Oxygen-modified poly(4-dicyanomethylene-4H-cyclopenta[2,1-b;3,4-b ']dithiophene): A tunable low band gap polymer

The reaction of n-doped poly(4-dicyanomethylene-4H-cyclopenta[2,1-b;3,4-b'] dithiophene) with O-2 at negative potentials (e.g., -0.60 V) produces chang es in the polymer's electrochemistry, electronic spectrum, and conductivity profile that indicate a lowering of its band gap. Cyclic voltammetry of O- 2-modified films shows increased currents in the voltage region correspondi ng to the band gap, between the original onsets of n and p doping. The onse t of electronic absorption shifts from 0.8 eV to lower energies with increa sing reaction time. In situ conductivity measurements show that the intrins ic conductivity increases by as much as a factor of 100 (to similar to 10(- 6) S cm(-1)), corresponding Do a reduction of the band gap to similar to 0. 2 eV. Thus by controlling the reaction time with O-2 the band gap of the po lymer can be tuned over the range of <0.2 to 0.8 eV. Raman spectra suggest that electrochemically driven substitution of the polymer with hydroxyl gro ups at the P-positions of the thiophene rings is responsible for the loweri ng of the band gap.