Synthesis and characterization of wide range light absorbing poly(dithieno[3,2-b:2′,3′-d]thiophene-alt-3,6-bis(thiophen-2-yl)-2, 5-di-n-octyl-pyrrolo[3,4-c]pyrrole-1,4-dione) for polymer solar cells

We designed and synthesized a new low band gap donor-acceptor copolymer (PDTDPP-Oct) composed of alternating dithieno[3,2-b:2′,3′-d] thiophene and bis-thienyl-diketopyrrolo[3,4-c]pyrrole units. The polymer films showed a very broad absorption band that extended up to 950 nm. Thermogravimetric analysis revealed good thermal stability and a high glass transition temperature of 128 °C. The highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) of the polymer were estimated to be -5.01 and -3.70 eV, respectively. Bulk heterojunction polymer solar cells were fabricated with the structure: ITO/PEDOT:PSS/polymer:PCBM/Al. The introduction of 1,8-octanedithiol as a processing additive to the polymer/PCBM blend solution significantly improved the device performance relative to devices prepared from the blend solution without the additive. This cell gave a short circuit current of -1.8 mA/cm², an open circuit voltage of 0.60 V, and a fill factor of 0.52, yielding a power conversion efficiency of 0.55%. The performance improvements were mainly attributed to the fine morphology of the blend film.