Nanoscale Photovoltaics Aldo Di Carlo Dipartimento di Ingegneria Elettronica Università di Roma “Tor Vergata” aldo.dicarlo@uniroma2.it

Example of photovoltaic systems PHOTOVOLTAIC CELL

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.

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

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

2007: Modern solar cell

Solar Energy Map

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

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.

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

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.

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

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

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

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

Max and module level efficiencies

Solar Cell Spectral Response

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

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

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.

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

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)

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

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

Competition Dynamic in DSSC (source: O’Regan)

Dyes (1) The optoelectronic properties (especially the absorption spectrum) can be tuned through the chemical design of novel dyes, even multicolored Efficiencies: max 10 - 11% (in labs) Lifetimes: few years Nikkei

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.

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

DSSC Fabrication: “cooking recepies” MOVIE: downloadable from http://www.freenergy.uniroma2.it

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!)

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

Is it possible to use printing technologies ?

Photovoltaic performance Absorption Spectra QE  70-80% Jsc = 15-20 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

DSSC performance

Source: M. McGhee

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

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

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.

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

Examples

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