Transcript L`innovazione nel fotovoltaico - Università degli Studi di Roma Tor

Nanoscale Photovoltaics
Aldo Di Carlo
Dipartimento di Ingegneria Elettronica
Università di Roma “Tor Vergata”
[email protected]
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Example of photovoltaic systems
PHOTOVOLTAIC CELL
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Componenti di un sistema fotovoltaico
Module
Cell
Array
The photovoltaic system is made of an array
of photovoltaic modules with additional
electronics like charge controllers, inverters
etc.
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Photovoltaic cell: working principle
Continuous
Current
P-type silicon
N-type silicon
“Conventional” photovoltaic cells are based p-n junction
between semiconductors.
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Photovoltaic cell: short history
1941
Russell Ohl (Bell Labs) discover the silicon p-n
junction and the effect of light on the junction
1954
Bell Labs researchers Pearson, Chapin,
e Fuller demonstrated the photovoltaic cell
with 4.5% efficiency
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2007:
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Modern solar cell
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Solar Energy Map
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Spectral power density [(W/m2)/nm]
Solar Spectrum
Wavelength [nm]
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Efficiency
One of the most important parameters of the photovoltaic cell is the efficiency
defined as:
EFFICIENCY = h =
Max electrical power produced by the cell
Total solar power impinging on the cell
Example:
10 W/dm2
h = 10%
1W
h = 20%
2W
1dm
1dm
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It is important to increase as much as possbile
the efficiency.
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Figures of merit
Important features of the I-V curves
· The intersection of the curve with the y-axis (current) is referred
to as the short circuit current ISC. ISC is the maximum current
the solar cell can put out under a given
illumination power without an external voltage source connected.
· The intersection with the x-axis (voltage) is called the open
circuit voltage (VOC). VOC is the maximum voltage a solar cell
can put out.
· IMP and VMP are the current and voltage at the point of
maximum power output of the solar cell. IMP and VMP can be
determined by calculating the power output P of the
solar cell (P=I*V) at each point between ISC and VOC and finding
the maximum of P.
Fill form factor
FF 
Pmax
I SCVOC

I MPVMP
I SCVOC
The overall efficiency of a solar cell is larger for larger FF
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Figures of merit
PHOTORESPONSIVITY
The photoresponsivity is defined as the photocurrent
extracted from the solar cell divided by the incident
power of the light at a certain wavelength.
EXTERNAL QUANTUM EFFICIENCY
The external quantum efficiency is defined as
the number of charges Ne extracted at the
electrodes divided by the number of photons Nph
of a certain wavelength incident on the solar cell
POWER CONVERSION EFFICIENCY
The power conversion efficiency is defined
as the ratio of the electric power output of
the cell at the maximum power point to the
incident optical power.
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Which are the factors influencing the cell efficiency ?
EFFICIENCY
MATERIALS
Silicon
GaAs
CdTe
…..
…..
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TECHNOLOGY
Single junctions
Multiple junctions
….
….
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Materials for photovoltaic cells
Bulk semiconductors
– Silicon
• Single crystal
• Multi crystalline
– Gallium arsemide (GaAs)
– Other III-V semiconductors
CdTe
Thin Films semiconductors
– Amorphous silicon (a-Si)
– Cadmium telluride (CdTe)
– Copper-Indium diselenide (CuInSe2, o CIS)
– Coper-Gallium-Indium diselenide (CIGS)
Organic and hybrid materials
- Small molecules
- Polymers
- Dye Sensitized Solal Cell
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Beyond the Shockley-Queisser limit
The maximum thermodynamic efficiency for the conversion
of unconcentrated solar irradiance into electrical free energy
in the radiative limit, assuming detailed balance, a single
threshold absorber, and thermal equilibrium between
electrons and phonons, was calculated by Shockley and
Queisser in 1961to be about 31%.
W. Shockley and H. J. Queisser. J. Appl. Phys. 32 (1961) 510.
What do we do to achieve efficiencies > 31 % ?
