Charge transfer and optoelectronic properties in the triarylamine-based donor–π bridge–acceptor dyes for dye-sensitised solar cells

The triarylamine–based donor–π bridge–acceptor dyes (namely, Ds-3, Ds-5 and Ds-6), with the higher conversion efficiency of sunlight to electricity, have been studied with quantum chemistry methods. The geometrical structure, frontier molecular orbital and electronic vertical excitation energies were calculated by using the density functional theory (DFT) and the time-dependent DFT with the Cam-B3LYP and PBE0 functional. From the calculated results, we perform a three-dimensional real-space analysis, which demonstrates that the lowest energy excited state of the triarylamine-based dye is a charge transfer (CT) excited state and electrons shift from triarylamine to cyanoacrylic acid group. The excited-state oxidation potentials and driving force energy are identified as the essential parameters to study the electron injection ability of the excited dyes. The evaluation of photochemical parameter and the visualised study of CT process provide the important information for revealing the relationship between structure and photochemical property of the triarylamine-based dyes.     

The effect of anchoring group number on molecular structures and absorption spectra of triphenylamine sensitizers: a computational study

The molecular structures and absorption spectra of triphenylamine dyes containing different numbers of anchoring groups (S1-S3) were investigated by density functional theory (DFT) and time-dependent DFT. The calculated geometries indicate that strong conjugation is formed in the dyes. The interfacial charge transfer between the TiO₂ electrode and S1-S3 are electron injection processes from the excited dyes to the semiconductor conduction band. The simulated absorption bands are assigned to π → π* transitions according to the qualitative agreement between the experimental and calculated results. The effect of anchoring group number on the molecular structures, absorption spectra and photovoltaic performance were comparatively discussed.     

Development of Next‐Generation Organic‐Based Solar Cells: Studies on Dye‐Sensitized and Perovskite Solar Cells

Next‐generation organic solar cells such as dye‐sensitized solar cells (DSSCs) and perovskite solar cells (PSCs) are studied at the National Institute of Advanced Industrial Science and Technology (AIST), and their materials, electronic properties, and fabrication processes are investigated. To enhance the performance of DSSCs, the basic structure of an electron donor, π‐electron linker, and electron acceptor, i.e., D–π–A, is suggested. In addition, special organic dyes containing coumarin, carbazole, and triphenylamine electron donor groups are synthesized to find an effective dye structure that avoids charge recombination at electrode surfaces. Meanwhile, PSCs are manufactured using both a coating method and a laser deposition technique. The results of interfacial studies demonstrate that the level of the conduction band edge (CBE) of a compact TiO₂ layer is shifted after TiCl₄ treatment, which strongly affects the solar cell performance. Furthermore, a special laser deposition system is developed for the fabrication of the perovskite layers of PSCs, which facilitates the control over the deposition rate of methyl ammonium iodide used as their precursor.     

Cyano or o-nitrophenyl? Which is the optimal electron-withdrawing group for the acrylic acid acceptor of D-π-A sensitizers in DSSCs? A density functional evaluation

We report a DFT, TDDFT and DFTB investigation of the performance of two donor-π-acceptor (D-π-A)-type organic dyes bearing different electron-withdrawing groups (EWG) for dye-sensitized solar cells (DSSCs) to evaluate which EWG is better for an acrylic acid acceptor, i.e., Cyano (–CN) or o-nitrophenyl (o-NO₂–Ph). A series of theoretical criteria applied successfully in our previous work to explain the different performance of organic dyes related to open-circuit photovoltage (V _oc) and short-circuit current density (J _sc) were used to evaluate the performance of the dyes with just different EWG. Our calculated results reveal that dye 2 with o-NO₂–Ph has a larger vertical dipole moment, more electrons transferred from the dye to the semiconductor and a lower degree of charge recombination, which could lead to larger V _oc; while the larger driving force and comparable light harvesting efficiency could lead to higher J _sc , highlighting the potential of o-NO₂–Ph as an EWG in an acrylic acid acceptor.

