Triarylamine N‐functionalized 3,6‐linked carbazole main chain polymers and copolymers: Preparation and physical properties

The preparation of triarylamine N‐functionalized 3,6‐linked carbazole homopolymers as well as alternating copolymers with 2,5‐diphenyl‐[1,3,4]oxadiazole and benzo[1,2,5]thiadiazole was undertaken using Suzuki cross‐coupling polymerization procedures associating 3,6‐bis(4,4,5,5‐tetramethyl‐[1,3,2]dioxaborolan‐2‐yl)‐9‐(bis[4‐(2‐butyl‐octyloxy)‐phenyl]‐amino‐phen‐4‐yl)‐carbazole and, respectively, 3,6‐dibromo‐9‐(bis[4‐(2‐butyl‐octyloxy)‐phenyl]‐amino‐phen‐4‐yl)‐carbazole, 2,5‐bis(4‐bromo‐phenyl)‐[1, 3,4]oxadiazole, and 4,7‐dibromo‐benzo[1,2,5]thiadiazole. Both the carbazole homopolymer and alternating copolymer with 2,5‐diphenyl‐[1,3,4]oxadiazole were found as wideband gap materials emitting in the blue part of the electromagnetic spectrum while the carbazole alternating copolymer with 4,7‐benzo[1,2,5]thiadiazole had a narrower band gap and emitted in the orange part of the electromagnetic spectrum. The new polymers are thermally stable up to 300 °C. A discussion of the electrochemical and optical properties of the new polymers is presented. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 5957–5967, 2007.     

Incorporation of different conjugated linkers into low band gap polymers based on 5,6‐Bis(octyloxy)‐2,1,3 benzooxadiazole for tuning optoelectronic properties

Four new 2,1,3‐benzooxadiazole‐based donor–acceptor conjugated polymers, namely poly{9‐(9‐heptadecanyl)‐9H‐carbazole‐alt‐5,6‐bis(octyloxy)‐4,7‐di(selenophen‐2‐yl)benzo[c][1,2,5]oxadiazole)}(PSBSC), poly{9‐(9‐heptadecanyl)‐9H‐carbazole‐alt‐5,6‐bis(octyloxy)‐4,7‐di(furan‐2‐yl)benzo[c][1,2,5]oxadiazole)}(PFBFC), poly{9,9‐dioctyl‐9H‐fluorene‐alt‐5,6‐bis(octyloxy)‐4,7‐di(selenophen‐2‐yl)benzo[c][1,2,5]oxadiazole)}(PSBSFL), and poly{9,9‐dioctyl‐9H‐fluorene‐alt‐5,6‐bis(octyloxy)‐4,7‐di(furan‐2‐yl)benzo[c][1,2,5]oxadiazole)}(PFBFFL), were synthesized via Stille polycondensation reaction. All polymers were found to be soluble in common organic solvents such as chloroform, tetrahydrofuran, and chlorobenzene. Their structures were verified by ¹H‐NMR and the molecular weights were determined by gel permeation chromatography (GPC). The polymer films exhibited broad absorption bands. Among all polymers, photovoltaic cells based on the device structure of ITO/PEDOT:PSS/PSBSC:PC₇₁BM(1:3, w/w)/LiF/Al revealed an open‐circuit voltage of 0.62 V, a short circuit current of 7.63 mA cm^?2 and a power conversion efficiency of 1.89%. This work demonstrates a good example for tuning absorption range, energy level, and photovoltaic properties of the polymers with different spacers and donor units can offer a simple and effective method to improve the efficiency of PSCs. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2016 , 54, 2459–2467     

New moderate bandgap polymers containing alkoxysubstituted‐benzo[c][1,2,5]thiadiazole and thiophene‐based units

Alkoxysubstituted benzo[c][1,2,5]thiadiazole electron accepting units were prepared and copolymerized with various thiophene‐based electron donating monomers to produce new low bandgap polymers P1–4 . The materials showed broad absorption in the range from 300 to 700 nm with bandgaps below 2 eV in solution. Efficiencies of over 1% were obtained from photovoltaic cells using P4 with PCBM as acceptor. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011     

