Synthesis, electroluminescence, and photovoltaic properties of dendronized poly(p-phenylene vinylene) derivatives

Two dendronized poly(p-phenylene vinylene) (PPV) derivatives, ED-PPV and BB-PPV, have been successfully synthesized according to the Gilch route. The obtained polymers possess excellent solubility in common solvents, good thermal stability with 5% weight loss temperature of more than 340 °C. The weight-average molecular weight (M_w) and polydispersity index (PDI) of ED-PPV and BB-PPV are in the range of (1.26-2.34)×10⁵ and 1.37-1.45, respectively. Polymer light-emitting diodes (PLEDs) with the configuration of ITO/PEDOT:PSS/polymer/Ca/Al devices were fabricated, and the PLEDs emitted green-yellow light. The turn-on voltages of the PLEDs based on ED-PPV and BB-PPV were approximately 4.3, and 4.5 V, respectively. The PLED devices of ED-PPV exhibited the maximum luminance of about 157 cd/m² at 10.5 V. Photovoltaic cells with the configuration of ITO/PEDOT:PSS/polymer:C₆₀ (1:1)/Al were also fabricated, and the energy conversion efficiency of the devices based on ED-PPV and BB-PPV was measured to be 0.58, and 0.014%, respectively, under the white light at 75 mW/cm².     

Synthesis of laterally attached side-chain liquid crystalline poly(p-phenylene vinylene) and polyfluorene derivatives for the application of polarized electroluminescence

Two series of poly(p-phenylene vinylene) and polyfluorene derivatives (PPV1-PPV4 and PF1-PF5) containing laterally attached penta(p-phenylene) mesogenes were synthesized and characterized. These polymers show nematic liquid crystalline behavior. The optical properties of the polymers were investigated by UV-vis absorption and photoluminescence spectrometers and these polymers were fabricated to form the polarized electroluminescent devices using poly(ethylenedioxythiophene)-poly(styrene sulfonic acid) (PEDOT-PSS) as an alignment layer. In the series of poly(p-phenylene vinylene) derivatives, polymer PPV4 offered the best EL device performance. It emitted yellow light at 588 nm at 4 V. The maximum brightness was about 1337 cd/m² at 9 V with a polarized ratio of 2.6. In another series of polyfluorene derivatives, PF4 offered the best EL device performance with the polarized ratio of 12.4 and a maximum luminescence of 1855 cd/m². In the case of polarized white light, as a consequence of blending small amount of PF4 and PF5 with a host polymer PF2, polarized ratio of up to 10.2 and a maximum brightness of 2454 cd/m² have been attained. The aligned films exhibited pronounced polarized ratio, implying that the polymers exhibit potential for linearly polarized LED application.     

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.     

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.     

Synthesis, characterization, and high gas permeability of poly(diarylacetylene)s having fluorenyl groups

Diarylacetylenes having fluorenyl groups and other substituents (trimethylsilyl, t-butyl, bromine, fluorine) (1a-1) were polymerized with TaCl₅-n-Bu₄Sn. Monomers 1a-l produced high molecular weight polymers 2a-l (M_w 5.1 × 10⁵-1.3 × 10⁶) in 12-59% yields. All of the polymers were soluble in common organic solvents, and gave tough free-standing membranes by the solution casting method. The onset temperatures of weight loss of polymers 2a-l in air were over 400 °C, indicating considerably high thermal stability. All the polymer membranes showed high gas permeability; e.g., the oxygen permeability coefficient (PO₂) of 2a was as large as 4800 barrers. Membrane 2d possessing two fluorine atoms at meta and para positions of the phenyl ring showed the highest oxygen permeability (PO₂ = 6600 barrers) among the present polymers.     

Novel red-emitting thieno-[3,4-b]-pyrazine derivatives suitable for vacuum evaporation and solution method to fabricate non-doped OLEDs

Two novel red-emitting thieno-[3,4-b]-pyrazine-cored molecules with phenyls (TP) or polyphenyls (Müllen type dendron, DTP) as peripheral groups were designed and synthesized. They have large Stokes shifts over 100 nm. DTP is thermally stable with decomposition temperature up to 458 °C. More importantly, it is amorphous with a remarkably high glass transition temperature of 262 °C. DTP can be made into thin films either by solution method or vacuum evaporation. Red OLEDs were fabricated using either spin coated or vacuum evaporated DTP film as emitting layer. The evaporated device exhibited a maximum brightness of 1753 cd m⁻² and a luminous efficiency of 0.74 cd A⁻¹, which are among the best data ever reported for thieno-[3,4-b]-pyrazine derivatives so far. In contrary, TP failed to produce satisfied red emission in its evaporated OLEDs.     

