Interfacial Solar Steam Generation Enables Fast-Responsive, Energy-Efficient, and Low-Cost Off-Grid Sterilization.

Steam sterilization is widely used as one of the most reliable sterilization methods for public health. However, traditional steam sterilization mainly relies on electricity, a constrained resource for many developing countries and areas. The lack of available and affordable sterilization techniques in these areas is exposing human beings to a high risk of various epidemic diseases, and calls for the development of off-grid sterilization solutions. For the first time, the kinetic advantages of interfacial solar steam generation is fundamentally revealed and it is demonstrated that interfacial solar steam generation can enable fast-responsive (as short as 8.4 min for a full sterilization cycle) and energy-efficient (100 J mL-1 for steam reaching 121 °C) sterilization, superior to those of the conventional sterilization techniques. The key solar absorber is made of low cost and widely available biochar. A proof-of-concept sterilization system with a 10.5 L solar autoclave is built with very low cost of whole life-cycle and operates with minimum carbon footprint. Effective sterilization (≈99.999999% inactivation of pathogen), exceeding the requirements of Food and Drug Administration is demonstrated, making the sterilization strategy a promising and complementary personalized sterilization solution, particularly beneficial for off-grid areas.

In Situ Measurement of Electric-Field Screening in Hysteresis-Free PTAA/FA0.83Cs0.17Pb(I0.83Br0.17)3/C60 Perovskite Solar Cells Gives an Ion Mobility of ∼3 × 10-7 cm2/(V s), 2 Orders of Magnitude Faster than Reported for Metal-Oxide-Contacted Perovskite Cells with Hysteresis.

We apply a series of transient measurements to operational perovskite solar cells of the architecture ITO/PTAA/FA0.83Cs0.17Pb(I0.83Br0.17)3/C60/BCP/Ag, and similar cells with FA0.83MA0.17. The cells show no detectable JV hysteresis. Using photocurrent transients at applied bias we find a ∼1 ms time scale for the electric field screening by mobile ions in these cells. We confirm our interpretation of the transient measurements using a drift-diffusion model. Using Coulometry during field screening relaxation at short circuit, we determine the mobile ion concentration to be ∼1 × 1018/cm3. Using a model with one mobile ion species, the concentration and the screening time require an ion mobility of ∼3 × 10-7 cm2/(V s). As far as we know, this article gives the first direct measurement of the ion mobility and concentration in a fully functional perovskite solar cell. The measured ion mobility is 2 orders of magnitude higher than the highest estimates previously determined using perovskite solar cells and perovskite thin films, and 3 orders of magnitude higher than is frequently used in modeling hysteresis effects. We provide evidence that the fast field screening is due to mobile ions, as opposed to dark injection and trapping of electronic carriers.

Investigating the Consistency of Models for Water Splitting Systems by Light and Voltage Modulated Techniques.

The optimization of solar energy conversion devices relies on their accurate and nondestructive characterization. The small voltage perturbation techniques of impedance spectroscopy (IS) have proven to be very powerful to identify the main charge storage modes and charge transfer processes that control device operation. Here we establish the general connection between IS and light modulated techniques such as intensity modulated photocurrent (IMPS) and photovoltage spectroscopies (IMVS) for a general system that converts light to energy. We subsequently show how these techniques are related to the steady-state photocurrent and photovoltage and the external quantum efficiency. Finally, we express the IMPS and IMVS transfer functions in terms of the capacitive and resistive features of a general equivalent circuit of IS for the case of a photoanode used for solar fuel production. We critically discuss how much knowledge can be extracted from the combined use of those three techniques.

Understanding the synergistic effect of WO3-BiVO4 heterostructures by impedance spectroscopy.

WO3-BiVO4 n-n heterostructures have demonstrated remarkable performance in photoelectrochemical water splitting due to the synergistic effect between the individual components. Although the enhanced functional capabilities of this system have been widely reported, in-depth mechanistic studies explaining the carrier dynamics of this heterostructure are limited. The main goal is to provide rational design strategies for further optimization as well as to extend these strategies to different candidate systems for solar fuel production. In the present study, we perform systematic optoelectronic and photoelectrochemical characterization to understand the carrier dynamics of the system and develop a simple physical model to highlight the importance of the selective contacts to minimize bulk recombination in this heterostructure. Our results collectively indicate that while BiVO4 is responsible for the enhanced optical properties, WO3 controls the transport properties of the heterostructured WO3-BiVO4 system, leading to reduced bulk recombination.

Germanium coating boosts lithium uptake in Si nanotube battery anodes.

Si nanotubes for reversible alloying reaction with lithium are able to accommodate large volume changes and offer improved cycle retention and reliable response when incorporated into battery anodes. However, Si nanotube electrodes exhibit poor rate capability because of their inherently low electron conductivity and Li ion diffusivity. Si/Ge double-layered nanotube electrodes show promise to improve structural stability and electrochemical kinetics, as compared to homogeneous Si nanotube arrays. The mechanism explaining the enhancement in the rate capabilities is revealed here by means of electrochemical impedance methods. The Ge shell efficiently provides electrons to the active materials, which increase the semiconductor conductivity thereby assisting Li(+) ion incorporation. The charge transfer resistance which accounts for the interfacial Li(+) ion intake from the electrolyte is reduced by two orders of magnitude, indicating the key role of the Ge layer as an electron supplier. Other resistive processes hindering the electrode charge-discharge process are observed to show comparable values for Si and Si/Ge array electrodes.

Relaxation of Electron Carriers in the Density of States of Nanocrystalline TiO2.

Band gap localized states and surface states play a dominant role in the application of nanocrystalline metal oxides to photovoltaics and solar fuel production. Electrons injected in nanocrystalline TiO2 by voltage or photogeneration are mainly located in band gap states. Therefore, charging a nanoparticulate semiconductor network allows one to recover the density of states (DOS) in the energy axis. However, shallow traps remain in equilibrium with the conduction band electrons, while deep traps do not. We show that the characteristic peak of the apparent DOS mixes an exponential DOS and a monoenergetic surface state. A model that incorporates the trap's kinetics proves to be very efficient to assess the important parameters that determine both contributions via variation of charging rate. Contrary to the common theory, we demonstrate that the peculiar capacitance peak of nanocrystalline TiO2 can be mainly attributed, in some cases, to deep traps in the exponential distribution.

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.

On the methods of calculation of the charge collection efficiency of dye sensitized solar cells.

The charge collection efficiency is one of the most critical parameters of photovoltaic devices. In this paper we provide the analysis and comparison between several approaches for the calculation of the collection efficiency of dye-sensitized solar cells. In addition, we point out that although it is reasonable to correlate transit time and recombination lifetime with respect to diffusion length, it is less physical to directly calculate collection efficiency only based on characteristic time constants.

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.

Equivalent Circuit of Electrons and Holes in Thin Semiconductor Films for Photoelectrochemical Water Splitting Applications.

A simple model for the kinetics of electrons and holes in a thin semiconductor film in photoelectrochemical water splitting conditions is discussed, with a focus to discriminate between trap-assisted recombination and charge-transfer processes. We formulate the kinetic model in terms of the measurements of impedance spectroscopy and discuss the application of the results for the interpretation of the current potential curve under photogeneration. We provide a rigorous structure of the fundamental equivalent circuit for photoelectrochemical water splitting systems including a new predicted feature that is a chemical capacitance of the minority carriers that can give rise, in combination with other standard features, to a total of three arcs in the complex plane.