Presentation on theme: "Nanoscale Photovoltaics"— Presentation transcript:
1Nanoscale Photovoltaics Aldo Di CarloDipartimento di Ingegneria ElettronicaUniversità di Roma “Tor Vergata”
2Example of photovoltaic systems PHOTOVOLTAIC CELL
3Componenti di un sistema fotovoltaico ModuleCellArrayThe photovoltaic system is made of an array of photovoltaic modules with additional electronics like charge controllers, inverters etc.
4Photovoltaic cell: working principle Continuous CurrentN-type siliconP-type silicon“Conventional” photovoltaic cells are based p-n junction between semiconductors.
5Photovoltaic cell: short history 1941Russell Ohl (Bell Labs) discover the silicon p-n junction and the effect of light on the junction1954Bell Labs researchers Pearson, Chapin,e Fuller demonstrated the photovoltaic cell with 4.5% efficiency
8Solar SpectrumSpectral power density [(W/m2)/nm]Wavelength [nm]
9EfficiencyOne of the most important parameters of the photovoltaic cell is the efficiency defined as:Max electrical power produced by the cellEFFICIENCY = h =Total solar power impinging on the cellExample:10 W/dm2h = 10%1 W1dmh = 20%2 W1dmIt is important to increase as much as possbile the efficiency.
10Figures 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 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 factorThe overall efficiency of a solar cell is larger for larger FF
11Figures 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 EFFICIENCYThe 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 cellPOWER CONVERSION EFFICIENCYThe 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.
12Which are the factors influencing the cell efficiency ? MATERIALSTECHNOLOGYSiliconGaAsCdTe…..Single junctionsMultiple junctions….
13Materials for photovoltaic cells Bulk semiconductorsSiliconSingle crystalMulti crystallineGallium arsemide (GaAs)Other III-V semiconductorsCdTeThin Films semiconductorsAmorphous 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
14Beyond 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 % ?ConcentrationMultijunctionNo thermal equilibriumNanotecnology
15Andamento dell’efficienza delle celle fotovoltaiche Max lab efficiency on small size solar cells4036Multijunctions (GaAs ed altri)Record ~40%3228Monocristalline Silicon24Multicristalline silicon20EFFICIENCY (%)1612CdTe8Organic: DSSCCIS e CIGS4Organic: polimera-Silicon1975198019851990199520002005YEAR
19MultiJunction a-Si solar Cells Amorphous silicon absorption coefficient is larger than Silicon. We can then use thin layers of a-Si (few microns).TCOpinaSi1 mmMultijunctions solar cells
20Photovoltaic generations First generation refers to high quality and hence lowdefect single crystal photovoltaic devices these havehigh efficiencies and are approaching the limitingefficiencies for single band gap devicesSecond generation technology involves low cost and low energyintensity growth techniques such as vapour deposition and electroplatingThird generation multiple energy threshold devices;modification of the incident spectrum; and use of excessthermal generation to enhance voltages or carrier collection.
21What about nanobjects ?Nanobjects can be use to avoid silicon in II generation photovoltaics and reduce the cost of the cellNanobjects play a fundamental role to develop III generation photovoltaics
22Structure of Dye Sensitized Solar Cells Glass SubstrateTransparent Conducting Oxide (ITO or SnO2:F)Catalyst (Platinum, graphite)Electrolyte I-/I-3Dye Molecules on TiO2nanocristalline TiO2Transparent Conducting Oxide (ITO or SnO2:F)Glass SubstrateWhy DSSC Nanocrystalline TiO2 Meas. Setup: Indoor Stability: Indoor HematineStructure of DSSC Assembling DSSC Meas. Setup: Outdoor Stability: OutdoorPrinciple of DSSC Final Assembling of DSSC Process Repeatability Enocyanine (E163)
23The “nano” object: Nanocristalline TiO2 E (V)S*E [LUMO (S*)] – EC [TiO2] > E exciton binding energy-0.5MonocrystalineNanostructuredStrong increase of optical density of the nanoporous film with respect to the monocrystalline film0.0Exciton0.5So/S+TiO2DyeVery large effective area available for dye-TiO2 interaction
24Principle of Dye Sensitized Solar Cells No permanent chemical transformation in the materials composing the cellTCOTiO2DyeElectrolyteCathodeInjectionS* (LUMO)Fermi Level in TiO2-0.5E (V)V Maxhυ0.03I-I-3Ox0.5So/S+ (HOMO)Load
26Dyes (1)The optoelectronic properties (especially the absorption spectrum) can be tuned through the chemical design of novel dyes, even multicoloredEfficiencies: max % (in labs)Lifetimes: few yearsNikkei
27Dyes (2)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)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.
28Conventional Cell Production Sistema di ricerca per la produzione di celle CISFornace industriale per la produzione di lingotti di silicioApparato industriale per la diffusioneApparati per la fabricazione di celle al silicio amorfo(Uni. Toledo)PECVD, hot-wire, sputtering13.56 MHz excitationGas handling for SiH4, CH4, PH3, B2H6, NH3Gas scrubber with toxic gas monitoring
29DSSC Fabrication: “cooking recepies” MOVIE: downloadable from
30How to create a DSSC1-2) Put TiO2 on ITO and oven 450 oC (Sintering)3) Sinterizer Impregnation (immerge the cell in the blackberries!)
31How to create a DSSC 4) Platinum on the counter electrode 5) Assemble the two pieces (25-50 mm distance)6) Fill with electrolyte KI/I27) Seal the solar cell
37Organic Photovoltaics DSSC Façade Systemat the CSIRO Energy CentreNewcastle, AustraliaCELLA FLESSIBILE SU PETKONARKA
38DSSC Inorganic Materials Concerns: Use of toxic metals like Cadmium Use of toxic gasses in the manufacturing of PV, silane, hydrogen selenideCan the materials be recycled or are they destined for landfills
39Photovoltaic with nanobjects Other approaches to exceed the Shockley-Queisser limit includehot 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 .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.
40Nanobjects for very high efficiency !!! There are two fundamental ways to utilize the hot carriers for enhancing the efficiency of photon conversion.Enhanced photovoltageCarriers 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.ISCcontactgapcontactVOCsemiconductorISCcontactgapcontactVOCIn 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
42Fundings and perspectives 20 x 20 x 20 EU ruleBy 2020 EU should reduce by 20% the CO2 emission and increase the 20% renewable energiesThis means $$ for research in this fieldModern 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