Dipole Moment Modulation of Terminal Groups Enables Asymmetric Acceptors Featuring Medium Bandgap for Efficient and Stable Ternary Organic Solar Cells.

This study puts forth a novel terminal group design to develop medium-bandgap Y-series acceptors beyond conventional side-chain engineering. We focused on the strategical integration of an electron-donating methoxy group and an electron-withdrawing halogen atom at benzene-fused terminal groups. This combination precisely modulated the dipole moment and electron density of terminal groups, effectively attenuating intramolecular charge transfer effect, and widening the bandgap of acceptors. The incorporation of these terminal groups yielded two asymmetric acceptors, named BTP-2FClO and BTP-2FBrO, both of which exhibited open-circuit voltage (VOC) as high as 0.96 V in binary devices, representing the highest VOCs among the asymmetric Y-series small molecule acceptors. More importantly, both BTP-2FClO and BTP-2FBrO exhibit modest aggregation behaviors and molecular crystallinity, making them suitable as a third component to mitigate excess aggregation of the PM6: BTP-eC9 blend and optimize the devices' morphology. As a result, the optimized BTP-2FClO-based ternary organic solar cells (OSCs) achieved a remarkable power conversion efficiency (PCE) of 19.34%, positioning it among the highest-performing OSCs. Our study highlights the molecular design importance on manipulating dipole moments and electron density in developing medium-bandgap acceptors, and offers a highly efficient third component for high-performance ternary OSCs.

Tailoring the Position of Ester Group on N-Alkyl Chains of Benzotriazole-based Small Molecule Acceptors for High-Performance Organic Solar Cells.

Side chain engineering plays a vital role in exploring high-performance small molecule acceptors (SMAs) for organic solar cells (OSCs). In this work, we designed and synthesized a series of A-DA'D-A type SMAs by introducing different N-substituted alkyl and ester alkyl side chains on benzotriazole (BZ) central unit and aimed to investigate the effect of different ester substitution positions on photovoltaic performances. All the new SMAs with ester groups exhibit lower the lowest unoccupied molecular orbital (LUMO) energy levels and more blue-shifted absorption, but relatively higher absorption coefficients than alkyl chain counterpart. After blending with the donor PM6, the ester side chain-based devices demonstrate enhanced charge mobility, reduced amorphous intermixing domain size and long-lived charge transfer state compared to the alkyl chain counterpart, which are beneficial to achieve higher short-circuit current density (Jsc ) and fill factor (FF), simultaneously. Thereinto, the PM6 : BZ-E31 based device achieves a higher power conversion efficiency (PCE) of 18.33 %, which is the highest PCE among the OSCs based on the SMAs with BZ-core. Our work demonstrated the strategy of ester substituted side chain is a feasible and effective approach to develop more efficient SMAs for OSCs.

Auxiliary sequential deposition enables 19%-efficiency organic solar cells processed from halogen-free solvents.

High-efficiency organic solar cells are often achieved using toxic halogenated solvents and additives that are constrained in organic solar cells industry. Therefore, it is important to develop materials or processing methods that enabled highly efficient organic solar cells processed by halogen free solvents. In this paper, we report an innovative processing method named auxiliary sequential deposition that enables 19%-efficiency organic solar cells processed by halogen free solvents. Our auxiliary sequential deposition method is different from the conventional blend casting or sequential deposition methods in that it involves an additional casting of dithieno[3,2-b:2',3'-d]thiophene between the sequential depositions of the donor (D18-Cl) and acceptor (L8-BO) layers. The auxiliary sequential deposition method enables dramatic performance enhancement from 15% to over 18% compared to the blend casting and sequential deposition methods. Furthermore, by incorporating a branched-chain-engineered acceptor called L8-BO-X, device performance can be boosted to over 19% due to increased intermolecular packing, representing top-tier values for green-solvent processed organic solar cells. Comprehensive morphological and time-resolved characterizations reveal that the superior blend morphology achieved through the auxiliary sequential deposition method promotes charge generation while simultaneously suppressing charge recombination. This research underscores the potential of the auxiliary sequential deposition method for fabricating highly efficient organic solar cells using environmentally friendly solvents.