Low‐bandgap donor–acceptor copolymers with 4,6‐bis(3′‐(2‐ethylhexyl)thien‐2′‐yl)thieno[3,4‐c][1,2,5]thiadiazole: synthesis,optical, electrochemical,and photovoltaic properties

Two new low‐bandgap alternating copolymers (CEHTF and CEHTP) consisting of 4,6‐bis(3′‐(2‐ethylhexyl)thien‐2′‐yl)thieno[3,4‐c][1,2,5] thiadiazole and 9,9‐bis(2‐ethylhexyl)fluorene or 2,5‐bis(isopentyloxy)benzene were synthesized by Suzuki coupling reaction of corresponding comonomers. Their optical, electrochemical, and photovoltaic (PV) properties were studied and are reported. Both the copolymers exhibited long‐wavelength absorption covering the whole visible spectral region, which is in CEHTP thin films extended up to near infrared region, ambipolar redox properties, and electrochromism. High‐electron affinities and low‐optical bandgap values, 1.37 and 1.15 eV, were determined for CEHTF and CEHTP, respectively. PV devices with bulk heterojunction made of blends of copolymers and fullerene derivative [6,6]‐phenyl‐C⁶¹‐butyric acid methyl ester ([60]PCBM) were prepared and characterized. Effects of intramolecular charge transfer strength and side‐chain nature and length on photophysical properties are discussed. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011     

Synthesis and photovoltaic performances of benzo[1,2‐b:4,5‐b']dithiophene‐alt‐2,3‐diphenylquinoxaline copolymers pending functional groups in phenyl rings

Two donor/acceptor (D/A)‐based benzo[1,2‐b:4,5‐b′]dithiophene‐alt‐2,3‐biphenyl quinoxaline copolymers of P 1 and P 2 were synthesized pending different functional groups (thiophene or triphenylamine) in the 4‐positions of phenyl rings. Their thermal, photophysical, electrochemical, and photovoltaic properties, as well as morphology of their blending films were investigated. The poly(4,8‐bis((2‐ethyl‐hexyl)oxy)benzo[1,2‐b:4,5‐b'] dithiophene)‐alt‐(2,3‐bis(4′‐bis(N,N‐bis(4‐(octyloxy) phenylamino)‐ 1,1′‐biphen‐4‐yl)quinoxaline) ( P 2) exhibited better photovoltaic performance than poly(4,8‐bis((2‐ethylhexyl)oxy)benzo[1,2‐b:4,5‐b'] dithiophene)‐alt‐(2,3‐bis(4‐(5‐octylthiophen‐2‐yl)phenyl)quinoxaline) ( P 1) in the bulk‐heterojunction polymer solar cells with a configuration of ITO/PEDOT:PSS/polymers: [6,6]‐phenyl‐C₇₁‐butyric acid methyl ester (PC₇₁BM)/LiF/Al. A power conversion efficiency of 3.43%, an open‐circuit voltage of 0.80 V, and a short‐circuit current of 9.20 mA cm^?2 were achieved in the P 2‐based cell under the illumination of AM 1.5, 100 mW cm^?2. Importantly, this power conversion efficiency level is 2.29 times higher than that in the P 1‐based cell. Our work indicated that incorporating triphenylamine pendant in the D/A‐based polymers can greatly improved the photovoltaic properties for its resulting polymers.     

[1,2,5]thiadiazolo[3,4‐g]quinoxaline acceptor‐based donor–acceptor–donor‐type polymers: Effect of strength and size of donors on the band gap

