Suppressing Defect Formation Pathways in the Direct C–H Arylation Polymerization of Photovoltaic Copolymers

High-mobility organic mixed conductors with a low synthetic complexity index via direct arylation polymerization

Through direct arylation polymerization, a series of mixed ion-electron conducting polymers with a low synthetic complexity index is synthesized. A thieno[3,2-b]thiophene monomer with oligoether side chains is used in direct arylation polymerization together with a wide range of aryl bromides with varying electronic character from electron-donating thiophene to electron-accepting benzothiadiazole. The obtained polymers are less synthetically complex than other mixed ion-electron conducting polymers due to higher yield, fewer synthetic steps and less toxic reagents. Organic electrochemical transistors (OECTs) based on a newly synthesized copolymer comprising thieno[3,2-b]thiophene with oligoether side chains and bithiophene exhibit excellent device performance. A high charge-carrier mobility of up to μ = 1.8 cm2 V-1 s-1 was observed, obtained by dividing the figure of merit [μC*] from OECT measurements by the volumetric capacitance C* from electrochemical impedance spectroscopy, which reached a value of more than 215 F cm-3.

Universal Design and Efficient Synthesis for High Ambipolar Mobility Emissive Conjugated Polymers

High ambipolar mobility emissive conjugated polymers (HAME-CPs) are perfect candidates for organic optoelectronic devices, such as polymer light emitting transistors. However, due to intrinsic trade-off relationship between high ambipolar mobility and strong solid-state luminescence, the development of HAME-CPs suffers from high structural and synthetic complexity. Herein, a universal design principle and simple synthetic approach for HAME-CPs are developed. A series of simple non-fused polymers composed of charge transfer units, π bridges and emissive units are synthesized via a two-step microwave assisted C-H arylation and direct arylation polymerization protocol with high total yields up to 61 %. The synthetic protocol is verified valid among 7 monomers and 8 polymers. Most importantly, all 8 conjugated polymers have strong solid-state emission with high photoluminescence quantum yields up to 24 %. Furthermore, 4 polymers exhibit high ambipolar field effect mobility up to 10-2 cm2 V-1 s-1, and can be used in multifunctional optoelectronic devices. This work opens a new avenue for developing HAME-CPs by efficient synthesis and rational design.

Synthesis of Organic Optoelectronic Materials Using Direct C-H Functionalization

Small molecules and polymers with conjugated structures can be used as organic optoelectronic materials. These molecules have conventionally been synthesized by cross-coupling reactions; however, in recent years, direct functionalization of C-H bonds has been used to synthesize organic optoelectronic materials. Representative reactions include direct arylation reactions (C-H/C-X couplings, with X being halogen or pseudo-halogen) and cross-dehydrogenative coupling (C-H/C-H cross-coupling) reactions. Although these reactions are convenient for short-step synthesis, they require regioselectivity in the C-H bonds and suppression of undesired homo-coupling side reactions. This review introduces examples of the synthesis of organic optoelectronic materials using two types of direct C-H functionalization reactions. In addition, we summarize our recent activities in the development of direct C-H functionalization reactions using fluorobenzenes as substrates. This review covers the reaction mechanism and material properties of the resulting products.

Constructing High-Performance Ternary Device Using Analogous Polymer Donors

Both ternary copolymerization and ternary blending are effective methods to fine-tune polymer structure and manipulate thin-film morphology to improve device performance. In this work, three D-A-A-A (D: donor, A: acceptor) terpolymer donors (FY1, FY2, and FY3) are synthesized by introducing BDD (1,3-bis(2-ethylhexyl)-5,7-di(thiophen-2-yl)benzo[1,2-c:4,5-c']dithiophene-4,8-dione) units into the D-A alternating copolymer PM6 backbone. Owing to the promoted conjugated planarity and excellent absorption of BDD, the obtained terpolymers display an extended absorption range and enhanced π-π stacking orientation, which is a promising third component in ternary device. As a result, the optimal FY1:PM6:BTP-eC9-based ternary device afforded an impressive power conversion efficiency (PCE) as high as 18.52%, owing to the efficient charge transport, negligible energy loss, and suitable domain size. The result provides an efficient method to obtain high-performance polymer solar cells by using analogous polymer donors in ternary device.

