Organic solar cells are diode structures, typically comprising both a light absorbing electron donor semiconductor and an electron accepting semiconductor, with a nanoscale blend microstructure or bilayer, creating an interfacial heterojunction. Fig. 1 illustrates the heterojunction energy diagram of a typical an organic solar cell.

The cell operates by firstly absorbing incoming photons through promotion of an electron in the highest occupied molecular orbital (HOMO) energy level into the lowest unoccupied molecular orbital (LUMO) energy level.  In conjugated systems, this is typically a π→π* transition.  The resulting bound electron-hole pair is referred to as an exciton, which can then diffuse from the p-type domain where it was formed to the interface with the organic n-type semiconductor.  Due to the lower lying LUMO energy level of the n-type semiconductor, it is energetically favourable for the electron to transfer to the n-type semiconductor.  In an optimal system, dissociated charges are then swept towards the electrodes by the built in cell potential.  Obviously the more photons that are absorbed, the greater the charge generation and the larger the current output.  Much research is being targeted at extending the absorption spectra of the light absorbing semiconductors to longer wavelengths in order to absorb more photons.

Bulk heterojunction photovoltaic devices, in which light absorbing, hole transporting semiconductor polymers phase separate at the nanometre length-scale from electron transporting semiconductors are particularly promising.  These devices achieve a compromise between power conversion efficiency and compatibility with the type of manufacturing processes that can enable low cost production and applications. Improvements in device efficiency are being achieved by focussing on new materials which have well designed electronic energy levels capable of generating larger cell voltages, absorbing more photons, improving charge separation, avoiding geminate and bimolecular recombinatilon losses, and having optimal transport properties.