Terpolymer Donor with Inside Alkyl Substituents on Thiophene π-Bridges toward Thiazolothiazole A2 -Unit Enables 18.21% Efficiency of Polymer Solar Cells

Recent developments in polymer semiconductors with excellent electron transport performances

Benefiting from molecular design and device innovation, electronic devices based on polymer semiconductors have achieved significant developments and gradual commercialization over the past few decades. Most of high-performance polymer semiconductors that have been prepared exhibit p-type performances, and records of their carrier mobilities are constantly being broken through. Although ambipolar and n-type polymers are necessary for constructing p-n heterojunctions and logic circuits, only a few materials show outstanding device performances, which leads to their developments lagging far behind that of p-type analogues. As a consequence, it is extremely significant to summarize polymer semiconductors with excellent electron transport performances. This review focuses on the design considerations and bonding modes between monomers of polymer semiconductors with high electron mobilities. To enhance electron transport performances of polymer semiconductors, the structural modification strategies are described in detail. Subsequently, the electron transport, thermoelectric, mixed ionic-electronic conduction, intrinsically stretchable, photodetection, and spin transport performances of high-electron mobility polymers are discussed from the perspective of molecular engineering. In the end, the challenges and prospects in this research field are presented, which provide valuable guidance for the design of polymer semiconductors with excellent electron transport performances and the exploration of more advanced applications in the future.

A High-Performance Organic Photovoltaic System with Versatile Solution Processability

Recently developed organic photovoltaic (OPV) materials have simultaneously closed the gaps in efficiency, stability, and cost for single-junction devices. Nonetheless, the developed OPV materials still pose big challenges in meeting the requirements for practical applications, especially regarding the prevalent issues of solution processability. Herein, a highly efficient polymer donor, named DP3, incorporating an electron-rich benzo[1,2-b:4,5-b']dithiophene unit as well as two similar and simple acceptor units is presented. Its primary objective is to enhance the interchain and/or intrachain interactions and ultimately fine-tune bulk-heterojunction microstructure. The DP3:L8-BO system demonstrates the highest power conversion efficiency (PCE) of 19.12%. This system also exhibits high-performance devices with over 18% efficiencies for five batches with various molecular weights (23.6-80.8 KDa), six different blend thicknesses (95-308 nm), differenced coating speeds (3.0-29.1 m min-1), with promising PCEs of 18.65% and 15.53% for toluene-processed small-area (0.029 cm2) cells and large-area (15.40 cm2) modules, thereby demonstrating versatile solution processability of the designed DP3:L8-BO system that is a strong candidate for commercial applications.

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.

A 3-Fluoropyridine Manipulating the Aggregation and Fibril Network of Donor Polymers for Eco-Friendly Solution-Processed Versatile Organic Solar Cells

The development of eco-friendly solvent-processed organic solar cells (OSCs) suitable for industrial-scale production should be now considered the imperative research. Herein, asymmetric 3-fluoropyridine (FPy) unit is used to control the aggregation and fibril network of polymer blends. Notably, terpolymer PM6(FPy = 0.2) incorporating 20% FPy in a well-known donor polymer poly[(2,6-(4,8-bis(5-(2-ethylhexyl-3-fluoro)thiophen-2-yl)-benzo[1,2-b:4,5-b']dithiophene))-alt-(5,5-(1',3'-di-2-thienyl-5',7'-bis(2-ethylhexyl)benzo[1',2'-c:4',5'-c']dithiophene-4,8-dione)] (PM6) can reduce the regioregularity of the polymer backbone and endow them with much-enhanced solubility in eco-friendly solvents. Accordingly, the excellent adaptability for fabricating versatile devices based on PM6(FPy = 0.2) by toluene processing is demonstrated. The resulting OSCs exhibit a high power conversion efficiency (PCE) of 16.1% (17.0% by processed with chloroform) and low batch-to-batch variation. Moreover, by controlling the donor-to-acceptor weight ratio at 0.5:1.0 and 0.25:1.0, semi-transparent OSCs (ST-OSCs) yield significant light utilization efficiencies of 3.61% and 3.67%, respectively. For large-area (1.0 cm2 ) indoor OSC (I-OSC), a high PCE of 20.6% is achieved with an appropriate energy loss of 0.61 eV under a warm white light-emitting diode (3,000 K) with the illumination of 958 lux. Finally, the long-term stability of the devices is evaluated by investigating their structure-performance-stability relationship. This work provides an effective approach to realizing eco-friendly, efficient, and stable OSCs/ST-OSCs/I-OSCs.

Direct Arylation Synthesis of Small Molecular Acceptors for Organic Solar Cells

In recent years, small molecular acceptors (SMAs) have extensively promoted the progress of organic solar cells (OSCs). The facile tuning of chemical structures affords SMAs excellent tunability of their absorption and energy levels, and it gives SMA-based OSCs slight energy loss, enabling OSCs to achieve high power conversion efficiencies (e.g., >18%). However, SMAs always suffer complicated chemical structures requiring multiple-step synthesis and cumbersome purification, which is unfavorable to the large-scale production of SMAs and OSC devices for industrialization. Direct arylation coupling reaction via aromatic C-H bonds activation allows for the synthesis of SMAs under mild conditions, and it simultaneously reduces synthetic steps, synthetic difficulty, and toxic by-products. This review provides an overview of the progress of SMA synthesis through direct arylation and summarizes the typical reaction conditions to highlight the field's challenges. Significantly, the impacts of direct arylation conditions on reaction activity and reaction yield of the different reactants' structures are discussed and highlighted. This review gives a comprehensive view of preparing SMAs by direct arylation reactions to cause attention to the facile and low-cost synthesis of photovoltaic materials for OSCs.