REU/RET Optics Research Workshop 2014 Workshop #3 Solar Energy, Solar Ovens Optical Detectors and Solar Cells Dr. Mike Nofziger Professor College of Optical Sciences University of Arizona Dr. Mike Nofziger 2014

Workshop #3 Outline: ● Solar Energy - Basics of Energy - Our Sun - The solar spectrum - The greenhouse effect - ● Solar Ovens ● Optical Detectors ● Solar Cells Dr. Mike Nofziger 2014 Workshop 3-1

Basics of Energy: ● Two fundamental types of energy: - Potential Energy : “stored” energy (work could be done with this available energy) - Kinetic Energy : “working” energy (work is being done with this energy) ● Forms of Energy: - Light (radiant) - Heat (thermal) - Motion (kinetic) - Electrical - Chemical - Nuclear - Gravitational Dr. Mike Nofziger 2014 Workshop 3-2

Basics of Energy: ● Renewable Energy Sources: - Solar energy → electricity or heat - Wind - Geothermal energy from heat inside the Earth - Biomass from plants - firewood, wood waste - ethanol from corn - biodiesel from vegetable oil - Hydropower from hydro-turbines at a dam ● Non-Renewable Energy Sources: - Fossil fuels - oil - natural gas - coal Dr. Mike Nofziger 2014 Workshop 3-3

Basics of Energy: ● Energy is conserved - Scientifically speaking, “Conservation of energy” does not mean “saving energy” “Law of Conservation of Energy” - The total amount of energy in a closed system remains constant. - Energy does not disappear, or “get used up.” - Energy is changed from one form to another when it is used. Dr. Mike Nofziger 2014 Workshop 3-4

Energy vs. Power: ● Energy is defined as the capacity for doing work. - Fundamental units of energy: - Joule, Calorie, British Thermal Unit (BTU) 1 J = calories 1 J = x10 -3 BTU ● Power is defined as the rate of using energy: - Fundamental units of power: - Watt, Horsepower 1 Watt ≡ 1 Joule/sec. 1 hp = 746 watts - Therefore, energy ≡ power x time - An equivalent unit of energy is: Watt·hour (Wh), kilo-Watt·hour (kWh) Units! (“love ‘em” or “hate ‘em”…..teach your students to “love ‘em”!) Dr. Mike Nofziger 2014 Workshop 3-5

Example of Power, Energy, and Photons/sec: Each photon of light carries a specific amount of energy: Energy and Power in a Laser Beam: “A typical red laser pointer emits 3-3 mW of power, at a wavelength of 650 nm. (For simplicity, assume the power is 1 mW)” How much energy is delivered by this laser beam, in 1 sec? How many photons per second are in this laser beam? Dr. Mike Nofziger 2014 Workshop 3-6 Energy per photon of light: where h is Planck’s constant h = 6.626x J-s

Our Sun: ● The “ROY G BIV” solar spectrum: ● The Ideal Blackbody (solar) spectrum: Dr. Mike Nofziger 2014 Workshop 3-7

Our Sun: ● The “Real” solar spectrum: ● The sun delivers ≈ 1000W/m 2 to the surface of the Earth! ● The Earth receives more energy from the Sun in just one hour than the world uses in a whole year. Dr. Mike Nofziger 2014 Workshop 3-8

Our Sun: ● The Ability to harness solar energy by concentrating it: Dr. Mike Nofziger 2014 Workshop 3-9

Our Sun: ● The Ability to harness solar energy by using solar cells: Dr. Mike Nofziger 2014 Workshop 3-10

Basis for the Heating in a Solar Oven (a.k.a. a “Greenhouse Effect”): The wavelength of peak output from a blackbody (an ideal emitter, much like our sun) is given by: where T(K) = T(°C) + 273° The surface temperature of our sun is ≈ 6000K The wavelength where our sun emits most energy is, therefore: 500 nm is in the green portion of the visible spectrum. The peak sensitivity of human (daylight) vision is at 550nm………?! Dr. Mike Nofziger 2014 Workshop 3-11

Basis for the Heating in a Solar Oven (a.k.a. a “Greenhouse Effect”): The wavelength of peak output from a solar oven cavity, at T≈ 400°F : Dr. Mike Nofziger 2014 Workshop 3-12

