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Electric field effect on multi-anchoring molecular architectures: Electron transfer process and opto-electronic property
Abstract
The molecular structures and optoelectronic properties of single/double-anchoring phenothiazine-based dyes (DCE1/DCE2) were calculated based on density functional theory (DFT) and time-dependent DFT (TD-DFT). There are two primary objectives: one is to study the influence of different number anchoring types on the overall efficiency, and the other is to shed light on how a local electric field affects the performance of double-anchoring dyes DCE2. The calculated results indicate that, DCE2 has larger harvesting efficiency (LHE), electron accepting power (ω⁺) and dipole moment (unormal) as well as lower reorganization energy (λtotal) and chemical hardness (η), which lead to a higher short circuit current density (Jsc) and open-circuit voltage (Voc) compared to DCE1. Under a condition of electric field, the unormal of DCE2 with the increase of electric field strength has increased as compared to nonelectric field environment, leading to a larger VOC. Furthermore, the electron accepting power (ω⁺) is increased and the chemical hardness (η) is decreased, which will further improve the Jsc. Therefore, the results show that the electric field is beneficial to improve the performance of double-anchoring DCE2, and the electric field is an important way to further enhance efficiency of DSSCs.
... Therefore, considering the obtained results, the dyes with the best properties are TPAZ7, TPAZ4, TPAZ3, and TPAZ5. The chemical hardness received particular attention in this prediction regarding previous studies reported [5,[88][89][90]. It can be recommended to synthesize and experimentally research dyes with azomethine on the π-bridge in DSSC. ...
Ten molecules were theoretically calculated and studied through density functional theory with the M06 density functional and the 6-31G(d) basis set. The molecular systems have potential applications as sensitizers for dye-sensitized solar cells. Three molecules were taken from the literature, and seven are proposals inspired in the above, including the azomethine group in the π-bridge expecting a better charge transfer. These molecular structures are composed of triphenylamine (donor part); different combinations of azomethine, thiophene, and benzene derivatives (π-bridge); and cyanoacrylic acid (acceptor part). This study focused on the effect that the azomethine group caused on the π-bridge. Ground-state geometry optimization, the highest occupied molecular orbital, the lowest unoccupied molecular orbital, and their energy levels were obtained and analyzed. Absorption wavelengths, oscillator strengths, and electron transitions were obtained via time-dependent density functional theory using the M06-2X density functional and the 6-31G(d) basis set. The free energy of electron injection (ΔGinj) was calculated and analyzed. As an important part of this study, chemical reactivity parameters are discussed, such as chemical hardness, electrodonating power, electroaccepting power, and electrophilicity index. In conclusion, the inclusion of azomethine in the π-bridge improved the charge transfer and the electronic properties of triphenylamine-based dyes.
... For DSSCs, the core component is the sensitizing agent, which is divided into two kinds: metal-free organic dyes and metallic dyes [8]. Among them, organic dyes have the characteristics of low price, high extinction coefficient, adjustable structure, and light absorption characteristics by molecular design [9,10]. In order to achieve higher PCE, development of new structures, materials, and technologies has become important for researchers to improve conversion efficiency. ...
This paper theoretically simulated (using DFT and TD-DFT in N,N-dimethylformamide (DMF) solvent) the photodynamic properties of three non-metallic dye molecules with D-π-A1-π-A2 structure. The total photoelectric conversion efficiency (PCE) could be evaluated by the following parameters: the geometric structures, the electronic structures, and the absorption spectra, the analyses of charge difference density (CDD) and natural bond orbitals (NBO), the analyses of ionization potential (IP) and electron affinity (EA) from electronic contribution capacity, the reorganization energies (λh, λe, and λtotal), and the chemical reaction parameter (h, ω, ω-, and ω+) for intramolecular charge transfer (ICT) processing, the excited lifetime (τ) and the vertical dipole moment (μnormol). The ∆Ginject, the ∆Gdyeregen, the light harvesting efficiencies (LHE) and the excited lifetime (τ) were used to explain experimental JSC. The experimental trend of VOC was explained by the calculation of ∆ECB and μnormol. Moreover, the 15 dyes were designed by adding the electron-donor groups (–OH, –NH2, and –OCH3) and the electron-acceptor groups (–CF3, –F, and −CN) to the LS-387 molecular skeleton, which improved electronic contribution, intramolecular charge transfer (ICT), and optoelectronic performance.
The photovoltaic characteristics and panchromatic photocatalysis based on monomers and bithiophene‐chlorophyll dyads is investigated. The optical properties and the electron transport capacity of the titled dyes are explicated by investigating the geometry, electronic structure, electron injection, and recombination, intramolecular charge transfer, regeneration process, dye aggregation, and light‐harvesting. The results manifest that the original 3,3′‐dithioalkyl‐2,2′‐bithiophene (SBT)‐based molecule behaves better with photovoltaic performance and an enhanced J SC and V OC . For molecular design, the designed dyads not only show the enhanced absorption range and prolonged excited lifetimes but also noticeable electron transfer ability. Moreover, the strong interfacial interaction, fast electron transfer process, long recombination distance, inhibition in dye aggregate, enhanced regenerative interaction energy are elucidated, which is beneficial for a better photocurrent response. However, the slight shift of the conduction band edge in dyads significantly reduces the V OC , especially for SBT‐Chl. Among dyads, SBT‐BChl exhibits outstanding photovoltaic performance of 14%. Besides, photocatalysis analysis manifests that the dyad composites behave with red‐shifted spectra, enhanced dipole moments, better interfacial interaction to ensure electron transfer, which significantly improved the photocatalytic performance of TiO 2 . Our study is expected to offer theoretical guidelines for further screening the promising D‐π‐A‐bacteriochlorin dyads in photoelectric functional materials.
This paper focuses on incorporation of graphene oxide (GO) to metal‐free phthalocyanine (MPc) through only hydrogen bonding and π‐π stacking. Briefly, Pc‐GO composites at various concentrations were prepared by self‐assembly method. The processing time was kept below 10 minutes to avoid covalent attachment and we aimed at answering the research question of what will happen if the conjugation is realized only through hydrogen bonding under extremely limited processing times. The as‐prepared MPc‐GO composites were characterized by FT‐IR, UV‐Vis, SEM and fluorescence analysis. We report that the interaction between MPc and GO could immediately be initiated upon mixing of corresponding solutions. Also, complete conjugation by hydrogen bonding and π‐π stacking could be reached even only in 5 minutes of sonication time. In addition, it was also determined that the prepared MPc‐GO composites are stable at room conditions and during dilution. Finally, the optoelectronic properties of MPc and MPc‐GO composites were also investigated experimentally and theoretically. Both experimental and theoretical results suggest that MPc‐GO composites exhibit improved optoelectronic properties as comparing to MPc, eventhough the conjugation of GO to MPc was only via hydrogen bonding without covalent attachment.
