Mechanisms of electron injection from retinoic acid and carotenoic acids to TiO2 nanoparticles and charge recombination via the T1 state as determined by subpicosecond to microsecond time-resolved absorption spectroscopy: dependence on the conjugation length

To examine the mechanisms of electron injection to TiO2 in retinoic acid (RA) and carotenoic acids (CAs), including RA5, CA6, CA7, CA8, CA9, and CA11 having the number of conjugated double bonds n = 5, 6, 7, 8, 9, and 11, respectively, their subpicosecond time-resolved absorption spectra were recorded free in solution and bound to TiO2 nanoparticles in suspension. The time-resolved spectra were analyzed by singular-value decomposition (SVD) followed by global fitting based on an energy diagram consisting of the 3A(g)(-), 1B(u)(-), 1B(u)(+), and 2A(g)(-) singlet excited states, whose energies had been determined as functions of 1/(2n + 1) by the use of carotenoids with n = 9-13. It was found that electron injection took place from both the 1B(u)(+) and 2A(g)(-) states in RA5, CA6, CA7, and CA8, whereas only from the 1B(u)(+) state in CA9 and CA11. The electron-injection efficiencies were determined, by the use of the relevant time constants determined by the SVD and global-fitting analyses, to be in the following order: RA5 approximately CA6 < CA7 > CA8 > CA9 > CA11. To determine the mechanism of charge recombination via the T(1) state, submicrosecond time-resolved absorption spectra of RA5, CA6, CA7, and CA8 bound to TiO2 nanoparticles in suspension were recorded. The SVD and global-fitting analyses lead us to a new scheme, which includes the formation of the D(0)(*+) - T(1) complex followed by transformation to both the D(0)(*+) and T(1) states. On the other hand, their one-electron oxidation potentials were determined, and their singlet and triplet levels were scaled to the conduction band edge (CBE) of TiO2. The T(1) level was lower than, but closest to, the CBE in RA5, and it became lower in the order RA5, CA6, CA7, and CA8. Consistent with the energy gap between the CBE and the T(1) levels, the generation of the T(1) state (or in other words, charge recombination) decreased in the order RA5 > CA6 > CA7 > CA8.     

SPECTRAL INHOMOGENEITY OF THE LIGHT-HARVESTING ANTENNA OF Rhodospirillum rubrum PROBED BY TRIPLET-MINUS-SINGLET SPECTROSCOPY AND SINGLET-TRIPLET ANNIHILATION AT LOW TEMPERATURES

–Triplet-minus-singlet (T-S) spectra and singlet-triplet annihilation was measured on Rhodospirillum rub-rum chromatophores at low temperatures (4–77K). The T-S spectrum of the LH-1 antenna was dependent on the wavelength of excitation and thus indicative of site-inhomogeneous broadening. The data cannot be understood adequately using the two pigment pool model (B880 + B896) but, in contrast, support a model for LH-1, in which the spectral properties are determined by site-inhomogeneous broadening. Consequently, the 4 K antenna domain for singlet-triplet annihilation in LH-1 is rather small. If the temperature is raised to 77 K the annihilation domain increases, and a fit of the fluorescence quenching and triplet yield data suggests that at 77 K the connectivity is between the extreme cases of the “lake” and the isolated “puddles” models. The implications of the observed antenna inhomogeneity on low temperature annihilation and fluorescence measurements are discussed.     

ZnO/graphene-oxide nanocomposite with remarkably enhanced visible-light-driven photocatalytic performance

