Synthesis,characterization and comparative photovoltaic behavior of diketopyrrolopyrrole (DPP)-containing poly(phenylene vinylene)s varying at N-alkyl or 3,6-aryl substituents in DPP units

Three new diketopyrrolopyrrole (DPP)-containing poly(phenylene vinylene)s varying at N-alkyl or 3,6-aryl substituents in DPP units, abbreviated as C6DPPDHPV, C6DTDPPDHPV, and C62DTDPPDHPV, were designed to comparatively study the structural effects on their photophysical, electrochemical properties, molecular organization and photovoltaic properties. The polymers exhibited remarkably broad UV–vis absorption spectra covering a major part of the wavelength range of visible light in the solar spectrum, and small bandgaps between 1.4 and 1.9 eV. X-ray diffraction and cyclic voltammetry measurements revealed that N-alkyl and 3,6-aryl in DPP unit played a key role in tuning their molecular organization as well as electrochemical behavior. The bulk heterojunction photovoltaic devices employing the three polymers as electron donors, together with PC₆₁BM as electron acceptor, were examined. C6DPPDHPV based device showed the highest energy conversion efficiency of 0.72%. Lower hole mobilities and relatively higher HOMO energy levels may be responsible for inferior performance of C6DTDPPDHPV and C62DTDPPDHPV-based devices. Moreover, the C6DPPDHPV was preliminarily evaluated as active material in organic thin film transistor.     

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.     

Electrosynthesis of 2,7-linked polycarbazole derivatives to realize low-bandgap electroactive polymers

Poly(2,7-carbazole) derivatives have been synthesized because they have more extended conjugation lengths and lower energy bandgaps than poly(3,6-carbazole)s; however, few studies regarding electrochemical synthesis of poly(2,7-carbazole)s have been reported. Here, a series of N-alkylated-2,7-di(2-furyl)carbazoles, N-butyl-2,7-di(2-thienyl)carbazole, and N-butyl-2,7-di(2-(3,4-ethylenedioxthienyl))carbazole, were synthesized to obtain electroactive films of poly(2,7-carbazole) derivatives by electrochemical polymerization. Cyclic voltammetry revealed that these monomers have excellent polymerization activity, due to their low oxidation potentials (<0.57 V vs. ferrocene (Fc/Fc⁺)), and the corresponding polymers exhibit good redox properties. The energy bandgaps of the polymers obtained from optical absorption spectra range from 2.1 to 2.3 eV. Among the polymers, poly[N-butyl-2,7-di(2-(3,4-ethylenedioxthienyl))carbazole] shows the lowest bandgap energy of 2.1 eV, which is lower than that of previously reported poly(3,6-carbazole) analogues (2.4 eV). This polymer exhibits a significant color change from red in the oxidized state to blue in the reduced state during an electrochemical redox process. The electrochemical and optical properties of the monomers are dependent on external heteroaromatic rings attached to the 2 and 7 positions of the carbazole unit.     

Photoexcitations in disubstituted polyacetylene: solitons and polarons

We studied the photoexcitations in films of disubstituted polyacetylene, a π-conjugated polymer with degenerate ground state. We found that the polymer in its pristine form supports charged and neutral topological soliton excitations. Photo-oxidized films or polymer/C₆₀ blends show, in addition, polaron excitations. At the same time the polymers show a strong emission band with high quantum efficiency, which leads to stimulated emission in thin films and lasing in cylindrical μ-cavities. These excitation properties are unique among polymers of both degenerate and non-degenerate ground state.     

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.     

Modulating spectroelectrochemical properties of [Ni(salen)] polymeric films at molecular level

Electroactive polymer films based on [Ni(salen)]-type complexes were fabricated and their electronic properties characterized using in situ UV–visible spectroelectrochemistry. The extent of π electronic delocalisation and electronic asymmetry were manipulated by introduction of different conjugated imine bridges. Measured electronic spectra were interpreted in terms of polaronic states in the band gap and metal-oxidized ligand charge transfer bands. Density functional theory (DFT) calculations for the monomers showed that the HOMO orbital (which governs oxidation potential) is ligand-dominated, and that substituents with greater electronic delocalisation in the diimine bridge decrease the HOMO–LUMO energy gap. Replacement of methyl by methoxyl substituents in the aldehyde moiety increases the calculated dipole moment. Substitution-driven variations in E_HOMO–E_LUMO for the monomers were reflected in the corresponding polymer band gaps, demonstrating that monomer electronic properties can be used predictively in the manipulation of polymer electronic properties. An important strategic aspect is the correlation of DFT predictions with the observed electronic properties of monomeric and polymeric materials; the extent to which such modelling can be used to optimise synthetic effort is discussed.     

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.     

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.     