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.     

Electrochemical study of stable N-alkoxyarylaminyl radicals

The electrochemical study of N-tert-butoxy-2,4-diphenyl-6-tert-butylphenylaminyl (1a), N-tert-butoxy-2,4-bis(4-chlorophenyl)-6-tert-butylphenylaminyl (1b), N-[2-(methoxycarbonyl)-2-propyl]-2,4-diphenyl-6-tert-butylphenylaminyl (2), and N-tert-butoxy-2,4,6-tris(4-chlorophenyl)phenylaminyl radicals (3) was performed by cyclic voltammetry using acetonitrile as the solvent and Bu₄NPF₆ as the supporting electrolyte. On cathodic scan (100 mV/s), all the radicals gave chemically reversible cyclic voltammograms, and the were determined to be −1.405 V (1a), −1.310 V (2a), −1.282 V (2b), and −1.195 V (3) (versus Fc⁺/Fc), respectively. On anodic scan (100 mV/s), on the other hand, 1a, 1b and 2 showed chemically reversible cyclic voltammograms, but 3 exhibited a partially reversible couple even on a scan rate of 500 mV/s, indicating that the cation species of 3 was less stable. The determined for 1a, 1b, 2 and 3 were 0.220, 0.280, 0.318 and 0.294 V (versus Fc⁺/Fc), respectively. The electrochemical data were compared with those of thioaminyl radicals, the corresponding sulfur analogues of 1-3.     

New amphiphilic fluorescent CBABC-type pentablock copolymers containing pyrene group by two-step atom transfer radical polymerization (ATRP) and its self-assembled aggregation

A series of novel amphiphilic fluorescent CBABC-type pentablock copolymers (Py-PMMA-PEG4600-PMMA-Py) were prepared from BAB-type amphiphilic triblock copolymer (PMMA-PEG4600-PMMA) as macroinitiator with various contents of 1-(methacryloyloxyethylamino-carboxylmethyl) pyrene (PyMOI) by atom transfer radical polymerization (ATRP) in toluene using CuBr/2,2-bipyridine as catalyst system. Triblock copolymer (PMMA-PEG4600-PMMA) was prepared by ATRP and obtained from Br-PEG4600-Br as macroinitiator with methyl methacrylate in tetrahydrofuran using the same catalyst. The molecular weights of pentablock copolymers which were reinitiated by PMMA-PEG4600-PMMA macroinitiator were calculated from ¹H NMR spectra up to 42,400 gmol⁻¹. The polydispersity of pentablock copolymers obtained from GPC analysis was narrow between 1.10 and 1.38. The crystallinity of triblock copolymer (PMMA-PEG4600-PMMA) was decreased slightly with incorporating PMMA segment. Introducing the bulky pyrene substituent into pentablock copolymer, the melting temperature was not observed and all pentablock copolymers showed amorphous patterns in wide-angle X-ray scattering (WAXS) due to decrease in the degree of crystallinity of polymer chain because of disturbing regular packing. The temperatures at 10% weight loss (Td₁₀), examined by TG analysis, showed values ranging from 265 to 323 °C in nitrogen and 264 to 313 °C in air. Fluorescence spectra of Py-PMMA-PEG4600-PMMA-Py exhibited stronger excimer emission at ca. 480 nm due to the aggregations of pyrene group formed via interaction of the hydrophobic chains. The more content of PyMOI segment in pentablock copolymers can obtain the higher emission intensity ca. 480 nm. When there were higher PyMOI contents (84.9 wt% PyMOI) in pentablock copolymers, they formed larger aggregates (210 nm) in SEM micrographs. On the other hand, while increasing the concentration of the polymer solution in THF, the morphology was changed from spherical (0.1 mg/mL) to chainlike (1.0 mg/mL) aggregates.     

