Introduction of Perylene Units for Enhanced Interchain Interaction in Conjugated Polymers for Organic Photovoltaic Devices

A series of semiconducting copolymers, poly[2,7-(9,9′-dioctylfluorene)-alt-5,5′-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole)] (PFDTBT), poly[2,2′-(9,9-dioctyl-9H-fluorene-2,7-diyl)dithiophene-alt-5,5′-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole)] (PFD2TBT), and their ter-polymers containing perylene units were synthesized using Suzuki coupling polymerization. The perylene units were introduced to improve the charge-transport ability by enhancing the π–π interaction between polymer chains. The resulting polymers were characterized by 1H NMR, elemental analysis, DSC, and TGA. The synthesized polymers were soluble in common organic solvents, and formed smooth and uniform spin-coated thin films. All of the polymers studied were found to exhibit good thermal stability, losing less than 5% of their weight upon heating to approximately 350 °C. Perylene- containing polymers showed higher field-effect mobilities than the corresponding PFDTBT or PFD2TBT polymers because of the enhanced π–π interaction between polymer chains upon the introduction of perylene units. Bulk heterojunction solar cells were fabricated with configuration of ITO/PEDOT:PSS/polymer:PC71BM/TiO x /Al. The devices using the perylene-containing polymers showed higher short-circuit currents, and fill factors than the corresponding PFDTBT or PFD2TBT devices. One of the fabricated devices using a perylene-containing copolymer showed a maximum power conversion efficiency of 3.16%, with a short circuit current density of 9.61 mA/cm2, open circuit voltage of 0.81 V, and fill factor of 41%.