(Z)-4-(thiophen-2-ylmethylene)-4H-thieno[2,3- b ]pyrrol-5(6H)-one based polymers for organic photovoltaics

This paper involves polymerization and study of optoelectronic properties novel (Z)-4-(thiophen-2-ylmethylene)-4H-thieno[2,3- b ]pyrrol-5(6H)-one based molecules. The novel monomer, (Z)-2-bromo-4-((5-bromothiophen-2-yl)methylene)-4H-thieno[2,3-b]pyrrol-5(6H)-one was polymerized with two different donors bis(trimethylstannyl) functionalized thienothiophene and bis(alkoxy)-benzodithiophene to form polymers P1 and P2 respectively. The synthesized polymers P1 and P2 showed respective absorption peaks at 664 nm and 665 nm respectively, measured through UV–visible Spectroscopy. P1 and P2 has greater thermal stability which were a main criterion for optoelectronic or photovoltaic applications. The DFT calculations showed that, the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy at -0.08319 and -0.19731 for the monomer and that of P1 was found to be -0.17888, -0.09767 and that of P2 was found at -0.17794 and -0.10263 respectively. The electrochemical energy gap for P2 was at 1.86 eV and optical band gap energy at 1.72 eV, a slight variation was observed in P2. Nevertheless, the electrochemical energy of P1 was 1.87 eV and optical band gap at 1.86 eV which were comparable. The solar cell devices based on the new polymers were fabricated and achieved a short circuit current density (J SC ) as 8.5, open circuit voltage (V OC ) as 0.64, fill factor (FF) as 0.52 and power conversion efficiency (PCE) as 9.06% for P2, whereas the J SC was at 8.0 with open circuit at 0.60 and fill factor 0.51 with 8.3% PCE for P1. Thus, the monomer and synthesized polymers were promising building blocks for photovoltaic applications.