Università di Roma Tor Vergata Maratea 2007 Nanoscale Photovoltaics Aldo Di Carlo Dipartimento di Ingegneria Elettronica Università di Roma Tor Vergata

Università di Roma Tor Vergata

Maratea 2007

Nanoscale Photovoltaics

Aldo Di Carlo

Dipartimento di Ingegneria ElettronicaUniversità di Roma “Tor Vergata”

[email protected]


Maratea 2007

Example of photovoltaic systems

PHOTOVOLTAIC CELL


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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.


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Photovoltaic cell: working principle

“Conventional” photovoltaic cells are based p-n junction between semiconductors.

N-type siliconP-type silicon

ContinuousCurrent


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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


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2007: Modern solar cell


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Solar Energy Map


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Solar SpectrumS

pec

tral

po

wer

den

sity

[(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:

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.


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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 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


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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


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Which are the factors influencing the cell efficiency ?

EFFICIENCY

MATERIALS

SiliconGaAsCdTe…..…..

TECHNOLOGY

Single junctionsMultiple junctions….….


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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


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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 Nanotecnology


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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%


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Max and module level efficiencies


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Solar Cell Spectral Response


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Multijunctions

Cell 1

Cella 2

Cella 3

Eg1

Eg2<Eg1

Eg3<Eg2

Eg=1.9eV

Eg=1.42eV

Eg=0.7eV


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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


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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

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


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-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


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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


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Competition Dynamic in DSSC

(source: O’Regan)


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• 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


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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)


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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


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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 m 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

• 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


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DSSC performance


Maratea 2007Source: M. McGhee


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Organic PhotovoltaicsDSSC Façade System at the CSIRO Energy CentreNewcastle, Australia

KONARKA

CELLA FLESSIBILE SU PET


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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


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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 !!!

– 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


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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

Documents

Università di Roma Tor Vergata Maratea 2007 Nanoscale Photovoltaics Aldo Di Carlo Dipartimento di Ingegneria Elettronica Università di Roma Tor Vergata