Energy and hole transfer between dyes attached to titania in cosensitized dye-sensitized solar cells

Cosensitization of broadly absorbing ruthenium metal complex dyes with highly absorptive near-infrared (NIR) organic dyes is a clear pathway to increase near-infrared light harvesting in liquid-based dye-sensitized solar cells (DSCs). In cosensitized DSCs, dyes are intimately mixed, and intermolecular charge and energy transfer processes play an important role in device performance. Here, we demonstrate that an organic NIR dye incapable of hole regeneration is able to produce photocurrent via intermolecular energy transfer with an average excitation transfer efficiency of over 25% when cosensitized with a metal complex sensitizing dye (SD). We also show that intermolecular hole transfer from the SD to NIR dye is a competitive process with dye regeneration, reducing the internal quantum efficiency and the electron lifetime of the DSC. This work demonstrates the general feasibility of using energy transfer to boost light harvesting from 700 to 800 nm and also highlights a key challenge for developing highly efficient cosensitized dye-sensitized solar cells.     

Absolute rates of hole transfer in DNA

Absolute rates of hole transfer between guanine nucleobases separated by one or two A:T base pairs in stilbenedicarboxamide-linked DNA hairpins were obtained by improved kinetic analysis of experimental data. The charge-transfer rates in four different DNA sequences were calculated using a density-functional-based tight-binding model and a semiclassical superexchange model. Site energies and charge-transfer integrals were calculated directly as the diagonal and off-diagonal matrix elements of the Kohn-Sham Hamiltonian, respectively, for all possible combinations of nucleobases. Taking into account the Coulomb interaction between the negative charge on the stilbenedicarboxamide linker and the hole on the DNA strand as well as effects of base pair twisting, the relative order of the experimental rates for hole transfer in different hairpins could be reproduced by tight-binding calculations. To reproduce quantitatively the absolute values of the measured rate constants, the effect of the reorganization energy was taken into account within the semiclassical superexchange model for charge transfer. The experimental rates could be reproduced with reorganization energies near 1 eV. The quantum chemical data obtained were used to discuss charge carrier mobility and hole-transport equilibria in DNA.     

Understanding the role of the sulfide redox couple (S2-/S(n)2-) in quantum dot-sensitized solar cells

The presence of sulfide/polysulfide redox couple is crucial in achieving stability of metal chalcogenide (e.g., CdS and CdSe)-based quantum dot-sensitized solar cells (QDSC). However, the interfacial charge transfer processes play a pivotal role in dictating the net photoconversion efficiency. We present here kinetics of hole transfer, characterization of the intermediates involved in the hole oxidation of sulfide ion, and the back electron transfer between sulfide radical and electrons injected into TiO(2) nanoparticles. The kinetic rate constant (10(7)-10(9) s(-1)) for the hole transfer obtained from the emission lifetime measurements suggests slow hole scavenging from CdSe by S(2-) is one of the limiting factors in attaining high overall efficiency. The presence of the oxidized couple, by addition of S or Se to the electrolyte, increases the photocurrent, but it also enhances the rate of back electron transfer.     

Side-chain engineering of high-efficiency conjugated polymer photovoltaic materials

In recent years,conjugated polymers have attracted great attention in the application as photovoltaic donor materials in polymer solar cells(PSCs).Broad absorption,lower-energy bandgap,higher hole mobility,relatively lower HOMO energy levels,and higher solubility are important for the conjugated polymer donor materials to achieve high photovoltaic performance.Side-chain engineering plays a very important role in optimizing the physicochemical properties of the conjugated polymers.In this article,we review recent progress on the side-chain engineering of conjugated polymer donor materials,including the optimization of flexible side-chains for balancing solubility and intermolecular packing(aggregation),electron-withdrawing substituents for lowering HOMO energy levels,and two-dimension(2D)-conjugated polymers with conjugated side-chains for broadening absorption and enhancing hole mobility.After the molecular structural optimization by side-chain engineering,the2D-conjugated polymers based on benzodithiophene units demonstrated the best photovoltaic performance,with powerconversion efficiency higher than 9%.     

