Synthesis and photovoltaic properties of donor–acceptor polymers incorporating a structurally-novel pyrrole-based imide-functionalized electron acceptor moiety

A structurally-novel pyrrole-based imide-functionalized electron accepting monomer unit, 4,6-dibromo-2,5-dioctylpyrrolo[3,4-c]pyrrole-1,3(2H,5H)-dione (DPPD), was prepared. The new DPPD unit was copolymerized with pyrrole-based electron rich monomers, such as thiophene-(N-alkyl)pyrrole-thiophene (TPT) and fused thiophene-(N-alkyl)pyrrole-thiophene (DTP) derivatives, to afford two new polymers, namely P(TPT-DPPD) and P(DTP-DPPD), respectively. The two polymers showed a strong absorption band at 300–600 nm and 300–650 nm, respectively, and their calculated optical band gaps were 2.09 eV and 1.89 eV, respectively. The electrochemical analysis reveals that the highest occupied molecular orbital (HOMO) energy levels of P(TPT-DPPD) and P(DTP-DPPD) were positioned at −5.55 eV and −5.24 eV, respectively, whereas their lowest unoccupied molecular orbital (LUMO) energy levels were positioned at −3.46 eV and −3.35 eV, respectively. The preliminary photovoltaic properties of the polymers, P(TPT-DPPD) and P(DTP-DPPD), were examined by fabricating polymer solar cells (PSCs) with each polymer as an electron donor and PC₇₁BM as an electron acceptor. The PSCs fabricated with the configuration of ITO/PEDOT:PSS/P(TPT-DPPD) or P(DTP-DPPD):PC₇₁BM/LiF/Al showed maximum power conversion efficiency (PCE) of 0.73% and 1.64%, respectively.     

Synthesis of conjugated polymers with broad absorption bands and photovoltaic properties as bulk heterojuction solar cells

Two new broad absorbing alternating copolymers, poly[1-(2,6-diisopropylphenyl)-2,5-bis(2-thienyl)pyrrole-alt-4,7-bis(3-octyl-2-thienyl)benzothiadiazole] (PTPTTBT-P1) and poly[1-(p-octylphenyl)-2,5-bis(2-thienyl)pyrrole-alt-4,7-bis(3-octyl-2-thienyl)benzothiadiazole] (PTPTTBT-P2), were prepared via Suzuki polycondensation with high yields. The two polymers were found to show characteristic absorption in the visible region of the solar spectrum. Interestingly the absorption of PTPTTBT-P1 was found to cover the visible region from 350 to 650 nm with the broad and flat absorption maximum from 440 to 510 nm in film and the absorption of PTPTTBT-P2 was found to cover the visible region from 350 to 950 nm with the relatively distinct absorption maxima at 425 and 522 nm and very weak absorption maximum at 832 nm in film. The electrochemical band gaps of the polymers were calculated to be 1.88 eV and 1.87 eV, respectively, while the optical band gaps of the polymers were calculated to be 1.94 eV and 1.87 eV, respectively. The photovoltaic properties of polymers were investigated with bulk heterojunction (BHJ) solar cells fabricated in ITO/PEDOT:PSS/polymer:PC₇₀BM(1:5 wt%)/TiOx/Al configurations. The maximum power conversion efficiency (PCE) of the solar cell composed of PTPTTBT-P1:PC₇₀BM as an active layer was 1.57% with current density (J_sc) of 8.17 mA/cm², open circuit voltage (V_oc) of 0.52 V and fill factor (FF) of 36%.     

Random vs. alternating donor-acceptor copolymers: A comparative study of absorption and field effect mobility

The influence of the arrangement of donor and acceptor units in a conjugated copolymer chain on the absorption and field effect mobilities was studied. We synthesized a target random copolymer and compared it with two structurally relevant alternating copolymers, all consisting of 2,1,3-Benzothiadiazole (BT) as acceptor and 3-Hexylthiophene (Th) as donor units. Especially, bifunctional AB-type monomers were developed to obtain the desired randomly linked copolymer r-BT-2Th. We chose AA/BB-type monomers as well to obtain relevant alternating copolymers a-BT-2Th and a-BT-1Th. The systematic structural variation enables us to compare the copolymers in a precise manner. In dilute solutions r-BT-2Th and a-BT-2Th resemble closely in absorption spectra and have similar oxidation potentials regardless of random or alternating arrangement of donor and acceptor. In thin films, a-BT-2Th shows the lowest optical gap and depicts the highest field effect hole mobility of 1.5 × 10⁻³ cm² V⁻¹ s⁻¹.     

