Photovoltaics F.-J. Haug Ecole Polytechnique Fédérale de Lausanne (EPFL), Institute of...
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Transcript of Photovoltaics F.-J. Haug Ecole Polytechnique Fédérale de Lausanne (EPFL), Institute of...
Photovoltaics
F.-J. HaugEcole Polytechnique Fédérale de Lausanne (EPFL), Institute of Microengineering (IMT),
Photovoltaics and Thin Film Electronics Laboratory (PV-Lab),
2000 Neuchâtel, Switzerland.
F.-J. Haug – Photovoltaics 2
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)
F.-J. Haug – Photovoltaics 3
The power source: Our Sun
• A low to medium size star
• Heated by fusion(essentially , processes for , etc.)
• Power output: (convert to energy)
• Delivers to the earth
• 10’000 the global energy consumption
F.-J. Haug – Photovoltaics 4
Solar irradiation atlas
SolarGIS
Central Europe: ca 1 kW/m2 under clear sky at noonYearly average: 1000 kWh/m2, ca 3 hours every day
F.-J. Haug – Photovoltaics 5
The solar spectrum
See www.pvlighthouse.com.au
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:
F.-J. Haug – Photovoltaics 6
Filtering by atmosphere: air mass (AM)
AM1.5: central Europe
AM2: 30° above horizon
AM3: 20° above horizon
Air mass number: , latitudePV standard: => , central Europe, Canada/US border
AM0: space
AM1: equator
400 600 800 10000.0
0.5
1.0
1.5
2.0 AM0 (data) AM1 AM1.5 (data) AM2 AM3
Spect
ral i
rradia
ton d
ensi
ty (W
/m2nm
)
Wavelength (nm)
WaterOzone
1000 W/m2
integrated: 1366 W/m2
F.-J. Haug – Photovoltaics 7
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 GWcover with solar cells of 10% efficiency => 8 GWp (alas, not continuously)
F.-J. Haug – Photovoltaics 8
Other “conventional” energy sources
Less obvious, but still impact on large area: dropping ground over coal shafts, shale-gas fracking, deep-sea oil drilling, etc.
MTR (mountain top removal) for Appalachian coal (USA)
Strip mining of tar sands in Alberta (Canada)
F.-J. Haug – Photovoltaics 9
Solar electricity (photovoltaics)
Roof top installation (Kaneka a-Si modules)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
Suitable roof top area in Switzerland: 138 km2 (residential, commercial, etc.)
(IEA)
F.-J. Haug – Photovoltaics 10
What is inside a solar cell?
thin film silicon solar cell p-i-n junction
glass
reflecting back electroden-layer
semiconductorabsorber layer
p-layer
transparent front contact
eh
glass
encapsulation
glass
metallic back contact
p-type wafer
n-contact
encapsulation
eh
glass
encapsulation
silicon wafer solar cell p-n junction
F.-J. Haug – Photovoltaics 11
What is inside a solar cell?
active part: semiconducting silicon (thin film)(also used: CdTe, CIGS etc.)