• Concentration
• Multijunction
• No thermal equilibrium
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Nanotecnology
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Andamento dell’efficienza delle celle fotovoltaiche
Max lab efficiency on small size solar cells
40
36
32
EFFICIENCY (%)
Record ~40%
Multijunctions (GaAs ed altri)
28
Monocristalline Silicon
24
Multicristalline silicon
20
16
12
CdTe
Organic: DSSC
8
4
CIS e CIGS
0
1975
Organic: polimer
a-Silicon
1980
1985
1990
1995
YEAR
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2000
2005
Max and module level efficiencies
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Solar Cell Spectral Response
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Multijunctions
Eg=1.9eV
Eg=1.42eV
Cell 1
Eg1
Eg=0.7eV
Cella 2
Eg2<Eg1
Cella 3
Eg3<Eg2
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MultiJunction a-Si solar Cells
Amorphous silicon absorption coefficient is larger than Silicon. We can then use thin
layers of a-Si (few microns).
TCO
p
TCO
p
aSi
i
1 mm
i
n
n
Multijunctions solar cells
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Photovoltaic generations
First generation refers to high quality
and hence low
defect single crystal photovoltaic
devices these have
high efficiencies and are approaching
the limiting
efficiencies for single band gap devices
Second generation technology involves
low cost and low energy
intensity growth techniques such as
vapour deposition and electroplating
Third generation multiple energy
threshold devices;
modification of the incident
spectrum; and use of excess
thermal generation to enhance
voltages or carrier collection.
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What about nanobjects ?
Nanobjects can be use to avoid silicon in II generation
photovoltaics and reduce the cost of the cell
Nanobjects play a fundamental role to develop III generation
photovoltaics
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Structure of Dye Sensitized Solar Cells
Glass Substrate
Transparent Conducting Oxide (ITO or SnO2:F)
Catalyst (Platinum, graphite)
Electrolyte I-/I-3
Dye Molecules on TiO2
nanocristalline TiO2
Transparent Conducting Oxide (ITO or SnO2:F)
Glass Substrate
Why DSSC
Structure of DSSC
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Principle
Tor Vergataof DSSC
Nanocrystalline TiO2
Meas. Setup: Indoor
Stability: Indoor
Assembling DSSC
Meas. Setup: Outdoor
Stability: Outdoor
Final Assembling of DSSC
Process Repeatability
Enocyanine (E163)
Maratea 2007
Hematine
The “nano” object: Nanocristalline TiO2
E (V)
S*
E [LUMO (S*)] – EC [TiO2] > E exciton binding energy
-0.5
Monocrystaline
0.0
Exciton
0.5
So/S+
TiO2
Dye
Nanostructured
Strong increase of
Very large
effective
optical
density
of
area
available forfilm
the nanoporous
dye-TiO
with respect
to the
2 interaction
monocrystalline
film
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Principle of Dye Sensitized Solar Cells
No permanent chemical transformation in the materials composing the cell
TCO
TiO2
Injection
Dye
Electrolyte
S* (LUMO)
Fermi Level in TiO2
-0.5
E (V)
V Max
0.0
hυ
3I-
Ox
I-3
0.5
So/S+ (HOMO)
Load
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Cathode
Competition Dynamic in DSSC
(source: O’Regan)
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Dyes (1)
• The optoelectronic properties
(especially the absorption spectrum)
can be tuned through the chemical
design of novel dyes, even
multicolored
• Efficiencies: max 10 - 11% (in labs)
• Lifetimes: few years
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Dyes (2)
Synthetic Dyes
Dyes synthesized with organic chemistry that have high
absorption coefficients in the visible region. These dye can be
dissolved in organic solvents. The optimal dye will absorb the
broadest range of sunlight spectrum
The molecule on the left:
cis-bis(isothiocyanato)bis(2,2-bipyridyl-4-carboxilicacid-4tetrabutylammonium carboxilate)ruthenium(II)
Biological Dyes: Anthocyanins are found in red wines,
blackberry etc. An anthocyanin has a carbohydrate (sugar,
usually glucose) esterified at the 3 position. An anthocyanidin,
termed the aglycone, does not have a sugar at the 3 position.
Naturally occurring pigments from grapes always have a sugar
bonded at the 3 position, though other compounds such as
hydroxycinnamates and acetate may be involved. The presence
of this sugar helps the anthocyanin maintain solubility in water.
Efficiencies are about an order of magnitude lower than with
synthetic dyes.
