Poly(3-hexylthiophene) bearing pendant fullerenes: aggregation vs. self-organization

Novel graft copolymers are reported based on poly(3-hexylthiophene) (P3HT) bearing side chains of poly(styrene-stat-chloromethylstyrene), onto which a fullerene C-60 or PCBM is covalently attached. P3HT was brominated at the 4-position to various extents (1-30 mol%), subsequently Suzuki-coupled with the boronic ester of 1-(4'-bromopheny1)-1-(2 '',2 '',6 '',6 ''-tetramethyl-1-piperidinyloxy)ethyl (tempo) to form a nitroxide-functionalized P3HT macroinitiator, which was then used to initiate the nitroxide-mediated radical polymerization (NMRP) of chloromethylstyrene (CMS)-stat-styrene (ST) side chains. CMS units were functionalized with azide units and used to attach fullerene. The polymers contained a relatively high mass content of fullerene (20-41 wt%). Photoluminescence of P3HT is strongly quenched by the fullerene. The absorption of P3HT maximum shifts toward shorter wavelengths with increasing graft density. Films of PCBM/C-60 graft copolymers form a bicontinuous morphology with feature sizes <5 nm. Grafting fullerene-bearing side chains directly to P3HT is found to reduce the semicrystallinity of the P3HT domains, reduce the hole charge mobility, and significantly reduce their photovoltaic activity. This is believed to be due to the poorer solubility of the fullerene units relative to the polymer chains which aggregate during film casting and restricts self-organization of the conjugated polymer.