Electrochromic polymers based on [1,2,5]thiadiazolo[3,4‐g]quinoxaline acceptor and thiophene, 3,4‐ethylenedioxythiophene and 3,3‐didecyl‐3,4‐proylenedioxythiophene donors, namely poly(6,7‐diphenyl‐4,9‐di(thiophen‐2‐yl)‐[1,2,5]thiadiazolo[3,4‐g]quinoxaline) ( P1 ), poly(4‐(2,3‐dihydrothieno[3,4‐b][1,4]dioxin‐5‐yl)‐9‐(2,3‐dihydrothieno[3,4‐b][1,4]dioxin‐7‐yl)‐6,7‐diphenyl‐[1,2,5]thiadiazolo[3,4‐g]quinoxaline) ( P2 ), and poly(4‐(3,3‐didecyl‐3,4‐dihydro‐2H‐thieno[3,4‐b][1,4]dioxepin‐6‐yl)‐9‐(3,3‐didecyl‐3,4‐dihydro‐2H‐thieno[3,4‐b][1,4]dioxepin‐8‐yl)‐6,7‐diphenyl‐[1,2,5]thiadiazolo[3,4‐g]quinoxaline) ( P3 ), respectively, were electrochemically and/or chemically synthesized and characterized. Electrochemical and optical properties of the polymers were then investigated. The results, which were obtained electrochemically and optically, indicate that the polymers bearing the same acceptor and different donor units have a band gap range of 0.59–1.24 eV depending on the strength and size of the donor units and band gap determination method. A significant finding in this study was the phenomenon that when the acceptor is physically huge, the general rule that a weak donor would have a high band gap whereas a strong donor would have low band gap can be broken due to the torsional angles/steric hindrances involved with physically large donor molecules. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2017 , 55, 3483–3493     

Synthesis and characterization of isoindigo‐based polymers using CH‐arylation polycondensation reactions for organic photovoltaics

A series of three new low bandgap donor–acceptor–donor–acceptor^/ (D–A–D–A^/) polymers have been successfully synthesized based on the combination of isoindigo as the electron‐deficient acceptor and 3,4‐ethylenedioxythiophene as the electron‐rich donor, followed by CH‐arylation with different acceptors (4,7‐dibromo[c][1,2,5]‐(oxa, thia, and/or selena)diazole ( 4a‐c )). These polymers were used as donor materials for photovoltaic applications. All of the polymers are highly stable and show good solubility in chlorinated solvents. The highest power conversion efficiency of 1.6% was achieved in the bulk heterojunction photovoltaic device that consisted of poly ((E)?6‐(7‐(benzo‐[c][1,2,5]‐thiadiazol‐4‐yl)?2,3‐dihydrothieno‐[3,4‐b][1,4]dioxin‐5‐yl)?6′‐(2,3‐dihydrothieno‐[3,4‐b][1,4]‐dioxin‐5‐yl)?1,1′‐bis‐(2‐octyldodecyl)‐[3,3′‐biindolinylidene]‐2,2′‐dione) as the donor and PC₆₁BM as the acceptor, with a short‐circuit current density (J_sc) of 8.10 mA/cm², an open circuit voltage (V_oc) of 0.56 V and a fill factor of 35%, which indicates that these polymers are promising donors for polymer solar cell applications. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2014 , 52, 2926–2933     

Synthesis and photovoltaic properties of 2,6‐bis(2‐thienyl) benzobisazole and 4,8‐bis(thienyl)‐benzo[1,2‐B:4,5‐B′]dithiophene copolymers

In an effort to design efficient low‐cost polymers for use in organic photovoltaic cells the easily prepared donor–acceptor–donor triad of a either cis‐benzobisoxazole, trans‐benzobisoxazole or trans‐benzobisthiazole flanked by two thiophene rings was combined with the electron‐rich 4,8‐bis(5‐(2‐ethylhexyl)‐thien‐2‐yl)‐benzo[1,2‐b:4,5‐b′]dithiophene. The electrochemical, optical, morphological, charge transport, and photovoltaic properties of the resulting terpolymers were investigated. Although the polymers differed in the arrangement and/or nature of the chalcogens, they all had similar highest occupied molecular orbital energy levels (?5.2 to ?5.3 eV) and optical band gaps (2.1–2.2 eV). However, the lowest unoccupied molecular orbital energy levels ranged from ?3.1 to ?3.5 eV. When the polymers were used as electron donors in bulk heterojunction photovoltaic devices with PC₇₁BM ([6,6]‐phenyl C₇₁‐butyric acid methyl ester) as the acceptor, the trans‐benzobisoxazole polymer had the best performance with a power conversion efficiency of 2.8%. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2016 , 54, 316–324     