The photophysical, electrochemical and photovoltaic properties of the title sensitizers as isolated monomers and co-sensitizers are systematically investigated for co-sensitized solar cells. Firstly, the monomers are clarified in a geometric configuration, optical property, and other microscopic parameters. Secondly, the heterodimers are simulated to reveal the electronic structures and multistep electron transfer mechanism in co-sensitized films. Thirdly, an evaluating model based on D-A-π-A dye coupled with a chlorophyll derivative (Chl) co-sensitized solar cells is established to evaluate short-circuit current (JSC), open-circuit voltage (VOC), fill factor (FF), and photoelectric conversion efficiency (PCE). Results demonstrate that MA1116 behaves as chief light harvest efficiency (LHE), superior electron transport capacity, suppressed aggregation and the energetical regeneration process among the monomers. The spectroscopic property, excited-state lifetime, and interfacial electron injection rate of the co-sensitized films are improved versus monomers. The LHE of MA1116-Chl is notably improved, and long-distance charge transfer can occur between Chl and MA1116. Furthermore, the estimated PCEs of monomers agree with the experimental data, and the co-sensitized solar cells achieve enhanced PCEs. Evaluating monomers and co-sensitized films from the overall photovoltaic properties and the heterodimer simulation can explain experiment results, providing a new protocol for co-sensitized solar cells to upgrade efficiency.
Modifying backbone, side chains and end groups of non-fullerene acceptors (NFAs) in organic photovoltaics (OPVs) can tune their properties, and then impact on performance. Hence, understanding how the different functional...
Electrochemical oxidation of phenothiazine (PTZ) was investigated in an acetonitrile/water (50/50, v/v) mixture using differential pulse voltammetry, cyclic voltammetry, choronopotentiometry, constant current electrolysis and controlled-potential coulometry. Our findings show that the oxidation pathway of PTZ at different pH values does not follow a simple pattern and varies with respect to pH. Our results show that the oxidation process at pH ≤ 1, occurs via two one-electron steps, while this process at 1 < pH ≤ 7 is a two-electron process. Surprisingly, this oxidation at pH values > 7 again turns into two one-electron steps. We have performed this study to achieve the following goals. (a) To gain in-depth insights into the oxidation mechanism of PTZ and (b) the development of a new strategy for PTZ dimerization. To achieve the second goal, we performed electrooxidation of PTZ at pH 3. Using the obtained electrochemical data, we succeeded in synthesizing two new types of PTZ dimer (D1 and D2). The structures of D1 and D2 are fully characterized by ¹H NMR, ¹³C NMR, mass spectrometry, and FTIR techniques. The synthesis of D1 and D2 is performed in a mixture of acetonitrile and water (50/50, v/v), without the use of the oxidant or catalyst, using an undivided cell and carbon electrode, under mild conditions, using an air-assisted electrochemical method.
Electrochemical properties of two new derivatives of phenothiazine, i.e., 3,7-bis(4-aminophenylamino)phenothiazin-5-ium chloride (PhTz-(NH2)2) and 3,7-bis(4-carboxyphenylamino)phenothiazin-5-ium chloride (PhTz-(COOH)2), has been investigated on glassy carbon electrode. The pH influence on their electrode reactions was specified. The compounds studied showed complex kinetics of electrode reactions. Heterogeneous constants of the electron transfer at pH = 7.0 calculated from the Tafel plot were equal to 8.3 × 10⁻⁴ and 7.4 × 10⁻⁴ cm s⁻¹ and transfer coefficients to 0.52 and 0.36 for PhTz-(NH2)2 and PhTz-(COOH)2, respectively. For carboxylate derivative, significant influence of the pH of working solution on the transfer coefficient was mentioned. Based on cyclic voltammetry, quartz crystal microbalance and spectroscopy of electrochemical impedance, deposition of oxidation products was found for PhTz-(NH2)2 and PhTz-(COOH)2 in multiple potential cycling. The carboxylic groups critically influenced electropolymerization of appropriate compound. The formation of bonds typical for electropolymerization was also confirmed by diffusion reflectance IR spectra. The electropolymerization product exerted high electrochemical activity within a broad pH range. Heterogeneous rate constant of the electropolymerized product was found to be 4.5 × 10⁻¹ s⁻¹.
Five D-π-A-A type of heterocyclic polycyclic aromatic hydrocarbons (hetero-PAHs) organic molecules with quinacridone (QA) derivatives as the core bridge connected with different donor groups of triarylamine (T), indoline derivative (W), and carbazole (K) and the auxiliary acceptor groups benzobisthiadiazole (B) and furan (F) have been designed. The potential application of designed sensitizers in solar cells and photocatalysis have been investigated. Microscopic major processes involve dye regeneration, electrons recombination, intramolecular charge transfer (ICT) properties and key parameters of molecular photoelectric performance. Besides, the coupling strength, energy gaps, dipole moments, molecular fluorescent lifetime and the bonding type between dye and TiO2 were estimated to reveal the nature of photocatalysis. Results indicated that introducing W or B unit should improve the photoelectric performance among all design strategies (especially, simultaneously introducing two moieties) due to the excellent electron-donating ability of W and the pull electrons' ability of B auxiliary acceptor. Designing [email protected]2 and [email protected]2 to have better photocatalytic properties owing to the stronger interaction and surface charge transfer, particularly for WQAB. Current molecular strategies using controlling moieties provide a choice for potential applications in solar cells and photocatalytic fields.
Herein, we introduce a novel redox mediator based ionic liquid for selective and sensitive detection of sulfite, which is one of the challenging areas of research in food quality control and environmental safety. A phenothiazine imidazolium ionic liquid with hexafluorophosphate counter anion (PTZ-IL) was designed and synthesized as molten salt and used as redox mediator in modified electrode preparation. PTZ-IL was immobilized onto multiwalled carbon nanotubes (MWCNT) deposited glassy carbon electrode (GCE) to fabricate PTZ-IL/MWCNT/GCE modified electrode. The electrochemical performance of PTZ-IL/MWCNT/GCE was examined using cyclic voltammetry (CV) and the fabricated sensor displayed a redox couple with anodic and cathodic peak potentials at +0.707 V and +0.594 V, attributed to phenothiazine/phenothiazine radical cation redox couple. The PTZ-IL/MWCNT/GCE showed impressive electrocatalytic activity towards the oxidation of sulfite, based on which an amperometric sensor was developed for sulfite detection. The nonenzymatic sensor thus developed has shown a wide linear range of 30–1177 μM with limit of detection of 9.3 μM and sensitivity of 282.2 μA mM⁻¹ cm⁻² for sulfite detection. Furthermore, the modified electrode has shown prominent stability and reproducibility.
Two new groups, cyclopentadipyrrole and dihydropyrroloindole, that has been applied for other relevant regions, are firstly employed as π group to compose the organic dyes, (E)-3-(6-(10-butyl-10H-phenothiazin-2-yl)-4,4-dimethyl-4,7-dihydro-1H-cyclopenta [2,1-b:3,4-b′]dipyrrol-2-yl)-2-cyanoacrylic acid (PCP3) and (E)-3-(7-(10-butyl-10H-phenothiazin-2-yl)-1,8-dihydropyrrolo[3,2-g]indol-2-yl)-2-cyanoacrylic acid (PBP4), in DSSCs along with phenothiazine as donor and cyanoacrylic acid as acceptor. Their performance should be screened out before it would be fabricated in device. When the frontier molecular orbitals (FMO) and absorption spectrum properties are studied, the performance of PCP3 and PBP4 is not better than that of (E)-3-(6-(10-butyl-10H-phenothiazin-2-yl)-4,4-dimethyl-4H-cyclopenta[2,1-b:3,4-b′]dithiophen-2-yl)-2-cyanoacrylic acid (PCT1) and (E)-3-(7-(10-butyl-10H-phenothiazin-2-yl)benzo[2,1-b:3,4-b′]dithiophen-2-yl)-2-cyanoacrylic acid (PBT2). Even when the dye-TiO2 adsorbed system is considered, the differences among four organic dyes are too small to distinguish them, which is also perplex for most of theoretical studies. To have a reliable and accurate results, the values of short-circuit current density (JSC), open-circuit voltage (VOC), and photon-to-electron conversion efficiency (PCE) for DSSCs on the basis of four organic dyes are accurately determined. Although the JSC of PCP3 is the smaller than that of PCT1, the PCE of PCP3 is comparable with that of PCT1 due to the larger VOC. The quantitative calculation is an effective method to make a distinction among every designed dyes.
