Nanoscale Photovoltaics

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Nanoscale Photovoltaics
Aldo Di Carlo
Dipartimento di Ingegneria Elettronica Università
di Roma Tor Vergata aldo.dicarlo_at_uniroma2.it
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Example of photovoltaic systems
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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
N-type silicon
P-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 Modern solar cell
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Solar Energy Map
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Solar Spectrum
Spectral power density (W/m2)/nm
Wavelength nm
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Efficiency
One of the most important parameters of the
photovoltaic cell is the efficiency defined as
Max electrical power produced by the cell
EFFICIENCY h
Total solar power impinging on the cell
Example
10 W/dm2
h 10
1 W
1dm
h 20
2 W
1dm
It is important to increase as much as
possbilethe 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 (PIV) at each
point between ISC and VOC and finding the maximum
of P.
Fill form factor
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
TECHNOLOGY
Single junctions Multiple junctions . .
Silicon GaAs CdTe .. ..
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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 gt 31 ?

• Concentration
• Multijunction
• No thermal equilibrium

Nanotecnology
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Andamento dellefficienza delle celle
fotovoltaiche
Max lab efficiency on small size solar cells
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Record 40
Multijunctions (GaAs ed altri)
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Monocristalline Silicon
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Multicristalline silicon
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EFFICIENCY ()
16
12
CdTe
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Organic DSSC
CIS e CIGS
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Organic polimer
a-Silicon
0
1975
1980
1985
1990
1995
2000
2005
YEAR
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Max and module level efficiencies
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Solar Cell Spectral Response
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Multijunctions
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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).
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 SnO2F)
Catalyst (Platinum, graphite)
Electrolyte I-/I-3
Dye Molecules on TiO2
nanocristalline TiO2
Transparent Conducting Oxide (ITO or SnO2F)
Glass Substrate
Why DSSC Nanocrystalline TiO2 Meas. Setup
Indoor Stability Indoor Hematine Structure
of DSSC Assembling DSSC Meas. Setup Outdoor
Stability Outdoor Principle of DSSC Final
Assembling of DSSC Process Repeatability
Enocyanine (E163)
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The nano object Nanocristalline TiO2
E (V)
S
E LUMO (S) EC TiO2 gt E exciton binding
energy
-0.5
0.0
Exciton
0.5
So/S
TiO2
Dye
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Principle of Dye Sensitized Solar Cells
No permanent chemical transformation in the
materials composing the cell
TCO
TiO2
Electrolyte
Cathode
Dye
Injection
S (LUMO)
Fermi Level in TiO2
-0.5
E (V)
V Max
h?
0.0
3I-
I-3
Ox
0.5
So/S (HOMO)
Load
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Competition Dynamic in DSSC
(source ORegan)
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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

Nikkei
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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(isothio
cyanato)bis(2,2-bipyridyl-4-carboxilicacid-4-tetra
butylammonium 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.uni
roma2.it
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How to create a DSSC
1-2) Put TiO2 on ITO and oven it _at_ 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
• ? 5-10

Voltage
Source J. Nelson

• Challenges
• Improving photocurrent dyes, light management
• Improving photovoltage minimise
  recombination alternative materials

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DSSC performance
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Source M. McGhee
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(No Transcript)
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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 !!!
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.
• 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.
• 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
  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.

ISC
VOC
contact
contact
gap
semiconductor
ISC
contact
VOC
contact
gap
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