Detecting Molecular Orientation in Organic Photovoltaic Thin Films using Infrared Spectroscopic Ellipsometry Supported by Density Functional Theory Calculations

Controlling molecular orientation in organic photovoltaics enhances device performance by maximizing the overlap between the electric-field vectors of incident light and the corresponding molecular absorptions. By combining infrared spectroscopic ellipsometry - implemented here on a prototype instrument using a quantum-cascade laser - with density functional theory calculations, we correlate the in-plane and out-of-plane (parallel and perpendicular to the substrate) polarization of measured IR absorptions with the average orientation of transition dipoles corresponding to the normal modes at the molecular level. Four-ring oligothiophene DCV4T-Et₂, changes from edge-on to face-on orientation with the use of an ellagic acid templating layer. This reorientation coincides with several IR absorption peaks changing their character from in-plane to out-of-plane. In contrast, furan-indole-based oligomeric absorber DCV-V-Fu-Ind-Fu-V assumes a face-on orientation. Resolving the spatial relationship between transition dipoles provides insights into the molecular orientation in organic thin films down to a few nm.