The molecular structures and photophysical properties of three D–A–π–A dyes (WS-2, WS-92 and WS-95) were calculated based on density functional theory (DFT) and time-dependent DFT (TD-DFT). The results show that different donor (D) groups and π-spacer bridges affect the photophysical properties. WS-95 presents the highest light harvesting efficiency (LHE), maximum absorption peak, dipole moment (μnormal), electron affinity (EA), electrophilicity (ω) and electron accepting power (ω⁺), as well as a small natural bond orbital (NBO) charge of the electron acceptor, ionization potential (IP) and chemical hardness (h). These critical parameters have a close relationship with the open-circuit photo-voltage (VOC) and the short-circuit current density (JSC), and lead to WS-95 exhibiting higher efficiency. In order to obtain an efficient dye, we designed a series of dyes based on WS-95 and analyzed their optical and electronic properties. It was found that upon introducing a CN group into WS-95 (as in the designed molecules 1, 2, 3, 7, 8 and 9), the band gap energies of the dyes were decreased, leading to an absorption peak red-shift compared to WS-95. The structural modifications also result in an increase in the electrophilicity and electron accepting power, and a decrease in the chemical hardness. When –NH2 is introduced into WS-95 (4, 5, 6, 10, 11 and 12), the opposite trend is observed compared to dyes 1, 2, 3, 7, 8 and 9. We hope that these results will be helpful for experiments to synthesize new and highly efficient dyes.
Optical active layers of the multi-polar and multi-branched organic solar cell dye molecules were analyzed by using DFT and TD-DFT methods. Optical active layers contain triphenylamine (TPA) as a donor section and diketopyrrolopyrrole (DPP) as a part of the molecular conjugated bridge. Relative optical and electronic properties have been calculated. The results showed that with the increasing of molecular structure, LUMO energy levels and energy gaps decrease. The decrease of energy gap is beneficial to the red-shift of absorption peak, and TPA(DPP-P)2 displays the maximum red-shift. Ionization potentials decrease and electron affinities increase with the increase of molecular structure. The hole and electron recombination energies of TPA(DPP-P)2 is the smallest, so TPA(DPP-P)2 has a better hole transport capability. For developing their application for DSSC, four molecules are designed on the basis of the original molecules, named after TPA–DPP-1, TPA–DPP-2, TPA(DPP-P)2-1, TPA(DPP-P)2-2, respectively. The effect of the number and the site of functional groups on the photoelectric properties of the designed molecules were studied. Absorption spectra, electron injection free energies, light harvesting efficiencies and the dye regeneration free energies were also calculated. The data shows that two –CNs were introduced into the molecules, energy gaps decrease and absorption peak makes red-shift. The introduction of a –CN is favorable to improve the electron injection free energies and dye regeneration free energies. The introduction of a side chain makes LHE increase; however, make absorption peak blue-shift. Through evaluating the absorption ability and charge transfer process for designed molecules, it is found that the photoelectric properties of the molecules can be regulated and improved by introducing appropriate acceptor group, which can provide valuable information for further experiment.
The technology to convert sunlight into electric current by employing organic photovoltaic systems has been an issue of an intensive research over the last two decades and nowadays receives an increasing industrial interest. Dye-sensitised solar cells (DSSCs) provide a technically and economically credible alternative to the present well-known p-n junction or thin film photovoltaic devices. In this Perspective, we review the basic concepts of DSSCs and we specifically discuss the recent theoretical research aimed at optimising DSSC's properties, as well as several of the controversial and emerging issues in this field.
Conceptual Density Functional Theory (DFT) has proposed a set of local descriptors to measure the reac-tivity on specific sites of a molecule, as an example dual descriptor has been successfully used in analyz-ing interesting systems to understand their local reactivity, however under the frozen orbital approximation (FOA), it is defined from non-degenerate frontier molecular orbitals (FMOs). In this work, the degeneration is taken into account to propose approximated expressions to obtain the dual descrip-tor, nucleophilic and electrophilic Fukui functions in closed-shell systems. The proposed expressions have been tested on molecules presenting degenerate FMOs.
Three donor-linker-acceptor triphenylamine-based cyanoacrylic acid organic dyes used for dye-sensitized solar cells (DSCs) have been examined with respect to their effect on the open-circuit voltage (V(oc)). Our previous study showed a decrease in V(oc) for DSCs based on dyes with increased molecular size (increased linker conjugation). In the present study, we investigate the origin of V(oc) with respect to (i) conduction band (E(CB)) positions of TiO(2) and (ii) degree of recombination between electrons in TiO(2) and electrolyte acceptor species at the interface. These parameters were studied as a function of dye structure, dye load, and I(2) concentration. Two types of behavior were identified: the smaller polyene dyes show a surface-protecting effect preventing recombination upon increased dye loading, whereas the larger dyes enhance the recombination. How the different dye structures affect the recombination is discussed in terms of dye surface blocking and intermolecular interactions between dyes and electrolyte acceptor species.
Three organic dyes (XY1, XY2 and XY3), synthesized by inserting a thiophene heterocycle and shifting the position of benzothiadiazole (BTZ) auxiliary acceptor on the cyclopentadithiophene(CPDT) π-bridge, are theoretically investigated using density functional theory (DFT) and time-dependent DFT (TD-DFT) for their potential applications in solar cells. The photoelectric paremeters (short-circuit current, open circuit voltage, conversion efficiency) are evaluated by the following factors: energy levels and the orbital compositions, the absorption spectra, NBO analysis from S0 to S1, the qualitative descriptions of DCT, Δr, S and ΔQ for intramolecular charge transfer(ICT) processing, the adsorption energy (Eads) and the excited lifetime (t), the shift of CB and vertical dipole moment (μnormal), the electron injection lifetime (τ), the Marcus rate of inner-path recombination, the simulation of outer-path recombination dye...I2 and dye regeneration [dye+•I-], etc. The Eads, the excited lifetime (t) and the electron injection lifetime (τ) explain the order of experimental JSC, and the trend of Voc is interpreted by the calcuations on the ΔECB, μnormal, the Marcus rate and the dye...I2 mode. Among three molecules, XY2 possesses the significance advantage in ICT mechanism and dye regeneration ability based on analysis of qualitative descriptions and two-step regeneration mechanism. Moreover, twelve dyes are designed by the substitution of π-bridge of XY2, which provide a strategy to improve the abilities of absorption and electron injection as well as photoelectric properties on the comparison of XY2.