In this work, a high-performance photocatalyst of ZnO/graphene-oxide (ZnO/GO) nanocomposite was synthesized via a facile chemical deposition route and used for the photodegradation of organic dye from water under visible light. The nanocomposite was characterized by X-ray diffraction, X-ray photoelectron spectroscopy, scanning electron microscopy, transmission electron microscopy, Brunauer-Emmett-Teller N(2) adsorption-desorption analysis, and UV-Vis diffusion reflectance spectroscopy. The ZnO/GO nanocomposite consisting of flower-like ZnO nanoparticles anchored on graphene-oxide sheets has a high surface area and hierarchical porosity, which is benefit to the adsorption and mass transfer of dye and oxygen species. For the photodegradation of organic dyes under visible light, ZnO/GO nanocomposite exhibited remarkably enhanced photocatalytic efficiency than graphene-oxide sheets and flower-like ZnO particles. Moreover, the photocatalytic efficiency of ZnO/GO nanocomposite could be further improved by annealing the product in N(2) atmosphere. The outstanding photocatalytic performance was ascribed to the efficient photosensitized electron injection and repressed charge carriers recombination in the composite with GO as electron collector and transporter, thus leading to continuous generation of reactive oxygen species for the degradation of methylene blue.     

Synthesis and Photovoltaic Properties of Boron β‐Ketoiminate Dyes Forming a Linear Donor‐π‐Acceptor Structure

Organoboron complexes are of interest as chromophores for dye sensitizers owing to their light‐harvesting and carrier‐transporting properties. In this study, compounds containing boron β‐ketoiminate (BKI) as a chromophore were synthesized and used as dye sensitizers in dye‐sensitized solar cells. The new dyes were orange or red crystals and showed maximum absorptions in the 410–450 nm wavelength region on titanium dioxide substrates. These electrodes exhibited maximum efficiencies of over 80% in incident photon‐to‐current conversion efficiency spectra, suggesting that the continuous process of light absorption‐excitation‐electron injection was effectively performed. Open‐circuit photovoltages were relatively high owing to the large dipole moments of the BKI dyes with a linear molecular structure. Thus, a maximum power conversion efficiency of 5.3% was successfully observed. Comparison of BKI dyes with boron β‐diketonate dyes revealed certain differences in solution stability, spectral properties, and photovoltaic characteristics.     

含中氮茚有机太阳能电池染料敏化剂的分子设计

采用密度泛函理论(DFT)和含时密度泛函理论(TD-DFT)方法研究了9个新的中氮茚[3, 4, 5-ab]异吲哚(INI)为给体的染料敏化剂性质.对影响电池效率的光捕获效率、电子注入、染料再生和电荷复合等重要因素与D5和D9染料进行了对比.计算表明,设计的INI系列敏化剂在440-500 nm内有最大吸收峰,表现出明显的电荷分离特征, INI2具有比D9染料更高的最大理论短路电流. Fukui反应指数计算指出INI2的亲核加成最易实现.染料分子在二氧化钛(101)面吸附计算表明,染料INI2以间接注入途径实现电子注入.综合计算结果,中氮茚INI染料有希望作为性能优良的染料敏化剂而得到应用.     

Photoinduced charge carrier dynamics of Zn-porphyrin-TiO2 electrodes: the key role of charge recombination for solar cell performance

Time resolved absorption spectroscopy has been used to study photoinduced electron injection and charge recombination in Zn-porphyrin sensitized nanostructured TiO(2) electrodes. The electron transfer dynamics is correlated to the performance of dye sensitized solar cells based on the same electrodes. We find that the dye/semiconductor binding can be described with a heterogeneous geometry where the Zn-porphyrin molecules are attached to the TiO(2) surface with a distribution of tilt angles. The binding angle determines the porphyrin-semiconductor electron transfer distance and charge transfer occurs through space, rather than through the bridge connecting the porphyrin to the surface. For short sensitization times (1 h), there is a direct correlation between solar cell efficiency and amplitude of the kinetic component due to long-lived conduction band electrons, once variations in light harvesting (surface coverage) have been taken into account. Long sensitization time (12 h) results in decreased solar cell efficiency because of decreased efficiency of electron injection.     

A Series of Iron(II)-NHC Sensitizers with Remarkable Power Conversion Efficiency in Photoelectrochemical Cells**

A series of six new Fe(II)NHC-carboxylic sensitizers with their ancillary ligand decorated with functions of varied electronic properties have been designed with the aim to increase the metal-to- surface charge separation and light harvesting in iron-based dye-sensitized solar cells (DSSCs). ARM130 scored the highest efficiency ever reported for an iron-sensitized solar cell (1.83 %) using Mg²⁺ and NBu₄I-based electrolyte and a thick 20 μm TiO₂ anode. Computational modelling, transient absorption spectroscopy and electrochemical impedance spectroscopy (EIS) revealed that the electronic properties induced by the dimethoxyphenyl-substituted NHC ligand of ARM130 led to the best combination of electron injection yield and spectral sensitivity breadth.     

