ZnO@SnO2 multilayered network was deposited on fluorine doped tin oxide (FTO) glass and applied as photoanode in dye sensitized solar cells whose functional performances are compared with single oxide-based photoanodes made of SnO2 nanoparticles and ZnO microparticles. Multi-oxide photoanodes provide for enhanced photoconversion efficiency (3.31%) as compared with bare SnO2 nanoparticles (1.06%) and ZnO microparticles (1.04%). Improved functional performances of the ZnO@SnO2 layered network are ascribable to partial inhibition of back electron transfer from SnO2 to the redox electrolyte, guaranteed by the ZnO, which acts as a capping layer for the underlying SnO2.

Yu Bai,†,‡ Ivań Mora-Sero,́ Filippo De Angelis, Juan Bisquert, and Peng Wang*,† †State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, China ‡Institute of Chemistry and Energy Material Innovation, Academy of Fundamental Interdisciplinary Sciences, Harbin Institute of Technology, Harbin 150080, China Photovoltaic and Optoelectronic Devices Group, Departament de Física, Universitat Jaume I, 12071 Castello,́ Spain Istituto CNR di Scienze e Tecnologie Molecolari, c/o Dipartimento di Chimica, Universita ̀ di Perugia, via Elce di Sotto 8, I-06123 Perugia, Italy

We report here the exploitation of ultrathin layers of Al2O3 deposited via atomic layer deposition (ALD) on SnO2 photoanodes used in dye-sensitized solar cells featuring the I3 /I couple as the redox electrolyte. We find that a single ALD cycle of Al2O3 increases the lifetimes of injected electrons by more than 2 orders of magnitude. The modified SnO2 photoanode yields nearly a 2-fold improvement fill factor and a greater than 2-fold increase in open-circuit photovoltage, with a slight increase in short-circuit photocurrent. The overall energy conversion efficiency increases by roughly 5-fold. The effects appear to arise primarily frompassivation of reactive, low-energy tin-oxide surface states, with bandedge shifts and tunneling based blocking behavior playing only secondary roles. SECTION Energy Conversion and Storage D ye-sensitized solar cells (DSSCs) have garnered considerable attention within the solar cell community. With comparatively low manufacturing cost, ease of fabrication, and fair solar-to-electrical efficiency, these cells are potential candidates to replace conventional Si-based solar cells in specialized applications. A standard DSSC employs a Ru-based dye, which has a moderate molar absorption coefficient. With this limitation on the absorption coefficient, the system needs a high surface area substrate for the dye;typically nanoparticulate TiO2;in order to achieve good light harvesting efficiency (LHE). Since electron transport is fairly slow in nanoparticulate TiO2, 4,5 the system also requires a slow redox shuttle (i.e., slow with respect to interception of injected electrons), usually I/I3 . Due to the otherwise sluggish reactivity of the shuttle, an energy of roughly 600mV is required for regenerating the initial state of the dye molecule at an acceptable rate. The inclusion of this driving force reduces the achievable open circuit voltage almost by half. Many groups have experimented with alternative redox shuttles to address this loss in voltage. Conversely, if another semiconductorwith higher electronmobilitywere used as the high-area substrate, electron transport would be faster, and a more reactive redox shuttle could be employed. Of the semiconductors that have been considered to replace TiO2, SnO2 is a recurring candidate. The use of SnO2 as the high area electrode is not without drawbacks. Namely, the rate of electron interception by the redox electrolyte is much faster with SnO2 than with traditional TiO2. Durrant and coworkers found that the back electron transfer rate from SnO2 (to the redox shuttle) is in the microsecond range, roughly 3 ordersmagnitude faster thanwith TiO2. 10 This is possibly due to the reactive, low-energy trap states in SnO2. 11 In principle, coating the electrode with a layer of a high-bandgap metal oxide such asMgO, ZrO2, 12 SiO2, 13 or Nb2O5 14 could passivate these states and make the rate of electron interception slower. Typically, the technique referred to as “dip-coating” has been employed to apply the barrier layer. However, this technique often creates layers with irregular, unpredictable thicknesses, as well as pinholes that reach the surface of the semiconductor. In contrast, atomic layer deposition (ALD) conformally coats high surface area materials. The selfterminating process in ALD enables angstrom-scale control over the coating thicknesses. The deposited layer's thickness is precisely controllable based on the number of ALD cycles. All of these benefitsmakeALD suitable for applying ultrathin barrier layers onphotoelectrodes. In fact, ALDhas beenutilized inmany recent alternative photoanode configurations. In this letter, we show that coating a SnO2 electrode with a fraction of a monolayer of redox-inactive Al2O3 via ALD effectively slows down the rate of electron recombination with triiodide by 2 to 3 orders of magnitude, while substantially boosting the attainable photovoltage.With the addition of Al2O3, SnO2 becomes viable as an electrode for DSSCs. In particular, because the conduction band-edge for SnO2 is ca. 0.4 V less negative than that for TiO2, incorporation of tin oxide in DSSCs should make possible the use of far-red-absorbing chromophores having excited states too low in energy to inject into TiO2. Figure 1 shows the effect that coating one cycle of Al2O3 has on DSSCs based on nanoparticulate SnO2 electrodes. The short-circuit current density from the naked electrode is nearly Received Date: March 18, 2010 Accepted Date: April 29, 2010