In this work, the vibration-resolved photoinduced electron transfer of an organic conjugated D∙∙A system subjected to an external electric field was theoretically investigated. The ground and excited state vibrational relaxation energies were quantitatively characterized. The effective high frequency, ωeff, could be estimated from the variation in energy of the excited-state equilibrium geometries of acceptor and donor sites as well as the analysis of the vibrational modes upon electron transfer. For the PCDTBT:PC70BM blend in an external electric field, the vibronic modes affected the charge separation process differently from the charge recombination process. The simulated results indicated that the vibrational quantum tunneling effect facilitated the charge recombination process to a large extent, supporting the mutual Columbic attraction between photogenerated extractable free charges. Thus, for electron transfer reactions, considering the vibrational excitation influence and perturbed nucleus-electron interactions is essential. These results provide a feasible way to enhance the efficiency in yielding the electron transfer process products.
Mono/double-anchored phenothiazine-based dyes (DCE1–DCE4) containing various N-substituents (12-crown-4-substituted phenyl, 4-hexoxyphenyl, and bare phenyl) have been synthesized and applied as the sensitizers for dye-sensitized solar cells (DSSCs). 12-Crown-4-substituted phenyl entity on the double-anchored dye effectively retards the dark current via Li+ chelation in the case of LiI redox mediator or steric blockade of Co-phen3+ in the case of cobalt redox mediator. The retarded dark current was further confirmed from the recombination rate constant for the reduction of the oxidized redox mediators at the photoanode/electrolyte interface by rotating disk electrode technique. The DCE2-based DSSC with iodide-based electrolyte exhibited a good cell efficiency up to 8.82%, which surpassed that of the N719-based DSSC (8.20%). With cobalt-based electrolyte, higher cell efficiencies of 10.12% and 11.17% were obtained at 1 sun and 0.1 sun, respectively, using a Pt-free and TCO-free counter electrode.
Three benzimidazole-based organic dyes, possessing the same triphenylamine donors and cyanoacrylic acid acceptors with the bithiophene ?-bridges combined in different nuclear positions of benzimidazole, were investigated in the utility of dye-sensitizer solar cells. The structure, molecular orbital and energy, absorption spectra and some important parameters (such as light harvesting efficiency (LHE), electron injection driving force, the electron injection time, chemical reactivity parameters, vertical dipole moment as well as interaction models of dye-I2) were obtained according to Newns?Anderson model and DFT calculation. The process and strength of charge transfer and separation were visualized with charge different density and index of spatial extent (S, D and ?q). Current work paid attention to the new T-shaped dyes to reveal the relation between the structure and photoelectric performance. Furthermore, nine dyes (substitution of alkyl chains and ?-bridges) have been designed and characterized to screen promising sensitizer candidates with excellent photo-electronic properties.
A series of dyes, containing thiophene and thieno[3,2-b]thiophene as π-conjugated bridges and six kinds of groups as electron acceptors, were designed for dye-sensitized solar cells. The ground and excited state properties of designed dyes were investigated via density functional theory and time-dependent DFT, respectively. Moreover, the parameters affecting the short-circuit current density and open circuit voltage were calculated to predict the photoelectric performance of dye. In addition, charge difference density were presented via three-dimensional (3D) real-space analysis method to investigate the electron injection mechanism in the complexes. Research shows that the longer conjugated bridge would inhibit the intramolecular charge transfer, thereby affecting the photoelectrical properties of DSSCs; owing to the lowest chemical hardness, largest electroaccepting power, μ_normal and 〖∆E〗_CB, the dye with (E)-3-(4-(benzo[c][1,2,5]thiadiazol-4-yl)phenyl)-2-cyanoacrylic acid (TCA) acceptor group would exhibit the prominent photoelectric property among the designed dyes.
Dye-sensitized solar cells (DSSCs) have shown significant potential for indoor and building integrated photovoltaic applications. Herein we present three new D-A-π-A organic sensitizers, XY1, XY2 and XY3, exhibiting high molar extinction coefficients and a broad absorption range. Molecular modifications of these dyes, featuring a benzothiadiazole (BTZ) auxillary acceptor, were achieved by introducing a thiophene heterocycle, as well as by shifting the position of BTZ on the conjugated bridge. The ensuing high molar absorption coefficients enable highly efficient thin-film solid-state dye-sensitized solar cells with only 1.3 μm mesoporous TiO2 layer. XY2 with a molar extinction coefficient of 6.66×104 M-1 cm-1 at 578 nm led to the best photovoltaic performance of 7.51%.
In dye-sensitized solar cells co-photosensitized with an alkoxysilyl-anchor dye and a carboxy-anchor organic dye , was revealed to work collaboratively by enhancing the electron injection from the light-excited dyes to the TiO2 electrodes, and the cells exhibited a high conversion efficiency of over 14% under one sun illumination.
D-A-π-A dyes have exhibited several excellent advantages including optimized energy levels, distinct improvement of photovoltaic performance and stability. By modulating the auxiliary acceptor and acceptor unit, the efficiency of D-A-π-A dyes based dye-sensitized solar cells can be further improved. Based on density functional theory methods, sixteen dimethoxyl-substituted triphenylamine based D-A-π-A dyes composed of different acceptor and auxiliary acceptor groups were designed, and other four referenced homologous D-π-A dyes were also involved to compare in this work. The properties of all the dyes, including intramolecular charge transfer, lighting harvesting efficiency, kinetic of electron injection, and vertical dipole moment, have been investigated theoretically to screen out the dyes which would produce high efficiency. Then the interaction between the selected dyes and the electron acceptor in electrolyte was discussed to reveal the interfacial charge recombination process. In comparison with other dyes, TNA4 would be a promising candidate for its better performance on key parameters and achieving a balance between competing factors. Compared with cyanoacrylic acid, 2-(1, 1-dicyanomethylene) rhodanine group can serve as an excellent acceptor for future DSSCs applications.
The ability of Time-Dependent Density Functional Theory (TD-DFT) to provide excited state geometries and reproduce emission energies of organic D-p-A dyes designed for DSSC applications is evaluated. The performance of six functionals (CAM-B3LYP, MPW1K, omega B97X-D, LC-BLYP, LC-omega PBE, and M06-HF) in combination with three basis sets (cc-pVDZ, 6-31+G(d,p), and 6-311+G(2d,p)) has been analyzed. Solvent effects have been taken into account by means of a Polarizable Continuum Model in both LR and SS formalisms. Our LR-PCM/TD-DFT results show that accurate emission energies are obtained only when solvent effects are included in the computation of excited state geometries and when a range separated hybrid functional is used. Vertical emission energies are reproduced with a mean absolute error of at most 0.2 eV. The accuracy is further improved using the SS-PCM formalism.
To rationalize the marked difference in the energy conversion efficiency of dye sensitized solar cells (DSSCs) based on organic dyes 1 and 2 different only in their π spacer, density functional theory (DFT) and time-dependent DFT calculations of the geometries, electronic structures and absorption spectra of the organic dyes before and after binding to titanium oxide were carried out. These enable us to determine factors such as dipole moments associated with the open-circuit photovoltage (Voc), and to quantify parameters such as the light harvesting efficiency, the electron injection efficiency associated with the short-circuit photocurrent density (Jsc). The results reveal that compared to 2 with a thiazole spacer, 1 with a thiophene spacer could cause a red shift of the absorption spectrum, increase the oscillator strength and improve the driving force for electron injection, thus leading to the larger Jsc, in good agreement with experimental data. As for Voc, our results stress that apart from the generally emphasized vertical dipole moment of the dyes pointing outward from the semiconductor surface, the number of photoinjected electrons from the dye to the semiconductor is also crucial to obtain high performance dyes with high Voc. After justifying the reliability of the quantum-chemical methods, we designed another four dyes with different π spacers to screen more efficient organic dyes. Fortunately, taking 1 as reference, we find that dye 4 with a thienothiophene spacer displays an enhanced Jsc and Voc, indicating that it will be a more efficient diarylamine-fluorene-based organic dye used in DSSCs, which will play a theoretical guiding role in the design and synthesis of new organic dyes.
