Direct and indirect electron transfer at a semiconductor surface with an adsorbate: Theory and application to Ag3Si(111):H

Quantum confinement effects on electronic photomobilities at nanostructured semiconductor surfaces: Si(111) without and with adsorbed Ag clusters

The conductivity of holes and electrons photoexcited in Si slabs is affected by the slab thickness and by adsorbates. The mobilities of those charged carriers depend on how many layers compose the slab, and this has important scientific and technical consequences for the understanding of photovoltaic materials. A previously developed general computational procedure combining density matrix and electronic band structure treatments has been applied to extensive calculations of mobilities of photoexcited electrons and holes at Si(111) nanostructured surfaces with varying slab thickness and for varying photon energies, to investigate the expected change in mobility magnitudes as the slab thickness is increased. Results have been obtained with and without adsorbed silver clusters for comparison of their optical and photovoltaic properties. Band states were generated using a modified ab initio density functional treatment with the PBE exchange and correlation density functionals and with periodic boundary conditions for large atomic supercells. An energy gap correction was applied to the unoccupied orbital energies of each band structure by running more accurate HSE hybrid functional calculations for a Si(111) slab. Photoexcited state populations for slabs with 6, 8, 10, and 12 layers were generated using a steady state reduced density matrix including dissipative effects due to energy exchange with excitons and phonons in the medium. Mobilities have been calculated from the derivatives of voltage-driven electronic energies with respect to electronic momentum, for each energy band and for the average over bands. Results show two clear trends: (a) adding Ag increases the hole photomobilities and (b) decreasing the slab thickness increases hole photomobilities. The increased hole populations in 6- and 8-layer systems and the large increase in hole mobility for these thinner slabs can be interpreted as a quantum confinement effect of hole orbitals. As the slab thickness increases to ten and twelve layers, the effect of silver adsorbates decreases leading to smaller relative enhancements to the conduction electron and hole mobilities, but the addition of the silver nanoclusters still increases the absorbance of light and the mobility of holes compared to their mobilities in the pure Si slabs.

Photoinduced electron transfer at a Si(111) nanostructured surface: effect of varying light wavelength, temperature, and structural parameters

We treat electronic dynamics at surfaces of nanostructured semiconductors induced by absorption of visible light using reduced density matrices and properties obtained from ab initio electronic structure calculations, to focus on two non-adiabatic phenomena: (a) how active electrons interacting non-adiabatically with atoms at the surface undergo electronic transitions and (b) how active electrons interacting by exchanging energy with excitons in the medium undergo a dissipative non-adiabatic dynamics. We test the effects on charge separation from varying oscillator strengths, non-adiabatic momentum couplings, the rates of relaxation of excited states coupled to the medium, temperature, and light wavelength. Varying the oscillator strength displays the interplay between competing relaxation and charge transfer dynamics. Varying the non-adiabatic momentum coupling between excited and final states demonstrates the importance of including enough vibrational levels to model the full dynamics of the system and further shows the interplay of relaxation and charge transfer from the final state to the excited state. Larger electron transfer probabilities and longer lasting charge separation occur when oscillator strength into the intermediate state decreases, or when it increases into the final state, and when temperature increases. Longer lasting charge separation also occurs when the non-adiabatic momentum coupling decreases, a somewhat unexpected result which is due to the combined effect of population relaxation and transitions among many vibronic states.