We report a transient absorption study of the kinetics of re-reduction of ruthenium bipyridyl dye cations adsorbed to nanocrystalline TiO2 films by iodide ions, employing a series of RuII(dcbpy)2X dyes, where dcpby ) 4,4′-dicarboxy-2,2′-bipyridyl and X ) (NCS)2, (CN)2, DTC (diethyldithiocarbamate), and Cl2. The data indicate that this re-reduction (or ‘regeneration’) reaction proceeds via a transient (dye+-iodide) intermediate complex formed by reaction of photogenerated dye cations with one iodide ion. The subsequent reaction of this complex with a second iodide ion, forming I2-, is shown to be the kinetically and thermodynamically limiting step in the overall regeneration reaction. The implications of this reaction scheme for the function of dye sensitized photoelectrochemical cells are discussed, considering in particular the high photovoltaic efficiencies obtained for such devices employing such ruthenium bipyridyl senstiser dyes in combination with iodide/tri-iodide based redox electrolytes.

We propose a passive factor, autocatalytic activity of some semiconductors (SnO2, WO3), for the loss of efficiency in dye-sensitized solar cells. On one hand, in the photoanode, SnO2 (or WO3) functions as a semiconductor to collect the photoelectrons that are generated by the sensitizer. On the other hand, SnO2 (or WO3) can also work as a catalyst to catalyze the recombination between the photoelectrons and the I3- ions, which results in a large current leakage, a low open-circuit voltage value, and a low efficiency. Some oxides, including, but not limited to, SnO2 and WO3, which have catalytic activity for I3- redn. or other oxidized species of the electrolyte are not suitable for use as semiconductor for the photoanode. In addn. to the semiconductors, phthalocyanine dyes also have catalytic activity for the recombination.

We observe a strong “light-soaking” effect in SnO2 based solid-state dye-sensitized solar cells (SDSCs). Both with and without the presence of UV light, the device's short-circuit photocurrent and efficiency increase significantly over 20–30 minutes, until steady-state is achieved. We demonstrate that this is not due to improved charge collection and investigate the charge generation dynamics employing optical-pump terahertz-probe spectroscopy. We observe a monotonic speeding-up of the generation of free-electrons in the SnO2 conduction band as a function of the light-soaking time. This improved charge generation can be explained by a positive shift in the conduction band edge or, alternatively, an increase in the density of states (DoS) at the energy at which photoinduced electron transfer occurs. To verify this hypothesis, we perform capacitance and charge extraction measurements which indicate a shift in the surface potential of SnO2 of up to 70 mV with light soaking. The increased availability of states into which electrons can be transferred justifies the increase in both the charge injection rate and ensuing photocurrent. The cause for the shift in surface potential is not clear, but we postulate that it is due to the photoinduced charging of the SnO2 inducing a rearrangement of charged species or loss of surface oxygen at the dye-sensitized heterojunction. Understanding temporally evolving processes in DSCs is of critical importance for enabling this technology to operate optimally over a prolonged period of time. This work specifically highlights important changes that can occur at the dye-sensitized heterojunction, even without direct light absorption in the metal oxide.

