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Charge Transport in Organic Semiconductors

GENERAL FRAMEWORK. Semiconductors based on organic compounds have been proposed as substitutes for silicon or germanium since the beginning of the electronics era, but they found only a limited group of applications until the beginning of the 90s. In the near future, organic electronics materials are expected to play an important role in applications that require low-cost, large-area and flexible electronic devices. The structure-property relations of these materials are not known and theory is expected to contribute to the determination of these relations and the development of new materials.

AIM. We try to compute the absolute charge mobility for all classes of organic materials (molecular crystal, liquid crystal, polymers) using a consistent bottom-up approach. We characterize the system (static or dynamic disorder, electronic Hamiltonian, electron-phonon coupling, morphology) before choosing or developing the appropriate charge transport mechanism. We use a variety of computational (molecular dynamics, quantum chemistry) methods and model Hamiltonian methods.


RESULT HIGHLIGHTS. We discovered the appropriate transport model to be used in molecular crystal at room temperature (dynamic disorder model) which conciliates apparently contrasting experimental observations and allows the computation of the mobility without adjustable parameters [28,35,53,60]. We are now developing a new theory for charge transport in polymers [98,105,110].


Theory of Organic Photovoltaics

GENERAL FRAMEWORK. Blends of organic electron donor and acceptor materials are promising candidates for the realization of low-cost plastic solar cell. The elementary processes taking place in the cell are not well understood, e.g. it is not known why positive and negative charges separate so well at the interface.

AIM. Understanding the mechanism of all elementary processes and predicting their rates by modelling at the same time the morphology of the material and its electronic properties.

RESULT HIGHLIGHTS. We studied the elementary processes in idealized junctions [68,78] and we have developed a realistic model of the junction for more accurate investigations. We have proposed a mechanism that explains why some polymer:fullerene blends are so effective in organic solar cell [83,89]. We are now considering quantum coherent effects [111] and optical properties [101].


Theory of Dye Sensitized Solar Cells

GENERAL FRAMEWORK. DSSCs represent another promising family of solar cells, potentially cheaper than the currently available silicon cells, based on dyes adsorbed on nanocrystalline titania. Also in this case the relation between chemical structure of the dye-electrolyte-additives and the solar cell efficiency is not well known.

AIM. Understanding the mechanism of all elementary processes in DSSC and predicting their rates by combining advanced electron propagator theory and state of the art surface science calculations.

RESULT HIGHLIGHTS. We developed a method to rapidly screen tens of dyes for their ability to efficiently inject an electron in the semiconductor [72]. We adapted the method to find out what is the best anchoring group for dyes [81]. We computed, for the first time, the charge recombination time in DSSC [80]. The combination of these results paves the way for ab initio desing of new dyes.


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