A new class of tribranched dye-sensitized solar cell (DSC) sensitizers carrying two conjugated donors and two acceptor/anchoring groups is introduced. The approach leads to significantly different optical properties and enhanced stability with respect to related di- and monobranched dyes and yields power conversion efficiencies of up to 5.05 %, which is possibly limited by the computed nonoptimal dye packing on the semiconductor surface.
We report a computational modeling study, based on DFT and time-dependent DFT techniques, to investigate the origin and the effect of local electric fields on the optical properties of organic dye-sensitized heterointerfaces, examining the case of the indoline D149 sensitizer on TiO2. On the one hand, we give precise information about the anchoring mode of D149 and its orientation with respect to the TiO2 surface, and on the other hand, we provide the computational framework model to interpret the Stark shifts experimentally observed by PIA spectroscopy. Our results show that the presence of oxidized dye molecules induces major spectral changes on the adjacent neutral dyes, which, along with the simulated effect of injected charge into TiO2, provide Stark shifts nicely reproducing the experimental observations.
A new multidonor and multianchoring asymmetric tribranched organic photosensitizer (i.e., TB-PT) for dye-sensitized solar cells containing three different (D–π–D, D–π1–A, and D–π2–A; D = electron-rich moiety, A = electron-poor moiety, π = conjugated bridge) polar branches, two donor cores, and two acceptor/anchoring groups to TiO2 was investigated and compared with the corresponding symmetric dyes. TB-PT combines the advantages arising from its π-extended tribranched architecture and the intramolecular cosensitization approach, which results in enhanced tailored optical and energetic properties and, eventually, photovoltaic performances.
The performance of dye sensitized solar cells is mainly based on the dye as a sensitizer. Natural dyes have become a viable alternative to expensive and rare organic sensitizers because of its low cost, easy attainability, abundance in supply of raw materials and no environment threat. Various components of a plant such as the flower petals, leaves and bark have been tested as sensitizers. The nature of these pigments together with other parameters has resulted in varying performance. This review briefly discusses the emergence, operation and components of dye sensitized solar cells together with the work done on natural dye based dye sensitized solar cells over the years.
Two mono anchoring dye molecules were bridged together to give a new di-anchoring bis-merocyanine dye which possessed a non-planar conformation on a TiO2 surface, a feature that impedes intermolecular aggregation of the dye in the adsorbed state. This dye also showed enhanced molar absorptivity and increased adsorption on TiO2. A dye sensitized solar cell based on the bis-merocyanine dye yielded enhanced power conversion efficiency of 6.1%.
The physical mechanisms which can produce second-order dielectric polarization are discussed on the basis of a simple extension of the theory of dispersion in ionic crystals. Four distinct mechanisms are described, three of which are related to the anharmonicity, second-order moment, and Raman scattering of the lattice. These mechanisms are strongly frequency dependent, since they involve ionic motions with resonant frequencies lower than the light frequency. The other mechanism is related to electronic processes of higher frequency than the light, and, therefore, is essentially flat in the range of the frequencies of optical masers. Since this range lies an order of magnitude higher than the ionic resonances, the fourth mechanism may be the dominant one. On the other hand, a consideration of the linear electro-optic effect shows that the lattice is strongly involved in this effect, and, therefore, may be very much less linear than the electrons. It is shown that the question of the mechanism involved in the second harmonic generation of light from strong laser beams may be settled by experiments which test the symmetry of the effect. The electronic mechanism is subject to further symmetry requirements beyond those for piezoelectric coefficients. In many cases, this would greatly reduce the number of independent constants describing the effect. In particular, for quartz and KDP there would be a single constant.
Since the late 1940s, the field of electron transfer processes has grown enonnously, both in chemistry and
biology. The development of the field, experimentally and theoretically, as well as its relation to the study
of other kinds of chemical reactions, represents to us an intriguing history, one in which many threads have
been brought together. In this lecture, some history, recent trends, and my own involvement in this research
are described.
The efficiency of electron injection from excited N3 dye (cis-bis-(4,4‘-dicarboxy-2,2‘-bipyridine) dithiocyanato ruthenium(II), Ru(dcbpy)2 (NCS)2), into various nanocrystalline semiconductor (ZrO2, TiO2, ZnO, Nb2O5, SnO2, In2O3) films was studied by transient absorption spectroscopy. For TiO2, ZnO, Nb2O5, SnO2, or In2O3 films, injection efficiencies were found to be very high; for ZrO2 film, the efficiency was very low. These findings indicate that electron injection occurs efficiently if the LUMO level of N3 dye is located sufficiently far above the bottom of the conduction band of the semiconductor film. On the basis of the results, we discuss the reason TiO2 exhibits higher solar cell performance than other materials.
All-organic dyes have shown promising potential as an effective sensitizer in dye-sensitized solar cells (DSSCs). The design concept of all-organic dyes to improve light-to-electric-energy conversion is discussed based on the absorption, electron injection, dye regeneration, and recombination. How the electron-donor-acceptor-type framework can provide better light harvesting through bandgap-tuning and why proper arrangement of acceptor/anchoring groups within a conjugated dye frame is important in suppressing improper charge recombination in DSSCs are discussed. Separating the electron acceptor from the anchoring unit in the donor-acceptor-type organic dye would be a promising strategy to reduce recombination and improve photocurrent generation.
A complete, functioning dye-sensitized solar cell made of popular indoline D149 sensitizer is studied by means of transient absorption in visible light in the time scale of nanoseconds to seconds. Photocurrent and photovoltage decays are also measured under the same experimental conditions. A local electric field causing a Stark shift of the D149 absorption band is found to strongly influence the transient spectra and kinetics. The presence of electrons in titania has a major contribution to the Stark shift and the effect disappears over many time scales with an average rate of 5 × 10(3) s(-1). This is much slower than the decay of the oxidized dye (2 × 10(6) s(-1)) but, on the other hand, significantly faster than the decay of electrons in titania nanoparticles (3 × 10(2) s(-1) at standard AM1.5 irradiation and open circuit conditions). Possible explanations of this phenomenon are discussed. Electron recombination from the titania conduction band to the oxidized dyes proceeds at an average rate of 2-16 × 10(4) s(-1), depending on the excitation energy density, and does not influence the efficiency of dye regeneration.
A localized one-electron orbital base, called bond-distorted orbital, is introduced to study hypothetically localized structures in the framework of valence bond theory. The use of valence bond method with bond-distorted orbitals allows us to evaluate the effects of hybridization and resonance on carbon−carbon bond lengths at the ab initio level. Valence bond self-consistent field studies on the delocalized and hypothetically localized structures of 1,3-butadiene and 1,3-butadiyne show that the theoretical C(sp2)−C(sp2) and C(sp)−C(sp) single bond lengths are 1.508 and 1.446 Å, respectively, and that the theoretical resonance energies of 1,3-butadiene and 1,3-butadiyne are −7.9 and −15.8 kcal/mol, respectively.
