36.16 - Physics at work: photovoltaic cells

Photovoltaic effect: Electron

flow caused by photons.

Solar cells are semiconductor devices that absorb

light and convert its energy into electricity. Since

sunlight is free and the operation of the cells has no

environmentally hazardous side effects, there is an

obvious appeal to their use. The challenge for

researchers and manufacturers is to lower their cost

and to raise their efficiency so that they may

effectively compete with oil, coal and other traditional

energy sources.

How do solar cells work? In Concept 1 we show a

typical n-on-p junction solar cell, the most common

type of cell. A wafer of p-type silicon has an element such as phosphorous diffused into

its upper surface. This results in an n-type material, one with mobile electrons, being

located above a p-type material, whose charge carriers are mobile holes. In short, this

is a p-n junction, or diode.

With solar cells, the concept of diffusion current is important. Near the junction, mobile

electrons from the n-region diffuse into the p-region, leaving positively charged donor

ions (holes) in their wake. These holes remain there and form a positive region.

Mobile holes also diffuse from the p-region in the other direction, into the n-region. This

flow of charge, a diffusion current, occurs for the same reason perfume diffuses across

a room, from where the concentration is higher to where the concentration is lower. The

result is a depletion zone near the junction. That is, the n-region is relatively depleted of

mobile electrons and the p-region is depleted of the same number of holes.

The effects of this diffusion are shown in Concept 2. After enough charges move,

equilibrium is reached: The result in the depletion zone is a built-in electric field that

points from the n-region toward the p-region. This field opposes any further motion of

charge, and the diffusion current quickly stops.

In an earlier section, we discussed how placing a p-n junction in a circuit with a battery

could create a depletion zone. The point here is that a depletion zone also forms

spontaneously when the materials are simply placed adjacent to one another.

Now let’s consider what happens when a photon strikes the solar cell. The only photons

relevant to our story are those that are energetic enough to promote an electron from

the valence to the conduction band. When such a photon strikes a valence electron in

the semiconductor, it increases the energy of the electron, promoting it to the higher

band.

The electron will flow toward the n-type semiconductor, since that side of the depletion

zone is positively charged, and the hole will flow to the negatively charged p-type

semiconductor. To put it another way, the electric field caused by the diffusion current

“pushes and pulls” the holes and electrons freed by photons striking the semiconducting

material. Press the refresh button in your browser to see this occur in Concept 3.

Since many photons will strike the material, many electrons and holes will flow: A

current is born. By placing metallic contacts on either side of the junction, this current

can be used to power a load. In Concept 4, you see the solar cell in a circuit powering a

home. There is a flow of negatively charged electrons out of the n-region into the circuit,

and a flow of positively charged holes out of the p-region into the circuit.

The price of photovoltaic cells has been steadily decreasing over the past 30 years.

However, the electrical power produced by such cells still costs more than power from

fossil fuels (coal and oil), or wind-generated power. To cite some approximate numbers

(since energy prices fluctuate), a kilowatt-hour produced by burning a fossil fuel costs

from 3.5 to 4.5 cents. Wind power costs just 4.5 to 5.5 cents per kilowatt-hour, although

there are issues with it: What do you do when the wind is not blowing?

In contrast, the cost of solar power is approximately 25 to 45 cents per kilowatt-hour.

Solar power costs more money, although it could be cost effective for supplying power

to remote locations, since power lines would not have to be run from distant power

plants.

Analysts in the environmental community do raise issues about the “true” costs of

different energy sources, such as the costs of environmental side effects, including

health issues, pollution and global warming. Fair enough! With those costs factored in,

Banks of photovoltaic cells populate a “solar energy farm”

that can produce hundreds of kilowatts of electric power.

Structure of photovoltaic cell

p-substrate with n-type material diffused

into surface

·Ap-n junction

When materials placed together

Electrons spontaneously diffuse, n to p

Holes spontaneously diffuse, p to n

When a photon strikes

A mobile electron/hole pair is formed

The electron moves to n-region

(^678) Copyright 2007 Kinetic Books Co. Chapter 36