glass
glass
encapsulation
glass
n-contact
encapsulation
glass
encapsulation
active part: semiconducting silicon (crystalline)
2-3 μm200-300 μm
F.-J. Haug – Photovoltaics 12
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
F.-J. Haug – Photovoltaics 13
Conductivity in metals
Flow of electrons similar to flow of waterHorizontal surface, no (very small) potential difference
Analogue:a pond of water
F.-J. Haug – Photovoltaics 14
Electronic conductivity in a semiconductor
Winter: the pond is covered with ice
F.-J. Haug – Photovoltaics 15
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
Winter: the pond is covered with ice
F.-J. Haug – Photovoltaics 16
Hole conductivity in a semiconductor
Imagine: blow air bubbles under the ice sheet
A new phenomenon, exclusive to semiconductors !Bubbles under the barrier move upwardsIn semiconductors: positive charge carriers called holes
F.-J. Haug – Photovoltaics 17
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+
F.-J. Haug – Photovoltaics 18
Absorption in semiconductors: bandgap
+
-
-
+
High bandgap:high el. potentiallow current
Low bandgap:low el. potentialhigh current
Photons with energy less than the band gap are not absorbedPhoton energy in excess of band gap is lost to thermalization
𝜂=𝑉 𝑜𝑐 ⋅ 𝑗𝑠𝑐 ⋅ 𝐹𝐹⏟¿ 0.7 ...0.8
F.-J. Haug – Photovoltaics 19
Limiting efficiency
0.5 1.0 1.5 2.0 2.50
10
20
30
40
CdTe
CuGaSe2
a-Si
CuInSe2
GaAs
Ge
Si
Effi
cien
cy (
%)
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)
Theoretical limit of ~33%, optimum bandgap 1.44 eV
F.-J. Haug – Photovoltaics 20
Better use of incident light: tandem cells
0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8
2.0
2.2
2.4
Top cell bandgap (eV)
Bot
tom
cel
l ban
dgap
(eV
)
0510152025303540
4 Terminal (no constraint on current matching)
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)
high gap
low gap
glue
F.-J. Haug – Photovoltaics 21
Wafers or thin films?Crystalline cells: size determined by wafer
e.g. c-Si:single-crystalline: max 30cm diametermulti-crystalline: typically cut to 15x15 cm2
Thin film modules: size determined by glass and machinery
e.g. thin film silicon tandem modules5.3 m2 (Applied Materials machinery)1.4 m2 (TEL Solar (ex Oerlikon) machinery)
high efficiency – high cost moderate efficiency – low cost
F.-J. Haug – Photovoltaics 22
Wafers or thin films?Crystalline material: perfect propertiesexcellent lifetime and collection
Polycrystalline or amorphous materials: defects and grain boundariesrecombination and short lifetime
Electric field: Electric field:
e-
h+
e- e-
h+ h+
e-
h+
F.-J. Haug – Photovoltaics 23
Module interconnection
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%)
SunPower
SaFlex/TEL Solar
Rowell, En. Env. Sci
F.-J. Haug – Photovoltaics 24
Thin film advantage
Semi-transparent a-Si modules (Schott)
Colorful TF-Si modules(PV-Lab prototypes)
F.-J. Haug – Photovoltaics 25
Thin film advantage
Roof tile laminate(Bressler)
Flexible a-Si modules(Flexcell)
(UniSolar)
F.-J. Haug – Photovoltaics 26
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
PV system cost (currently ~5 $/W)
includes items that scale with area
(mount, wiring, labour, etc.)
For given power kWp, a larger area is needed, higher cost takes longer for amortisation
3.5$/W in 3.5 y
Balance of System (BOS)“cost baseline”
F.-J. Haug – Photovoltaics 27
PV landscape in Switzerland
PV-Centre
Neuchâtel
dye sensitizedsolar cells
flexible CIGS
3S Laminators
MB wafer sawing
Pasan sun simulators
Roth and Rau (CH)
PV-Labsystens
module testing
PV-Labc-Si, TF-Si
dye cells
inverters
Equipment(end 2014)
F.-J. Haug – Photovoltaics 28
What we do at PV-Lab
TF-Sichallenge: weak absorberption especially for long wavelengths (red light)
Solution: add back reflector => 2x absorptionadd texture for scattering => up to 50x
TCOp
i
n
TCO
p-i-n (usually on rigid glass)
Superstrate
(glass)
back reflector
grow
th
F.-J. Haug – Photovoltaics 29
Textured ZnO:B electrodes
Requirements:- conductivity: add B-dopant (diborane)- surface texture for light scatteringExamples:
ZnO:B (PV-Lab std.)SnO2:F (commercial, Asahi)
W. Wenas, Jap. J. Appl. Phys (1991)S. Faÿ, EU PV Conf. (2000)
F.-J. Haug – Photovoltaics 30
Take care with texturing!
Collision of growth fronts on facets yields defective material !
H. Sakai, Jap. J. Appl. Phys. (1990)Y. Nasuno, Jap. J. Appl. Phys. (2001) M. Python, J. Non-Cryst. Sol., (2008)
Here: μc-Si, but also observed in a-Si
F.-J. Haug – Photovoltaics 31
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)
F.-J. Haug – Photovoltaics 32
Heterojunction solar cells
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)
Surfacepassivation
A. Descoedre, EU-PV Conf., (2012)
F.-J. Haug – Photovoltaics 33
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
34F.-J. Haug – Photovoltaics
• 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
Thanks for your attention