Organic solar cells (OSCs) based on nonfullerene acceptors (NFAs) have made significant breakthrough in their device performance, now achieving a power conversion efficiency of ≈18% for single junction devices, driven by the rapid development in their molecular design and device engineering in recent years. However, achieving long‐term stability remains a major challenge to overcome for their commercialization, due in large part to the current lack of understanding of their degradation mechanisms as well as the design rules for enhancing their stability. In this review, the recent progress in understanding the degradation mechanisms and enhancing the stability of high performance NFA‐based OSCs is a specific focus. First, an overview of the recent advances in the molecular design and device engineering of several classes of high performance NFA‐based OSCs for various targeted applications is provided, before presenting a critical review of the different degradation mechanisms identified through photochemical‐, photo‐, and morphological degradation pathways. Potential strategies to address these degradation mechanisms for further stability enhancement, from molecular design, interfacial engineering, and morphology control perspectives, are also discussed. Finally, an outlook is given highlighting the remaining key challenges toward achieving the long‐term stability of NFA‐OSCs.

have especially attracted great attention as the demand for low-power electronic devices is increasing rapidly with the advent of the Internet of Things, radiofrequency identification, Bluetooth low energy, etc. requiring ≈10 µW to ≈1 mW of electrical power to communicate between wireless electronic devices. [8] Indoor OPVs utilize organic semiconductors as the photoactive material in indoor energy harvesting devices. This allows for optical band gap control to ensure a good match with the visible emission spectra (300-800 nm) of indoor lighting, such as light emitting diodes (LEDs), fluorescent lamps, or halogen lamps. [7,9] Freunek et al. reported theoretical maximum PCE limits of photovoltaic devices under indoor lighting conditions as a function of optical band gap of photoactive materials. [10] In case of white RGB LEDs, for example, theoretical maximum PCE limits of over 50% can be achieved when a photoactive material with an optical band gap of 1.90 eV is used. This emphasizes the importance of the spectral overlap to achieve high PCEs in indoor photovoltaics, exemplifying the applicability of OPVs to indoor applications. Additional to the optical band gap, the frontier molecular orbital energy levels of organic semiconductors can be controlled by adjusting molecular structure. Therefore, unlike inorganic photovoltaic devices, both a high short-circuit current density (J SC ) and opencircuit voltage (V OC ) can be achieved. Although promising, there are some important considerations when using OPVs for indoor light applications. For example, unlike use under solar radiation (1 Sun), typical light intensities of indoor conditions (e.g., office, supermarket, etc.) are very low, ≈1000 lux. Due to the low light intensity, the photocurrent density of OPVs is also extremely low, typically around hundreds of µA cm −2 . Therefore, minimizing leakage currents and reducing recombination losses are essential strategies to achieve highly efficient indoor OPVs. [5,6,11,12] Most research related to indoor OPVs utilizes a bulk-heterojunction (BHJ) photoactive layer. BHJs have been widely used to overcome the limitations of organic semiconductors, namely a large exciton binding energy, and short exciton diffusion lengths (L D ). [13] Randomly intermixed donor and acceptor domains in BHJs facilitate exciton dissociation at the interface between donor and acceptor, leading to high photocurrent generation. However, this nano-structured morphology can induce unwanted energy losses by trapped charge carriers Indoor organic photovoltaics (OPVs) are a potential niche application for organic semiconductors due to their strong and well-matched absorption with the emission of indoor lighting. However, due to extremely low photocurrent generation, the device parameters critical for efficient indoor OPVs differ from those under 1 Sun conditions. Herein, these critical device parameters-recombination loss and shunt resistance (R sh )-are identified and it is demonstrated that bilayer OPVs are suitable for indoor PV applications. Co...

Crystalline molybdenum oxide layers as efficient and stable hole contacts in organic photovoltaic devices Ahmadpour, Mehrad; Cauduro, A. L. F.; Méthivier, C.; Kunert, B. ; Labanti, C.; Resel, R.; Engmann

Non-fullerene acceptors (NFAs) for organic solar cells (OSCs) have significantly developed over the past five years with continuous improvements in efficiency now over 18%. However, a key challenge still remains in order to fully realize their commercialization potential: the need to extend device lifetime and to control degradation mechanisms. Herein, we investigate the effect of two different molecular engineering routes on the widely utilized ITIC NFA, to tune its optoelectronic properties and interactions with the donor polymer in photoactive blends. Heavier selenium (Se) atoms substitute sulfur (S) atoms in the NFA core in either outer or inner positions, and methyl chains are attached to the end groups. By investigating the effects of these structural modifications on the long-term operational stability of bulk-heterojunction OSC devices, we identify outer selenation as a powerful strategy to significantly increase device lifetime compared to ITIC. Combining outer selenation and methylation results in an impressive 95% of the initial OSC efficiency being retained after 450 h under operating conditions, with an exceptionally long projected half-lifetime of 5600 h compared to 400 h for ITIC. We find that the heavier and larger Se atoms at outer-core positions rigidify the molecular structure to form highly crystalline films with low conformational energetic disorder. It further enhances charge delocalization over the molecule, promoting strong intermolecular interactions among acceptor molecules. Upon methylation, this strong intermolecular interaction stabilizes acceptor domains in blends to be resilient to light-induced morphological changes, thereby leading to superior device stability. Our results highlight the crucial role of NFA molecular structure for OSC operational stability and provide important NFA design rules via heteroatom position and end-group control.