In hybrid halide perovskite, the effectiveness of charge transport in relation to film microstructure and processing has remained elusive. In this study we succeeded in tuning grain size and grain boundary chemistry through solvent vapor annealing, which resulted in an increase in charge-carrier mobility by one order of magnitude. To understand the mechanism responsible for the enhanced charge transport, we performed a series of complementary measurements. Atomic force microscopy revealed an increase in grain size and uniformity, and optical microscopy showed a macroscopic reorganization of the film structure. X-ray diffraction measurements of the MAPbI3-xClx films confirmed the removal of preferential orientation after 20 min of solvent annealing at room temperature, in N,N-dimethylformamide. The presence of additional peaks was assigned to the formation of the solvent complex MAI:DMF:PbI2 and the PbI2:DMF ligand, and the content of these phases was monitored as a function of annealing time. Charge-carrier mobility was evaluated from field-effect transistor measurements in devices with gold top contacts and SiO2 bottom-gate dielectric. We obtained ambipolar transport, with both hole and electron mobility exceeding 10cm2/Vs at room temperature. We propose that this remarkable enhancement in electrical properties resulted from an increase in the grain size and passivation of grain boundaries via formation of intermediate solvent complexes formed from unreacted material. This work has allowed us to gain unprecedented insight into the impact of film morphology on charge transport in perovskite materials, an important milestone towards achieving high-performance optoelectronic devices such as transistors, photovoltaics, light emitting diodes, and photodetectors.