Photochemical Decomposition of Y-Series Non-Fullerene Acceptors Is Responsible for Degradation of High-Efficiency Organic Solar Cells

Organic photovoltaic cells that employ Y-series non-fullerene acceptors (NFAs) have recently achieved impressive power-conversion efficiencies (>18%). To fulfill their commercial promise, it is important to quantify their operational lifetimes and understand their degradation mechanisms. In this work, the spectral-dependent photostability of films and solar cells comprising several Y-series acceptors and the donor polymer PM6 is investigated systematically. By applying longpass filters during aging, it is shown that UV/near-UV photons are responsible for the photochemical decomposition of Y-series acceptors; this degradation is the primary driver of early solar cell performance losses. Using mass spectrometry, the vinylene linkage between the core and electron-accepting moieties of Y-series acceptors is identified as the weak point susceptible to cleavage under UV-illumination. Employing a series of device characterization, along with numerical simulations, the efficiency losses in organic photovoltaic cells are attributed to the formation of traps, which reduces charge extraction efficiency and facilitates non-radiative recombination as the Y-series acceptors degrade. This study provides new insights for molecular degradation of organic photovoltaic absorber materials and highlights the importance of future molecular design and strategies for improved solar cell stability.