In the past few years, the emergence of nonfullerene (n-type organic semiconductor (n-OS)) acceptors combining with the rational design of conjugated polymer donor materials has dominated the rapid development of highly efficient polymer solar cells (PSCs). Relative to the traditional fullerene derivative acceptors, narrow bandgap n-OS acceptors with a fused ring core possess the advantages of strong absorption in the visible-near infrared region and tunable electronic energy levels. On the other hand, the anisotropic conjugated structures of n-OS acceptors make it more challenging to match them with conjugated polymer donors. Therefore, it is critical to rationally design conjugated polymer donors with complementary absorption and matching electronic energy levels with nonfullerene acceptors for realizing high performance PSCs. Here we review the recent development of state-of-the-art wide-bandgap conjugated polymer donor materials, including D–A copolymers with BDT (benzodithiophene) as the D-unit and BDD (1,3-bis(thiophen-2-yl)-5,7-bis(2-ethylhexyl)benzo[1,2-c:4,5-c′]dithiophene-4,8-dione), FBTA (fluorinated benzotriazole) or quinoxaline as the A-unit, etc., for efficient PSCs based on narrow bandgap n-OS acceptors. Our focus is mainly on the design strategies, chemical structure–property relationships, and photovoltaic performance of the polymer donors. We hope that this review article could contribute to better understanding of the design strategies of high-performance conjugated polymer donors for nonfullerene PSCs.

High-performance conjugated polymer donor materials for polymer solar cells with narrow-bandgap nonfullerene acceptors

There is little question that the "electronic revolution" of the 20th century has impacted almost every aspect of human life However, the emergence of solid-state electronics as a ubiquitous feature of an advanced modern society is posing new challenges such as the management of electronic waste (e-waste) that will remain through the 21st century In addition to developing strategies to manage such e-waste, further challenges can be identified concerning the conservation and recycling of scarce elements, reducing the use of toxic materials and solvents in electronics processing, and lowering energy usage during fabrication methods In response to these issues, the construction of electronic devices from renewable or biodegradable materials that decompose to harmless by-products is becoming a topic of great interest Such "green" electronic devices need to be fabricated on industrial scale through low-energy and low-cost methods that involve low/non-toxic functional materials or solvents This review highlights recent advances in the development of biodegradable materials and processing strategies for electronics with an emphasis on areas where green electronic devices show the greatest promise, including solar cells, organic field-effect transistors, light-emitting diodes, and other electronic devices

Biodegradable Materials and Green Processing for Green Electronics.

The pursuit of low-cost, flexible, and lightweight renewable power resources has led to outstanding advancements in organic solar cells (OSCs). Among the successful design principles developed for synthesizing efficient conjugated electron donor (ED) or acceptor (EA) units for OSCs, chlorination has recently emerged as a reliable approach, despite being neglected over the years. In fact, several recent studies have indicated that chlorination is more potent for large-scale production than the highly studied fluorination in several aspects, such as easy and low-cost synthesis of materials, lowering energy levels, easy tuning of molecular orientation, and morphology, thus realizing impressive power conversion efficiencies in OSCs up to 17%. Herein, an up-to-date summary of the current progress in photovoltaic results realized by incorporating a chlorinated ED or EA into OSCs is presented to recognize the benefits and drawbacks of this interesting substituent in photoactive materials. Furthermore, other aspects of chlorinated materials for application in all-small-molecule, semitransparent, tandem, ternary, single-component, and indoor OSCs are also presented. Consequently, a concise outlook is provided for future design and development of chlorinated ED or EA units, which will facilitate utilization of this approach to achieve the goal of low-cost and large-area OSCs.

Design Principles and Synergistic Effects of Chlorination on a Conjugated Backbone for Efficient Organic Photovoltaics: A Critical Review.

1Center for Excellence in Nanoscience (CAS), Key Laboratory of Nanosystem and Hierarchical Fabrication (CAS), National Center for Nanoscience and Technology, Beijing 100190, China 2Key Laboratory of Low-grade Energy Utilization Technologies and Systems (MOE), School of Energy and Power Engineering, Chongqing University, Chongqing 400044, China 3Hunan Key Laboratory of Super Microstructure and Ultrafast Process, School of Physics and Electronics, Central South University, Changsha 410083, China 4Department of Materials Science and Engineering, University of Science and Technology of China, Hefei 230026, China 5School of Materials and Energy, University of Electronic Science and Technology of China, Chengdu 611731, China 6Institute for Energy Research, Jiangsu University, Zhenjiang 212013, China 7Key Laboratory of Polar Materials and Devices (MoE), School of Physics and Electronic Science, East China Normal University, Shanghai 200241, China 8College of Chemistry, Sichuan University, Chengdu 610064, China 9School of Physical Science and Technology, Lanzhou University, Lanzhou 730000, China

