p-Types, n-Types, and the Electric Field
Although both materials are electrically neutral, n-type silicon has excess electrons and p-type silicon has excess holes. Sandwiching these together creates a p/n junction at their interface, thereby creating an electric field.
When the p-type and n-type semiconductors are sandwiched together, the excess electrons in the n-type material flow to the p-type, and the holes thereby vacated during this process flow to the n-type. (The concept of a hole moving is somewhat like looking at a bubble in a liquid. Although it's the liquid that is actually moving, it's easier to describe the motion of the bubble as it moves in the opposite direction.) Through this electron and hole flow, the two semiconductors act as a battery, creating an electric field at the surface where they meet (known as the "junction"). It's this field that causes the electrons to jump from the semiconductor out toward the surface and make them available for the electrical circuit. At this same time, the holes move in the opposite direction, toward the positive surface, where they await incoming electrons.
We'll use silicon as an example because crystalline silicon was the semiconductor material used in the earliest successful PV devices, it's still the most widely used PV material, and, although other PV materials and designs exploit the PV effect in slightly different ways, knowing how the effect works in crystalline silicon gives us a basic understanding of how it works in all devices.
- An Atomic Description of Silicon - The Silicon Molecule
- Introducing Phosphorous - Boron - Other Semiconductor Materials
Absorption and Conduction
To make an efficient solar cell, we try to maximize absorption, minimize reflection and recombination, and thereby maximize conduction.
efficiency of a PV cell is the proportion of sunlight energy that the cell
converts to electrical energy. This is very important when discussing PV
devices, because improving this efficiency is vital to making PV energy
competitive with more traditional sources of energy (e.g., fossil fuels).
Naturally, if one efficient solar panel can provide as much energy as two
less-efficient panels, then the cost of that energy (not to mention the
space required) will be reduced. For comparison, the earliest PV devices
converted about 1%-2% of sunlight energy into electric energy. Today's
PV devices convert 7%-17% of light energy into electric energy. Of course,
the other side of the equation is the money it costs to manufacture the
PV devices. This has been improved over the years as well. In fact, today's
PV systems produce electricity at a fraction of the cost of early PV systems.