Time and Frequency Resolved Studies of Photoelectrochemical Kinetics

Distribution of Relaxation Times Based on Lasso Regression: A Tool for High-Resolution Analysis of IMPS Data in Photoelectrochemical Systems

Intensity-modulated photocurrent spectroscopy (IMPS) has been largely employed in semiconductor characterization for solar energy conversion devices to probe the operando behavior with widely available facilities. However, the implementation of IMPS data analysis to complex structures, whether based on the physical rate constant model (RCM) or the assumption-free distribution of relaxation times (DRT), is generally limited to a semi-quantitative description of the charge carrier kinetics of the system. In this study, a new algorithm for the analysis of IMPS data is developed, providing unprecedented time resolution to the investigation of μs to s charge carrier dynamics in semiconductor-based systems used in photoelectrochemistry and photovoltaics. The algorithm, based on the previously developed DRT analysis, is herein modified with a Lasso regression method and available to the reader free of charge. A validation of this new algorithm is performed on a α-Fe2O3 photoanode for photoelectrochemical water splitting, identified as a standard platform in the field, highlighting multiple potential-dependent charge transfer paths, otherwise hidden in the conventional IMPS data analysis.

Accurate determination of the charge transfer efficiency of photoanodes for solar water splitting

The charge transfer efficiency,η_t, is important to distinguish between bulk and surface recombination losses, but it is difficult to determine it accurately. We provide a comparison of methods and give recommendations, how to obtain reliable values forη_t.

Black Phosphorus Based Photocathodes in Wideband Bifacial Dye-Sensitized Solar Cells

Ultrasmall black phosphorus quantum dots (BPQDs) serve as the near-infrared light absorber and charge transfer layer in the photocathode of a bifacial n-type dye sensitized solar cell. Wideband light absorption and ≈20% enhancement in the light-to-electron efficiency are accomplished due to the fast carrier transfer and complementary light absorption by the BPQDs demonstrating that BP has large potential in photovoltaics.

Empirical in operando analysis of the charge carrier dynamics in hematite photoanodes by PEIS, IMPS and IMVS

In this Perspective, we introduce intensity modulated photocurrent/voltage spectroscopy (IMPS and IMVS) as powerful tools for the analysis of charge carrier dynamics in photoelectrochemical (PEC) cells for solar water splitting, taking hematite (α-Fe2O3) photoanodes as a case study. We complete the picture by including photoelectrochemical impedance spectroscopy (PEIS) and linking the trio of PEIS, IMPS and IMVS, introduced here as photoelectrochemical immittance triplets (PIT), both mathematically and phenomenologically, demonstrating what conclusions can be extracted from these measurements. A novel way of analyzing the results by an empirical approach with minimal presumptions is introduced, using the distribution of relaxation times (DRT) function. The DRT approach is compared to conventional analysis approaches that are based on physical models and therefore come with model presumptions. This work uses a thin film hematite photoanode as a model system, but the approach can be applied to other PEC systems as well.

New insights into electrolyte-component biased and transfer- and transport-limited charge recombination in dye-sensitized solar cells

Charge recombination takes place, respectively, within the frameworks of transfer- and transport-limited recombination mechanisms, at low and high electron density.

Photoelectrochemical water splitting at semiconductor electrodes: fundamental problems and new perspectives

Some fundamental aspects of light-driven water splitting at semiconductor electrodes are reviewed along with recent experimental and theoretical progress. The roles of thermodynamics and kinetics in defining criteria for successful water-splitting systems are examined. An overview of recent research is given that places emphasis on new electrode materials, theoretical advances and the development of semi-quantitative experimental methods to study the dynamics of light-driven water-splitting reactions. Key areas are identified that will need particular attention as the search continues for stable, efficient and cost-effective light-driven photoelectrolysis systems that exploit electron/hole separation in semiconductor/electrolyte junctions.

Photoelectrochemical characterization of electrodepositedZnOthin films sensitized by octacarboxymetallophthalocyanine derivatives

Hybrid thin films of crystalline zinc oxide ( ZnO ) modified by different octacarboxymetallophthalocyanines (MOCPc) were prepared by the readsorption method. Homogeneously blue or green thin films were formed. The photoelectrochemical characteristics of the electrodes were studied by time-resolved photocurrent measurements. Zinc(II) 2,3,9,10,16,17,23,24-octacarboxyphthalocyanine ( ZnOCPc ) showed considerably large quantum efficiency in sensitization of ZnO , one of the highest quantum efficiencies obtained so far with phthalocyanine-type sensitizers on nanocrystalline ZnO films.

Direct Measurement of the Internal Electron Quasi-Fermi Level in Dye Sensitized Solar Cells Using a Titanium Secondary Electrode

The spatial dependence of the electron quasi-Fermi level (QFL) in the interior of dye sensitized nanocrystalline solar cells (DSC) under short circuit conditions can be inferred from calculations based on a diffusive electron transport model. The calculations predict that the difference in the QFL between the electrolyte and contact sides of the TiO(2) layer under short circuit conditions at 1 sun could be as much as 0.5-0.7 eV. The predicted QFL profiles depend on assumptions made about energy positions, electron mobility, and the conduction band density of states. In this work, the position of the QFL at the electrolyte side of the dye sensitized TiO(2) film in a DSC has been measured using a thin passivated titanium contact deposited on top of the nanocrystalline TiO(2) by evaporation. The method allows changes in the electron QFL at all points on the IV characteristic of the cell to be monitored under dark and photostationary conditions. In addition, cells incorporating the titanium electrode can give information about the behavior of the QFL under dynamic conditions.

Capacitance and Field-Driven Electron Transport in Electrochemically Self-Assembled Nanoporous ZnO/Dye Hybrid Films

Electrodeposited nanoporous ZnO/eosin Y hybrid films have been investigated in view of their potential applications in dye-sensitized solar cells and supercapacitors. Intensity-modulated photocurrent spectra were measured at different electrode potentials at films of different thicknesses. It was found that the results represent either the RC constant of the cell and surface recombination of photogenerated holes with electrons or the diffusion of photogenerated electrons and are dependent on the electron concentration in the ZnO, which is influenced by the film thickness, the electrode potential, and the light intensity. The results suggest that the porosity of the electrodeposited ZnO increases with the film thickness and the films therefore consist of two parts, a less porous part deposited in the first few minutes that exhibits field-driven electron transport and a more porous outer part where electron transport is by diffusion. The results are supported by frequency-dependent capacitance measurements, which also show that the material is suitable for supercapacitors.

Dye-sensitized TiO2 nanotube solar cells: fabrication and electronic characterization

TiO2 nanotubes (TNTs) with large aspect ratio and large specific surface area were prepared from P25 (Nippon Aerosil) and applied to dye-sensitized titanium dioxide solar cells (DSSCs). Optimization of fabrication conditions, i.e., pH of the starting paste, sintering temperature for the TiO2 electrodes, electrolyte compositions of DSSCs gave the high conversion efficiency with improved open circuit voltage (V(oc)) and fill factor (FF) when compared to DSSCs made of P25. The evaluation of dye adsorption and the photo-injected electron transport such as electron diffusion coefficient (D) and electron lifetime (tau) in TNTs electrodes revealed that the higher efficiency resulted from increase of electron density with keeping much longer tau in TNTs electrodes than in P25 electrodes.