Low bandgap EDOT-quinoxaline and EDOT-thiadiazol-quinoxaline conjugated polymers: Synthesis, redox, and photovoltaic device

Three new low bandgap conjugated copolymers with 3,4-ethylenedioxythiophene (EDOT) as donor and 2,3-bis(4-octyloxyphenyl)-quinoxaline (P1), 2,3-bis(4-octyloxyphenyl)-thiadiazol-quinoxaline (P2, P3) as acceptors were synthesized by Stille cross-coupling reaction, and their optical and electrochemical properties were studied. These polymers exhibited optical bandgap of 1.77, 1.29 and 1.13 eV, for P1, P2 and P3, respectively. Photovoltaic cells with device configuration of ITO/PEDOT: PSS/Copolymer: PCBM (1:4 w/w)/LiF/Al were fabricated. The measurements revealed an open-circuit voltage (V_oc) of 0.52 V, short-circuit current density (J_sc) of 3.24 mA/cm² and power conversion efficiency (PCE) of 0.60% for P1, and showed a V_oc of 0.33 V, J_sc of 2.11 mA/cm², PCE of 0.39% for P2.     

Synthesis, electrochemical and optical studies of new cyanopyridine based conjugated polymers as potential fluorescent materials

In this communication we describe the design and synthesis of four new conjugated polymers (P1-P4) carrying cyanopyridine in their backbone via Wittig condensation technique. Their structures were well established by FTIR, ¹H NMR, elemental analysis and gel permeation chromatographic techniques. They exhibited good thermal stability with the onset decomposition temperature around 300-350 °C in nitrogen atmosphere. Further, their electrochemical, linear and third-order non-linear optical properties have been investigated. The polymers showed low electrochemical band gaps in the range of 1.77-1.99 eV and displayed very good fluorescence emission property in various polar solvents. The fluorescence quantum yields of polymers P1-P4 in tetrahydrofuran (THF) were found to be 35, 42, 38 and 34%, respectively. The effective two-photon absorption coefficients (β) for the polymers were determined by Z-Scan technique and were found to be 1.34 × 10⁻¹¹, 1.24 × 10⁻¹¹, 3.04 × 10⁻¹¹ and 1.85 × 10⁻¹¹ m/W for polymers P1-P4, respectively.     

Synthesis, characterization and third-order non-linear optical properties of novel fluorene monomers and their cross-conjugated polymers

We designed and synthesized two novel fluorene monomers of D-A-D (donor-acceptor-donor) type (M1 and M2), and their two corresponding polymers (PM1 and PM2) and a copolymer (CPM). These high molecular weight, film-forming polymers were obtained from metal-free, superacid-catalyzed reactions of the monomers with N-phenylisatin. The cubic NLO response (χ⁽³⁾) for these new compounds, in solid thin films, was measured through the use of third-harmonic generation (THG) Maker-Fringes technique at IR wavelengths given values of the order of 10⁻¹² esu from which, the corresponding second hyperpolarizabilities (γ) were estimated to be of the order of 10⁻³³ esu for monomers and 10⁻³¹ esu for polymers. Second hyperpolarizabilities have also been estimated theoretically at B3LYP/6-31G(d) level of theory in gas phase and related with the electronic structure of the synthesized molecules.     

