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The research funded by the ACS-PRF grant # 46772-AC6 focused on the fundamental physical and chemical processes that occur in semiconductor quantum dots (QD) that provide the basis for a novel type of solar cells. These solar cells have a great potential for creating an additional source of energy and alleviating the economic pressure on petroleum production. Advantages of QDs over other materials used in solar cells, such as bulk silicon, conjugated polymers or dye-sensitized metal oxides, include the tunability of QD optical properties with size, low cost, optical stability and excellent heterojunction with solid-state electrodes. In addition, QD solar cells have a unique potential to produce photon-to-electron quantum yields greater than one, increasing the photocurrent, and to utilize hot carriers, increasing the photovoltage.

The research results obtained during the second year of PRF funding were reported in 17 invited conference talks, 7 invited university seminars, 13 regular articles and 2 invited reviews.  With the help of the ACS-PRF support we were able to generate preliminary results, apply and succeed in obtaining a DOE grant on “Atomistic Time-Domain Simulations of Light-Harvesting and Charge-Transfer Dynamics in Novel Nanoscale Materials for Solar Hydrogen Production”.  The support allowed us to collaborate with scientists at the Los Alamos National Lab, to have several students carry out summer research there, and to be a member of the DOE Center for Integrated Nanotechnologies in Los Alamos.

Specific research projects funded by ACS-PRF in the 2^nd year include:

Symmetric band structure and asymmetric ultrafast electron-hole relaxation in silicon and germanium quantum dots: time-domain ab initio simulation. State-of-the-art time domain density functional theory and non-adiabatic (NA) molecular dynamic simulations were used to study phonon-induced relaxation of photoexcited electrons and holes in Ge and Si quantum dots (QDs). The relaxation competes with productive processes and causes energy and voltage losses in QD solar cells. The ab initio calculations showed that quantum confinement makes the electron and hole density of states (DOS) more symmetric in Si and Ge QDs compared to bulk. Surprisingly, in spite of the symmetric DOS, the electron and hole relaxations are quite asymmetric: the electrons decayed faster than the holes. The asymmetry arises due to stronger NA coupling in the conduction band (CB) than in the valence band (VB). The stronger NA coupling of the electrons compared to the holes was rationalized by the larger contribution of the high-frequency Ge–H and Si–H surface passivating bonds to the CB relative to the VB. Linear relationships between the electron and hole relaxation rates and the CB and VB DOS were found in agreement with Fermi’s golden rule. The faster relaxation of the electrons compared to the holes in the Ge and Si QDs was unexpected and was in contrast with the corresponding dynamics in the majority of binary QDs, such as CdSe. It suggested that Auger processes would transfer energy from holes to electrons rather than in the opposite direction as in CdSe, and that a larger fraction of the photoexcitation energy would be transferred to phonons coupled with electrons rather than holes. The difference in the phonon-induced electron and hole decay rates was larger in Ge than Si, indicating that the Auger processes should be particularly important in Ge QDs. The simulations provided direct evidence that the high-frequency ligand modes on the QD surface play a pivotal role in the electron–phonon relaxation dynamics of semiconductor QDs.

Phonon-induced dephasing of excitons in silicon quantum dots: multiple exciton generation, fission and luminescence. Phonon-induced dephasing processes that govern optical line widths, multiple exciton (ME) generation (MEG), and ME fission (MEF) in semiconductor quantum dots (QDs) were investigated by ab initio molecular dynamics simulation. Using Si QDs as an example, we proposed that MEF occured by phonon-induced dephasing and, for the first time, estimated its time scale to be 100 fs. In contrast, luminescence and MEG dephasing times were all sub-10 fs. Generally, dephasing was faster for higher-energy and higher-order excitons and increased temperatures. MEF was slow because it was facilitated only by low-frequency acoustic modes. Luminescence and MEG coupled to both acoustic and optical modes of the QD, as well as ligand vibrations. The detailed atomistic simulation of the dephasing processes advanced understanding of exciton dynamics in QDs and other nanoscale materials.

Quantum dot charging quenches multiple exciton generation: first-principles calculations on small PbSe clusters. We demonstrated using symmetry adapted cluster theory with configuration interaction (SAC-CI) that charging of small PbSe nanocrystals (NCs) greatly modified their electronic states and optical excitations. Conduction and valence band transitions that were not available in neutral NCs dominated low energy electronic excitations and showed weak optical activity. At higher energies these transitions mixed with both single excitons (SEs) and multiple excitons (MEs) associated with transitions across the band-gap. As a result, both SEs and MEs were significantly blue-shifted, and ME generation was drastically hampered. The overall contribution of MEs to the electronic excitations of the charged NCs was small even at very high energies. The calculations supported the recent view that the observed strong dependence of the ME yields on the experimental conditions was likely due to the effects of NC charging.

Temperature dependence of hot-carrier relaxation in a PbSe quantum dot: An ab initio study. Temperature-dependent dynamics of phonon-assisted relaxation of hot carriers, both electrons and holes, was studied in a PbSe nanocrystal using ab initio time-domain density-functional theory. The electronic structure was first calculated, showing that the hole states were denser than the electron states. Fourier transforms of the time-resolved energy levels showed that the hot carriers couple to both acoustic and optical phonons. At higher temperature, more phonon modes in the high-frequency range participated in the relaxation process due to their increased occupation number. The phonon-assisted hot-carrier relaxation time was predicted using nonadiabatic molecular dynamics, and the results clearly showed a temperature-activation behavior. The complex temperature dependence was attributed to the combined effects of the phonon occupation number and the thermal expansion.