Highly efficient synthesis of thieno[3,4-b]thiophene derivatives and (opto)electrochemical properties of new low bandgap conjugated polymers

Various thieno[3,4-b]thiophene derivatives functionalized by n-octyl, 4-tert-butylphenyl, 4-n-butylphenyl, and 4-n-pentylphenyl were synthesized in a concise and efficient way. Previously reported synthetic processes were modified to produce target molecules in relatively high yields. Electrochemical and optical properties of polymers were examined by cyclic voltammetry and Vis–NIR spectrophotometry. The bandgap of electrochemically prepared polymers varied with substituents, ranging from 0.91 eV to 0.98 eV. While HOMO of conjugated polymers was raised by inductive effect of alkyl substituents, the bandgap was mainly determined by resonance stabilization of phenyl substituents.     

Low band gap conjugated polymers consisting of alternating dodecyl thieno[3,4-b]thiophene-2-carboxylate and one or two thiophene rings: Synthesis and photovoltaic property

In order to develop conjugated polymers with low band gaps and deep HOMO levels, copolymers consisting of alternating dodecyl thieno[3,4-b]thiophene-2-carboxylate and one (DTT-T₁) or two thiophene rings (DTT-T₂) were synthesized. The estimated optical band gap and HOMO level of DTT-T₁ and DTT-T₂ were ～1.58/1.61 and ～?5.15/?5.20 eV, respectively, indicating that the polymers have relatively low band gaps and deep HOMO levels, as compared to many other reported polymers. Photovoltaic devices were fabricated using DTT-T₂ and a fullerene derivative (PCBM), and whose power conversion efficiency was 0.21% under the illumination of AM 1.5 (100 mW/cm²). The low conversion efficiency of the devices was attributed to the inefficiency of exciton formation due to the low absorption coefficient and self-quenching of the polymer as well as the un-optimized device conditions.     

Phenyl-, carbazolyl- and fluorenyl-substituted derivatives of indolo[3,2-b]carbazole as hole-transporting glass forming materials

A series of new derivatives of indolo[3,2-b]carbazole containing phenyl, fluorenyl and carbazolyl substituents at the nitrogen atoms were synthesized by Ullmann coupling of 6-pentyl-5,11-dihydroindolo[3,2-b]carbazole with the different aryl halogenides. The optical, photophysical, photoelectrical and thermal properties of the materials obtained were studied. All the synthesized compounds can be transformed into the amorphous phase with the glass transition temperatures ranging from 0 to 154 °C. The ionization potentials of the newly synthesized derivatives of indolo[3,2-b]carbazole are in the range of 5.22–5.48 eV. The lowest energy absorption edges and the lowest ionization potentials were observed for carbazolyl-substituted derivatives. Charge transport properties of the synthesized materials were estimated by the time-of-flight technique. The highest hole drift mobilities were observed for the fluorenyl-substituted derivative. For the molecular glass of 5,11-bis(9,9-dibutyl-9H-fluoren-2-yl)-6-pentyl-5,11-dihydroindolo[3,2-b]carbazole they exceed 10^?3 cm²/V s at an electric field of 3.6 × 10⁵ V/cm.     

New tunable thieno[3,4-b]pyrazine-based materials

The effect of electron-donating and electron-withdrawing side chains on the electronic properties of thieno[3,4-b]pyrazines and their corresponding homopolymers have been investigated. Contrary to common trends in polythiophene materials, the addition of electron-donating groups results in higher HOMO–LUMO energies in the monomers and higher E_g values in the resulting polymers. The use of electron-withdrawing groups, however, provides reduced HOMO–LUMO energies and lower E_g values. The application of electron-withdrawing functionalized thieno[3,4-b]pyrazines appears to be a promising new approach for the production of low E_g materials.     