Natural chlorophyll-related porphyrins and chlorins for dye-sensitized solar cells

Natural-chlorophyll-related porphyrins, including (2H, Zn, Cu)-protoporphyrin IX (Por-1) and Zn-mesoporphyrin IX (Por-2), and chlorins, including chlorin e? (Chl-1), chlorin e? (Chl-2), and rhodin G? (Chl-3), have been used in dye-sensitized solar cells (DSSCs). For porphyrin sensitizers that have vinyl groups at the β-positions, zinc coordinated Por-1 gives the highest solar-energy-to-electricity conversion efficiency (h) of up to 2.9%. Replacing the vinyl groups of ZnPor-1 with ethyl groups increases the open-circuit voltage (V(oc)) from 0.61 V to 0.66 V, but decreases the short-circuit current (J(sc)) from 7.0 mA·cm?2 to 6.1 mA·cm?2 and the value of h to 2.8%. Density functional theory (DFT) and time-dependent DFT (TD-DFT) calculations suggest that the higher J(sc) values of Zn-based porphyrin sensitizers result from the favorable electron injection from the LUMO at higher energy levels. In the case of the chlorin sensitizers, the number of carboxyl protons has a large effect on the photovoltaic performance. Chl-2 with two carboxyl protons gives much higher values of J(sc), V(oc), and h than does Chl-1 with three carboxyl protons. Replacing the protons of Chl-1 with sodium ions can substantially improve the photovoltaic performance of Chl-1-based solar cells. Furthermore, the sodium salt of Chl-3 with an aldehyde group at the C7 position shows poorer photovoltaic performance than does the sodium salt of Chl-1 with methyl groups at the C7 position. This is due to the low light-harvesting capability of Chl-3.     

Effect of the anchoring group (carboxylate vs phosphonate) in Ru-complex-sensitized TiO2 on hydrogen production under visible light

We synthesized six Ru-bipyridyl complexes having di-, tetra-, and hexacarboxylate (C2, C4, and C6) and di-, tetra-, and hexaphosphonate (P2, P4, and P6) as the anchoring group, prepared six different sensitized TiO2 samples by using them, and then systematically tested their visible light reactivity for hydrogen production in aqueous suspension (with EDTA as an electron donor) under lambda > 420 nm illumination. The properties and efficiencies of C- and P-complexes as a sensitizer depended on the number and kind of anchoring groups in very different ways. The adsorption of P-complexes on TiO2 is strong enough not to be hampered by the presence of competing adsorbates (EDTA), whereas that of C-complexes is significantly inhibited. As a result, P-TiO2 exhibited much higher activity for the hydrogen production than C-TiO2, although the visible light absorbing capabilities are comparable among C- and P-complexes. Among the six sensitizers, P2 was the most active one for the H2 production. The hydrogen production activities of C-TiO2 and P-TiO2 depended on the concentration of sensitizers and electron donors in different ways as well. How the sensitizing activity for hydrogen production is influenced by the anchoring group and the experimental conditions was investigated and discussed in detail. It is also notable that the effects of the anchoring group on the sensitized production of hydrogen were drastically different from those on the dye-sensitized solar cell we recently reported for the same set of six sensitizers.     

