In a molecular semiconductor, a charged molecule experiences a lattice relaxation which reorganizes it into a cation or an anion-radical. This species is not, in general, a polaron. By using calculations of the geometry and the electronic structure both ab initio and at the semi-empirical levels, we have explored the conditions of polaron formation in a molecular model system: oligophenylene-vinylenes, PVs of increasing sizes. The symmetry breaking occurs at sizes larger than five monomers for the anion and six monomers for the cation. The driving force for this process is primarily the charge-lattice coupling through the stretching mode at 1600 cm^-1