An optical study of a distyrylbenzene derivative with bulky substituents

A distyrylbenzene derivative with two tert-butyl groups and one benzyloxy group as bulky substituents in each outer phenylene ring (b-DSB) was synthesized. The compound dispersed in the polycarbonate (PC) matrix showed an emission in the blue region of 400–500 nm with a maximum intensity at 432 nm. When it was dispersed with 2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (PBD) in the PC matrix, an efficient energy transfer occurred from the photo-excited PBD to b-DSB in the ground state, resulting in a significant enhancement in the photoluminescence intensity of b-DSB. For example, up to five-fold increase of photoluminescence intensity of b-DSB was observed from the film containing 5 wt.% b-DSB and 30 wt.% PBD and four-fold increase from the film containing 20 wt.% b-DSB and 30 wt.% PBD.     

Investigation of electrochromic properties of poly(3,5-bis(4-methoxyphenyl)dithieno[3,2-b;2′,3′-d]thiophene)

Electropolymerization of 3,5-bis(4-methoxyphenyl)dithieno[3,2-b;2′,3′-d]thiophene BMPhDTT, having strong electron-donating methoxy groups, was performed, utilizing potentiodynamic method. The homopolymer was characterized by cyclic voltammetry (CV), Fourier transform infrared (FTIR) and UV–vis spectroscopy. Spectroelectrochemical and electrochromic properties of the homopolymer film were investigated and a PBMPhDTT/PEDOT device was constructed to understand its characteristics in detail. It was revealed that the potential range of 0.0–2.0 V is suitable for operating the device between yellow and blue colors. It indicated a good open circuit memory and stability.     

Polymer infrared photo-detector with high sensitivity up to 1100 nm

We reported a low band-gap conjugated polymer, poly[2,3-bis(4-(2-ethylhexyloxy)phenyl)-5,7-di(thiophen-2-yl)thieno[3,4-b]pyrazine] (PDTTP), was studied for the near infrared (NIR) photo-detector application. PDTTP shows intense absorption in NIR wavelength (to 1000 nm) and the estimated optical and electrochemical band-gaps of PDTTP are quite small around 1.15 eV and 1.08 eV, respectively. The low band-gap and the extended long wavelength absorption originates from the introduction of alternating TP units when its parent polythieno[3,4-b]pyrazine shows excellent narrow band-gap properties. Therefore, the relatively low band-gap and intense absorption in long wavelength of PDTTP make itself a promising candidate for near-infrared photo-detector. The hole mobility of the PDTTP measured from the bottom contact field effect transistor is around 1.40 × 10^?3 cm²/V s with a on/off ratio of 2100. The photo-detector based on bulk hetero-junction PDTTP and (6,6)-phenyl-C61-butyric acid methyl ester blend (PCBM) has the incident photon-to-electron conversion efficiency 28.9% at 1000 nm (?5 V) and 6.2% at 1100 nm (?5 V). This photo-detector can be operated at a high-speed of 1 MHz. The experimental result suggests the potential applications of low band-gap conjugated polymers on near-infrared photo-detectors.     

Synthesis and characterization of soluble narrow band gap conducting polymers based on diketopyrrolopyrrole and propylenedioxythiophenes

A series of novel electro-active conjugated polymers containing 2,5-dialkyl-3,6-di(thiophen-2-yl)pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-diones (DPPs) and 3,4-dihydro-3,3-dialkyl-2H-thieno[3,4-b][1,4]dioxepines (dialkyl-ProDOTs) were synthesized using Stille coupling reaction in presence of CuO. The molecular weights of the synthesized polymers were found to be in the range of 18,000–45,000. Incorporation of the electron deficient DPP units and the electron rich dialkyl-ProDOT units in the conjugated backbone leads to low band gap polymers. All the polymers were found to be highly soluble in most chlorinated organic solvents as well THF and toluene with excellent film forming properties. From the UV–vis spectra, the band gap of the polymers was determined as 1.40–1.42 eV which is lower than the poly(dialkylProDOT)s. From the electrochemical study, highest occupied molecular orbital (HOMO) energy levels of the synthesized polymers were found to be in the range of 5.54–5.51 eV. Because of such high HOMO level, the resulting polymers were found to be more oxidatively stable. Polymers are thermally stable till 325–346 °C with only 5% weight loss which was confirmed from thermogravimetric analysis (TGA). The polymers were found to be moderately conducting with maximum conductivity up to 0.2–6.0 S/cm.     