Synthesis and photovoltaic properties of low-bandgap polymers based on N-arylcarbazole

Low-bandgap poly(2,7-carbazole) derivatives with variable N-substituent of ethyl (PEtCzBT), phenyl (PPhCzBT) and 4-diphenylaminophenyl (PTPACzBT) on the carbazoles, were synthesized through Suzuki coupling reaction. The polymers show excellent solubility in organic solvents (readily soluble in chloroform, THF and toluene etc.), good thermal stability (5% weight loss temperature of more than 417 °C), and electrochemical properties (reversible redox process with narrow bandgap), and deep HOMO energy levels (∼5.1 eV), allowing them promising candidates in the solar cell fabrication. Bulk-heterojunction solar cells with these polymers as electron donor and (6,6)-phenyl-C₇₁-butyric acid methyl ester (PC₇₁BM) as electron acceptor exhibit high V_oc (0.91-0.95 V) and good power conversion efficiency (PCE) of 1.69% for PEtCzTB, 2.01% for PPhCzTB, and 2.42% for PTPACzTB.     

Thermally cross-linkable hyperbranched polymers containing triphenylamine moieties: Synthesis, curing and application in light-emitting diodes

This paper demonstrates synthesis of hyperbranched polymers (HTP and HTPOCH₃), containing triphenylamine moieties in main chain and thermally cross-linkable periphery or terminal vinyl groups, and application as hole-transporting layer (HTL) in multilayer light-emitting diodes. Absorption and photoluminescence (PL) spectroscopy, cyclic voltammetry (CV) and differential scanning calorimetry (DSC) were employed to investigate their photophysical, electrochemical properties and thermal curing behaviors, respectively. The hyperbranched HTP and HTPOCH₃ were readily cross-linked by heating scan, with the exothermic peaks being at 221 and 210 °C respectively. The glass-transition temperatures (T_g) of the hyperbranched polymers were higher than 140 °C after thermal cross-linking at 210 °C for 30 min. Multilayer light-emitting diodes (ITO/PEDOT:PSS/HTL/MEH-PPV/Ca/Al), using HTP and HTPOCH₃ as HTL, were readily fabricated by successive spin-coating. The performance of MEH-PPV device (maximum luminance: 9310 cd/m², luminance efficiency: 0.26 cd/A) was effectively enhanced by inserting the thermally cross-linked HTP or HTPOCH₃ as HTL (HTP: 12610 cd/m², 0.32 cd/A; HTPOCH₃: 14060 cd/m², 0.33 cd/A). This indicates that these thermally cross-linkable hyperbranched HTP and HTPOCH₃ are very suitable for the fabrication of multilayer PLEDs using solution processes.     

Highly efficient electroluminescent biphenylyl-substituted poly(p-phenylenevinylene)s through fine tuning the polymer structure

A series of novel biphenylyl-substituted PPV derivatives, polymers 1-4, with different substitution patterns, has been synthesized and characterized. These polymers possess excellent solubilities, good thermal stabilities, and high-photoluminescent efficiencies. ¹H NMR measurements indicated that the polymers contain negligible tolane-bisbenzyl (TBB) structural defects. Light-emitting diodes fabricated from the four polymers with the configuration of ITO/PEDOT:PSS (50 nm)/polymer (80 nm)/LiF (0.4 nm)/Ca (20 nm)/Ag emitted a saturated green light and demonstrated maximum current efficiencies of 5.1, 4.5, 4.7, and 1.4 cd/A for polymers 1-4, respectively. The much higher current efficiencies of polymers 1-3 than polymer 4 are ascribed to more balanced charge transport in the polymer layers of the three polymers, which has been confirmed by time of flight (TOF) charge mobility measurement. The hole mobilities of the polymers at the applied electric field of 2.0×10⁵ V/cm are 4.70×10⁻⁶, 3.83×10⁻⁶, 7.21×10⁻⁶, and 1.76×10⁻⁵ cm²/Vs for polymers 1-4. This research indicated that fine tuning the substitution pattern of the polymer side chains is an effective way to optimize the LED device performance by controlling the structural defects as well as balancing the charge mobility of the polymers.     

