We measure the time- and temperature-dependent aggregation of the organic photovoltaic polymer poly(3-hexylthiophene) in o-dichlorobenzene solution using small-angle neutron scattering and rheometry. Results from this study provide guidance for formulation stability assessment and process-dependent morphology optimization. The aggregates are similar to the supramolecular crystalline structures found in annealed films and poor solvents. The addition of phenyl-C61-butyric acid methyl ester (PCBM) molecules to the polymer solution slows the aggregation rate by about 10-fold. PCBM addition also appears to reduce the overall extent of crystallization in solution. Dynamic theological measurements show thermoreversible mechanical stiffening of the solutions into gel-like solids with a physical cross-link density that depends on temperature and oscillation frequency.