Side‐Chain‐Bulk Effects on the Molecular Packing and Photovoltaic Performance of Benzotrithiophene–Benzooxadiazole Conjugated Copolymers

Abstract

Two alternating donor-acceptor conjugated polymers, PBTTBO-C13 C11 and PBTTBO-C13 C8 , comprising 5-alkylbenzo[1,2-b:3,4-b':5,6-d'']trithiophene (BTT) as the donor and 4,7-bis(4-dodecylthien-2-yl)benzo[1,2,5]oxadiazole (BO) as the acceptor, with different alkyl side-chain architectures on their BTT units are synthesized, and their bulk heterojunction photovoltaic properties when blended with the fullerene PC71 BM are characterized. Even a slight change in the length of the alkyl chain of the BTT units influences the steric bulk to such a degree that it substantially affects the molecular packing of the polymers and the performance of their photovoltaic devices. The bulkier side chains of the polymer PBTTBO-C13 C11 not only prevent its crystallization, but also suppress its light-absorption coefficient relative to that of PBTTBO-C13 C8 , as evidenced by X-ray diffraction and UV/Vis absorption studies, presumably because of weakened intermolecular interactions. Moreover, the polymer bearing bulkier side chains, PBTTBO-C13 C11 , is less miscible with PC71 BM than PBTTBO-C13 C8 , and this characteristic determines the morphology of their annealed blended films, as shown by TEM studies. The best efficiency is obtained with a device containing an annealed PBTTBO-C13 C8 /PC71 BM (1/2 w/w) active layer that was maintained at 120 °C for 10 min, which shows a power conversion efficiency of 6.2 %, an open-circuit voltage of 0.75 V, a short-circuit current density of 12.6 mA cm(-2) , and a fill factor of 66 %.

Side chain structure affects the molecular packing and photovoltaic performance of oligothiophene-based solution-processable small molecules

“…40,41 For polymeric systems, side-chain engineering inuences the degree of molecular packing. [42][43][44][45][46] For solution-processed small molecules, the position [47][48][49] and also the length [50][51][52] of the side chain on the molecule can greatly impact its molecular crystallinity and the performance of its corresponding devices. In spite of these efforts, the studies of side-chain engineering for solutionprocessed small molecules OPV are still limited.…”