New host copolymers containing pendant triphenylamine and carbazole for efficient green phosphorescent OLEDs

A series of vinyl copolymers (P1-P6) containing pendant hole-transporting triphenylamine (11-88 mol%) and carbazole chromophores were synthesized by radical copolymerization to investigate the influence of triphenylamine groups upon optoelectronic properties. The copolymers were readily soluble in common organic solvents and their weight-average molecular weights (M_ws) were between 1.41 × 10⁴ and 2.24 × 10⁴. They exhibited moderate thermal stability with T_d = 402-432 °C at 5% weight loss. The emission spectra (both PL and EL) of the blends [P1-P6 with 4 wt% Ir(ppy)₃] showed dominant green emission (517 nm) attributed to Ir(ppy)₃ due to efficient energy transfer from P1-P6 to Ir(ppy)₃. The HOMO levels of P1-P6, estimated from onset oxidation potentials in cyclic voltammeter, were −5.42 to −5.18 eV, which are much higher than −5.8 eV of conventional poly(9-vinylcarbazole) (PVK) host owing to high hole-affinity of the triphenylamine groups. The optoelectronic performances of phosphorescent EL devices, using P1-P6 as hosts and Ir(ppy)₃ as dopant (ITO/PEDOT:PSS/P1-P6:Ir(ppy)₃ (4 wt%):PBD (40 wt%)/BCP/Ca/Al), were greatly improved relative to that of PVK. The best performance was obtained with P4 device, in which the maximum luminance and luminance efficiency were 11?501 cd/m² and 10.6 cd/A, respectively.     

Conjugated copolymers based on dihexyl-benzimidazole moiety for organic photovoltaics

In this study, we report the synthesis, characterization, and photovoltaic properties of a series of four conjugated polymers based on donor-acceptor (D-A) structure. New polymers, P1, P2, P3 and P4 utilizing a new accepter, dihexyl-2H-benzimidazole, were synthesized using Stille coupling reaction. Even with two bithiophene units in the copolymers to facilitate absorption at the longer wavelength, the incorporation of dihexyl-substituent on 2H-benzimidazole enables the polymers to have good solubility. The spectra of the solid films show absorption bands with maximum peaks at about 442-479 nm and the absorption onsets at 529-575 nm, corresponding to band gaps of 2.16-2.34 eV. The device with P4:PC₇₁BM blend demonstrated an open-circuit voltage (V_OC) of 0.59 V, a short-circuit current (J_SC) of 6.43 mA/cm², and a fill factor (FF) of 0.39, leading to the efficiency of 1.46%.     

Synthesis, optical and electrochemical properties of luminescent polymers containing 1,2-diphenylmaleimide and thiophene segments

In an attempt to balance energy barriers of hole and electron injection we prepared and characterized homopolymer containing electron-transporting 1,2-diphenylmaleimide chromophores (P1) and copolymers consisting of 1,2-diphenylmaleimide and hole-transporting 2,5-thiophene moieties (P2, P3) via dehalogenation polycondensation. The copolymers are amorphous materials with decomposition temperature greater than 450 °C. Absorption and fluorescence spectra were employed to investigate their optical properties both in solution and film state. Photoluminescence maxima of P1, P2 and P3 films are 564, 559 and 558 nm, respectively. The HOMO and LUMO energy levels have been estimated from their cyclic voltammograms. The HOMO levels of P1, P2, and P3 were readily raised with increasing thiophene content (−5.99, −5.59, and −5.43 eV, respectively), whereas their LUMO levels were very similar (−3.61 to −3.65 eV). Double-layer light-emitting diodes (Al/PEDOT:PSS/P1-P3/ITO) were fabricated to evaluate their optoelectronic characteristics. Incorporation of thiophene units successfully reduced the turn-on electric field from 11.0×10⁵ to 2.9×10⁵ V/cm, but it decreased the luminescent efficiency and the maximum brightness.     

