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Simulation of Charge Carrier Dynamics at Interface of Two Semiconductors in Photoelectrochemical Cell Using Nonadiabatic Molecular Dynamics Approach

Mehdipour, Hamid | 2019

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  1. Type of Document: Ph.D. Dissertation
  2. Language: Farsi
  3. Document No: 52452 (04)
  4. University: Sharif University of Technology
  5. Department: Physics
  6. Advisor(s): Moshfegh, Alireza; Taeifeh Rezakhani, Ali
  7. Abstract:
  8. Nowadays, the population growth and industrial development have caused pollution of air, water, and soil, which this turned into in a global challenge. Thus, replacing the running-out fossil fuels with renewable sources of energy is an important issue facing research and scientific communities. One of the methods to generate clean energy is hydrogen production through water splitting reaction inside photoelectrochemical cells using an appropriate semiconductor. Upon the light illumination, rates of charge transfer and recombination in semiconductors heterostructure (as photoanodes) are determining factors for efficiency of these cells. If electron transfer occurs faster (in a shorter time period) than charge recombination process, the more efficient the cell would be expected. In this research we will two semiconductor heterostructures, namely: 1) cadmium sulfide/titanium dioxide (CdS/TiO2) and 2) Zn-Phthalocyanine/few-layer graphene (ZnPc/FLG), which have become known as suitable candidates for high-efficiency photoelectrochemical cells. Motivated by observations of sophisticated and accurate experimental measurements, such as transient absorption spectroscopy, and results of photocurrent density generated from water splitting, we have used nonadiabatic molecular dynamics simulation to model the dynamics of charge carriers (generated as a result of light illumination) in CdS/TiO2 and ZnPc/FLG semiconductor heterostructures. Faster electron transfer (in 260 fs) observed at the interfaces of CdS/TiO2 heterostructures is interpreted based on stronger nonadiabatic coupling of 2.1 meV formed between the orbitals participating in the electron transfer process and long coherence of 47 fs between these orbitals. In contrast, the recombination across the interface takes place slowly (in 18 ps) due to weak coupling and rapid decoherence between edge states of the conduction and valence bands. For the ZnPc/FLG system, calculations show that faster electron transfer from ZnPc molecule to single layer graphene (580 fs) than double layer graphene (810 fs) can be rationalized by stronger coupling formed between donor and acceptor states in the molecule and single layer graphene (4.0 meV). Furthermore, the ZnPc out-of-plane bending mode at 1100 cm-1 and an overtone of fundamental mode in graphene at 2450 cm-1 are the main driving forces for the electron transfer at the ZnPc/FLG interfaces. The results obtained are in good agreement with experimental observations, and accurate understandings of the underlying mechanisms governing the charge transfer and recombination processes are obtained using this simulation. Such understanding would enable researchers to predict the material combinations with similar or superior properties suitable for photoelectrochemical applications
  9. Keywords:
  10. Inhomogeneous Structure ; Charge Transfer Compounds ; Nonadiabatic Molecular Dynamics ; Time Dependent Density Functional Theory ; Lattice Vibrations ; Charge Recombination

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