Synthesis and characterization of dithienothiophene vinylene based co-polymer for bulk heterojunction photovoltaic cells

An electron-donor–acceptor type co-polymer, PDTTBBO, composed of 3,5-dihexyldithieno [3,2-b:2′3′-d] dithiophene as donor and 2,6-dimethyl benzo[1,2-d; 5,4-d′] bisoxazole as an acceptor unit was synthesized by Horner–Wadsworth–Emmons (HWE) olefination reaction via multi-step procedure. The UV–vis absorption and photoluminescence emission peaks of the co-polymer in chlorobenzene were observed in a range from UV to near 700 nm. The lowest unoccupied molecular orbital (LUMO) and highest unoccupied molecular orbital (HOMO) levels of the co-polymer were estimated at ?5.71 eV and ?3.46 eV, respectively, corresponding to a band-gap of 2.25 eV. Bulk heterojunction photovoltaic cells were fabricated using a blend of PDTTBBO and PCBM with a ratio of 1:1. The short-circuit current density (J_sc), open-circuit voltage (V_oc) and fill factor (FF) of the device were estimated to be 1.306 mA/cm², 0.628 and 0.37, respectively, corresponding to power conversion efficiency (PCE) of 0.31% under AM 1.5 illumination.     

A novel 1,5-naphthylenediamine derivative as potential organic blue light-emitting material

A novel 1,5-naphthylenediamine derivative, 1,5-bis[N-(1-naphthyl)-N-phenyl]naphthalene diamine (NND), was designed and synthesized. This amine exhibited high glass transition temperature (T_g=127 °C) and hole transporting ability. The device with a structure of ITO/N,N′-bis(naphthalene-1-yl)-N,N′-diphenyl-benzidine(NPB)/NND/2-(4-tert-butylphenyl)-5-(4-biphenyl)-1,3,4-oxadiazole (PBD)/Mg:Ag was fabricated and bright blue light emission was obtained with a peak wavelength of 432 nm, and the color coordinate in CIE chromaticity is (0.172, 0.126). The brightness of 250 cd/m² at 14 V was achieved.     

Photoluminescent and electroluminescent properties of DCM-1 dispersed poly(p-phenylene vinylene) derivatives

The single layer devices utilizing poly[2-(carbazol-9-yl)-5-(2-ethylhcxyloxy)-1,4-phenylene vinylene] (CzEh-PPV) and poly[2-{4-[5-(4-tert-butylphenyl)-l,3,4-oxadiazolyl]-phenyl}-5-(2-ethythexyloxy)-l,4-phenylene vinylene] (OxdEh-PPV) doped with varying weight percent of 4-(dicyanomethylene)-2-methyl-6-[p-(dimethylamino)styryl]-4H-pyran (DCM-1) were fabricated and their photo luminescence (PL) and electroluminescence (EL) properties were discussed in this investigation. The PL spectra of DCM-1 doped polymers show that the emission is mostly from DCM-1 and Förster energy transfer may occur between DCM-1 and the two polymers. On the other hand, field-dependence of the emission spectra of EL devices was observed in detail. For the CzEh-PPV/DCM-1, the emission at the wavelength of 534 nm remains unchanged as the level of DCM-1 increases, whereas the peak at 572 nm is intensified with increasing both the additive level and applied electric field. For the OxdEh-PPV/DCM-1, the main peak is red-shift as the level of DCM-1 increases and blue-shift as the applied electric field does.     