We have investigated the effect of ZnO nanoparticle properties on the dye-sensitized solar cell performance. Nanoparticles with different sizes and optical properties were considered. We found that there is a complex relationship between native defects, dye adsorption, charge transport and solar cell performance. The presence of a high concentration of nonradiative defects was found to be detrimental to photovoltaic performance, whereas for radiative defects, samples displaying orange-red defect emission exhibited better performance compared to samples with green defect emission (when the samples had similar emission intensities). Detailed discussion of the nanoparticle properties and their relationship with dye adsorption, electron injection, electron lifetime, electron transport time, and solar cell performance is given.

We developed an equivalent circuit for dye-sensitized solar cells, which provides a more accurate presentation and a better fit to the experimental I-V curves and Nyquist plots than earlier literature results, as verified by simulation. A simple expression for the estimation of the Warburg coefficient from the experimental Nyquist plot is also proposed in this letter. The simulated I-V curves and Nyquist plots are in good agreement with the experimental data. The interfacial charge transfer and recombination losses at the oxide/dye/electrolyte interface are found to be the most influential factor on the overall conversion efficiency.

Triadic photoanodes have been prepared based on nanoporous films of the metal oxides ZrO2, TiO2, and SnO2, sensitizer [Ru(bpy)2(dpbpy)]2+ (P2), and polyoxometalate water oxidation catalyst [{Ru4O4(OH)2(H2O)4}(γ-SiW10O36)2]10– (1) and investigated for their potential utility in water-splitting dye-sensitized photoelectrochemical cells. Transient visible and mid-IR absorption spectroscopic studies were carried out to investigate the charge separation dynamics of these systems, indicating that electron transfer from photoexcited P2 to TiO2 and SnO2 is still the main excited state quenching pathway in the presence of 1. Furthermore, the accelerated recovery of the P2 ground state bleach in the presence of 1 results from ultrafast (nanosecond) electron transfer from catalyst to oxidized dye. Catalyst loading appears to depend largely on the point of zero charge of the supporting oxide and as such is significantly lower on SnO2 than on TiO2: nonetheless, the rate of recovery of the ground state bleach is simila...

Titanium dioxide (TiO2)-layered fluorine doped tin oxide (FTO) powder was synthesized and applied as the photoanode in dye-sensitized solar cells (DSSCs). FTO powders are connected to form a direct electron pathway for the efficient extract of injected electrons, while the TiO2 layer serves as an energy barrier prohibiting the charge combination with oxidized dye or I3(-). The electrochemical impedance spectroscopy (EIS) analyses suggest that electrons have a longer combination lifetime (τe = 233 ms) than that of the electron in the DSSCs using traditional P25 photoanodes (τe = 28 ms). The DSSCs using 5 μm thick TiO2@FTO as photoanodes eventually give a respectable and long-term stable photovoltaic performance with a current density of 23.8 mA/cm(2), an open circuit voltage of 0.69 V, and power conversion efficiency of 7.4%. The results are received on a low dye loading level (0.25 × 10(-7) mol/cm(2)), which is (1)/10 of that for traditional photoanode (2.79 × 10(-7) mol/cm(2)).

