Dr. Mike Nofziger Professor College of Optical Sciences University of Arizona

REU/RETOptics Research Workshop 2014

Workshop #3

Solar Energy, Solar OvensOptical Detectors and Solar Cells

Dr. Mike NofzigerProfessor

College of Optical SciencesUniversity 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 2014Workshop 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 2014Workshop 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 2014Workshop 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 2014Workshop 3-4

www.eia.gov

Energy vs. Power:

● Energy is defined as the capacity for doing work. - Fundamental units of energy:

- Joule, Calorie, British Thermal Unit (BTU)1 J = 0.23889 calories1 J = 0.947816x10-3 BTU

● Power is defined as the rate of using energy: - Fundamental units of power:

- Watt, Horsepower1 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 2014Workshop 3-5

1Wh 1W 1hour 1J s 1hour 60min/h 60sec /min 3600J

energyPtime

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 2014Workshop 3-6

Energy per photon of light: where h is Planck’s

constant h = 6.626x10-34 J-s

3 -33

1W 1J s 1mJ J1mW 1s 10 J 1mJ ; =10 =1mW10 mW 1W 1s s

E P

-1

134 81 915

3

Photons/sec = power energy photon energy sec photon energy = photons/sec

6.626 10 3 10 /1 / 10= 1 .001 3.26 10 sec10 650 1

Js m sW J s hc J nmmW photonsmW W s nm m

hcE h

Our Sun:

● The “ROY G BIV” solar spectrum:

● The Ideal Blackbody (solar) spectrum:

Dr. Mike Nofziger 2014Workshop 3-7

81 0 71 0 61 0 51 0 41 0 31 0 21 0 1 0 11 0 - 21 0 - 31 0 - 41 0 - 51 0 - 61 0 - 71 0 - 81 0 - 91 0 - 101 0 - 1 11 0 - 121 0 - 1 31 0 - 141 0 - 151 0 - 1 61 0 -1

700 600 500 400W a v e le n g th ( in n a n o m e te rs)

V is ib l e sp e c tr u m

W a v e le n g th (m )

F r eq u e n c y (H z )10 21 0 31 0 41 0 51 0 61 0 71 0 810 91 0 1010 111 0 121 0 131 0 1 41 0 151 0 161 0 1710 1810 191 0 201 0 211 0 221 0 231 0 2410

R a d io w av e s X ra y s G a m m a r ay s

Our Sun: ● The “Real” solar spectrum:

● The sun delivers ≈ 1000W/m2 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. http://org.ntnu.no/solarcells/pages/Chap.2.php Dr. Mike Nofziger 2014

Workshop 3-8

Our Sun: ● The Ability to harness solar energy by concentrating it:

Dr. Mike Nofziger 2014Workshop 3-9

Our Sun: ● The Ability to harness solar energy by using solar cells:

Dr. Mike Nofziger 2014Workshop 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 2014Workshop 3-11

max

max

3000

3000

m K T

m KT

max

3000 10000.5 5006000 1

m K nmm nmK m

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 2014Workshop 3-12

max

5[ ] 400 32 273 4779

30006

477

T K C C K

m Km

K

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

2 4 6 8 10 12 14 16 18-0.1

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

Wavelength (m)

Tran

smis

sion

(%)

Xerox Overhead Transparency Transmission Spectra (8 cm-1)

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 2014Workshop 3-14

Photoconductors—Physical Construction:

Ref: “Photoconductive Cells”

Dr. Mike Nofziger 2014Workshop 3-15

Photoconductors—Current-Voltage Characteristics:

dynamic

V I RVRI

dVRdI

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 2014Workshop 3-16

Photodiodes—Physical Construction:

A 40 Gb/s “Optical Receiver” !!

“Official Symbol”

Dr. Mike Nofziger 2014Workshop 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 2014Workshop 3-18

Photodiodes—Basic Physics:

ghch E

max1.24

g g

hcE E

max

For Si, 1.12

1.1 mgE eV

u

METAL CONTACT

N-TYPE BULK SILICON

A-R COAT

ACTIVE AREA

SiO 2

P+ DIFFUSION

DEPLETION REGION

Dr. Mike Nofziger 2014Workshop 3-19

Photodiodes—Current-Voltage Characteristics:

1qV kTdark oI I e

p e d

e

I qE Ahc

qhc

1qV kTTOTAL o pI I e I

The “Shockley diode equation”

Io is the reverse saturation currentV is the voltage across the junction

Photocurrent generated by irradiance Ee (W/m2)

Photocurrent generated by optical power ϕe (W)

Dr. Mike Nofziger 2014Workshop 3-20

Photodiodes—Current-Voltage Characteristics:

1qV kTTOTAL o pI I e I

Dr. Mike Nofziger 2014Workshop 3-21

Photodiodes—Use in an Electrical Circuit:

Operated at V = 0 “zero-bias”: Output is very linear over 7-decades of fluxOperated at –V “reverse-bias”: Capacitance decreases, speed increases

Operated at I≈0 “open-circuit”: The open-circuit voltage is logarithmic with flux:

o

phooc I

IIqkTV ln

NOT the preferred way to operate a photodiode!

Dr. Mike Nofziger 2014Workshop 3-22

Transimpedance Amplifier (“TIA”) • converts current to voltage

Photodiodes—Use in a Commercial “TIA”:

Dr. Mike Nofziger 2014Workshop 3-23

Thorlabs PDA 200Photodiode 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 2014Workshop 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:

1qVkT

o phI I e I

ph ph e e dI q q q E Ahc hc

Dr. Mike Nofziger 2014Workshop 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.

www.keithley.com

Dr. Mike Nofziger 2014Workshop 3-26

P V I