Two Channels of Charge Generation in Perylene Monoimide Solid‐State Dye‐Sensitized Solar Cells

The mechanism of charge generation in solid‐state dye‐sensitized solar cells using triarylamine‐substituted perylene monoimide dyes is studied by vis‐NIR broadband pump‐probe transient absorption spectroscopy. The experiments demonstrate that photoinduced electron injection into the TiO₂ can only occur in regions where Li⁺, from the commonly used Li‐TFSI additive salt, is present on the TiO₂ surface. Incomplete surface coverage by Li⁺ means that some dye excitons cannot inject their electron into the TiO₂. However it is observed in the solar cell structure that some of the dye excitons that cannot directly inject an electron still contribute to free charge generation by the previously hypothesized reductive quenching mechanism (hole transfer to the solid‐state hole transporter followed by electron injection from the dye anion into the TiO₂). The contribution of reductive quenching to the quantum efficiency of charge generation is significant, raising it from 68% to over 80%. Optimization of this reductive quenching pathway could be exploited to maintain high quantum efficiency in dyes with greater NIR absorption to achieve overall enhancements in device performance. It is demonstrated that broadband NIR transient spectroscopy is necessary to obtain population kinetics in these systems, as strong Stark effects distort the population kinetics in the visible region.     

Similar or Totally Different: the Adjustment of the Twist Conformation Through Minor Structural Modification,and Dramatically Improved Performance for Dye‐Sensitized Solar Cell

A novel approach for enhancing the performance of dye‐sensitized solar cells is presented. It is based on the analysis of five sensitizers by utilizing triarylamine as donor, thiophene benzothiadiazole as chromophore and substituted thienyl linked with cyanoacrylic acid as the anchoring group (LI‐80‐LI‐84). Accompanied with the increasing steric hindrance of the substituents on the thienyl isolation group, the conformation of the dyes, in particular the angle between the chromophore and the anchoring group, becomes more and more twisted. Surprisingly, sensitizers with poorer conjugation effects (the higher twisted conformation) achieve better photovoltaic performances, showing a contrary trend to the traditional donor‐(π‐spacer)‐acceptor dyes with a better co‐planarity. On the basis of the preceding fundamental comprehensions, an empirical method is successfully applied to a new phenyl‐based system (LI‐85 and LI‐86) to improve their performances. The systematical investigation indicates that the twisted structures can contribute to the E_CB of the TiO₂ film, electron lifetime and resistance at the TiO₂/dye/electrolyte interface. Thereby, the efficiency of the initial LI‐80‐based cell has been dramatically improved to 2.45 times higher for LI‐86‐based cell, paving a new way for the design of better sensitizers with higher device performances.     

Performance of density functionals for the structure and energetics of (M–O)0,± (M=Al,Si, Sc–Zn)

We report the results of the performance of 20 exchange–correlation functionals of density functional theory (DFT) in the structure (Metal–Oxygen bond length) and energetical properties (bond dissociation energy, adiabatic ionisation energy, and adiabatic electron affinity) of twelve metal monoxides (M–O, M=Al, Si, Sc–Zn). The calculated results show that the selected DFT functionals have the ability to reproduce the M–O bond length with a mean deviation of 0.01–0.05 Å, the energy values are reproduced with a mean deviation of 0.20–1.00?eV. In general, the functionals with significant HF exchange show decent performance in the calculation of bond length and harmonic vibrational frequency. These functionals show poor performance in energetics. Our calculated results show that the M06-L, B3LYP, and TPSSh functionals give good performance in both structure and energetical properties of metal monoxides. These functionals are recommended for the studies of structure and energetics in metal oxide systems. Further, our studies indicate that M06-L can be used for the studies in larger molecular systems. Among the 20 DFT functionals, the recently developed N12 functional gives poor performance in the studies of metal monoxides. Hence this functional is not recommended for the studies of structure and energetics in metal oxide systems.     

Decolorization of azo dyes using Basidiomycete strain PV 002

Summary Basidiomycete PV 002, a recently isolated white-rot strain from decomposed neem waste displayed high extracellular peroxidase and rapidly decolorized azo dyes. In this study, the optimal culture conditions for efficient production of ligninolytic enzymes were determined with respect to carbon and nitrogen. An additional objective was to determine the efficiency of PV 002 to degrade the azo dyes. White-rot strain PV 002 efficiently decolorized Ranocid Fast Blue (96%) and Acid Black 210 (70%) on day 5 and 9 respectively under static conditions. The degradation of azo dyes under different conditions was strongly correlated with the ligninolytic activity. The optimum growth temperature of strain PV 002 was 26 °C and pH 7.0.     

