Ultrafast photophysics in Mn-doped semiconductor quantum dots for optoelectronic application

Recent years witnessed a substantial improvement of solar cell technologies with the power conversion efficiencies of a variety of devices that can approach the thermodynamic limits (Shockley–Queisser Limit). However, in order to finally overcome the Shockley–Queisser Limit (≈33% for a standard cell with bandgap 1.4 eV), any process that will induce the loss of harvested photon energy should be well prevented. On the one hand, defects trapping of the excited charge carriers in the photovoltaic materials will result in non-radiative recombination and reduce the quantum efficiency of solar cells. On the other hand, rapid photo-generated hot‐carrier cooling is another primary channel for heat loss. The reduced defects concentration and diminished HCs cooling in solar absorbers are, therefore, critical features for realizing highly efficient solar cells to break the Shockley–Queisser Limit. Quantum dots (QDs) are among such promising candidates in solar cells as the size-tunable optical properties can be utilized for materials engineering to achieve the above objectives.

Meanwhile, the emerging metal ions doping in colloidal QDs raises new opportunities to overcome SQ limitations. Such doping states could modulate not only the electronic structures but also the phonon structures of the QDs. If the electronic structure and phonon structure can be well-tailored by the doping in QDs, we can expect enhanced efficiency for QDs solar cells. The main objective of the thesis is thereby to seek the feasibility of photophysical modulation on QDs by transition metal doping.

In the first work of the thesis, we studied the influence of the Mn dopant on the photo-induced charge carrier dynamics in Mn-doped CsPbCl₃ perovskite QDs by steady-state and time-resolved spectroscopies. We found the Mn-doping not only adds extra electronic states in the QDs, but also significantly modulates the defect state of the materials. The energy levels of those possible defect states were thoroughly analyzed using Density Functional Theory (DFT) calculations. As the Mn concentration increases, the exciton photoluminescence quantum yield (PLQY) decreases and the Mn dopant emission QY first increases and then decreases. The experiments and calculations reveal that Mn2+ doping qualitatively changes the type of defects from antisites Pb_Cs (undoped) to interstitials Cli (doped). The competition between exciton to Mn/dopant energy transfer and defect trapping at early timescale (< 100 ps) determines the final PLQY of the CsPbCl₃ QDs.

In the second work, we study hot carriers (HCs) relaxation dynamics in Mn-doped LHPs CsPbI₃ nanocrystals (NCs) combining femtosecond transient absorption spectroscopy and DFT calculations with a particular focus on the influence of Mn-doping on the electronic and phononic structures. We demonstrate that Mn²⁺ doping 1) enlarges the LO-acoustic phonon bandgap, 2) enhances the electron-LO phonon coupling strength, and 3) adds HCs relaxation pathways (LO is longitudinal optical mode). The first two factors are associated with the local distortion after Mn²⁺ replacement of Pb²⁺ in the lattice, while the third effect is attributed to the location of Mn orbitals within the bands of LHPs. The spectroscopic study shows that the HCs cooling process is decelerated after doping under band-edge excitation due to the dominant effect of enlarged LO-acoustic phonon bandgap. When the excitation photon energy is much larger than the optical bandgap and the Mn²⁺ transition gap, the doping accelerates the cooling rate owing to the dominant effect of enhanced carrier-phonon coupling and relaxation pathways. The enhanced electron-phonon coupling and efficient thermalization of HCs at high energy together with delayed heat dissipation after thermalization with HCs at low energy are optimal for the HCSC application. Our results establish a straightforward methodology to control the HCs dynamics by doping.