Balancing the Efficiency and Synthetic Accessibility of Organic Solar Cells with Isomeric Acceptor Engineering

Multiple-Asymmetric Molecular Engineering Enables Regioregular Selenium-Substituted Acceptor with High Efficiency and Ultra-low Energy Loss in Binary Organic Solar Cells

Asymmetric molecular engineering is utilized for developing efficient small molecular acceptors (SMAs), whereas adopting multiple asymmetric strategies at the terminals, side chains, and cores of efficient SMAs remains a challenge, and effects on reducing energy loss (Eloss) have been rarely investigation. Herein, four regioregular multiple-asymmetric SMAs (DASe-4F, DASe-4Cl, TASe-2Cl2F, and TASe-2F2Cl) are constructed by delicately manipulating the number and position of F and Cl on end groups. Triple-asymmetric TASe-2F2Cl not only exhibits a unique and most compact 3D network crystal stacking structure but also possesses excellent crystallinity and electron mobility in neat film. Surprisingly, the PM1:TASe-2F2Cl-based binary organic solar cells (OSCs) yield a champion power conversion efficiencies (PCEs) of 19.32%, surpassing the PCE of 18.27%, 17.25%, and 16.30% for DASe-4F, DASe-4Cl, and TASe-2Cl2F-based devices, which attributed to the optimized blend morphology with proper phase separation and more ordered intermolecular stacking and excellent charge transport. Notably, the champion PCE of 19.32% with ultralow nonradiative recombination energy loss (ΔE3) of 0.179 eV marks a record-breaking result for selenium-containing SMAs in binary OSCs. Our innovative multiple-asymmetric molecular engineering of precisely modulating the number and position of fluorinated/chlorinated end groups is an effective strategy for obtaining highly-efficient and minimal ΔE3 of selenium-substituted SMAs-based binary OSCs simultaneously.

The Critical Isomerization Effect of Core Bromination on Nonfullerene Acceptors in Achieving High-Performance Organic Solar Cells with Low Energy Loss

Highly efficient nonfullerene acceptors (NFAs) for organic solar cells (OSCs) with low energy loss (Eloss) and favorable morphology are critical for breaking the efficiency bottleneck and achieving commercial applications of OSCs. In this work, quinoxaline-based NFAs are designed and synthesized using a synergistic isomerization and bromination approach. The π-expanded quinoxaline-fused core exhibits different bromination sites for isomeric NFAs, namely AQx-21 and AQx-22. Theoretical and experimental analyses reveal that the isomerization effect of core bromination significantly influences molecular intrinsic properties, including electrostatic potentials, polarizability, dielectric constant, exciton binding energy, crystallinity, and miscibility with donor materials, thereby improving molecular packing and bulk-heterojunction morphology. Consequently, the AQx-22-based blend exhibits enhanced crystallinity, reduced domain size, and optimized phase distribution, which facilitates charge transport, suppresses charge recombination, and improves charge extraction. The AQx-22-treated OSCs obtain an impressive efficiency of 19.5% with a remarkable open-circuit voltage of 0.970 V and a low Eloss of 0.476 eV. This study provides deep insights into NFA design and elucidates the potential working mechanisms for optimizing morphology and device performance through isomerization engineering of core bromination, highlighting its significance in advancing OSC technology.

Polyfluoride Acceptor with Limited Molecular Diffusion Enables Efficient and Stable Ternary Organic Solar Cells

Due to the slow diffusion of photovoltaic molecules, in particular, small-molecule acceptors (SMAs), under light and heating, the morphology of the active layer in organic solar cells (OSCs) prefers to deviate from the favorably metastable status, leading to the challenge of stability during long-term operation. Employing materials with a high glass transition temperature (Tg) as the third component to suppress molecular diffusion is an efficient method to achieve the balance of efficiency and stability of OSCs. Herein, a dimerized small-molecule acceptor denoted as F6D is synthesized by introducing a polyfluoride moiety as the linker to enhance the Tg. Benefitting from a rational molecular design, F6D not only exhibits a higher Tg, complementary absorption, and cascade energy levels with the host materials of the polymer donor PM6 and the SMA Y6 but also has excellent miscibility and multiple intermolecular interactions with Y6. As a result, a champion power conversion efficiency of 17.52% is achieved in the optimal PM6:Y6:F6D-based device. More importantly, the ternary device exhibits superior stability under continuous heating and lighting compared with the binary device.

Electron Transporting Polymeric Materials with Partial Quaternization for High-Performance Organic Solar Cells

Efficient cathode interfacial layers (CILs) have become a crucial component of organic solar cells (OSCs). Charge extraction barriers, interfacial trap states, and significant transport resistance may be induced due to the unfavorable cathode interlayer, limiting the device performance. In this study, poly(4-vinylpyridine) is used as the CIL for OSCs, and a new type of CIL named P4VP-I is synthesized through the quaternization strategy. Compared to P4VP, P4VP-I CIL exhibits enhanced conductivity and optimized work function. OSCs employing the P4VP-I ETL demonstrate prolonged carrier lifetime, suppressed charge recombination, and achieve higher power conversion efficiencies (PCE) than the commonly used ETLs such as PFN-Br and Phen-NaDPO.

Rational molecular and device design enables organic solar cells approaching 20% efficiency

For organic solar cells to be competitive, the light-absorbing molecules should simultaneously satisfy multiple key requirements, including weak-absorption charge transfer state, high dielectric constant, suitable surface energy, proper crystallinity, etc. However, the systematic design rule in molecules to achieve the abovementioned goals is rarely studied. In this work, guided by theoretical calculation, we present a rational design of non-fullerene acceptor o-BTP-eC9, with distinct photoelectric properties compared to benchmark BTP-eC9. o-BTP-eC9 based device has uplifted charge transfer state, therefore significantly reducing the energy loss by 41 meV and showing excellent power conversion efficiency of 18.7%. Moreover, the new guest acceptor o-BTP-eC9 has excellent miscibility, crystallinity, and energy level compatibility with BTP-eC9, which enables an efficiency of 19.9% (19.5% certified) in PM6:BTP-C9:o-BTP-eC9 based ternary system with enhanced operational stability.

Terminally Chlorinated and Thiophene-linked Acceptor-Donor-Acceptor Structured 3D Acceptors with Versatile Processability for High-efficiency Organic Solar Cells

To exploit the potential of our newly developed three-dimensional (3D) dimerized acceptors, a series of chlorinated 3D acceptors (namely CH8-3/4/5) were reported by precisely tuning the position of chlorine (Cl) atom. The introduction of Cl atom in central unit affects the molecular conformation. Whereas, by replacing fluorinated terminal groups (CH8-3) with chlorinated terminal groups (CH8-4 and CH8-5), the red-shift absorption and enhanced crystallization are achieved. Benefiting from these, all devices received promising power conversion efficiencies (PCEs) over 16 % as well as decent thermal/photo-stabilities. Among them, PM6:CH8-4 based device yielded a best PCE of 17.58 %. Besides, the 3D merits with multi alkyl chains enable their versatile processability during the device preparation. Impressive PCEs of 17.27 % and 16.23 % could be achieved for non-halogen solvent processable devices prepared in glovebox and ambient, respectively. 2.88 cm2 modules also obtained PCEs over 13 % via spin-coating and blade-coating methods, respectively. These results are among the best performance of dimerized acceptors. The decent performance of CH8-4 on small-area devices, modules and non-halogen solvent-processed devices highlights the versatile processing capability of our 3D acceptors, as well as their potential applications in the future.