Alkylthienyl Side Chains Research Articles

P-Type Conjugated Polymers Containing Electron-Deficient Pentacyclic Azepinedione.

Bisthienoazepinedione (BTA) has been reported for constructing high-performing p-type conjugated polymers in organic electronics, but the ring extended version of BTA is not well explored. In this work, we report a new synthesis of a key building block to the ring expanded electron-deficient pentacyclic azepinedione (BTTA). Three copolymers of BTAA with benzodithiophene substituted by different side chains are prepared. These polymers exhibit similar energy levels and optical absorption in solution and solid state, while significant differences are revealed in their film morphologies and behavior in transistor and photovoltaic devices. The best-performing polymers in transistor devices contained alkylthienyl side chains on the BDT unit (pBDT-BTTA-2 and pBDT-BTTA-3) and demonstrated maximum saturation hole mobilities of 0.027 and 0.017 cm2 V-1 s-1. Blends of these polymers with PC71BM exhibited a best photovoltaic efficiency of 6.78% for pBDT-BTTA-3-based devices. Changing to a low band gap non-fullerene acceptor (BTP-eC9) resulted in improved efficiency of up to 13.5%. Our results are among the best device performances for BTA and BTTA-based p-type polymers and highlight the versatile applications of this electron-deficient BTTA unit.

Benzo[d]thiazole Based Wide Bandgap Donor Polymers Enable 19.54% Efficiency Organic Solar Cells Along with Desirable Batch-to-Batch Reproducibility and General Applicability.

The limited selection pool of high-performance wide bandgap (WBG) polymer donors is a bottleneck problem of the nonfullerene acceptor (NFA) based organic solar cells (OSCs) that impedes the further improvement of their photovoltaic performances. Herein, a series of new WBG polymers, namely PH-BTz, PS-BTz, PF-BTz, and PCl-BTz, are developed by using the bicyclic difluoro-benzo[d]thiazole (BTz) as the acceptor block and benzo[1,2-b:4,5-b']dithiophene (BDT) derivatives as the donor units. By introducing S, F, and Cl atoms to the alkylthienyl sidechains on BDT, the resulting polymers exhibit lowered energy levels and enhanced aggregation properties. The fluorinated PBTz-F not only exhibits a low-lying HOMO level, but also has stronger face-on packing order and results in more uniform fibril-like interpenetrating networks in the related PF-BTz:L8-BO blend. A high-power conversion efficiency (PCE) of 18.57% is achieved. Moreover, PBTz-F also exhibits a good batch-to-batch reproducibility and general applicability. In addition, ternary blend OSCs based on the host PBTz-F:L8-BO blend and PM6 guest donor exhibits a further enhanced PCE of 19.54%, which is among the highest values of OSCs.

Direct Arylation Polycondensation of β-Fluorinated Bithiophenes to Polythiophenes: Effect of Side Chains in C–Br Monomers

Six polythiophenes (PThs) were synthesized via direct arylation polycondensation (DArP) with β-fluorinated bithiophenes, that is, 4,4′-difluoro-2,2′-bithiophene (2FBT) and 3,3′,4,4′-tetrafluoro-2,2′-bithiophene (4FBT), as the C–H monomers and 5,5′-dibromo-2,2′-bithiophenes DCBT-2Br, DABT-2Br, and DTBT-2Br that carry alkoxycarbonyl, alkyl, and alkylthienyl side chains at 4,4′-positions, respectively, as the C–Br monomers. Both 2FBT and 4FBT are highly reactive for DArP, and high molecular weight PThs with unobservable chain defects were successfully synthesized from 2FBT/DCBT-2Br, 2FBT/DABT-2Br, 2FBT/DTBT-2Br, and 4FBT/DTBT-2Br. However, the DArP of 4FBT with DCBT-2Br and DABT-2Br produced PThs containing C–H/C–H homo-coupling defects along with relatively lower molecular weights. Detailed theoretical analysis suggests that the occurrence of C–Br reduction-provoked C–H/C–H homo-coupling for 4FBT/DCBT-2Br and 4FBT/DABT-2Br monomer pairs originated from the poor selectivity between the cis- and trans-forms of transition states in the concerted metalation–deprotonation (CMD) process is responsible for the formation of structural defects and depressed molecular weights. This research reveals that a C–H monomer with high reactivity and selectivity is not sufficient for the synthesis of well-defined conjugated polymers via DArP, and the structure of the C–Br monomer also has a significant impact on the results of DArP via influencing the catalytic cycles.

Novel Pyrrolo [3,4-b] Dithieno [3, 2-f:2″,3″-h] Quinoxaline-8,10 (9H)-Dione Based Wide Bandgap Conjugated Copolymers for Bulk Heterojunction Polymer Solar Cells.