β‐Functionalized Push–Pull opp‐Dibenzoporphyrins as Sensitizers for Dye‐Sensitized Solar Cells

A novel class of β‐functionalized push–pull zinc opp ‐dibenzoporphyrins were designed, synthesized, and utilized as sensitizers for dye‐sensitized solar cells. Spectral, electrochemical, and computational studies were systematically performed to evaluate their spectral coverage, redox behavior, and electronic structures. These porphyrins displayed much broader spectral coverage and more facile oxidation upon extension of the π conjugation. Free‐energy calculations and femtosecond transient absorption studies (charge injection rate in the range of 10¹¹ s⁻¹) suggested efficient charge injection from the excited singlet state of the porphyrin to the conduction band of TiO₂. The power conversion efficiency (η ) of YH3 bearing acrylic acid linkers (η =5.9 %) was close to that of the best ruthenium dye N719 (η =7.4 %) under similar conditions. The superior photovoltaic performance of YH3 was attributed to its higher light‐harvesting ability and more favorable electron injection and collection, as supported by electrochemical impedance spectral studies. This work demonstrates the exceptional potential of benzoporphyrins as sensitizers for dye‐sensitized solar cells.     

Biradicals stabilized by intramolecular charge transfer: properties of heterosubstituted pentalene and cyclooctatetraene biradicals

Intramolecular charge transfer can lead to substantial stabilization of singlet ground state and a corresponding increase of the singlet-triplet gap for molecules isoelectronic with the dianions of antiaromatic hydrocarbons. The formal biradicals 2,5-di-heterosubstituted-pentalenes and 1,5-di-heterosubstituted-cyclooctatetraenes are theoretically predicted to have the potential to be stable, persistent non-Kekulé molecules, as supported by high-level quantum chemical calculations. The singlet-triplet energy gaps and the S(0)-S(1) excitation energies of these molecules are similar to those of aromatic molecules rather than standard biradicals. These formal biradicals have a pronounced zwitterionic character, having a singlet ground state. The marked stabilization of the ground-state singlet for these non-Kekulé molecules is accompanied by a significant destabilization of the highest occupied molecular orbital (HOMO), leading to a low ionization potential (IP). This apparent inconsistency is explained by analyzing the electronic structure of the molecules. In the case of di-aza-pentalene, the energy of the first electronic excited state is only slightly lower than the ionization potential, making it a candidate for molecular autoionization.     

Photocurrents of solar cells sensitized by aggregate-forming polyenes: Enhancement due to suppression of singlet–triplet annihilation by lowering of dye concentration or light intensity

Titania-based Grätzel-type solar cells were fabricated by the use of polyene dyes with various transition dipole moments. In the dye having the largest transition dipole among the samples, an aggregate was readily formed through dispersive interaction, and the photocurrent was increased when the dye concentration or the light intensity was lowered. This observation was ascribed to the suppression of the singlet–triplet annihilation reaction. In the dye having the smallest transition dipole, there was no sign of aggregate formation, and the photocurrent was decreased when the dye concentration or the light intensity was lowered.     

Quantum dot-sensitized solar cells incorporating nanomaterials

Quantum dot-sensitized solar cells (QDSSCs) are interesting energy devices because of their (i) impressive ability to harvest sunlight and generate multiple electron/hole pairs, (ii) ease of fabrication, and (iii) low cost. The power conversion efficiencies (η) of most QDSSCs (typically <4%) are, however, less than those (up to 12%) of dye-sensitized solar cells, mainly because of narrow absorption ranges and charge recombination occurring at the QD-electrolyte and TiO(2)-electrolyte interfaces. To further increase the values of η of QDSSCs, it will be necessary to develop new types of working electrodes, sensitizers, counter electrodes and electrolytes. This Feature Article describes the nanomaterials that have been used recently as electronic conductors, sensitizers and counter electrodes in QDSSCs. The nature, size, morphology and quantity of these nanomaterials all play important roles affecting the efficiencies of electron injection and light harvesting. We discuss the behavior of several important types of semiconductor nanomaterials (sensitizers, including CdS, Ag(2)S, CdSe, CdTe, CdHgTe, InAs and PbS) and nanomaterials (notably TiO(2), ZnO and carbon-based species) that have been developed to improve the electron transport efficiency of QDSSCs. We point out the preparation of new generations of nanomaterials for QDSSCs and the types of electrolytes, particularly iodide/triiodide electrolytes (I(-)/I(3)(-)), polysulfide electrolytes (S(2-)/S(x)(2-)), and cobalt redox couples ([Co(o-phen)(3)(2+)/(3+)]), that improve their lifetimes. With advances in nanotechnology, we foresee significant improvements in the efficiency (η > 6%) and durability (>3000 h) of QDSSCs.     