An Isomeric Solid Additive Enables High-Efficiency Polymer Solar Cells Developed Using a Benzo-Difuran-Based Donor Polymer.

Currently, nearly all high-efficiency organic photovoltaic devices use donor polymers based on the benzo-dithiophene (BDT) unit. To diversify the choices of building blocks for high-performance donor polymers, the use of benzo-difuran (BDF) units is explored, which can achieve reduced steric hindrance, stronger molecular packing, and tunable energy levels. In previous research, the performance of BDF-based devices lagged behind those of BDT-based devices. In this study, a high efficiency (18.4%) is achieved using a BDF-based polymer donor, which is the highest efficiency reported for BDF donor materials to date. The high efficiency is enabled by a donor polymer (D18-Fu) and the aid of a solid additive (2-chloronaphthalene), which is the isomer of the commonly used additive 1-chloronaphthalene. These results revealed the significant effect of 2-chloronaphthalene in optimizing the morphology and enhancing the device parameters. This work not only provides a new building block that can achieve an efficiency comparable to dominant BDT units but also proposes a new solid additive that can replace the widely used 1-chloronaphthalene additive.

Temperature Induced Aggregation of Organic Semiconductors.

One important feature of organic semiconductors is their solution processability, which allows researchers to tune their aggregation states in solution and solid states and to control the processing conditions to reach desirable electronic and optoelectronic properties. Temperature is one of the most important processing parameters of organic semiconductors and has been studied extensively particularly for those conjugated small- and macro- molecules with strong temperature-dependent aggregation properties. This minireview summarizes the temperature-induced aggregation behaviors of organic semiconductors in solution, during solution casting and upon thermal annealing post-treatment of solid-state thin films. The influences of different aggregation states on the optoelectronic properties, in particular the photovoltaic properties, are discussed. The conclusions in this work will provide a rational guide to precisely control the aggregation states of organic semiconductors to fabricate high-performance optoelectronic devices.

Rh-Catalyzed aminative dearomatization of 2-naphthols.

A direct aminative dearomatization of 2-naphthols was achieved. In the presence of 1 mol% Rh2(esp)2 and 3 equivalents of O-(2,4-dinitrophenyl)hydroxylamine (DPH) as readily available aminating reagents, the reactions of 2-naphthols afforded unprotected α-amino-ß-naphthalenones in good yields under mild reaction conditions. The conditions were compatible with gram-scale reaction, and the product could undergo diverse transformations.

Synthesis of C3-Methyl-Substituted Pyrroloindolines and Furoindolines via Cascade Dearomatization of Indole Derivatives with Methyl Iodide.

A highly efficient method for constructing C3-methyl-substituted pyrroloindolines and furoindolines via cascade dearomatization reaction of indole derivatives with methyl iodide was developed. This protocol offers a direct approach to a wide range of C3 methyl substituted pyrroloindolines under mild conditions. The utility of this method was further demonstrated in the concise synthesis of (±)-esermethol.

Copper(I)-Catalyzed Asymmetric Dearomatization of Indole Acetamides with 3-Indolylphenyliodonium Salts.

The rapid and direct asymmetric synthesis of 3-(3a-indolyl)hexahydropyrroloindoline motifs is an extremely important part of the total synthesis of several alkaloid structures. Herein, an intermolecular, asymmetric cascade dearomatization reaction of indole acetamides with 3-indolylphenyliodonium salts has been developed. This protocol provides a straightforward access to 3-(3a-indolyl)hexahydropyrroloindolines bearing an all-carbon quaternary stereocenter at the C3 position of the indoline ring with high enantioselectivities. The utility of the protocol has been demonstrated by the formal asymmetric synthesis of folicanthine.

Enantioselective Synthesis of 3a-Amino-Pyrroloindolines by Copper-Catalyzed Direct Asymmetric Dearomative Amination of Tryptamines.

A direct asymmetric dearomative amination of tryptamines with O-(2,4-dinitrophenyl)hydroxylamine (DPH) was achieved using CuBr-bisoxazoline complex as a catalyst, affording 3a-amino-pyrroloindolines in good to excellent enantioselectivity under mild reaction conditions. Furthermore, the synthetic value of this method was demonstrated in the total synthesis of (-)-psychotriasine in a highly concise manner.