Computational design of molecules for dye sensitized solar cells and nano electronics

The huge increase in computational power has enabled the use of high-throughput computational screening methods for many purposes. In combination with more detailed computational studies, this provides a powerful tool in the search for new materials and molecules useful for e.g. photovoltaics. This is illustrated in this thesis, where a high-throughput Density Functional Theory study of a total of 5145 porphyrin based dye molecules is presented. Initially, the structures of the dyes are optimized and the frontier energy orbital energies calculated. Following this, the dyes are scored for use in a dye sensitized solar cell (DSSC) in terms of a loss-less level alignment quality. This scoring only takes into account a simplified absorption spectrum of the dye in combination with the alignment between the molecular levels, the semi-conductor conduction band edge and the redox mediator. To improve on this, the effect of the free energy barrier associated with the dye regeneration reaction is included through extensive molecular dynamics simulations for a simple model dye, followed by an extrapolation of the result to the 5145 porphyrins. This model succeeds in rediscovering high efficiency dyes and suggests that the next generation of high efficiency porphyrin dyes may utilize a titanium metal center. Furthermore, the large internal reorganization energies calculated for the octahedral cobalt complexes, used as redox mediators, lead to the requirement of a large driving force for the regeneration reaction. Hence, using redox mediators with a lower internal reorganization energy would allow for a less constrained choice of dye, possibly boosting the efficiency.
The obtained data is furthermore used to search for suitable pairs of porphyrins for a novel type of DSSC schemes, using two dyes in a molecular two-photon tandem approach. Here, a high current is sacrificed for a larger voltage. As a smaller current is however often associated with a better fill-factor, the proposed scheme may lead to an increase in the efficiency. Specific candidates for the different schemes are identified and the resulting setups have theoretically obtainable open-circuit voltages exceeding 1.5 V.
Creating a metal-molecule-metal junction allows tuning the conductance through the junction by manipulating the molecular energy levels. In this thesis a computational approach to model the conductance as a function of the applied bias voltage, shifting the molecular levels, for a redox active molecule is presented and compared to experimental results. Here, it is shown that shifting the molecular energy levels in and out of resonance with the Fermi level of the electrodes, allows for a standard tuning of the conductance. In addition to this, reversibly changing the redox state, allows for switching quantum interference on and off, shifting the conductance by an order of magnitude. The simple computational model used is furthermore qualitatively found to be in very good agreement with experiments.
A different way of tuning the conductance through a molecular junction, is by controlling the junction geometry. This is achieved by designing a molecule with two sets of anchor groups, which bind to gold with significantly different strengths. Hence, it is proposed that the geometry can be controlled by chemical passivisation of one type of anchor group. Using a simple computational model, this experimental hypothesis is verified and the change in conductance upon changing junction geometry is reproduced.