Film-depth-dependent crystallinity for light transmission and charge transport in semitransparent organic solar cells

In semiconductor organic thin films, molecular crystallinity simultaneously influences the optical (light transmission) and electronic (transport energy level) properties. In this work, two isomeric acceptors with different crystallization capabilities in combination with different solvent additives are utilized to broadly tune the film-depth-dependent crystallinity for donor:acceptor bulk heterojunction films. It is found that moderate crystallinity with weak film-depth-dependence contributes to an optimized photovoltaic efficiency and optical transparency. However, higher crystallinity leads to a red-shift of absorption peaks and induces the generation of more excitons in the vicinity of the electrode surface. Moreover, large variations of the crystallinity along the film-depth-direction increase the spatial variations of transport levels and inevitably form low energy trap sites, deteriorating the charge transport and degrading the photovoltaic performance. Upon optimizing the film-depth-dependent optical and electronic properties by manipulating the crystallinity and its spatial distribution, a high power conversion efficiency of 7.6–11.1% with an optical average transparency of 11.7–15.0% and a human-eye visual-sensitivity transparency up to 12.3–15.2% is realized.