[ Instrument Network Instrument Development ] Recently, Wu Kaifeng, a researcher of the Photoelectric Materials Dynamics Research Group of the Dalian Institute of Chemical Physics, Chinese Academy of Sciences, made new progress in the study of thermal electron relaxation dynamics of semiconductor quantum dots, and observed copper-doped quantum dots for the first time. The "phonon bottleneck" effect of thermal electron relaxation.
Dalian Institute of Chemicals first observed the "phonon bottleneck" dynamics in doped quantum dots
In most inorganic semiconductor materials, hot carriers with higher energy than the semiconductor band gap collide with the lattice (phonon), and the fast (sub-picosecond level) relaxes to the band edge, resulting in higher solar photons than the semiconductor band. The energy of the gap is dissipated as heat energy. This is one of the main reasons for the Shockley-Queisser efficiency limit in a single solar cell. If the hot carriers can be effectively utilized, this limit can be broken, and the theoretical energy conversion efficiency of the solar cell can be increased to 66%. In addition, the efficient use of hot carriers is also important for improving photocatalytic efficiency and sensitizing photochemical reactions. However, the sub-picosecond level relaxation of hot carriers poses a huge challenge for efficient use of hot carriers.
Researchers have predicted that semiconductor nanocrystals (or quantum dots) may have long-lived hot carriers because quantum confinement effects cause quantum dots to have discrete energy levels of atoms, and the energy difference between these energy levels is as high as several hundred. meV makes it difficult for hot carriers to relax by emitting phonons. This is the famous "phonon bottleneck" phenomenon. However, various quantum dots (except nuclear/shell structures) reported so far have exhibited sub-picosecond thermal electron relaxation, and no "phonon bottleneck" effect has been observed. For the II-VI quantum dots, the researchers speculated that the thermal electrons in the conduction band will quickly relax to the band edge by the confinement-enhanced Auger-type electron-hole energy transfer mechanism; and because the valence band energy level is small, The hot holes are coupled to the phonons and relaxed to the band edge. Therefore, if the electron-hole can be effectively separated and the Auger-type energy transfer is suppressed, it is expected to prolong the life of the hot electron.
Based on this understanding, the team compared the thermoelectron relaxation kinetics of copper-doped and undoped CdSe quantum dots with simple structure. Ultrafast spectral analysis shows that cuprous ions can rapidly capture photogenerated holes in the valence band at the femtosecond time scale (< 390fs), weakening electron-hole coupling and making the 1Pe hot electron lifetime in quantum dots from ~0.25ps. Extending to ~8.6ps (>30 times), high-efficiency hot electron extraction is expected. In addition, based on the previous understanding of the non-adiabatic interaction between hot electrons and quantum dot surface ligands (Chem. Sci., 2019), the team also found that the capture of photogenerated holes may also shrink the electron wave function in the conduction band, thereby inhibiting The hot electrons are relaxed by the non-adiabatic action induced by the surface ligand molecules. This double suppression effect makes the thermal electron relaxation rate of copper-doped quantum dots not only significantly slower than undoped quantum dots, but also more than 20 times slower than various types of perovskite materials that have received much attention recently.
For the first time, the "phonon bottleneck" effect of thermal electron relaxation was observed in copper-doped quantum dots with simple structure, which has guiding significance for improving solar cell efficiency, improving photocatalytic efficiency and sensitizing photochemical reaction.
The work was funded by the National Key Research and Development Program, the Strategic Pilot Research Program of the Chinese Academy of Sciences, the National Natural Science Foundation, and the Liaoning Xingliao Talents Program. It was recently published in Nat. Commun.
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