Direct heteroarylation polymerization of a π-conjugated polymer with degradable 1,2,4-oxadiazole linkers

The synthesis of π-conjugated polymers containing a degradable 1,2,4-oxadiazole linker by direct heteroaryl polymerization (DHAP) is reported.

Polymers based on thieno[3,4-c]pyrrole-4,6-dione and pyromellitic diimide by CH–CH arylation reaction for high-performance thin-film transistors

Three homopolymers were successfully synthesized by direct CH–CH arylation polymerization of thieno[3,4-c]pyrrole-4,6-dione or pyromellitic diimide derivatives affording highly purified polymers with high molecular weights (43.0–174.7 K). Thieno[3,4-c]pyrrole-4,6-dione and pyromellitic diimide derivatives are considered as electron-withdrawing units. The synthesized homopolymers P1, P2, and P3 showed band gaps in the range of 2.13–2.08 eV, respectively. The electron mobilities of the three homopolymers have been investigated. The thin film transistor for P1 prepared by the eutectic-melt-assisted nanoimprinting method achieved an electron mobility of 2.11 × 10⁻³ cm² s⁻¹ V⁻¹. Based on the obtained results, the synthesized polymers can be used as potential electron acceptors in solar cell applications.

The homopolymers P1, P2 and P3 were successfully synthesized by direct CH–CH arylation polymerization in an eco-friendly one-step coupling reaction. They present n-type properties for potential applications as acceptor polymers.

The role of excited-state character, structural relaxation, and symmetry breaking in enabling delayed fluorescence activity in push-pull chromophores

Thermally activated delayed fluorescence (TADF) is a current promising route for generating highly efficient light-emitting devices. However, the design process of new chromophores is hampered by the complicated underlying photophysics. In this work, four closely related donor-π-acceptor-π-donor systems are investigated, two of which were synthesised previously, with the aim of elucidating their varying effectiveness for TADF. We outline that the frontier orbitals are insufficient for discriminating between the molecules. Subsequently, a detailed analysis of the excited states at a correlated ab initio level highlights the presence of a number of closely spaced singlet and triplet states of varying character. Results from five density functionals are compared against this reference revealing dramatic changes in, both, excited state energies and wavefunctions following variations in the amount of Hartree-Fock exchange included. Excited-state minima are optimised in solution showing the crucial role of structural variations and symmetry breaking for producing a strongly emissive S₁ state. The adiabatic singlet-triplet gaps thus obtained depend strongly on the range separation parameter used in the hybrid density functional calculations. More generally, this work highlights intricate differences present between singlet and triplet excited state wavefunctions and the challenges in describing them accurately.

Molecular Encapsulation of Naphthalene Diimide (NDI) Based π-Conjugated Polymers: A Tool for Understanding Photoluminescence

Conjugated polymers are an important class of chromophores for optoelectronic devices. Understanding and controlling their excited state properties, in particular, radiative and non‐radiative recombination processes are among the greatest challenges that must be overcome. We report the synthesis and characterization of a molecularly encapsulated naphthalene diimide‐based polymer, one of the most successfully used motifs, and explore its structural and optical properties. The molecular encapsulation enables a detailed understanding of the effect of interpolymer interactions. We reveal that the non‐encapsulated analogue P(NDI‐2OD‐T) undergoes aggregation enhanced emission; an effect that is suppressed upon encapsulation due to an increasing π‐interchain stacking distance. This suggests that decreasing π‐stacking distances may be an attractive method to enhance the radiative properties of conjugated polymers in contrast to the current paradigm where it is viewed as a source of optical quenching.

