The :T-stacking of oxidized thiophene oligomers is investigated using extensive ab initio quantum chemistry methods. Dimers of singly charged oligothiophenes are unstable in the gas phase but can be stabilized as bound dications in the singlet state by a polarizable solvent such as acetonitrile. Our calculations provide a detailed description of the mechanisms and the energetics involved in the dimerization phenomenon and highlight the role and importance of the environment in the stabilization of the stacks. The need for accurate treatments of electronic correlations and of solvation effects for a realistic description of these materials is underscored.