Effects of the Reduction and/or Fluorination of the TT-Units in BDT-TT Polymers on the Photostability of Polymer:Fullerene Solar Cells

Polymer solar cells have a promising future for applications in our day to day usage of energy in small appliances and portable devices. However, their performance in terms of efficiency is limited by a number of factors, among which is their low open circuit voltage (V-oc). It is, therefore, understandable that much effort is channeled by the scientific community in improving the V-oc. One way to achieve this goal is the development of novel materials, for example, polymers, through chemical structure modification. Typical examples are addition (chlorination, fluorination, or sulfonylation) and/or reduction (from alkyl-ester to ketone substituents) mechanisms. This paper reports on the study of the effect of these structural changes for V-oc enhancement on the performance of the polymers in polymer:fullerene solar cells. In particular, it looks at seven polymers of the polybenzodithiophene-thienothiophene family, identifying the structural changes in the thienothiophene units or their moieties as a function of V-oc behavior in relation to their UV-stability. The findings reveal that the fluorination of the TT-units or having alkyl-ester groups as substituents on the TT-units is bad for photostability. However, when these alkyl-ester groups are reduced into ketone substituents, the photostability behavior improves.