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Title: Theoretical studies of the dynamics of electron transfer in dye sensitized solar cells and of the mechanism of singlet fission in organic crystalline materials
Author: Ambrosio, Francesco
ISNI:       0000 0004 5364 2154
Awarding Body: University of Warwick
Current Institution: University of Warwick
Date of Award: 2014
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We present a theoretical and computational study on two different processes of interest for the development of third generation solar cells: (i) electron injection in dye sensitized solar cells (DSSCs); (ii) mechanism of singlet fission (SF) in organic crystalline materials, commonly employed in organic solar cells. (i) Electron injection rates in DSSCs are computed through a combination of ab initio calculations and theoretical modeling. In particular, rates are calculated using a matrix partitioning approach, in conjunction with the propagation of Green’s functions. We are able to separate the entire dye-semiconductor surface in smaller sub-systems, whose study is less computational demanding. We prove that this approach is not only capable of simulating experimental results, but, due to its flexibility, can be used in a predictive way. We propose three possible applications of our method: a) a comparative study of different organic dyes sharing the same anchoring group; b) an investigation aimed to identify the optimal anchoring group for a DSSC dye, by screening 15 potential candidates in terms of adsorption strength on the TiO2 surface and electron injection properties; c) an extension of our approach for the study of metal-organic dyes, attaching the TiO2 surface through a variety of binding modes and with multiple anchoring groups. (ii) SF is studied focusing on the multi-excitonic (ME) intermediate, playing a pivotal role in the mechanism of the process. A model configuration interaction Hamiltonian is presented for the study of the electronic structure of a linear cluster of molecules undergoing to SF. The analysis of its electronic structure shows that different distances between the pseudo-triplets composing the ME produce energetically distinguishable states. In particular, we can separate the ME in two types: bound stabilized ME states whose pseudo-triplets are located on neighboring molecules, and unbound ME states, whose energy is almost equal to the energy of two triplets. We also demonstrate that, while singlet excitons are delocalized, ME states are localized. Dynamics of the ME←S1 radiationless decay is studied with Fermi golden rule, where a model is proposed to simulate explicitly the vibronic ME states. Effect of different choices of model parameters is studied, along with an analysis of the branching of the rates, showing that the transition occurs almost completely toward bound ME states.
Supervisor: Not available Sponsor: European Research Council (ERC)
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: QD Chemistry