Impact of Local Electric Fields on Charge-Transfer Processes at the TiO₂/Dye/Electrolyte Interface

Photoinduced electron-transfer processes at the TiO₂/dye/electrolyte interface are vital for various emerging technologies. Here, the impact of the local electric field at this interface on the charge-transfer processes was investigated in two aspects: (a) charge recombination between the electrons accumulated within TiO₂ and the photoxidized dye and (b) regeneration of the dyes by the cobalt bipyridyl redox mediators. The amplitude of the local electric field was changed by use of different cations in the electrolytic environment, in the order <i>E</i>_Ca²⁺ > <i>E</i>_Mg²⁺ > <i>E</i>_Na⁺ > <i>E</i>_Li⁺ characterized by the transient absorption spectroscopy. For the charge recombination process, the kinetic time constant showed a remarkable linear correlation with the relative electric field strength, while for the regeneration process, no evident dependence was observed. These results collectively suggest the spatial confinement of the effects of the local electric field on the interfacial electron-transfer processes

Carrier Dynamics of Dye Sensitized-TiO₂ in Contact with Different Cobalt Complexes in the Presence of Tri(p-anisyl)amine Intermediates

Heterogeneous charge transfer processes at sensitized wide bandgap semiconductor surfaces are imperative for both fundamental knowledge and technical applications. Herein, we focus on the investigation of carrier dynamics of a triphenylamine-based dye, LEG4, sensitized TiO₂ (LEG4/TiO₂) in contact with two types of electrolyte systems: pure cobalt-based electrolytes and in combination with an organic donor, tri­(p-anisyl)­amine (TPAA). Four different cobalt redox systems with potentials spanning a 0.3 V range were studied, and the carrier recombination and regeneration kinetics were monitored both at low and at high TiO₂ (e^–) densities (1.3 × 10¹⁸ and 1.3 × 10¹⁹ cm^–3, respectively). The results reveal that the introduction of the TPAA intermediate more effectively suppress the recombination loss of TiO₂ (e^–) under high charge conditions, close to open-circuit, as compared to low charge conditions. As a result, the charge transfer from the cobalt complexes to the oxidized dyes is significantly improved by the addition of TPAA. Dye-sensitized solar cells fabricated with the TPAA-containing electrolytes demonstrate remarkable improvement in both <i>V</i>_OC and <i>J</i>_SC and lead to more than 25% increase of the light-to-electricity conversion efficiency. Furthermore, an unprecedented detrimental impact of TPAA on the device performance was identified when the redox potential of the TPAA donor and the cobalt complexes are close. This is ascribed to the formation of TPAA^•+ which can act as an active recombination centers and thus lower the solar cell performance. These insights point at a strategy to enhance the lifetimes of electrons generated in sensitized semiconductor electrodes by overcoming the charge recombination between TiO₂ and the oxidized dye under high carrier densities in the semiconductor substrate and offer practical guidance to the design of future efficient electrolyte systems for dye-sensitized solar cells

Mesoporous TiO₂ Microbead Electrodes for Cobalt-Mediator-Based Dye-Sensitized Solar Cells

Light scattering, porosity, surface area, and morphology of TiO₂ working electrode can affect the power conversion efficiency of dye -sensitized solar cells dramatically. Here mesoporous TiO₂ microbeads were tested as working electrode in dye-sensitized solar cells based on cobalt tris-bipyridine electrolyte. Power conversion efficiencies up to 6.4% were obtained with D35 dye adsorbed onto the light-scattering microbeads. Electron transport, studied using small light perturbation methods, was found to be significantly faster in the microbead films than in standard mesoporous TiO₂ films. This was attributed to the favorable assembly of nanocrystals in the microbeads, which can increase the electron diffusion coefficient in the conduction band. Electron lifetimes were similar in both types of film. While solar cell performance of microbead films was comparable to that of standard mesoporous films in acetonitrile-based electrolytes, a significant improvement was found when the more viscous 3-methoxypropionitrile was used as solvent for electrolyte

