Incorporation of Pyrene Units to Improve Hole Mobility in Conjugated Polymers for Organic Solar Cells

Solution-processable semiconducting copolymers, poly[N-9'-heptadecanyl-2,7-carbazole-alt-,'-(4',7'-di-2-thienyl- 2',1',3'-benzothiadiazole)] (PCDTBT) and poly[4,8-bis(2-ethylhexyl-2-thenyl)-benzo[1,2-b:4,5-b']dithiophene-alt-5,5'-(4',7'-di-2- thienyl-2',1',3'-benzothiadiazole)] (PBDTDTBT), and their pyrene-containing terpolymers were synthesized using Suzuki or Stille coupling. Pyrene units were introduced to improve the charge-transporting abilities of the polymers. The resulting polymers were found to be soluble in common organic solvents and formed smooth and uniform spin coated thin films. They also exhibited good thermal stability and lost <5% of their weight upon heating to similar to 350 degrees C. Solution-processed field-effect transistors fabricated using these polymers showed p-type organic thin-film transistor characteristics. The pyrene-containing terpolymers showed higher field-effect mobilities than their corresponding parent polymers, and their mobility increased with increasing pyrene content Furthermore, they had lower HOMO energy levels than the corresponding PCDTBT or PBDTDTBT polymers. Bulk heterojunction solar cells with an ITO/PEDOT:PSS/polymer:PC71BM/Ca/Al configuration fabricated using the pyrene-containing polymers had higher power conversion efficiencies than those using the corresponding parent polymers.