Composites of low bandgap conducting polymer-wrapped MWNT and poly(methyl methacrylate) for low percolation and high transparency

The composites of multi-walled carbon nanotubes (MWNT) wrapped with low bandgap conjugated polymer and poly(methyl methacrylate) (PMMA) were prepared for transparent conductive films. NIR-absorbing poly(ethyl thieno[3,4-b]thiophene-2-carboxylate) (PTTEt) with E_g of 1.0 eV was used in this study. Upon hybridization with MWNT, PTTEt in an insulating state became partially conductive due to electron transfer from PTTEt to MWNT, meaning that PTTEt can function as conductive glue interconnecting MWNT in a PMMA matrix. The electrical conduction of the composites (PTTEt-MWNT/PMMA), consisting of PTTEt-wrapped MWNT (PTTEt-MWNT/PMMA) and PMMA, showed the percolation at 0.10 wt% MWNT loading, which was ca. 0.18 wt% lower than the composites of MWNT and PMMA (MWNT/PMMA). The maximum conductivity of PTTEt-MWNT/PMMA, on the other hand, was one order of magnitude lower than that of MWNT/PMMA, suggesting that PTTEt incorporation onto MWNT for transparent conductive films is effective within a specific range of MWNT loadings (i.e., between percolation thresholds of MWNT/PMMA and PTTEt–MWNT/PMMA). The comparison of transmittance of PTTEt–MWNT/PMMA (0.18 wt% MWNT) with MWNT/PMMA (0.32 wt% MWNT), possessing the same conductivities (3 × 10^?3 S cm^?1), showed ca. 10% enhanced transmittance at 550 nm. These results imply that hybridization of low bandgap conjugated polymers with carbon nanotubes can be utilized for the reduction of percolation threshold and the increase of optical transparency without sacrificing conductivities at low MWNT loadings.     

Thermal aging of single-layer polymer light-emitting diodes composed of a carbazole and oxadiazole containing copolymer doped with singlet or triplet emitters

An electron-transporting monomer was synthesized that was structurally and energetically similar to the small molecule 2-biphenyl-4-yl-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (tBu-PBD). The monomer was copolymerized with 2-(9H-carbazol-9-yl)ethyl 2-methylacrylate and the resulting copolymer was utilized in organic light emitting devices which employed fluorescent coumarin 6 (C6) or phosphorescent tris(2-phenylpyridine)iridium(III) [Ir(ppy)₃] emitters. The copolymer devices exhibited a mean luminance of ca. 400 and 3552 cd/m² with the C6 and Ir(ppy)₃ emitters, respectively, that was stable with thermal aging at temperatures ranging from 23 °C to 130 °C. Comparable poly(9-vinyl-9H-carbazole)/tBu-PBD blend devices exhibited more pronounced variations in performance with thermal aging.     

Synthesis,structure and properties of a novel quinoid compound

A novel quinoid compound α,α,α′,α′-tetra(4-tert-butylphenyl)-1,3,4-oxadiazole-quinodimethane (TPOQ) was synthesized through oxidative dehydrogenation in basic condition. The structure was characterized by elemental analysis, ¹H NMR, and mass spectrum. UV–vis spectra, photoluminescence (PL) spectra, and electron spin resonance (ESR) measurements showed that the compound had a small bandgap and π–π^* transition process produced an increase of polarity. Pure red emission at near 670 nm was observed from a single layer electroluminescence (EL) device, the emission color was unchanged with increase of applied voltage, which showed that TPOQ could be used as red emission material.     

Highly efficient bilayer blue phosphorescent organic light-emitting devices

We have developed highly efficient blue phosphorescent organic light-emitting devices comprising of two organic layers. Hole transporting 1,1-bis[(di-4-tolylamino)phenyl]cyclohexane (TAPC) was used as an emitting host for iridium(III)bis[(4,6-di-fluorophenyl)-pyridinato-N,C²′]picolinate (FIrpic) guest. In our bilayer system, the host–guest energy transfer process leads to a low optimal doping concentration of 2 wt%, while the better charge balance is achieved by the better electron injection into the host layer from electron transport layer. Using these bilayer structures, we demonstrate a maximum current efficiency of 34 cd/A in the device structure of ITO/TAPC: FIrpic (30 nm, 2 wt%)/3-(4-biphenyl-yl)-4-phenyl-5-(4-tert-butylphenyl)-1,2,4-triazole (50 nm)/LiF/Al.     

