Diffusion-Recombination Impedance Model for Solar Cells with Disorder and Nonlinear Recombination

The diffusion–recombination model is a key tool in understanding the photovoltaic operation of solar cells. Dye-sensitized solar cells, organic solar cells, and inorganic semiconductor solar cells are systems affected by disorder that are often characterized with impedance spectroscopy. In this paper, we extend the previous theory of diffusion–recombination impedance including traps and nonlinear recombination. We show the transmission line equivalent circuit representation, and we describe the physical meaning of a number of model parameters that can be obtained: the chemical capacitance, ; the recombination resistance, ; the transport resistance, ; the electron lifetime, ; the electron conductivity, ; the chemical diffusion coefficient of electrons, ; and the diffusion length, . At most, three of these parameters are independent, but if the diffusion length is short, the impedance model collapses to a function that has one degree of freedom less, known as the Gerischer impedance. We show the connection of the two parameters that remain to the diffusion length and the lifetime

Photoelectrochemical and Impedance Spectroscopic Investigation of Water Oxidation with “Co–Pi”-Coated Hematite Electrodes

Uniform thin films of hematite (α-Fe2O3) deposited by atomic layer deposition (ALD) coated with varying amounts of the cobalt phosphate catalyst, “Co–Pi,” were investigated with steady-state and transient photoelectrochemical measurements and impedance spectroscopy. Systematic studies as a function of Co–Pi thickness were performed in order to clarify the mechanism by which Co–Pi enhances the water-splitting performance of hematite electrodes. It was found that under illumination, the Co–Pi catalyst can efficiently collect and store photogenerated holes from the hematite electrode. This charge separation reduces surface state recombination which results in increased water oxidation efficiency. It was also found that thicker Co–Pi films produced increased water oxidation efficiencies which is attributed to a combination of superior charge separation and increased surface area of the porous catalytic film. These combined results provide important new understanding of the enhancement and limitations of the Co–Pi catalyst coupled with semiconductor electrodes for water-splitting applications

Interpretation of cyclic voltammetry measurements of thin semiconductor films for solar fuel applications

A simple model is proposed that allows interpretation of the cyclic voltammetry diagrams obtained experimentally for photoactive semiconductors with surface states or catalysts used for fuel production from sunlight. When the system is limited by charge transfer from the traps/catalyst layer and by detrapping, it is shown that only one capacitive peak is observable and is not recoverable in the return voltage scan. If the system is limited only by charge transfer and not by detrapping, two symmetric capacitive peaks can be observed in the cathodic and anodic directions. The model appears as a useful tool for the swift analysis of the electronic processes that limit fuel production.The research leading to these results is supported by Universitat Jaume I Project P1·1B2011-50

Quantification of charge carriers and acetate diffusion lengths in intermittent electro-active biofilms using Electrochemical Impedance Spectroscopy

Intermittent anode potential regimes have been used to increase the concentration of charge carriers in electro-active biofilms (EABfs). Even though this increased number of carriers is typically correlated to higher current densities, estimating the concentration of charge carriers in EABfs and linking it to measured current density has never been done. In this study, Electrochemical Impedance Spectroscopy (EIS) and Optical Coherence Tomography (OCT) were used to estimate charge carriers and to study mass transfer limitations in intermittently polarized anodic EABfs. Intermittent potential steps of 20, 60, and 300 s were applied and EABf equilibration times were measured. These times were in the order of 10 s and correlated to the diffusion times obtained from EIS. Acetate consumption rates 100 times faster than the diffusion time of acetate into the EABfs were also estimated with EIS, indicating that current was diffusion limited. Using the capacitance and considering the measured volume of EABf, concentrations of charge carriers ranging from 0.05 molcharge carriers m−3EABf at current densities of 1 A m−2 up to 0.2 molcharge carriers m−3EABf at current densities higher than 2 A m−2 were calculated. This study shows that EIS can be used to study developing EABfs in a non-destructive way and in real-time.</p

Quantification of charge carriers and acetate diffusion lengths in intermittent electro-active biofilms using Electrochemical Impedance Spectroscopy

