Defect segregation and its effect on the photoelectrochemical properties of Ti-doped hematite photoanodes for solar water splitting

Optimising the photoelectrochemical performance of hematite photoanodes for solar water splitting requires better understanding of the relationships between dopant distribution, structural defects and photoelectrochemical properties. Here, we use complementary characterisation techniques including electron microscopy, conductive atomic force microscopy (CAFM), Rutherford backscattering spectroscopy (RBS), atom probe tomography (APT) and intensity modulated photocurrent spectroscopy (IMPS) to study this correlation in Ti-doped (1 cat.%) hematite films deposited by pulsed laser deposition (PLD) on F:SnO2 (FTO) coated glass substrates. The deposition was carried out at 300 {\deg}C, followed by annealing at 500 deg C for 2 h. Upon annealing, Ti was observed by APT to segregate to the hematite/FTO interface and into some hematite grains. Since no other pronounced changes in microstructure and chemical composition were observed by electron microscopy and RBS after annealing, the non-uniform Ti redistribution seems to be the reason for a reduced interfacial recombination in the annealed films, as observed by IMPS. This results in a lower onset potential, higher photocurrent and larger fill factor with respect to the as-deposited state. This work provides atomic-scale insights into the microscopic inhomogeneity in Ti-doped hematite thin films and the role of defect segregation in their electrical and photoelectrochemical properties

Reconsidering figures of merit for performance and stability of perovskite photovoltaics

The development of hybrid organic-inorganic halide perovskite solar cells (PSCs) that combine high performance and operational stability is vital for implementing this technology. Recently, reversible improvement and degradation of PSC efficiency have been reported under illumination-darkness cycling. Quantifying the performance and stability of cells exhibiting significant diurnal performance variations is challenging. We report the outdoor stability measurements of two types of devices showing either reversible photo-degradation or reversible efficiency improvement under sunlight. Instead of the initial (or stabilized) efficiency and T as the figures of merit for the performance and stability of such devices, we propose using the value of the energy output generated during the first day of exposure and the time needed to reach its 20% drop, respectively. The latter accounts for both the long-term irreversible degradation and the reversible diurnal efficiency variation and does not depend on the type of process prevailing in a given perovskite cell

Roadmap on commercialization of metal halide perovskite photovoltaics

Perovskite solar cells (PSCs) represent one of the most promising emerging photovoltaic technologies due to their high power conversion efficiency. However, despite the huge progress made not only in terms of the efficiency achieved, but also fundamental understanding of the relevant physics of the devices and issues which affect their efficiency and stability, there are still unresolved problems and obstacles on the path toward commercialization of this promising technology. In this roadmap, we aim to provide a concise and up to date summary of outstanding issues and challenges, and the progress made toward addressing these issues. While the format of this article is not meant to be a comprehensive review of the topic, it provides a collection of the viewpoints of the experts in the field, which covers a broad range of topics related to PSC commercialization, including those relevant for manufacturing (scaling up, different types of devices), operation and stability (various factors), and environmental issues (in particular the use of lead). We hope that the article will provide a useful resource for researchers in the field and that it will facilitate discussions and move forward toward addressing the outstanding challenges in this fast-developing field

Real-time detection of ammonium in soil pore water

Abstract The development of technologies for continuous measurement of nitrogen forms in the soil is essential for optimizing the application of fertilizers in agriculture and preventing water-resource pollution. However, there is no effective commercial technology available for continuous monitoring of ammonium species in soil pore water. This work investigates an approach for real-time measurement of ammonium in soil water using near-infrared transmission spectroscopy and partial least squares regression (PLSR) for spectral analysis. The PLSR model was trained using soil pore water collected from various soils spiked with ammonium to achieve a wide concentration range. The monitoring approach was then validated through transport experiments in a soil column. The results demonstrated capabilities for real-time tracking of the temporal variation in soil ammonium concentration and potential utilization in agronomical or environmental sensing

Photoconductance of ITO/Conductive Polymer Junctions in the UV and Visible Ranges

Controlling charge transfer at indium-doped tin oxide (ITO)/conductive polymer junctions is of special importance for organic photovoltaic (OPV) devices and organic light emitting diodes (OLEDs), where ITO is often the transparent electrode of choice. Light induced conductance enhancement, i.e., photoconductance, can allow such control. ITO/conductive polymer junctions are shown herein to exhibit photoconductance under UV illumination mostly due to photoinduced decrease of an electron barrier at the ITO–polymer interface by discharging of ITO extrinsic surface states, related to the adsorption of oxygen species. Furthermore, we show that ITO surface modification by photoactive porphyrin adsorption can sensitize the ITO/conductive polymer junctions, extending the photoconductance to the visible range, to which ITO is transparent. This process is ascribed mostly to discharging of ITO adsorbate states by recombination with photogenerated holes in the photoexcited molecules. Such sensitization is highly relevant for organic optoelectronic devices utilizing ITO interfaced with photoactive organic species and operating in the visible range, such as OPV and OLED devices, and might be applicable also to other UV-photoconductive metal oxide electrodes