Grain size dependent bandgap shift of SnO2 nanofibers

SnO₂ nanofibers with various grain sizes ranging from 18.5 to 31.6 nm in diameter were fabricated by electrospinning a polymeric solution and subsequent controlled calcination of the as-spun fibers. The calcined fibers were polycrystalline and composed of densely packed nano-sized SnO₂ grains. The effect of the nanograin size on the optical bandgap of SnO₂ nanofibers was examined by ultraviolet-visible spectroscopy. The bandgap showed a strong dependence on the nanograin size. The bandgap decreased with increasing nanograin size. Some calculations were performed to understand the relationship between the experimentally obtained bandgaps of the SnO₂ nanofibers and the theoretical ones. Quantum confinement and lattice strain of the SnO₂ nanofibers are likely responsible for the bandgap shift. This suggests that optimization of the nanograin size is essential not only for achieving the required optical properties of oxide nanofibers, but also to secure superior working properties of electronic devices that are fabricated with electrospinning-synthesized oxide nanofibers.     

Terthiophene-cyanovinylene π-conjugated polymers as donor material for organic solar cells

Conjugated polymers of hybrid structure containing cyanovinylene linkages have been synthesized by Knoevenagel condensation of diformyl terthienyls with para-dicyanomethylbenzene. UV–vis and cyclic voltammetric data show that these polymers combine reduced band gap, improved light-harvesting properties and low lying HOMO level. Whereas the very low solubility of the polymers did not allow the fabrication of bulk heterojunction solar cells, bilayer heterojunction solar cells have been realized using thermally evaporated films of fullerene C₆₀ as acceptor material. The best devices show a maximum external quantum efficiency of ～20% and a power conversion efficiency of 0.40% under simulated AM 1.5 solar illumination.     

Synthesis of selenopheno[3,2-c]thiophene derivatives and (opto)electrochemical properties of new low bandgap conjugated polymers

Various selenopheno[3,2-c]thiophene (STh) derivatives functionalized by 4-n-butylphenyl, 4-n-pentylphenyl, 4-tert-butylphenyl, and n-octyl were newly synthesized in a concise and efficient way. Electrochemical and optical properties of polymers were examined by cyclic voltammetry and visible-near infrared (Vis-NIR) spectrophotometry. When compared with polymers of thieno[3,4-b]thiophene, poly(selenopheno[3,2-c]thiophene) (PSTh) was more easily oxidized by ca. 0.2 V (higher HOMO). On the other hand, the bandgap of electrochemically prepared PSTh varied with substituents, but showed similar values to those of poly(thieno[3,4-b]thiophene). While the resonance effect of phenyl substituents slightly lowered HOMO, the combination of resonance and inductive effects decreased LUMO more effectively, resulting in the lowest bandgap of 0.91 eV for 4-tert-butylphenyl functionalized PSTh.     

Synthesis and characterization of low bandgap poly(dithienosilole vinylene) derivatives

Two low bandgap conjugated polymers containing vinylene unit on their main chains, poly(dithienosilole vinylene) derivatives P1 and P2, were synthesized by Pd-catalyzed Stille-coupling method. The two polymers show good solubility in common organic solvents, broad absorption bands from 500 nm to 700 nm and lower energy bandgap of 1.72 eV for P1 and 1.77 eV for P2. The electronic energy levels of the polymers were measured by electrochemical cyclic voltammetry. The HOMO and LUMO energy levels of P1 and P2 are at ?5.03 eV, ?5.18 eV and ?3.39 eV, ?3.43 eV, respectively. The polymers P1 and P2 containing dithienosilole units show lower HOMO and LUMO energy levels, in comparison with other poly(thienylene vinylene) derivatives.     

Synthesis and photovoltaic performance of new diketopyrrolopyrrole (DPP) dyes for dye-sensitized solar cells

Two new metal-free organic dyes (DPP-I and DPP-II) with diketopyrrolopyrrole (DPP) core were designed and synthesized, in which triphenylamine or N,N-bis(4-methoxyphenyl)benzenamine moieties was used as the electron donor, DPP units as the π-conjugated bridge, and carboxylic acid group as the electron acceptor. Photophysical and electrochemical properties of two dyes were investigated by UV–vis spectrometry and cyclic voltammetry. Electrochemical measurement data indicate that the tuning of the HOMO and LUMO energy levels can be conveniently accomplished by alternating the donor moiety. The DSSC based on dye DPP-I showed better photovoltaic performance: a maximum monochromatic incident photon-to-current conversion efficiency (IPCE) of 80.6% corresponding to an overall conversion efficiency of 2.68%. Although the power conversion efficiencies are not so high, this work explores new donor–π-accepter–π-donor models and the effects of molecular design on optical properties.     

Effect of acceptor substituents on photophysical and photovoltaic properties of triphenylamine–carbazole alternating copolymers

Three alternative copolymers of carbazole (Cz) and triphenylamine (TPA) with substituent containing acceptor group of aldehyde (PCzTPA-CHO), mono-cyano (PCzTPA-CN) or di-cyano (PCzTPA-DCN), were designed and synthesized. Through manipulating the acceptor group attached to TPA unit, the electronic properties and energy levels of the copolymers were effectively tuned. The presence of pendant cyano groups leads to a decrease in the LUMO levels, but influences the HOMO levels very little, so that results in a lower bandgap and red-shift of its absorption spectrum. These results suggest a simple and effective approach for tuning the bandgap in a conjugated polymer through modification of the pendant acceptor groups. Their optoelectronic properties are largely determined by the nature of pendant substituent. Polymer solar cell based on PCzTPA-CN as donor and PC₇₀BM as acceptor demonstrates a power conversion efficiency of 0.81% with a high V_oc of 0.93 V.     

