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Università di Roma Tor Vergata
Maratea 2007
Nanoscale Photovoltaics
Aldo Di Carlo
Dipartimento di Ingegneria ElettronicaUniversità di Roma “Tor Vergata”
aldo.dicarlo@uniroma2.it
Università di Roma Tor Vergata
Maratea 2007
Example of photovoltaic systems
PHOTOVOLTAIC CELL
Università di Roma Tor Vergata
Maratea 2007
Componenti di un sistema fotovoltaico
CellModule
Array
The photovoltaic system is made of an array of photovoltaic modules with additional electronics like charge controllers, inverters etc.
Università di Roma Tor Vergata
Maratea 2007
Photovoltaic cell: working principle
“Conventional” photovoltaic cells are based p-n junction between semiconductors.
N-type siliconP-type silicon
ContinuousCurrent
Università di Roma Tor Vergata
Maratea 2007
Photovoltaic cell: short history
Russell Ohl (Bell Labs) discover the silicon p-n junction and the effect of light on the junction
Bell Labs researchers Pearson, Chapin, e Fuller demonstrated the photovoltaic cell with 4.5% efficiency
1941
1954
Università di Roma Tor Vergata
Maratea 2007
2007: Modern solar cell
Università di Roma Tor Vergata
Maratea 2007
Solar Energy Map
Università di Roma Tor Vergata
Maratea 2007
Solar SpectrumS
pec
tral
po
wer
den
sity
[(W
/m2 )
/nm
]
Wavelength [nm]
Università di Roma Tor Vergata
Maratea 2007
Efficiency
One of the most important parameters of the photovoltaic cell is the efficiency defined as:
EFFICIENCY = = Max electrical power produced by the cell
Total solar power impinging on the cell
10 W/dm2
Example:
1dm
1dm
= 10% 1 W
= 20% 2 W
It is important to increase as much as possbilethe efficiency.
Università di Roma Tor Vergata
Maratea 2007
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 givenillumination 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 thesolar cell (P=I*V) at each point between ISC and VOC and finding the maximum of P.
Fill form factorOCSC
MPMP
OCSC VIVI
VIPFF max
The overall efficiency of a solar cell is larger for larger FF
Università di Roma Tor Vergata
Maratea 2007
PHOTORESPONSIVITY
EXTERNAL QUANTUM EFFICIENCY
POWER CONVERSION EFFICIENCY
The photoresponsivity is defined as the photocurrent extracted from the solar cell divided by the incident power of the light at a certain wavelength.
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
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.
Figures of merit
Università di Roma Tor Vergata
Maratea 2007
Which are the factors influencing the cell efficiency ?
EFFICIENCY
MATERIALS
SiliconGaAsCdTe…..…..
TECHNOLOGY
Single junctionsMultiple junctions….….
Università di Roma Tor Vergata
Maratea 2007
Materials for photovoltaic cells Bulk semiconductors
– Silicon• Single crystal • Multi crystalline
– Gallium arsemide (GaAs)– Other III-V semiconductors
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
CdTe
Università di Roma Tor Vergata
Maratea 2007
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 Nanotecnology
Università di Roma Tor Vergata
Maratea 2007
Andamento dell’efficienza delle celle fotovoltaiche
0
4
8
12
16
20
24
28
32
1975 1980 1985 1990 1995 2000 2005
EF
FIC
IEN
CY
(%
)
YEAR
Multijunctions (GaAs ed altri)
Monocristalline Silicon
Multicristalline silicon
CIS e CIGS
a-Silicon
CdTe
Organic: polimer
Organic: DSSC
36
Max lab efficiency on small size solar cells40
Record ~40%
Università di Roma Tor Vergata
Maratea 2007
Max and module level efficiencies
Università di Roma Tor Vergata
Maratea 2007
Solar Cell Spectral Response
Università di Roma Tor Vergata
Maratea 2007
Multijunctions
Cell 1
Cella 2
Cella 3
Eg1
Eg2<Eg1
Eg3<Eg2
Eg=1.9eV
Eg=1.42eV
Eg=0.7eV
Università di Roma Tor Vergata
Maratea 2007
MultiJunction a-Si solar CellsAmorphous silicon absorption coefficient is larger than Silicon. We can then use thin layers of a-Si (few microns).
Multijunctions solar cells
TCO
p
i
n
n
p
i
TCO
aSi
1 m
Università di Roma Tor Vergata
Maratea 2007
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.
