Advances in layer-by-layer processing for efficient and reliable organic solar cells

Layer-by-layer (LBL) deposition using solution processing is a promising technique for fabricating organic solar cells (OSCs) with high efficiency and stability. In comparison with bulk-heterojunction (BHJ) structures, in the LBL method, the donor (D) and acceptor (A) materials are deposited sequentially, presenting many distinct advantages including a p–i–n-like configuration (D/D:A/A) that may create well-defined and controllable nanostructures to facilitate charge generation and extraction. This concept is an advisable option to fabricate pseudo-bilayer configurations in the active layer of OSCs. At present, high power conversion efficiencies (PCEs) of over 19% in ternary LBL processed OSCs and 11.97% in the processed module (11.52 cm2) have been successfully realized, indicating that the selection of an appropriate ternary system is an effective strategy to improve the morphology of the active layer towards efficient and stable OSCs. Moreover, the unique merits of LBL configuration in individual layer processing enable it to be a promising approach for large-scale printing and further commercialization of OSCs. In this article, we summarize the recent advances in LBL OSCs, focusing on the selection of materials, solvents, processing parameters, device architectures, and stability. We also discuss the key strategies used for further improvement of LBL OSCs from the perspectives of structural design, performance, and scalability. Finally, we discuss the current limitations of BHJ devices and the prospect of LBL OSCs as a promising alternative for high-performance and stable OSCs. We also highlight some key research directions that can help enhance the efficiency and stability of LBL OSCs for their potential applications in the future.