Our Energy Challenge Billion people Billion people Can Nuclear Power Provide Energy for the Future? The answer is no! Number of nuclear reactors: 459 contributing less than 7% electricity (2525 billion kWh) but produces 66,500 tonnes of waste If the entire present world electricity demand were to be provided by nuclear power it requires 6557 reactors that produce 950,000 tons of waste. Even if we build 50 reactors/year it requires 130 years. The available ores would be exhausted within ten years Source: Nuclear Power: the Energy Balance by Jan-Willem Storm van Leeuwen and Philip Smith The solution is solar energy. Only 10 minutes of solar irradiation on the Earth’s surface is equal to the total yearly human consumption. Therefore, if we could accomplish harvesting merely a fraction of the solar energy reaching the Earth, we would solve many problems associated with the energy, and the global environment.

Total Primary Power vs Year 1990: 12 TW 2050: 28 TW (in the U.S. in 2002) 1-4 ¢ ¢ 6-8 ¢ 5-7 ¢ Production Cost of Electricity Cost, ¢/kW-h Unit = cent CO 2 Emissions Data from Vostok Ice Core Solar heat Solar energy Solar light

Solar Spectral Irradiance Irradiance (W/m 2 /µm Wavelength (µm) Air Mass 1.5 (solar zenith angle 48.19°) Ref. Power level in outer space (AM 0): 140 mW/cm 2 Power level on earth at sea level, with the sun at zenith (AM1.5): 100 mW/cm 2 Atmosphere can cut solar energy reaching earth by 50% and more

1839: Edmond Becquerel discovered photoconductivity phenomenon between the two electrodes immersed in an electrolyte solution under irradiation. 1876: Heinrich Hertz observed similar phenomenon from selenium : Research and development for the commercialization of photovoltaic system 1954: Bell Lab first introduced crystalline silicon sloar cell with  = 4% 1970s: 1 st Energy Crisis 1980s: Development of CdTe, CuInSe 2, and dye-sensitized solar cell (DSSC). 1990s: Polymer and organic cells History Types of Solar Cells Semiconductor Solar Cells –silicon solar cells single crystalline silicon solar cell polycrystalline silicon solar cell amorphous silicon solar cell –compound semiconductor solar cells III-V Group : GaAs, InP, GaAlAs, GaInAs etc. II-VI Group : CuInSe2, CdS, CdTe, ZnS etc. Dye-Sensitized Solar Cells Polymer-Based Solar Cells Organic Solar Cells Single Poly Crystalline High  Amorphous Si Compound Semiconductor Chemical II-VI (CdS, CdTe) III-V (GaAs, InP) I-III-VI (CuInSe 2 ) Organic Dye-sensitized (Grätzel cell) Polymer/QD-based Low cost Solar Cell High 

Bandgap and Theoretical Solar Cell Efficiency Bandgap and Solar Spectrum Theoretical Solar Cell Efficiency E g ~ 1.4 eV for optimum efficiency Solar generated electricity is dc, which can be used directly for appliances like lights and electric heaters For ac appliances, dc can be converted to ac using an inverter Extra energy can be stored in batteries or sold back to the electric company through the power grid

Solar cell efficiency - I SC -Im-Im -I 0 V m V oc V Maximum power (P max ) Rectangle V oc : Open circuit voltage I sc : Short circuit current V m, I m : Operating voltage at maximum power FF : fill factor (Efficiency,  ) (  ) = (P max / P in ) ×100 (%) P in = input power of light per second P max I m V m P in I sc V oc < 1 FF ≡ = (1240/λ = eV) Incident Photon-to-Current Conversion Efficiency Overall Efficiency Efficiency (%) Year crystalline Si amorphous Si DSSC CIS/CIGS CdTe Efficiencies

Solar Cell Efficiency Records Efficiencies measured under the global AM1.5 spectrum (1,000 W/m 2 ) at 25 ºC 1,000,000 mW/10,000 cm 2 = 100 mW/cm 2

p-n Junction Solar Cell This is the basis for all photovoltaic devices When the two types are brought into contact, electrons and holes are exchanged over a short region, the interface, known as depletion or junction region Front contact grid Anti-reflective coating n-type Back Contact p-type Junction Silicon Solar Cell

p-n Junction diode

p-n Junction Solar Cell J V V oc J sc Increasing optical power pn R Light (h ) V OC - + E

