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.

Contorted polycyclic aromatic molecules, such as corannulenes and sumanenes, have been studied as host molecules for fullerenes due to their complementary geometries. It has been suggested that stronger associations with fullerenes can be attained by extending their π surface or by incorporating heteroatoms, as unsubstituted corannulene and sumanene do not readily complex with fullerenes in solution. Here, we design and synthesize πextended buckybowls derived from contorted hexabenzocoronene and pentabenzocoronene with either pentyl or butoxy side chains. These coronene-based buckybowls have approximately twice the π surface compared to unsubstituted corannulene and sumanene. Buckybowls with pentyl side chains complex with C 60 in a preferred 1:1 buckybowl-to-C 60 stoichiometry, with association constants on the order of 10 3 M −1 . Complexation with C 60 further strengthens with the addition of heteroatoms in buckybowls bearing butoxy side chains. We find a preferred 1:2 association stoichiometry with fullerene and association constants on the order of 10 4 M −1 . Density functional theory calculations suggest the larger dipole moment on the butoxy-substituted buckybowls relative to their pentyl-substituted counterparts is responsible. This greater dipole moment polarizes C 60 , leading to a preferred 1:2 stoichiometry and larger association constants in the complexation of buckybowls with butoxy side chains and C 60 .