White organic light-emitting diodes: Status and perspective – published!

The above mentioned paper has finally shaped to its final form. White organic light-emitting diodes: Status and perspective has been published today in Reviews of Modern Physics (RMP). It contains more than 40 pages of densely packed details on white OLEDs including the main concepts of realizing white emission (from both polymers and/or small molecules), in depth discussions of respective efficiency limits, and promising strategies for enhanced light outcoupling. Here is an editorial summary (source RMP):

The conversion of electricity into a photon flux with color quality resembling natural sunlight is desirable for artificial illumination. The ubiquitous incandescent light bulb, introduced in 1880 by Thomas Edison, satisfies this color quality requirement but suffers from a rather poor energy conversion efficiency of approximately 5%. This review focuses on device strategies to produce efficient organic white light-emitting diodes based on small molecular and polymeric semiconductors, which in the context of next generation lighting have a considerable technological promise.

If you like it, please spread the word. Here is an exciting side note: This paper is published in the same issue of RMP that also contains the great Nobel lectures of Serge Haroche and David J. Wineland, who share the 2012 Nobel Prize for Physics. Another good reason to check out the current issue of RMP.

America, let’s talk energy efficiency!

Two years ago, when we moved to New England and finally found an apartment to stay, we were fairly sure that we didn’t want to use the one air conditioning (AC) unit that was sticking out of our living room wall. It just didn’t seem right to us. – Now, two summers later we are in the middle of a period of serious heat (triple digit, at least when you measure in Fahrenheit). And, despite still not feeling right about it, we are here with two AC units running – simply because the heat (night and day) is unbearable.

I am bringing this up, because we are constantly discussing energy efficiency (where most of my professional overlap is with solid state lighting and photovoltaics), anxiously pointing our fingers at the global warming caused by mankind. If I am looking left and right, I vastly see residential houses that have a concrete base and are otherwise build from mostly wood and drywalls – thermal insulation often missing. Now this is problematic because at the end of the day, your house is thermalizing with the environment very “efficiently” – whether its toe freezing cold or ice cube melting hot. I don’t mind local electrical heaters or window AC units in general, but in my view, the real problem is that they only give you the effect while running – simply because the average apartment can’t store the heat or the cold. Thus, these things are constantly running, to keep our modern comfort, not only adding to our utility bill but also unnecessarily hurting the environment.

A back-of-the-envelope calculation for the US: With about

  • 300 million inhabitants (wikipedia.org, entry “USA”, last retrieved 7/21/2013),
  • every second person actually in need of cooling and heating due to the local climate (that is a vague estimate, could be quite off…),
  • about 4 person per household,
  • and 1 AC unit and two electrical heaters per household (Note, this is stuff we use in addition to central heat. Also, looking at houses I see daily, these are conservative estimates for the AC units.),

we get about 37.5 million AC units and 75 million heaters in use. Now further, taking our AC unit as representative, having an energy consumption of 5100 BTU (British thermal unit – yet another funny, old school unit. It converts to about 1.5 kWh) and a similar value (1.5 kWh) for a small room heater, we end up with an energy use per hour of 4.5 kWh per average US household. Lets consider three month of annual cooling with 12 hours per day and 5 month of heating with 8 hours a day. This puts us to 2280 hours of annual use per household. This translates to 385 billion kWh of energy use. That is 385,000,000,000 kWh.

For comparison, a standard, two-slot toaster (750 Watts) consumes about 1 kWh of energy, if you would let it run constantly for 80 minutes straight. – Thats a lot of toast. Another comparison: The nuclear power plant in Plymouth, MA produces 685 MWe (a fairly average value – wikipedia.org, entry “List of nuclear reactors”, last retrieved 7/21/2013). Considering constant run that is about 6,000,000 MWh or 6,000,000,000 kWh annually. In other words, these two inefficient factors – heating in winter and cooling in summer – account for a sadly high annual energy use that would require more than 50 nuclear power plants to run. The US consumed 25,778,000,000,000 kWh of energy in 2010 (wikipedia.org, entry “Energy of the USA”, last retrieved 7/21/2013). Thus, with about 1.5% of the total annual energy use, the two sources significantly contribute to the energy consumption.

So clearly, a similar movement as the one we see with SSL to more efficient use of energy could make a difference. Try to reduce energy consumption that adds to general central heat. Go, build some brick walls!

Disclaimer: I hope, I got these numbers right. With such unimaginable numbers, it is easy to loose a couple of zeros on the way.

PS: I am sweating because the ACs are off for a while.