Low energetic disorder molecules: A new paradigm for breaking the trade-off of voltage and current in organic solar cells

The power conversion efficiency (PCE) of organic solar cells (OSCs) has rapidly approached and surpassed 20% owing to continuous advances in non-fullerene acceptors (NFAs). However, further progress remains fundamentally constrained by the intrinsic trade-off between short-circuit current density (JSC) and open-circuit voltage (VOC), particularly in narrow-band-gap systems. Although narrowing the band gap effectively broadens photon harvesting, the reduced charge-transfer-state energies and associated voltage losses in narrow-band-gap systems often limit the achievable VOC. Considerable efforts have been devoted to suppressing energy losses in OSCs through molecular engineering, optimization of donor-acceptor combinations, and blend morphology. These approaches have successfully lowered energetic disorder and non-radiative voltage losses, yet simultaneously achieving a narrow band gap, low energetic disorder, and efficient near-infrared photon harvesting remains a major challenge.

To address this challenge, Tao et al. report a compelling strategy by designing a quinoxaline-based acceptor, Qx-Se-NF, that combines a narrow band gap (Eg ≈ 1.31 eV) with exceptionally low energetic disorder (Figure 1A). By integrating a rigid naphthyl terminal group and a selenium-substituted core, the authors demonstrate that reducing electron-vibration coupling and energetic disorder can effectively suppress non-radiative recombination, thereby mitigating voltage loss without sacrificing current. This work establishes energetic disorder as a key descriptor governing the efficiency limits of OSCs.

Unlike inorganic semiconductors, OSCs inherently exhibit significant energetic disorder arising from molecular conformational variability and intermolecular packing fluctuations. This disorder leads to band tailing and enhanced non-radiative recombination, which dominates total energy loss. This work reveals that the Qx-Se-NF-based blend exhibits the lowest Urbach energy (Eu ≈ 22.75 meV), indicating reduced energetic disorder compared with those of Y6 and related derivatives. The total disorder (σT) is decomposed into static disorder (σS) and dynamic disorder (σD) (Figure 1B), which are related to the structural heterogeneity and electron-vibration coupling and reorganization energy (λ), respectively. The authors demonstrate that both σS and σD are minimized in Qx-Se-NF, primarily due to the enhanced molecular rigidity imparted by the naphthyl terminal group, reduced vibrational contributions by suppressing C-C stretching modes, and improved planarity and intermolecular packing. This dual suppression leads to a significant reduction in non-radiative energy loss (ΔEnr), validating energetic disorder as a unifying parameter linking molecular structure to device physics.