Fluorene‐based alternating polymers containing electron‐withdrawing bithiazole units: Preparation and device applications

We report here the synthesis via Suzuki polymerization of two novel alternating polymers containing 9,9‐dioctylfluorene and electron‐withdrawing 4,4′‐dihexyl‐2,2′‐bithiazole moieties, poly[(4,4′‐dihexyl‐2,2′‐bithiazole‐5,5′‐diyl)‐alt‐(9,9‐dioctylfluorene‐2,7‐diyl)] (PHBTzF) and poly[(5,5′‐bis(2″‐thienyl)‐4,4′‐dihexyl‐2,2′‐bithiazole‐5″,5″‐diyl)‐alt‐(9,9‐dioctylfluorene‐2,7‐diyl)] (PTHBTzTF), and their application to electronic devices. The ultraviolet–visible absorption maxima of films of PHBTzF and PTHBTzTF were 413 and 471 nm, respectively, and the photoluminescence maxima were 513 and 590 nm, respectively. Cyclic voltammetry experiment showed an improvement in the n‐doping stability of the polymers and a reduction of their lowest unoccupied molecular orbital energy levels as a result of bithiazole in the polymers' main chain. The highest occupied molecular orbital energy levels of the polymers were ?5.85 eV for PHBTzF and ?5.53 eV for PTHBTzTF. Conventional polymeric light‐emitting‐diode devices were fabricated in the ITO/PEDOT:PSS/polymer/Ca/Al configuration [where ITO is indium tin oxide and PEDOT:PSS is poly(3,4‐ethylenedioxythiophene) doped with poly(styrenesulfonic acid)] with the two polymers as emitting layers. The PHBTzF device exhibited a maximum luminance of 210 cd/m² and a turn‐on voltage of 9.4 V, whereas the PTHBTzTF device exhibited a maximum luminance of 1840 cd/m² and a turn‐on voltage of 5.4 V. In addition, a preliminary organic solar‐cell device with the ITO/PEDOT:PSS/(PTHBTzTF + C₆₀)/Ca/Al configuration (where C₆₀ is fullerene) was also fabricated. Under 100 mW/cm² of air mass 1.5 white‐light illumination, the device produced an open‐circuit voltage of 0.76 V and a short‐circuit current of 1.70 mA/cm². The fill factor of the device was 0.40, and the power conversion efficiency was 0.52%. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 1845–1857, 2005     

Synthesis of phenylquinoxaline oligomers containing pendant electron‐donating and electron‐withdrawing groups

A series of extended 6‐substituted quinoxaline AB monomer mixtures, 2‐(4‐fluorophenyl)‐3‐[4‐(4‐hydroxyphenoxy)phenyl]‐6‐substituted quinoxaline and 3‐(4‐fluorophenyl)‐2‐[4‐(4‐hydroxyphenoxy)phenyl]‐6‐substituted quinoxaline, were prepared and polymerized to afford phenylquinoxaline oligomers. High‐molecular‐weight polymers could not be obtained because of the formation of cyclic oligomers. On the basis of matrix‐assisted laser desorption/ionization time‐of‐flight analysis and molecular modeling results, the formation of a cyclic dimer could be a favorable process resulting in low‐molecular‐weight oligomers. They were completely soluble and amorphous, with glass‐transition temperatures varying from 165 to 266 °C, and they had thermooxidative stability, with samples displaying 5% weight loss temperatures of 419–511 °C in nitrogen. The thermal properties of the monomers and resultant polymers dramatically depended on the polarity of the substituents. The monomers and resultant oligomers displayed high fluorescence in tetrahydrofuran solutions and N‐methyl‐2‐pyrrolidinone solutions, respectively. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 6465–6479, 2005     

Acceptor–acceptor conjugated copolymers based on perylenediimide and benzothiadiazole for all‐polymer solar cells

