Slow charge recombination in dye-sensitised solar cells (DSSC) using Al2O3 coated nanoporous TiO2 films

The conformal growth of an overlayer of Al2O3 on a nanocrystalline TiO2 film is shown to result in a 4-fold retardation of interfacial charge recombination, and a 30% improvement in photovoltaic device efficiency.     

Charge separation versus recombination in dye-sensitized nanocrystalline solar cells: the minimization of kinetic redundancy

In this paper we focus upon the electron injection dynamics in complete dye-sensitized nanocrystalline metal oxide solar cells (DSSCs). Electron injection dynamics are studied by transient absorption and emission studies of DSSCs and correlated with device photovoltaic performance and charge recombination dynamics. We find that the electron injection dynamics are dependent upon the composition of the redox electrolyte employed in the device. In a device with an electrolyte composition yielding optimum photovoltaic device efficiency, electron injection kinetics exhibit a half time of 150 ps. This half time is 20 times slower than that for control dye-sensitized films covered in inert organic liquids. This retardation is shown to result from the influence of the electrolyte upon the conduction band energetics of the TiO2 electrode. We conclude that optimum DSSC device performance is obtained when the charge separation kinetics are just fast enough to compete successfully with the dye excited-state decay. These conditions allow a high injection yield while minimizing interfacial charge recombination losses, thereby minimizing "kinetic redundancy" in the device. We show furthermore that the nonexponential nature of the injection dynamics can be simulated by a simple inhomogeneous disorder model and discuss the relevance of our findings to the optimization of both dye-sensitized and polymer based photovoltaic devices.     

Charge transport versus recombination in dye-sensitized solar cells employing nanocrystalline TiO2 and SnO2 films

We report a comparison of charge transport and recombination dynamics in dye-sensitized solar cells (DSSCs) employing nanocrystalline TiO(2) and SnO(2) films and address the impact of these dynamics upon photovoltaic device efficiency. Transient photovoltage studies of electron transport in the metal oxide film are correlated with transient absorption studies of electron recombination with both oxidized sensitizer dyes and the redox couple. For all three processes, the dynamics are observed to be 2-3 orders of magnitude faster for the SnO(2) electrode. The origins of these faster dynamics are addressed by studies correlating the electron recombination dynamics to dye cations with chronoamperometric studies of film electron density. These studies indicate that the faster recombination dynamics for the SnO(2) electrodes result both from a 100-fold higher electron diffusion constant at matched electron densities, consistent with a lower trap density for this metal oxide relative to TiO(2), and from a 300 mV positive shift of the SnO(2) conduction band/trap states density of states relative to TiO(2). The faster recombination to the redox couple results in an increased dark current for DSSCs employing SnO(2) films, limiting the device open-circuit voltage. The faster recombination dynamics to the dye cation result in a significant reduction in the efficiency of regeneration of the dye ground state by the redox couple, as confirmed by transient absorption studies of this reaction, and in a loss of device short-circuit current and fill factor. The importance of this loss pathway was confirmed by nonideal diode equation analyses of device current-voltage data. The addition of MgO blocking layers is shown to be effective at reducing recombination losses to the redox electrolyte but is found to be unable to retard recombination dynamics to the dye cation sufficiently to allow efficient dye regeneration without resulting in concomitant losses of electron injection efficiency. We conclude that such a large acceleration of electron dynamics within the metal oxide films of DSSCs may in general be detrimental to device efficiency due to the limited rate of dye regeneration by the redox couple and discuss the implications of this conclusion for strategies to optimize device performance.     

Molecular control of recombination dynamics in dye-sensitized nanocrystalline TiO2 films: free energy vs distance dependence

