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Bromination: An Alternative Strategy for Non-Fullerene Small Molecule Acceptors.

ABSTRACT: The concept of bromination for organic solar cells has received little attention. However, the electron withdrawing ability and noncovalent interactions of bromine are similar to those of fluorine and chlorine atoms. A tetra-brominated non-fullerene acceptor, designated as BTIC-4Br, has been recently developed by introducing bromine atoms onto the end-capping group of 2-(3-oxo-2,3-dihydro-1H-inden-1-ylidene) malononitrile and displayed a high power conversion efficiency (PCE) of 12%. To further improve its photovoltaic performance, the acceptor is optimized either by introducing a longer alkyl chain to the core or by modulating the numbers of bromine substituents. After changing each end-group to a single bromine, the BTIC-2Br-m-based devices exhibit an outstanding PCE of 16.11% with an elevated open-circuit voltage of V oc = 0.88 V, one of the highest PCEs reported among brominated non-fullerene acceptors. This significant improvement can be attributed to the higher light harvesting efficiency, optimized morphology, and higher exciton quenching efficiencies of the di-brominated acceptor. These results demonstrate that the substitution of bromine onto the terminal group of non-fullerene acceptors results in high-efficiency organic semiconductors, and promotes the use of the halogen-substituted strategy for polymer solar cell applications.

SUBMITTER: Wang H

PROVIDER: S-EPMC7201261 | biostudies-literature | 2020 May

REPOSITORIES: biostudies-literature

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Bromination: An Alternative Strategy for Non-Fullerene Small Molecule Acceptors.

Wang Huan H Liu Tao T Zhou Jiadong J Mo Daize D Han Liang L Lai Hanjian H Chen Hui H Zheng Nan N Zhu Yulin Y Xie Zengqi Z He Feng F

Advanced science (Weinheim, Baden-Wurttemberg, Germany) 20200228 9

The concept of bromination for organic solar cells has received little attention. However, the electron withdrawing ability and noncovalent interactions of bromine are similar to those of fluorine and chlorine atoms. A tetra-brominated non-fullerene acceptor, designated as BTIC-4Br, has been recently developed by introducing bromine atoms onto the end-capping group of 2-(3-oxo-2,3-dihydro-1H-inden-1-ylidene) malononitrile and displayed a high power conversion efficiency (PCE) of 12%. To further ...[more]

PMID: 32382488

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Project description:The active layer in a solution processed organic photovoltaic device comprises a light absorbing electron donor semiconductor, typically a polymer, and an electron accepting fullerene acceptor. Although there has been huge effort targeted to optimize the absorbing, energetic, and transport properties of the donor material, fullerenes remain as the exclusive electron acceptor in all high performance devices. Very recently, some new non-fullerene acceptors have been demonstrated to outperform fullerenes in comparative devices. This Account describes this progress, discussing molecular design considerations and the structure-property relationships that are emerging. The motivation to replace fullerene acceptors stems from their synthetic inflexibility, leading to constraints in manipulating frontier energy levels, as well as poor absorption in the solar spectrum range, and an inherent tendency to undergo postfabrication crystallization, resulting in device instability. New acceptors have to address these limitations, providing tunable absorption with high extinction coefficients, thus contributing to device photocurrent. The ability to vary and optimize the lowest unoccupied molecular orbital (LUMO) energy level for a specific donor polymer is also an important requirement, ensuring minimal energy loss on electron transfer and as high an internal voltage as possible. Initially perylene diimide acceptors were evaluated as promising acceptor materials. These electron deficient aromatic molecules can exhibit good electron transport, facilitated by close packed herringbone crystal motifs, and their energy levels can be synthetically tuned. The principal drawback of this class of materials, their tendency to crystallize on too large a length scale for an optimal heterojunction nanostructure, has been shown to be overcome through introduction of conformation twisting through steric effects. This has been primarily achieved by coupling two units together, forming dimers with a large intramolecular twist, which suppresses both nucleation and crystal growth. The generic design concept of rotationally symmetrical aromatic small molecules with extended π orbital delocalization, including polyaromatic hydrocarbons, phthalocyanines, etc., has also provided some excellent small molecule acceptors. In most cases, additional electron withdrawing functionality, such as imide or ester groups, can be incorporated to stabilize the LUMO and improve properties. New calamitic acceptors have been developed, where molecular orbital hybridization of electron rich and poor segments can be judiciously employed to precisely control energy levels. Conformation and intermolecular associations can be controlled by peripheral functionalization leading to optimization of crystallization length scales. In particular, the use of rhodanine end groups, coupled electronically through short bridged aromatic chains, has been a successful strategy, with promising device efficiencies attributed to high lying LUMO energy levels and subsequently large open circuit voltages.

Dataset Information

Bromination: An Alternative Strategy for Non-Fullerene Small Molecule Acceptors.

Publications

Bromination: An Alternative Strategy for Non-Fullerene Small Molecule Acceptors.

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