Donor–acceptor (D–A) conjugated copolymers are one of known classes of organic optoelectronic materials and have been well developed. However, less attention has been paid on acceptor–acceptor (A–A) conjugated analogs. In this work, two types of A–A conjugated copolymers, namely P1‐Cn and P2‐Cn (n is the carbon number of their alkyl side chains), were designed and synthesized based on perylenediimide ( PDI ) and 2,1,3‐benzothiadiazole ( BT ). Different from P1‐Cn , P2‐Cn polymers have additional acetylene π‐spacers between PDI and BT and thus hold a more planar backbone configuration. Property studies revealed that P2‐Cn polymers possess a much red‐extended UV–vis absorption spectrum, stronger π–π interchain interactions, and one‐order larger electron mobility in their neat film state than P1‐Cn . However, all‐polymer solar cells using P1‐Cn as acceptor component and poly(3‐hexyl thiophene) or poly(2,7‐(9,9‐didodecyl‐fluoene)‐alt?5,5′‐(4,7‐dithienyl‐2‐yl‐2,1,3‐benzothiadiazole) as donor component exhibited much better performance than those based on P2‐Cn . Apart from their backbone chemical structure, the side chains were found to have little influence on the photophysical, electrochemical, and photovoltaic properties for both P1‐Cn and P2‐Cn polymers. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2014 , 52, 1200–1215     

High‐performance fluorine‐containing BDT‐based copolymer for organic solar cells with a high open circuit voltage

A new donor–acceptor (D‐A) conjugated copolymer (PBDTT(ff)‐ttTPD) based on fluorine‐substituted benzodithiophene (BDT) and 6‐alkylthienothienyl thieno[3,4‐c]pyrrole‐4,6‐dione (ttTPD) has been synthesized via a Stille cross‐coupling reaction. As a control, the nonfluorinated BDT‐based ttTPD copolymer (PBDTT‐ttTPD) was also synthesized by using the same polymerization method. The number‐average molecular weights (M _n) of PBDTT(ff)‐ttTPD and PBDTT‐ttTPD were found to be 48,000 g/mol (? = 2.2) and 43,000 g/mol (? = 2.1), respectively. The HOMO levels of PBDTT(ff)‐ttTPD and PBDTT‐ttTPD were calculated to be ?5.65 and ?5.45 eV, respectively. The inclusion of fluorinated BDT units is a very effective approach to lowering the polymer's HOMO level. The SCLC mobilities of PBDTT(ff)‐ttTPD and PBDTT‐ttTPD were determined to be 5.9 × 10^?4 and 3.0 × 10^?4 cm²/Vs, respectively. Polymer solar cell devices prepared with PBDTT(ff)‐ttTPD and PBDTT‐ttTPD as their active layers were found to exhibit power conversion efficiencies of 7.45 and 6.79% with open circuit voltages of 0.98 and 0.84 V, respectively. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2017 , 55 , 2506–2512     

Synthesis and characterization of thiazolothiazole‐based polymers and their applications in polymer solar cells

A novel series of thiazolothiazole (Tz)‐based copolymers, poly[9,9‐didecylfluorene‐2,7‐diyl‐alt‐2,5‐bis‐(3‐hexylthiophene‐2‐yl)thiazolo[5,4‐d]thiazole] (P1), poly[9,9‐dioctyldibenzosilole‐2,7‐diyl‐alt‐2,5‐bis‐(3‐hexylthiophene‐2‐yl)thiazolo[5,4‐d]thiazole] (P2), and poly[4,4′‐bis(2‐ethylhexyl)‐dithieno[3,2‐b:2′,3′‐d]silole‐alt‐2,5‐bis‐(3‐hexylthiophene‐2‐yl)thiazolo[5,4‐d]thiazole] (P3), were synthesized for the use as donor materials in polymer solar cells (PSCs). The field‐effect carrier mobilities and the optical, electrochemical, and photovoltaic properties of the copolymers were investigated. The results suggest that the donor units in the copolymers significantly influenced the band gap, electronic energy levels, carrier mobilities, and photovoltaic properties of the copolymers. The band gaps of the copolymers were in the range of 1.80–2.14 eV. Under optimized conditions, the Tz‐based polymers showed power conversion efficiencies (PCEs) for the PSCs in the range of 2.23–2.75% under AM 1.5 illumination (100 mW/cm²). Among the three copolymers, P1, which contained a fluorene donor unit, showed a PCE of 2.75% with a short‐circuit current of 8.12 mA/cm², open circuit voltage of 0.86 V, and a fill factor (FF) of 0.39, under AM 1.5 illumination (100 mW/cm²). © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011     

