Gabriel Bernardo

Understanding and controlling morphology evolution via DIO plasticization in PffBT4T-2OD/PC71BM devices.

We demonstrate that the inclusion of a small amount of the co-solvent 1,8-diiodooctane in the preparation of a bulk-heterojunction photovoltaic device increases its power conversion efficiency by 20%, through a mechanism of transient plasticisation. We follow the removal of 1,8-diiodooctane directly after spin-coating using ellipsometry and ion beam analysis, while using small angle neutron scattering to characterise the morphological nanostructure evolution of the film. In PffBT4T-2OD/PC71BM devices, the power conversion efficiency increases from 7.2% to above 8.7% as a result of the coarsening of the phase domains. This coarsening process is assisted by thermal annealing and the slow evaporation of 1,8-diiodooctane, which we suggest, acts as a plasticiser to promote molecular mobility. Our results show that 1,8-diiodooctane can be completely removed from the film by a thermal annealing process at temperatures ≤100 °C and that there is an interplay between the evaporation rate of 1,8-diiodooctane and the rate of domain coarsening in the plasticized film which helps elucidate the mechanism by which additives improve device efficiency.

Recent progress in the understanding and manipulation of morphology in polymer : fullerene photovoltaic cells

The morphology of the active layer in OPV devices is widely recognized as being crucial for their photovoltaic performance [1-4]. The physics of the system dictates that excitons must dissociate efficiently at a donor-acceptor interface, and that sufficient pathways for charge transport to the electrodes are also required. Conjugated polymer crystals are considered to be the primary hole carrier and thus are essential for effective charge transport. With this in mind, the ideal morphology for an organic photovoltaic BHJ film was often considered until a few years ago to be a bicontinuous, interpenetrating network morphology composed of pure P3HT and pure PCBM phases, with both phases of order ∼20 nm in size [5, 6] and numer‐ ous cartoon depictions have helped to propagate this view, as the one shown in Figure 1.

Low temperature solid state processing of pure P3HT fibers

Fibers of pure poly(3-hexylthiophene) (P3HT) of 1 and 2 mm diameter, were produced from solid state processing at temperatures of 200, 150 and 100oC, i.e. up to more than 100 oC below the melting point of P3HT (∼240 oC), using a small device reminiscent with the early stage polymer extrusion machines. The fibers produced by this method are continuous and mechanically robust. WAXS results show that the macromolecular chains of P3HT are preferably oriented along the fiber axis. This study clearly shows, for the first time, that P3HT fibers can be processed without solvents well below the polymers melting temperature.

Does 1,8-diiodooctane affect the aggregation state of PC71BM in solution?

1,8-Diiodooctane (DIO) is an additive used in the processing of organic photovoltaics and has previously been reported, on the basis of small-angle X-ray scattering (SAXS) measurements, to deflocculate nano-aggregates of [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) in chlorobenzene. We have critically re-examined this finding in a series of scattering measurements using both X-rays and neutrons. With SAXS, we find that the form of the background solvent scattering is influenced by the presence of DIO, that there is substantial attenuation of the X-rays by the background solvent and that there appears to be beam-induced aggregation. All three factors call into question the suitability of SAXS for measurements on these samples. By contrast, small-angle neutron scattering (SANS) measurements, performed at concentrations of 15 mg ml−1 up to and including 40 mg ml−1, show no difference in the aggregation state for PC71BM in chlorobenzene with and without 3% DIO; we find PC71BM to be molecularly dissolved in all solvent cases. In situ film thinning measurements of spin-coated PC71BM solution with the DIO additive dry much slower. Optical imaging shows that the fullerene films possess enhanced molecular mobility in the presence of DIO and it is this which, we conclude, improves the nanomorphology and consequently solar cell performance. We propose that any compatible high boiling solvent would be expected to show the same behaviour.