Molecular engineering of organic sensitizers for dye-sensitized solar cell applications

Novel unsymmetrical organic sensitizers comprising donor, electron-conducting, and anchoring groups were engineered at a molecular level and synthesized for sensitization of mesoscopic titanium dioxide injection solar cells. The unsymmetrical organic sensitizers 3-(5-(4-(diphenylamino)styryl)thiophen-2-yl)-2-cyanoacrylic acid (D5), 3-(5-bis(4-(diphenylamino)styryl)thiophen-2-yl)-2-cyanoacrylic acid (D7), 5-(4-(bis(4-methoxyphenylamino)styryl)thiophen-2-yl)-2-cyanoacrylic acid (D9), and 3-(5-bis(4,4'-dimethoxydiphenylamino)styryl)thiophen-2-yl)-2-cyanoacrylic acid (D11) anchored onto TiO2 and were tested in dye-sensitized solar cell with a volatile electrolyte. The monochromatic incident photon-to-current conversion efficiency of these sensitizers is above 80%, and D11-sensitized solar cells yield a short-circuit photocurrent density of 13.90 +/- 0.2 mA/cm(2), an open-circuit voltage of 740 +/- 10 mV, and a fill factor of 0.70 +/- 0.02, corresponding to an overall conversion efficiency of 7.20% under standard AM 1.5 sun light. Detailed investigations of these sensitizers reveal that the long electron lifetime is responsible for differences in observed open-circuit potential of the cell. As an alternative to liquid electrolyte cells, a solid-state organic hole transporter is used in combination with the D9 sensitizer, which exhibited an efficiency of 3.25%. Density functional theory/time-dependent density functional theory calculations have been employed to gain insight into the electronic structure and excited states of the investigated species.     

Photophysical and Electrochemical Properties,and Molecular Structures of Organic Dyes for Dye‐Sensitized Solar Cells

Dye‐sensitized solar cells (DSSCs) based on organic dyes adsorbed on oxide semiconductor electrodes, such as TiO₂, ZnO, or NiO, which have emerged as a new generation of sustainable photovoltaic devices, have attracted much attention from chemists, physicists, and engineers because of enormous scientific interest in not only their construction and operational principles, but also in their high incident‐solar‐light‐to‐electricity conversion efficiency and low cost of production. To develop high‐performance DSSCs, it is important to create efficient organic dye sensitizers, which should be optimized for the photophysical and electrochemical properties of the dyes themselves, with molecular structures that provide good light‐harvesting features, good electron communication between the dye and semiconductor electrode and between the dye and electrolyte, and to control the molecular orientation and arrangement of the dyes on a semiconductor surface. The aim of this Review is not to make a list of a number of organic dye sensitizers developed so far, but to provide a new direction in the epoch‐making molecular design of organic dyes for high photovoltaic performance and long‐term stability of DSSCs, based on the accumulated knowledge of their photophysical and electrochemical properties, and molecular structures of the organic dye sensitizers developed so far.     

Engineering of highly efficient tetrahydroquinoline sensitizers for dye-sensitized solar cells

Four novel tetrahydroquinoline dyes by inserting isophorone and/or thiophene moieties as π bridge between the electron donating unit of substituted tetrahydroquinoline and the electron withdrawing unit of cyano carboxylic acid have been synthesized and successfully applied to dye-sensitized solar cells. Among them, DSCs sensitized by HYTIC, which shows the simplest molecular structure, exhibit improved efficiency of 7.0%. This by now is the highest efficiency for the reported tetrahydroquinoline sensitizers and comparable to the performance of N719-sensitized solar cells under the conditions employed here.     