Titanium dioxide (TiO2) is widely used for photocatalysis and solar cell applications, and the electronic structure of bulk TiO2 is well understood. However, the surface structure of nanoparticulate TiO2, which has a key role in properties such as solubility and catalytic activity, still remains controversial. Detailed understanding of surface defect structures may help explain reactivity and overall materials performance in a wide range of applications. In this work we address the solubility problem and surface defects control on TiO2 nanoparticles. We report the synthesis and characterization of ∼4 nm TiO2 anatase spherical nanoparticles that are soluble and stable in a wide range of organic solvents and water. By controlling the temperature during the synthesis, we are able to tailor the density of defect states on the surface of the TiO2 nanoparticles without affecting parameters such as size, shape, core crystallinity, and solubility. The morphology of both kinds of nanoparticles was determined by TEM. EPR experiments were used to characterize the surface defects, and transient absorption measurements demonstrate the influence of the TiO2 defect states on photoinduced electron transfer dynamics.

Tin oxide is a promising candidate for the replacement of titanium dioxide in dye‐sensitized solar cells (DSSCs), due to its excellent electron transport properties. The performance of TiO2‐based DSSCs can be improved either by modifying the morphology of SnO2 nanostructures, or by doping. Here, we investigate the influence of different dopants (Mg, Zn, and Ag) to SnO2 anode on the DSSC performance. We found that the addition of the dopant affects not only the electron lifetime, but also the dye adsorption as well. The best performance was obtained for zinc doped SnO2 anodes, with the power conversion efficiency of 3.4%.

This study carefully examines carrier dynamics in highly efficient dye-sensitized solar cells (DSSCs) based on a series of recently developed dicarboxyterpyridine Ru(II) dyes (PRT-11-15). Comprehensive spectral response analyses, transient photovoltage and photocurrent measurements, and transient absorption techniques are exploited to investigate the effects of various functionalized terpyridines on carrier injection efficiency (η(inj)), electron diffusion length (L) and dye-regeneration efficiency (η(reg)). The resulting parameters are fully comprehended, which are then correlated with the origins of the device performance such as short circuit current density J(sc), open circuit voltage V(oc) and hence the overall conversion efficiency η in these Ru(II) based DSSCs.

This paper reports visible-light sensitization of TiO2 nanoparticles by surface modification with Mn(II)−terpyridine complexes, as evidenced by UV−vis spectroscopy of colloidal thin films and aqueous suspensions. Photoexcitation of the [MnII(H2O)3(catechol-terpy)]2+/TiO2 (terpy = 2,2‘:6,2‘ ‘-terpyridine) complex, attached to the TiO2 surface, leads to interfacial electron transfer within 300 fs as indicated by ultrafast optical pump−terahertz probe transient measurements and computational simulations. Photoinduced interfacial electron transfer is accompanied by Mn(II) → Mn(III) photooxidation. The half-time for regeneration of the Mn(II) complex is ca. 23 s (at 6 K), as monitored by time-resolved measurements of the Mn(II) EPR signal.

This paper presents an overview of the research carried out by a European consortium with the aim to develop and test new and improved ways to realise dye‐sensitized solar cells (DSC) with enhanced efficiencies and stabilities. Several new areas have been explored in the field of new concepts and materials, fabrication protocols for TiO2 and scatterlayers, metal oxide blocking layers, strategies for co‐sensitization and low temperature processes of platinum deposition. Fundamental understanding of the working principles has been gained by means of electrical and optical modelling and advanced characterization techniques. Cost analyses have been made to demonstrate the potential of DSC as a low cost thin film PV technology. The combined efforts have led to maximum non‐certified power conversion efficiencies under full sunlight of 11% for areas <0ċ2 cm2 and 10ċ1% for a cell with an active area of 1ċ3 cm2. Lifetime studies revealed negligible device degradation after 1000 hrs of accelerated tests under thermal stress at 80°C in the dark and visible light soaking at 60°C. An outlook summarizing future directions in the research and large‐scale production of DSC is presented. Copyright © 2006 John Wiley & Sons, Ltd.

