Synergistic Engineering of Substituents and Backbones on Donor Polymers: Toward Terpolymer Design of High-Performance Polymer Solar Cells

Fine-Tuning Intra/Inter-Molecular Interaction via Ternary Copolymerization Strategy to Obtain Efficient Polymer Donors

Incorporating a third component through ternary copolymerization strategy has proven to be a promising and effective approach for further improving the device performance of polymer donors. However, terpolymer donors typically exhibit negative effects on molecular stacking and weaken charge transport due to the irregular distribution of the polymer skeleton. Herein, two terpolymers PBBQ-5 (5% ff-Qx) and PBBQ-10 (10% ff-Qx) are developed by introducing the difluoro-2-(3-hexyldecyloxy) quinoxaline (ff-Qx) to the main chain of PM6. The introduction of ff-Qx unit not only enables the terpolymers to possess N···S noncovalent intramolecular interactions that contribute to enlarge molecular planarity and enhance intramolecular interaction but also can fine-tuning electrostatic potential (ESP) values of the related polymers, thereby regulate intermolecular interactions with acceptor by electrostatic force. After blending with L8-BO, the PBBQ-5:L8-BO-based blend film exhibits appreciated miscibility, desirable phase separation sizes, and more ordered molecular packing than the analogs. Consequently, an impressive power conversion efficiency (PCE) as high as 19.54% is realized in PBBQ-5-based devices, which set a new record for Qx-based polymer solar cells. The work emphasizes the drawbacks of terpolymers and provides a new pathway for developing high-performance polymer donors by optimizing intra/inter-molecular interaction.

Synthesis and Characterization of the Donor-Acceptor Conjugated Polymer PBDB-T Implementing Group IV Element Germanium

Here, we synthesized and characterized a novel two-dimensional (2D) conjugated electron donor-acceptor (D-A) copolymer (PBDB-T-Ge), wherein the substituent of triethyl germanium was added to the electron donor unit of the polymer. The Turbo-Grignard reaction was used to implement the group IV element into the polymer, resulting in a yield of 86%. This corresponding polymer, PBDB-T-Ge, exhibited a down-shift in the highest occupied molecular orbital (HOMO) level to -5.45 eV while the lowest unoccupied molecular orbital (LUMO) level was -3.64 eV. The peaks in UV-Vis absorption and the PL emission of PBDB-T-Ge were observed at 484 nm and 615 nm, respectively.

Spectroscopic study of D1-A-D2-A terpolymer films for optoelectronic applications

Several strategies have been considered in search of more efficient organic materials for charge transfer in photovoltaic devices. Among them, the integration of donor-acceptor (D-A) functional units on a conjugated copolymer has been widely applied. In this framework, we evaluated four terpolymers made up of donor moieties derived from 9,9-dioctylfluorene and 9-(heptadecan-9-yl)-9H-carbazole combined with 2,1,3-benzothiadiazole, the acceptor moiety, in different monomer ratios and polymerization routes (block and random microstructures). The preferred molecular orientation and charge transfer dynamics of the polymeric films were assessed by near edge X-ray absorption fine structure spectroscopy (NEXAFS) and resonant Auger electron spectroscopy (RAES) around the sulfur K-edge. Charge transfer times (τ_CT) were estimated by the Core-Hole Clock (CHC) method. Films with a high degree of organization were identified for the block terpolymer and random terpolymers with uneven amounts of donor units, showing a preferred orientation of the benzothiadiazole (BT) molecular plane parallel to the substrate surface. The values of τ_CT measured for all terpolymers were higher than those for typical polymers used in photovoltaic devices, which is not desirable for this type of optoelectronic application, but this may be correlated to the strong acceptor character of BT, the unit probed. To investigate the effect of film formation on the excited state behavior, steady-state and time-resolved photoluminescence measurements were also conducted. X-ray photoelectron spectroscopy (XPS) was employed to characterize the surface chemical composition of the terpolymer films. Based on the spectroscopic data the block copolymer appears to be the most suitable for the desired application.