The narrow optical gap conjugated polymer poly[N-9″-heptadecanyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole)] (PCDTBT) has been used as the electron donor material in efficient solution-processed bulk-heterojunction (BHJ) photovoltaic and photodetector devices when blended with fullerene derivatives. It was found that the solution viscosity used to form the active films could be controlled by the cooling rate of a hot solution of the materials and that fast-cooled solutions afforded more efficient bulk-heterojunction solar cells than their slow-cooled counterparts. Viscometry measurements showed that the rheological behavior of the solutions is modeled by the Martin equation for different PCDTBT molecular weights and temperatures. The Martin constant KM that describes the interpolymer interactions in solution was found to increase with polymer molecular weight and decrease with increasing temperature in an analogous manner to the Flory-Huggins interaction parameter χ. Small-angle neutron scattering (SANS) was used to show that when hot solutions of the polymer were cooled, phase separation into polymer-rich clusters and solvent-rich domains occurred. Similar phase separation was observed in the case of blend solutions. In addition, the fast-cooled solutions trapped more 70-PCBM in the polymer-rich phase, which in turn made the structure of the polymer more rodlike in the clusters. The results provide an explanation as to why fast-cooled solutions lead to devices with greater efficiency.