Combining Modulated Techniques for the Analysis of Photosensitive Devices.

Small-perturbation techniques such as impedance spectroscopy (IS), intensity-modulated photocurrent spectroscopy (IMPS), and intensity-modulated photovoltage spectroscopy (IMVS) are useful tools to characterize and model photovoltaic and photoelectrochemical devices. While the analysis of the impedance spectra is generally carried out using an equivalent circuit, the intensity-modulated spectroscopies are often analyzed through the measured characteristic response times. This makes the correlation between the two methods of analysis generally unclear. In this work, by taking into consideration the absorptance and separation efficiency, a unified theoretical framework and a procedure to combine the spectral analysis of the three techniques are proposed. Such a joint analysis of IS, IMPS, and IMVS spectra greatly reduces the sample space of possible equivalent circuits to model the device and allows obtaining parameters with high reliability. This theoretical approach is applied in the characterization of a silicon photodiode to demonstrate the validity of this methodology, which shows great potential to improve the quality of analysis of spectra obtained from frequency domain small-perturbation methods.

Negative Capacitance and Inverted Hysteresis: Matching Features in Perovskite Solar Cells.

Negative capacitance in the low-frequency domain and inverted hysteresis are familiar features in perovskite solar cells, which origin is still under discussion. Here we use impedance spectroscopy to analyze these responses in methylammonium lead bromide cells treated with lithium cations at the electron-selective layer/perovskite interface and in iodide devices exposed to different relative humidity conditions. Employing the surface polarization model, we obtain a time constant associated with the kinetics of the interaction of ions/vacancies with the surface, τkin, in the range of 100-102 s for all the cases exhibiting both features. These interactions lead to a decrease in the overall recombination resistance, modifying the low-frequency perovskite response and yielding a flattening of the cyclic voltammetry. As a consequence of these results we find that negative capacitance and inverted hysteresis lead to a decrease in the fill factor and photovoltage values.

TiO2 Nanotubes for Solar Water Splitting: Vacuum Annealing and Zr Doping Enhance Water Oxidation Kinetics.

Herein, we report the cooperative effect of Zr doping and vacuum annealing on the carrier dynamics and interfacial kinetics of anodized TiO2 nanotubes for light-driven water oxidation. After evaluation of different Zr loads and different annealing conditions, it was found that both Zr doping and vacuum annealing lead to a significantly enhanced light harvesting efficiency and photoelectrochemical performance. The substitution of Zr4+ by Ti4+ species leads to a higher density of surface defects such as oxygen vacancies, facilitating electron trapping on Zr4+, which reduced the charge recombination and hence boosted the charge transfer kinetics. More importantly, vacuum annealing promoted the presence of surface defects. Furthermore, the mechanistic study through impedance spectroscopy revealed that both charge transfer and surface conductivity are significantly enhanced due the presence of an oxygen-deficient TiO2 surface. These results represent an important step forward in the optimization of nanostructured TiO2-based photoelectrodes, with high potential in photocatalytic applications, including solar fuel production.

In†situ Biofilm Quantification in Bioelectrochemical Systems by using Optical Coherence Tomography.

Detailed studies of microbial growth in bioelectrochemical systems (BESs) are required for their suitable design and operation. Here, we report the use of optical coherence tomography (OCT) as a tool for in situ and noninvasive quantification of biofilm growth on electrodes (bioanodes). An experimental platform is designed and described in which transparent electrodes are used to allow real-time, 3D biofilm imaging. The accuracy and precision of the developed method is assessed by relating the OCT results to well-established standards for biofilm quantification (chemical oxygen demand (COD) and total N content) and show high correspondence to these standards. Biofilm thickness observed by OCT ranged between 3 and 90 µm for experimental durations ranging from 1 to 24 days. This translated to growth yields between 38 and 42 mgCODbiomass gCODacetate -1 at an anode potential of -0.35 V versus Ag/AgCl. Time-lapse observations of an experimental run performed in duplicate show high reproducibility in obtained microbial growth yield by the developed method. As such, we identify OCT as a powerful tool for conducting in-depth characterizations of microbial growth dynamics in BESs. Additionally, the presented platform allows concomitant application of this method with various optical and electrochemical techniques.