Synthesis and electro-optical properties of helical polyacetylenes carrying carbazole and triphenylamine moieties

Novel chiral acetylene monomers bearing carbazole and triphenylamine groups, namely, (S)-3-butyn-2-yl 2-(9-carbazolyl)ethyl carbonate (1) and (S)-3-butyn-2-yl 4-(diphenylamino)benzoate (2) were synthesized, and polymerized with Rh⁺(nbd)[η⁶-C₆H₅B⁻(C₆H₅)₃] catalyst to give the corresponding polymers with moderate molecular weights (M_n 13.0 × 10³ and 15.5 × 10³) in good yields (86% and 88%). CD spectroscopic studies revealed that poly(1) and poly(2) took predominantly one-handed helical structure in CHCl₃. The helical structures of poly(1) and poly(2) were very stable against heating and addition of MeOH. The solution of poly(1) and poly(2) emitted fluorescence in 0.52% and 7.2% quantum yields, which were lower than those of the corresponding monomers 1 and 2 (22.5% and 76.5%). The cyclic voltammograms of the polymers indicated that the oxidation potentials of the polymers were lower than those of the monomers. The polymers showed electrochromism and changed the color from pale yellow to pale blue by application of voltage, presumably caused by the formation of polaron at the carbazole and triphenylamine moieties. The onset temperatures of weight loss of poly(1) and poly(2) were 225 and 270 °C under air.     

Synthesis of a pyridine substituted polycarbodiimide and its use as a solid support for chemical reagents

Optically active, polycarbodiimides 3(a, b & c) with pyridine pendant groups were synthesized using [(R) - 2,2′- binaphthoxy] (di-isopropoxy) titanium(IV) catalyst. The polymers were characterized by ¹H and ¹³C NMR, and IR. Thermal stability of these polymers (up to 162 °C by TGA), allows thermally demanding chemical transformations on their side chains without decomposition. Advantages include fine-tunability of the other pendant group of the carbodiimide monomer. This allows one to optimize the properties of the polymer without undergoing copolymerization or further post-polymerization modifications. Borane (BH₃) was coordinated to poly 3 (a & b) to prepare the functional polymers 4 (a & b) respectively. A strong IR signature peak at 2368 cm⁻¹ supports BH₃ coordination. Gravimetric analysis indicates 97-99% borane complexation of the pyridine units. In addition, the thermal stability increased to 194 °C in poly 4a is consistent with the incorporation of BH₃ to the pendant pyridine of the helical polycarbodiimide 3a. Poly 4 (a & b) can be used as supported reagents and successfully reduced the carbonyl compounds (5 a-e) in moderate to excellent yields (60-100%) and are shown to be efficient, non-volatile, stable, and mild supported-reducing reagents. Upon completion of the reduction reaction, the polymer support was quantitatively recycled as required for a green solid catalyst support.     

Synthesis and characterization of chiral polythiophenes: Poly [(R)-(−) and (S)-(+)-2-(3′-thienyl)ethyl N-(3″,5″-dinitrobenzoyl)-α-phenylglycinate]

(R)-(−) (1) and (S)-(+)-2-(3′-Thienyl)ethyl N-(3″,5″-dinitrobenzoyl)-α-phenylglycinate (2) monomers were synthesized, characterized, and polymerized in chloroform using FeCl₃ as an oxidizing agent. Molecular weights of 2.6 × 10⁴ and 3.2 × 10⁴ for poly1 and poly2, respectively, were determined by SEC analysis. FTIR spectra of the polymers indicated the coupling of monomers through the α positions. UV-vis spectra showed absorption bands at λ_max = 226 and 423 nm for poly1 and poly2, ascribed to transitions of side groups and polythiophene backbone, respectively. Poly1 and poly2 remained stable up to 210 °C. At higher temperatures, a two step weight loss degradation process was observed for both polymers by TGA analysis. ¹H NMR, in the presence of Eu(tfc)₃, and optical rotation measurements indicate the chiral properties of the monomers 1 ([α]_D²⁸ = −76.2) and 2 ([α]_D²⁸ = +76.0), and the maintenance of chirality after polymerization (poly1 [α]_D²⁸ = −29.0 and poly2 [α]_D²⁸ = +28.4, c = 2.5 in THF). According to scanning electron microscopic analysis, the polymers are highly porous.     

