Nemal S. Gobalasingham

Palladium-catalyzed oxidative direct arylation polymerization (Oxi-DArP) of an ester-functionalized thiophene

Poly(hexyl thiophene-3-carboxylate), an alkyl ester side-chain functionalized thiophene polymer herein referred to as poly(3-hexylesterthiophene) (P3HET), is synthesized via a novel palladium-catalyzed oxidative dehydrogenative polycondensation method. The ester promotes the generation of high molecular weight polymers as a directing group, enabling direct hetero C–H/C–H coupling and bypassing the functionalization requirements of traditional direct arylation (halogenation) and transition metal-catalyzed cross-coupling (halogenation and metalation) methods. Despite lacking functional groups in the 2- and 5-position, these unique reaction conditions achieve good regioregularity (around 85%) through the utilization of a phosphine ligand. To evaluate this new polymerization method, poly(hexyl thiophene-3-carboxlates) and poly(3-hexylthiophene) polymers are synthesized via Oxi-DArP, traditional DArP, and Stille polycondensation and subsequently characterized by electrochemical HOMO determination, UV-Vis, GIXRD, and space-charge limited current (SCLC) hole mobilities. High quality polymers via Oxi-DArP were only acquired when the ester functional group was present, whereas molecular weight, regioregularity, and yields suffered with 3-hexylthiophene. Optimized Oxi-DArP P3HET exhibited absorption coefficients, electrochemical HOMO levels, and semi-crystallinity comparable to DArP and Stille P3HET. This work expands on an emerging synthetic method and develops attractively simple and mild conditions toward the generation of high quality polymers promoted by carbonyl directing groups.

Influence of Surface Energy on Organic Alloy Formation in Ternary Blend Solar Cells Based on Two Donor Polymers

The compositional dependence of the open-circuit voltage (Voc) in ternary blend bulk heterojunction (BHJ) solar cells is correlated with the miscibility of polymers, which may be influenced by a number of attributes, including crystallinity, the random copolymer effect, or surface energy. Four ternary blend systems featuring poly(3-hexylthiophene-co-3-(2-ethylhexyl)thiophene) (P3HT75-co-EHT25), poly(3-hexylthiophene-co-(hexyl-3-carboxylate)), herein referred to as poly(3-hexylthiophene-co-3-hexylesterthiophene) (P3HT50-co-3HET50), poly(3-hexylthiophene-thiophene-diketopyrrolopyrrole) (P3HTT-DPP-10%), and an analog of P3HTT-DPP-10% with 40% of 3-hexylthiophene exchanged for 2-(2-methoxyethoxy)ethylthiophen-2-yl (3MEO-T) (featuring an electronically decoupled oligoether side-chain), referred to as P3HTTDPP-MEO40%, are explored in this work. All four polymers are semicrystalline and rich in rr-P3HT content and perform well in binary devices with PC61BM. Except for P3HTTDPP-MEO40%, all polymers exhibit similar surface energies (∼21-22 mN/m). P3HTTDPP-MEO40% exhibits an elevated surface energy of around 26 mN/m. As a result, despite the similar optoelectronic properties and binary solar cell performance of P3HTTDPP-MEO40% compared to P3HTT-DPP-10%, the former exhibits a pinned Voc in two different sets of ternary blend devices. This is a stark contrast to previous rr-P3HT-based systems and demonstrates that surface energy, and its influence on miscibility, plays a critical role in the formation of organic alloys and can supersede the influence of crystallinity, the random copolymer effect, similar backbone structures, and HOMO/LUMO considerations. Therefore, we confirm surface energy compatibility as a figure-of-merit for predicting the compositional dependence of the Voc in ternary blend solar cells and highlight the importance of polymer miscibility in organic alloy formation.

