1. Nanoscale Photovoltaics
Aldo Di Carlo
Dipartimento di Ingegneria Elettronica
Università di Roma “Tor Vergata”

2. Example of photovoltaic systems
PHOTOVOLTAIC CELL

3. 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.

4. Photovoltaic cell: working principle
Continuous Current
N-type silicon
P-type silicon
“Conventional” photovoltaic cells are based p-n junction between semiconductors.

5. 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

6. 2007: Modern solar cell

7. Solar Energy Map

8. Solar Spectrum
Spectral power density [(W/m2)/nm]
Wavelength [nm]

9. 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 possbile the efficiency.

10. Figures of merit Fill form factor
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
The overall efficiency of a solar cell is larger for larger FF

11. 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.

12. Which are the factors influencing the cell efficiency ?
MATERIALS
TECHNOLOGY
Silicon
GaAs
CdTe
…..
Single junctions
Multiple junctions ….

13. 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

14. 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

15. Andamento dell’efficienza delle celle fotovoltaiche
Max lab efficiency on small size solar cells 40 36
Multijunctions (GaAs ed altri)
Record ~40% 32 28
Monocristalline Silicon 24
Multicristalline silicon 20
EFFICIENCY (%) 16 12
CdTe 8
Organic: DSSC
CIS e CIGS 4
Organic: polimer
a-Silicon
1975
1980
1985
1990
1995
2000
2005
YEAR

16. Max and module level efficiencies

17. Solar Cell Spectral Response

18. Multijunctions Eg=1.9eV Eg=1.42eV Cell 1 Eg1 Eg=0.7eV Cella 2

19. 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 i n
aSi
1 mm
Multijunctions solar cells

20. 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.

21. 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

22. 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 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)

23. The “nano” object: Nanocristalline TiO2
E (V) S*
E [LUMO (S*)] – EC [TiO2] > E exciton binding energy
-0.5
Monocrystaline
Nanostructured
Strong increase of optical density of the nanoporous film with respect to the monocrystalline film
0.0
Exciton
0.5
So/S+
TiO2
Dye
Very large effective area available for dye-TiO2 interaction

24. Principle of Dye Sensitized Solar Cells
No permanent chemical transformation in the materials composing the cell
TCO
TiO2
Dye
Electrolyte
Cathode
Injection
S* (LUMO)
Fermi Level in TiO2
-0.5
E (V)
V Max hυ
0.0
3I-
I-3 Ox
0.5
So/S+ (HOMO)
Load

25. Competition Dynamic in DSSC
(source: O’Regan)

26. Dyes (1)
The optoelectronic properties (especially the absorption spectrum) can be tuned through the chemical design of novel dyes, even multicolored
Efficiencies: max % (in labs)
Lifetimes: few years
Nikkei

27. 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-4-tetrabutylammonium 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.

28. 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

29. DSSC Fabrication: “cooking recepies”
MOVIE: downloadable from

30. How to create a DSSC
1-2) Put TiO2 on ITO and oven 450 oC (Sintering)
3) Sinterizer Impregnation (immerge the cell in the blackberries!)

31. 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

32. Is it possible to use printing technologies ?

33. Photovoltaic performance
Absorption Spectra
QE  70-80%
Jsc = 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

34. DSSC performance

35. Source: M. McGhee

37. Organic Photovoltaics
DSSC Façade System
at the CSIRO Energy Centre
Newcastle, Australia
CELLA FLESSIBILE SU PET
KONARKA

38. 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

39. 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].
A. J. Nozik. Annu. Rev. Phys. Chem. 52 (2001) 193.
R. T. Ross and A. J. Nozik. J. Appl. Phys. 53 (1982) 3813.
D. S. Boudreaux, F. Williams, and A. J. Nozik. J. Appl. Phys. 51 (1980) 2158.
P. T. Landsberg, H. Nussbaumer, and G. Willeke. J. Appl. Phys. 74 (1993) 1451.
S. Kolodinski, J. H. Werner, T. Wittchen, and H. J. Queisser. Appl. Phys. Lett. 63 (1993) 2405.
M. A. Green. Third Generation Photovoltaics. (Bridge Printery, Sydney) 2001.

40. 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
contact
gap
contact
VOC
semiconductor
ISC
contact
gap
contact
VOC
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

41. Examples

42. 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