In this paper we address the dependence of the charge recombination dynamics in dye-sensitized, nanocrystalline TiO2 films upon the properties of the sensitizer dye employed. In particular we focus upon dependence of the charge recombination kinetics upon the dye oxidation potential E0(D+/D), determined electrochemically, and the spatial separation r of the dye cation HOMO orbital from the metal oxide surface, determined by semiempirical calculations. Our studies employed a series of ruthenium bipyridyl dyes in addition to porphyrin and phthalocyanine dyes. A strong correlation is observed between the recombination dynamics and the spatial separation r, with variation in r by 3 A resulting in a more than 10-fold change in the recombination half-time t(50%). This correlation is found to be in agreement with electron tunneling theory, t(50%) proportional, variant exp(-betar) with beta = 0.95 +/- 0.2 A-1. In contrast, the recombination dynamics were found to be relatively insensitive to variations in E0D+/D), indicative of the recombination reaction lying near the peak of the Marcus free energy curve, DeltaG approximately lambda, and with lambda approximately 0.8 eV. A correlation is also observed between the recombination half-time and the temporal shape of the kinetics, with faster recombination dynamics being more dispersive (less monoexponential). Comparison with numerical Monte Carlo type simulations suggests this correlation is attributed to a shift from fast recombination dynamics primarily limited by dispersive electron transport within the metal oxide film to slower dynamics primarily limited by the interfacial electron-transfer reaction. We conclude that the primary factor controlling the charge recombination dynamics in dye-sensitized, nanocrystalline TiO2 films is the spatial separation of the dye cation from the electrode surface. In particular, we show that for the Ru(dcbpy)2NCS2 dye series, the use of X = NCS rather than X = CN results in a 2 A shift in the dye cation HOMO orbital away from the electrode surface, causing a 7-fold retardation of the recombination dynamics, resulting in the remarkably slow recombination dynamics observed for this sensitizer dye.     

Interfacial electron transfer on cytochrome-c sensitised conformally coated mesoporous TiO2 films

Hybrid protein films incorporating Cyt-c immobilized on TiO(2) films were prepared and characterised optically with UV-visible spectroscopy and electrochemically with cyclic voltammetry, and their conductivity properties were studied in detail. In addition the effects of a thin overlayer coating of a second metal oxide such as SiO(2), Al(2)O(3), ZrO(2) and MgO(2) were studied and the effects over the electrochemical properties of the hybrid working electrodes were discussed.     

Multistep electron transfer processes on dye co-sensitized nanocrystalline TiO2 films

We report a method for achieving multilayer co-sensitization of nanocrystalline TiO2 films. The method is based upon an aluminum isopropoxide treatment of the monosensitized film prior to deposition of a second sensitizer. Appropriate selection of sensitizer dyes allows vectorial, multistep, electron transfer processes, resulting in a suppression of interfacial charge recombination and a significantly improved photovoltaic device performance relative to single-layer co-sensitization devices.     

Interfacial charge recombination between e(-)-TiO2 and the I(-)/I3(-) electrolyte in ruthenium heteroleptic complexes: dye molecular structure-open circuit voltage relationship

A series of heteroleptic ruthenium(II) polypyridyl complexes containing phenanthroline ligands have been designed, synthesized, and characterized. The spectroscopic and electrochemical properties of the complexes have been studied in solution and adsorbed onto semiconductor nanocrystalline metal oxide particles. The results show that for two of the ruthenium complexes, bearing electron-donating (-NH2) or electron-withdrawing (-NO2) groups, the presence of the redox-active I(-)/I3(-) electrolyte produces important changes in the interfacial charge transfer processes that limit the device performance. For example, those dyes enhanced the electron recombination reaction between the photoinjected electrons at TiO2 and the oxidized redox electrolyte. In an effort to understand the details of such striking observations, we have monitored the charge transfer reactions taking place at the different interfaces of the devices using time-resolved single photon counting, laser transient spectroscopy, and light-induced photovoltage measurements.     

Effects of Metal Oxide Modifications on Photoelectrochemical Properties of Mesoporous TiO2 Nanoparticles Electrodes for Dye-Sensitized Solar Cells

Mesoporous TiO₂ (m-TiO₂) nanoparticles were used to prepare the porous film electrodes for dye-sensitized solar cells, and a second metal oxide (MgO, ZnO, Al₂O₃, or NiO) modifi-cation was carried out by dipping the m-TiO₂ electrode into their respective nitrate solution followed by annealing at 500 oC. Experimental results indicated that the above second metal oxide modifications on m-TiO₂ electrode are shown in all cases to act as barrier layer for the interfacial charge transfer processes, but film electron transport and interfacial charge recombination characteristics under applied bias voltage were dependent significantly on the existing states and kinds of these second metal oxides. Those changes based on sec-ond metal oxide modifications showed good correlation with the current-voltage analyses of dye-sensitized solar cell, and all modifications were found to increase the open-circuit photo-voltage in various degrees, while the MgO, ZnO, and NiO modifications result in 23%, 13%, and 6% improvement in cell conversion efficiency, respectively. The above observations indi-cate that controlling the charge transport and recombination is very important to improve the photovoltaic performance of TiO₂-based solar cell.     