Two D-A copolymers consisting of fused ring pyrrolo-dithieno-quinoxaline acceptors are synthesized with different donor units, i.e., benzodithiophene (BDT) with alkylthienyl (P134) and 2-ethylhexyloxy (P117) side chains. These copolymers are used as donors and a narrow bandgap acceptor Y6 to fabricate bulk heterojunction polymer solar cell devices. Owing to the strong electron-deficient fused ring pyrrolo-bithieno-quinoxaline and weak alkyl thienyl side chains in BDT, the polymer solar cells (PSCs) based on P134:Y6 attain the power conversion efficiency (PCE) of 15.42%, which is higher than the P117:Y6 counterpart (12.14%). The superior value of PCE for P134:Y6 can be associated with more well-adjusted charge transport, weak charge recombination, proficient exciton generation, and dissociation into free charge carriers and their subsequent charge collection owing to the dense π-π stacking distance and more considerable crystal coherence length for the P134:Y6 thin films. This investigation confirms the great potential of a strong acceptor-weak donor tactic for developing efficient D-A copolymers consists of quinoxaline acceptor for PSCs.

Enhanced photovoltaic performance of quinoxaline-based donor-acceptor type polymers with monocyano substituent

A series of quinoxaline-based polymers with CN group are synthesized to elucidate the effect of strong electron-withdrawing CN groups on the photovoltaic properties of the polymers and to clarify the influence of CN group on the photovoltaic properties of conjugated polymers for manufacturing high-performance polymer solar cells. The reference polymer (PAB-Qx) without CN is prepared by the polymerization between electron-donating benzodithiophene (BDT) and electron-accepting 2,3-diphenylquinoxaline (DPQ) monomer. Subsequently, a CN group is substituted at the 6-position of the DPQ unit of the reference polymer to yield PAB-CNQx. An additional target polymer (PTB-CNQx) is prepared by replacing two 2-ethylhexyloxy groups on the BDT unit in PAB-QxCN with 2-ethylhexyl thiophene side chains. The photovoltaic properties of the polymers are examined by constructing a nonfullerene inverted-type PSCs. Owing to the positive contributions of the strong electron-withdrawing CN group and alkylthienyl side chains, the power conversion efficiencies of the devices are significantly improved in the following order: PAB-Qx (4.35%), PAB-CNQx (8.66%), and PTB-CNQx (13.3%). Therefore, this study confirms the great potential of electron-withdrawing CN groups for developing high-performance quinoxaline-based polymers for photovoltaic applications.

BODIPY bearing alkylthienyl side chains: a new building block to design conjugated polymers with near infrared absorption for organic photovoltaics

A new benzene-fused BODIPY unit for designing polymer donors with near-infrared absorption for organic photovoltaics.

Synthesis of Trifluoromethylated Quinoxaline-Based Polymers for Photovoltaic Applications.

A series of quinoxaline-based conjugated polymers, in which the electron-donating benzodithiophene (BDT) unit is linked to the electron-accepting 6,7-difluorinated quinoxaline (DFQ) derivatives by a thiophene bridge, is synthesized. To investigate their effects on the intrinsic properties of polymers, strong electron-withdrawing trifluoromethyl (CF3 ) groups were incorporated into the meta-position of the phenyl ring at the 2,3-positions of the DFQ unit of the reference polymer, labelled PEhB-FQx, to yield the target polymer PEhB-FQxCF3. In addition, the 2-ethylhexyloxy substituents on the BDT donor in PEhB-FQxCF3 are changed to the more planar 2-ethylhexyl thiophene units to produce another target polymer PThB-FQxCF3. Owing to the significant contributions of the CF3 moiety, PEhB-FQxCF3 exhibits quite discernible optical and electrochemical properties along with significant enhancement in photovoltaic performances compared to the reference polymer PEhB-FQx. Furthermore, the incorporation of the alkylthienyl side chains on the BDT moiety confers on the resultant PThB-FQxCF3 to possess the maximum power conversion efficiency of 7.26% with an open circuit voltage of 0.88V, short-circuit current density of 12.20mA cm-2 , and fill factor of 67.80%.