Uniform dispersion of triplet emitters in multi-layer solution-processed organic light-emitting diodes

The influence of the iridium complex solubility on the efficiency of multi-layer solution-processed organic light-emitting diode is demonstrated by synthesized orange triplet iridium complexes with the same core. The solubility of the iridium(III) bis(4-phenylthieno[3,2-c]pyridinato-N,C²′) acetylacetonate is increased and uniform dispersion of iridium complex in polymer host poly(vinylcarbazole) is achieved by tert-butyl and n-hexyl group modification. Blade coating technique is utilized to achieve tri-layer structures with a polymer hole-transporting layer poly[(9,9-dioctylfluorenyl-2,7-diyl)-co-(4,4′-(N-(4-s-butylphenyl))diphenylamine)], a host–guest emissive layer, and small-molecule hole-blocking layer 1,3,5-tris(N-phenylbenzimidazol-2-yl) benzene. The efficiency as high as 20 cd/A is achieved for orange-emitting device.     

Indolo[3,2-b]carbazole derivatives as hole transporting materials for electrophotography

Synthesis, thermal, optical and photoelectrical properties of N,N-diarylated indolo[3,2-b]carbazole derivatives, bearing six strategically placed alkoxy chains of different length, are reported. Electron photoemission spectra of the synthesized derivatives have been recorded and the ionization potentials have been established. It was found that the best charge transport properties were shown by 5,11-di(1,2,3-trioctadecyloxyphenyl)indolo[3,2-b]carbazole. Room temperature hole-drift mobility in its 50% solid solution in bisphenol Z polycarbonate established by the xerographic time of flight technique was found to exceed 10^?6 cm² V^?1 s^?1 at an electric field of 10⁶ V cm^?1.     

Light-emitting diodes based on alternating copolymers containing fluorene and oligo(p-phenylenevinylene)

We synthesized poly[(9,9-di-n-hexylfluorene-2,7-diyl)-alt-co-(2,5-bis(4′-cyanostyryl)benzene-1,4-diyl)] [P(FOPV-CN)] and poly[(9,9-di-n-hexylfluorene-2,7-diyl)-alt-co-(2,5-bis(4′-diphenylaminostyryl)benzene-1,4-diyl)] [P(FOPV-Am)]. These polymers have two axes of longitudinal π-conjugation: one in the direction of the polymer backbone and the other in the direction of the oligo(p-phenylenevinylene) chain. From the spectroscopic study, the emissive color of poly[(9,9-di-n-hexylfluorene-2,7-diyl)-alt-co-(benzene-1,4-diyl)] changed from blue to green by substituting cyanostyryl or diphenylaminostyryl group at the 2,5-positions of the benzene ring. The electronic structures were significantly influenced by the substituents at the ends of oligo(p-phenylenevinylene) moiety. The diphenylamino substituent which has more extended π-conjugation resulted in red-shifted electronic absorption and emission spectra compared to the cyano substituent groups. In the light-emitting diodes (LEDs) based on these two polymers, the turn-on voltage (ca. 9 V) of the device based on P(FOPV-Am) was lower than that of P(FOPV-CN) (ca. 12 V) due to the electron-donating effect of the amino groups. The maximum brightness of the P(FOPV-Am)-based LED was 3100 cd m⁻² at 22 V which is much higher than that of the P(FOPV-CN)-based LED (27 cd m⁻² at 26 V).     