Donor–acceptor‐structured naphtodithiophene‐based copolymers for organic thin‐film transistors

New donor–acceptor (D‐A) polymers, poly(4,5‐bis(2‐octyldodecyloxy)naphto[2,1‐b:3,4‐b']dithiophenebenzo[c][1,2,5]thiadiazole) (PNDT‐B) and poly(4,5‐bis(2‐octyldodecyloxy)naphto [2,1‐b:3,4‐b′]dithiophene‐4,7‐di(thiophen‐2‐yl)benzo[c][1,2,5]thiadiazole) (PNDT‐TBT), with the extended π‐electron delocalization of naphtho[2,1‐b:3,4‐b']dithiophene, were successfully synthesized by Suzuki and Stille coupling reactions. The structure and physical properties of polymers were characterized by DFT calculation, UV–vis absorption, cyclovoltammetry, TGA and DSC analyses. X‐ray diffraction studies indicated a relatively highly ordered intermolecular structure in PNDT‐TBT after annealing. This high degree of molecular order resulted from the crystallinity and increasing planarity, provided by the thiophene linker groups and the interdigitation of the long alkoxy side chains. The new D‐A polymer, PNDT‐TBT, exhibited a p‐type carrier mobility of 0.028 cm²/Vs and an on/off ratio of 5.9 × 10³. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2016 , 54, 525–531     

Suppressing charge recombination by incorporating 3,6‐carbazole into poly[9‐(heptadecan‐9‐yl)‐9H‐carbazole‐2,7‐diyl‐alt‐(5,6‐bis‐(octyloxy)‐4,7‐di(thiophen‐2‐yl)benzo[1,2,5]‐thiadiazole)‐5,5‐diyl]

A series of poly[9‐(heptadecan‐9‐yl)‐9H‐carbazole‐2,7‐diyl‐alt‐(5,6‐bis‐(octyloxy)‐4,7‐di(thiophen‐2‐yl)benzo‐[1,2,5]‐thia‐diazole)‐5,5‐diyl] compositions containing various ratios of 3,6‐carbazole was synthesized for testing in a polymer solar cell. An appropriate amount of 3,6‐carbazole units incorporated into the copolymer improved intermolecular charge transport, whereas excess amount of 3,6‐carbazole units temporarily seized on the partial negative charge generated in the conjugation breaks. We extensively studied the effects of the incorporated 3,6‐carbazole units on the intermolecular interactions, which can affect nongeminated recombination in bulk heterojunction‐polymer solar cells. These properties were investigated using photocurrent‐ and light intensity‐dependent measurements and electrochemical impedance spectroscopy. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2014 , 52, 2047–2056     

Novel Donor–Acceptor Polymer Containing 4,7‐Bis(thiophen‐2‐yl)benzo[c][1,2,5]thiadiazole for Polymer Solar Cells with Power Conversion Efficiency of 6.21%

In order to improve the solution processability of 4,7‐bis(thiophen‐2‐yl)benzo[c][1,2,5]thiadiazole (DTBT)‐based polymers, novel donor–acceptor polymer PTOBDTDTBT containing DTBT and benzo[1,2‐b:4,5‐b′]dithiophene (BDT) with conjugated side chain is designed and synthesized with narrow band gap 1.67 eV and low lying HOMO energy level −5.4 eV. The blend film of PTOBDTDTBT and PC₇₁BM exhibits uniform and smooth film with root‐mean‐square (RMS) surface roughness 1.15 nm because of the excellent solubility of PTOBDTDTBT when six octyloxy side chains are introduced. The hole mobility of the blend film is measured to be 4.4 × 10⁻⁵ cm² V⁻¹s⁻¹ by the space‐charge‐limited current (SCLC) model. The optimized polymer solar cells (PSCs) based on PTOBDTDTBT /PC₇₁BM exhibits an improved PCE of 6.21% with V_oc = 0.80 V, J_sc = 11.94 mA cm⁻² and FF = 65.10%, one of the highest PCE in DTBT containing polymers.

