Numerical Simulation of the Current−Voltage Curve in Dye-Sensitized Solar Cells

A theoretical model based on the numerical integration of the continuity equation for electrons with trap-limited density-dependent diffusion and recombination constants is implemented to describe the functioning of dye-sensitized solar cells (DSSC). The application of the model combines recent theory on charge transport in nanocrystalline materials with parameters extracted from five simple measurements: the UV/vis spectrum of the dye in solution, the steady-state current−voltage curve, the open circuit photovoltage versus light intensity curve, photocurrent transient upon switching on an illumination source, and open-circuit voltage decay upon switching off the illumination source. As a novel feature not previously included in this kind of calculations, the model includes an additional term that accounts for the charge transfer from the transparent conducting oxide (TCO) substrate to the electrolyte solution. The general applicability of the model is illustrated by applying it to two different types of cell: a TiO2-based solar cell with an organic solvent electrolyte and a ZnO-based solar cell with a solvent-free electrolyte. It is found that the numerical model is capable of adequately fitting all data for both systems, resulting in quantitative estimates for the main parameters controlling solar cell functioning and efficiency. The results show that it is possible to provide a global description of DSSCs based on fundamental theories for trap-limited transport and recombination using simple experimental techniques available to every solar cell laboratory. The present paper tries to help fill the gap between pure theoreticians and experimentalists working on this kind of system

Origin of Nonlinear Recombination in Dye-Sensitized Solar Cells: Interplay between Charge Transport and Charge Transfer

Electron transfer between nanostructured semiconductor oxides and redox active electrolytes is a fundamental step in many processes of technological interest, such as photocatalysis and dye-sensitized solar cells. It has been shown that the transfer kinetics in the dye-sensitized solar cell are determined simultaneously by trap-limited transport and by the relative energetics of donor and acceptor states in the semiconductor and electrolyte. In this work, the transport and recombination mechanisms of photogenerated electrons in dye-sensitized solar cells are modeled by random walk numerical simulations with explicit description of the electron transfer process in terms of the Marcus–Gerischer model. The recombination rate is computed as a function of Fermi level in order to extract the electron lifetime and its influence on the electron diffusion length. The simulation method allows one to relate the recombination reaction order to the trap distribution parameter, the band edge position, and the reorganization energy. The results show that a model involving electron transfer from both shallow and deep traps, coupled with transport via shallow states, adequately reproduces all the experimental phenomena, including the dependence of the electron lifetime and the electron diffusion length on the open-circuit voltage when either the conduction band or the redox potential are displaced. Nonlinear recombination is predicted when the electron diffusion length increases with Fermi level, which is correlated with a reaction order different from one, in an open-circuit voltage decay “experiment”. The results reported here are relevant to the understanding of the effect of using new electrolyte compositions and novel redox shuttles in dye-sensitized solar cells

Electron Diffusion and Back Reaction in Dye-Sensitized Solar Cells: The Effect of Nonlinear Recombination Kinetics

The electron collection efficiency in dye-sensitized solar cells (DSCs) is usually related to the electron diffusion length, <i>L</i> = (<i>D</i>τ)^1/2, where <i>D</i> is the diffusion coefficient of mobile electrons and τ is their lifetime, which is determined by electron transfer to the redox electrolyte. Analysis of incident photon-to-current efficiency (IPCE) spectra for front and rear illumination consistently gives smaller values of <i>L</i> than those derived from small amplitude methods. We show that the IPCE analysis is incorrect if recombination is not first-order in free electron concentration, and we demonstrate that the intensity dependence of the apparent <i>L</i> derived by first-order analysis of IPCE measurements and the voltage dependence of <i>L</i> derived from perturbation experiments can be fitted using the same reaction order, γ ≈ 0.8. The new analysis presented in this letter resolves the controversy over why <i>L</i> values derived from small amplitude methods are larger than those obtained from IPCE data

Pseudohalogens for Dye-Sensitized TiO₂ Photoelectrochemical Cells

In this paper, we report on the preparation and characterization of two pseudohalogen redox couples for dye-sensitized TiO2 photoelectrochemical cells. The equilibrium potentials of the (SeCN)2/SeCN- and (SCN)2/SCN- couples are respectively 0.19 and 0.43 V more positive than for the I3-/I- couple, providing the opportunity to determine the influence of the redox potential on the open circuit photovoltage. With the sensitizer cis-Ru(dcb)2(NCS)2 (N3), the incident photon-to-current conversion efficiency was 20% for the (SeCN)2/SeCN- couple and 4% for the (SCN)2/SCN- couple. Transient absorbance measurements showed that the quantum yield for electron injection is independent of the pseudohalogen redox couple and that the regeneration rates of the dye decrease in the order I- > SeCN- > SCN-. The effects of the redox potential on open circuit photovoltage were determined by independent measurement of the dependence of the sensitized TiO2 working electrode and the platinum counter electrode potentials on the cell voltage