电极功函数对TiO2/PbS平面异质结激子太阳电池性能影响的理论模拟

采用一维微光电子结构分析模型(AMPS-1D)软件模拟分析了由TiO2纳米晶薄膜与PbS量子点薄膜组成的平面异质结激子太阳电池中电极功函数对电池性能的影响. 通过在界面上引入厚度为2 nm的激子发生分离的自由载流子产生层, 得到电池的电流-电压曲线、 电子电流及空穴电流的空间分布等信息. 模拟结果表明, 透明导电氧化物电极的功函数可以在一定区间内变化而不影响电池效率, 但是金属电极功函数的变化则会明显影响电池输出性能, 这是因为PbS与金属电极之间的肖特基(Schottky)接触会对电池性能产生负面作用.     

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.     

Time—resolved Microwave Conductivity Studies on the Chemical Treatment of the Nanocrystalline Porous TiO2 Films

Effect of Ti(iso-C3H7O)4 treatment on the photoinduced charge carrier kinetics of nanocrystalline porous TiO2 films is studied by time-resolved microwave conductivity measurements. Analysis of the transient photoconductivity decays indicates that Ti(iso-C3H7O)4 treatment leads to an increased concentration of photogenerated charge carriers and a fast interfacial transfer rate of holes via the surface modification of the fleshly growing TiO2 nanocrystallites.     

Effects of electron transfer between TiO2 films and conducting substrates on the photocatalytic oxidation of organic pollutants

TiO2 films on Al alloy (Al), indium-tin oxide glass (ITO/glass), and glass were prepared by a dip-coating method. ITO is found to have a higher work function, while the work function for Al is lower than that of TiO2 films. An electron transfer is indicated to occur in the interfaces between TiO2 films and conducting substrate Al or ITO, which results in an Ohm contact or Schottky barrier under the transient equilibrium UV radiation conditions. Photocatalytic measurements showed that the TiO2 films on Al have a higher activity for photocatalytic oxidation of C2H4, but the activity for photocatalytic degradation of oleic acid is lower as compared with TiO2 films on glass. Alternatively, TiO2 films on ITO give completely contrary photocatalytic performance to those on Al. These observations could be associated with the electron transfer, in which Al acts as an electron donor and offers electrons to TiO2, allowing photocatalytic oxidation of ethylene to proceed by the photogenerated electrons, while ITO could be an acceptor for the photogenerated electrons, which is beneficial to photocatalytic degradation of oleic acid by the photogenerated holes. This electron-transfer model could be extended to other photocatalytic systems.     

Fe3+/V5+/TiO2复合纳米微粒光催化性能的研究

采用溶胶凝胶法制备了Fe^3 /V^5 /TiO2复合纳米微粒作为光催化剂。光降解反应结果表明，其掺杂催化剂Fe^3 /V^5 /TiO2的光催化活性明显提高。光电化学研究显示，铁离子可以成为电荷陷阱，促进空穴的界面传递反应。适量钒离子掺杂使TiO2电极的光电流升高，导带中电子浓度的增大，加快了界面的电子传递反应。共掺杂催化剂中，Fe^3 、V^5 分别提供了空穴与电子的陷阱，同时加快了电子与空穴的界面传递反应，从 更有效地提高光催化活性。双组份共掺杂为提高TiO2光催化活性提供新的途径。     

The influence of dye structure on charge recombination in dye-sensitized solar cells

Replacing the nonyl groups on the solar cell dye Ru(4,4'-carboxylic acid-2,2'-bipyridine)(4,4'-dinonyl-2,2'-bipyridine)(NCS)(2) (Z-907) with amino groups results in a marked decrease in solar cell performance. This is despite the fact that the amino derivative (Z-960) has more favourable light absorption characteristics than Z-907 when used with thick nanocrystalline TiO(2) layers. Electron transfer to the electrolyte from the exposed fluorine-doped tin oxide (FTO) substrate is particularly fast in cells employing the Z-960 dye if a compact TiO(2) blocking layer is not used. The kinetics of electron transfer from the nanocrystalline TiO(2) layer in DSCs employing Z-960 are comparable to those of bare TiO(2) and ca. 2 to 5 times faster than for cells employing Z-907. The faster charge recombination in cells employing Z-960 lowers open-circuit photovoltage and results in very significant charge collection losses that lower short-circuit photocurrent. Voltammetric measurements show that surface modification of FTO electrodes with Z-960 results in slightly more facile charge transfer to acceptor species in triiodide/iodide electrolytes in the dark. A simpler molecule, p-aminobenzoic acid, more dramatically catalyses this charge transfer reaction. Conversely, chemical modification of FTO electrodes with Z-907 or p-toluic acid retards charge transfer kinetics. Similar results are obtained for nanocrystalline TiO(2) electrodes modified with these benzoic acid derivatives. These results strongly imply that surface adsorbed molecules bearing amino groups, including dye molecules, can catalyse charge recombination in dye-sensitized solar cells.     