Laccase-catalyzed decolorization of the synthetic azo-dye diamond black PV 200 and of some structurally related derivatives

The kinetics of laccase-catalyzed transformation of the azo-dye Diamond Black PV 200 (CI Mordant Black 9) and various related synthesized derivatives were analyzed for dependence on pH and substrate structure. The reaction mixture of Diamond Black PV 200 was analyzed by HPLC/MS–MS and it was shown that upon laccase oxidation, reactive chinoid fragments of lower molecular weight were formed. These may further oligomerize as indicated by the appearance of a number of compounds with increased molecular weight. The pH optimum for the decolorization was pH 5 for Diamond Black PV 200 which did not change significantly when the substitution pattern of its basic structure was varied. Biodegradability, however, was strongly dependent on the structure of the dyes.     

Excitation Dynamics in Low Band Gap Donor–Acceptor Copolymers and Blends

Donor–acceptor (D–A) type copolymers show great potential for the application in the active layer of organic solar cells. Nevertheless the nature of the excited states, the coupling mechanism and the relaxation pathways following photoexcitation are yet to be clarified. We carried out comparative measurements of the steady state absorption and photoluminescence (PL) on the copolymer poly[N‐(1‐octylnonyl)‐2,7‐carbazole]‐alt‐5,5‐[4′,7′‐di(thien‐2‐yl)‐2′,1′,3′‐benzothiadiazole] (PCDTBT), its building blocks as well as on the newly synthesized N‐(1‐octylnonyl)‐2,7‐bis‐[(5‐phenyl)thien‐2‐yl)carbazole (BPT‐carbazole). The high‐energy absorption band (HEB) of PCDTBT was identified with absorption of carbazoles with adjacent thiophene rings while the low‐energy band (LEB) originates instead from the charge transfer (CT) state delocalized over the aforementioned unit with adjacent benzothiadiazole group. Photoexcitation of the HEB is followed by internal relaxation prior the radiative decay to the ground state. Adding PC₇₀BM results in the efficient PL quenching within the first 50 ps after excitation. From the PL excitation experiments no evidence for a direct electron transfer from the HEB of PCDTBT towards the fullerene acceptor was found, therefore the internal relaxation mechanisms within PCDTBT can be assumed to precede. Our findings indicate that effective coupling between copolymer building blocks governs the photovoltaic performance of the blends.     

Impact of UV‐Visible Light on the Morphological and Photochemical Behavior of a Low‐Bandgap Poly(2,7‐Carbazole) Derivative for Use in High‐Performance Solar Cells

This paper reports on the photochemical behavior upon exposure to UV‐visible light of a poly(2,7‐carbazole) derivative for use in high‐performance solar cells. Poly[N‐9′‐hepta‐decanyl‐2,7‐carbazole‐alt‐5,5‐(4′,7′‐di‐2‐thienyl‐2′,1′,3′‐benzothiadiazole)] (PCDTBT) is one of a relatively large class of push‐pull carbazole‐based copolymers that have been synthesized to better harvest the solar spectrum. The 2,7‐carbazole building block of PCDTBT is also used with different electron‐accepting units in a large variety of low‐band‐gap polymers. The photochemical and morphological behavior of PCDTBT thin films is investigated from the molecular scale to the nanomechanical properties. The photo‐oxidation mechanism is shown to be governed by chain‐scission and cross‐linking reactions. It results in dramatic evolution of the morphology, roughness and stiffness of thin PCDTBT films. Based on the identification of several photoproducts formed along the macromolecular chains or released into the gas phase, the main pathways of PCDTBT photochemical evolution are discussed. These processes first involve the scission of the C–N bond between the carbazole group and the tertiary carbon atom bearing the alkyl side‐chain. Modifications of the chemical structure of PCDTBT, the evolution of its UV‐visible absorbance, and its nanomechanical properties initiated by light irradiation are shown to be closely related.     

DFT and TDDFT study on the electronic structure and photoelectrochemical properties of dyes derived from cochineal and lac insects as photosensitizer for dye-sensitized solar cells

Essential parameters related to the photoelectrochemical properties, such as ground state geometries, electronic structures, oxidation potential and electron driving force, of cochineal insect dyes were investigated by DFT and TDDFT at the B3LYP/6-31+G(d,p) level of the theory. The results show that the major charge flow dynamic for all dyes is the HOMO→LUMO transition. The bi-coordinated binding mode, in which the dye uses one carboxyl- and hydroxyl oxygen bound to Ti(IV), is found for all dye-TiO₂ systems. Additionally, the doubly bi-coordinated binding mode in which the dye used both carboxyl groups bound to two Ti(IV) is also possible due to high energy distribution occupied at anchoring groups. This study highlights that most of these insect dyes can be good photosensitizers in dye-sensitized solar cells based on their strong binding to the TiO₂ surface, good computed excited state oxidation potential and thermodynamically favored electron driving force.     