Possibility to Use Hydrothermally Synthesized CuFeS<Subscript>2</Subscript> Nanocomposite as an Acceptor in Hybrid Solar Cell

Here we have approached the plausible use of CuFeS₂ nanocomposite as an acceptor in organic–inorganic hybrid solar cell. To produce CuFeS₂ nanocomposite, hydrothermal strategy was employed. The room-temperature XRD pattern approves the synthesized material as CuFeS₂ with no phase impurity (JCPDS Card no: 37-0471). The elemental composition of the material was analyzed from the TEM-EDX data. The obtained selected area electron diffraction (SAED) planes harmonized with the XRD pattern of the synthesized product. Optical band gap (4.14 eV) of the composite from UV–Vis analysis depicts that the synthesized material is belonging to wide band gap semiconductor family. The HOMO (? 6.97 eV) and LUMO (? 2.93 eV) positions from electrochemical study reveal that there is a possibility of electron transfer from MEH-PPV to CuFeS₂. The optical absorption and photoluminescence spectra of MEH-PPV:CuFeS₂ (donor:acceptor) composite were recorded sequentially by varying weight ratios. The monotonic blue shifting of the absorption peak position indicated the interaction between donor and acceptor materials. The possibility of electron transfer from donor (MEH-PPV) to acceptor (CuFeS₂) was approved with photoluminescence analysis. Subsequently, we have fabricated a hybrid solar cell by incorporating CuFeS₂ nanocomposite with MEH-PPV in open atmosphere and obtained 0.3% power conversion efficiency.     

Electronic structure of conducting π-electron systems

We will discuss the application of one-electron band theory to several systems where π-electron delocalization has been experimentally established. We will use polyacetylene as our prototype and extend results on it to related polymers such as (SN)_x, polypyrrole and poly(p-phenylene). We will particularly concentrate on those aspects of chemical bonding and symmetry which govern whether a given system will be semiconducting or metallic, and which scale its transport properties.     

Effect of donor moiety in organic sensitizer on spectral response,electrochemical and photovoltaic properties

This paper describes the effects of donating ability in a D (donor)–π–A (acceptor) structured organic dye on the HOMO and LUMO energy levels and photovoltaic performance in dye-sensitized solar cell. Two different dyes were synthesized based on the cyanoacetic acid as an acceptor and the phenyl-thiophene as a π-conjugated bridge, where one contains a diethyl amino group (P2) and the other has an ethoxy group (P5) as the donor. The HOMO–LUMO energy gap was significantly influenced by donor group, where change of ethoxy donor with stronger amino one in donating ability decreased the energy gap from 2.84 eV to 2.21 eV. Moreover, it was also found that HOMO energy level was more sensitive to change in donor group than LUMO one. Photovoltaic performances of P2- and P5-dye-sensitized solar cells were systematically investigated in terms of the effects of co-adsorbent concentration, TiO₂ film thickness and TiCl₄ post-treatment. Under the best condition, the P5 dye showed the conversion efficiency of 2.62%, while the P2 dye exhibited much higher efficiency of 5.89% due to broader absorption.     

Investigations on the configuration of new arenemethylenes for the synthesis of low bandgap polymers

The new arenemethylenes 9 and 13 have been synthesized by a Knoevenagel-type condensation, as precursor molecules for polyarenemethylenes (PAMs, 1) which are, based on theoretical calculations, low bandgap polymers. The configuration of the monomers was investigated, because this is a criterion to determine whether an electrochemical polymerization of the compounds could lead to linear polymers. All of the annulated polyarenemethylidenes 1 have a Z,Z-configuration, using the methods of preparation shown.     

Electronic properties of mislinked polypyrrole and polythiophene

Electronic properties of polypyrrole and polythiophene involving α-β linkages have been studied using the one-dimensional tight-binding SCF-CO (self consistent field-crystal orbital) method. It has been found that in such polymers the degree of π-conjugation is reduced and that the nature of the band gap varies compared with polymers consisting of only α-α′ (normal) linkages. It is mentioned that formation of the bipolaron structure along several pyrrole or thiophene rings with α-β linkages will be difficult.     

Hückel theory applied to large linear and cyclic conjugated π-systems. Part II

The band gap in ideal heteropolyarenes has been calculated by Hückel molecular orbital theory. The previously described method, in which the actual calculations are carried out on hypothetical ring compounds with Hückel or Möbius topology from the smallest repeating units of the polymer, has been used. Non-planar polymers are considered as well as the question of the repeating unit. Simple methods for the evaluation of π-electron bandwidths are suggested. Calculations of band gaps in polymers in which the repeating units are linked by more than one bond have been carried out.