Excited Electron–Hole States in Molecular Chains

The eigenvalues and the eigenfunctions of molecular excitons, charge-transfer excitons, and electron–hole pairs have been found in the approximation of electron and hole transfer between the lowest unoccupied and highest occupied orbitals in a rigid molecular chain of identical photosensitive molecules, the recognized model of organic solar cells. It has been shown that as the Coulomb binding energy decreases, the wave functions become superposition of functions of the increasing number of sites and the decay time, determined by electron or hole transitions, is shorter that the transfer time of the exciton as a whole, so that energy transfer and charge transfer become interrelated processes.     

Contributions of the distance-dependent reorganization energy and proton-transfer to the hole-transfer process in DNA

A kinetic study of the single-step hole transfer in DNA was performed by measuring time-resolved transient absorption. DNA molecules with various sequences were designed and conjugated with naphthalimide (NI) and phenothiazine (PTZ) to investigate the sequence and distance dependence of the single-step hole transfer between guanines (Gs). Hole injection into DNA was accomplished by excitation of the NI site with a 355 nm laser pulse, and the kinetics of the hole-transfer process were investigated by monitoring the transient absorption of the PTZ radical cation (PTZ.+). Kinetic analysis of the time profile of PTZ.+ based on the kinetic model showed that the distance dependence of the hole-transfer process was significantly influenced by the DNA sequence. Results of temperature- and isotope-effect experiments demonstrated that the activation energy increased as the number of bridge bases separating the Gs increased. This is because of the distance-dependent reorganization energy and contribution of the proton-transfer process to the hole transfer in DNA.     

Photosynthetic models with maximum entropy production in irreversible charge transfer steps

Steady-state bacterial photosynthesis is modelled as cyclic chemical reaction and is examined with respect to overall efficiency, power transfer efficiency, and entropy production. A nonlinear flux–force relationship is assumed. The simplest two-state kinetic model bears complete analogy with the performance of an ideal (zero ohmic resistance of the P–N junction) solar cell. In both cases power transfer to external load is much higher than the 50% allowed by the impedance matching theorem for the linear flux–force relationship. When maximum entropy production is required in the transition with a load, one obtains high optimal photochemical yield of 97% and power transfer efficiency of 91%. In more complex photosynthetic models, entropy production is maximized in all irreversible electron/proton (non-slip) transitions in an iterative procedure. The resulting steady-state is stable with respect to an extremely wide range of initial values for forward rate constants. Optimal proton current increases proportionally to light intensity and decreases with an increase in the proton-motive force (the backpressure effect). Optimal affinity transfer efficiency is very high and nearly perfectly constant for different light absorption rates and for different electrochemical proton gradients. Optimal overall efficiency (of solar into proton-motive power) ranges from 10% (bacteriorhodopsin) to 19% (chlorophyll-based bacterial photosynthesis). Optimal time constants in a photocycle span a wide range from nanoseconds to milliseconds, just as corresponding experimental constants do. We conclude that photosynthetic proton pumps operate close to the maximum entropy production mode, connecting biological to thermodynamic evolution in a coupled self-amplifying process.     

Coupled Solar Battery with 6.9 % Efficiency

Solar-to-electrochemical energy storage in solar batteries is an important solar utilization technology comparable to solar-to-electricity (solar cells) and solar-to-fuel (photocatalytic cells) conversion. Unlike the indirect approach of integrated solar flow batteries combining photoelectrodes with redox-electrodes, coupled solar batteries enable direct solar energy storage, but are hampered by low efficiency due to rapid charge recombination of materials and misaligned energy levels between electrodes. Herein, we propose a design for a coupled solar battery that intercouples two photo-coupled ion transfer (PCIT) reactions through electron-ion transfer upon co-photo-pumping of photoelectrochemical storage cathode and anode. We used a representative covalent organic framework (COF) to achieve efficient charge separation and directional charge transfer between two band-matched photoelectrochemical storage electrodes, with a photovoltage sufficient for COF dual-redox reactions. By pumping these electrodes, the coupled solar battery stores solar energy via two synergistic PCIT reactions of electron-proton-relayed COF oxidation and reduction, and the stored solar energy is released as electrochemical energy during COF regeneration in discharge while interlocking the loops. A breakthrough in efficiency (6.9 %) was achieved, adaptive to a large-area (56 cm²) tandem device. The presented photo-intercoupled electron-ion transfer (PIEIT) mechanism provides expandable paths toward practical solar-to-electrochemical energy storage.     

