Solar: Why and How? Opportunities and Challenges ELCT 101 Solar Module Kickoff

Problem Statement There is too much dependence on foreign oil!! Energy Production Increased production More sustainable production Energy Usage More efficient systems- More responsible usage-clean?

Global warming, environmental fallout 10’s years infrastructure The Green Economy >107 years Carbon emissions Fossil Fuels Ethanol Global warming, environmental fallout Sun Water cycle Water Vapor emissions Hydrogen Production Photovoltaics/wind 10’s years infrastructure Production -days CLEAN, RENEWABLE ENERGY!! Fuel storage transportation, power distribution

SOLAR ENERGY PRODUCTION

Photoelectrochemical? The Dream Shine light on it and you get lots of clean fuel/power How much? How quickly? What kind? Payback time? Other? Photoelectrochemical? Hydrogen Photovoltaic Electricity

Solar Energy Production Light Absorption in Semiconductors Energy re-emitted Light/heat or both Ec Elight Bandgap Eg If Elight >Bandgap ABSORPTION! Ev h+ e- Electric field is simplest way to separate charge Pn junction is traditional way Efficient absorber/emitter of light-Direct Gap Bandgap determines color of light Absorbed light→ electricity vs. heat/light

Solar Energy Production Charge Separation in Semiconductors ! Electric field is simplest way to separate charge Pn junction is traditional way Recombine Electrical energy released! Key is charge separation in electric field How to produce this field?

Producing an Electric Field-Junctions Charge separation at interface liquid Solid e- p-type e- n-type Redox energies in liquid e.g. H2O h+ h+ Solid-liquid junction Single solid type Liquid fills in all gaps continuous interface Solid-solid junction Same material different impurity levels Interface of two electrically different materials Potential Hill-like water in gravity Perfection/abruptness of interface desirable

Solid/Solid vs. Solid/Liquid Junction TiO2 -Barbe et al., Cahen et. al Challenges include Contacting each powder together Chemical stability of surface- Solid-solid more robust- difficult to produce 8 year energy payback time (single crystal 12-30%) Liquid can fill in gaps easily-can use powders Greater engineering challenge to achieve 10-20%

Solar Hydrogen Production Elight =hv H+/H2 H2O/O2 EF Ec Ev Non-corrosive counter electrode Semiconductor Water redox potentials h+ O2 evolved H2 evolved h+ Discovered with TiO2 electrodes in 1972* Now looking at making methanol from CO2 in water. *Fujishima, Honda, Nature, 1972, 238, 37

Dye-Sensitized Solar Cells-Gratzel Cells Elight dye e- mediator S solid semiconductor liquid Dye can change color sensitivity-photo film Everything is regenerated-can start again >10% efficient like a-Si solar cells

EFFICIENT ENERGY USAGE

Energy Usage EFFICIENCY! But coupled with payback time? Solid-state GaN single crystal LED lighting efficiency comparable to compact fluorescent (CFL) ~9-15% Much poorer payback time! SiC electrical power conversion devices gaining traction-TranSiC, Cree, Toyota Small devices-reasonable payback time Energy economies of scale? SiC suitable for harsh environments e.g. engine Emissions monitoring/controls

LIGHTING

Compact Fluorescent Lamps UV Blue Green Red UV emitted from Hg sources UV downconverted to visible using phosphors Well balanced white light required Efficiency of CFL limited by phosphors 254nm vs. 365nm→ 4.9eV vs. 3.4 eV (GaN) improve efficiency by ~20%

GaN-powder Based Phosphors Pump Elight_in UV Elight_out RED GaN:Eu Shi, Chandrashekhar et. al, J. Cryst. Gr. (2007) Elight_in Ec Fast energytransfer to impurity-lost as heat Bandgap Eg Impurity Elight_out Ev e- h+ Pump with above bandgap light-UV Emits light at lower energy red (Eu)/green (Er)/Blue (pure)=WHITE!

Novel “Solid-State” GaN Lighting Electroluminescent lamp with DC Electroluminescent lamp with AC Steckl et. al, IEEE STQE (2002) Cheap electrophoretically deposited electrodes/phosphors Solid/liquid OR solid/solid junction DC or AC voltage electroluminescent lamps Geometries similar to single crystal-performance?

EMISSIONS

Gas Sensing and Efficiency/Emissions Percent by volume (Building) Percent by Volume (automotive) Short Term Occupational Exposure Limit (ppm) Long Term Occupational Exposure Limit (ppm) Enforceable limit NO2 * <1ppm 1-10 ppm 20 1 10 CO2 <1% 20% H2O* <5% 10% O2 21% 3% CO* 2000 ppm 1200 50 <30 Higher O2 intake→ Greater Engine Efficiency Higher O2 intake → Higher NO2 emissions NO2 1ppm is considered harmful-enforceable? Europe deploys Urea for NO2 removal-controls

Graphene/SiC for Sensing Polar Molecules Graphene resistance changes in response to molecule on surface Electrical Probe Capacitances change in response to molecule on surface Graphene Single molecule sensitivity to NO2 observed in small samples of graphene/SiO2 Large area graphene films produced on SiC Other molecules can be sensed also H2O, CO etc.

Graphene/SiC for Sensing NO2 Chandrashekhar et. al, IEEE Sensors (2007) Sensed<1ppm NO2 using capacitive means Suitable for environmental/emissions controls Challenges-cross-sensitivity, durability?