IR–Spectrophotoelectrochemical Characterization of Mesoporous Semiconductor Films

A combined IR–spectroscopic and electrochemical approach for the study of photo- and bias-induced reactions at the semiconductor/electrolyte interface is presented. Information on the electronic structure of a mesoporous semiconductor nanoparticle network, concretely the density and distribution of band gap states, as well as the nature of solution species are analyzed in situ. It has been shown that under appropriate conditions the electrode potential determines the quasi-Fermi level throughout the mesoporous film and thus the occupation of IR-active band gap states, independently of the type of external perturbation, i.e., application of a bias voltage or electrode exposure to photons exceeding the semiconductor band gap at open circuit. Importantly, electronic properties of the semiconductor and vibrational properties of solution species can be addressed simultaneously by IR–spectroscopy. In addition, electrochemical methods provide a means for the active manipulation (in potentiostatic measurements) or the passive tracking (during open circuit potential decay) of the quasi-Fermi level in the mesoporous film together with the possibility of electron quantification (by charge extraction experiments)

Charge-Transfer Reductive in Situ Doping of Mesoporous TiO₂ Photoelectrodes: Impact of Electrolyte Composition and Film Morphology

Some material properties not only depend on synthesis and processing parameters but also may significantly change during operation. This is particularly true for high-surface-area materials. We used a combined electrochemical and spectroscopic approach to follow the changes in the photoelectrocatalytic activity and in the electronic semiconductor properties of mesoporous TiO₂ films upon charge-transfer reductive doping. Shallow donors (i.e., electron/proton pairs) were introduced into the semiconductor by the application of an external potential or, alternatively, by band gap excitation under open circuit conditions. In the latter case, the effective open circuit doping potential depends critically on the electrolyte composition (e.g., the presence of electron or hole acceptors). Transient charge accumulation (electrons and protons) in nanoparticle electrodes results in a photocurrent enhancement that is attributed to the deactivation of recombination centers. In nanotube electrodes, the formation of a space–charge layer results in an additional decrease of charge recombination at positive potentials. Doping is transient in nanoparticle films but turns out to be stable for nanotube arrays

Electrons in the Band Gap: Spectroscopic Characterization of Anatase TiO₂ Nanocrystal Electrodes under Fermi Level Control

Macroscopic properties of semiconductor nanoparticle networks in functional devices strongly depend on the electronic structure of the material. Analytical methods allowing for the characterization of the electronic structure in situ, i.e., in the presence of an application-relevant medium, are therefore highly desirable. Here, we present the first spectral data obtained under Fermi level control of electrons accumulated in anatase TiO₂ electrodes in the energy range from the MIR to the UV (0.1–3.3 eV). Band gap states were electrochemically populated in mesoporous TiO₂ films in contact with an aqueous electrolyte. The combination of electrochemical and spectroscopic measurements allows us for the first time to determine both the energetic location of the electronic ground states as well as the energies of the associated optical transitions in the energetic range between the fundamental absorption threshold and the onset of lattice absorption. On the basis of our observations, we attribute spectral contributions in the vis/NIR to d–d transitions of Ti³⁺ species and a broad MIR absorption, monotonically increasing toward lower wavenumbers, to a quasi-delocalization of electrons. Importantly, signal intensities in the vis/NIR and MIR are linearly correlated. Absorbance and extractable charge show the same exponential dependence on electrode potential. Our results demonstrate that signals in the vis/NIR and MIR are associated with an exponential distribution of band gap states

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

How Important is Working with an Ordered Electrode to Improve the Charge Collection Efficiency in Nanostructured Solar Cells?

The collection efficiency of carriers in solar cells based on nanostructured electrodes is determined for different degrees or morphological one-dimensional order. The transport process is modeled by random walk numerical simulation in a mesoporous electrode that resembles the morphology of nanostructured TiO₂ electrodes typically used in dye-sensitized solar cells and related systems. By applying an energy relaxation procedure in the presence of an external potential, a preferential direction is induced in the system. It is found that the partially ordered electrode can almost double the collection efficiency with respect to the disordered electrode. However, this improvement depends strongly on the probability of recombination. For too rapid or too slow recombination, working with partially ordered electrodes will not be beneficial. The computational method utilized here makes it possible to relate the charge collection efficiency with morphology. The collection efficiency is found to reach very rapidly a saturation value, meaning that, in the region of interest, a slight degree of ordering might be sufficient to induce a large improvement in collection efficiency

The Redox Pair Chemical Environment Influence on the Recombination Loss in Dye-Sensitized Solar Cells

Reduction of recombination losses in dye-sensitized solar cells (DSC) is vital to fabricate efficient devices. The electron recombination lifetime depends on the relative energetics of the semiconductor and the redox pair and on the chemical nature of the electrolyte (hole conductor). In this work, the behavior of the electron lifetime in DSC devices prepared with various solvents (acetonitrile, valeronitrile, ethylene carbonate, pure ionic liquids), additives (lithium ions, TBP), and redox pairs (iodide/iodine, Co­(II)/Co­(III)) is thoroughly studied using high-extinction dyes. Lifetimes were extracted by means of small-perturbation electrochemical techniques (impedance spectroscopy, intensity-modulated photovoltage spectroscopy) and open-circuit voltage decays. To ensure a safe inner comparison and a proper interpretation, all devices were constructed using the same type of TiO₂ electrode and the same dyes (C101 and Z907 for iodide/iodine and cobalt-based electrolytes, respectively). Furthermore, small-perturbation techniques and voltage decay provided consistent results. The lifetime shows a clear change of behavior when iodide/iodine electrolytes in organic solvents are compared to iodide/iodine in ionic liquids and with cobalt electrolytes. In the first case, the lifetime–voltage semilogarithmic plot exhibits a curvature, whereas in the second case the behavior is purely exponential. This observation is consistent with previous theoretical predictions based on the multiple-trapping model and the Marcus–Gerischer theory, which predict an exponential law for large reorganization energies and a curvature for small ones. The obtained results show that solvents or ligands that interact strongly with the redox mediator originate larger reorganization energies and lead to devices with shorter lifetimes. This can be interpreted as an enhancement of extra routes for electron recombination as a consequence of a wider overlap in energies between donor and acceptor states for strongly interacting chemical environments