The Impact of the Electrical Nature of the Metal Oxide on the Performance in Dye-Sensitized Solar Cells: New Look at Old Paradigms

The process of primary charge separation in Ru-based dye-sensitized solar cells (DSSCs) based on both TiO₂ and ZnO photoanodes has been studied in fully working devices by time-resolved measurements, including emission and transient absorption (from femtoseconds to microseconds), and electrochemical techniques (current–voltage characteristics and impedance spectroscopy). By studying the effect of different electrolyte compositions using potential-determining additives (Li⁺ ions, 4-<i>tert</i>-butylpiridine), we have been able to provide novel insights into the mechanisms of electron injection and dye regeneration across the oxide/dye/electrolyte system. In this respect, the shift of the conduction band that is commonly reported for TiO₂ in the presence of additives is not observed for ZnO. In addition, both injection and regeneration for ZnO-based cells are shown to be much slower than those for TiO₂ and independent of the electrolyte composition. The slower injection and regeneration for ZnO and its lower sensitivity with respect to the addition of potential-determining additives strongly suggest that the electrical nature of the oxide is crucial to facilitate charge separation across the oxide/dye/electrolyte interface. In addition, electron injection from singlet and triplet states has been identified for both metal oxides. The former process occurs on the ultrafast time scale (<100 fs), while the latter extends over many time scales from single ps to single ns. The present work provides novel insights about the processes of electron injection and dye regeneration in DSSCs as well as the reasons for the lower performance of ZnO-based dye solar cells

Optical, Electrochemical, and Photoelectrochemical Behavior of Copper Pyrovanadate: A Unified Theoretical and Experimental Study

Copper vanadates have been important to the development of new-generation photoelectrodes for solar water splitting and hydrogen generation. Of these, copper pyrovanadate (β-Cu2V2O7), the 2:1 ternary compound derived from CuO and V2O5, has been of particular interest to the solar fuel community. This n-type semiconductor has shown the highest photocurrent for water oxidation at 1.23 V versus reversible hydrogen electrode and exhibits good photostability in aqueous media of pH 9.2. However, further successful application of this material in photoelectrochemical (PEC) devices hinges on a comprehensive understanding of its optical, electrochemical, and optoelectronic attributes. This was done in this study by a combination of density-functional theory (for the structural, magnetic, and optical characterization) and experiments, with the latter using both small-signal (intensity-modulated photocurrent spectroscopy, IMPS) and large-signal, transient photocurrent (TP) analyses. Both IMPS and TP measurements yielded complementary and self-consistent insights into the rate constants for hole transfer and carrier recombination at the irradiated β-Cu2V2O7/electrolyte interface, specifically their dependence on the applied bias potential. The information from PEC data analyses was also self-consistent with that garnered from the optical (diffuse reflectance spectroscopy) data

Antifungal Coatings Based on Ca(OH)₂ Mixed with ZnO/TiO₂ Nanomaterials for Protection of Limestone Monuments

The presence and deteriorating action of microbial biofilms on historic stone buildings have received considerable attention in the past few years. Among microorganisms, fungi are one of the most damaging groups. In the present work, antimicrobial surfaces were prepared using suspensions of Ca­(OH)₂ particles, mixed with ZnO or TiO₂ nanoparticles. The antimicrobial surfaces were evaluated for their antifungal activity both in the dark and under simulated natural photoperiod cycles, using <i>Penicillium oxalicum</i> and <i>Aspergillus niger</i> as model organisms, and two limestone lithotypes commonly used in construction and as materials for the restoration of historic buildings. Both Ca­(OH)₂-ZnO and Ca­(OH)₂-TiO₂ materials displayed antifungal activity: ZnO-based systems had the best antifungal properties, being effective both in the dark and under illumination. In contrast, TiO₂-based coatings showed antifungal activity only under photoperiod conditions. Controls with coatings consisting of only Ca­(OH)₂ were readily colonized by both fungi. The antifungal activity was monitored by direct observation with microscope, X-ray diffraction (XRD), and scanning electron microscopy (SEM), and was found to be different for the two lithotypes, suggesting that the mineral grain distribution and porosity played a role in the activity. XRD was used to investigate the formation of biominerals as indicator of the fungal attack of the limestone materials, while SEM illustrated the influence of porosity of both the limestone material and the coatings on the fungal penetration into the limestone. The coated nanosystems based on Ca­(OH)₂-50%ZnO and pure zincite nanoparticulate films have promising performance on low porosity limestone, showing good antifungal properties against <i>P</i>. <i>oxalicum</i> and <i>A</i>. <i>niger</i> under simulated photoperiod conditions