Università di Roma Tor Vergata
Maratea 2007
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
Università di Roma Tor Vergata
Maratea 2007
Structure of Dye Sensitized Solar Cells
Dye Molecules on TiO2
Glass Substrate
Electrolyte I-/I-3
Catalyst (Platinum, graphite)
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)
Transparent Conducting Oxide (ITO or SnO2:F)
Transparent Conducting Oxide (ITO or SnO2:F)
nanocristalline TiO2
Università di Roma Tor Vergata
Maratea 2007
-0.5
0.0
0.5
E (V)
TiO2 Dye
S*
So/S+
E [LUMO (S*)] – EC [TiO2] > E exciton binding energy
Exciton
The “nano” object: Nanocristalline TiO2
Very large effective area available for dye-TiO2 interaction
Monocrystaline
Nanostructured
Strong increase of optical density of the nanoporous film with respect to the monocrystalline film
Università di Roma Tor Vergata
Maratea 2007
TiO2 Electrolyte CathodeTCO
Injection
V Max
Ox
So/S+ (HOMO)
S* (LUMO)
Dye
hυ
Principle of Dye Sensitized Solar Cells
Load
-0.5
0.0
0.5
E (
V)
3I- I-3
Fermi Level in TiO2
No permanent chemical transformation in the materials composing the cell
Università di Roma Tor Vergata
Maratea 2007
Competition Dynamic in DSSC
(source: O’Regan)
Università di Roma Tor Vergata
Maratea 2007
• Efficiencies: max 10 - 11% (in labs)• Lifetimes: few years
• The optoelectronic properties (especially the absorption spectrum) can be tuned through the chemical design of novel dyes, even multicolored
Dyes (1)
Nikkei
Università di Roma Tor Vergata
Maratea 2007
Dyes (2)
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.
Synthetic DyesDyes 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 spectrumThe molecule on the left:cis-bis(isothiocyanato)bis(2,2-bipyridyl-4-carboxilicacid-4-tetrabutylammonium carboxilate)ruthenium(II)
Università di Roma Tor Vergata
Maratea 2007
Conventional Cell Production
Fornace industriale per la produzione di lingotti di silicio
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
Sistema di ricerca per la produzione di celle CIS
Apparato industriale per la diffusione
Università di Roma Tor Vergata
Maratea 2007
DSSC Fabrication: “cooking recepies”
MOVIE: downloadable from http://www.freenergy.uniroma2.it
Università di Roma Tor Vergata
Maratea 2007
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!)
Università di Roma Tor Vergata
Maratea 2007
How to create a DSSC
4) Platinum on the counter electrode
5) Assemble the two pieces (25-50 m distance)
6) Fill with electrolyte KI/I2
7) Seal the solar cell
Università di Roma Tor Vergata
Maratea 2007
Is it possible to use printing technologies ?
Università di Roma Tor Vergata
Maratea 2007
Photovoltaic performance
• QE 70-80%
• Jsc = 15-20 mA cm-2
• Voc = 0.8 V
= 5-10%
• Challenges:– Improving photocurrent: dyes, light management– Improving photovoltage : minimise recombination
alternative materials
Voltage
Source: J. Nelson
Absorption Spectra
Università di Roma Tor Vergata
Maratea 2007
DSSC performance
Università di Roma Tor Vergata
Maratea 2007Source: M. McGhee
Università di Roma Tor Vergata
Maratea 2007
Università di Roma Tor Vergata
Maratea 2007
Organic PhotovoltaicsDSSC Façade System at the CSIRO Energy CentreNewcastle, Australia
KONARKA
CELLA FLESSIBILE SU PET
Università di Roma Tor Vergata
Maratea 2007
DSSC
Inorganic MaterialsConcerns:
Use of toxic metals like CadmiumUse of toxic gasses in the manufacturing of PV, silane, hydrogen selenideCan the materials be recycled or are they destined for landfills
Università di Roma Tor Vergata
Maratea 2007
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.
Università di Roma Tor Vergata
Maratea 2007
Nanobjects for very high efficiency !!!
– 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.
There are two fundamental ways to utilize the hot carriers for enhancing the efficiency of photon conversion.
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
cont
act contact
semiconductor
VOCgap
cont
act contact
VOCgap
ISC
ISC
Università di Roma Tor Vergata
Maratea 2007
Examples
Università di Roma Tor Vergata
Maratea 2007
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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