Energy Conversion Efficiency by LED’s used in Solid State Lighting

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 since this will directly impact cost per Watt or cost per Lumen. 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 a LED. A LED is typically constructed from a classical p-n 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]. By putting the LED in a forward bias the electrons and holes that arrive in the active zone can recombine in two different ways:

–         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] 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.

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[i] See for an explanation: http://en.wikipedia.org/wiki/Direct_and_indirect_band_gaps

[ii] O. Heikkila, J. Oksanen, J. Tulki, Ultimate limit and temperature dependency of light-emitting diode efficiency,  Journal of Applied Physics 105, 093119 (2009)

©  Copyright John Schmitz 2010

 

How Technology Can Help – Solid State Lighting

As I said earlier, technology by itself can not solve our global warming problem. On the contrary, sometimes new technologies make things only worse (bio fuels for instance). But there are also many cases where technology certainly can help. A good example is that of solid state lighting (SSL). The light source in SSL is a light emitting diode (LED). Let me explain that a bit.

A LED is a device made out of semiconductor materials (for example Gallium Nitride, chemically noted as GaN). It is basically a diode with which you would normally rectify AC current. But by proper geometrical shaping and the right applied voltages the diode can also generate light. The principle of light generating in the LED (electron-hole recombination) allows that high efficiencies can be achieved because heat generation is strongly reduced compared to gas discharge or incandescent bulbs.

About 20% of all electrical energy is used for lighting. For a very long time we used the incandescent bulbs. They have a very low efficiency of converting the electrical energy into light: less than 2%, thus most of the electrical power going into the lamp is converted into heat. The introduction of fluorescent lamps (where a gas discharge is invoked) improves the efficiency considerable up to about 25% (see for instance  https://secondlawoflife.wordpress.com/2008/10/05/compact-fluorescence-lamps/).  An LED can improve this light/power efficiency with about a factor of two (so up to 50%), but can also exhibit a dramatic increase in life time (say a multiple of 10,000 hours versus 1000 hours for an incandescent bulb). Massive research is still ongoing to improve performance and color characteristics of the LED further. Generally it is expected that in the next 10 to 20 years there will be a major shift towards LED based lighting.

The US Department of Energy hopes that by 2025 the total electrical energy needed for lighting can thus be reduced up to 50%, which is of course a substantial number. This can eventually lead to a 10% reduction in overall electrical power requirement of the society.