Evaluating structure–function relationships toward three-component conjugated polymers via direct arylation polymerization (DArP) for Stille-convergent solar cell performance

The utilization of direct arylation polymerization (DArP) for the tin-free synthesis of three-component semi-random copolymers is rare, as several assumptions derived from Stille polycondensation—such as functional group assignment, monomer reactivity, defect tendencies, and practical performance—are not necessarily applicable across a broad range of DArP conditions and monomer substrates. Through a comparative analysis of conditions on increasingly complex model systems ranging from P3HT (where highly successful but distinct conditions are compared for the first time herein) to a three-component polymer architecture featuring ubiquitous diketopyrrolopyrrole (DPP), reaction conditions are optimized to generate polymers that perform similarly to Stille reference polymers in solar cells. Subsequently, these optimized conditions are then applied to the synthesis of several novel semi-random polymers, exploring a variety of both electron-poor and electron-rich substrates, such as (E)-2-(2-(thiophen-2-yl)vinyl)thiophene (TvT), 4,4′-dimethyl-2,2′-bithiazole (BTz), and 3,4-ethylenedioxythiophene (EDOT) as well as distinct acceptor units, such as pentacyclic aromatic lactam, 4,10-bis(2-ethylhexyl)thieno[2′,3′:5,6]pyrido[3,4-g]thieno[3,2-c]isoquinoline-5,11(4H,10H)-dione (TPTI) and 2,3-bis(3-(octyloxy)phenyl)quinoxaline (QX), further demonstrating the broad compatibility of these DArP conditions and expanding the semi-random toolkit toward achieving polymers with a variety of highly tunable optoelectronic properties. Polymers are characterized by NMR analysis, electrochemical HOMO level determination, UV-Vis, GIXRD, DSC, SCLC hole mobilities, and are also implemented into polymer solar cells.

Synthesis of random poly(hexyl thiophene-3-carboxylate) copolymers via oxidative direct arylation polymerization (oxi-DArP)

Oxidative direct arylation polymerization has recently emerged as a simple, mild, and atom economical homopolymerization method; however, the application of this promising method to unsymmetrical or binary monomer systems, where different C–H reactivities are present, is limited. Two random copolymer families of poly(hexyl thiophene-3-carboxylate), herein referred to as poly(3-hexylesterthiophene) (P3HET), featuring either thieno[3,4-c]pyrrole-4,6-dione (TPD) or 4,4′-dimethyl-2,2′-bithiazole (BTz) comonomers are synthesized by oxidative direct arylation polymerization (oxi-DArP) conditions that enable high regioregularities despite the lack of preactivation of the monomers. Through refinement of the reaction parameters and minimization of auxiliary reagent loadings, polymers with good molecular weights are achieved and the feed ratio is closely correlated to the polymer composition. These random copolymers are evaluated by 1H NMR, SEC, UV-Vis, GIXRD, and SCLC hole mobility analyses to determine the compatibility of this emerging synthetic method with increasingly popular random copolymer architectures.

Conjugated Polymers Via Direct Arylation Polymerization in Continuous Flow: Minimizing the Cost and Batch-to-Batch Variations for High-Throughput Energy Conversion

Continuous flow methods are utilized in conjunction with direct arylation polymerization (DArP) for the scaled synthesis of the roll-to-roll compatible polymer, poly[(2,5-bis(2-hexyldecyloxy)phenylene)-alt-(4,7-di(thiophen-2-yl)-benzo[c][1,2,5]thiadiazole)] (PPDTBT). PPDTBT is based on simple, inexpensive, and scalable monomers using thienyl-flanked benzothiadiazole as the acceptor, which is the first β-unprotected substrate to be used in continuous flow via DArP, enabling critical evaluation of the suitability of this emerging synthetic method for minimizing defects and for the scaled synthesis of high-performance materials. To demonstrate the usefulness of the method, DArP-prepared PPDTBT via continuous flow synthesis is employed for the preparation of indium tin oxide (ITO)-free and flexible roll-coated solar cells to achieve a power conversion efficiency of 3.5% for 1 cm2 devices, which is comparable to the performance of PPDTBT polymerized through Stille cross coupling. These efforts demonstrate the distinct advantages of the continuous flow protocol with DArP avoiding use of toxic tin chemicals, reducing the associated costs of polymer upscaling, and minimizing batch-to-batch variations for high-quality material.

