Low Bandgap Copolymers with (Alkylsulfanyl) Bithiophene Unit for Efficient Polymer Solar Cells

Polymer solar cells have evolved as a promising cost-effective alternative to silicon-based ones. However, low efficiency of these plastic devices limits their feasibility for commercial use. The efficiencies of polymer photovoltaic cells got a major boost with the introduction of the bulk heterojunction (BHJ) configuration consisting of an interpenetrating network of electron donor and acceptor materials. Recently, benzodithiophene based polymers has been used as electron donors in polymer solar cells showing very promising properties such as an increasing charge transport and a red shift of the absorption spectra. In this work the synthesis, the spectroscopic and photovoltaic characterization of a copolymer based on (octylsulfanyl)bithiophene and benzo[1,2-b:4,5-b’]dithiophene units and a polymer based on (butylsulfanyl)bithiophene unit are reported. BHJ solar cells using these polymers as donor materials blended with [70]PCBM as acceptor were prepared and investigated. The geometry of the device is: glass/ITO/PEDOT:PSS/blend/Ca/Al. The electrical performances of the cells were compared in order to study the influence of the benzodithiophene unit on the polymer backbone. The power conversion efficiency of the best realized polymer solar cell is 2.3% under 100 mW/cm2 AM 1.5G illumination