Basis for the Heating in a Solar Oven (a.k.a. a “Greenhouse Effect”): Typical “window” material for a student solar oven is a single sheet of Mylar (Xerox Overhead Transparency): High transmission in the visible/near IR spectrum Low transmission in the thermal IR spectrum (i.e. at 6 μm) - The cavity absorbs visible light but has trouble emitting (radiating) thermal energy at 6 μm, therefore the cavity heats up. Basic “Greenhouse Effect” (car interiors, the Earth, etc.): Dr. Mike Nofziger 2014 Workshop 3-13 Fig. A.3. Infrared transmission spectrum of one layer of Mylar for wavelengths from 2 to 18 micrometers

Photoconductors—Physical Construction: Known as: “Photoresistors”, “Light-Dependent Resistors (LDR)” “Photocells” …………. Photoconductors! “Official Symbol” Most common semiconductor material used for detection of visible light is CdS (also CdSe). Dr. Mike Nofziger 2014 Workshop 3-14

Photoconductors—Physical Construction: Ref: “Photoconductive Cells”Photoconductive Cells Dr. Mike Nofziger 2014 Workshop 3-15

Photoconductors—Current-Voltage Characteristics: The resistance is the inverse of the slope of this curve. …where fc refers to “foot-candles” (a measure of the amount of visible light per unit area) Dr. Mike Nofziger 2014 Workshop 3-16

Photodiodes—Physical Construction: A 40 Gb/s “Optical Receiver” !! “Official Symbol” Dr. Mike Nofziger 2014 Workshop 3-17

Photodiodes—Basic Properties: ● p-n junction (p-side ≡ “anode”, n-side ≡ “ cathode”) ● Built-in electric field (depletion region) separates the electrons and holes (electrons → p-side, holes → n-side) ● Photons absorbed (ideally in or near the depletion region) create electron-hole pairs ● Built-in electric field separates the electrons and holes before they recombine, producing a photocurrent (electrons → n-side, holes → p-side) ● I-V curve is very non-linear ● The photocurrent is linear with photon flux over 7-decades! ● Most common semiconductor material used to make photodiodes (for detection of visible light) is Silicon (Si). Dr. Mike Nofziger 2014 Workshop 3-18

Photodiodes—Basic Physics: Dr. Mike Nofziger 2014 Workshop 3-19

Photodiodes—Current-Voltage Characteristics: The “Shockley diode equation” I o is the reverse saturation current V is the voltage across the junction Photocurrent generated by irradiance E e (W/m 2 ) Photocurrent generated by optical power ϕ e (W) Dr. Mike Nofziger 2014 Workshop 3-20

Photodiodes—Current-Voltage Characteristics: Dr. Mike Nofziger 2014 Workshop 3-21

Photodiodes—Use in an Electrical Circuit: Operated at V = 0 “zero-bias”: Output is very linear over 7-decades of flux Operated at –V “reverse-bias”: Capacitance decreases, speed increases Operated at I≈0 “open-circuit”: The open-circuit voltage is logarithmic with flux: NOT the preferred way to operate a photodiode! Dr. Mike Nofziger 2014 Workshop 3-22 Transimpedance Amplifier (“TIA”) converts current to voltage

Photodiodes—Use in a Commercial “TIA”: Dr. Mike Nofziger 2014 Workshop 3-23 Thorlabs PDA 200 Photodiode Amplifier (“TIA”)

Basics of Solar Cells: ● A solar cell is a Photovoltaic (“PV”) detector: - is made of Silicon (not silicone!!) - absorbs light from ≈ 350nm – 1100nm - the absorption of light “frees up” electrons - This creates a voltage at the terminals of the cell (the “Open-Circuit” voltage) - If a load resistor is connected to the cell, a current will flow (the “Photocurrent”) - If the cell’s terminals are shorted, the maximum current will flow (the “Short-Circuit” current) Dr. Mike Nofziger 2014 Workshop 3-24

Basics of Solar Cells: ● The IV Curve of a PV detector is given by: ● The Photocurrent of a PV detector is given by: Dr. Mike Nofziger 2014 Workshop 3-25

Basics of Solar Cells: ● The Power (Watts) that the cell can produce is given by: ● Because of internal resistance in the cell, the maximum power you can generate is across a load resistance equal to the internal resistance. Dr. Mike Nofziger 2014 Workshop 3-26