Ultralow-temperature high-sensitivity optical spectroscopies for disentangling structural and thermal disorder in non-fullerene acceptors for organic solar cells

Organic solar cells have advanced rapidly, yet their photovoltaic performance remains limited by pronounced structural and thermally induced disorder compared with more ordered inorganic semiconductors. In this contribution, we employ ultralow-temperature, high-sensitivity spectroscopy to quantitatively disentangle structural and thermal disorder across diverse photovoltaic systems, from fullerene and polymeric blends to small molecule, dimeric, and macrocyclic non-fullerene acceptor blends, benchmarking against crystalline perovskites. Coupling these disorder metrics with comprehensive optical characterizations reveals the physical origin of disorder in organic solids. Our results demonstrate that conformationally locked giant molecule acceptors (dimer and macrocyclic) exhibit systematically reduced thermal disorder relative to their monomeric analogues. Specifically, macrocyclic acceptor layers exhibit the lowest overall disorder among the organic systems investigated, minimizing both structural and thermal contributions, yet remaining distinctly above the crystalline perovskite limit. Further analysis shows that the extracted thermal disorder correlates with phonon energy, exciton-phonon coupling, and vibrational energy distribution, implicating molecular vibrations as its molecular origin. The reduction of thermal disorder can be associated with suppressed non-radiative voltage losses, which in turn enables improvements in device performance. Overall, this work elucidates the physical origin of thermal disorder in organic semiconductors and establishes molecular design principles to further enhance photovoltaic performance.