Conducting polymer poly(3,4-ethylenedioxythiophene):polystyrenesulfonate (PEDOT:PSS) is gaining technological importance for the fabrication of organic and organic–inorganic heterostructure devices. The conductivity of PEDOT:PSS can be improved by the addition of co-solvents. Here, we show that the simple addition of a suitable wt% of a co-solvent, either ethylene glycol (EG) or dimethyl sulfoxide (DMSO), in PEDOT:PSS can significantly enhance the performance of hybrid solar cells. We provide a morphological model to explain the influence of the co-solvents in PEDOT:PSS, in which the co-solvent modifies the internal crystalline ordering of individual PEDOT nanocrystals that increases the crystal size and forms closely packed nanocrystals, and it also facilitates rearrangement of PSS that reduces its surface chain networks to enhance the polymer conductivity and hybrid solar cell properties. A hybrid solar cell made of EG 7 wt% modified PEDOT:PSS on planar Si exhibits an exceptionally high power conversion efficiency exceeding 12% for the first time.