Proton conducting polymers containing 1H‐1,2,3‐triazole moieties

Polyacrylates containing a different number of 1H‐1,2,3‐triazole groups per repeat unit have been synthesized via conventional free radical polymerization. These polymers were characterized by nuclear magnetic resonance spectroscopy (NMR), gel permeation chromatography (GPC), thermogravimetric analysis (TGA), and differential scanning calorimetry (DSC). Proton conductivity measurements were made using impedance spectroscopy. Introduction of more than one triazole per repeat unit did not result in an increase in conductivity as there was an accompanying increase in glass transition temperature (T_g). A maximum conductivity of 17.5 μS/cm was obtained at 200 °C under anhydrous condition. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 188–196, 2009     

Silafluorene‐based polymers for electrochromic and polymer solar cell applications

In this study, four novel silafluorene (SiF) and benzotriazole (Btz) bearing conjugated polymers are synthesized. In the context of electrochemical and optical studies, these polymers are promising materials both for electrochromic device (ECD) and polymer solar cell (PSC) applications. All of the polymers are ambipolar (both p‐ and n‐dopable) and multichromic. Electrochemistry experiments indicate that incorporation of selenophene instead of thiophene unit increases the HOMO energy level of the polymers. Power conversion efficiency of the PSCs reached 1.75% for PTBTSiF, 1.55% for PSBSSiF, 2.57% for PBTBTSiF, and 1.82% for PBSBSSiF. The hole mobilities of the polymers are estimated through space charge limited current (SCLC) model. PBTBTSiF has the highest hole mobility as 2.44 × 10^?3 cm² V s^?1. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2015 , 53, 1541–1547     

Imide‐functionalized naphthodithiophene based donor‐acceptor conjugated polymers for solar cells

Donor‐acceptor conjugated polymers containing a new imide‐functionalized naphthodithiophene (INDT) as the acceptor unit and a 2,2'‐bithiophene with varied substituents as the donor unit have been synthesized. The bandgaps of these polymers depend strongly on the dihedral angle of the 2,2'‐bithiophene unit. The 3,3'‐dialkoxy substitution (polymers PDOR / PBOR ) leads to near planar bithiophene conformation due to the well‐known S–O short contact, while the 3,3'‐dialkyl substitution (polymer PDR ) results in significant twisting due to the steric effect. Consequently PDOR / PBOR shows the lowest bandgap of 1.82/1.85 eV while PDR has a bandgap of 2.38 eV. Bulk‐heterojunction solar cells of the polymer/fullerene blends have been fabricated. Preliminary results show that PBOR gives the best device performance with power conversion efficiencies as high as 2.45% in air without any thermal annealing treatment, indicating the promising potential of INDT‐containing conjugated polymers for efficient solar cells. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2013 , 51, 3818–3828     

Benzo[2,1‐b;3,4‐b′]dithiophene‐based low‐bandgap polymers for photovoltaic applications

The synthesis of four alternating copolymers using benzo[2,1‐b;3,4‐b′]dithiophene (BDP) as the common donor unit is presented. Before the synthesis, theoretical calculations that we performed predicted that the incorporation of BDP, which consists of fused dithiophene units with a benzene ring, into these polymers would produce a low‐lying highest occupied molecular orbital (HOMO) energy level. Low‐lying HOMO levels are desirable to produce high open circuit voltages (V_OC) in organic bulk heterojunction (BHJ) photovoltaic devices. The polymers' structural characterization, as well as the preliminary results of their performance in BHJ devices, using (6,6)‐phenyl C₆₁‐butyric acid methyl ester as the electron acceptor, is presented. The V_OC values follow the expected trend: increasing with decreasing HOMO level of the polymer. High V_OC values of 0.81 and 0.82 V have been obtained from two polymers: PBDPBT and PBDPDPP. The initial power conversion efficiency achieved in these unoptimized devices was 1.11% because of relatively low J_SC values. The variation observed in the J_SC values between the four polymers is discussed. Device performance is expected to increase with optimization of processing conditions for the devices. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011     

