title: The good host: formation of discrete one-dimensional fullerene "channels" in well-ordered poly(2,5-bis(3-alkylthiophen-2-yl)thieno[3,2-b]thiophene) oligomers.
abstract: Due to the unique crystallinity of poly(2,5-bis(3-alkylthiophen-2-yl)thieno[3,2-b]thiophene) (PBTTT), it is an excellent model polymer to study the structure-property relationship in organic devices, especially those relying on junctions of electron- and hole-transporting materials. Here, we report the synthesis and characterization of a series of monodisperse PBTTT oligothiophenes (n = 1-5) and systematically examine the evolution of crystalline behavior, morphology, and interaction with [6,6]-phenyl C61-butyric acid methyl ester (PCBM) as the molecular conjugation length increases. We discovered that fullerene intercalation occurs when there is enough free volume between the side chains to accommodate the fullerene molecule. The intercalation of PCBM is observed beyond BTTT-2 and longer oligomers, likely similar to that of PBTTT. Interestingly, both experiments and molecular simulations show that PCBM intercalation also appears to "catalyze" a more efficient packing of the BTTT-2 dimers. Crystal structure analysis revealed that the straight BTTT-2 side chains form one-dimensional (1D) channels that could perfectly host PCBM but, in the pure material, accommodate the interdigitated side chains from adjacent layers. In the blend with PCBM, these channels are maintained and enable the cocrystallization and intercalation of PCBM. This is the first time the actual sublattice cell of PCBM has been determined from the X-ray data, and demonstration the utility of the oligomers as model systems for their polymer counterparts. Among the organic photovoltaic devices (OPVs) made from the BTTT oligomers and [6,6]-phenyl C71-butyric acid methyl ester (PC71BM) blends, the ones containing the BTTT-2 dimer exhibit the highest performance.
authors.affiliation: Department of Polymer Science and Engineering, University of Massachusetts , 120 Governors Drive, Amherst, Massachusetts 01003, United States.
authors.forename: Lei
authors.initials: L
authors.lastname: Zhang
authors.forename: Feng
authors.initials: F
authors.lastname: Liu
authors.forename: Ying
authors.initials: Y
authors.lastname: Diao
authors.forename: Hilary S
authors.initials: HS
authors.lastname: Marsh
authors.forename: Nicholas S
authors.initials: NS
authors.lastname: Colella
authors.forename: Arthi
authors.initials: A
authors.lastname: Jayaraman
authors.forename: Thomas P
authors.initials: TP
authors.lastname: Russell
authors.forename: Stefan C B
authors.initials: SC
authors.lastname: Mannsfeld
authors.forename: Alejandro L
authors.initials: AL
authors.lastname: Briseno