Exciton dynamics in alternating polyfluorene/fullerene blends

Exciton dynamics in alternating copolymer/fullerene solar cell blends have been investigated using femtosecond transient absorption spectroscopy. The acceptor concentrations have been varied over a wide range. Experimental data, kinetic modeling and simulations, all indicate that the efficiency of exciton conversion to charges is 100% even at acceptor concentrations as low as 20 wt%. The reported dependence of solar cell efficiency on fullerene concentration may thus arise from other factors. However, there exists an acceptor concentration threshold (5 wt%) below which a substantial fraction of the excitations remain unquenched. The results, we believe are very relevant to optimization of performance efficiency by clever manipulation of morphology. We have also observed exciton–exciton energy transfer in these blends at low acceptor concentrations.     

环聚炔苯和环聚炔吡啶组成的盘状液晶中的电荷转移

用电子转移的半经典模型在量子化学B3LYP/6-31G(d)水平(对单体)和B3LYP/STO-3G水平(对二聚物)对环聚炔苯和环聚炔吡啶组成的盘状液晶体系的电荷转移性质进行了研究. 盘状液晶体系的电荷转移速率主要依赖于重组能和电荷转移矩阵元, 重组能越小, 电荷转移矩阵元越大, 则电荷转移速率常数越大. 计算结果表明, 这些大环化合物比目前广泛研究和应用的苯并菲衍生物组成的液晶有较小的重组能, 所以有更好的电荷转移性质. 计算结果对有效地设计和合成高效的光导材料和载流子输送材料是有帮助的.     

How TiO(2) crystallographic surfaces influence charge injection rates from a chemisorbed dye sensitiser

High-energy metal oxide surfaces are considered to be promising for applications involving surface-adsorbate electron transfer, such as photocatalysis and dye-sensitised solar cells. Here, we compare the efficiency of electron injection into different TiO(2) anatase surfaces. We model the adsorption of a carboxylic acid (formic acid) on anatase (101), (001), (100), (110) and (103) surfaces using density functional theory calculations, and calculate electron injection times from a model dye into these surfaces. We find that the different positions of the conduction band edge of these surfaces determine the rate of electron injection (which is faster for the surfaces with lower-lying conduction band, among them the most stable (101) surface). However, if the dye's injection energy is enforced to be at a fixed energy deep inside each surface's conduction band, then several anatase surfaces, such as the synthetically achievable (001) surface, show rates of injection comparable or faster than the (101) surface. Moreover, because of their higher-lying conduction bands, these minority surfaces are likely to offer higher open-circuit voltages in dye-sensitised solar cells. Therefore, synthetically accessible high-energy anatase surfaces, such as (001)-oriented nanostructures, may be promising candidates for use in dye-sensitised solar cells.     

Design Principles for Maximizing Hole Utilization of Semiconductor Quantum Wires toward Efficient Photocatalysis

Maximizing hole-transfer kinetics—usually a rate-determining step in semiconductor-based artificial photosynthesis—is pivotal for simultaneously enabling high-efficiency solar hydrogen production and hole utilization. However, this remains elusive yet as efforts are largely focused on optimizing the electron-involved half-reactions only by empirically employing sacrificial electron donors (SEDs) to consume the wasted holes. Using high-quality ZnSe quantum wires as models, we show that how hole-transfer processes in different SEDs affect their photocatalytic performances. We found that larger driving forces of SEDs monotonically enhance hole-transfer rates and photocatalytic performances by almost three orders of magnitude, a result conforming well with the Auger-assisted hole-transfer model in quantum-confined systems. Intriguingly, further loading Pt cocatalyts can yield either an Auger-assisted model or a Marcus inverted region for electron transfer, depending on the competing hole-transfer kinetics in SEDs.     