     

Pyrrolo[3,4‐c]pyrrole‐1,3‐dione‐based large band gap polymers containing benzodithiophene derivatives for highly efficient simple structured polymer solar cells

Pyrrolo[3,4‐c]pyrrole‐1,3(2H,5H)‐dione (DPPD)‐based large band gap polymers, P(BDT‐TDPPDT) and P(BDTT‐TDPPDT), are prepared by copolymerizing electron‐rich 4,8‐bis(2‐ethylhexyloxy)benzo[1,2‐b:4,5‐b′]dithiophene (BDT) or 4,8‐bis(5‐(2‐ethylhexyl)thiophen‐2‐yl)benzo[1,2‐b:4,5‐b′]dithiophene (BDTT) unit with novel electron deficient 2,5‐dioctyl‐4,6‐di(thiophen‐2‐yl)pyrrolo[3,4‐c]pyrrole‐1,3(2H,5H)‐dione (TDPPDT) unit. The absorption bands of polymers P(BDT‐TDPPDT) and P(BDTT‐TDPPDT) cover the region from 300 to 600 nm with an optical band gap of 2.11 eV and 2.04 eV, respectively. The electrochemical study illustrates that the highest occupied/lowest unoccupied molecular orbital energy levels of P(BDT‐TDPPDT) and P(BDTT‐TDPPDT) are ?5.39 eV/?3.28 eV and ?5.44 eV/?3.40 eV, respectively. The single layer polymer solar cell (PSC) fabricated with a device structure of ITO/PEDOT:PSS/P(BDT‐TDPPDT) or P(BDTT‐TDPPDT):PC70BM+DIO/Al offers a maximum power conversion efficiency (PCE) of 6.74% and 6.57%, respectively. The high photovoltaic parameters such as fill factor (～72%), open circuit voltage (V_oc, ～0.90 V), incident photon to collected electron efficiency (～76%), and PCE obtained for the PSCs made from polymers P(BDT‐TDPPDT) and P(BDTT‐TDPPDT) make them as promising large band gap polymeric candidates for PSC application. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2014 , 52, 3564–3574     

Control of vertical distribution of thiophene‐based copolymers containing 4,7‐Dithien‐2‐yl‐benzo[C][1,2,5]thiadiazole and 3,6‐Dithien‐2‐yl‐pyrrolo[3,4‐C]pyrrole‐1,4(2H,5H)‐dione as Side Groups for Photovoltaics

Four new D—A type copolymers with 2D‐conjugated side‐chain identified PfToBT, PbToBT, PfTDPP and PbTDPP, containing two acceptors 4,7‐dithien‐2‐yl‐benzo[c][1,2,5]thiadiazole (DTBT), and diketopyrrolopyrrole (DPP) linked by thiophene donors, are obtained using Pd‐catalyzed Stille‐coupling reaction. These polymers show a broad visible‐near‐infrared absorption band (E_g = 1.79–1.66 eV) and possess a relatively low‐lying HOMO level at ?5.34 to ?5.12 eV. All the polymer:PC₇₀BM blend films showed edge‐on structure and have similar d_π‐spacing values. According to the structure of conjugated side‐chain, the vertical distributions of polymer chains and PC₇₀BM within the BHJ (bulk heterojunction) were different. When DPP used as an acceptor, conjugated side chains of the polymer coexisted with PC₇₀BM in same position. The BHJ film prepared from PfToBT, PbToBT had a discontinuous network between polymer and PC₇₀BM, whereas films from PfTDPP and PbTDPP formed continuous and evenly distributed network between them. This optimized vertical morphology promotes hole transport along respective pathways of polymers and fullerenes in the vertical direction, leading to high J_SC. PbTDPP shows PCE up to 2.9% (Jsc of 9.4 mA/cm², Voc of 0.68 V, and FF of 0.44). © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2016 , 54, 2746–2759     

Efficient organic photovoltaic cells based on thiazolothiazole and benzodithiophene copolymers with π‐conjugated bridges