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 ruthenium-based 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 °C, the least stable device composed of Z907/HSE showed an efficiency degradation rate of ∼14%/1000 h, while the Ru505/L-12 system retained 96% of its initial efficiency by losing ∼1% 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.

Electron injection and scaffold effects in perovskite solar cells.

In spite of the impressive efficiencies reported for perovskite solar cells (PSCs), key aspects of their working principles, such as electron injection at the contacts or the suitability of the utilization of a specific scaffold layer, are not yet fully understood. Increasingly complex scaffolds attained by the sequential deposition of TiO2 and SiO2 mesoporous layers onto transparent conducting substrates are used to perform a systematic characterization of both the injection process at the electron selective contact and the scaffold effect in PSCs. By forcing multiple electron injection processes at a controlled sequence of perovskite-TiO2 interfaces before extraction, interfacial injection effects are magnified and hence characterized in detail. An anomalous injection behavior is observed, the fingerprint of which is the presence of significant inductive loops in the impedance spectra with a magnitude that correlates with the number of interfaces in the scaffold. Analysis of the resistive and capacitive behavior of the impedance spectra indicates that the scaffolds could hinder ion migration, with positive consequences such as lowering the recombination rate and implications for the current-potential curve hysteresis. Our results suggest that an appropriate balance between these advantageous effects and the unavoidable charge transport resistive losses introduced by the scaffolds will help in the optimization of PSC performance.

Surface Polarization Model for the Dynamic Hysteresis of Perovskite Solar Cells.

The dynamic hysteresis of perovskite solar cells consists of the occurrence of significant deviations of the current density-voltage curve shapes depending on the specific conditions of measurement such as starting voltage, waiting time, scan rate, and other factors. Dynamic hysteresis is a serious impediment to stabilized and reliable measurement and operation of the perovskite solar cells. In this Letter, we formulate a model for the dynamic hysteresis based on the idea that the cell accumulates a huge quantity of surface electronic charge at forward bias that is released on voltage sweeping, causing extra current over the normal response. The charge shows a retarded dynamics due to the slow relaxation of the accompanying ionic charge, that produces variable shapes depending on scan rate or poling value and time. We show that the quantitative model provides a consistent description of experimental results and allows us to determine significant parameters of the perovskite solar cell for both the transient and steady-state performance.

Surface Recombination and Collection Efficiency in Perovskite Solar Cells from Impedance Analysis.

The large diffusion lengths recurrently measured in perovskite single crystals and films signal small bulk nonradiative recombination flux and locate the dominant carrier recombination processes at the outer interfaces. Surface recombination largely determines the photovoltaic performance, governing reductions under short-circuit current and open-circuit voltage. Quantification of recombination losses is necessary to reach full understanding of the solar cell operating principles. Complete impedance model is given, which connects capacitive and resistive processes to the electronic kinetics at the interfaces. Carrier collection losses affecting the photocurrent have been determined to equal 1%. Photovoltage loss is linked to the decrease in surface hole density, producing 0.3 V reduction with respect to the ideal radiative limit. Our approach enables a comparison among different structures, morphologies, and processing strategies of passivation and buffer layers.

Carbon Counter-Electrode-Based Quantum-Dot-Sensitized Solar Cells with Certified Efficiency Exceeding 11.

The mean power conversion efficiency (PCE) of quantum-dot-sensitized solar cells (QDSCs) is mainly limited by the low photovoltage and fill factor (FF), which are derived from the high redox potential of polysulfide electrolyte and the poor catalytic activity of the counter electrode (CE), respectively. Herein, we report that this problem is overcome by adopting Ti mesh supported mesoporous carbon (MC/Ti) CE. The confined area in Ti mesh substrate not only offers robust carbon film with submillimeter thickness to ensure high catalytic capacity, but also provides an efficient three-dimension electrical tunnel with better conductivity than state-of-art Cu2S/FTO CE. More importantly, the MC/Ti CE can down shift the redox potential of polysulfide electrolyte to promote high photovoltage. In all, MC/Ti CEs boost PCE of CdSe0.65Te0.35 QDSCs to a certified record of 11.16% (Jsc = 20.68 mA/cm(2), Voc = 0.798 V, FF = 0.677), an improvement of 24% related to previous record. This work thus paves a way for further improvement of performance of QDSCs.

