Semiconductors play an important role in solving the planet's energy issues.
There are two distinct, but related, phenomena: the conversion of (sun)
light in electricity and the conversion of electrical power in visible
light. The first conversion is known as the photo-voltaic (PV) technology
and the second conversion is the one that is used by Light Emitting Diodes
(LED's) used in Solid State Lighting applications. Both conversions enjoy
considerable interest from scientists, governments, energy companies as well
as citizens. Clear is that both energy conversions can contribute
substantially in solving the availability and distribution of energy around
the planet.

A key factor for the successful acceptance (at least in terms of
economically feasibility) of both PV and LED's is the efficiency of these
two types of energy conversions. Indeed, the question arises are there
fundamental limitations to these energy conversions? For PV cells it has
been reported that the upper efficiency of on silicon based cells will run
at about 30%. For LED's there has not been reported so far a fundamental
barrier that would keep the LED away from 100% efficiency (however, the fact
that the device heats up during operation hints already to a less than 100%
efficient light conversion).

On the efficiency of PV cells I will come back in a future contribution, for
now I would like to focus on the efficiency of an LED. A LED is typically
constructed from a classical pn junction but in the LED case the p and n
material are separated by what is called an active zone that can be either
doped or intrinsic. The semiconductor material must be a direct band gap
semiconductor in order to have sufficient conversion efficiency[i]
asteword.htm?ver=327-1235-syntaxhighlighter2.3.6#_edn1> . By putting the LED
in a forward bias the electrons and holes that arrive in the active zone can
recombine in two different ways:

- a radiative recombination. It is this recombination that fuels the
light emission from the LED.

- Several other non-radiative recombination processes occur as well.
These reduce the amount of holes and electrons available for light emission.

There are other loss (non-radiative) mechanisms operating (such as
absorption of the photons by the semiconductor) that further reduce the
light generation efficiency.

Recently an article in the Journal of Applied Physics[ii]
asteword.htm?ver=327-1235-syntaxhighlighter2.3.6#_edn2> appeared that gives
good insight in the different factors that influence the power-light
conversion efficiency. An important factor is the so-called wall plug
efficiency, defined as follows:

Wall Plug Efficiency = emission power/electrical power

a pretty straightforward definition. In the article all the different
recombination and loss mechanisms are mathematically described and then put
together in one model for the LED. This model can then calculate the
behavior (and thus wall plug efficiency) of the LED device in terms of
operating conditions (temperature, current, voltage), material properties
(semiconductor material such as GaN or GaAS and doping) and LED structure
(thickness of the different layerings, metal contacts and lay out of the
active layer). This is of great help when optimizing the LED device for
conversion efficiency.

Let me summarize a few important conclusions from the article:

- There is not a fundamental reason why the power-light conversion
cannot be 100%. Even stronger, the conversion can be more than 100% (see
next point for explanation)! However, the high efficiencies may not always
in a practical operating window (for instance at the current densities the
LED needs to run because of a certain required light output per surface area
semiconductor).

- The energy of the photon may come not only because from the band
gap energy difference but phonons (thermal energy from the lattice) may
contribute as well. In that case the LED can act as a heat pump: the device
cools actually and can in that way extract heat from the environment and
achieve efficiency better than 100% (using the above wall plug efficiency
definition).

- Further improvements will be possible to increase the light output
of LED's.

Thus, we can expect to see in the coming years more developments coming to
improve the Solid State Light technology and this will be a very valuable
contribution to our energy strategy.