Presentation on theme: "Università di Roma Tor Vergata Maratea 2007 Nanoscale Photovoltaics Aldo Di Carlo Dipartimento di Ingegneria Elettronica Università di Roma Tor Vergata."— Presentation transcript:
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 Example of photovoltaic systems PHOTOVOLTAIC CELL
Università di Roma Tor Vergata Maratea 2007 Componenti di un sistema fotovoltaico Cell Module 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 silicon P-type silicon Continuous Current
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
Università di Roma Tor Vergata Maratea : Modern solar cell
Università di Roma Tor Vergata Maratea 2007 Solar Energy Map
Università di Roma Tor Vergata Maratea 2007 Solar Spectrum Spectral power density [(W/m 2 )/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/dm 2 Example: 1dm = 10% 1 W = 20% 2 W It is important to increase as much as possbile the 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 I SC. I SC 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 (V OC ). V OC is the maximum voltage a solar cell can put out. · I MP and V MP are the current and voltage at the point of maximum power output of the solar cell. I MP and V MP can be determined by calculating the power output P of the solar cell (P=I*V) at each point between I SC and V OC and finding the maximum of P. Fill form factor 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 N e extracted at the electrodes divided by the number of photons N ph 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 Silicon GaAs CdTe ….. TECHNOLOGY Single junctions Multiple 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 dellefficienza delle celle fotovoltaiche EFFICIENCY (%) 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 cells 40 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
Università di Roma Tor Vergata Maratea 2007 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 TCO p i n n p i 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 TiO 2 Glass Substrate Electrolyte I - /I - 3 Catalyst (Platinum, graphite) Glass Substrate Why DSSC Nanocrystalline TiO 2 Meas. Setup: Indoor Stability: Indoor Hematine Structure of DSSCAssembling DSSC Meas. Setup: Outdoor Stability: Outdoor Principle of DSSC Final Assembling of DSSC Process Repeatability Enocyanine (E163) Transparent Conducting Oxide (ITO or SnO2:F) nanocristalline TiO 2
Università di Roma Tor Vergata Maratea E (V) TiO 2 Dye S* S o /S + E [LUMO (S*)] – E C [TiO 2 ] > E exciton binding energy Exciton The nano object: Nanocristalline TiO2 Very large effective area available for dye-TiO 2 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 TiO 2 Electrolyte Cathode TCO Injection V Max OxOx S o /S + (HOMO) S* (LUMO) Dye hυhυ Principle of Dye Sensitized Solar Cells Load E (V) 3I - I-3I-3 Fermi Level in TiO 2 No permanent chemical transformation in the materials composing the cell
Università di Roma Tor Vergata Maratea 2007 Competition Dynamic in DSSC (source: ORegan)
Università di Roma Tor Vergata Maratea 2007 Efficiencies: max % (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 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)
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 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
Università di Roma Tor Vergata Maratea 2007 How to create a DSSC 1-2) Put TiO 2 on ITO and oven 450 o C (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/I 2 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% J sc = mA cm -2 V oc = 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 2007 Source: M. McGhee
Università di Roma Tor Vergata Maratea 2007
Università di Roma Tor Vergata Maratea 2007 Organic Photovoltaics DSSC Façade System at the CSIRO Energy Centre Newcastle, Australia KONARKA CELLA FLESSIBILE SU PET
Università di Roma Tor Vergata Maratea 2007 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
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 . 1.A. J. Nozik. Annu. Rev. Phys. Chem. 52 (2001) R. T. Ross and A. J. Nozik. J. Appl. Phys. 53 (1982) D. S. Boudreaux, F. Williams, and A. J. Nozik. J. Appl. Phys. 51 (1980) P. T. Landsberg, H. Nussbaumer, and G. Willeke. J. Appl. Phys. 74 (1993) S. Kolodinski, J. H. Werner, T. Wittchen, and H. J. Queisser. Appl. Phys. Lett. 63 (1993) 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 contact semiconductor V OC gap contact V OC gap I SC
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