This Progress Report highlights recent developments in dye-sensitized solar cells composed of both liquid electrolytes and solid-state hole transport materials. The authors discuss and review the present understanding of and recent developments in the operational processes, such as charge generation, transport, recombination, and charge collection. Also, the merits and challenges of alternative device approaches are discussed, including extremely thin absorber cells, devices containing inorganic p-type hole-transporters and non-TiO 2 mesoporous metal-oxide elctrodes employed in dye-sensitized solar cells.

The prosperity of human society largely relies on safe energy supply, and fossil fuel has been serving as the most reliable energy source. However, as a non-renewable energy source, the exhaustion of fossil fuel is inevitable and imminent in this century. To address this problem, renewable energy especially solar energy has attracted much attention, because it directly converts solar energy into electrical power leaving no environment affect. In the past, various photovoltaic devices like organic, inorganic, and hybrid solar cells were fabricated in succession. In spite of high conversion rate of silicon based solar cells, the high module cost and complicated production process restricted their application solely to astronautic and aeronautic technology. For domestic and other commercial applications, research has been focused on organic solar cells for their inherent low module cost and easy fabrication. In addition, organic solar cells have their lightweight and flexibility advantage over conventional silicon-based crystalline solar cells. Among all the organic solar cells, dye-sensitized solar cells (DSSCs) are the most efficient and easily implemented technology. Here, this study examines the working principle, present development and future prospectus for this novel technology.

The photovoltaic performance of liquid electrolyte and solid-state dye sensitized solar cells, employing a squarilium methoxy cyanide dye, are evaluated in terms of interfacial electron transfer kinetics. Dye adsorption to the metal oxide film resulted in a mixed population of aggregated and monomeric sensitizer dyes. Emission quenching data, coupled with transient absorption studies, indicate that efficient electron injection was only achieved by the monomeric dyes, with the aggregated dye population having an injection yield an order of magnitude lower. In liquid electrolyte devices, transient absorption studies indicate that photocurrent generation is further limited by slow kinetics of the regeneration of monomeric dye cations by the iodide/iodine redox couple. The regeneration dynamics are observed to be too slow (≫ 100 µs) to compete effectively with the recombination of injected electrons with dye cations. In contrast, for solid-state devices employing the organic hole conductor spiro-OMeTAD, the regeneration dynamics are fast enough (≪ 1 µs) to compete effectively with this recombination reaction, resulting in enhanced photocurrent generation.

The most commonly used redox mediators in dye-sensitized solar cells (DSCs), iodide/triiodide and cobalt trisbipyridine ([Co(bpy)3]2+/3+), were successfully replaced by bis(2,9-dimethyl-1,10-phenanthroline)copper(I/II) ([Cu(dmp)2]1+/2+). The use of the copper complex based electrolyte led to an exceptionally high photovoltaic performance of 8.3% for LEG4-sensitized TiO2 solar cells, with a remarkably high open-circuit potential of above 1.0 V at 1000 W m–2 under AM1.5G conditions. The copper complex based redox electrolyte has higher diffusion coefficients and is considerably faster in dye regeneration than comparable cobalt trisbipyridine based electrolytes. A driving force for dye regeneration of only 0.2 eV is sufficient to obtain unit yield, pointing to new possibilities for improvement in DSC efficiencies. The interaction of the excited dye with components of the electrolyte was monitored using steady-state emission measurements and time-correlated single-photon counting (TC-SPC). Our results indicat...

The dye-sensitized solar cell, developed in the 1990s, is a non-conventional solar technology that has attracted much attention owing to its stability, low cost, and device efficiency. Power-conversion efficiencies of over 11% have been achieved for devices that contain liquid electrolytes, whereas solid-state devices that do not require a liquid electrolyte display an overall efficiency of 5%. Improvement of the efficiency of solid-state dye-sensitized solar cells requires optimization of their various components, such as the hole-transport material, sensitizer, mesoporous TiO2 film, and the blocking layer. This Minireview highlights the current state of the art and future directions of solid-state dye-sensitized solar cell technology.