Synthesis of soluble poly(para‐phenylene) with a long polymer chain: Characteristics of regioregular poly(1,4‐phenylene)

Soluble poly(para‐phenylene) having a long polymer chain (more than six repeat units) was synthesized with a tert‐butyl end‐group (t‐PPP) and was found to have improved solubility and excellent optical properties. Poly(1,3‐cyclohexadiene) (PCHD) consisting of only 1,4‐cyclohexadiene (1,4‐CHD) units was synthesized with a tert‐butyl end‐group (t‐PCHD), and completely dehydrogenated to obtain t‐PPP. This end‐group effectively prevented the crystallization of t‐PPP, and polymers containing up to 16 repeat units were soluble in tetrahydrofuran. Soluble t‐PPP obtained had an ability to form a tough thin film prepared by spin‐coating method. Optical analyses of t‐PPP provided strong evidence for a linear polymer chain structure. A block copolymer of t‐PPP and a soluble polyphenylene (PPH) was then synthesized, and the excellent optical properties were retained by this block copolymer along with its solubility. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 5223–5231, 2008     

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     

Synthesis and FET characterization of novel ambipolar and low‐bandgap naphthalene‐diimide‐based semiconducting polymers

A novel series of naphthalene‐diimide‐based semiconducting polymers ( P1–P4 ) containing benzodithiophene or dithienopyrrole were successfully synthesized for ambipolar semiconducting materials showing near infrared absorptions. The incorporation of a 3‐hexylthiophene (3HT) spacer extended the intramolecular charge‐transfer (ICT) peak from λ_onset = 739 nm ( P1 ) to 785 nm ( P3 ). Moreover, about 250 nm red‐shift of the ICT peaks was observed in P2 and P4 compared to P1 and P3 due to the increased high‐lying HOMO energy levels. The grazing incidence X‐ray scattering of the P3 and P4 films proved the slightly improved crystalline order in the π?π stacking direction, indicating that the planar backbone is probably due to the introduced 3HT. The P1–P4 ‐based field‐effect transistor showed n‐type dominant ambipolar characteristics. The P2 and P4 showed higher electron mobilities up to 1.5 × 10^?2 cm² V^?1 s^?1 than P1 and P3 , which might be influenced by the orientation of the polymer backbone and the intermolecular orbital overlap. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2016 , 54, 359–367     

Synthesis and applications of low‐bandgap conjugated polymers containing phenothiazine donor and various benzodiazole acceptors for polymer solar cells

A series of soluble donor‐acceptor conjugated polymers comprising of phenothiazine donor and various benzodiazole acceptors (i.e., benzothiadiazole, benzoselenodiazole, and benzoxadiazole) sandwiched between hexyl‐thiophene linkers were designed, synthesized, and used for the fabrication of polymer solar cells (PSC). The effects of the benzodiazole acceptors on the thermal, optical, electrochemical, and photovoltaic properties of these low‐bandgap (LBG) polymers were investigated. These LBG polymers possessed large molecular weight (M_n) in the range of 3.85?5.13 × 10⁴ with high thermal decomposition temperatures, which demonstrated broad absorption in the region of 300?750 nm with optical bandgaps of 1.80?1.93 eV. Both the HOMO energy level (?5.38 to ?5.47 eV) and LUMO energy level (?3.47 to ?3.60 eV) of the LBG polymers were within the desirable range of ideal energy level. Under 100 mW/cm² of AM 1.5 white‐light illumination, bulk heterojunction PSC devices containing an active layer of electron donor polymers mixed with electron acceptor [6,6]‐phenyl‐C₆₁‐butyric acid methyl ester (PC₆₁BM) or [6,6]‐phenyl‐C₇₁‐butyric acid methyl ester (PC₇₁BM) in different weight ratios were investigated. The best performance of the PSC device was obtained by using polymer PP6DHTBT as an electron donor and PC₇₁BM as an acceptor in the weight ratio of 1:4, and a power conversion efficiency value of 1.20%, an open‐circuit voltage (V_oc) value of 0.75 V, a short‐circuit current (J_sc) value of 4.60 mA/cm², and a fill factor (FF) value of 35.0% were achieved. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2010     