Synthesis and properties of various poly(diphenylacetylenes) containing tert-amine moieties

The polymerization of diphenylacetylene derivatives possessing tert-amine moieties, such as triphenylamine, N-substituted carbazole and indole, was examined in the presence of TaCl₅-n-Bu₄Sn (1:2) catalyst. A polymer with high molecular weight (M_w = 570 × 10³) was obtained in good yield by the polymerization of diphenylamine-containing monomer 1b, whereas the isopropylphenylamine derivative (1c) gave a polymer with relatively low molecular weight (M_w = 2.4 × 10³). The polymerization of monomer 1d containing cyclohexylphenylamine group did not proceed; however, carbazolyl- and indolyl-containing monomers also produced polymers. Poly(1b), poly(2f) and poly(4b) could be fabricated into free-standing membranes by casting toluene solutions of these polymers. The gas permeability of poly(1b) was too low to be evaluated accurately whereas poly(4b) possessing two chlorine atoms in the repeating unit showed higher gas permeability than that of poly(1b); furthermore, poly(2f) having trimethylsilyl and 3-methylindolyl groups exhibited relatively high gas permeability (). In the cyclic voltammograms of diphenylamino group-containing polymers, poly(1b) and poly(2b), the intensities of oxidation and reduction peaks decreased more than those of carbazolyl-containing poly(2a). The molar absorptivity (?) of poly(1b) at ∼700 nm increased with increasing applied voltage in the UV-vis spectrum.     

Optical and electroluminescent properties of polyfluorene copolymers and their blends

Light emitting properties of several polyfluorene (PF) copolymers (P1-P4) and their blends have been investigated. Light emitting diodes were fabricated with the configuration of ITO/PEDOT:PSS/polymer/Ca/Al. The EL peak wavelengths were 421 nm (violet), 505, 513 nm (green) and 570 nm (yellow) for PF copolymers and 510, 535 nm (green) for P1/P2 and P1/P3 blends, respectively. Förster energy transfer in the photoluminescence and electroluminescence of the polymer blends P1/P2 and P1/P3 was studied. The LED using the polymer blend P1/P2 showed a turn-on voltage of 2.5 V and a brightness of 5×10⁴ cd/m² at 7 V. The highest external quantum efficiency was observed to be 3.71% at 5 V. Upon addition of 20 wt% of the green emitter P2 to the violet emitter P1, the device efficiency increased from 1.18 to 3.71%.     

Structure-property relationships for a series of polyimide copolymers with sulfonated pendant groups

A series of high molecular weight, sulfonated polyimide copolymers (8a-f) with controlled acid contents have been obtained using 2,2′-bis(4-sulfobenzyloxy)benzidine (14) prepared via a flexible synthetic route. This series of novel sulfonated polyimide membranes were found to possess higher hydrolytic stability than polyimides in which the sulfonic acid groups are bound directly to the polymer main chain. An in-depth analysis of conductivity data was also performed for 8 and compared to the results for Nafion^® (1), sulfonated poly(ether ether ketone) (2) and a main-chain sulfonated polyimide (3). In order to remove the influence of acid strength, the proton mobility value for 8 at infinite dilution was calculated and found to be 1.2(±0.6) × 10⁻³ cm² s⁻¹ V⁻¹. A catalyst-coated membrane (CCM)-MEA based on a polyimide incorporating 60% sulfonated monomer (8d) was found to exhibit comparable beginning-of-life fuel cell performance as a Nafion^®-based CCM MEA at 50 °C.     

Copolymers containing pendant styryltriphenylamine and carbazole groups: Synthesis, optical, electrochemical properties and its blend with Ir(ppy)3