New donor–π–acceptor (D–π–A) type conjugated copolymers, poly[(4,8‐bis((2‐hexyldecyl)oxy)benzo[1,2‐b:4,5‐b′]dithiophene)‐alt‐(2,5‐bis(4‐octylthiophen‐2‐yl)thiazolo[5,4‐d]thiazole)] (PBDT‐tTz), and poly[(4,8‐bis((2‐hexyldecyl)oxy)benzo[1,2‐b:4,5‐b′]dithiophene)‐alt‐(2,5‐bis(6‐octylthieno[3,2‐b]thiophen‐2‐yl)thiazolo[5,4‐d]thiazole)] (PBDT‐ttTz) were synthesized and characterized with the aim of investigating their potential applicability to organic photovoltaic active materials. While copolymer PBDT‐tTz showed a zigzagged non‐linear structure by thiophene π‐bridges, PBDT‐ttTz had a linear molecular structure with thieno[3,2‐b]thiophene π‐bridges. The optical, electrochemical, morphological, and photovoltaic properties of PBDT‐tTz and PBDT‐ttTz were systematically investigated. Furthermore, bulk heterojunction photovoltaic devices were fabricated by using the synthesized polymers as p‐type donors and [6,6]‐phenyl‐C₇₁‐butyric acid methyl ester as an n‐type acceptor. PBDT‐ttTz showed a high power conversion efficiency (PCE) of 5.21% as a result of the extended conjugation arising from the thienothiophene π‐bridges and enhanced molecular ordering in the film state, while PBDT‐tTz showed a relatively lower PCE of 2.92% under AM 1.5 G illumination (100 mW/cm²). © 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2018 , 56, 1978–1988     

Donor–acceptor‐integrated conjugated polymers based on carbazole[3,4‐c:5,6‐c]bis[1,2,5]thiadiazole with tight π–π stacking for photovoltaics

An original strategy to construct a new donor–acceptor (D–A)‐integrated structure by directly imposing “pull” unit on the “push” moiety to form fused ring architecture has been developed, and poly{N‐alkyl‐carbazole[3,4‐c:5,6‐c]bis[1,2,5]thiadiazole‐alt‐thiophene} (PCBTT) with D–A‐integrated structure, in which two 1,2,5‐thiadiazole rings are fixed on carbazole in 3‐, 4‐ and 5‐, 6‐position symmetrically and thiophene is used as bridge, has been synthesized. The interaction between pull and push units has fine tuned the HOMO/LUMO energy levels, and the resulting copolymer covers the solar flux from 300 to 750 nm. The interaction between pull and push units is worth noting that due to the fused five rings inducing strong intermolecular interaction, an extremely short π–π stacking distance of 0.32 nm has been achieved for PCBTT both in powder and solid states. This is the shortest π–π stacking distance reported for conjugated polymers. Additionally, an obvious intramolecular charge transfer and energy transfer from donor units to acceptor units have been detected in this D–A integration. A moderate‐to‐high open‐circuit voltage of ～0.7 V in PCBTT:[6,6]‐phenyl‐C61 butyric acid methyl ester (PCBM) (w/w = 1/2) solar cells is achieved due to the low‐lying HOMO energy level of PCBTT. © 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2013     

Benzo[1,2‐c:3,4‐c']bis[1,2,5]selenadiazole, [1,2,5]selenadiazolo‐[3,4‐e]‐2,1,3‐benzothiadiazole,furazanobenzo‐2,1,3‐thiadiazole,furazanobenzo‐2,1,3‐selenadiazole and related heterocyclic systems

The first synthesis of benzo[1,2‐c:3,4‐c']bis[1,2,5]selenadiazole has been developed starting from commercially available 4‐nitrobenzo‐2,1,3‐selenadiazole. Improved syntheses of the related heterocycles [1,2,5]selenadiazolo[3,4‐e]‐2,1,3‐benzothiadiazole, furazanobenzo‐2,1,3‐thiadiazole and furazanobenzo‐2,1,3‐selenadiazole are also reported.     

