The structural order and aggregation of non‐fullerene acceptors (NFA) are critical toward light absorption, phase separation, and charge transport properties of their photovoltaic blends with electron donors, and determine the power conversion efficiency (PCE) of the corresponding organic solar cells (OSCs). In this work, the fibrillization of small molecular NFA L8‐BO with the assistance of fused‐ring solvent additive 1‐fluoronaphthalene (FN) to substantially improve device PCE is demonstrated. Molecular dynamics simulations show that FN attaches to the backbone of L8‐BO as the molecular bridge to enhance the intermolecular packing , inducing 1D self‐assembly of L8‐BO into fine fibrils with a compact polycrystal structure. The L8‐BO fibrils are incorporated into a pseudo‐bulk heterojunction (P‐BHJ) active layer with D18 as a donor, and show enhanced light absorption, charge transport, and collection properties, leading to enhanced PCE from 16.0% to an unprecedented 19.0% in the D18/L8‐BO binary P‐BHJ OSC, featuring a high fill factor of 80%. This work demonstrates a strategy for fibrillating NFAs toward the enhanced performance of OSCs.

Fibrillization of Non‐Fullerene Acceptors Enables 19% Efficiency Pseudo‐Bulk Heterojunction Organic Solar Cells

Interfacial layers (ILs) are prerequisites to form the selective charge transport for high-performance organic photovoltaics (OPVs), but mostly result in considerable parasitic absorption loss. Trimming the ILs down to a mono-molecular level via the self-assembled monolayer (SAM) is an effective strategy to mitigate parasitic absorption loss. However, such strategy suffers from inferior electrical contact with low surface coverage on rough surface and the poor producibility. To address these issues, here we develop the self-assembled interlayer (SAI) strategy, which involves a thin layer of 2-6 nm to form a full coverage on substrate via both covalent and Van de Waals bonds by using a self-assembled molecule of 2-(9H-carbazol-9-yl) (2PACz). Via the facile spin coating without further rinsing and annealing process, it not only optimizes the electrical and optical properties of OPVs, which enables a world-record efficiency of 20.17% (19.79% certified), but also simplified the tedious processing procedure. Moreover, the SAI strategy is especially useful to improve the absorbing selectivity for semi-transparent OPVs, which enables a record light utilization efficiency of 5.34%. Our work provides an effective strategy of SAI to optimize the optical and electrical properties of OPVs for high performance and solar window applications. This article is protected by copyright. All rights reserved.

Self-Assembled Interlayer Enables High-performance Organic Photovoltaics with Power Conversion Efficiency Exceeding 20.

Isomeric non-fullerene acceptor guest with large π-conjugation skeleton is used to construct stable organic solar cells with efficiencies over 19%.

Isomerization strategy on a non-fullerene guest acceptor for stable organic solar cells with over 19% efficiency

The limited selection pool of high‐performance wide bandgap (WBG) polymer donors is a bottleneck problem of the nonfullerene acceptor (NFA) based organic solar cells (OSCs) that impedes the further improvement of their photovoltaic performances. Herein, a series of new WBG polymers, namely PH‐BTz, PS‐BTz, PF‐BTz, and PCl‐BTz, are developed by using the bicyclic difluoro‐benzo[d]thiazole (BTz) as the acceptor block and benzo[1,2‐b:4,5‐b′]dithiophene (BDT) derivatives as the donor units. By introducing S, F, and Cl atoms to the alkylthienyl sidechains on BDT, the resulting polymers exhibit lowered energy levels and enhanced aggregation properties. The fluorinated PBTz‐F not only exhibits a low‐lying HOMO level, but also has stronger face‐on packing order and results in more uniform fibril‐like interpenetrating networks in the related PF‐BTz:L8‐BO blend. A high‐power conversion efficiency (PCE) of 18.57% is achieved. Moreover, PBTz‐F also exhibits a good batch‐to‐batch reproducibility and general applicability. In addition, ternary blend OSCs based on the host PBTz‐F:L8‐BO blend and PM6 guest donor exhibits a further enhanced PCE of 19.54%, which is among the highest values of OSCs.