Microscopic Investigation of Degradation Processes in a Polyfluorene Blend by Near-Field Scanning Optical Microscopy

We have studied the degradation of the photoluminescence (PL) of a phase-separated film of a polyfluorene blend, F8BT/PFO, on the submicron length scale using near-field scanning optical microscopy, visualizing the PL of blend compositions that do not exist macroscopically in equilibrium. In the initial scans, the topography and the PL were anticorrelated, as the emission was dominated by the PFO-rich phase. This behavior changed at longer illumination times, where the emission was dominated by the F8BT-rich phase; i.e., the topography and PL were correlated. Using macroscopic investigation of the mechanisms that govern the PL, we could explain the time dependence of the PL spatial distribution: while the degradation of F8BT was driven by photobleaching, both faster absorption degradation and photobleaching processes dominate the degradation of PFO. In addition, we found that energy transfer does not protect the PFO from degradation and does not improve its resistance to oxidation

Lead iodide as a buffer layer in UV-induced degradation of CH3NH3PbI3 films

Encapsulated CH3NH3PbI3 films, the 'work horse' of the organic–inorganic perovskite-based photovoltaics, grown by one step- and two step- deposition methods, were used to study the effect of the film preparation method on their photostability. Time dependent light absorption decay under exposure to concentrated sunlight was used to estimate the degradation of the films. Films deposited by one step showed a significant decrease in the CH3NH3PbI3 absorbance when illuminated through the substrate, while films obtained through two step deposition exhibited almost no photodegradation under similar sunlight exposure. On the other hand, both types of films degraded significantly when irradiated through the top encapsulation. Unreacted PbI2 present near the substrate is suggested to be responsible for enhancing the photostability of the films obtained by two step deposition. Here, remnant PbI2 works as a UV filter and reduces UV light-induced degradation. The results demonstrate the significance of the preparation method in determining photochemical stability of the perovskite films, due to favorable property of remnant PbI2 in the absorber as a UV-protective layer

Application of luminescence downshifting materials for enhanced stability of CH3NH3PbI3(1-x)Cl3x perovskite photovoltaic devices

The application of luminescent down shifting (LDS) layers as alternative UV filters for CH3NH3PbI3(1-x)Cl3x perovskite solar cell (PSC) devices is reported. A combination of photo-absorption measurements and of device decay measurements during light soaking are used to verify the stability. The application of a UV filter or LDS layer was able to significantly retard photo-induced degradation with ∼18% drop in device power conversion efficiency (PCE) observed over 30 h for non-encapsulated devices, which is compared to ∼97% for an un-filtered device, also without encapsulation. Whilst the PCE of the PSC device decreases with the application of the LDS layer, the drop is not as significant as when a commercial UV filter is used. Considering that UV filters will be essential for the commercialization of PSCs, the work provides evidence that the LDS layer can act as an alternative UV filter in PSCs and can limit the drop in PCE that can be expected from the inclusion of a UV filter, thus providing an added benefit over commercial UV filters

Models of Surface Morphology and Electronic Structure of Indium Oxide and Indium Tin Oxide for Several Surface Hydroxylation Levels

Indium oxide (IO) and indium tin oxide (ITO) are important metal oxide materials with a wide array of applications. Particularly, ITO is employed as a transparent conductive electrode in photovoltaic systems. While bulk metal oxides are typically well characterized, their surfaces, especially in real-life applications, can be hydroxylated and intrinsically disordered to a level that a structure–function prediction becomes a daunting task. We tackle this problem by carrying out simulations based on Density Functional Theory. We propose IO and ITO hydroxylated surfaces derived from the bcc and rombohedral IO polymorphs (100%, 66%, 33%, and 0% hydroxylation coverages were considered). By correlating computed quantities such as surface partial density of states, work functions, and surface dipole strength, a clear picture of the structure–function relationships in these model systems emerges. In line with conclusions drawn from experiments, we find that the density of states of 100% hydroxylated surfaces and bulk models are unaltered by Sn doping, with the only difference being the position of the Fermi level. The partially hydroxylated surfaces, instead show a rich array of behaviors, including appearance of surface states in the gap and appearance of interesting morphologies, such as chemisorbed molecular oxygen. We also find that the hydroxylation level affects surface dipoles in a systematic way, that is, the higher the hydroxylation level, the higher the surface dipole (screening/reducing the work function). Furthermore, models with In-atom vacancies show a relatively small decrease in surface dipole with hydroxyl coverage due to surface distortions