Tuning of Conductivity and Density of States of NiO Mesoporous Films Used in p‑Type DSSCs

Nickel oxide has been used as the mesoporous electrode material for p-type dye sensitized solar cell (DSSC) for many years, but no high efficiency cells have been obtained yet. The poor results are commonly attributed to the lack of conductivity of the NiO film. In this paper we studied the electrical conduction of NiO mesoporous film with cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). We used unsensitized NiO on FTO as an electrode with no dye adsorbed on the surface. Tests made with a DSSC device-like cell (FTO-Pt-I^–/I₃^–-NiO-FTO) showed a surprisingly high Faradaic current (20 mA/cm^–2 at 1 V), proving a good electrical conductivity of mesoporous NiO. We also used lithium as dopant to improve the electrical properties of the film. The Li-doping resulted in widening the inert (not conductive) window in the CV plot. The EIS analysis clarified that this behavior is due to a strong dependence of the valence band shape and position with respect to the Li-doping concentration. Our results show that DSSC performance does not need to be limited by the conductivity of mesoporous NiO, which encourages more effort in p-type DSSC research based on this material

Improved Morphology Control Using a Modified Two-Step Method for Efficient Perovskite Solar Cells

A two-step wet chemical synthesis method for methylammonium lead­(II) triiodide (CH₃NH₃PbI₃) perovskite is further developed for the preparation of highly reproducible solar cells, with the following structure: fluorine-doped tin oxide (FTO)/TiO₂ (compact)/TiO₂ (mesoporous)/CH₃NH₃PbI₃/spiro-OMeTAD/Ag. The morphology of the perovskite layer could be controlled by careful variation of the processing conditions. Specifically, by modifying the drying process and inclusion of a dichloromethane treatment, more uniform films could be prepared, with longer emission lifetime in the perovskite material and longer electron lifetime in solar cell devices, as well as faster electron transport and enhanced charge collection at the selective contacts. Solar cell efficiencies up to 13.5% were obtained

Unraveling the Effect of PbI₂ Concentration on Charge Recombination Kinetics in Perovskite Solar Cells

CH₃NH₃PbI₃ perovskite solar cells have rapidly risen to the forefront of emerging photovoltaic technologies. A solution-based, two-step method was reported to enhance the reproducibility of these solar cells. In this method, first a coating of PbI₂ is applied by spin-coating onto a TiO₂-coated substrate, followed by a dip in a methylammonium iodide solution, leading to conversion to CH₃NH₃PbI₃. The concentration of PbI₂ in the spin-coating solution is a very important factor that affects the infiltration of the perovskite and the amount deposited. The best solar cell performance of 13.9% was obtained by devices prepared using 1.0 M of PbI₂ in dimethylformamide. These devices also had the longest electron lifetime and shortest carrier transport time, yielding lowest recombination losses. Rapid quenching of the perovskite emission is found in device-like structures, suggesting reasonably good efficient carrier extraction at the TiO₂ interface and quantitative extraction at the spiro–OMeTAD interface

Cation-Dependent Photostability of Co(II/III)-Mediated Dye-Sensitized Solar Cells

The electrolyte composition has a significant effect on the performance and stability of cobalt-based, dye-sensitized solar cells (DSSCs). The stability of DSSCs incorporating Co­(II/III) tris­(bipyridine) redox mediator has been investigated over 1000 h under full solar irradiation (with UV cutoff) at a temperature of 60 °C, the main focus being on monitoring the photovoltaic performance of the device and analyzing the internal charge-transfer dynamics in the presence of different cation coadditives (preferably added as tetracyanoborate salts). A clear cation-dependence is shown, not only of the early light-induced performance but also of the long-term photostability of the photovoltage of the device. These light-induced changes, which are attributed to the promotion of electron injection and less electron recombination loss, by transient spectral and electrochemical studies at the TiO₂/dye/electrolyte interface, indicate that the main cation effects involve the TiO₂ surface electric field and energy-state distribution. By examining the stability of adsorbed and solvated dye during aging, it has been found that the dye photodegradation is probably responsible for the decline in the photovoltage and that this is extremely dependent on the nature of the cation coadditives in the electrolyte. It is therefore suggested that optimization of the electrolyte cation composition is essential for improving the stability of cobalt-based DSSCs

