A new means of illumination should be commercially available later this year according to its developer Dr David Carroll at US Wake Forest University. Similar to organic light emitting diodes (OLEDs) and light emitting capacitors (LECs), the new flat-panel source is being touted by Dr Carroll as a replacement for fluorescents, LECs and OLEDs, which could dismay manufacturers who have sunk millions into developing OLED technology.
The new light source is a flexible panel that emits light using field-induced polymer electroluminescence – fipel for short (see new plastic light source, January/February 2013 issue page 42).
There are similarities between the three flat-panel light sources, but the researchers who developed fipel lighting say its luminous efficacy is on a par with LED efficacy. They also say it is more efficient, has a longer life and produces better quality light than OLED.
The right to commercialise fipel sources has been secured by US-based CeeLite Technologies, a relatively new ‘clean technology’ company that already markets LECs. However, LEC technology has been around since early last century and with an efficacy of only 20 lumens per watt (lm/W) it seems unlikely to ever compete with either OLED or fipel sources – if the fipel claims prove accurate. LED efficacy has already surpassed 100 lm/W and OLEDs continue to become more efficient.
Konica Minolta currently claims the highest OLED efficacy saying its proprietary materials deliver a luminous efficacy of 45 lm/W. It says the quality of the light is soft, uniform and glare-free, with constant colours and brightness across a wide range of viewing angles.
Philips expects its OLED Lumiblade luminaires to become more efficient than fluorescent lamps eventually, generating up to 140 lm/W – 15 times more efficient than conventional GLS lamps.
The developers of fipel sources claim they emit full-spectrum light similar to that of the sun, but when the new source was announced the claims were not backed up with figures. They also criticised OLEDs saying they start to melt under higher current loads. They further claimed they had a fipel lamp working in the lab for ten years so the technology is not new, but may be recently improved.
Only last year OLEDs were heralded as starting a new age of plastic lighting because they had already opened up new avenues for designers with their sleek wafer-thin profile. OLEDs can be installed where other luminaires cannot fit and be made into unusual new luminaires, but there are cost and performance issues to get over before they become mainstream. While it is still relatively early days in OLED development, its future is not looking as bright as it was if the claims of fipel advocates prove to be true.
How they work
All three sources are thin ‘sandwiches’ of materials consisting of a substrate, an anode, conductive and emissive layers and a cathode. OLEDs work in a similar way to LEDs. When a current flows from anode to cathode via the semiconductor conductive layer, the electrons flow into ‘holes’ in the molecules of the emissive polymer. This causes changes in atomic energy levels which are accompanied by the emission of light. OLED substrates can be made from flexible and transparent plastic or ordinary glass.
The reference to the word organic in the term OLED is misleading. Apart from carbon, much of the chemicals used in OLEDs are not naturally occurring, but are specifically synthesised polymers used for making the electroluminescent emissive layer. These polymers are called polyfluorenes and different colour-emitting polyfluorenes can be mixed to give different shades of white or coloured light.
The most promising innovation in OLED manufacture is an ‘inkjet’ printing technique which prints the luminescent layers directly onto the substrates. This enables larger panels to be made, as the emissive chemicals are dissolved in solvent and sprayed onto substrates by printers similar to newspaper presses. This printing method could give OLEDs an advantage over LEDs, which despite being more energy-efficient, are more expensive to make as they have to be manufactured in sterile cleanrooms. The LEC construction is similar to OLED but they have more layers and at 1 mm thick are not as slim as OLEDs. Like OLEDs, fipel sources are made from layered polymer sheets but fipel layers contain small amounts of multi-walled carbon nanotubes as well as electroluminescent chemicals, which emit light when activated by alternating current. While the basic fipel technology has been around for a decade the addition of carbon nanotubes is claimed to increase light output dramatically.
Differences
OLEDs can be manufactured with anode, cathode and polymer layers made from flexible and transparent materials. This means in future windows might automatically light up indoors after dark because they can be made with an emissive side and a non-emissive side while still providing transparency. Some current OLEDs are around 85 percent transparent whereas ‘top-emitting’ OLEDs with an opaque substrate emit light only on one side.
Flexible OLEDs are very light and durable and are already seen as a better material for flat screen TVs as they do not break easily. White OLEDs emit light that is more uniform than fluorescent lamps although their luminous efficacy is still lower. Despite this, some have a colour rendering index (CRI) of over Ra 90. However, the current CRI is not suited to describing LED and OLED colour quality (see New CRI on the way? Page 48).
Experimental OLEDs have already been made with an efficacy of 90 lm/W emitting 1,000 candelas per square metre and researchers say there is potential to increase this to over 120 lm/W.
As OLEDs are cool to the touch they have already been made as touch-screen luminaires and further research aims to switch and dim OLEDs with hand movement alone. If development continues at the current rate this should not take too long.
Fipel light sources, like OLEDs, are also shatter proof, flicker-free and contain no mercury. The claims that these lamps are at least twice as efficient as fluorescents and emit full-spectrum light remain unsubstantiated.