金属氧化物(Fe2O3, CuO,NiO)改性对TiO2纳米管阵列光电催化活性的增强效应

采用阳极氧化法和阴极电沉积法制备了Fe2O3,CuO和NiO纳米粒子改性的高度有序的TiO2纳米管(TiO2-NT)阵列.运用场发射扫描电子显微镜(FE-SEM),透射电子显微镜(TEM),X射线衍射(XRD)和紫外-可见漫反射光谱等手段对Fe2O3/TiO2-NT、CuO/TiO2-NT和NiO/TiO2-NT复合电极进行表征.以苯酚为模拟污染物,考察复合电极的光电性能.结果表明,金属氧化物(Fe2O3,CuO,NiO)纳米粒子成功沉积在TiO2-NTs的管口、内壁和管底.金属氧化物改性复合电极的光电催化活性比未改性的TiO2-NTs提高了2倍以上.Fe2O3/TiO2-NTs在可见光区显示出最高的吸收强度.以Fe2O3/TiO2-NTs为阳极处理苯酚废水,光照120min后苯酚去除率达到96%,而未改性的TiO2-NTs的苯酚去除率只有41%.此外,Fe2O3/TiO2-NTs在生成低毒中间产物方面表现出良好的性能.较高的复合电极光电催化活性主要是由于TiO2纳米管和过渡金属氧化物纳米粒子间构筑的高界面面积异质纳米结构,有效地促进了电子转移,抑制了光生电子-空穴对的复合.     

Correlating interface heterostructure, charge recombination, and device efficiency of poly(3-hexyl thiophene)/TiO2 nanorod solar cell

The charge recombination rate in poly(3-hexyl thiophene)/TiO(2) nanorod solar cells is demonstrated to correlate to the morphology of the bulk heterojunction (BHJ) and the interfacial properties between poly(3-hexyl thiophene) (P3HT) and TiO(2). The recombination resistance is obtained in P3HT/TiO(2) nanorod devices by impedance spectroscopy. Surface morphology and phase separation of the bulk heterojunction are characterized by atomic force microscopy (AFM). The surface charge of bulk heterojunction is investigated by Kelvin probe force microscopy (KPFM). Lower charge recombination rate and lifetime have been observed for the charge carriers in appropriate heterostructures of hybrid P3HT/TiO(2) nanorod processed via high boiling point solvent and made of high molecular weight P3HT. Additionally, through surface modification on TiO(2) nan,orod, decreased recombination rate and longer charge carrier lifetime are obtained owing to creation of a barrier between the donor phases (P3HT) and the acceptor phases (TiO(2)). The effect of the film morphology of hybrid and interfacial properties on charge carrier recombination finally leads to different outcome of photovoltaic I-V characteristics. The BHJ fabricated from dye-modified TiO(2) blended with P3HT exhibits 2.6 times increase in power conversion efficiency due to the decrease of recombination rate by almost 2 orders of magnitude as compared with the BHJ made with unmodified TiO(2). In addition, the interface heterostructure, charge lifetime, and device efficiency of P3HT/TiO(2) nanorod solar cells are correlated.     

Toward exceeding the Shockley-Queisser limit: photoinduced interfacial charge transfer processes that store energy in excess of the equilibrated excited state