Direct Arylation Polycondensation toward Water/Alcohol-Soluble Conjugated Polymers: Influence of Side Chain Functional Groups

Direct arylation of 2,7-dibromofluorene with n-octyl, 6-diethoxylphosphorylhexyl, 6-(N,N-diethylamino)hexyl or 6-bromohexyl side chains and 1,2,4,5-tetrafluorobenzene (TFB) were conducted to investigate the effect of side chain functional groups on the coupling, and the resulting TFB-substituted fluorene derivatives were used as C-H monomers for the synthesis of water/alcohol soluble conjugated polymers (WSCPs) by direct arylation polycondensation (DArP). The direct arylation and DArP of the monomers carrying phosphonate and amino groups went on smoothly in typical DArP conditions, that is, Pd(OAc)₂/P^tBu₂Me-HBF₄/base/DMAc and Pd₂(dba)₃·CHCl₃/P(o-MeOPh)₃/pivalic acid/base/THF, and high molecular weight polymers with these groups were successfully synthesized. However, for fluorene-monomers with bromohexyl side chains, the target products could not be obtained from the above conditions but could be prepared in the absence of carboxylic acid additives in low polar solvents. With the above DArP-made polymers as cathode interfacial layers, high performance organic solar cells (OSCs) were successfully fabricated.

Synthesis and Characterization of a 2,3-Dialkoxynaphthalene-Based Conjugated Copolymer via Direct Arylation Polymerization (DAP) for Organic Electronics

Poly[(5,5'-(2,3-bis(2-ethylhexyloxy)naphthalene-1,4-diyl)bis(thiophene-2,2'-diyl))-alt-(2,1,3-benzothiadiazole-4,7-diyl)] (PEHONDTBT) was synthesized for the first time and through direct arylation polymerization (DAP) for use as p-donor material in organic solar cells. Optimized reaction protocol leads to a donor-acceptor conjugated polymer in good yield, with less structural defects than its analog obtained from Suzuki polycondensation, and with similar or even higher molecular weight than other previously reported polymers based on the 2,3-dialkoxynaphthalene monomer. The batch-to-batch repeatability of the optimized DAP conditions for the synthesis of PEHONDTBT was proved, showing the robustness of the synthetic strategy. The structure of PEHONDTBT was corroborated by NMR, exhibiting good solubility in common organic solvents, good film-forming ability, and thermal stability. PEHONDTBT film presented an absorption band centered at 498 nm, a band gap of 2.15 eV, and HOMO and LUMO energy levels of -5.31 eV and -3.17 eV, respectively. Theoretical calculations were performed to understand the regioselectivity in the synthesis of PEHONDTBT and to rationalize its optoelectronic properties. Bilayer heterojunction organic photovoltaic devices with PEHONDTBT as the donor layer were fabricated to test their photovoltaic performance, affording low power-conversion efficiency in the preliminary studies.

Recent advances in non-fullerene organic solar cells: from lab to fab

The key factors for OSC materials toward application mainly include high performance, thickness tolerance, low cost, simple fabrication processing, high stability, and an environmentally-friendly nature.

Conjugated copolymers bearing 2,7-dithienylphenanthrene-9,10-dialkoxy units: highly soluble and stable deep-blue emissive materials

Excellent yields, high stability and solubility. M_w = 36.5–152.0 kDa and Đ = 2.5–3.0. Deep-blue emission with quantum yields up to 17%.

Significant Difference in Semiconducting Properties of Isomeric All-Acceptor Polymers Synthesized via Direct Arylation Polycondensation

The direct arylation polycondensation (DArP) appeared as an efficient method for producing semiconducting polymers but often requires acceptor monomers with orienting or activating groups for the reactive carbon-hydrogen (C-H) bonds, which limits the choice of acceptor units. In this study, we describe a DArP for producing high-molecular-weight all-acceptor polymers composed of the acceptor monomers without any orienting or activating groups via a modified method using Pd/Cu co-catalysts. We thus obtained two isomeric all-acceptor polymers, P1 and P2, which have the same backbone and side-chains but different positions of the nitrogen atoms in the thiazole units. This subtle change significantly influences their optoelectronic, molecular packing, and charge-transport properties. P2 with a greater backbone torsion has favorable edge-on orientations and a high electron mobility μ_e of 2.55 cm²  V^-1  s^-1 . Moreover, P2-based transistors show an excellent shelf-storage stability in air even after the storage for 1 month.