Abstract:

Owing to the advantages of broad absorption, semitransparency, and large-area solution processing, organic solar cells based on a nonfullerene blend system have attracted wide attention and become an important aspect of clean energy. At present, the power conversion efficiency of organic solar cells based on nonfullerene blends is more than 19% because of the molecular design, device structure optimization, and morphology regulation. Organic solar cells consist of a cathode, an anode, the corresponding interface layers, and the active layer. Research shows that the morphology of the active layer has significant influence on the device performance. For example, the phase separation structure affects the charge transport, exciton diffusion efficiency is dependent on the domain sizes of the donor and acceptor, crystallinity has a considerable impact on photon absorption and carrier mobility, and molecular orientation affects the dissociation of the charge-transfer state and carrier mobility. Owing to the rigidity of conjugated molecules, the coupling of crystallization between the donor and acceptor always occurs during the film-forming and/or post-annealing processes. Moreover, crystallization and phase separation are inclined to occur simultaneously, leading to poor morphology control. Although many methods, such as post-annealing, solution-state, solvent or solid additive, and solvent engineering, have been exploited, forming the ideal structure morphology of the active layer is still difficult. This is particularly challenging in nonfullerene blends owing to the asymmetric phase separation behavior. This feature article summarizes the recently developed crystallization kinetics strategy in morphology control, which made precise morphology control possible. In this strategy, the interpenetrating network can be constructed by applying modified film-forming kinetics, which inhibits the liquid–liquid phase separation and induces liquid–solid phase separation. The domain size can be reduced by employing sequential crystallization, where the donor and acceptor crystallize in different stages through the combination of the solution-state and post-annealing treatments, surpassing the driving force of phase separation. In addition, the crystallinity of small nonfullerene molecules in the polymer/nonfullerene blends can be effectively enhanced by prioritizing their crystallization. This shift in crystallization priority can reduce the confinement of crystalline framework polymers and benefit the diffusion of the small nonfullerene molecules. Moreover, the ordered stacking of molecules in crystals can be improved by regulating the matching degree between the crystal nucleation rate and growth rate. Molecular orientation can be regulated by combining the motion scale and heterogeneous nucleation. The optimized morphology is beneficial to device performance as it suppresses exciton quenching, recombination of the charge-transfer state, and bimolecular recombination and improves charge mobility, thereby laying the foundation for high-performance organic solar cells.

Key words: Organic solar cells, Morphology, Interpenetrating network, Rate of crystallization, Sequence of crystallization, Diffusivity