A series of vinyl copolymers (PVKST12-PVKST91) and homoploymer PVST containing pendant hole-transporting 4-(4-oxystyryl)triphenylamine (12-100 mol%) and carbazole chromophores were synthesized by radical copolymerization and employed as host for Ir(ppy)₃ phosphor to tune emission color. They were characterized using the ¹H NMR, FT-IR, absorption and photoluminescence spectra, elemental analysis, GPC, cyclic voltammetric and thermal analysis (DSC, TGA). Their weight-average molecular weights (M_w) and decomposition temperatures (T_d) were 1.46-5.68 × 10⁴ and 356-399 °C, respectively. The HOMO levels of PVKST12-PVKST91 and PVST, estimated from the onset oxidation potentials in cyclic voltammograms, were −5.40 to −5.14 eV, which are much higher than −5.8 eV of the conventional host poly(9-vinylcarbazole) (PVK) owing to high hole-affinity of the 4-(4-oxystyryl)triphenylamine groups. Therefore, copolymers PVKST are effective in reducing hole-injection barrier between the PEDOT:PSS and emitting layer. Electroluminescent devices [ITO/PEDOT:PSS/PVKST:Ir(ppy)₃:PBD/BCP/Ca/Al] using the hole-transporting PVKST as host were fabricated to tune the emission color. Their EL spectra showed a major emission at 515 nm and a minor peak at 435 nm attributed to Ir(ppy)₃ and 4-(4-oxystyryl)triphenylamine, respectively. The C.I.E. 1931 coordinates shift from (0.29, 0.61) for PVK to (0.33, 0.42) for PVST with an increase in 4-(4-oxystyryl)triphenylamine content.     

Characterization of manganese tetraarylthiosubstituted phthalocyanines self assembled monolayers

Manganese tetraarylthiosubstituted phthalocyanines (complexes 1-5) have been deposited on Au electrode surfaces through the self assembled monolayer (SAM) technique. SAM characteristics reported in this work are: ion barrier factor (∼1); interfacial capacitance (303-539 μF cm⁻²) and surface coverage (1.06 × 10⁻¹⁰-2.80 × 10⁻¹⁰ mol cm⁻²). Atomic force microscopy was employed in characterizing a SAM. SAMs of complexes 1-5 were employed to detect l-cysteine (with limit of detection ranging from 2.83 × 10⁻⁷ to 3.14 × 10⁻⁷ M at potentials of 0.68-0.75 V vs. Ag|AgCl) and nitrite (limit of detection ranging from 1.78 × 10⁻⁷ to 3.02 × 10⁻⁷ M at potentials of 0.69-0.76 V vs. Ag|AgCl).     

Polybinaphthyls incorporating chiral 2,2′-binaphthyl and isoquinoline moieties by Sonogashira reaction

Polymers P-1, P-2, P-3, P-4 and P-5 were synthesized by the polymerization of 5,8-bis(ethynyl)isoquinoline (M-1) with (R)-3,3′-diiodo-2,2′-bisbutoxy-1,1′-binaphthyl ((R)-M-2), (S)-3,3′-diiodo-2,2′-bisbutoxy-1,1′-binaphthyl ((S)-M-2), (R)-6,6′-dibromo-2,2′-bisbutoxy-1,1′-binaphthyl ((R)-M-3), (S)-6,6′-dibromo-2,2′-bisbutoxy-1,1′-binaphthyl ((S)-M-3), and rac-6,6′-dibromo-2,2′-bisbutoxy-1,1′-binaphthyl (M-4) under Sonogashira reaction, respectively. Both monomers and polymers were analyzed by NMR, MS, FT-IR, UV-vis spectroscopy, DSC-TGA, fluorescence spectroscopy, GPC and circular dichroism (CD) spectroscopy. CD spectra of polymers P-1 and P-2, P-3 and P-4 are almost identical except that they gave opposite signals at each wavelength. The long wavelength CD effect of P-1 and P-2 can be regarded as the more extended conjugated structure in the repeating unit and the helical backbone in the polymer chain. All five polymers have strong blue-green fluorescence due to the efficient energy migration from the extended π-electronic structure of the repeating unit of the polymers to the chiral binaphthyl core and are expected to provide understanding of structure-property relationships of the chiral conjugated polymers.     

Pd-catalyzed hydrosilylation polymerization of a dihydrosilane with diyne/triyne mixed systems affording crosslinked silylene-divinylene polymers and their properties

Treatment of a dihydrosilane (methylphenylsilane, 1) with mixtures of a diyne (p- or m-diethynylbenzene, 2a or 2b) and a triyne (1,3,5-triethynylbenzene, 3a or B,B′,B″-triethynyl-N,N′,N″-trimethylborazine, 3b; 1:2:3=100:95:5, 100:90:10, 100:80:20) in the presence of Pd-PCy₃ (Cy=cyclohexyl) catalyst gave new crosslinked silylenedivinylene polycarbosilanes. In TGA the resulting crosslinked polymers tended to show higher Td₅ values and higher char yields than the corresponding linear polymers. On the other hand, UV/vis absorption spectra of the crosslinked polymers obtained in the reactions of 2a or 2b with 3a exhibited increased broad peaks around 390 nm for 2a or 360 nm for 2b. Coincidently, their fluorescence spectra showed significant increase of the emission peaks in 400-550 nm. The crosslinked polymer derived from 2a and 3b, however, showed decrease of the absorption peak around 390 nm and profound depression of fluorescence peaks in 400-550 nm.     

