In this work the effects of Co3O4 concentration on the properties of agarose polymer electrolyte and the corresponding DSSCs are studied with/without magnetic field treatments. The surface morphology, ion concentration, and ionic conductivity of agarose electrolytes with different Co3O4 concentrations are compared under 0 and 35 mT magnetic field using various characterization methods. The photovoltaic performances of quasi-solid-state DSSCs with Co3O4 modified polymer electrolyte are examined. After applying magnetic fields, the ionic conductivity and photo-to-electric conversion efficiency of DSSCs are obviously improved in the Co3O4 concentrations below 4 wt.%. The highest ionic conductivity of 6.00 mS cm− 1 is obtained with electrolyte modified by 3 wt.% Co3O4. The DSSCs with the optimal Co3O4 content show the best conversion efficiency under both 0 and 35 mT magnetic fields.