A chlorinated copolymer donor demonstrates a 18.13% power conversion efficiency

Recent advances in molecular design of functional conjugated polymers for high-performance polymer solar cells

New wide‐bandgap D–A–π copolymers based on an asymmetric bithiophene with one carboxylate substituent were synthesized. The asymmetric structure unit is flexible and versatile, which allows the absorption, energy levels and morphology of the blend films to be adjusted easily. D‐A‐p copolymers produced a high power conversion efficiency of 10.0% for halogen solvent‐processed OSCs and 9.55% for non‐halogen solvent‐processed devices.

Nonhalogen Solvent‐Processed Asymmetric Wide‐Bandgap Polymers for Nonfullerene Organic Solar Cells with Over 10% Efficiency

Fluorobenzotriazole (FTAZ)‐Based Polymer Donor Enables Organic Solar Cells Exceeding 12% Efficiency

Alkylsilyl Functionalized Copolymer Donor for Annealing‐Free High Performance Solar Cells with over 11% Efficiency: Crystallinity Induced Small Driving Force

Two novel copolymers based on benzothiadiazole (BT) or difluorobenzothiadizole (ffBT) with 2,2'-(perfluoro-1,4-phenylene)dithiophene (2TPF4), namely PBT-2TPF4 and PffBT-2TPF4, are synthesized for applications in polymer solar cells (PSCs). A noticeably high open-circuit voltage (Voc ) of 1.017 and 0.87 V are achieved for PffBT-2TPF4 and PBT-2TPF4-based devices, respectively. Although only a moderate efficiency (5.7%) of PBT-2TPF4-based devices is obtained, it is first demonstrated that 2TPF4 is a promising acceptor block for construction of the donor copolymers which possess high Voc , prominent crystallinity, and long-term stability, simultaneously. Besides, two thienyl flanking the tetrafluorophenylene can decrease torsion angle between conjugated units, resulting in a high coplanar structure of copolymers to enhance their charge carrier mobility. The findings may open a promising and practical way to accelerate the commercialization of PSCs by developing a series of new donor copolymers for efficient and long-term stable thickness bulk heterojunction PSCs.

Novel Copolymers Based Tetrafluorobenzene and Difluorobenzothiadiazole for Organic Solar Cells with Prominent Open Circuit Voltage and Stability.

Fused-ring electron acceptor (FREA) based ternary organic solar cells (OSCs) have made significant progress and attracted considerable attention due to their simple device architecture and broad absorption range in devices. There are three key parameters that need to be fine-tuned in ternary OSCs including absorption, energy level and morphology in order to realize high efficiencies. Herein, a series of FREAs with diverse electron-rich cores or electron-deficient terminals are developed and rationally combined to achieve high performance ternary OSCs. The dipole moment of FREAs’ terminals has been unveiled as an important factor and its working mechanism has been thoroughly investigated by systematically studying six ternary OSCs. These ternary blends all exhibit complementary absorption and cascade energy levels, which can facilitate efficient light-harvesting and charge transfer. Additionally, the morphological effects on ternary OSCs are eliminated through comparative studies while demonstrating distinctively different performance. The preliminary results show that compatible dipole moment between two FREAs is critical in ternary blends. Specifically, the performance of the ternary system with two FREAs having quite different dipole moment terminals is worse compared to that with similar terminal dipole moments. The pair with larger difference in the dipole moment will also negatively impact device performance. This interesting phenomenon is likely due to the fact that very different dipole moments of terminals in FREAs can significantly decrease the electron mobility as well as induce unbalanced hole/electron transport. Consequently, it results in increased charge recombination and reduced charge collection efficiency. This finding demonstrates that the dipole moment of FREAs should be taken into account in designing ternary OSCs.

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Yongkang An

Papers

Nonhalogen Solvent‐Processed Asymmetric Wide‐Bandgap Polymers for Nonfullerene Organic Solar Cells with Over 10% Efficiency

Fluorobenzotriazole (FTAZ)‐Based Polymer Donor Enables Organic Solar Cells Exceeding 12% Efficiency

Alkylsilyl Functionalized Copolymer Donor for Annealing‐Free High Performance Solar Cells with over 11% Efficiency: Crystallinity Induced Small Driving Force

Novel Copolymers Based Tetrafluorobenzene and Difluorobenzothiadiazole for Organic Solar Cells with Prominent Open Circuit Voltage and Stability.

The role of dipole moment in two fused-ring electron acceptor and one polymer donor based ternary organic solar cells