Thin-film field-effect transistors showing n- and p-type conduction under different bias conditions are produced from solution-processable ambipolar blends of thieno[2,3-b]thiophene terthiophene polymer and phenyl C61 butyric acid methyl ester (see Figure). Balanced charge transport in this blend is achieved by treating the insulator interface with alkyl-chain silanes. Complementary-like inverters have been fabricated on a single substrate, showing a maximum gain of 65.