Low Boiling Point Solvent Additives for Improved Photooxidative Stability in Organic Photovoltaics

AbstractThe efficiency of organic photovoltaic (OPV) cells is rapidly increasing. However, many of the highest performance OPV devices are also the most unstable, including the two widely used benzodithiophene‐co‐thieno[3,4‐b]thiophene polymers, PTB7 and PTB7‐Th. Here it is shown that both the choice of pendant side chain, and solvents used for film deposition, have an important effect on active layer stability. Specifically, polymers with alkylthienyl side chains are found to be less stable than polymers with alkoxy side chains. Secondly, high boiling point solvent additives such as 1,8‐diiodooctane are found to dramatically accelerate photooxidation unless they are removed fully during device processing, which may be attributed to increased oxygen diffusion in films containing residual solvent. Replacing these higher boiling point solvent additives with lower boiling additives can greatly enhance the polymer stability without requiring high vacuum treatment, making these systems more scalable and compatible with roll‐to‐roll printing. Based on this finding, the non‐halogenated solvent system o‐xylene with 2% N‐methylpyrrolidone is explored as a route to achieving higher active layer stability, while maintaining the favorable, intermixed morphology achieved with higher boiling point, halogenated solvent additives. This result offers a promising way forward in the development of scalable, stable, and high‐efficiency organic solar cells.

Investigation of Conjugated Polymers Based on Naphtho[2,3-c]thiophene-4,9-dione in Fullerene-Based and Fullerene-Free Polymer Solar Cells

Two new conjugated polymers based on the naphtho[2,3-c]thiophene-4,9-dione unit with different chemical structures but similar photoelectric properties were designed and synthesized. The differences and the related issues in their photovoltaic properties were well studied. The results show that both polymers can work well with PC71BM, recording PCEs of 7.2% for PBDT-DTN and 4.9% for PIDT-DTN. However, when blended with ITIC, PBDT-DTN can get a PCE of 8.3% compared with that of 1.1% for PIDT-DTN. It was found that PBDT-DTN with alkylthienyl side chains possesses stronger π–π interactions than PIDT-DTN with alkylphenyl side chains, as the latter’s side chains are of higher steric hindrance. In addition, due to the similar constituent moiety to ITIC, PIDT-DTN could exhibit much better miscibility with ITIC than PBDT-DTN. In that case, compared with PIDT-DTN, it is easier for PBDT-DTN to form suitable phase separation with either PC71BM or ITIC, which is beneficial for improving exciton dissociation efficiency, reducing bimolecular recombination, and getting more balanced charge mobilities. Overall, homologous polymers with different chemical structures may demonstrate comparable photovoltaic properties in fullerene-based PSCs but extremely different photovoltaic behaviors in fullerene-free PSCs. This important information provides the guidance for molecular design of polymeric donors to achieve high efficiency in fullerene-free PSCs.

Side chain engineering and conjugation enhancement of benzodithiophene and phenanthrenequnioxaline based conjugated polymers for photovoltaic devices

ABSTRACTA series of donor‐acceptor conjugated polymers incorporating benzodithiophene (BDT) as donor unit and phenanthrenequnioxaline as acceptor unit with different side chains have been designed and synthesized. For polymer P1 featuring the BDT unit and alkoxy chains substituted phenanthrenequnioxaline unit in the backbone, serious steric hindrance resulted in quite low molecular weight. The implementation of thiophene ring spacer in polymer P2 greatly suppressed the interannular twisting to extend the effective conjugation length and consequently gave rise to improved absorption property and device performance. In addition, utilizing the alkylthienyl side chains to replace the alkyl side chains at BDT unit in polymer P3 further enhanced the photovoltaic performance due to the increased conjugation length. For polymer P4, translating the alkoxy side chains at the phenanthrenequnioxaline ring into the alkyl side chains at thiophene linker group enhanced molecular planarity and strengthened π−π stacking. Consequently improved absorption property and increased hole mobility were achieved for polymer P4. Our results indicated that side chain engineering not only can influence the solubility of polymer but also can determine the polymer backbone planarity and hence the photovoltaic properties. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2015, 53, 1915–1926

Side-chain engineering of benzodithiophene-fluorinated quinoxaline low-band-gap co-polymers for high-performance polymer solar cells.

A new series of donor-acceptor co-polymers based on benzodithiophene and quinoxaline with various side chains have been developed for polymer solar cells. The effect of the degree of branching and dimensionality of the side chains were systematically investigated on the thermal stability, optical absorption, energy levels, molecular packing, and photovoltaic performance of the resulting co-polymers. The results indicated that the linear and 2D conjugated side chains improved the thermal stabilities and optical absorptions. The introduction of alkylthienyl side chains could efficiently lower the energy levels compared with the alkoxyl-substituted analogues, and the branched alkoxyl side chains could deepen the HOMO levels relative to the linear alkoxyl chains. The branched alkoxyl groups induced better lamellar-like ordering, but poorer face-to-face packing behavior. The 2D conjugated side chains had a negative influence on the crystalline properties of the co-polymers. The performance of the devices indicated that the branched alkoxyl side chains improved the Voc, but decreased the Jsc and fill factor (FF). However, the 2D conjugated side chains would increase the Voc, Jsc, and FF simultaneously. For the first time, our work provides insight into molecular design strategies through side-chain engineering to achieve efficient polymer solar cells by considering both the degree of branching and dimensionality.