Syntheses and characterization of new low-band gap polymers containing 4H-cyclopenta[def]phenanthrene unit and 4,7-di(thien-2-yl)-2H-benzimidazole-2-spirocyclohexane for photovoltaic device

Two new low-band gap polymers, poly[(2,6-(4,4-bis(2′-ethylhexyl)-4H-cyclopenta[def]phenanthrene))-alt-(5,5-(4′,7′-di(thien-2-yl)-2H-benzimidazole-2′-spirocyclohexane))] (PCPP-DTCHBI) and poly[(2,6-(4,4-bis(4-((2-ethylhexyl)oxy)phenyl)-4H-cyclopenta[def]phenanthrene))-alt-(5,5-(4′,7′-di(thien-2-yl)-2H-benzimidazole-2′-spirocyclohexane))] (PBEHPCPP-DTCHBI), were synthesized and characterized for the photovoltaics. These polymers showed typical characteristics of low-band gap polymers through the internal charge transfer (ICT) between 4H-cyclopenta[def]phenanthrene as the electron-rich unit and di(thien-2-yl)-2H-benzimidazole-2′-spirocyclohexane as the electron-deficient unit. The highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels are ?5.52 and ?3.82 eV for PCPP-DTCHBI, and ?5.36 and ?3.76 eV for PBEHPCPP-DTCHBI, respectively. Optical band gaps of PCPP-DTCHBI and PBEHPCPP-DTCHBI are 1.70 and 1.60 eV, respectively. As compared to the case of poly[(2,6-(4,4-bis(2′-ethylhexyl)-4H-cyclopenta[def]phenanthrene))-alt-(5,5-(4′,7′-di(thien-2-yl)-2,1,3-benzothiadiazole))] (PCPP-DTBT), PCPP-DTCHBI shows deeper HOMO energy levels by 0.12 eV and lower band gap by 0.3 eV. The FET mobilities of PCPP-DTCHBI and PBEHPCPP-DTCHBI are 1.19 × 10^?4 and 5.11 × 10^?5 cm²/V s, respectively, and the power conversion efficiencies of the solar cell devices of the PCPP-DTCHBI and PBEHPCPP-DTCHBI blended with [6,6]phenyl-C₇₁-butyric acid methyl ester (PC₇₁BM) are 1.01% and 0.53%, respectively. The newly designed DTCHBI unit can be used as the electron-deficient moiety inducing efficient ICT for low band gap generation while keeping deep HOMO energy level of the polymer.     

Synthesis,optical and electrochemical properties of novel hole-blocking poly(biphenylene-1,3,4-oxadiazole) containing electron-withdrawing trifluoromethyl groups

New poly(biphenylene-1,3,4-oxadiazole) P1 and poly(biphenylene hydrazide) P2 containing electron-withdrawing trifluoromethyl group at the 2 and 2′ positions of biphenyl moiety were synthesized. The biphenyl is forced to adapt a non-coplanar conformation due to the bulky trifluoromethyl group. High quality polymer P1 thin film can be easily obtained by thermal cyclodehydration from its soluble polyhydrazide precursor P2. The polymer P1 exhibited good thermal stability with glass transition temperature of 234 °C and 5% decomposition temperature of 469 °C. The optical and electrochemical properties were investigated by UV–vis spectroscopy, photoluminescence spectroscopy and cyclic voltammetry. For polymer P1 film, two absorption peaks at 370 and 414 nm were observed. It also exhibited a photoluminescent peak at 555 nm when excited by 414 nm light. The cyclic voltammetric studies revealed that polymer P1 had extremely low HOMO (?6.85 eV) and LUMO (?3.71 eV) energy levels due to the presence of strong electron-withdrawing trifluoromethyl group. It also exhibited a large energy gap (3.14 eV) which is an indication of short conjugation length resulted from non-coplanar biphenyl structure. Its HOMO energy was even lower than that of widely used hole-blocking material, 2-(4-biphenyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole PBD (?6.30 eV). The low LUMO energy of polymer P1 could allow the easier electron injection from air-stable cathode such as aluminum. The good electron transfer ability was also shown by measuring the current density of electron-only devices with the structures of Al/P1 (150 nm)/Alq₃ (50 nm)/Al and Al/Alq₃ (200 nm)/Al. Combined with high thermal stability and amorphous morphology, polymer P1 would be a promising candidate as the hole-blocking material for organic light-emitting diodes.