Block copolymer-templated bulk heterojunctions
Lal Busher Azad’s PhD Project
Unlike the flat pn junction in silicon solar cells, the pn junction in organic solar cells is extremely rough in order to harness the large amount of surface area to generate free electrons. A bulk heterojunction is the interface between p- and n-type materials when they are finely dispersed. The size of dispersed structures is limited by the exciton diffusion length, which is ~10 nm. Whilst it is possible to cleverly process a p-type polymer and n-type fullerene into a bulk heterojunction with ~10-nm features, these structures are highly susceptible to coarsening during normal operation of the solar cell. Above the glass transition temperature of the polymer, the bulk heterojunction phases can flow like a viscous liquid. Thermodynamic driving forces favor a lower surface energy, consequently the finely structured bulk heterojunction coarsens, cause the solar cell to underperform.
Block copolymers self assemble into thermodynamically stable structures ~10 nm, depending on the molecular weight of the chain. These inherently stable structure provide an ideal template for structuring a bulk heterojunction. This project seeks to determine whether a block copolymer can be used to host the materials for a plastic solar cell; i.e. one block is designed to absorb the p-type polymer whilst the other block absorbs the n-type fullerene. There are myriad electronic considerations, e.g. Will separating the p- and n-type materials within a host matrix allow for sufficient interfacial contact? Nevertheless, the concept provides an ideal way to produce thermally stable solar cells.
Poly-3-hexylthiophene (P3HT) is the p-type polymer used in this study, phenyl-C61-butyric methyl acid is the n-type fullerene, and polystyrene-block-pmma (PS-b-PMMA) is the block copolymer template. To date, Lal has examined the effects of adding P3HT to the nanostructure of PS-b-PMMA using atomic force microscopy (AFM). At room temperature, toluene is a good solvent for PS-b-PMMA, and a poor solvent for P3HT; consequently, the P3HT slowly precipitates from solution as nanofibril crystals. The age of the solution is controlled, and it is spin coated onto UV ozone-treated silicon wafers. Figure 1a shows a 50-nm film as cast; the disordered fibrils are characteristic of P3HT. Figure 1b shows the same film annealed at 140°C for 1 hour; the resulting structure is regular but disordered and may reflect the underlying structure of the block copolymer. It is unclear where the P3HT fibrils segregate within the film; there is even a very faint possibility that they dissolve into one of the blocks. (Generally polymers do not mix do to enthalpic repulsions.) Lal is continuing to investigate the location of the P3HT fibrils using cross-sectional transmission electron microscopy, and the effects of concentration and film thickness upon the structures that emerge.