Synthesis and electroluminescent properties of fluorene‐based copolymers containing electron‐withdrawing thiazole derivatives

We synthesized two fluorene‐based copolymers poly[(2,5‐bis(4‐hexylthiophen‐2‐yl)thiazolo[5,4‐day]thiazole‐5,5′‐diyl)‐alt‐(9,9′‐dioctylfluorene‐2,7‐diyl)] ( PF‐TTZT), and poly[(5,5′‐bis(4‐hexylthiophen‐2‐yl)‐2,2′‐bithiazole‐5,5′‐diyl)‐alt‐(9,9′‐dioctylfluorene‐2,7‐diyl)] (PF‐TBTT), which contain the electron‐withdrawing moieties, thiazolothiazole, and bithiazole, respectively. Through electrochemical studies, we found that these two polymers exhibit stable reversible oxidation and reduction behaviors. Moreover, the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels of PF‐TBTT are lower than those of PF‐TTZT, and the bandgap of PF‐TBTT is smaller than that of PF‐TTZT. Thus the bithiazole moiety in PF‐TBTT is more electron‐withdrawing than the thiazolothiazole moiety in PF‐TTZT. Light‐emitting devices with indium tin oxide (ITO)/poly(3,4‐ethylene dioxythiophene):poly(styrenesulfonate)(PEDOT)/polymer/bis(2‐methyl‐8‐quinolinato)‐4‐phenylphenolate aluminum (BAlq)/LiF/Al configurations were fabricated. The performance of the PF‐TBTT device was found to be almost three times better than that of the PF‐TTZT device, which is because electron injection from the cathode to PF‐TBTT is much easier than for PF‐TTZT. We also investigated the planarity and frontier orbitals of the electron donor‐acceptor (D‐A) moieties with computational calculations using ab initio Hartree–Fock with the split‐valence 6‐31G* basis set. These calculations show that TBTT has a more nonplanar structure than TTZT and that the bithiazole moiety is more electron‐withdrawing than thiazolothiazole. These calculations are in good agreement with the experimental results. © Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 7148–7161, 2008     

Silole‐containing polymers for high‐efficiency polymer solar cells

Silole‐containing conjugated polymers ( P1 and P2 ) carrying methyl and octyl substituents, respectively, on the silicon atom were synthesized by Suzuki polycondensation. They show strong absorption in the region of 300–700 nm with a band gap of about 1.9 eV. The two silole‐containing conjugated polymers were used to fabricate polymer solar cells by blending with PC₆₁BM and PC₇₁BM as the active layer. The best performance of photovoltaic devices based on P1 /PC₇₁BM active layer exhibited power conversion efficiency (PCE) of 2.72%, whereas that of the photovoltaic cells fabricated with P2 /PC₇₁BM exhibited PCE of 5.08%. 1,8‐Diiodooctane was used as an additive to adjust the morphology of the active layer during the device optimization. PCE of devices based on P2 /PC₇₁BM was further improved to 6.05% when a TiO_x layer was used as a hole‐blocking layer. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011     

Synthesis of polymers containing 1,2,4‐oxadiazole as an electron‐acceptor moiety in their main chain and their solar cell applications