Novel fluorene-based light-emitting copolymers containing cyanophenyl pendants and carbazole-triphenylamines: Synthesis, characterization and their PLED application

A series of novel blue light-emitting copolymers PCC-1, PCC-2, and PCC-3, composed of different ratios of electron-withdrawing segments (spirobifluorene substituted with cyanophenyl groups) and electron-donating segments (carbazole-triphenylamines), has been synthesized and characterized. In order to investigate the effect of hole/electron charge transporting segments, two reference polymers PSF and PCF, containing only one charge transporting moiety in the polymer backbone, were also synthesized. Incorporation of the rigid spirobifluorene units substituted with cyanophenyl groups into the polymer backbone improved not only the thermal stabilities but also the photoluminescence efficiencies. The polymers except PSF possess similar hole injection barriers but different hole transporting abilities. With the device configuration of ITO/PEDOT:PSS/polymers:PBD/CsF/Ca/Al, PCC-2 showed the best performance with the lowest turn-on voltage of 3.1 V, the highest luminance of 6369 cd/m², the highest current efficiency of 1.97 cd/A, and the best power efficiency of 1.40 lm/w.     

Photosensitive poly(benzoxazole) based on precursor from diphenyl isophthalate and bis(o-aminophenol)

A new synthetic method for the preparation of poly(benzoxazole) (PBO) precursor, poly(o-hydroxyamide) (7) from bis(o-aminophenol) (5) and diphenyl isophthalate (6) has been developed. Polymer 7 was prepared by the polycondensation of 5 and 6 in 1-methyl-2-pyrrolidinone (NMP) at 185-205 °C. Model reactions were carried out in detail to elucidate appropriate conditions for the formation of 2-hydroxybenzanilide (3) from o-aminophenol (1) and phenyl benzoate (2). The photosensitive (PBO) precursor based on polymer 7 containing a 22% of benzoxazole unit and 30 wt% 1-{1,1-bis[4-(2-diazo-1-(2H)naphthalenone-5-sulfonyloxy)phenyl]ethyl}-4-{1-[4-(2-diazo-1(2H)naphthalenone-5-sulfonyloxy)phenyl]methylethyl}benzene (S-DNQ) showed a sensitivity of 110 mJ cm⁻² and a contrast of 5.0 when it was exposed to 436 nm light followed by developing with a 2.38 wt% aqueous tetramethylammonium hydroxide solution at room temperature. A fine positive image featuring 8 μm line and space patterns was observed on the film of the photoresist exposed to 200 mJ cm⁻² of UV-light at 436 nm by the contact mode.     

Synthesis and characterization of alternating fluorene-based copolymers containing diaryl- and non-substituted bithiophene units

A series of soluble alternating fluorene-based copolymers containing diaryl- and non-substituted bithiophene units are synthesized by palladium-catalyzed Suzuki coupling reaction. All polymers demonstrate green colors of photoluminescence (PL) in chloroform, good thermal stability (with decomposition temperatures above 436 °C), and high glass transition temperatures (in the range of 120-144 °C). Owing to the large steric hindrance of diaryl substituents on bithiophenes in the polymers (P2-P4), the aggregation of solids is reduced as well as the solubility is improved, so the performance of their PLED devices are superior to that of the non-substituted polymer (P1). Compared with P1, the introduction of substitutents at 3,3′-position of bithiophene in P2-P4 has significant effects on the photophysical properties of resulting polymers in solution and solid states. Though the PL quantum yield of P1 is much higher than those of diaryl-substituted polymers (P2-P4), the PLED device of P1 has the worst electroluminescence (EL) properties due to the poor solubility of P1. Consequently, among these polymers, the device made of P3 as an emitter has the highest luminance of 2590 cd/m² at 9.5 V. For optimum device performance, a device of P3 blended with PVK can be further enhanced to a brighter luminance of 4284 cd/m² at 18 V.     

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%.