better organic active layer materials, such as those star materials of P3HT (2005 by Kim et al.), [2c] DR3TBDTT (2013 by Chen et al.), [2e] ITIC (2015 by Zhan et al.), [2f ] PM6 (2015 by Hou et al.), [2g] Y6 (2019 by Zou et al.), [2h] D18 (2018 by Ding et al.). [2i] Especially in 2019, the rapidly developing OSCs stepped into a new era of PCE exceeding 15% due to the emergence of a super-star acceptor Y6. [2g] As we all known that two determining factors for PCE of OSCs are the active layer materials and its morphology. Therefore, there have been main two approaches for higher performance, one is to get better molecules, and the latter one is to achieve better morphology. While we are always exploring more efficient active layer materials, the journey has been long and challenging. Therefore, it would be much easier to tune the morphology for better performance using some existing materials to achieve their best intrinsic performance. Up to now, tremendous efforts in tuning morphology have been made in the Y6 analogs-based OSCs, such as more suitable coating solvent, [3] solvent [4] or low boiling point solid additives, [5] post treatments of thermal or solvent annealing, [6] rendering the PCE over 18%. [7] In spite of the impressive PCE achieved, the continuously increasing performance of Y6 analogs-based OSCs indicates that the intrinsic best performance might not have been achieved.The above strategies of morphology regulation, such as more suitable coating solvent, solvent or low boiling point solid additives, post treatments of thermal or solvent annealing, can adapt a favorable morphology upon the quick solvent volatile process or post-treatment process by controlling the crystallization and phase separation process. In addition, a multicomponent strategy, which introduces a guest component into the host systems in OSCs, has been proved as an effective way to further improve the device performance recently. [7a,8] The application of a multicomponent strategy can break the limits of light absorption of the existing host systems, optimize the morphology and energy level alignment for the active layer, and thus improve the efficiency of OSC devices. Thus, a feasible multicomponent strategy based on a state-of-the-art system is expected to guide superior device performances. In the past decade, the numerous oligomer-like donors-based organic A wide bandgap oligomer-like donor CNS-6-8 is synthesized and incorporated into the host PM6:Y6:PC 71 BM system to tune the morphology of the active layer for better device performance. Due to the good miscibility of CNS-6-8 with both host donor (PM6) and acceptors (Y6 and PC 71 BM), an optimized morphology is achieved with the appropriate phase separation size and enhanced crystallinity, which ultimately leads to more efficient exciton dissociation, charge transport, and lower nonradiative energy loss. As a result, the quaternary device achieves an improved efficiency of 18.07%, with a simultaneously increased open circuit voltage of 0.868 V, fill factor of 78....

Halogenation of terminal of acceptors has been shown to give dramatic improvements in power conversion efficiencies (PCEs) of organic solar cells (OSCs). Similar significant results could be expected from the halogenation of the central units of state-ofthe-art Y-series acceptors. Herein, a pair of acceptors, termed CH6 and CH4, featuring a conjugation-extended phenazine central unit with and without fluorination, have been synthesized. The fluorinated CH6 has enhanced molecular interactions and crystallinity, superior fibrillar network morphology and improved charge generation and transport in blend films, thus affording a higher PCE of 18.33 % for CH6-based binary OSCs compared to 16.49 % for the non-fluorinated CH4. The new central site offers further opportunities for structural optimization of Y-series molecules to afford better-performed OSCs and reveals the effectiveness of fluorination on central units.

It has been proposed that isotope effects could effectively downshift intramolecular vibrational frequencies of lightharvesting materials, thereby reducing the non-radiative recombination from the charge-transfer (CT) state to the ground state (GS) and achieving a smaller non-radiative energy loss (ΔE loss non-rad ) theoretically in organic solar cells (OSCs). However, there are no systematic experimental studies to address such a crucial issue: can isotope effects enable OSCs to achieve a smaller ΔE loss non-rad and why? Herein, we constructed 29 non-fullerene acceptors (NFAs) by isotope substitution on different functional groups based on four high-performance NFA systems and further investigated their photovoltaic performance systematically. Large-scale statistical experimental and theoretical analyses indicate no significant difference of PCE and ΔE loss non-rad due to the intrinsically very weak electron-vibration coupling between the CT state and GS (EVC CT-GS ) and largely unimpacted coupling strength (t CT-LE ) between the CT and local exciton states. Also based on theoretical results from the Huang−Rhys factor, although different vibration modes could have different influences on the strength of EVC CT-GS , all are quite small. Both experimental and theoretical results suggest that an isotope strategy may not be a feasible way to significantly improve PCEs of high-performance OSCs by reducing ΔE loss non-rad at the current stage. Article pubs.acs.org/cm

Molecular innovation is highly desirable to achieve efficient all‐small‐molecule organic solar cells (SM‐OSCs). Herein, three small‐molecule donors (SMDs) with alkylated thiazole side groups (namely BO‐1, HD‐1, and OD‐1), which differ only in the alkyl side chain are reported. Although these SMDs possess similar absorption profiles and molecular energy levels, their crystallinity and miscibility with BTP‐eC9 slightly decrease along with the elongation of the alkyl side chain. After blending with BTP‐eC9, different miscibility leads to different degrees of phase separation. Among these SM‐OSCs, the HD‐1‐based device shows a decent bulk‐heterojunction (BHJ) morphology with proper phase separation and more dynamic carrier behavior. Thus, compared to the BO‐1 and OD‐1‐based devices, the HD‐1‐based device achieves a higher short‐circuit current of 26.04 mA cm−2 and a fill factor of 78.46%, leading to an outstanding PCE of 17.19%, which is one of the highest values among SM‐OSCs. This work provides a rational design strategy of SMDs for highly efficient SM‐OSCs.

Peripheral halogen regulations can endow non-fullerene acceptors (NFAs)with enhanced features as organic semi-conductors and further boost efficient organic solar cells (OSCs). Herein, based on a remarkable molecular platform of CH14 with more than six halogenation positions, a preferred NFA of CH23 is constructed by synergetic halogen swapping on both central and end units, rendering the overall enlarged molecular dipole moment, packing density and thus relative dielectric constant. Consequently, the CH23-based binary OSC reaches an excellent efficiency of 18.77% due to its improved charge transfer/transport dynamics, much better than that of 17.81% for the control OSC of CH14. This work demonstrates the great potential for further achieving state-of-the-art OSCs by delicately regulating the halogen formula on these newly explored CH-series NFAs.