Structural Design of Benzo[1,2-b:4,5-b′]dithiophene-Based 2D Conjugated Polymers with Bithienyl and Terthienyl Substituents toward Photovoltaic Applications

In this contribution, six conjugated polymers consisting of benzo[1,2-b:4,5-b′]dithiophene–bithiophene (BDT-BT) and benzo[1,2-b:4,5-b′]dithiophene–benzothiadiazle (BDT-BTD) as building blocks in the main chain were synthesized by coupling polymerization and utilized for photovoltaic applications. By directly attaching three kinds of alkylthienyl side chains to the conjugated main chain, the resulted two-dimensional configuration revealed a broader absorption range due to the ground state electron transition of their corresponding alkylthienyl units and polymer backbone. Temperature-dependent absorbance, emission spectra, and thermal annealing further verify that the shoulder band(s) were originated from the aggregated (crystalline) species of polymers. The photovoltaic properties of the donor–acceptor polymers revealed well-defined side chain geometries, physical, and electronic structures and showed the highest power conversion efficiency of 4.25% among polymer solar cells based on two-dimensional (2-D) bithienyl- or terthienyl-substituted benzodithiophene.

Structure, band gap and energy level modulations for obtaining efficient materials in inverted polymer solar cells

Two new donor–acceptor (D–A) polymers composed of benzo[1,2-b:4,5-b′]dithiophene (BDT) as donor and thiadiazolo[3,4-c]pyridine (PyTZ) as acceptor were designed and synthesized. Compared to the polymer based on BDT and 4,7-dithien-2-yl-2,1,3-benzothiadiazole (DTBT), the planarity, energy levels, and band-gaps of the new polymers were fine adjusted by incorporating conjugated alkylthienyl side chains to BDT and substitute a stronger acceptor PyTZ for 2,1,3-benzothiadiazole. The new polymers exhibit broad absorption from 300 to 800nm in both solution and film state. The polymer band gaps and energy levels are close to the optimal values. As a result, power conversion efficiencies (PCEs) of 4.84% and 5.11% were obtained for inverted polymer solar cells based on these new polymers. The PCEs are significantly higher than those of the BDT–DTBT based polymers (2–4%).

Efficient Polymer Solar Cells with a High Open Circuit Voltage of 1 Volt

AbstractA series of polymers containing benzo[1,2‐b:4,5‐b′]dithiophene and N‐alkylthieno[3,4‐c]pyrrole‐4,6‐dione are designed. By incorporating different alkylthienyl side chains, the fill factor (FF) and open circuit voltage (Voc) of the copolymers are further improved. The experimental results and theoretical calculations show that the size and topology of the side chains can influence the polymer solubility, energy levels, and intermolecular packing by altering the molecular coplanarity. As a result of improved morphology and fine‐tuned energy levels, an increased FF and a high Voc of 1.00 V are achieved, as well as a power conversion efficiency of 6.17%, which is the highest efficiency ever reported for polymer solar cells with a Voc over 1 V.

Design of benzodithiophene-diketopyrrolopyrrole based donor–acceptor copolymers for efficient organic field effect transistors and polymer solar cells

Three low-band-gap donor–acceptor (D–A) copolymers containing benzo(1,2-b:4,5-b′)dithiophene (BDT), 3,6-di(thiophen-2-yl)pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione (TDP) or 3,6-di(furan-2-yl)pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione (FDP) were designed and synthesized. Their thermal stability, optical and electrochemical properties, device performances for organic field effect transistors (OFETs) and polymer solar cells were systematically investigated. The device performances were significantly enhanced by the introduction of conjugated alkylthienyl side chains to the BDT core and the substitution of thiophene with furan moieties in polymer backbone. Compared to alkoxy side chains, conjugated alkylthienyl chains resulted in higher coplanarity, increased thermal stability (Td increased from 364 °C to 417 °C) and a lower HOMO level (from −5.10 eV to −5.24 eV). The incorporation of furan evidently improved the polymer solubility, leading to a finer phase separation morphology as proved by atomic force microscopy and transmission electron microscopy. After optimization, the designed polymer showed excellent performance in both OFETs and PSCs with an optimal hole mobility of 0.16 cm2 V−1 s−1 and a high power conversion efficiency of 5.54%.