A novel strategy to improve the operational and outdoor stability of non-fullerene acceptor-based organic solar cells

Abstract

Humankind is living in a climate emergency that requires immediate action. However, in the meantime, the world’s energy demand is continuously increasing; thus, making clean energy generation of paramount importance. Photovoltaics is at the forefront of this clean energy movement with organic photovoltaics emerging as a promising candidate to address this issue and meet this demand. Having said that, organic semiconductors came a long way from fullerene-based solar cells to state-of-the-art non-fullerene acceptor-based (NFAs) cells that show promise as next-generation photovoltaic devices. Organic solar cells employing such materials have already surpassed 20% in power conversion efficiencies (PCEs) after less than a decade of research, excluding initial attempts to outclass their fullerene counterparts in the 90s.[1] Much is done to realize high PCEs by the research community; nonetheless, the urgency to unravel the underlying mechanisms crippling the stability of the photovoltaic devices based on such material systems picked up only in recent years. Even so, a thorough understanding or a guideline is still lacking to tackle this limitation once and for all. In this work, we implement a novel strategy, which we refer to as donor dilution, to improve the operational and outdoor stability of NFA-based systems in an archetypal donor:acceptor blend, namely, PTB7-Th:IEICO-4F.[2,3] Organic solar cells exploiting this material system with decreasing polymer content in the polymer:small molecule blend progressively perform superior both in operational and outdoor stability tests. Unencapsulated donor-diluted devices retain 85% of their initial PCE under continuous 1 sun equivalent metal halide lamp illumination at 40⁰C after 1000 hours. Furthermore, the in-house developed encapsulation procedure enables these devices to endure the harsh outdoor conditions (~23⁰C and ~64% relative humidity on average throughout winter) for an extended period in the Kingdom of Saudi Arabia. These findings underpin the universal effort to improve the organic photovoltaic device stability, provide a practical strategy as a proof-of-concept to enhance the lifetime of organic photovoltaic devices, and bring this technology a step closer to commercialization. Future work consists of further examination of the universality of the donor dilution approach and is underway by testing various donor:acceptor blends with different classes of non-fullerene acceptors, such as state-of-the-art Y-series molecules.