Intermittent anode potential regimes have been used to increase the concentration of charge carriers in electro-active biofilms (EABfs). Even though this increased number of carriers is typically correlated to higher current densities, estimating the concentration of charge carriers in EABfs and linking it to measured current density has never been done. In this study, Electrochemical Impedance Spectroscopy (EIS) and Optical Coherence Tomography (OCT) were used to estimate charge carriers and to study mass transfer limitations in intermittently polarized anodic EABfs. Intermittent potential steps of 20, 60, and 300 s were applied and EABf equilibration times were measured. These times were in the order of 10 s and correlated to the diffusion times obtained from EIS. Acetate consumption rates 100 times faster than the diffusion time of acetate into the EABfs were also estimated with EIS, indicating that current was diffusion limited. Using the capacitance and considering the measured volume of EABf, concentrations of charge carriers ranging from 0.05 molcharge carriers m−3EABf at current densities of 1 A m−2 up to 0.2 molcharge carriers m−3EABf at current densities higher than 2 A m−2 were calculated. This study shows that EIS can be used to study developing EABfs in a non-destructive way and in real-time.</p

Analysis of bio-anode performance through electrochemical impedance spectroscopy

In this paper we studied the performance of bioanodes under different experimental conditions using polarization curves and impedance spectroscopy. We have identified that the large capacitances of up to 1 mF·cm− 2 for graphite anodes have their origin in the nature of the carbonaceous electrode, rather than the microbial culture. In some cases, the separate contributions of charge transfer and diffusion resistance were clearly visible, while in other cases their contribution was masked by the high capacitance of 1 mF·cm− 2. The impedance data were analyzed using the basic Randles model to analyze ohmic, charge transfer and diffusion resistances. Increasing buffer concentration from 0 to 50 mM and increasing pH from 6 to 8 resulted in decreased charge transfer and diffusion resistances; lowest values being 144 Ω·cm2 and 34 Ω·cm2, respectively. At acetate concentrations below 1 mM, current generation was limited by acetate. We show a linear relationship between inverse charge transfer resistance at potentials close to open circuit and saturation (maximum) current, associated to the Butler–Volmer relationship that needs further exploration.The authors wish to acknowledge funding from the European Union Seventh Framework Programme (FP7/2012-2016) project ‘Bioelectrochemical systems for metal production, recycling, and remediation’ under grant agreement no. 282970. AtH is supported by a NWO VENI grant no. 13631. OS was supported by the French environmental agency ADEME, by the Region Bretagne and by Rennes Metropole when doing the experiments. This work was performed in the cooperation framework of Wetsus, Centre of Excellence for Sustainable Water Technology (www.wetsus.nl). Wetsus is co-funded by the Dutch Ministry of Economic Affairs and Ministry of Infrastructure and Environment, the European Union Regional Development Fund, the Province of Fryslân, and the Northern Netherlands Provinces

New Views on Carrier Diffusion and Recombination by Combining Small Perturbation Techniques: Application to BiVO4 Photoelectrodes

Impedance spectroscopy (IS), intensity-modulated photocurrent spectroscopy (IMPS), and intensity-modulated photovoltage spectroscopy (IMVS) are well-established powerful modulated techniques to characterize optoelectronic devices. Their combined use has proven to provide an understanding of the behavior and performance of these systems, far beyond the output obtained from their independent analysis. However, this combination is shown to be challenging when applied to complex systems. Herein, IS, IMPS, and IMVS are cooperatively used, for the first time, to study the distributed photogeneration, diffusion, and recombination processes in a photoanode of zircon-doped bismuth vanadate. The use of this methodology reveals that the carriers that determine the response of the device are the electrons when the device is illuminated from the hole-collector side (electrolyte) and the holes when the illumination reaches the device from the electron-collector side. Detailed quantitative information is obtained for each carrier, including recombination lifetime, diffusion coefficient and collectrion and separation efficiencies, identifying the latter as the main limitation of this device. This methodology is a powerful tool that can be used for the characterization and understanding of the operating processes of other photoconversion devices.Funding for open access charge: CRUE-Universitat Jaume

Stability of dye-sensitized solar cells under extended thermal stress

In the last few decades, dye-sensitized solar cell (DSC) technology has been demonstrated to be a promising candidate for low cost energy production due to cost-effective materials and fabrication processes. Arguably, DSC stability is the biggest challenge for making this technology appealing for industrial exploitation. This work provides further insight into the stability of DSCs by considering specific dye–electrolyte systems characterized by Raman and impedance spectroscopy analysis. In particular, two rutheniumbased dyes, Z907 and Ru505, and two commercially available electrolytes, namely, the high stability electrolyte (HSE) and solvent-free Livion 12 (L-12), were tested. After 4700 h of thermal stress at 85 1C, the least stable device composed of Z907/HSE showed an efficiency degradation rate of B14%/1000 h, while the Ru505/L-12 system retained 96% of its initial efficiency by losing B1% each 1000 h. The present results show a viable route to stabilize the DSC technology under prolonged annealing conditions complying with the IEC standard requirements