Nanocrystalline (anatase), mesoporous TiO2 thin films were functionalized with [Ru(bpy)2(deebq)](PF6)2, [Ru(bq)2(deeb)](PF6)2, [Ru(deebq)2(bpy)](PF6)2, [Ru(bpy)(deebq)(NCS)2], or [Os(bpy)2(deebq)](PF6)2, where bpy is 2,2'-bipyridine, bq is 2,2'-biquinoline, and deeb and deebq are 4,4'-diethylester derivatives. These compounds bind to the nanocrystalline TiO2 films in their carboxylate forms with limiting surface coverages of 8 (+/- 2) x 10(-8) mol/cm2. Electrochemical measurements show that the first reduction of these compounds (-0.70 V vs SCE) occurs prior to TiO2 reduction. Steady state illumination in the presence of the sacrificial electron donor triethylamine leads to the appearance of the reduced sensitizer. The thermally equilibrated metal-to-ligand charge-transfer excited state and the reduced form of these compounds do not inject electrons into TiO2. Nanosecond transient absorption measurements demonstrate the formation of an extremely long-lived charge separated state based on equal concentrations of the reduced and oxidized compounds. The results are consistent with a mechanism of ultrafast excited-state injection into TiO2 followed by interfacial electron transfer to a ground-state compound. The quantum yield for this process was found to increase with excitation energy, a behavior attributed to stronger overlap between the excited sensitizer and the semiconductor acceptor states. For example, the quantum yields for [Os(bpy)2(dcbq)]/TiO2 were phi(417 nm) = 0.18 +/- 0.02, phi(532.5 nm) = 0.08 +/- 0.02, and phi(683 nm) = 0.05 +/- 0.01. Electron transfer to yield ground-state products occurs by lateral intermolecular charge transfer. The driving force for charge recombination was in excess of that stored in the photoluminescent excited state. Chronoabsorption measurements indicate that ligand-based intermolecular electron transfer was an order of magnitude faster than metal-centered intermolecular hole transfer. Charge recombination was quantified with the Kohlrausch-Williams-Watts model.     

A nuclear isotope effect for interfacial electron transfer: excited-state electron injection from Ru ammine compounds to nanocrystalline TiO2

The coordination compounds Ru(deeb)(NH3)4(PF6)2 and Ru(deeb)(NH2(CH2)2NH2)4(PF6)2, where deeb is 4,4'-(CO2CH2CH3)2-2,2'-bipyridine, were synthesized and attached to optically transparent nanocrystalline (anatase) TiO2 films. The compounds were found to be nonemissive in fluid acetonitrile and when attached to TiO2 with excited-state lifetimes <10 ns. Infrared measurements showed the expected isotopic substitution of the deuterated compounds on TiO2 thin films. A small 10-15 mV shift in the RuIII/II reduction potentials was measured upon deuteration. Metal-to-ligand charge-transfer (MLCT) excitation resulted in interfacial electron transfer into the TiO2 semiconductor with quantum yields that were dependent on the excitation wavelength and deuteration of the ammine ligands. The quantum yields were optimized with blue light excitation (417 nm) and deuterium substitution. In contrast, the kinetic rate constants for charge recombination were insensitive to deuteration and the excitation wavelength. Control experiments with Ru(deeb)(bpy)2(PF6)2 indicated that deuteration of the TiO2 surface alone does not affect the injection or recombination processes. A model is proposed wherein electron injection occurs in competition with vibrational relaxation and/or intersystem crossing of the excited states. Exchange of hydrogen by deuterium slows vibrational relaxation and/or intersystem crossing, resulting in higher injection yields.     

Photochemical reduction of oxygen adsorbed to nanocrystalline TiO(2) films: a transient absorption and oxygen scavenging study of different TiO(2) preparations

Transient absorption spectroscopy (TAS) has been used to study the interfacial electron-transfer reaction between photogenerated electrons in nanocrystalline titanium dioxide (TiO(2)) films and molecular oxygen. TiO(2) films from three different starting materials (TiO(2) anatase colloidal paste and commercial anatase/rutile powders Degussa TiO(2) P25 and VP TiO(2) P90) have been investigated in the presence of ethanol as a hole scavenger. Separate investigations on the photocatalytic oxygen consumption by the films have also been performed with an oxygen membrane polarographic detector. Results show that a correlation exists between the electron dynamics of oxygen consumption observed by TAS and the rate of oxygen consumption through the photocatalytic process. The highest activity and the fastest oxygen reduction dynamics were observed with films fabricated from anatase TiO(2) colloidal paste. The use of TAS as a tool for the prediction of the photocatalytic activities of the materials is discussed. TAS studies indicate that the rate of reduction of molecular oxygen is limited by interfacial electron-transfer kinetics rather than by the electron trapping/detrapping dynamics within the TiO(2) particles.     

Direct observation of reactive trapped holes in TiO2 undergoing photocatalytic oxidation of adsorbed alcohols: evaluation of the reaction rates and yields

To investigate the primary process of photocatalytic oxidation of TiO2, interfacial charge-transfer reaction of trapped holes formed in nanocrystalline TiO2 films by UV irradiation was directly measured by highly sensitive femtosecond and nanosecond transient absorption spectroscopy under low intensity excitation condition to avoid fast electron-hole recombination. Accordingly, the rates and yields of photocatalytic oxidation of several alcohols adsorbed on TiO2 were evaluated successfully.