Photodetectors capture images in digital cameras and medical imagers by converting light into electrical signals. When building new photodetectors, the challenge is to create a device that is highly sensitive to ultraviolet, infrared, as well as visible light. Among the two common types of photodetectors, photodiodes quickly and efficiently convert light to charges, while phototransistors are more sensitive, because each photon generates multiple charge carriers, resulting in signal amplification or “gain.”

Two-dimensional materials like graphene can be good phototransistors because their inherent good electron mobility generates high gain. But because these materials do not absorb light very well, graphene-based photodetectors are often covered with a light-absorbing polymer, a perovskite layer, or quantum dots that carries electrical charges to the graphene. In 2012, Frank H.L. Koppens, Gerasimos Konstantatos, and colleagues at the The Institute of Photonic Sciences (ICFO) in Spain built the first high-gain graphene photodetector by covering graphene with a layer of colloidal lead sulfide quantum dots (see the abstract in Nature Nanotechnology). The resulting photodetector was 10 orders of magnitude more responsive than a plain graphene photodetector. But the quantum efficiency, a measure of how many incident photons become electrons, was limited by the thickness of the quantum dot layer. Increasing the thickness of the layer could have increased the quantum efficiency, Konstantatos says, but the light-generated charges would recombine before they reached the graphene, eliminating the signal in the device.

Now the researchers have redesigned and developed a new hybrid photodetector to improve quantum efficiency and other characteristics. To build this new device, they placed graphene between palladium electrodes on a silica substrate. A layer of collodial PbS quantum dots covers the graphene, and a transparent indium tin oxide (ITO) electrode covers the quantum dots. After visible, near-infrared, or short-wave infrared light hits the quantum dots, a voltage is applied between the ITO electrode and the graphene, driving holes toward the graphene and electrons to the ITO electrode. This active charge separation increases the chances of collecting the charges in the graphene layer before they recombine, resulting in an increased quantum efficiency of more than 70%. Extending the lifetime of the charges in the quantum dot layer also enhanced graphene’s role as a phototransistor, which improved linear dynamic range of the hybrid photodetector.

This device is useful because it can detect infrared light as well as visible light, Konstantatos says. Current infrared photodetectors used for remote sensing, night vision, biomedical imaging, and food screening are made from III–V InGaAs produced using molecular epitaxy, an expensive manufacturing method. This new device could be manufactured more easily and integrated with existing electronics, as well as future flexible devices, he says.

Emily Weiss, at Northwestern University, likes that the hybrid photodetector only has one junction between different materials, which is simpler than most well-functioning multifunctional devices. By combining materials of different dimensions and compositions, the resulting material has properties different than either starting material, she says. In this case, electrons flow between the graphene and quantum dots even before light shines on it.

Read the abstract in Nature Communications.