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 50mM and increasing pH from 6 to 8 resulted in decreased charge transfer and diffusion resistances; lowest values being 144 Ω·cm(2) and 34 Ω·cm(2), 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.

Competitive photoelectrochemical methanol and water oxidation with hematite electrodes.

Photocatalytic water and methanol oxidation were studied at thin film hematite electrodes synthesized by atomic layer deposition (ALD). Systematic photoelectrochemical characterization along with O2 evolution measurements were carried out in order to better understand the mechanisms of both water and methanol oxidation at hematite electrodes. When both water and methanol are present in the solution, they are oxidized competitively with each other, allowing the detection and assignment of distinct surface states characteristic to each process. The measurement of different surface states for methanol and water oxidation, along with the absence of measurable surface states in an inert acetonitrile electrolyte, clearly shows that the detected surface states are chemically distinct reaction intermediates of water or methanol oxidation.

General working principles of CH3NH3PbX3 perovskite solar cells.

Organometal halide perovskite-based solar cells have recently realized large conversion efficiency over 15% showing great promise for a new large scale cost-competitive photovoltaic technology. Using impedance spectroscopy measurements we are able to separate the physical parameters of carrier transport and recombination in working devices of the two principal morphologies and compositions of perovskite solar cells, viz. compact thin films of CH3NH3PbI(3-x)Clx and CH3NH3PbI3 infiltrated on nanostructured TiO2. The results show nearly identical spectral characteristics indicating a unique photovoltaic operating mechanism that provides long diffusion lengths (1 µm). Carrier conductivity in both devices is closely matched, so that the most significant differences in performance are attributed to recombination rates. These results highlight the central role of the CH3NH3PbX3 semiconductor absorber in carrier collection and provide a new tool for improved optimization of perovskite solar cells. We report for the first time a measurement of the diffusion length in a nanostructured perovskite solar cell.

Role of the Selective Contacts in the Performance of Lead Halide Perovskite Solar Cells.

The effect of electron- and hole-selective contacts in the final cell performance of hybrid lead halide perovskite, CH3NH3PbI3, solar cells has been systematically analyzed by impedance spectroscopy. Complete cells with compact TiO2 and spiro-OMeTAD as electron- and hole-selective contacts have been compared with incomplete cells without one or both selective contacts to highlight the specific role of each contact. It has been described how selective contacts contribute to enhance the cell FF and how the hole-selective contact is mainly responsible for the high Voc in this kind of device. We have determined that the recombination rate is mainly governed by the selective contacts. This fact has important implication for the future optimization of perovskite solar cells. Finally, we have developed a method to analyze the results obtained, and it has been applied for three different electron-selecting materials: TiO2, ZnO, and CdS.

Selective contacts drive charge extraction in quantum dot solids via asymmetry in carrier transfer kinetics.

Colloidal quantum dot solar cells achieve spectrally selective optical absorption in a thin layer of solution-processed, size-effect tuned, nanoparticles. The best devices built to date have relied heavily on drift-based transport due to the action of an electric field in a depletion region that extends throughout the thickness of the quantum dot layer. Here we study for the first time the behaviour of the best-performing class of colloidal quantum dot films in the absence of an electric field, by screening using an electrolyte. We find that the action of selective contacts on photovoltage sign and amplitude can be retained, implying that the contacts operate by kinetic preferences of charge transfer for either electrons or holes. We develop a theoretical model to explain these experimental findings. The work is the first to present a switch in the photovoltage in colloidal quantum dot solar cells by purposefully formed selective contacts, opening the way to new strategies in the engineering of colloidal quantum dot solar cells.

Mechanism of carrier accumulation in perovskite thin-absorber solar cells.

Photovoltaic conversion requires two successive steps: accumulation of a photogenerated charge and charge separation. Determination of how and where charge accumulation is attained and how this accumulation can be identified is mandatory for understanding the performance of a photovoltaic device and for its further optimization. Here we analyse the mechanism of carrier accumulation in lead halide perovskite, CH3NH3PbI3, thin-absorber solar cells by means of impedance spectroscopy. A fingerprint of the charge accumulation in high density of states of the perovskite absorber material has been observed at the capacitance of the samples. This is, as far as we know, the first observation of charge accumulation in light-absorbing material for nanostructured solar cells, indicating that it constitutes a new kind of photovoltaic device, differentiated from sensitized solar cells, which will require its own methods of study, characterization and optimization.

