REE9: Solar energy: organic photovoltaics

Researcher:  Dr James Kirkpatrick
Team Leader(s): Dr Chris Breward & Prof. John Ockendon
Collaborators: Dr Henry Snaith

Project suspended June 6, 2012, due to recommence October 2013

Background

Organic molecules are promising materials for solar cells, as they could be manufactured inexpensively using existing technology from the plastic industry, such as roll-to-roll printing. These types of solar cells are not yet competitive in the market place because they are not as efficient as traditional materials in converting sunlight to electrical power. The nature of charges and excitations in organic materials is different from traditional semiconductors. For example, in crystalline silicon, the absorption of light leads to the formation of free, delocalised charges, but in an organic molecule, all important processes are mediated by electronic species that are localised on individual molecules.

Techniques and Challenges

The localised nature of electronic species leads to new design requirements, chiefly the fact that different molecules must be used to carry the positive and negative charges. This in turn leads to new modelling challenges.  The main aim of this project was to develop mathematical models of these novel devices from the bottom up, which requires knowledge of electronic structure theory to understand the microscopic processes underpinning device operation.

Results

For organic solar cells made of two flat layers we showed that, in order for the drift-diffusion equation to lead to diode-like current voltage behaviour, it is necessary for interfacial recombination to be the slowest process in the device. Also, we described and solved the time-dependent response of a solar cell to a small light perturbation. The resulting analysis was used to experimentally measure important physical characteristics of a solar cell, for example, the charge collection efficiency. 

For dye-sensitised cells containing phthalocyanines, we modelled light absorption using time-dependent density functional theory and explained why some of these dyes do not lead to increased photocurrents. We also modelled hole-hopping in ruthenium-containing dye-sensitised solar cells. Specifically, we modelled how a charge on a dye might be solvated by ionic species, explaining the large reorganisation energies of the dyes studied.

This work is funded by the James Martin 21st Century School