Progress in Non-Fullerene Acceptors: Evolution from Small to Giant Molecules.

With the rapid development of non-fullerene acceptors (NFAs), the power conversion efficiency (PCE) of organic solar cells (OSCs) is increasing. According to their different chemical structures, NFAs can initially be divided into two categories: small molecule acceptors (SMAs) and polymerized small molecule acceptors (PSMAs). Due to the strong absorption capacity and controllable energy levels, the PCE of devices based on SMAs has approached 20%. Compared with SMAs, PSMAs have advantages in stability and flexibility, and the PCE of PSMA-based devices has exceeded 18%. However, the higher synthesis cost and lower batch repeatability hinder its further development. Recently, the concept of giant molecule acceptors (GMAs) has been proposed. These materials have a clear molecular structure and are considered novel acceptor materials that combine the advantages of SMAs and PSMAs. Currently, the PCE of devices based on GMAs has exceeded 19%. In this review, we will introduce the latest developments in SMAs, PSMAs, and GMAs. Then, the advantages of GMAs and the relationship between their structure and performance will be analyzed. In the end, perspectives on the opportunities and challenges of these materials are provided, which could inspire further development of NFAs for advanced OSCs.

Thermodynamic Phase Transition of Three-Dimensional Solid Additives Guiding Molecular Assembly for Efficient Organic Solar Cells.

Fine-tuning the thermodynamic self-assembly of molecules via volatile solid additives has emerged to be an effective way to construct high-performance organic solar cells. Here, three-dimensional structured solid molecules have been designed and applied to facilitate the formation of organized molecular assembly in the active layer. By means of systematic theory analyses and film-morphology characterizations based on four solid candidates, we preselected the optimal one, 4-fluoro-N,N-diphenylaniline (FPA), which possesses good volatility and strong charge polarization. The three-dimensional solids can induce molecular packing in active layers via strong intermolecular interactions and subsequently provide sufficient space for the self-reassembly of active layers during the thermodynamic transition process. Benefitting from the optimized morphology with improved charge transport and reduced energy disorder in the FPA-processed devices, high efficiencies of over 19 % were achieved. The strategy of three-dimensional additives inducing ordered self-assembly structure represents a practical approach for rational morphology control in highly efficient devices, contributing to deeper insights into the structural design of efficient volatile solid additives.