Characterizing the Thickness Distribution of Donor and Acceptor in Organic Photovoltaics via Quasi-Monochromatic Light Transmittance

The performance of organic solar cells is sensitive to the film thickness and the uniformity of the donor and acceptor distribution in the photoactive layer. Especially, in the quasi-planar heterojunction fabricated by layer-by-layer process, the donor and acceptor materials may be non-homogeneously distributed across the entire photoactive layer. Visualization of the spatial distributions of donor and acceptor components sheds light on the relationship between nanostructures and device performances. Based on the distinct infrared absorption characteristics of the organic donor and acceptor, a quasi-monochromatic light transmittance (QMLT) technique is proposed to quantify the donor and/or acceptor thickness in the photoactive layer blend across the entire device area with nanometer precision. The variation in thickness of donor and acceptor can be clearly and separately revealed. Accordingly, the impacts of spin coating techniques (such as dynamic or static dispensing, and on-center or off-center spin coating), as well as solvent choices, on film morphology and device performance can be correlated by the noncontact, nondestructive, and convenient QMLT method. A quasi-monochromatic light transmittance method is proposed to visualize the spatial thickness distribution of each component in a semitransparent blend film in a large area. This method provides an effective way to understand the relationship between active layer nanostructures and the performances of organic photovoltaics and sheds light on choices of processing methods and solvents for high-performance layer-by-layer processed devices.image