Conventional and inverted organic solar cells of poly[9,9-dioctyl-fluorene-co-bithiophene] (F8T2) as liquid-crystal semiconducting polymer and fullerene as electron acceptor were fabricated and characterized. An effect of thermal treatment of the films on annealing condition near glass transition was investigated for tuning optimization and improving the photovoltaic and optical properties. Annealing treatment below the glass transition improved the photovoltaic performance and carrier diffusion in crystal growth of active layer. The X-ray diffraction patterns indicate a crystalline structure with molecular order of F8T2 in crystal index, 100 as a layer distance between sheets of F8T2 chains. The photovoltaic properties were based on molecular interactions with molecular ordering in active layer at crystal state. As the photovoltaic mechanisms, the F8T2 thin film as p-type semiconducting polymer worked for electron-donor layer to support light-induced generation, carrier diffusion and charge transfer near interface in active layer. Copyright (c) 2014 John Wiley & Sons, Ltd.