Effect of donor-acceptor molecular orientation on charge photogeneration in organic solar cells

The relative orientation of an electron donor and electron acceptor, which significantly affects charge photogeneration in an organic solar cell, is investigated here. The effects of the molecular orientations at the donor-acceptor heterojunction are examined using bilayer solar cells comprising a fixed acceptor layer and donor polymer layers that assume a variety of orientations. The orientation of the conjugated polymer is controlled during film formation using solvents with slow or fast drying rates. Although the donor polymer layers show similar light-harvesting and nongeminate recombination dynamics, photocurrent generation is more efficient at the face-on donor-acceptor interface than at the edge-on interface. Photophysical analysis reveals that the efficient charge generation at the face-on interface originates from enhanced exciton diffusion toward the donor-acceptor interface and reduced geminate recombination of charge pairs. These findings offer clear evidence that the separation efficiency of an interfacial charge pair is affected by the relative orientations of the donor and acceptor molecules. This orientation should be controlled to maximize the PCE of an organic solar cell.