Green Apples Are Crisp

“Apple has been criticized by some environmental organizations for not being a leader in removing toxic chemicals from its new products, and for not aggressively or properly recycling its old products,” said Jobs. “Upon investigating Apple’s current practices and progress towards these goals, I was surprised to learn that in many cases Apple is ahead of, or will soon be ahead of, most of its competitors in these areas.”

“Fortunately, all iPod displays already use LEDs for illumination, and therefore contain no mercury,” wrote Jobs. “We plan to introduce our first Macs with LED backlight technology in 2007. Our ability to completely eliminate fluorescent lamps in all of our displays depends on how fast the LCD industry can transition to LED backlighting for larger displays.”

Enter: OLED

The first light-emitting polymer device involved a single layer of poly. Multilayer OLEDs can have more than two layers to improve device efficiency. As well as conductive properties, layers may be chosen to aid charge injection at electrodes by providing a more gradual electronic profile, or block a charge from reaching the opposite electrode and being wasted.

A voltage is applied across the OLED such that the anode is positive with respect to the cathode. This causes a current of electrons to flow through the device from cathode to anode. Thus, the cathode gives electrons to the emissive layer and the anode withdraws electrons from the conductive layer; in other words, the anode gives electron holes to the conductive layer.

Soon, the emissive layer becomes negatively charged, while the conductive layer becomes rich in positively charged holes. Electrostatic forces bring the electrons and the holes towards each other and they recombine. This happens closer to the emissive layer, because in organic semiconductors holes are more mobile than electrons (unlike in inorganic semiconductors). The recombination causes a drop in the energy levels of electrons, accompanied by an emission of radiation whose frequency is in the viable region. That is why this layer is called emissive.

 

Just like passive-matrix LCD versus active-matrix LCD, OLEDs can be categorized into passive-matrix and active-matrix displays. Active-matrix OLEDs (AMOLED) can make higher resolution and larger size displays possible. For a high resolution display like a TV, a TFT backplane is necessary to drive the pixels correctly.

OLEDs enable a greater range of colors, brightness, and viewing angle than LCDs, because OLED pixels directly emit light. OLED pixel colors appear correct and unshifted, even as the viewing angle approaches 90 degrees from normal. LCDs use a backlight and cannot show true black, while an “off” OLED element produces no light and consumes no power. Energy is also wasted in LCDs because they require polarizers which filter out about half of the light emitted by the backlight. Additionally, color filters in color LCDs filter out two-thirds of the light.

OLEDs also have a faster response time than standard LCD screens. Whereas a standard LCD currently has an average of 4-8 millisecond response time, an OLED can have less than 0.01ms response time. 

The biggest technical problem for OLEDs is the limited lifetime of the organic materials. In particular, blue OLEDs historically have had a lifetime of around 14,000 hours (5 years at 8 hours a day) when used for flat-panel displays, which is lower than typical lifetime of LCD, LED or PDP technology—each currently rated for about 60,000 hours, depending on manufacturer and model. Toshiba and Panasonic have come up with a way to solve this problem with a new technology that can double the lifespan of OLED displays, pushing its expected life past that of LCD displays. A metal membrane helps deliver light from polymers in the substrate throughout the glass surface more efficiently than current OLED’s. The result is the same picture quality with half the brightness and a doubling of the screens expected life.

In 2007, experimental PLEDs were created which can sustain 400 cd/m² of luminance for over 198,000 hours for green OLEDs and 62,000 hours for blue OLEDs.

The intrusion of water into displays can damage or destroy the organic materials. Therefore, improved sealing processes are important for practical manufacturing and may limit the longevity of more flexible displays.