Carrier multiplication refers to a process where the kinetic energy of a carrier relaxes by generating additional excitons. This phenomenon has gained the attention of experts since it can help boost the yield of generated electron-hole pairs. However, the carrier multiplication yield is limited by Coulomb interactions.
New Insights About Spin in Quantum Dots
To address the challenge of limited carrier multiplication yield, a group of researchers from Los Alamos integrated magnetic dopants into colloidal quantum dots specially engineered to be semiconductor crystals at nanoscale size. Led by head researcher Victor Klimov, the team was able to generate effects that could power solar cell technology, photo-detectors, and applications that rely on light to trigger chemical reactions.
The team used quantum dots composed of a lead-selenide core and a cadmium-selenide shell to make this possible. In this setup, the manganese ions serve as small magnets with magnetic spins that strongly interact with the quantum dots' core and shell. During these interactions, the spin-exchange can transfer energy to and from the manganese ion.
Spin-exchange carrier multiplication allows a single absorbed photon to generate two excitons due to the spin-flip relaxation of a manganese ion that goes through the excited state. In the study made by the nanotechnology team, the spin-exchange interactions happened at a speedy rate. As a result, the magnetically doped quantum dots showed a three-fold enhancement in the carrier multiplication yield compared to undoped quantum dots with similar structures.
The result of the study also reveals substantial enhancement in the range of photon energies within the solar spectrum. This provides new insight into boosting applications that depend on energy conversion from light.
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Benefits of Carrier Multiplication
In a normal setting, a photon that gets absorbed by a semiconductor generates an electron in the conduction band and a space in the valence band known as a 'hole.' Such a procedure determines the operation of image sensors, photo-diodes, and solar cells, where the generated charge carriers are extracted as a photo-current. The photo-generated electrons and holes facilitate redox reactions involving transferring electrons from one particle to another.
Every photo-conversion scheme can benefit from the carrier multiplication process as it generates a 'hot' carrier with large kinetic energy. This energy vanishes in a collision process with a valence-band electron. This results in a new electron-hole pair and the original pair produced by the absorbed photon.
However, the competing energy losses brought by the interactions with lattice vibrations make the carrier multiplication inefficient in bulk solids. The Los Alamos researchers demonstrated that this effect could be enhanced in colloidal quantum dots that are chemically synthesized. The frequency of collision between each electron is increased by the tiny size of the colloidal quantum dots, facilitating carrier multiplication.
Another setback is that even in the quantum dots, the carrier multiplication efficiency is not high enough to bring an appreciable effect on the performance of practical photo-conversion schemes. For bulk crystals, energy losses become the major limitation due to the fast emission of lattice vibrations leading to nonproductive heating of a crystal lattice.
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