The device performance and microstructure of a series of PTB7-based polymers with varied molecular weight and degree of fluorination are investigated. Although the energy level of the highest occupied molecular orbital is found to increase with degree of fluorination, a strong molecular weight dependence of device performance dominates any underlying fluorination-dependent trend on overall performance. Microstructural investigation using a combination of X-ray techniques reveals a striking effect of polymer molecular characteristics on film morphology, with the size of PC71BM domains systematically decreasing with increasing polymer molecular weight. Furthermore, the relative purity of the mixed PTB7:PC71BM domain is found to systematically decrease with increasing molecular weight. Comparing domain sizes with and without the use of the solvent additive diiodooctane (DIO), the effectiveness of DIO in reducing PC71BM domain sizes is also found to be strongly dependent on the molecular weight of the polymer. It is found that molecular weights of at least 150 kg/mol are required for DIO to be effective in reducing the PC71BM domain size in order to produce high short-circuit current densities. Finally, it is shown that relatively high device efficiencies can be achieved with low degrees of fluorination, with an efficiency of 4.6% achieved for a degree of fluorination of only 5.3%.