A triphenylamine (TPA) dye with two hexyl groups at the opposable 3,3′-positions of a bithiophene linker (1a) was prepared and compared to a dye with a regio regular 4,3′-dihexylbithiophene unit (1b). The absorption spectrum of 1a showed a blue shift in comparison to 1b, suggesting the structure of 1a was twisted in comparison to 1b. The twisted structure agreed with the structure optimized by DFT calculations. By replacing one thiophene unit of 1a and 1b with a pyridine ring (2a and 2b, respectively), a further blue shift was observed. Dye-sensitized solar cells (DSSCs) were prepared from these dyes and a conventional Ru dye (N719). Under one sun conditions, DSSCs/2a showed comparable or higher open-circuit voltage (Voc) than did DSSCs/N719. The high Voc was attributed solely to long electron lifetime in the DSSCs/2a. A previous study has suggested that TPA dyes with long π-conjugation unit suffer from larger dispersion forces between the dyes and acceptors, I3− and/or I2, causing short electron lifetime and thus low Voc. The present study shows that this problem can be overcome by increasing steric hindrance by attaching obstacle units to the π-linker without a significant increase of polarizability. The obstacle unit is to increase the intermolecular distance between the π-linker and acceptor species in electrolytes. Twisted structure is suggested to be one strategy to add such an effect.
We designed and synthesized novel organic dyes of double electron acceptor type based on phenothiazine framework, as photosensitizers for the dye-sensitized solar cell (DSC). The density functional theory (DFT) and time-dependent density functional theory (TD-DFT) calculations were used to estimate the photovoltaic properties of the dyes in the design stage. The molecular structure having two electron acceptors on both sides of phenothiazine moiety provided the efficient electron extraction paths from electron donor part, which was demonstrated by the analysis of the electronic structures on donor and acceptor, and the excitations between HOMOs and LUMOs. In accordance, the measurements of photovoltaic properties of the DSCs prepared in the laboratory scale showed that the organic dyes of double electron acceptor type gave about 20% higher performace than their counterparts of single electron acceptor type. The effect of the kind of electron acceptor (C, cyanocrylic acid and R, rhodanine-acetic acid) on the performance of DSC was studied as well. We found that the organic dyes with R as acceptor gave much lower efficiencies, compared to those with C, for both single and double electron acceptor type. It was attributed to the electronic decoupling between anchoring and TiO2 conduction band accompanying the lack of π-conjugation of carboxylic group as anchoring on R, despite that the dyes with R had relatively broad and intense absorption spectra in the visible region.
In this review, we address the materials design parameters that control the processes of charge separation, and thereby device efficiency, in dye-sensitized photoelectrochemical solar cells. The review starts with an overview of the structure, energetics and kinetics of dye-sensitized solar cells. It then goes on to consider in more detail the parameters determining the efficiency of the two primary charge separation steps in these devices: electron injection from the dye excited state into the metal oxide electrode, and regeneration of the dye ground state by the redox electrolyte. We consider the kinetic competition between these desired charge separation steps and the undesired loss pathways of excited state decay to ground and electron recombination with dye cations. The review avoids detailed mathematical and spectroscopic discussion, but rather tries to summarize the key conclusions relevant to materials design. A recurring theme of the review is the energy cost of achieving charge separation, and how this limits device performance. A further factor addressed in this review is real as opposed to ideal materials behavior, including, for example, consideration of the implications of empirical observations of an exponential density of acceptor states in the metal oxide, as well as identification of unresolved issues in our current understanding.Keywords: dye-sensitized solar cell; electron transfer; photoelectrochemistry; electron injection; mesoporous
Until now, photovoltaics--the conversion of sunlight to electrical power--has been dominated by solid-state junction devices, often made of silicon. But this dominance is now being challenged by the emergence of a new generation of photovoltaic cells, based, for example, on nanocrystalline materials and conducting polymer films. These offer the prospect of cheap fabrication together with other attractive features, such as flexibility. The phenomenal recent progress in fabricating and characterizing nanocrystalline materials has opened up whole new vistas of opportunity. Contrary to expectation, some of the new devices have strikingly high conversion efficiencies, which compete with those of conventional devices. Here I look into the historical background, and present status and development prospects for this new generation of photoelectrochemical cells.
Representative atomic and molecular systems, including various inorganic and organic molecules with covalent and ionic bonds, have been studied by using density functional theory. The calculations were done with the commonly used exchange-correlation functional B3LYP followed by a comprehensive analysis of the calculated highest-occupied and lowest-unoccupied Kohn-Sham orbital (HOMO and LUMO) energies. The basis set dependence of the DFT results shows that the economical 6-31+G* basis set is generally sufficient for calculating the HOMO and LUMO energies (if the calculated LUMO energies are negative) for use in correlating with molecular properties. The directly calculated ionization potential (IP), electron affinity (EA), electronegativity (c), hardness (h), and first electron excitation energy (t) are all in good agreement with the available experimental data. A generally applicable linear correlation relationship exists between the calculated HOMO energies and the experimental/calculated IP's. We have also found satisfactory linear correlation relationships between the calculated LUMO energies and experimental/calculated EA's (for the bound anionic states), between the calculated average HOMO/LUMO energies and c values, between the calculated HOMO-LUMO energy gaps and h values, and between the calculated HOMO-LUMO energy gaps and experimental/calculated first excitation energies. By using these linear correlation relationships, the calculated HOMO and LUMO energies can be employed to semi-quantitatively estimate ionization potential, electron affinity, electronegativity, hardness, and first excitation energy.
The geometric and electronic structures and photophysical properties of anilido-pyridine boron difluoride dyes 1-4, a series of scarce 4,4-difluoro-4-bora-3a,4a-diaza-s-indacene (BODIPY) derivatives with large Stokes shift, are investigated by employing density functional theory (DFT) and time-dependent DFT (TD-DFT) calculations to shed light on the origin of their large Stokes shifts. To this end, a suitable functional is first determined based on functional tests and a recently proposed index-the charge-transfer distance. It is found that PBE0 provides satisfactory overall results. An in-depth insight into Huang-Rhys (HR) factors, Wiberg bond indices, and transition density matrices is provided to scrutinize the geometric distortions and the character of excited states pertaining to absorption and emission. The results show that the pronounced geometric distortion due to the rotation of unlocked phenyl groups and intramolecular charge transfer are responsible for the large Stokes shift of 1 and 2, while 3 shows a relatively blue-shifted emission wavelength due to its mild geometric distortion upon photoemission, although it has a comparable energy gap to 1. Finally, compound 4, which is designed to realize the rare red emission in BODIPY derivatives, shows desirable and expected properties, such as high Stokes shift (4847 cm(-1) ), red emission at 660 nm, and reasonable fluorescence efficiency. These properties give it great potential as an ideal emitter in organic light-emitting diodes. The theoretical results could complement and assist in the development of BODIPY-based dyes with both large Stokes shift and high quantum efficiency.
