Effect of Surface Roughness on Light-Absorber Orientation in an Organic Photovoltaic Film

Atomistic nonequilibrium molecular dynamics simulations have been used to model the orientation of a side-chain substituted dicyanovinyl oligothiophene, DCV4T-Et2, in thin films formed by vacuum deposition as used in organic photovoltaics. The orientation of the DCV4T-Et2 molecules was analyzed in neat layers deposited onto smooth or rough C60 substrates. The average angle between the long axis of DCV4T-Et2 and the horizontal was 21 ± 1° in the layer deposited on smooth C60, in agreement with experimental measurements of layers deposited on silicon. In the layer deposited on rough C60, the average angle was 25 ± 2°, which could lead to a decrease of 6% in the efficiency of light absorption compared to the layer on the smooth substrate. Thus, the simulations suggest that surface roughness must be considered in the design of model systems in order to maintain relevance to devices. Importantly, the simulations provide the ability to analyze the orientation of each individual molecule as opposed to experiments that provide an average across a film. The orientation of the thiophene rings of DCV4T-Et2 and the variation in orientation with respect to distance from the substrate were also analyzed and found to be similar for films on smooth and rough substrates. In addition, we show that X-ray diffraction patterns of the layers can be constructed from the simulations.