Synthesis and properties of poly(heteroaryleneethynylene)s consisting of electron‐accepting benzothiadiazole/quinoxaline units and electron‐donating alkyl thiophene units

Low‐band‐gap π‐conjugated polymers composed of π‐excessive thiophene and π‐deficient benzothiadiazole and quinoxaline units were prepared in high yields by a polycondensation method using palladium cross‐coupling reactions of alkylthiophene diacetylenes, 4,7‐dibromo‐2,1,3‐benzothiadiazole, and 5,8‐dibromo‐2,3‐dipyridine‐2‐ylquinoxaline. The copolymers were characterized by NMR, IR, UV, gel permeation chromatography, and elemental analysis. High‐molecular‐weight (weight‐average molecular weight up to 82,600 g/mol), thermostable, soluble, and film‐forming materials were obtained. The polymers were photoluminescent in chloroform and showed metallic luster in the solid state. The absorption and emission in solution and in the solid state of the polymers revealed that the polymers generated a π‐stacked structure in the solid state, and the polymer molecules in the film were ordered. Thin films of poly[3‐dodecylthiophen‐2,5‐diylethynylene‐(benzo[1,2,5]thiadiazole‐4,7‐diyl)ethynylene] ( P‐1 ), poly[3,4‐di dodecylthiophen‐2,5‐diylethynylene‐(benzo[1,2,5]thiadiazole‐4,7‐diyl)ethynylene] ( P‐2 ), poly[3‐dodecylthiophene‐2,5‐diylethynylene‐(2,3‐dipyridine‐2‐ylquinoxaline‐5,8‐diyl)ethynylene] ( P‐3 ), and poly[3,4‐didodecylthiophene‐2,5‐diylethynylene‐(2,3‐dipyridine‐2‐ylquinoxaline‐5,8‐diyl)‐ethynylene] ( P‐4 ) exhibited an optical band gap of ～1.85–2.08 eV. The highest occupied molecular orbital and lowest unoccupied molecular orbital levels of the polymers were determined from electrochemical measurements. In the absorption and emission spectra of these polymers in chloroform/methanol mixtures, all the polymers revealed solvatochromic effects, which were related to the formation of aggregates, as confirmed by temperature‐dependence absorption investigations. The absorption spectra of P‐2 and P‐4 at different temperatures also revealed significant effects of the structure on the molecular interactions. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 6445–6454, 2005     

Synthesis and characterization of cyclopentadithiophene‐based low bandgap copolymers containing electron‐deficient benzoselenadiazole derivatives for photovoltaic devices

We have synthesized two cyclopentadithiophene (CDT)‐based low bandgap copolymers, poly[(4,4‐bis(2‐ethyl‐hexyl)‐4H‐cyclopenta[2,1‐b:3,4‐b′]dithiophene‐2,6‐diyl)‐alt‐(benzo[c][1,2,5]selenadiazole‐4,7‐diyl)] (PCBSe) and poly[(4,4‐bis(2‐ethyl‐hexyl)‐4H‐cyclopenta[2,1‐b:3,4‐b′]dithiophene‐2,6‐diyl)‐alt‐(4,7‐dithiophen‐2‐yl‐benzo[c][1,2,5]selenadiazole‐5,5′‐diyl)] (PCT2BSe), for use in photovoltaic applications. Through the internal charge transfer interaction between the electron‐donating CDT unit and the electron‐accepting benzoselenadiazole, we realized exceedingly low bandgap polymers with bandgaps of 1.37–1.46 eV. The UV–vis absorption maxima of PCT2BSe were subjected to larger hypsochromic shifts than those of PCBSe, because of the distorted electron donor–acceptor (D–A) structures of the PCT2BSe backbone. These results were supported by the calculations of the D–A complex using the ab initio Hartree‐Fock method with a split‐valence 6‐31G* basis set. However, PCT2BSe exhibited a better molar absorption coefficient in the visible region, which can lead to more efficient absorption of sunlight. As a result, PCT2BSe blended with [6,6]‐phenyl‐C₆₁‐butyric acid methyl ester (PC₆₁BM) exhibited a better photovoltaic performance than PCBSe because of the larger spectral overlap integral with respect to the solar spectrum. Furthermore, when the polymers were blended with PC₇₁BM, PCT2BSe showed the best performance, with an open circuit voltage of 0.55 V, a short‐circuit current of 6.63 mA/cm², and a power conversion efficiency of 1.34% under air mass 1.5 global illumination conditions. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 1423–1432, 2010     