Benzo[d]thiazole Based Wide Bandgap Donor Polymers Enable 19.54% Efficiency Organic Solar Cells Along with Desirable Batch‐to‐Batch Reproducibility and General Applicability

Non-fullerene acceptor with asymmetric structure and phenyl-substituted alkyl side chain for 20.2% efficiency organic solar cells

Suppressed recombination loss in organic photovoltaics adopting a planar–mixed heterojunction architecture

Power conversion efficiency and long-term stability are two critical metrics for evaluating the commercial potential of organic photovoltaics. Although the field has witnessed a rapid progress of efficiency towards 19%, the intrinsic trade-off between efficiency and stability is still a challenging issue for bulk-heterojunction cells due to the very delicate crystallization dynamics of organic species. Herein, we developed a class of non-fullerene acceptors with varied side groups as an alternative to aliphatic chains. Among them, the acceptors with conjugated side groups show larger side-group torsion and more twisted backbone, however, they can deliver an efficiency as high as 18.3% in xylene-processed cells, which is among the highest values reported for non-halogenated solvent processed cells. Meanwhile, decent thermal/photo stability is realized for these acceptors containing conjugated side groups. Through the investigation of the geometry-performance-stability relationship, we highlight the importance of side-group steric hinderance of acceptors in achieving combined high-performance, stable, and eco-friendly organic photovoltaics. 

/pdf/importance-of-structural-hinderance-in-performance-stability-e3km1nal.pdf

Importance of structural hinderance in performance–stability equilibrium of organic photovoltaics

Intramolecular chlorine-sulfur (Cl-S) non-covalent interaction is introduced to modify the molecular backbone of a benchmark small-molecule donor (SMD) BTR to help planarize the molecular framework for improving charge transport. Theoretical simulations and temperature-variable NMR experiments clearly validate the existence of Cl-S non-covalent interaction in BP-Cl and BM-Cl and explain its important role in enhancing planarity and rigidity of the molecules for enhancing their crystallinity. Moreover, the asymmetric isomerization of side-chains by shifting the alkyl group from para - to meta -position on the phenyl ring further optimizes the molecular orientation and surface energy of BM-Cl to strike a balance between its crystallinity and miscibility. This carefully manipulated molecular design helps result in increased carrier mobility and suppressed charge recombination to obtain simultaneously enhanced J SC and FF and a very high power conversion efficiency (PCE) of 15.73% in binary all-small-molecule organic solar cells (ASM-OSCs). This work demonstrates a rational molecular design approach in improving the performance of ASM-OSCs through delicately balanced crystallinity and miscibility of SMDs. 

Intramolecular Chloro–Sulfur Interaction and Asymmetric Side‐Chain Isomerization to Balance Crystallinity and Miscibility in All‐Small‐Molecule Solar Cells

The current research investigates the structure features and intermolecular interactions of nonfullerene acceptors (NFAs) in single crystal and thin films, as well as their solar cell applications. Guiding parameters and key intermolecular forces that lead to 2D brickwork or 3D web packing are identified. The atomic modification is shown as the key to induce hydrogen bonding or π–π stacking column, which results in different crystalline packing. The molecular assembly in thin film is initiated by hydrogen bonding, and completed by π–π stacking reorganization. The packing energy is seen as a guiding parameter that dictates the NFA crystalline morphology in blended thin films. The crystalline packing motif is not directly related with device efficiency. However, the crystalline morphology is the key parameter to influence exciton/carrier dynamics and device performance. A broader picture on the scaling behavior of organic semiconductor crystals ranging from oligoacenes to NFAs is established. 

The Crystalline Behavior and Device Function of Nonfullerene Acceptors in Organic Solar Cells

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Suppressed recombination loss in organic photovoltaics adopting a planar–mixed heterojunction architecture

Importance of structural hinderance in performance–stability equilibrium of organic photovoltaics

Intramolecular Chloro–Sulfur Interaction and Asymmetric Side‐Chain Isomerization to Balance Crystallinity and Miscibility in All‐Small‐Molecule Solar Cells

The Crystalline Behavior and Device Function of Nonfullerene Acceptors in Organic Solar Cells