Morphology Control in Organic Solar Cells

Organic solar cells (OSCs) can directly convert the sunlight into electrical energy and present some advantages, such as low cost, light weight, flexibility, semitransparency, and roll-to-roll large-area fabrication. Due to the short diffusion length of exciton (approximate to 10 nm) in organic semiconductor materials, the ideal nanoscale phase separation in the active layer is one of the crucial factors for achieving efficient exciton dissociation and charge transport. The morphology of the active layer is mainly determined by the nature of donors and acceptors (e.g., solubility, crystallinity, and miscibility), the film processing, the device configuration, and so on. In general, it is very hard to obtain ideal morphology in the as-cast films. Therefore, it is usually essential to take measures to achieve the active layer with good molecular stacking, proper domain size, high domain purity, and suitable vertical phase separation. In this review, recent developments in morphology control and morphology characterization are summarized and analyzed. This review might help the community to decipher active layer morphology at multiple length scales and to achieve ideal morphology toward high-performance OSCs.