Bimolecular Crystals of Fullerenes in Conjugated Polymers and the Implications of Molecular Mixing for Solar Cells

Abstract

The performance of polymer:fullerene bulk heterojunction solar cells is heavily influenced by the interpenetrating nanostructure formed by the two semiconductors because the size of the phases, the nature of the interface, and molecular packing affect exciton dissociation, recombination, and charge transport. Here, X‐ray diffraction is used to demonstrate the formation of stable, well‐ordered bimolecular crystals of fullerene intercalated between the side‐chains of the semiconducting polymer poly(2,5‐bis(3‐tetradecylthiophen‐2‐yl)thieno[3,2‐b]thiophene. It is shown that fullerene intercalation is general and is likely to occur in blends with both amorphous and semicrystalline polymers when there is enough free volume between the side‐chains to accommodate the fullerene molecule. These findings offer explanations for why luminescence is completely quenched in crystals much larger than exciton diffusion lengths, how the hole mobility of poly(2‐methoxy‐5‐(3′,7′‐dimethyloxy)‐p‐phylene vinylene) increases by over 2 orders of magnitude when blended with fullerene derivatives, and why large‐scale phase separation occurs in some polymer:fullerene blend ratios while thermodynamically stable mixing on the molecular scale occurs for others. Furthermore, it is shown that intercalation of fullerenes between side chains mostly determines the optimum polymer:fullerene blending ratios. These discoveries suggest a method of intentionally designing bimolecular crystals and tuning their properties to create novel materials for photovoltaic and other applications.

Charge generation in polymer–fullerene bulk-heterojunction solar cells

“…For example, it has been demonstrated in many combinations that an intermixing phase with both polymer and fullerene is crucial for efficient charge generation. [133][134][135][136][137][138] This delocalisation mechanism does not seem to give a direct answer to this …”

On the morphology of polymer‐based photovoltaics

“…12 for detail). 77,78 Once the PCBM intercalated in polymer side chains, the (100) d-spacing shifted to a larger size with higher orders of reflections observed, indicating the formation of cocrystals. This cocrystalization process takes large amount of PCBM thus a higher loading ratio is required to form interpenetrating acceptor phase.…”

Silaindacenodithiophene Semiconducting Polymers for Efficient Solar Cells and High-Mobility Ambipolar Transistors

“…10 The high optimal ratio of fullerene, particularly in the P-1 devices, indicates that it is likely that the fullerene has a high incorporation within the polymers. 11 The low lying HOMO energy level of P-1 promotes the larger V oc exhibited by the P-1 devices, shown in Figure 3a, and with the low bandgap also contributing to good current density, a relatively large PCE was obtained.…”