Abstract:

Bulk-heterojunctions (BHJ) Organic solar cells (OSCs) have garnered considerable attention in the past two decades due to their advantages, including mechanical flexibility, lightweight, low-cost solution-processability, and semitransparency. The recent years have witnessed rapid progress in the realm of OSCs centered around non-fullerene acceptors (NFAs) thanks to the distinct merits of NFAs's stronger and broader absorption, highly adjustable energy levels, and easily modification of molecular structures. The power conversion efficiency (PCE) of single-junction OSCs has now reached high values of over 19%, which brings them closer to the threshold for commercial viability. This increase in PCE is attributed to advancements in active layer materials, device engineering, and a deeper comprehension of device physics. Nevertheless, achieving high PCE is not the only requirement for commercialization, while the stability of the devices is equally pivotal. The photovoltaic performance degradation of BHJ OSCs devices has been widely observed, however, the research on the stability of NFA-based OSCs has received less attention than efforts directed towards improving PCE through novel material development. The instability of NFA-OSCs has been one of the key obstacles limiting their transition to commercial applications. Various factors, encompassing both external and intrinsic variables, influence their stability. External factors, such as light, heat, water, and stress, exert significant impact on the stability of OSCs. Effective encapsulation can protect the devices from contact with oxygen and water, curtailing degradation. Encapsulated OSCs have demonstrated encouraging operational lifetimes of several years under certain degradation environments, in stark contrast to unencapsulated OSCs which often succumb to rapid degradation, losing their performance within a matter of minutes to days. Intrinsic degradation processes within NFA-OSCs involve material stability, morphology stability of BHJ and interface stability. The degradation of NFA based cells was not fully investigated as has been done on the fullerene based OSCs. There remains a lack of concrete molecular design rules to enhance the stability of NFA based OSCs. Therefore, it is still highly needed to further understand the intrinsic degradation processes of NFA-OSCs and to find proper ways to suppress these degradation processes. Herein, in this context, we present a comprehensive review encompassing the intrinsic fundamentals of instability including photo-oxidation of NFAs, interlayer-induced degradation of NFAs and the blend film morphology. Furthermore, we summarize recent advancements in strategies aimed at enhancing the stability of NFA-OSCs. These include stable NFA molecular design, mitigation of interfacial chemical reactions, and implementation of ternary strategies. It is our aspiration that this concise review will serve as a valuable resource for researchers interested in stability considerations, providing a guiding framework for future endeavors in achieving both efficient and stable NFA-OSCs.

Key words: Organic solar cell, Non-fullerene acceptor, Inherent instability, Photooxidative degradation, Morphology instability, Interfacial layer