of band gap is lost to thermalization
High bandgap:
high el. potential
low current
𝜂 = 𝑉𝑜𝑐 ⋅ 𝑗𝑠𝑐 ⋅
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𝐹𝐹
=0.7...0.8
Low bandgap:
low el. potential
high current
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Limiting efficiency
40
Si
Efficiency (%)
30
GaAs
Theoretical limit of ~33%,
optimum bandgap 1.44 eV
Ge
CdTe
20
CuInSe2
CuGaSe2
a-Si
10
0
0.5
1.0
1.5
2.0
2.5
Band gap energy (eV)
certified records:
28.8%: crystalline GaAs (Alta Devices)
25.6%: crystalline silicon (Panasonic)
20.5%: thin film Cu(In.Ga)Se2, (Solibro)
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Better use of incident light: tandem cells
Bottom cell bandgap (eV)
2.4
4 Terminal
2.2 (no constraint on
2.0 current matching)
1.8
1.6
1.4
1.2
1.0
high gap
0
5
10
15
20
25
30
35
40
glue
low gap
0.8
0.6
0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4
Top cell bandgap (eV)
Theoretical limit of ~43%,
optimum bandgaps 1.1 and 1.7 eV
certified records (triples):
37.9: crystalline (In,Ga)P/GaAs/(In,Ga)As (Sharp)
13.4%: thin film a-Si/nc-Si/nc-Si (LG)
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Wafers or thin films?
Crystalline cells:
size determined by wafer
Thin film modules:
size determined by glass and machinery
e.g. c-Si:
single-crystalline: max 30cm diameter
multi-crystalline: typically cut to 15x15 cm2
e.g. thin film silicon tandem modules
5.3 m2 (Applied Materials machinery)
1.4 m2 (TEL Solar (ex Oerlikon) machinery)
high efficiency – high cost
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moderate efficiency – low cost
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Wafers or thin films?
Crystalline material:
perfect properties
excellent lifetime and collection
e-
e-
h+
h+
1𝑉
Electric field: 𝐸 ≈ 200 𝜇𝑚
F.-J. Haug – Photovoltaics
Polycrystalline or amorphous materials:
defects and grain boundaries
recombination and short lifetime
e-
eh+
h+
1𝑉
Electric field: 𝐸 ≈ 200 𝑛𝑚
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Module interconnection
SaFlex/TEL Solar
SunPower
Wafer based: solder cells into strings
(e.g. SunPower: 22.4%)
Thin film: 3 x laser scribing during process
(e.g. TEL Solar: 12.2%)
Rowell, En. Env. Sci
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Thin film advantage
Colorful TF-Si modules
(PV-Lab prototypes)
Semi-transparent a-Si modules
(Schott)
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Thin film advantage
Roof tile laminate
(Bressler)
Flexible a-Si modules
(Flexcell)
(UniSolar)
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Thin film dilemma
PV modules are sold by power, currently:
~0.8 $/W crystalline
~0.6 $/W thin film
3.5$/W in 3.5 y
3.5$/W in 3.5 y
PV system cost (currently ~5 $/W)
includes items that scale with area
(mount, wiring, labour, etc.)
For given power kWp, a larger area is
needed, higher cost takes longer for
amortisation
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Balance of System (BOS)
“cost baseline”
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PV landscape in Switzerland
PV-Lab
systens
inverters
3S Laminators
MB wafer sawing
Pasan sun simulators
Roth and Rau (CH)
flexible CIGS
Neuchâtel
Equipment
(end 2014)
PV-Lab
c-Si, TF-Si
PV-Centre
dye cells
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dye sensitized
solar cells
module testing
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What we do at PV-Lab
TF-Si
challenge: weak absorberption
especially for long
wavelengths (red light)
Solution:
add back reflector => 2x absorption
add texture for scattering => up to 50x
Superstrate
(glass)
TCO
p
growth
i
n
TCO
back reflector
p-i-n (usually on rigid glass)
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Textured ZnO:B electrodes
Requirements:
- conductivity: add B-dopant (diborane)
- surface texture for light scattering
Examples:
SnO2:F (commercial, Asahi)
ZnO:B (PV-Lab std.)
W. Wenas, Jap. J. Appl. Phys (1991)
S. Faÿ, EU PV Conf. (2000)
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Take care with texturing!
Collision of
growth fronts on facets
yields defective material !
Here: μc-Si, but also
observed in a-Si
F.-J. Haug – Photovoltaics
H. Sakai, Jap. J. Appl. Phys. (1990)
Y. Nasuno, Jap. J. Appl. Phys. (2001)
M. Python, J. Non-Cryst. Sol., (2008)
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Cells on flexible plastic substrate
Nano-imprinting used for texturing of plastic (here: periodic grating structure)
T. Söderström, Appl. Phys. Lett, (2009)
K. Söderström, Prog. in PV, (2010)
J. Escarré, J. Optics, (2012)
R. Biron, Sol. En. Mat. (2013)
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Heterojunction solar cells
Surface
passivation
 c-Si is a perfect crystal; cell-performance is not bulk- but surface-limited
 Intrinsic (undoped) a-Si:H provides excellent passivation of c-Si surface
 Charge is extracted through passivating a-Si:H bi-layers
 High efficiency cells >22% (certified)
F.-J. Haug – Photovoltaics
A. Descoedre, EU-PV Conf., (2012)
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Research on hetero-junction cells
• Thin a-Si layers: max 5 nm
• Pure a-Si layers: avoid epitaxy on underlying c-Si
• Chemical annealing: alternate SiH4 and H2 plasma
• Other:
– Texture etch (avoid sharp valleys)
– Highly transparent TCOs (very high transmission at 1100 nm)
– Replace screen printing of Ag contacts by galvanic Cu
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Thanks for your attention
• Acknowledgement for funding
– Swiss Federal Office for Energy
EU FP6 and FP7, CTI, FNS, CCEM-CH, Swiss Electric Research,
Axpo Naturstromfonds, Velux-Stiftung, IBM…
– Bosch, Oerlikon Solar, Pasan, Flexcell, Solvay, Dupont, Metalor,
Meyer-Burger (3S Moduletec, Roth and Rau), Indeotec SA, …
•
Thanks also to the PV-Lab members
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