Tiny crystals, known as quantum dots, have enabled an international team to achieve greater than 100% quantum efficiency in the photocurrent generated in an inorganic-organic hybrid semiconductor.
Perovskites are exciting semiconductors for light-harvesting applications and have already shown impressive performance in solar cells. But improvements in photo-conversion efficiency are needed to extend this technology to a wider market.
Light comes in the form of packets of energy called photons. When a semiconductor absorbs a photon, the electromagnetic energy is transferred to a negatively charged electron and its positively charged counterpart, called a hole. An electric field can sweep these particles in opposite directions, allowing a current to flow. This is the basic operation of a solar cell. It may sound simple, but maximizing quantum efficiency, or getting as many electron-hole pairs as possible from incoming photons, is a long-standing goal.
One cause of inefficiency is that if the photon has more energy than it takes to create the electron-hole pair, the excess energy is usually lost as heat. But nanomaterials offer a solution. Small particles, such as nanocrystals or quantum dots, can convert high-energy photons into multiple electron-hole pairs.
Jun Yin and Omar Mohammed of KAUST worked with Yifan Chen and Mingjie Li of Hong Kong Polytechnic University and colleagues to demonstrate this so-called multiple exciton generation (MEG) in tin halide perovskite nanocrystals and lead. “We have demonstrated photocurrent quantum efficiency exceeding 100% by exploiting MEG in perovskite nanocrystal devices,” Yin says.
In the past, MEG has been observed in perovskite nanocrystals with a large band gap: that is, those semiconductors that can only absorb high-energy photons.
Narrower bandgap materials present a greater challenge because the excited electron-hole pairs expand or cool too quickly to extract into a working solar cell device. “Effective MEG in bandgap perovskite nanocrystals and verification of their inherent MEG in practical optical devices has not been reported,” Yin says.
Chen, Yin and the team synthesized a semiconductor material composed of tiny particles of lead iodide and formamidinium tin perovskite – made from small amounts of tin – embedded in tin-free FAPbI3. The team thinks the introduction of the tin helps to slow down the “cooling”. “We will be able to further optimize the perovskite nanocrystal by modifying its composition to achieve higher MEG performance and improved light-to-power conversion,” Yin says.
The research has been published in Nature Photonics.
Helping Semis Find a Cooler Way to Relax
Yifan Chen et al, Generation of multiple excitons in tin-lead halide perovskite nanocrystals for photocurrent quantum efficiency enhancement, Nature Photonics (2022). DOI: 10.1038/s41566-022-01006-x
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