Block copolymer optoelectronic property versus bridge and morphology

Abstract

Block copolymers containing electron rich conjugated donor block and electron poor conjugated acceptor block connected covalently via non-conjugated bridge units are promising for potential light harvesting applications due to fast photo induced charge separation and slow/controllable charge recombinations. Potential bicontineous and ordered nano-domain donor/acceptor phase separated morphologies of the block copolymer systems are also favorable for Frenkel type exciton dissociations. In this work, a DBfA type block copolymer is being evaluated for light harvesting applications where D is a conjugated donor block, fA is a conjugated and fluorinated acceptor block, and B is non conjugated bridge chain containing different number of methylene units. This study reveals that the photoluminescence PL quenching as well as photoelectric power conversion efficiencies of DBfA type block copolymers decreases from the bridge unit containing one methylene unit up to six methylene units, i.e., the shorter the chain the more efficient photoelectric conversions. However, in a DfA block copolymer where there is no bridge unit, the PL quenching or the photoelectric conversions are not better than the shortest bridge containing one methylene unit, i.e., the photo induced charge separation in DfA still fall in weak electronic coupling regime. This may be accounted for by a twist angle existence between the D and fA conjugated plains. Morphological and optoelectronic correlation studies reveals that the surface roughness of such block copolymer varies nonlinearly with thermal annealing temperature, and that a medium roughness exhibit best or optimal optoelectronic property.