Development of Deuterated Non-Fullerene Acceptors for Use in Selective Tracking of Morphological Changes in Organic Photovoltaic Active Layer Blends

Organic semiconducting polymers and small molecules have shown a remarkable ability to act as photoactive components in organic photovoltaic (OPV) devices over the past decade, with widespread efficiencies of 18-19% for binary OPV systems which have rapidly approached the fabled 20% efficiency threshold needed for OPV technology commercialisation. Overall, there has been a paradigm shift in OPV device performance from 8% in 2012 to over 20% PCE in 2022.¹ This step-change has been propelled by the development of novel electron-accepting (n-type) materials, moving away from fullerene-based acceptors (PC₆₀BM/PC₇₀BM) to emerging third-generation OPV materials, known as non-fullerene acceptors (NFAs). These NFAs are typically composed of rigid electron-rich fused aromatic ladder-type cores with electron-deficient end groups such as ITIC and IDTBR, most notably with the recent emergence of a new family of high-performance Y6 NFA materials. However, the monumental increase in device efficiency has not been in lock-step with donor polymer:NFA solar cell device stability, which often show poor tolerance to oxygen, UV-light, thermal stimuli, and water which lead to photo-oxidative and chemical degradation, alongside thermodynamic instabilities in the photoactive blend, which remains a critical bottleneck towards real-world commercialisation of OPV technologies.²<br/><br/>This work presents a series of NFAs with deuterated side chains (d-NFAs), based on O-IDTBR and EH-IDTBR hydrogenated acceptors (h-NFAs), which share the same physical and optoelectronic properties as their hydrogenated counterparts and allow them to act as ideal proxies for accurately representing the h-NFAs within the chosen polymer:NFA blend morphologies. Understanding the morphological and chemical stability of donor polymer:NFA OPV blend systems is critical to optimising the delicate balance between the donor and acceptor blend mixing in devices, which is crucial for both long-term operational stability and high performance. With this in mind, the d-NFAs were selectively blended with the donor polymers Poly(3-hexylthiophene) (P3HT) and PffBT4T-2OD (PCE11) in OPV devices which were then sealed in degradation chambers under N₂ and placed under environmental stress, either via light soaking under AM 1.5 G or heating at 80 °C in the dark for 100-1300 hrs with continuous tracking of the device’s J-V performance. This was done in order to decouple chemical and morphological changes in the active layer blend due to photo-degradation and thermal morphological demixing processes, which were then selectively investigated by ToF-SIMS (Time-of-Flight Secondary Ion Mass Spectrometry) and photoluminescence measurements.<br/><br/>Both sets of P3HT:d-IDTBR and PCE11:d-IDTBR active layer systems demonstrated excellent long-term thermal stability in the dark, with no significant losses in PCE, which was further corroborated through vertical and lateral ToF-SIMS measurements which showed no discernable phase separation of the d-NFAs from the donor polymers. Similarly for the light-aged devices, there were no notable losses in PCE which was further confirmed by ToF-SIMS measurements. The results suggest that overall P3HT:d-IDTBR and PCE11:d-IDTBR active layer blend systems are chemically and morphologically stable and that using deuterated NFAs is an effective methodology for probing blend morphologies of systems with polymers and NFAs that possess very similar chemical structures and heteroatoms. Future work would involve utilising d-NFAs as third components in ternary OPV blends with d-Y6 and similarly tracking the morphological evolution of this component in active layer blends as a function of thermal and light aging conditions.<br/><br/>1. Zheng, Z. et al. Tandem Organic Solar Cell with 20.2% Efficiency. Joule 6, 171–184 (2022).<br/>2. Speller, E. M. et al. From fullerene acceptors to non-fullerene acceptors: prospects and challenges in the stability of organic solar cells. J. Mater. Chem. A 7, 23361–23377 (2019).