Two novel dyes TPAR3 and BTDR2 based on triphenylamine and benzothiadiazole, respectively, with multiple electron acceptors were synthesized and characterized by FT-IR, 1H NMR, TGA and thermomechanical analysis (TMA). They carried terminal cyanoacrylic acid electron acceptors/anchoring moieties, which were connected with the central unit through a thiophene ring. The absorption bands of the dyes were extended up to ∼570 nm with long-wave absorption maximum at 425–455 nm and optical band gap of 2.10–2.17 eV. The dyes emitted yellow–orange light with photoluminescence maximum at 547–615 nm. We have investigated the photovoltaic properties of quasi solid state dye sensitized solar cells (DSSCs) based on these metal free organic dyes. It has been found that the power conversion efficiency of the DSSCs based on composite zinc titanium oxide (ZTO) nanocrystalline photoelectrode is higher than that for TiO2 based DSSCs. This has been attributed to the longer electron lifetime and more negative conduction band edge of ZTO. The overall power conversion efficiency of the DSSCs based on TPAR3 and BTDR2 employing ZTO photoelectrode is 6.3% and 3.6%, respectively. These results indicate that both the acceptor moiety of metal free organic dyes and ZTO photoelectrode have an effect on the photovoltaic performance of DSSCs.
We report the synthesis and photophysical/electrochemical properties of triphenylamine (TPA)-based multiple electron acceptor dyes (TPAR1, TPAR2, and TPAR3) as well as their applications in dye-sensitized solar cells (DSSCs). In these dyes, the TPA group and the rhodanine-3-acetic acid play the role of the basic electron donor unit and the electron acceptor, respectively. It was found that introduction of two rhodanine-3-acetic acid groups into the TPA unit (TPAR2) exhibited better photovoltaic performance due to the increase with a red shift and broadening of the absorption spectrum. The monolayer of these TPA-based dyes was adsorbed on the surface of nanocrystalline TiO2 mesoporous electrode with the thickness of ∼6 μm, polyethylene oxide (PEO) used as the matrix of gel electrolyte, and 4-nm thick Pt used as a counter-electrode. Photovoltaic device can be realized in a single quasi-solid-state DSSC. TPAR2-based gel DSSC had an open circuit voltage and short circuit current density of about 541 and 10.7 mA cm−2, respectively, at 1-sun.
This paper provides an overview of the title paper by Miertus, Scrocco and Tomasi, including the impact that it has had on
the theoretical description of solvation by means of continuum models.
The single crystals of methyl p-hydroxy benzoate (MPHB) are grown from methanol solution by a low temperature solution growth technique. The SHG efficiency is tested using Q-switched Nd:YAG laser of wavelength λ at 1064 nm, which is approximately 1.2 times that of urea. Vibrational spectral analysis using NIR-FT Raman and FT-IR spectra is carried out to understand the structural and electronic contributions to hyperpolarizability in MPHB. The DFT computations are also performed at B3LYP/6-311G(d,p) level to derive equilibrium geometry, vibrational wavenumbers and intensities. The results of ab initio calculations at HF/6-311G(d,p) level show that the vibrational contribution for the second-order electro-optic coefficient in MPHB is about 19.5%. Vibrational spectral studies also provide evidence for the charge transfer interaction between the donors and the acceptor group through the π-system. The π-electron cloud movement from donor to acceptor can make the molecule highly polarized and the intramolecular charge transfer interaction must be responsible for the nonlinear optical properties of MPHB. The splitting of the carbonyl mode may be attributed to the intramolecular association based on CO⋯H type hydrogen bonding in the molecule. The conjugation and influence of intermolecular hydrogen bonding (CO⋯H) type network in the crystal results in lowered CO stretching wavenumber.
A new hybrid exchange–correlation functional named CAM-B3LYP is proposed. It combines the hybrid qualities of B3LYP and the long-range correction presented by Tawada et al. [J. Chem. Phys., in press]. We demonstrate that CAM-B3LYP yields atomization energies of similar quality to those from B3LYP, while also performing well for charge transfer excitations in a dipeptide model, which B3LYP underestimates enormously. The CAM-B3LYP functional comprises of 0.19 Hartree–Fock (HF) plus 0.81 Becke 1988 (B88) exchange interaction at short-range, and 0.65 HF plus 0.35 B88 at long-range. The intermediate region is smoothly described through the standard error function with parameter 0.33.
Despite the remarkable thermochemical accuracy of Kohn–Sham density-functional theories with gradient corrections for exchange-correlation [see, for example, A. D. Becke, J. Chem. Phys. 96, 2155 (1992)], we believe that further improvements are unlikely unless exact-exchange information is considered. Arguments to support this view are presented, and a semiempirical exchange-correlation functional containing local-spin-density, gradient, and exact-exchange terms is tested on 56 atomization energies, 42 ionization potentials, 8 proton affinities, and 10 total atomic energies of first- and second-row systems. This functional performs significantly better than previous functionals with gradient corrections only, and fits experimental atomization energies with an impressively small average absolute deviation of 2.4 kcal/mol.
The first calculations on polyenes and the attention given to the issues of bond length alternation and ordering of the lowest singlet excited state served as impetus for the description of the electronic structure of π-conjugated materials. Initially, the goal of most calculations was to determine the nature of the (unrelaxed) excited states playing a role in the second-order and third-order molecular polarizabilities. Later on, the relaxation effects in the excited states and impact of intermolecular interactions drew significant interest. These and other related works point to the increased significance of the dynamic processes taking place in π-conjugated materials, such as charge transport, charge recombination, exciton formation, exciton diffusion, or exciton dissociation.
Dye-sensitized solar cells (DSCs) offer the possibilities to design solar cells with a large flexibility in shape, color, and transparency. DSC research groups have been established around the world with biggest activities in Europe, Japan, Korea, China, and Australia. The sun emits light with a range of wavelengths from the ultraviolet and visible to the infrared. It peaks in the visible, resembling the spectrum of a blackbody at a temperature of 5760 K. It is, however, influenced by atmospheric absorption and the position of the sun. The advent of heteroleptic ruthenium complexes furnished with an antenna function has taken the performance of the DSC to a new level. Two examples of these dyes are Z991 and C101. Compared with the classical DSC Ru dyes, their extinction coefficients are higher and the spectral response is shifted to the red. The positions of the energy levels at the oxide/dye/electrolyte interface are fundamentally important to the function of the DSC.
The dye-sensitized solar cell (DSC) challenges conventional photovoltaics with its potential for low-cost production and its flexibility in terms of color and design. Transient absorption spectroscopy is widely used to unravel the working mechanism of DSCs. A surprising, unexplained feature observed in these studies is an apparent bleach of the ground-state absorption of the dye, under conditions where the dye is in the ground state. Here, we demonstrate that this feature can be attributed to a change of the local electric field affecting the absorption spectrum of the dye, an effect related to the Stark effect first reported in 1913. We present a method for measuring the effect of an externally applied electric field on the absorption of dye monolayers adsorbed on flat TiO(2) substrates. The measured signal has the shape of the first derivative of the absorption spectra of the dyes and reverses sign along with the reversion of the direction of the change in dipole moment upon excitation relative to the TiO(2) surface. A very similar signal is observed in photoinduced absorption spectra of dye-sensitized TiO(2) electrodes under solar cell conditions, demonstrating that the electric field across the dye molecules changes upon illumination. This result has important implications for the analysis of transient absorption spectra of DSCs and other molecular optoelectronic devices and challenges the interpretation of many previously published results.
