Normally, human eyes are not able to see the near-infrared light (780-3000 nm) for wavelength range of visible light is 350-780 nm. This leads to a question: How to see near-infrared light? The traditional method is to convert light signals into electrical signals, then through complex signal processing to get the visible image. This method is indirect and expensive, therefore people are trying to find direct ways.

Upconversion photodetector, one method to see infrared light directly, has been developing quickly these decades. Here “upconversion” means infrared photons with lower frequencies and energies are converted to visible photons with higher frequencies and energies. This is realized by combining an infrared photodetector and a visible light emitting diode (LED) in a monolithic structure. However, the overall photon-photon conversion efficiency of the infrared up-conversion device is relatively low. On the other hand, most of the infrared up-conversion devices reported so far need to be prepared by a vacuum deposition method, which increases the manufacturing cost.

Colloidal quantum dots show excellent infrared light harvesting capability and visible light emitting character, and they can be fabricated by solution process. An idea naturally came up: fabricating upconversion devices using quantum dots. However, it is not easy to do this: there are many layers and the layer below may be damaged when depositing the upper layer. After trying many times, we find suitable solvents and deposition crafts to deposit each layer. Finally we successfully fabricated upconversion devices based on quantum dots using solution-processed method, which are cheap and easy processing.

The external quantum efficiencies (EQE) of traditional photodiodes can not exceed 100%. If they are used in upconversion devices, the photon-to photon efficiencies will be limited. In order to improve the efficiency, we designed a photodiode type photodetector with gain, which is realized by doping Ag nanoparticles to the electron transport layer. Without IR illumination, the energy barrier prevents the injection of carriers and the dark current is small; When IR light illuminates, holes can tunnel across the energy barrier and generate large photocurrent. As a result, an EQE of 8000% and a detectivity of 6×10¹² Jones can be got. We then used this photodetector in the upconvertion devices and got a photon to photon efficiency of 6.5%. The turn-on voltage is 2.5 V, which is the smallest value of the reported upconversion devices, which is beneficial to flexible devices. We showed a prototype flexible device that can work well under bending condition. We also illustrate the potential application of our upconversion device for bio-imaging. Cancerous tissue and normal tissue can be distinguished under infrared light (Figure 1).

    Figure 1   Application of solution-processed upconversion devices based on quantum                      dots.

For more details, please see our recent publication  in Nature Electronics: “solution-processed upconversion photodetectors based on quantum dots” (https://www.nature.com/articles/s41928-020-0388-x).