Contribution from a hole-conducting dye to the photocurrent in solid-state dye-sensitized solar cells

The hole transporting medium in solid-state dye-sensitized solar cells can be utilized to harvest sunlight. Herein we demonstrate that a triphenylamine-based dye, used as hole-transporting medium, contributes to the photocurrent in a squaraine-sensitized solid-state dye-sensitized solar cell. Steady-state photoluminescence measurements have been used to distinguish between electron transfer and energy transfer processes leading to energy conversion upon light absorption in the hole-transporting dye.     

Charge hopping in DNA.

The efficiency of charge migration through stacked Watson-Crick base pairs is analyzed for coherent hole motion interrupted by localization on guanine (G) bases. Our analysis rests on recent experiments, which demonstrate the competition of hole hopping transitions between nearest neighbor G bases and a chemical reaction of the cation G(+) with water. In addition, it has been assumed that the presence of units with several adjacent stacked G bases on the same strand leads to the additional vibronic relaxation process (G(+)G...G) --> (GG...G)(+). The latter may also compete with the hole transfer from (G(+)G...G) to a single G site, depending on the relative positions of energy levels for G(+) and (G(+)G...G). A hopping model is proposed to take the competition of these three rate steps into account. It is shown that the model includes two important limits. One corresponds to the situation where the charge relaxation inside a multiple guanine unit is faster than hopping. In this case hopping is terminated by several adjacent G bases located on the same strand, as has been observed for the GGG triple. In the opposite, slow relaxation limit the GG...G unit allows a hole to migrate further in accord with experiments on strand cleavage exploiting GG pairs. We demonstrate that for base pair sequences with only the GGG triple, the fast relaxation limit of our model yields practically the same sequence- and distance dependencies as measurements, without invoking adjustable parameters. For sequences with a certain number of repeating adenine:thymine pairs between neighboring G bases, our analysis predicts that the hole transfer efficiency varies in inverse proportion to the sequence length for short sequences, with change to slow exponential decay for longer sequences. Calculations performed within the slow relaxation limit enable us to specify parameters that provide a reasonable fit of our numerical results to the hole migration efficiency deduced from experiments with sequences containing GG pairs. The relation of the results obtained to other theoretical and experimental studies of charge transfer in DNA is discussed. We propose experiments to gain a deeper insight into complicated kinetics of charge-transfer hopping in DNA.     

Improving the performance of solid-state dye-sensitized solar cell using MgO-coated TiO2 nanoporous film

An ultrathin overlayer of MgO on TiO2 is shown to drastically improve the stability of solid-state dye-sensitized solar cell using CuI as a hole conductor in addition to solar energy conversion efficiency.     

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.     

Adiabatic transfer of population in a dense fluid: the role of dephasing statistics

We report the results of simulation studies of the statistics of vibrational dephasing of a YCl (Y=H, D, T, and I) diatom in dense fluid Ar at two temperatures, including the effect of strong field driving on the energy level modulation statistics. The distribution of energy level modulations is found to be non-Gaussian with a high energy tail. Aspects of stimulated Raman adiabatic passage (STIRAP) between the vibrational levels of HCl in dense fluid Ar have been investigated. For HCl with nearly degenerate v=0-->v=1 and v=1-->v=2 transitions, the combined effect of modulation and power broadening reduces the STIRAP efficiency for population transfer from v=0 to v=2 of the order of 30%. However, if the transitions used have very different frequencies, as in the original model studied by Demirplak and Rice [J. Chem. Phys. 116, 8028 (2002)], the STIRAP efficiency for population transfer remains high, of the order of 80%, even with non-Gaussian modulation of energy levels.     