Photophysical studies were performed with [Ru(dtb)(2)(dcb)](PF(6))(2) and cis-Ru(dcb)(dnb)(NCS)(2,) where dtb is 4,4'-(C(CH(3))(3))(2)-2,2'-bipyridine, dcb is 4,4'-(COOH)(2)-2,2'-bipyridine, and dnb is 4,4'-(CH(3)(CH(2))(8))(2)-2,2'-bipyridine), anchored to anatase TiO(2) particles ( approximately 15 nm in diameter) interconnected in a mesoporous, 10 mum thick film immersed in Li(+)-containing CH(3)CN electrolytes with iodide or phenothiazine donors. Pulsed-laser excitation resulted in rapid excited-state injection and donor oxidation to yield TiO(2)(e(-))s and oxidized donors, while the metal-to-ligand charge-transfer (MLCT) absorption spectrum of the Ru(II) coordination compounds differed from that which was initially excited. The spectral data were consistent with an underlying Stark effect and indicated that the surface electric field was not completely screened from the molecular sensitizer. The magnitude of the electric field was estimated to be approximately 270 MV/m from Li(+) titration experiments, corresponding to a approximately 40 mV potential drop. With iodide donors, the amplitude of the Stark effect decreased over time periods where charge recombination was absent, behavior attributed to "screening" of the electric field by interfacial ionic reorganization. The screening kinetics were nonexponential but were well described by the Kohlrausch-Williams-Watts model, from which a characteristic rate constant, tau(o)(-1), of approximately 1.5 x 10(5) s(-1) was abstracted. At least seven other sensitizers and five different cations, as well as on SnO(2) nanoparticle films, exhibited similar transient absorption behavior with iodide donor molecules indicating that the effect was quite general. In the presence of phenothiazine donors (or in the absence of an external donor), there was no clear evidence for screening, and the Stark effect disappeared concurrent with interfacial charge recombination. Complementary spectroelectrochemical studies of these same sensitized films displayed similar absorption spectra when the TiO(2) thin film was partially reduced with a forward bias. Spectral modeling in the absence of donor molecules as well as studies of TiO(2) thin films sensitized with two different Ru(II) compounds demonstrated that the electric field created by excited-state injection from one sensitizer influenced the absorption spectra of other sensitizers that had not undergone photoinduced electron injection.
We employed the bisthienothiophene conjugated linker along with a hydrophobic triphenylamine electron-donor and a hydrophilic cyanoacrylic acid electron-acceptor to construct a high molar extinction coefficient organic photosensitizer, exhibiting a power conversion efficiency of 8.0% measured under irradiation of air mass 1.5 global (AM 1.5G) full sunlight.
Three antipyrine derivatives of o-, m- and p-chlorobenzylideneaminoantipyrines were characterized by spectral techniques and density functional calculations. The optimized configurations are very close to the XRD values and are used as foundations to investigate the molecular properties. The spectral assignments were attempted to ascribe to the vibrational modes of the detailed substructures with the aid of theoretical calculations because of the satisfactory consistencies between the experimental and theoretical spectra for each of the studied compounds. Raman spectral ascriptions represent that the pi-conjugated moieties linked by Schiff base imines are responsible for the excellent Raman scattering activities of these compounds. The linear polarizabilities and first hyperpolarizabilities of the studied molecules indicate that the compounds are good candidates of nonlinear optical materials. The statistical thermodynamic functions and their correlations with temperatures obtained from the theoretical vibrations are similar to each other among the isomers.
A correlation-energy formula due to Colle and Salvetti [Theor. Chim. Acta 37, 329 (1975)], in which the correlation energy density is expressed in terms of the electron density and a Laplacian of the second-order Hartree-Fock density matrix, is restated as a formula involving the density and local kinetic-energy density. On insertion of gradient expansions for the local kinetic-energy density, density-functional formulas for the correlation energy and correlation potential are then obtained. Through numerical calculations on a number of atoms, positive ions, and molecules, of both open- and closed-shell type, it is demonstrated that these formulas, like the original Colle-Salvetti formulas, give correlation energies within a few percent.
The ruthenium complexes [Ru(dcbpyH(2))(2)(Cl)(2)] (1), [Ru(dcbpyH(2))(2)(NCS)(2)] (2), (Bu(4)N)(4)[Ru(dcbpy)(2)(NCS)(2)] (3), and (Bu(4)N)(2)[Ru(dcbpyH)(2)(NCS)(2)] (4) were synthesized and characterized by cyclic voltammetry, UV-vis absorption, and emission, IR, Raman, and NMR spectroscopy. The absorption and emission maxima of these complexes red shifted with decreasing pH, and showed pH-dependent excited-state lifetimes. The ground-state pK(a) values were determined by spectrophotometeric methods, and the dissociation of protons was found to occur in two steps (pK(a) = 3 and 1.5). The Ru(II)/(III) couple in the complex (Bu(4)N)(4)[Ru(dcbpy)(2)(NCS)(2)] is shifted ca. 290 mV negatively with regard to that of the complex [Ru(dcbpyH(2))(2)(NCS)(2)] due to the replacement of H(+) by tetrabutylammonium cation. The negative shift for the dcbpy-based reduction potential is even larger, i.e., about 600 mV compared to that of the complex [Ru(dcbpyH(2))(2)(NCS)(2)]. The effect of deprotonation on the performance of these complexes as photosensitizers for nanocrystalline titania was investigated.
This contribution explores the relation between molecular second hyperpolarizabilities (gamma) and molecular architecture in phenylacetylene dendrimers using the semiempirical molecular orbital method, that is, INDO/S method. The orientationally averaged gamma of a large-size phenylacetylene dendrimer, which is composed of 24 units of phenylacetylenes and is referred to as D25, is found to be about 50 times as large as that of the diphenylacetylene monomer. In contrast, the gamma(s)() value of D25 is found to be about 6 times as small as that of the para-substituted phenylacetylene oligomer (L25) composed of 24 units of phenylacetylenes. To investigate the structure-property relation in gamma for D25 and L25, we examine the spatial contributions of electrons to gamma values using gamma density analysis. The present analysis reveals that the dominant contributions of electrons to gamma of D25 are localized in the linear-leg regions parallel to the applied electric field and the contributions are also well segmented at the meta-connected points (benzene rings), while the spatial distribution of the gamma density of L25 is extended over the whole region of the chain, and the dominant contribution stems from the both-end regions. It is found for D25 that the magnitude of contributions to gamma in the internal region is more enhanced than that in the outer region. We further found that the magnitudes of contributions in internal linear-leg regions of D25 are somewhat larger than those of the same-size isolated linear-leg molecules. This suggests that the slightly remaining pi-conjugations via the meta-branching points still enhance the contributions to gamma localized in the linear-leg regions. These features of spatial contributions to gamma of D25 are found to originate in the fractal architecture, in which pi-conjugation lengths in the linear-leg region increase on going from the periphery to the core. Finally, fractal antenna dendrimers are expected to be promising novel nonlinear optical (NLO) substances with a controllability of the magnitude and spatial contribution of the third-order NLO properties.
The quality of human life depends to a large degree on the availability of energy. This is threatened unless renewable energy resources can be developed in the near future. Chemistry is expected to make important contributions to identify environmentally friendly solutions of the energy problem. One attractive strategy discussed in this Forum Article is the development of solar cells that are based on the sensitization of mesoscopic oxide films by dyes or quantum dots. These systems have already reached conversion efficiencies exceeding 11%. The underlying fundamental processes of light harvesting by the sensitizer, heterogeneous electron transfer from the electronically excited chromophore into the conduction band of the semiconductor oxide, and percolative migration of the injected electrons through the mesoporous film to the collector electrode will be described below in detail. A number of research topics will also be discussed, and the examples for the first outdoor application of such solar cells will be provided.