Effects of alkyl side chain length of low bandgap naphtho[1,2‐c:5,6‐c′]bis[1,2,5]thiadiazole‐based copolymers on the optoelectronic properties of polymer solar cells

Two donor‐π‐acceptor (D‐π‐A) type naphtho[1,2‐c:5,6‐c′]bis[1,2,5]thiadiazole (NT)‐based conjugated copolymers (CPs), namely, PBDT‐TT‐DTNT‐HD and PBDT‐TT‐DTNT‐OD, containing different side chain length (2‐hexyldecyl, HD and 2‐octyldodecyl, OD) anchoring to thiophene π‐bridge between the two‐dimensional (2D) 5‐((2‐butyloctyl)thieno[3,2‐b]thiophen‐2‐yl)benzo[1,2‐b:4,5‐b′]dithiophene (BDT‐TT) unit and NT moiety are developed and fully characterized. The resultant two copolymers exhibited broader absorption in wide range of 300–820 nm and obviously deepened E_HOMO of approximately −5.50 eV. The effects of side chain length on film‐forming ability, absorption, energy levels, aggregation, dielectric constant (ɛ_r), mobility, morphology, and photovoltaic properties are further systematically investigated. It was found that the side chain length had little impact on solution‐processability, absorption, energy levels, and aggregation in CB solution of resultant CPs. However, tinily increasing side chain length promoted to form the more ordered structure of neat polymer film even if the corresponding ɛ_r decreased. As a result, the side‐chain‐extended PBDT‐TT‐DTNT‐OD:PC₇₁BM‐based device achieved 32% increased FF than that of PBDT‐TT‐DTNT‐HD:PC₇₁BM and thus the PCE was significantly raised from 3.99% to 5.21%, which were benefited from 2 times higher SCLC hole mobility, more favorable phase separation, and improved exciton dissociation. These findings could provide an important and valuable insight by side chain modulation for achieving efficient PSCs. © 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2018 , 56, 2059–2071     

Conjugated random terpolymers based on benzodithiophene,diketopyrrolopyrrole, and 8,10‐bis(thiophen‐2‐yl)‐2,5‐di(nonadecan‐3‐yl)bis[1,3]thiazolo[4,5‐f:5′,4′‐h]thieno[3,4‐b]quinoxaline for Efficient Polymer Solar Cell

A series of novel donor–acceptor (D–A) random conjugated terpolymers P2‐P4 along with the homopolymers P1 (BDT‐DPP) and P5 (BDT‐BTDQ) were designed and synthesized by copolymerizing a benzo[1,2‐b:4,5‐b]dithiophene (BDT) donor with an electron‐deficient diketopyrrolo[3,4‐c]pyrrole (DPP) unit and a benzothiadiazolo[3,4‐e]quinoxaline (BTDQ) moieties of different electron‐withdrawing strengths, and the resultant terpolymers showed broad absorption profile ranging from 300 to 1200 nm. The HOMO levels of the polymers were adjusted from ?5.23 to ?5.11 eV, and the optical bandgaps were controlled from 1.32 to 1.13 eV by changing the molar ratio of DPP and BTDQ acceptors. These terpolymers were used as a donor along with PC₇₁BM as an acceptor for the creation of polymer solar cells, and the performance was optimized via variable the donor to acceptor ratio and solvent vapor annealing. The polymer solar cells made from the random terpolymer P3 showed the highest overall power conversion efficiency of (9.27%), which is higher than that for the corresponding homo‐polymers counterparts, that is, P1 (7.27%) and P5 (7.68%). The results demonstrate that the designing of random D‐A1‐D‐A2 terpolymers may be the best approach for efficient polymer solar cells. © 2019 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2019, 57, 1478–1485