Carbazole-based copolymers via direct arylation polymerization (DArP) for Suzuki-convergent polymer solar cell performance

Although direct arylation polymerization (DArP) has recently emerged as an alternative to traditional cross-coupling methods like Suzuki polymerization, the evaluation of DArP polymers in practical applications like polymer solar cells (PSCs) is limited. Because even the presence of minute quantities of defects can dramatically influence the solar cell, performance of DArP polymers offers critical insight alongside other structural and optoelectronic comparisons. Even via traditional methods, carbazole-based donors are frequently prone to homocoupling defects, which has been shown to—along with β-defects—compromise performance. Through defect minimization with the bulky and affordable neodecanoic acid (NDA) mixture, we report the synthesis of DArP poly[(9-(heptadecan-9-yl)-9H-carbazole)-alt-(4,7-di(thiophen-2-yl)benzo[c][1,2,5]thiadiazole)] (PCDTBT) that outperforms Suzuki PCDTBT with similar molecular weights. Expanding beyond this model system, carbazole-based polymers featuring 2,5-diethylhexyl-3,6-di(thiophen-2-yl)-2,5-dihydropyrrolo[3,4-c]pyrrole-1,4-dione (DPP), 4,10-bis(diethylhexyl)-thieno[2′,3′:5,6]pyrido[3,4-g]thieno[3,2-c]isoquinoline-5,11-dione (TPTI), 5-octyl-1,3-di(thiophen-2-yl)-4H-thieno[3,4-c]pyrrole-4,6(5H)-dione (DT-TPD), and 2,5-bis(2,3-dihydrothieno[3,4-b][1,4]dioxin-5-yl)pyridine (EDOT-Pyr) are generated. Polymers are characterized by 1H NMR, cyclic voltammetry, UV-Vis, GIXRD, SCLC hole mobilities, and are implemented into polymer solar cells fabricated in air under ambient humidity. We demonstrate that DArP polymers perform comparably to Suzuki in practical applications.

Investigation of green and sustainable solvents for direct arylation polymerization (DArP)

Direct arylation polymerization (DArP) is an emerging method for conjugated polymer synthesis. It alleviates typical synthetic routes from toxic, hazardous materials, such as pyrophorphoric organolithium or highly-toxic stannane reagents. The progress and development of synthetic methodologies for DArP have allowed for the preparation of conjugated polymers with a minimization or exclusion of undesired couplings, such as branching (β) defects and donor–donor or acceptor–acceptor homocouplings. This has allowed for conjugated polymers prepared using DArP to converge upon or surpass the performance of polymers prepared using conventional polymerization methods, e.g. Stille or Suzuki, when integrated into polymer bulk-heterojunction solar cells. Considering that DArP has the potential to become the industrial-scale method for conjugated polymer synthesis, determining the compatability of environmentally benign, non-hazardous, and low-cost solvents with DArP is imperative. Herein, we report the application of green and sustainable solvents, such as 2-methyltetrahydrofuran, cyclopentyl methyl ether, diethylcarbonate, and γ-valerolactone, for DArP towards the preparation of poly[(2,5-bis(2-hexyldecyloxy)phenylene)-alt-(4,7-di(thiophen-2-yl)benzo[c][1,2,5]thiadiazole) (PPDTBT) and poly(3-hexylthiophene) (P3HT), where optimal conditions are derived based on the molecular weight, yield, and characterization (NMR, XRD, and UV-vis) for the aforementioned polymers. We find that cyclopentyl methyl ether (CPME) provides the best polymerization products with an Mn up to 41 kDa for PPDTBT and with yields up to 98%, which is the highest reported to our knowledge for this polymer prepared using DArP. Application of CPME to P3HT resulted in Mn values of 12 kDa with 93% regio-regularity (RR) and no detectable β-defects.