Parallel connection of cells, Series connection of cells, Series connection of 34–36 crystalline silicon cells are used in stand-alone applications for the charging of batteries. The series connection of these cells produces an open circuit voltage of around 18 V (depending on the detail of the cell design) and a maximum power point voltage of around 14–15 V. Connection of solar cells

Dye-Sensitized Photochemical Cell Transparent conducting glass (TCO) Electrolyte TiO 2 nanoparticles, ~20 nm, sintered to electrode Dye monolayer e-e- Electrode Counter Electrode 1. photon is absorbed in the dye layer creating an excited state electron transfers to the TiO 2 conducts through the particles to the electrode 2. electron returns from the counter electrode by ionic conduction through the electrolyte to the dye The diagram is schematic only. The layer thickness is about 15  m or about 750 times the particle diameter. Dye surface area is about 2000 times greater than the cell area. Transparent conducting glass (TCO) e-e-

The regenerative process in the dye solar cell consists of five steps: a ) The sensitizer S o absorbs a photon, and an electron is transferred to a higher lying energy level. The sensitizer is in the excited state S*. b) Injection of the excited electron into the conduction band of the semiconductor occurs within a femtosecond time scale. c) The electron percolates through the porous TiO 2 layer to the conductive support and passes the external load (electrical work) to the counter electrode. d) At the counter electrode the electron is transferred to triiodide to yield iodide. f) The iodide reduces the oxidized dye (S + ) to its original state (S o ). The device operates in a regenerative mode. B. O’Regan, M. Graetzel, Nature 353, 737 (1991) Nano-crystalline TiO 2 (particle size nm) High surface, roughness factor > 1000

Dye sensitized nanocrystalline solar cell (DSSC) FF = % Efficiency = 10.4 %

IPCE curves RuL'(NCS) 3 RuL 2 (NCS) 2 TiO2 (N3) Black dye % AM1.5 Efficiency 11.04% 65% AM % AM1.5 Efficiency 11.18% Efficiency 10.87% Potential (V) Current (mA/cm 2 ) I-V Curve & Overall efficiency Dyes N3, N719: 3 TBA + Black dye

Dye-sensitized solar cell Advantages ease of processing low cost large area flexible (unique properties), etc. Challenges efficiency stability manufacturing issues understanding device function Organic solar cell Si Solar cell

Recent developments Materials J sc (mA/cm 2 ) V oc (V) Fill factor Efficiency (%) Peak QE & Wavelength (nm) References Doped pentacene Heterojunction Doped pentacene Homojunction Cu-PC/C60 MDMO-PPV/PCBM Dye-sensitized cell with OMeTAD HTL a-Si Single-crystal Si nm nm nm nm nm ~ 90 % > 90 % J. H. Schon et al. Synth. Met. (2001) J. H. Schon et al. Nature 403 (2000) P. Peumans et al. APL 79, 126 (2001) S. E. Shaheen et al. APL 78, 841 (2001) J. Kruger et al. APL 79, 2085 (2001)

Structures of organic solar cells Glass ITO Single crystal Cathode Glass ITO D + A blend Cathode Glass ITO Acceptor (A) Cathode Donor (D) Glass ITO Exciton blocking layer Cathode D A  p = 2.5 % Sean E. Shaheen, et al., Appl. Phys. Lett. 78, 841 (2001) Interpenetrating networks of D/A bulk heterojunctions

Processes in an organic solar cell Incident photons separated charges at electrodes Conversion stepsLoss mechanism light absorption exciton (e-h pair) creation exciton diffusion (~ 10 nm) charge separation charge transport charge collection reflection, transmission recombination of exciton recombination of charge carriers limited mobility of charge carriers recombination near electrodes barriers at electrodes

Absorption and Solar Spectral Irradiance