To explore the aptitude of 1,2,4‐oxadiazole‐based electron‐acceptor unit in polymer solar cell applications, we prepared four new polymers (P1–P4) containing 1,2,4‐oxadiazole moiety in their main chain and applied them to solar cell applications. Thermal, optical, and electrochemical properties of the polymers were studied using thermogravimetric, absorption, and cyclic voltammetry analysis, respectively. All four polymers showed high thermal stability (5% degradation temperature over 335 °C), and the optical band gaps were calculated to be 2.20, 1.72, 1.37, and 1.74 eV, respectively, from the onset wavelength of the film‐state absorption band. The energy levels of the polymers were found to be suitable for bulk heterojunction (BHJ) solar cell applications. The BHJ solar cells were prepared by using the synthesized polymers as a donor and PC₇₁BM as an electron acceptor with the configuration of ITO/PEDOT:PSS/polymer:PC₇₁BM (1:3 wt %)/LiF/Al. One of the polymers was found to show the maximum power conversion efficiency of 1.33% with a Jsc of 4.95 mA/cm², a V_oc of 0.68 V, and a FF of 40%, measured using AM 1.5 G solar simulator at 100 mW/cm₂ light illumination. © 2013 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2013     

Tunable light‐harvesting polymers containing embedded dipolar chromophores for polymer solar cell applications

A series of light‐harvesting conjugated polymers were designed and synthesized for polymer solar cells. These newly designed polymers comprise an unusual two‐dimensional conjugated structure with an electron‐rich thiophene–triphenylamine backbone and stable planar indacenodithiophene π‐bridges terminated with tunable electron acceptors. It was found that the electron‐withdrawing strength of the acceptor could be used to manipulate the energy level of the lowest unoccupied molecular orbital and bandgap (as much as 0.3 eV), generating derivatives with complementary absorbance in the visible spectrum. This approach provides great flexibility in fine tuning the electronic and optical properties of the resultant polymers and facilitates the investigation of how these chemical modifications alter the subsequent photovoltaic properties of these materials. © 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2012     

Jacket‐like structure of donor–acceptor chromophores‐based conducting polymers for photovoltaic cell applications

Through the Stille coupling polymerization, a series of soluble acceptor/donor quinoxaline/thiophene alternating conducting polymers with a hole‐transporting moiety of carbazole as a side chain ( PCPQT ) has been designed, synthesized, and investigated. The UV–vis measurement of the charge‐transferred type PCPQT s of different molecular weights with low polydispersity exhibits a red shifting of their absorption maximum from 530 to 630 nm with increasing chain length (M_n: from 1100 to 19,200). The HOMO and LUMO energy levels of PCPQT can be determined from the cyclic voltammetry measurement to be ?5.36 and ?3.59 eV, respectively. Solar cells made from PCPQT/PCBM bulk heterojunction show a high open‐circuit voltage, V_oc of ～0.75 V, which is significantly higher than that of a solar cell made from conventional poly(3‐hexyl thiophene)/ PCBM as the active polymer PCPQT has lower HOMO level. Further improvements are anticipated through a rational design of the new low band‐gap and the structurally two‐dimensional donor–acceptor conducting polymers. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 1607–1616, 2010     

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     

Synthesis and characterization of naphtho[2,1‐b:3,4‐b′]dithiophene‐based polymers with extended π‐conjugation systems for use in bulk heterojunction polymer solar cells

Two novel polymeric semiconductor materials based on naphtho[2,1‐b:3,4‐b']dithiophene (NDT), PNDT‐TTT and PNDT‐TET , were designed and synthesized. These synthesized polymers were tested in bulk heterojunction solar cells as blends with the acceptor [6,6]‐phenyl‐C71‐butyric acid methyl ester (PC₇₁BM). PNDT‐TTT contained tri‐thiophene units, and PNDT‐TET contained bi‐thiophene units coupled by ethylenic linkages. Comparison to the properties of PNDT‐T , which contained single thiophene units, these polymers exhibit red‐shifted absorption spectra as a result of the enhanced conjugation lengths. These effects resulted in high short circuit currents (J_SC) in the organic solar cells. The PNDT‐TET ‐ and PNDT‐TTT ‐based devices exhibited considerably better photovoltaic performances, with power conversion efficiencies of 3.5 and 3.3%, respectively, compared to the PNDT‐T ‐based device (1.3%). © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2013 , 51, 4742–4751