Nanofiber‐Based Bulk‐Heterojunction Organic Solar Cells Using Coaxial Electrospinning

Nanofibers consisting of the bulk heterojunction organic photovoltaic (BHJ–OPV) electron donor–electron acceptor pair poly(3‐hexylthiophene):phenyl‐C₆₁‐butyric acid methyl ester (P3HT:PCBM) are produced through a coaxial electrospinning process. While P3HT:PCBM blends are not directly electrospinnable, P3HT:PCBM‐containing fibers are produced in a coaxial fashion by utilizing polycaprolactone (PCL) as an electrospinnable sheath material. Pure P3HT:PCBM fibers are easily obtained after electrospinning by selectively removing the PCL sheath with cyclopentanone (average diameter 120 ± 30 nm). These fibers are then incorporated into the active layer of a BHJ–OPV device, which results in improved short‐circuit current densities, fill factors, and power‐conversion efficiencies (PCE) as compared to thin‐film devices of identical chemical composition. The best‐performing fiber‐based devices exhibit a PCE of 4.0%, while the best thin‐film devices have a PCE of 3.2%. This increase in device performance is attributed to the increased in‐plane alignment of P3HT polymer chains on the nanoscale, caused by the electrospun fibers, which leads to increased optical absorption and subsequent exciton generation. This methodology for improving device performance of BHJ–OPVs could also be implemented for other electron donor–electron acceptor systems, as nanofiber formation is largely independent of the PV material.     

Controlling light absorption and photoelectric properties of coumarin-triphenylaminedye by different acceptor functional groups

The ground state and excited state properties of three coumarin dyes, ZCJ1, ZCJ2 and ZCJ3, including ground state structures, energy levels, absorption spectra and driving forces of electron injection, were investigated via density functional theory (DFT) and time-dependent density functional theory (TD-DFT). In addition, five new molecules ZCJ3-1, ZCJ3-2, ZCJ3-3, ZCJ3-4 and ZCJ3-5 were designed through the introduction of a –CN group into molecule ZCJ3. The ground state and excited state properties of the five designed molecules were also calculated and compared with that of the original molecule, aiming to investigate the effect of different position of –CN groups on the optical and electrical properties of dye molecules. Moreover, the external electric field was taken into account. The results indicated that all three original molecules have better absorption within the visible-light range, and the molecule with a thiophene–thiophene conjugated bridge enables a red shift of the absorption spectrum. The molecule with a thiophene–benzene ring conjugated bridge enables the increase of driving force of electron injection. The energy levels, spectra and driving force of electron injection for the designed molecules are discussed in terms of studying their potential utility in dye-sensitized solar cells.     

Biodegradation of carbazole by Ralstonia sp. RJGII.123 isolated from a hydrocarbon contaminated soil

The use of microorganisms for bioremediation of contaminated soils may be enhanced with an understanding of the pathways involved in their degradation of hazardous compounds. Ralstonia sp. strain RJGII.123 was isolated from soil located at a former coal gasification plant, based on its ability to mineralize carbazole, a three-ring N-heterocyclic pollutant. Experiments were carried out with strain RJGHII.123 and 14C-carbazole (2 mg/L and 500 mg/L) as the sole organic carbon source. At 15 days, 80% of the 2 mg/L carbazole was recovered as CO2, and <1% remained as undegraded carbazole, while 24% of the 500 mg/L carbazole was recovered as CO2 and approximately 70% remained as undegraded carbazole. Several stable intermediates were formed during this time. These intermediates were separated by high performance liquid chromatography (HPLC) and were characterized using high resolution mass spectroscopy (HR-MS) and gas chromatography - mass spectroscopy (GC-MS). At least 10 ring cleavage products of carbazole degradation were identified; four of these were confirmed as anthranilic acid, indole-2-carboxylic acid, indole-3-carboxylic acid, and (1H)-4-quinolinone by comparison with standards. These data indicate that strain RJGII.123 shares aspects of carbazole degradation with previously described Pseudomonas spp., and may be useful in facilitating the bioremediation of NHA from contaminated soils.     