Post-polymerization modification and organocatalysis using reactive statistical poly(ionic liquid)-based copolymers

Copoly(ionic liquid)s (coPILs) based on poly(styrene)-co-poly(4-vinylbenzylbutylimidazolium) with different anions (Cl⁻ and HCO₃⁻), denoted as PS-co-PVBnBuImCl 1 and PS-co-PVBnBuImHCO₃2, were used as reactive polymers for the purpose of post-polymerization modification and organic catalysis. While coPIL 1 could be derived into the corresponding poly(N-heterocyclic carbene)-silver transition metal complex referred to as poly(NHC–Ag) by a simple deprotonation/metallation sequence utilizing Ag₂O, coPIL 2 was found to quantitatively react with various electrophiles, including CS₂, isothiocyanate and transition metals (based on Pd and Au) upon formal loss of “H₂CO₃, affording the post-functionalized poly(NHC-CS₂), poly(NHC-isothiocyanate) and poly(NHC-Met) (Met = Au, Pd) copolymers. The catalytic activity of both coPILs 1 and 2 was also examined in cyclic carbonate formation by reaction between CO₂ and propylene oxide and in the benzoin condensation, respectively.     

Investigations on the electrochemical properties of new conjugated polymers containing benzo[c]cinnoline and oxadiazole moieties

A four-step route was designed to synthesize 3,8-benzo[c]cinnoline dicarboxylic acid (4). New conjugated polymers, POXD (I) and POXD (T), containing benzo[c]cinnoline and oxadiazole moieties, were obtained by thermal cyclodehydration of their soluble polyhydrazide precursors PHA (I) and PHA (T), respectively. Two reduction peaks were observed for these new conjugated polymers during CV cathodic scan. From the CV voltammograms combined with the results from molecular simulation, we concluded that the first reduction occurred at oxadiazole moiety and benzo[c]cinnoline moiety was responsible for the second reduction. It indicates that oxadiazole has stronger electron affinity than benzo[c]cinnoline. We proposed a mechanism to explain this two-stage reduction process. Due to the planar and electron-accepting ability of benzo[c]cinnoline and oxadiazole moieties, POXD (I) and POXD (T) exhibited very low LUMO (−3.42 and −3.45 eV) and HOMO (−6.23 and −6.27 eV) energy levels. They can be used as hole-blocking or electron-injection layers for OLED applications.     

High-performance poly(2,3-diphenyl-1,4-phenylene vinylene)-based polymer light-emitting diodes by blade coating method

A new series of super high brightness and luminance efficient poly(2,3-diphenyl-1,4-phenylene vinylene) (DP-PPV)-based electroluminescent (EL) polymers containing methoxy or long branched alkoxy chains were synthesized via Gilch polymerization. The branched alkoxy groups were introduced to enhance solubility for blade and spin-coating processes. Monomers of DMeO-PPV and m-Ph-PPV were used to increase steric hindrance and prevent close packing of the main chain. By controlling the feeding ratio of different monomers during polymerization, DP-PPV derivatives with high molecular weight were obtained. All synthesized polymers possess high glass transition temperatures and thermal stabilities. The maximum photoluminescent emissions of the thin films are located between 544 and 547 nm. Cyclic voltammetry analysis reveals that the band gaps of these light-emitting materials are in the range of 2.75-2.84 eV. Blade coating was used to fabricate multilayer polymer light-emitting diodes. A multilayer electroluminescent device with the configuration of ITO/PEDOT:PSS/TFB/P1/TPBi/LiF/Al exhibited a very high luminescence efficiency (10.96 cd A⁻¹). The maximum brightness of the multilayer EL device ITO/PEDOT:PSS/TFB/P3/CsF/Al reached up to 78,050 cd m⁻² with a low turn-on voltage (4.0 V). For further investigation, polymer P3 was blended with DPPFBNA to achieve white light-emitting device; the multilayer devices generated a maximum brightness of 1085 cd m⁻² and a luminance efficiency of 0.75 cd A⁻¹, with CIE coordinates (0.28, 0.33) at 11 V.