Side-chain fluorination on the pyrido[3,4-b]pyrazine unit towards efficient photovoltaic polymers

A series of new polymer donors （PT-PP, PT-2fPP and PT-4fPP） were synthesized based on alkylthiophene substituted benzodithiophene （BDT-T） and pyrido[3,4-b]pyrazine （PP） building blocks and the effects of fluorination on the polymer properties were explored. Photophysical properties, charge mobilities and morphologies of the three polymers have been intensively investigated. The results indicated that the introduction of the fluorine atom at meta-positions of phenyl substituted PP unit hardly affected their highest occupied molecular orbital （HOMO） level. More importantly, controlling the degree of side-chain fluorination in the polymers is crucial for optimizing the blend morphology. Three polymers showed different photovoltaic properties. The polymer solar cell （PSC） based on the single layer device structure of ITO/PEDOT：PSS/PT-4fPP：PC71BM （1：1, w：w）/ZrAcac/Al demonstrates a high power conversion efficiency （PCE） of 7.61% under the illumination of AM 1.5G,100 mW cm-2, which is the highest value for PP-based PSCs.