Organic Solar Cells
Organic semiconductors are plastic-like materials that combine novel semiconducting optoelectronic properties with scope for simple fabrication. Organic light-emitting diodes are now commercial display devices, and are being developed towards lighting applications. Organic solar cells are also very promising and an important field of research in the SUPA energy theme. The simple processing of these materials – for example by printing processes means that plastic solar cells could be mass produced at low cost and with low embedded energy. Organic semiconductors are very strong absorbers of light, so a film only 100 nm thick can absorb most of the light falling on it. SUPA research into organic solar cells can look at all aspects of materials and devices, from fundamental photophysics through structural studies to device performance and testing.
When a photon is absorbed in the organic semiconductor it generates a bound, neutral excited state, an exciton. In order to generate a current the exciton must be separated into its constituent electron and hole and the charges extracted from the device. This is achieved by blending the organic semiconductor with an electron acceptor, such as a fullerene. The key steps in the operation of an organic solar cell are then: light absorption, exciton diffusion, charge separation and charge extraction. We develop materials and measurements to understand and improve each of these steps. For example, most organic solar cell materials have visible band gaps, and so lower band gap materials are in development to absorb more of the solar spectrum. We have developed sophisticated measurements of exciton diffusion and are now applying these to understand the physics of exciton transport and the factors controlling it. Laser-based measurements also allow us to study charge transport and recombination. By combining materials, photophysical, structural and device measurements we can understand and improve organic solar cell materials and devices.