Our work deals with the improvement of "light harvesting" in organic photovoltaic cells by using photonic nanostructures. We have theoretically studied a periodically nanostructured poly
(3-hexylthiophene)(P3HT)/6,6-phenyl C61-butyric acid methyl ester (PCBM) thin film in order to increase its absorption in the near infrared spectral range. We have used a software, based on the FDTD (Finite-Difference Time-Domain) method, to calculate the absorption of light in organics solar cells. We have also considered the nanostructured photoactive layer of solar cells as a photonic crystal
and we have computed band diagrams to study the dispersion curves of this structure. We have first studied a blend (bulk heterojunction) with the same proportions of P3HT and PCBM. This material
provide at this time the best results in terms of photovoltaic efficiency. Nevertheless, in order to improve the
transport of charges to the electrodes, a model with P3HT and PCBM independently nanostructured (ordered
heterostructure) was also used. Moreover, this periodic nanostructuration allows "slow Bloch modes" to be coupled
inside the device with a low group velocity of electromagnetic waves. Thus, the interaction duration between light and
organics materials is improved.
The P3HT/PCBM photonic crystal parameters have been adjusted to maximize the density of Bloch modes and to obtain
flat dispersion curves. We have found that the light matter interaction was strongly enhanced which resulted in a 35.6%
increase of absorption in the 600 nm to 700 nm spectral range. In order to realize nanostructured organic solar cells, we
are also developing an experimental prototype, based on a patented process, which allows to nanostructure several kinds of polymers.