One of the most exciting applications of organic semiconductors is in photovoltaic devices. Their solution processability makes them a promising technology for delivering cheap solar power. They are very strong light absorbers, with a 100 nm or so thick film capable of absorbing most of the incident light. We make solar cells and test them with a solar simulator, and use complementary measurements of photophysics and charge transport to understand their operation. Absorbed photons generate a bound, neutral excited state, an exciton. In order to generate a current from these 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, but as excitons only diffuse about 10 nm during their lifetime the morphology of the blend needs to be controlled and optimised if most of the excitons are to be harvested. Understanding the processes that contribute towards the operation of organic solar cell is crucial for their development.

Exciton Diffusion

The quenching of excitons in the P3HT film by the TiO2 results in a faster decay of the photoluminescence when compared to a film on a non-quenching substrate such as fused silica. Fits to the data are shown for a diffusion coefficient of 1.8x10-3 cm2s-1.

The performance of organic solar cells is greatly influenced by the dynamics of excitons. In particular the distance an exciton is able to travel during its lifetime is of critical importance and knowledge of this parameter can aid the design and optimisation of materials and devices. This is known as the exciton diffusion length and time-resolved fluorescence measurements offer a reliable method for determining it.

One such technique is the surface quenching technique. A polymer film is spin coated onto a quencher, which results in exciton dissociation at the interface. When compared to a similar film on a non-quenching substrate the photoluminescence decay is faster for the film on the quencher.

The output from the diffusion model for a 15 nm thick film with the quencher at z = 0. This greatly reduces the number of excitons in the surrounding film.

By modelling the motion of the excitons in the polymer film as a diffusion process it is possible to extract a value of the diffusion coefficient by fitting to the PL decays. By repeating the measurement for a range of film thickness a reliable estimate can be obtained. A detailed description of the experiment on the polymer P3HT is reported in Adv. Mat., 20, 3516 (2008).