  In organic optoelectronics, order of conjugated molecules is required for
good charge transport, but strong aggregation behavior may generate grain
boundaries and trapping, opposing those benefits. Side chains on a polymer's
backbone are major reason for and also tool to modify its morphological
characteristics. In this report, we show on the example
poly(9,9-dioctylfluorenyl-co-bithiophene) (F8T2) that by a combination of two
types of side-chains on the backbone of equal number of carbons, one promoting
crystallization, another hindering it, organization of the main chains can be
controlled, without changing its major properties. We compare the traditional
F8T2 derivative with octyl substituent with two modified species, one
containing solely 2-ethylhexyl side-chains and another with both types randomly
distributed. Thermal characteristics, photophysics and morphology are compared
and effects on film formation and charge transport in bulk-heterojunction
blends demonstrated on photovoltaic devices utilizing F8T2s as donor and the
fullerene derivative ICBA as acceptor material.