Kinetic Evidence of Two Pathways for Charge Recombination in NiO-Based Dye-Sensitized Solar Cells

Mesoporous nickel oxide has been used as electrode material for p-type dye-sensitized solar cells (DSCs) for many years but no high efficiency cells have yet been obtained. One of the main issues that lowers the efficiency is the poor fill factor, for which a clear reason is still missing. In this paper we present the first evidence for a relation between applied potential and the charge recombination rate of the NiO electrode. In particular, we find biphasic recombination kinetics: a fast (15 ns) pathway attributed to the reaction with the holes in the valence band and a slow (1 ms) pathway assigned to the holes in the trap states. The fast component is the most relevant at positive potentials, while the slow component becomes more important at negative potentials. This means that at the working condition of the cell, the fast recombination is the most important. This could explain the low fill factor of NiO-based DSCs

Effect of Different Hole Transport Materials on Recombination in CH₃NH₃PbI₃ Perovskite-Sensitized Mesoscopic Solar Cells

We report on perovskite (CH₃NH₃)­PbI₃-sensitized solid-state solar cells using spiro-OMeTAD, poly­(3-hexylthiophene-2,5-diyl) (P3HT) and 4-(diethylamino)­benzaldehyde diphenylhydrazone (DEH) as hole transport materials (HTMs) with a light to electricity power conversion efficiency of 8.5%, 4.5%, and 1.6%, respectively, under AM 1.5G illumination of 1000 W/m² intensity. Photoinduced absorption spectroscopy (PIA) shows that hole transfer occurs from the (CH₃NH₃)­PbI₃ to HTMs after excitation of (CH₃NH₃)­PbI₃. The electron lifetime (τ_e) in these devices are in the order Spiro-OMeTAD > P3HT > DEH, while the charge transport time (<i>t</i>_tr) is rather similar. The difference in τ_e can therefore explain the lower efficiency of the devices based on P3HT and DEH. This report shows that the nature of the HTM is essential for charge recombination and elucidates that finding an optimal HTM for the perovskite solar cell includes controlling the perovskite/HTM interaction. Design routes for new HTMs are suggested

Studies on the Interfacial Electric Field and Stark Effect at the TiO₂/Dye/Electrolyte Interface

Interfaces of dye-sensitized TiO₂ nanoparticles with electrolytes or hole conductors have been widely applied in photoelectrochemical cells. However, the fundamental understanding of their properties and function is still poor. Herein, we demonstrate that the spectral changes that occur in the visible spectrum of dye-sensitized TiO₂ films upon (a) Li⁺ titration, (b) potentiostatic electron accumulation in mesoporous TiO₂, and (c) photoinduced electron injection into TiO₂ can be explained by the Stark effect, which can then be used to characterize the change in the local electric field at the TiO₂/dye/electrolyte interface. A quantitative analysis of the Stark effect indicates that the compact (Helmholtz) layer capacitance at the TiO₂/dye/electrolyte interface strongly affects the strength of the local electric field. Systematic studies show that the Helmholtz layer capacitance depends strongly on the Li⁺ concentration and surface dye coverage but is independent of the concentrations of other electrolytic species and the light intensity. These results illustrate the potential of Stark spectroscopy for the in situ study of the TiO₂/dye/electrolyte interfaces and provide substantial new insights into these widely applied interfaces related to photoelectrochemistry and other research fields