Typically, a maximum efficiency in PHJs is expected for ~10 nm of active material – the value of the exciton diffusion length in most conjugated polymers. The idea behind this statement is that if an exciton is generated at a distance greater than its diffusion length from the hetero-interface of the PHJ, it would be very unlikely to be able to contribute to the photocurrent.

Searching through the literature, the researchers noted that their device was not the only P3HT-based PHJ device to display anomalous behaviour. Another design, this time with PCBM molecules functioning as the acceptor material, was also reported to show a similar trend in photocurrent versus P3HT thickness.

Predicting device behaviour

The Strano team decided to explore these results in more detail by combining both an optical T-matrix and a Kinetic Monte Carlo (KMC) model. The optical model determines where photons are being absorbed in the device and shows that interference plays a much larger role in the P3HT/PCBM device than in the P3HT/SWNT device because the reflecting electrode is positioned directly beneath the active layer. Once it is known where photons are absorbed (and hence excitons are generated), the KMC simulation focuses on tracking the path of each exciton.

By taking into account the recent experimental observation that excitons can also dissociate in the bulk of the active layer, not just at the hetero-interface, the model accurately predicts the trend for the P3HT/SWNT device. The bulk exciton dissociation is more pronounced for smaller thicknesses of P3HT (due to a corresponding stronger electric field), which shifts the maximum photocurrent to larger values of the P3HT thickness.

In the P3HT/PCBM case this effect is offset by the increased positive optical interference at lower thicknesses. Here the model teaches us that the shift of the maximum is caused by PCBM molecules interdiffusing into the P3HT upon annealing.

Based on the results of this model it will be possible to improve the design of nanostructured photovoltaics.

Further information can be found in the journal Nanotechnology.