State-to-state rotational relaxation rate constants for CO+Ne from IR-IR double-resonance experiments: comparing theory to experiment

IR-IR double-resonance experiments were used to study the state-to-state rotational relaxation of CO with Ne as a collision partner. Rotational levels in the range Ji=2-9 were excited and collisional energy transfer of population to the levels Jf=2-8 was monitored. The resulting data set was analyzed by fitting to numerical solutions of the master equation. State-to-state rate constant matrices were generated using fitting law functions. Fitting laws based on the modified exponential gap (MEG) and statistical power exponential gap (SPEG) models were used; the MEG model performed better than the SPEG model. A rate constant matrix was also generated from scattering calculations that employed the ab initio potential energy surface of McBane and Cybulski [J. Chem. Phys. 110, 11 734 (1999)]. This theoretical rate constant matrix yielded kinetic simulations that agreed with the data nearly as well as the fitted MEG model and was unique in its ability to reproduce both the rotational energy transfer and pressure broadening data for Ne-CO. The theoretical rate coefficients varied more slowly with the energy gap than coefficients from either of the fitting laws.     

Characterization of photoinduced self-exchange reactions at molecule-semiconductor interfaces by transient polarization spectroscopy: lateral intermolecular energy and hole transfer across sensitized TiO2 thin films

Transient anisotropy measurements are reported as a new spectroscopic tool for mechanistic characterization of photoinduced charge-transfer and energy-transfer self-exchange reactions at molecule-semiconductor interfaces. An anisotropic molecular subpopulation was generated by photoselection of randomly oriented Ru(II)-polypyridyl compounds, anchored to mesoscopic nanocrystalline TiO(2) or ZrO(2) thin films, with linearly polarized light. Subsequent characterization of the photoinduced dichromism change by visible absorption and photoluminescence spectroscopies on the nano- to millisecond time scale enabled the direct comparison of competitive processes: excited-state decay vs self-exchange energy transfer, or interfacial charge recombination vs self-exchange hole transfer. Self-exchange energy transfer was found to be many orders-of-magnitude faster than hole transfer at the sensitized TiO(2) interfaces; for [Ru(dtb)(2)(dcb)](PF(6))(2), where dtb is 4,4'-(C(CH(3))(3))(2)-2,2'-bipyridine and dcb is 4,4'-(COOH)(2)-2,2'-bipyridine, anchored to TiO(2), the energy-transfer correlation time was θ(ent) = 3.3 μs while the average hole-transfer correlation time was <θ(h+)> = 110 ms, under identical experimental conditions. Monte Carlo simulations successfully modeled the anisotropy decays associated with lateral energy transfer. These mesoscopic, nanocrystalline TiO(2) thin films developed for regenerative solar cells thus function as active "antennae", harvesting sunlight and transferring energy across their surface. For the dicationic sensitizer, [Ru(dtb)(2)(dcb)](PF(6))(2), anisotropy changes indicative of self-exchange hole transfer were observed only when ions were present in the acetonitrile solution. In contrast, evidence for lateral hole transfer was observed in neat acetonitrile for a neutral sensitizer, cis-Ru(dnb)(dcb)(NCS)(2), where dnb is 4,4'-(CH(3)(CH(2))(8))(2)-2,2'-bipyridine, anchored to TiO(2). The results indicate that mechanistic models of interfacial charge recombination between electrons in TiO(2) and oxidized sensitizers must take into account diffusion of the injected electron and the oxidized sensitizer. The phenomena presented herein represent two means of concentrating potential energy derived from visible light that could be used to funnel energy to molecular catalysts for multiple-charge-transfer reactions, such as the generation of solar fuels.     

Hole transfer dynamics from dye molecules to p-type NiO nanoparticles: effects of processing conditions

Hole transfer dynamics of Atto647N sensitized p-type NiO nanoparticle (NP) thin films is investigated using both ensemble-averaged and single-molecule spectroscopy techniques. The rate of hole transfer is dependent on the processing conditions and is enhanced when the NiO is pre-annealed in air as compared to vacuum. This is possibly due to an upward shift of the valence band of the semiconductor and an increase in the free energy for hole transfer as more Ni(2)O(3) are formed in the presence of air. The stretched exponential fluorescence decay profile of Atto647N on NiO NP suggests the presence of a distribution of hole transfer rates. This is in agreement with the observed emission lifetime and intensity fluctuations and non-monoexponential fluorescence decays for individual Atto647N molecules on NiO NP films. A plausible explanation for the heterogeneous hole transfer rates is an inhomogeneous distribution of (defect) sites on the metal oxide due to the processing conditions and a fluctuation in the intermolecular interaction.