Solar Electricity

Light energy, one photon at a time

Outline

Photoelectric Effect– The existence of photons– Light energy in bundles– Light energy creating an electric potential

Photovoltaic cells– Semiconductors– Energy band gaps– Silicon, Gallium Arsenide, Thin Films

Light is a wave, or is it?

Waves carry energy in proportion to their amplitude

Light wave amplitude is Intensity.So light energy should depend on intensityThe brighter the light, the more energy it

carries.

The Photoelectric Effect.

The Experiment.

Light source

Metal Plate

electrons

ElectricCurrent

Light making a current

Light shining on a metal can create a current: The Photoelectric effect.

Experiment:– VARY the light frequency (or wavelength) but

keep the intensity the same.– OBSERVE the Voltage required to stop a

current from flowing (the “stopping voltage”)– This should be a measure of the energy the

electrons have as they leave the metal plate.

Photoelectric effect and light frequency

Frequency vs. Stopping Voltage

0

1

2

3

4

5

6

0.00E+00 2.00E+14 4.00E+14 6.00E+14 8.00E+14 1.00E+15

Frequency (Hz)

Vol

ts

Analysis

Observations:– For each metal, there was a certain frequency below

which no amount of light could cause a current.

– Above that frequency, there was some current no matter how weak the light source.

– The energy the electrons in the current had directly depended on the frequency of the light

– Although MORE CURRENT could be produced with higher intensity light, the STOPPING VOLTAGE depended only on the frequency.

Einstein’s explanation

Published in 1905; Nobel Prize in 1921.Light energy comes in packets, called “photons”Each photon carries an energy equal to:

– Eph = h f, where f is the frequency, and h = 6.63 x 10 -34 J*s is called Planck’s constant.

The electrons absorb one photon at a timeThe electrons need a minimum of energy to escape

the metal surface, hence the minimum f.The photon energy above the minimum goes into

the energy of the current, hence the increasing V.

Photon Energy

Eph = h f– Red light has f = 4.5 x 1014 Hz– Ered = (6.63 x 10-34 J*s ) (4.5 x 1014 Hz)

• = 2.98 x 10-19 Joules!

We can’t feel that, but what about an electron?A new unit of energy:

– 1 eV, the amount of energy required to move a single electron across a 1 Volt electric potential.

– 1 eV = 1.6 x 10-19 Joules

Ered = 1.86 eV– Red photons have enough energy to charge up a 1.8 V battery!

The light energy spectrum

Light frequency corresponds to photon energy.

The most convenient unit is eV. h = 6.63 x 10-34J*s = 4.15 x 10-15eV*sVisible light from 1.8 (red) to 3 eV (violet)

Photovoltaic Cells

n-type Silicon

p-type Silicon

+ + + + +

- - - - - 1.5-2 V

electron

The p-n junction

PV-types and applications

Each cell produces .5 to 1.5 V, 8-24 cells in series produce 12 V. Highest efficiency – 23%

Types:– Crystalline Silicon: More efficient/expensive– Amorphous Silicon: Less efficient/expensive– Thin Film: Silicon or other semiconductors

Applications:– Remote locations– Homes more than ¼ of mile from electric grid.– Watches, calculators