The solution Tang’s team arrived at starts with the way infrared light is registered in the first place.
Selective quantum dots
Bulk semiconductors routinely used in IR detectors have continuous energy bands, which means they absorb a broad, undifferentiated swath of the infrared spectrum. This doesn’t preserve much information about which specific wavelength arrived. The mercury telluride colloidal quantum dots the Chinese team picked for their device behave differently. Due to their tiny size, roughly 4 nanometers across, quantum confinement breaks their energy levels into discrete steps rather than a continuous chunk. Photons of different infrared wavelengths and intensities excite different electronic transitions, moving an electron between discrete energy levels within the dot, rather than just producing more or less of a single signal.
At wavelengths of around 2 micrometers, the longest the team tested, incoming photons have just enough energy to bump an electron across the dot’s fundamental bandgap, producing a modest number of charge carriers, either negatively charged electrons or positively charged holes that move around the dot.
At shorter infrared wavelengths, photons carry more energy per particle, and they can access additional, higher-energy electronic transitions inside the dot. In some cases the excess energy in a carrier can be enough to kick loose more than one electron-hole pair per photon. These processes open extra channels for generating charge carriers. The effect is that shorter infrared wavelengths and more intense infrared light push more positively charged holes out of the quantum dot layer and toward the OLED side of the device.
But getting a variable number of holes dependent on the wavelength and intensity of infrared light out of the detector was only half the problem. The other half was translating this signal into a color image at the other side.
The barrier
The team achieved this by building the OLED with two separate emissive layers stacked on top of each other. One layer, closer to where the holes enter, was doped with a red-emitting phosphor. The other one, positioned farther away, was doped with phosphor-emitting cyan light. The key component that made full-color vision work was an energy barrier of about 0.82 electron volts between these two layers.
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