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. The orientation of polymer molecules must be controlled to maximize the power conversion efficiency of organic solar cells, according to researchers in South Korea. Organic solar cells can be constructed by combining two different polymeric molecules, known as the donor and the acceptor, between which the photogenerated electrons flow. The relative orientation of these molecules was thought to be important for optimising solar cell efficiency, but to date evidence has been based on overall device performance. Directly observing the influence of orientation at the donor–acceptor interface has proven difficult. Kilwon Cho from Pohang University of Science and Technology and co-workers created bilayer solar cells with a fixed acceptor layer and donor polymer layers of controllable orientation. This enabled them to quantify how the molecular orientation affected charge transfer and photocurrent generation. The relative molecular orientation at the donor–acceptor interface is a key factor that determines photocurrent generation in organic photovoltaics. The effects of this orientation on the photocurrent generation are investigated using bilayer devices with a sharp and unmixed donor–acceptor heterojunction. Improved exciton diffusion and charge separation are observed at the face-on donor–acceptor interface than at the edge-on interface