A novel photoanode architecture of dye-sensitized solar cells based on TiO2 hollow sphere/nanorod array double-layer film

A novel TiO(2) double-layer (DL) film consisting of TiO(2) hollow spheres (HSs) as overlayer and single-crystalline TiO(2) nanorod arrays (RAs) as underlayer was designed as the photoanode of dye-sensitized solar cells (DSSCs). This new-typed TiO(2) HS/RA DL film could significantly improve the efficiency of DSSCs owing to its synergic effects, i.e. the relatively large specific surface area of TiO(2) HSs for effective dye adsorption, enhanced light harvesting capability originated from TiO(2) RA film, and rapid interfacial electron transport in one-dimensional TiO(2) nanorod arrays. The overall energy-conversion efficiency of 4.57% was achieved by the formation of TiO(2) DL film, which is 16% higher than that formed by TiO(2) HS film and far larger than that formed by TiO(2) RA film (η=0.99%). The light absorption and interfacial electron transport, which play important roles in the efficiency of DSSCs, were investigated by UV-vis absorption spectra and electrochemical impedance spectra.     

Substituent Effect on the π Linkers in Triphenylamine Dyes for Sensitized Solar Cells: A DFT/TDDFT Study

A series of metal‐free organic donor–π bridge–acceptor dyes are studied computationally using density functional theory (DFT) and time‐dependent DFT (TDDFT) approaches to explore their potential performances in dye‐sensitized solar cells (DSSCs). Taking triphenylamine (TPA) and cyanoacrylic acid moieties as donor and acceptor units, respectively, the effects of different substituents of the π linkers in the TPA‐based dyes on the energy conversion efficiency of the DSSCs are theoretically evaluated through optimized geometries, charge distributions, electronic structures, simulated absorption spectra, and free energies of injection. The results show that the molecular orbital energy levels and electron‐injection driving forces of the TPA dyes can be tuned by the introduction of substituents with different electron‐withdrawing or ‐donating abilities. The electron‐withdrawing substituent always lowers the energies of both frontier orbitals, while the electron‐donating one heightens them simultaneously. The efficiency trend of these TPA derivatives as sensitizers in DSSCs is also predicted by analyzing the light‐harvesting efficiencies and the free energies of injection. The following substituents are shown to increase the efficiency of the dye: OMe, OEt, OHe, and OH.     

有机染料敏化剂JK16和JK17的几何结构、电子结构及相关性质

运用密度泛函理论中的杂化泛函B3LYP研究了高效太阳能电池新型染料敏化剂JK16和JK17的几何结构、电子结构、极化率和超极化率, 并用含时密度泛函理论(TDDFT)研究了电子吸收谱. 基于含时密度泛函理论计算结果和实验结果的定性符合, 指认了在可见和近紫外区的吸收属于π→π*跃迁. 计算结果还表明JK16和JK17激发能最低的三个跃迁都与光诱导电荷转移过程有关, 而且二-二甲基芴氨基苯并噻吩基团对光电转换过程的敏化起主要作用, 发生于染料敏化剂JK16、JK17和TiO2界面之间的电荷转移是由染料分子激发态向半导体导带的电子注入过程. 此外, 通过对JK16和JK17的比较, 分析了亚乙烯基对几何结构、电子结构和谱学特性的影响.     

新型四硫富瓦烯-三苯胺类光敏染料理论研究

在简单结构的D-π-A三苯胺光敏染料(YD1)中引入不同数量的四硫富瓦烯(TTF)单元作为次级电子给体以增强有机光敏染料的给电子能力,设计了两个结构分别为D-D-π-A (YD2)以及2D-D-π-A (YD3)的光敏染料分子,并且采用密度泛函理论(DFT)和含时密度泛函理论(TD-DFT)分别模拟计算了纯光敏剂分子及其吸附二氧化钛团簇后的几何构型、电子结构以及光物理性能。采用周期性密度泛函理论模拟计算光敏染料分子在二氧化钛(101)面吸附的表面形貌以及态密度(DOS)。计算结果表明,TTF单元的引入不仅可以有效减少光敏染料分子的团聚,还可以提升其吸收性能。此外,光吸收效率(LHE)、电子注入驱动力(ΔG^inject)以及DOS的计算结果显示,YD2和YD3理论上可以呈现出比YD1更高的短路电流密度(J_sc)以及开路电压(V_oc)。因此,通过本文的理论研究表明,TTF单元可以作为有机光敏染料中的次级电子给体来改善光敏染料的性能。