Elucidating Transport-Recombination Mechanisms in Perovskite Solar Cells by Small-Perturbation Techniques

Solar cells using perovskite as semiconducting pigment have recently attracted a surge of interest owing to their remarkable solar-to-electric conversion efficiencies and ease of processing. In this direction various device architectures and materials have been employed, and attempts were made to elucidate the underlying working principles. However, factors governing the performance of perovskite devices are still obscure. For instance, the interpretation of electrochemical impedance spectroscopy (EIS) is not straightforward, and the complexity of the equivalent circuits hinders the identification of transport and recombination mechanisms in devices, especially those that determine the performance of the device. Here in we carried out a comprehensive and complementary characterization of perovskite solar cells by using an array of small-perturbation techniques: EIS and intensity-modulated photocurrent and photovoltage spectroscopy (IMPS/IMVS). The employment of IMPS allowed us to identify two transport times separated by 2 orders of magnitude and with opposite voltage dependences. For recombination, well agreement was found between lifetimes obtained by IMVS and EIS. The feature associated with recombination and charge accumulation in an impedance spectrum through correlation to the IMVS response was experimentally identified. This correlation paves the way to reconstruct the current–voltage curve using a continuity equation model for transport and recombination in the working device. The adopted methodology demonstrates that complementary techniques facilitate the interpretation of EIS results in perovskite solar cells, allowing us for the identification of the transport-recombination mechanisms and providing new insights into the efficiency-determining steps

Universal Features of Electron Dynamics in Solar Cells with TiO₂ Contact: From Dye Solar Cells to Perovskite Solar Cells

The electron dynamics of solar cells with mesoporous TiO₂ contact is studied by electrochemical small-perturbation techniques. The study involved dye solar cells (DSC), solid-state perovskite solar cells (SSPSC), and devices where the perovskite acts as sensitizer in a liquid-junction device. Using a transport-recombination continuity equation we found that mid-frequency time constants are proper lifetimes that determine the current–voltage curve. This is not the case for the SSPSC, where a lifetime of ∼1 μs, 1 order of magnitude longer, is required to reproduce the current–voltage curve. This mismatch is attributed to the dielectric response on the mid-frequency component. Correcting for this effect, lifetimes lie on a common exponential trend with respect to open-circuit voltage. Electron transport times share a common trend line too. This universal behavior of lifetimes and transport times suggests that the main difference between the cells is the power to populate the mesoporous TiO₂ contact with electrons

Universal Features of Electron Dynamics in Solar Cells with TiO₂ Contact: From Dye Solar Cells to Perovskite Solar Cells

The electron dynamics of solar cells with mesoporous TiO₂ contact is studied by electrochemical small-perturbation techniques. The study involved dye solar cells (DSC), solid-state perovskite solar cells (SSPSC), and devices where the perovskite acts as sensitizer in a liquid-junction device. Using a transport-recombination continuity equation we found that mid-frequency time constants are proper lifetimes that determine the current–voltage curve. This is not the case for the SSPSC, where a lifetime of ∼1 μs, 1 order of magnitude longer, is required to reproduce the current–voltage curve. This mismatch is attributed to the dielectric response on the mid-frequency component. Correcting for this effect, lifetimes lie on a common exponential trend with respect to open-circuit voltage. Electron transport times share a common trend line too. This universal behavior of lifetimes and transport times suggests that the main difference between the cells is the power to populate the mesoporous TiO₂ contact with electrons

Comparison of TiO₂ and ZnO Solar Cells Sensitized with an Indoline Dye: Time-Resolved Laser Spectroscopy Studies of Partial Charge Separation Processes

Time-resolved laser spectroscopy techniques in the time range from femtoseconds to seconds were applied to investigate the charge separation processes in complete dye-sensitized solar cells (DSC) made with iodide/iodine liquid electrolyte and indoline dye D149 interacting with TiO₂ or ZnO nanoparticles. The aim of the studies was to explain the differences in the photocurrents of the cells (3–4 times higher for TiO₂ than for ZnO ones). Electrochemical impedance spectroscopy and nanosecond flash photolysis studies revealed that the better performance of TiO₂ samples is not due to the charge collection and dye regeneration processes. Femtosecond transient absorption results indicated that after first 100 ps the number of photoinduced electrons in the semiconductor is 3 times higher for TiO₂ than for ZnO solar cells. Picosecond emission studies showed that the lifetime of the D149 excited state is about 3 times longer for ZnO than for TiO₂ samples. Therefore, the results indicate that lower performance of ZnO solar cells is likely due to slower electron injection. The studies show how to correlate the laser spectroscopy methodology with global parameters of the solar cells and should help in better understanding of the behavior of alternative materials for porous electrodes for DSC and related devices