Spectroscopic ellipsometry characterization of vacuum-deposited organic films for the application in organic solar cells

Variable angle spectroscopic ellipsometry (VASE) in the wavelength range from 245 to 1680 nm has been applied to determine the optical properties of the recently developed electron donor α,ω-bis-dicyanovinylene-sexithiophene (DCV6T), an efficient absorber material in organic solar cells (OSCs). To ensure uniqueness of the evaluation results interference enhanced substrates are used and comparison to simple native silicon substrates is presented. Similar as applied in OSC, DCV6T was deposited both as a pure single layer as well as in a mixed heterojunction with C60. For both cases, the in-plane refractive indices and extinction coefficients were higher than the out-of-plane ones, revealing that the DCV6T molecules in the films are preferentially horizontally oriented. This rough indication was further quantified by the so called molecular orientation parameter. Moreover, it is shown that annealing initiates molecular reorganization of the films, which leads to a higher birefringence and more defined horizontal orientation in the single layer. However, in the mixed layer annealing seems to reduce anisotropy. These effects and the consequences for the performance of organic solar cells are discussed.