TD-DFT investigation of D–π–A organic dyes with thiophene moieties as π-spacers for use as sensitizers in DSSCs

The geometrical, conformational, and electronic properties of a series of D–π–A metal-free dyes designed for use as sensitizers in DSSCs were studied using DFT and TD-DFT methods. A substituted triphenylamine moiety was used as the donor group and 2-cyanoacrylic acid as the acceptor group in these dyes. They also contained conjugated bridging π-linker groups containing two or more thiophene rings to enhance the intramolecular charge transfer. The B3LYP, M06-HF, ωB97XD and CAM-B3LYP functionals were utilized in combination with the 6-31G(d,p) basis set for the calculations. The dye solvation process was taken into account via the polarizable continuum model. To rationalize the relationships between dye structure and the photochemical properties of the dyes when used as sensitizers in DSSCs, the vertical excitation energies, the light-harvesting efficiencies, the free-energy changes during the process of injecting an electron into the surface of a TiO₂ nanocrystalline semiconductor, and the open-circuit potentials were calculated for all of the dyes in the solvent THF using the above methods. The results of these computations are discussed and compared with the available corresponding experimental data.     

Polyoxometalate–biomolecule conjugates: A new approach to create hybrid drugs for cancer therapeutics

Some polyoxometalate (POM) clusters have demonstrated attractive anticancer properties. Unfortunately, their cytotoxicity upon normal cell is one of fateful side effects obstructing their further clinic application as inorganic drugs. In this communication, we report a new approach to create hybrid drugs potentially for cancer therapeutics. At first, the POM cluster bioconjugates were created by attaching the bioactive ligands on an amine grafted POM via simple amidation reaction. The cytotoxicity study with breast cancer cells (MCF-7 and MDA-MB-231) and non-cancerous breast epithelial cell (MCF-10A) showed that rationally selected ligands with cancer-cell targeting ability on POM–biomolecule conjugates can impart enhanced anti-tumor activity and selectivity, thus representing a new concept to develop novel POM–biomolecule hybrid drugs with the potential synergistic effect: increased bioactivity and lower side effect.     

Antibody binding modulates the dynamics of the membrane-bound prion protein

Misfolding of the cellular prion protein (PrP^C) is associated with lethal neurodegeneration. PrP^C consists of a flexible tail (residues 23–123) and a globular domain (residues 124–231) whose C-terminal end is anchored to the cell membrane. The neurotoxic antibody POM1 and the innocuous antibody POM6 recognize the globular domain. Experimental evidence indicates that POM1 binding to PrP^C emulates the influence on PrP^C of the misfolded prion protein (PrP^Sc) while the binding of POM6 has the opposite biological response. Little is known about the potential interactions between flexible tail, globular domain, and the membrane. Here, we used atomistic simulations to investigate how these interactions are modulated by the binding of the Fab fragments of POM1 and POM6 to PrP^C and by interstitial sequence truncations to the flexible tail. The simulations show that the binding of the antibodies restricts the range of orientations of the globular domain with respect to the membrane and decreases the distance between tail and membrane. Five of the six sequence truncations influence only marginally this distance and the contact patterns between tail and globular domain. The only exception is a truncation coupled to a charge inversion mutation of four N-terminal residues, which increases the distance of the flexible tail from the membrane. The interactions of the flexible tail and globular domain are modulated differently by the two antibodies.     

Molecularly imprinted microspheres as SPE sorbent for selective extraction of four Sudan dyes in catsup products

A highly selective molecularly imprinted solid-phase extraction (MISPE) coupled with high performance liquid chromatography (HPLC) ultraviolet-visible detection was developed for the simultaneous isolation and determination of four Sudan dyes (I, II, III and IV) in catsup products. The novel molecularly imprinted microspheres (MIM) were synthesized by aqueous suspension polymerization using phenylamine and naphthol as template, which showed high affinity to Sudan dyes in aqueous solution. In order to develop a selective extraction protocol for simultaneous determination the four Sudan dyes from catsup products, the molecular recognition properties of MIM as a SPE sorbent were evaluated. Under the optimized condition, good linearity was obtained from 0.01 to 2.5 μg g(-1) (r(2)≥ 0.9990) with the relative standard deviations of less than 3.4%. This proposed MISPE-HPLC procedure eliminated the effect of template leakage on quantitative analysis and could be applied to direct determination of four Sudan dyes in complicated food samples.