REE9: Solar energy: organic photovoltaics

If energy production is to be made carbon free and sustainable, photovoltaic cells are likely to be a major source of energy. At the moment, even though the solar industry is growing at 40%, greater usage is hampered by the high costs. In conventional semiconductors, such as silicon, a large proportion of this cost is material as producing high purity crystals is an expensive undertaking. Organic semiconductors offer the possibility of significant material cost reductions through the use of high throughput processing techniques such as printing, but unfortunately their efficiencies are still too low for commercialisation.

Another great opportunity that organic semiconductors offer is chemical tailoring of properties. Using chemical synthesis, absorption coefficients and electron diffusion coefficients can be modified. Unfortunately, simple models of the role of the material properties in determining current voltage characteristics and hence maximum efficiencies have not been achieved.

Organic photovoltaic cells differ from their inorganic counterparts in several key issues:                                           

• Basic operation mechanism. In an  organic solar cell two different materials are used. When one of them is excited by light, the resulting excited state must travel to an interface to be separated into electrons and holes.  Charges are generated by jumps in electrochemical potential between the different materials. Charges are therefore segregated in different regions of the device.
• Disorder. Unlike single crystalline devices, the materials are electronically disordered. Transport occurs by hopping between localised states. This creates an effective diffusion coefficient of the charges which is dependent on charge density, electric field and temperature.
• Morphology. The thickness that is necessary to absorb a significant proportion of sunlight is much greater than the distance excited states are able to diffuse before recombining. This leads to the need to use highly convoluted structures to ensure high charge collection. 

The aim of this project is to obtain a model for organic solar cells that is capable of explaining the current voltage curves as a function of material and physical parameters.  Device operation is modelled by solving (asymptotically) the current continuity equations for electron and holes in the steady state, subject to jump conditions at the interface between the two materials.

In order to make the present model applicable to more realistic scenarios, expressions for the charge mobility and charge recombination will be extended so that they depend on the electric field. The resulting model will be comparable with real life data on bi-layer devices fabricated by our experimental collaborators

The next stage will be the introduction of simple idealised 2 dimensional morphologies, we will consider the case of interpenetrating columns of one material into another, as illustrated in the figure.

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