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Conventional Cell Production
Sistema di ricerca per la
produzione di celle CIS
Fornace industriale per
la produzione di lingotti
di silicio
Apparato industriale
per la diffusione
Apparati per la fabricazione
di celle al silicio amorfo
(Uni. Toledo)
•PECVD, hot-wire, sputtering
•13.56 MHz excitation
•Gas handling for SiH4, CH4, PH3, B2H6, NH3
•Gas scrubber with toxic gas monitoring
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DSSC Fabrication: “cooking recepies”
MOVIE: downloadable from http://www.freenergy.uniroma2.it
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How to create a DSSC
1-2) Put TiO2 on ITO and oven it @ 450 oC
(Sintering)
3) Sinterizer Impregnation
(immerge the cell in the
blackberries!)
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How to create a DSSC
4) Platinum on the counter electrode
5) Assemble the two pieces (25-50 mm distance)
6) Fill with electrolyte KI/I2
7) Seal the solar cell
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Is it possible to use printing technologies ?
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Photovoltaic performance
Absorption Spectra
• QE  70-80%
• Jsc = 15-20 mA cm-2
• Voc = 0.8 V
Voltage
• h = 5-10%
• Challenges:
– Improving photocurrent: dyes, light management
– Improving photovoltage : minimise recombination
alternative materials
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Source: J. Nelson
DSSC performance
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Source: M. McGhee
Maratea 2007
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Organic Photovoltaics
DSSC Façade System
at the CSIRO Energy Centre
Newcastle, Australia
CELLA FLESSIBILE SU PET
KONARKA
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DSSC
Inorganic Materials
Concerns:
Use of toxic metals like Cadmium
Use of toxic gasses in the manufacturing of
PV, silane, hydrogen selenide
Can the materials be recycled or are they
destined for landfills
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Photovoltaic with nanobjects
Other approaches to exceed the Shockley-Queisser limit
include
– hot carrier solar cells [1-3],
– solar cells producing multiple electron-hole pairs per photon
through impact ionization [4,5],
– multiband and impurity solar cells [6,7],
– thermophotovoltaic/thermophotonic cells [6].
1.
A. J. Nozik. Annu. Rev. Phys. Chem. 52 (2001) 193.
2.
R. T. Ross and A. J. Nozik. J. Appl. Phys. 53 (1982) 3813.
3.
D. S. Boudreaux, F. Williams, and A. J. Nozik. J. Appl. Phys. 51 (1980) 2158.
4.
P. T. Landsberg, H. Nussbaumer, and G. Willeke. J. Appl. Phys. 74 (1993) 1451.
5.
S. Kolodinski, J. H. Werner, T. Wittchen, and H. J. Queisser. Appl. Phys. Lett. 63 (1993) 2405.
6.
M. A. Green. Third Generation Photovoltaics. (Bridge Printery, Sydney) 2001.
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Nanobjects for very high efficiency !!!
gap
contact
ISC
contact
There are two fundamental ways to utilize the hot carriers for enhancing the
efficiency of photon conversion.
– Enhanced photovoltage
» Carriers need to be extracted from the photoconverter before they
cool.
VOC
» The rates of photogenerated carrier separation, transport, and
interfacial transfer across the semiconductor interface must all be
fast compared to the rate of carrier cooling.
gap
contact
ISC
contact
semiconductor
– Enhanced photocurrent.
» Energetic hot carriers to produce a second (or more) electron-hole
pair through impact ionization —a process that is the inverse of an
VOC
Auger process whereby two electron-hole pairs recombine to
produce a single highly energetic electron-hole pair.
» The rate of impact ionization is greater than the rate of carrier
cooling and forward Auger processes.
In recent years, it has been proposed, and experimentally verified in some cases, that the
relaxation dynamics of photogenerated carriers may be markedly affected by quantization
effects in the semiconductor (i.e., in semiconductor quantum wells, quantum wires, quantum
dots, superlattices, and nanostructures). Specifically, the hot carrier cooling rates may be
dramatically reduced, and the rate of impact ionization could become competitive with the
rate of carrier cooling
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Examples
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Fundings and perspectives
20 x 20 x 20 EU rule
By 2020 EU should reduce by 20% the CO2
emission and increase the 20% renewable
energies
This means $$ for research in this field
Modern Physics and Nanotechnology should now
(re)consider the photovoltaic problem with new
innovative solutions. There is a plenty of space
for basic and advanced research
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