Binder-free titania pastes with high viscosity were developed for the preparation of electrodes for dye-sensitized solar cells on plastic substrates. The rheological behaviour of the ethanol-based titania pastes with the addition of ammonia, hydrochloric acid and water was investigated. The change in the viscosity could be correlated with the measured zeta-potential of the colloidal titania pastes. A newly developed nano-scratch test was used to evaluate the mechanical strength or the inter-particle connectivity of the titania films prepared under different conditions. Improved inter-particle connectivity and solar cell performance were found for the pastes containing acid or water, but no such improvements were seen for the pastes containing ammonia. A maximum light-to-electrical energy conversion efficiency of 4.9% (under one sun - 100 mW cm-2) was obtained for the plastic-based dye-sensitized solar cells fabricated by the modified titania pastes.