Three dimensional-TiO(2) nanotube array photoanode architectures assembled on a thin hollow nanofibrous backbone and their performance in quantum dot-sensitized solar cells.

Facile synthesis of TiO2 nanotube branched (length ∼0.5 µm) thin hollow-nanofibers is reported. The hierarchical three dimensional photoanodes (H-TiO2-NFs) (only ∼1 µm thick) demonstrate their excellent candidature as photoanodes in QD-sensitized solar cells, exhibiting ∼3-fold higher energy conversion efficiency (η = 2.8%, Jsc = 8.8 mA cm(-2)) than that of the directly grown nanotube arrays on a transparent conducting oxide (TCO) substrate (η = 0.9%, Jsc = 2.5 mA cm(-2)).

Temperature effects in dye-sensitized solar cells.

In the standard solar cell technologies such as crystalline silicon and cadmium telluride, increments of temperature in the cell produce large variations in the energy conversion efficiency, which decreases at a constant rate. In dye solar cells the efficiency remains roughly constant with a maximum at around 30-40 °C and further decays above this temperature. In this work, the origin of this characteristic behavior is explained. Data show that under illumination recombination kinetics in the active layer of the cell is the same between -7 and 40 °C. Consequently, the efficiency of the cell remained virtually constant, with only small differences in the fill factor associated with changes in the series resistance. A further increase in temperature up to 70 °C produces an increase in recombination kinetics yielding lower photopotential and device performance. Finally, it is emphasized that at the normal operating temperatures of solar cells, the gap among the conversion efficiency of different technologies is much smaller than generally acknowledged.

Elucidating capacitance and resistance terms in confined electroactive molecular layers.

Electrochemical analyses on confined electroactive molecular layers, herein exemplified with electroactive self-assembled monolayers, sample current contributions that are significantly influenced by additional nonfaradaic and uncompensated resistance effects that, though unresolved, can strongly distort redox analysis. Prior work has shown that impedance-derived capacitance spectroscopy approaches can cleanly resolve all contributions generated at such films, including those which are related to the layer dipolar/electrostatic relaxation characteristics. We show herein that, in isolating the faradaic and nonfaradaic contributions present within an improved equivalent circuit description of such interfaces, it is possible to accurately simulate subsequently observed cyclic voltammograms (that is, generated current versus potential patterns map accurately onto frequency domain measurements). Not only does this enable a frequency-resolved quantification of all components present, and in so doing, a full validation of the equivalent circuit model utilized, but also facilitates the generation of background subtracted cyclic voltammograms remarkably free from all but faradaic contributions.

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.

Harnessing Infrared Photons for Photoelectrochemical Hydrogen Generation. A PbS Quantum Dot Based "Quasi-Artificial Leaf".

Hydrogen generation by using quantum dot (QD) based heterostructures has emerged as a promising strategy to develop artificial photosynthesis devices. In the present study, we sensitize mesoporous TiO2 electrodes with in-situ-deposited PbS/CdS QDs, aiming at harvesting light in both the visible and the near-infrared for hydrogen generation. This heterostructure exhibits a remarkable photocurrent of 6 mA·cm(-2), leading to 60 mL·cm(-2)·day(-1) hydrogen generation. Most importantly, confirmation of the contribution of infrared photons to H2 generation was provided by the incident-photon-to-current-efficiency (IPCE), and the integrated current was in excellent agreement with that obtained through cyclic voltammetry. The main electronic processes (accumulation, transport, and recombination) were identified by impedance spectroscopy, which appears as a simple and reliable methodology to evaluate the limiting factors of these photoelectrodes. On the basis of this TiO2/PbS/CdS heterostructrure, a "quasi-artificial leaf" has been developed, which has proven to produce hydrogen under simulated solar illumination at (4.30 ± 0.25) mL·cm(-2)·day(-1).