Energy Loss in a Solar Cell

Visible light is only part of the electromagnetic spectrum.
Electromagnetic rad­iation is not monochromatic -- it is made
up of a range of different wavelengths, and therefore energy
levels. (See How Special Relativity Works for a good
discussion of the electromagnetic spectrum.)

Light can be separated into different wavelengths, and we
can see them in the form of a rainbow. Since the light that
hits our cell has photons of a wide range of energies, it
turns out that some of them won't have enough energy to form
an electron-hole pair. They'll simply pass through the cell
as if it were transparent. Still other photons have too much
energy. Only a certain amount of energy, measured in electron
volts (eV) and defined by our cell material (about 1.1 eV for
crystalline silicon), is required to knock an electron loose.
We call this the band gap energy of a material. If a photon
has more energy than the required amount, then the extra
energy is lost (unless a photon has twice the required
energy, and can create more than one electron-hole pair, but
this effect is not significant). These two effects alone
account for the loss of around 70 percent of the radiation
energy incident on our cell.

Why can't we choose a material with a really low band gap, so
we can use more of the photons? Unfortunately, our band gap
also determines the strength (voltage) of our electric field,
and if it's too low, then what we make up in extra current
(by absorbing more photons), we lose by having a small
voltage. Remember that power is voltage times current. The
optimal band gap, balancing these two effects, is around 1.4
eV for a cell made from a single material.

We have other losses as well. Our electrons have to flow from
one side of the cell to the other through an external circuit.
We can cover the bottom with a metal, allowing for good
conduction, but if we completely cover the top, then photons
can't get through the opaque conductor and we lose all of our
current (in some cells, transparent conductors are used on
the top surface, but not in all). If we put our contacts only
at the sides of our cell, then the electrons have to travel
an extremely long distance (for an electron) to reach the
contacts. Remember, silicon is a semiconductor -- it's not
nearly as good as a metal for transporting current. Its
internal resistance (called series resistance) is fairly
high, and high resistance means high losses. To minimize
these losses, our cell is covered by a metallic contact grid
that shortens the distance that electrons have to travel
while covering only a small part of the cell surface. Even
so, some photons are blocked by the grid, which can't be too
small or else its own resistance will be too high.