08 February 2019
USC Dornsife chemists have found a cheaper way to light up smartphone and TV screens, which could save manufacturers and consumers money without affecting visual quality – “copper is the answer”.
“The current technology that is in every Samsung Galaxy phone, high-end Apple iPhone and LG TV relies on iridium compounds for the colours and light on OLED screens,” says Mark Thompson, a chemist at USC Dornsife College.
“We have been using iridium because you get a highly efficient light emission, but it is the rarest naturally-occurring element on Earth. One of our challenges has been to come up with an alternative that is more abundant.”
Prior attempts to generate a copper-based OLED failed. The copper complexes in those studies had weaker structures. The molecules were unstable, with shorter lifetimes than the iridium compounds.
Copper definitely solves the problem of availability since it is a plentiful metal worldwide. Iridium, on the other hand, is found in only a few places.
The most widely-accepted hypothesis that explains iridium’s scarcity and its origins is that it traveled here on a meteor – the same one that wiped out the dinosaurs 65 million years ago.
Unless another meteor like that hits Earth, iridium will continue to dwindle in supply. Demand for it is only increasing as smartphones, TVs and other devices that feature OLED screens gain popularity.
OLEDs have come to replace LED LCD screens. In an OLED screen, each pixel generates light, while in the LCD screens, pixels are illuminated by an LED backlight.
Besides its scarcity, iridium has another drawback: weaker molecules for generating blue light. When the molecules from the iridium compounds are excited, they generate two of the OLED screen’s primary colours – red and green – very efficiently, quickly and in devices that give very long operational lifetimes, says Thompson.
The third requisite colour, blue, has been the bane of OLED technology because blue emissive OLEDs have a short lifetime. Thompson explained that the bonds within the blue molecules tend to break down. Blue molecules also require more electricity than the green and red molecules to energise them. Since blue is among the primary colours for OLED, its poor performance can affect a range of colours that you see on a screen that contain any blue.
Thompson’s team may have solved that with their new copper complex – a more rigid molecular complex than the prior, failed types of copper compounds, which were weaker. The new compound’s rate of light emission also matches iridium’s, so the energy is converted efficiently into light and colour, the chemists found.
“Our paper lays out the basic design rules for obtaining iridium-like emission efficiencies out of copper, with colours ranging from blue to green and yellow,” said Rasha Hamze, the study’s lead author.
“Achieving efficient blue emission out of copper compounds opens up entirely new possibilities for tackling the problem of short lifetimes in blue devices.”
The team at USC has submitted a patent application for their copper compound.
Thompson says that next, he wants to see if these copper compounds could also lead to the creation of more energy-efficient lighting.
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