PV Panels and P N Junctions

How PV Panels work Or

An Introduction to the World of Microelctronics

What are PV Panels?

PV Panels are thin layers of Silicon that have been designed to convert solar energy into electricity.

Silicon is a semi conductor. Semi conductors are elements that

under certain conditions conduct electricity and under others do not.

Periodic Table of Elements

Silicon basics Second most

abundant element on Earth.

Elemental silicon transmits more than 95% of all wavelengths of infrared light

The essentials Name: silicon Symbol: Si Atomic number: 14 Atomic weight:

28.0855 (3) Group number: 14 Period number: 3

Complete Valence Bands All elements want to have their

outermost band complete, or no missing electrons.

For all elements with the exception of Hydrogen and Helium that outermost band will require 8 electrons.

Due to the Heisenberg Uncertainty Principle the atoms are able to share electrons in their outermost shells.

Crystal Lattices

Silicon is solid at room temperature and its atoms are arranged in a lattice type of structure.

Since Silicon has 4 electrons in its outer band it is looking to share or borrow 4 electrons from another atom or atoms so that it has its complete outer band of 8 electrons.

Silicon Crystals

Silicon Crystals Multicolored

Silicon Crystals and Light

Can That Really Happen?

This will happen but only under specific conditions (Si is a semi conductor). This EHP (electron hole pair) can be

caused by light or heat to name a few. In Si the bad gap is 1.12 eV. At room temperature there will be 145

trillion EHPs at any one time.

Types of Doping Impurities are added on purpose to

increase electron mobility or flow. B, In are Group III dopants. (green)

These elements have only 3 electrons in their outer band so a hole is created. P-type Si.

P, As, Sb are Group V dopants. (blue) These elements have 5 electrons in their

outer shells so a electron is free to move. N-type Si.

Doped Silicon Crystals

Types of Electrical Flow

Current is the measure of the flow of electrons.

There are two ways that electricity can flow. Conduction band electron current Valence band hole current.

PN Junctions When P-type Si is placed next to N-

type Si you have what is called a PN junction.

PN junctions are at the heart of things like PV panels and electrical diodes.

The band gaps in N-type and P-type Si are different however due to the presence of different dopants.

Band Gaps Different dopants mean that there are

different band gaps.

Boron Doped Phosphorus Doped

PN Junctions When the two Si types are placed

next to one another a depletion region is formed. The depletion region is created when

the free electrons from the N-type side rush over to fill the holes left in the P-type side.

The depletion regions have no free carriers and therefore resist electron flow.

Depletion Regions

Depletion Regions If the size of the depletion region is

“large” enough then the electrons will not be able to cross the gap. Think of it like the Grand Canyon vs.

rain water rushing towards the storm drain.

No one* would consider jumping the Grand Canyon, but a rain water stream most can handle.

The same is true of electrons.

Biasing the Junctions

If the junction is biased (a voltage is applied to either side) then current will flow. This is where the photons come in to play in the PV panels.

Applying the voltage changes the electric Grand Canyon into that small storm water stream.

Biased PN Junctions By applying a

positive voltage to the P-type side and a negative voltage to the N-type side the depletion region shrinks dramatically.

This is because more free carriers are added to the system.

Silicon Crystals and Light

Sunlight and PV panels The photons that are raining down on us

from the sun are little packets of energy. There are some packets which just

happen to be at exactly 1.12 eV. The photons from the sun can therefore

be harnessed to increase the number of free carriers in the Si.

More free carriers means the junction is biased and current can flow across the gap.