On the Way to Optoionics

Discussions with Michael Grätzel, Ursula Röthlisberger, Robert A. Evarestov and Bettina V. Lotsch are gratefully acknowledged.Based on the recent finding of significant ion conduction enhancement in iodide perovskites upon illumination, the potential of an emerging field ‘opto-ionics’ – that we define in parallelism to ‘opto-electronics’ – is explored. We emphasize that the major prerequisite is the identification of appropriate stable materials which can act as light-tunable electrolytes, permeation membranes, or electrodes. In this way, classic, but light-tunable electrochemical devices would be in reach. We also touch upon related issues such as sensing, switching, and catalysis, in which light effects on ionic charge carriers are also expected to be important.Institute of Solid State Physics, University of Latvia as the Center of Excellence has received funding from the European Union’s Horizon 2020 Framework Programme H2020-WIDESPREAD-01-2016-2017-TeamingPhase2 under grant agreement No. 739508, project CAMART

Defect chemistry of methylammonium lead iodide

The present work deals with the defect chemistry and charge transport properties in halide perovskites, and in particular in the archetypal methylammonium lead iodide. These materials are extensively researched due to their very promising application as light-harvesters in solar cells and in other optoelectronic devices. Notwithstanding the numerous studies dealing with these materials (especially with their optical and electronic properties, and with device application), a significant portion of the underlying physics and chemistry is still poorly understood. Indeed, the physico-chemical features behind their exceptional photo-electrochemical properties are still largely unknown. Moreover, these materials suffer from severe degradation processes presently impeding their practical application. In addition, the charge transport in these materials is not purely electronic, but rather shows a significant ionic portion due to mobile ionic defects. The nature of such ion conduction, alongside its effect on the photo-electrochemical properties and on the materials stability, has never been systematically investigated. The study of these aspects is the aim of this thesis, where: At first, we study the charge transport properties of methylammonium lead iodide, with particular attention to the ionic contribution, and we perform a defect chemical study of the compound. We show how the ionic conductivity, in equilibrium conditions, can be unambiguously assigned to mobile iodine vacancies, with an electronic contribution due to electron holes or conduction electron depending on the iodine activity. Secondly, we investigate the light effect on the charge transport previously characterized. Alongside an expected increase of the electronic contribution, we observe a striking enhancement of ionic conductivity in methylammonium lead iodide upon illumination. This remarkable observation is of fundamental relevance for both photovoltaics and solid state ionics fields. Here we also discuss a mechanism for such photo-enhanced ion conduction that relies on electron-ion interaction. Subsequently, we analyze methylammonium lead iodide and other hybrid halide perovskites under oxygen exposure, both in the dark and under illumination. We show that light strongly affects the kinetics of oxygen interaction, so much so that under illumination methylammonium lead iodide completely degrades, while it is metastable in the dark. The oxygen, when incorporated in a sufficient amount in the lattice, also acts as acceptor dopant, greatly varying the ionic and electronic conductivities. We then investigate stability of several halide perovskites with respect to temperature, oxygen, and illumination through thermodynamic considerations. Here we show how many of these processes are expected to be extremely severe for some of the compounds, underlining an important -and intrinsic- bottleneck for the application of these materials. At last, we investigate the short-range ion dynamics in methylammonium lead iodide, since these are linked to stability and electronic transport properties. Here, we show that methylammonium dynamics conforms well to a fast bi-axial rotation becoming isotropic in the cubic phase. In the inorganic lattice, a strong nuclear coupling between Pb and I is present, alongside highly active iodine dynamics

First-principles calculations of iodine-related point defects in CsPbI3

Many thanks to A. Lushchik, A. Popov and R. Merkle for numerous fruitful discussions. This study was partly supported by the Latvian Council for Science (grant LZP-2018/1-0147 to EK). R.A.E acknowledges the assistance of the University Computer Center of Saint-Petersburg State University for high-performance computations.We present here first principles hybrid functional calculations of the atomic and electronic structure of several iodine-related point defects in CsPbI3, a material relevant for photovoltaic applications. We show that the presence of neutral interstitial I atoms or electron holes leads to the formation of di-halide dumbbells of I2− (analogous to the well-known situation in alkali halides). Their formation and one-electron energies in the band gap are determined. The formation energy of the Frenkel defect pair (I vacancies and neutral interstitial I atoms) is found to be ∼1 eV, and as such is smaller than the band gap. We conclude that both iodine dumbbells and iodine vacancies could be, in principle, easily produced by interband optical excitation.Latvian Council for Science (grant LZP-2018/1-0147 to EK); Institute of Solid State Physics, University of Latvia as the Center of Excellence has received funding from the European Union’s Horizon 2020 Framework Programme H2020-WIDESPREAD-01-2016-2017-TeamingPhase2 under grant agreement No. 739508, project CAMART

Detection and relevance of ion conduction in hybrid organic-inorganic halide perovskites for photovoltaic applications

In recent years, hybrid organic-inorganic halide perovskites have attracted much attention with respect to their potential use as sensitizers in solar cells.[1] These materials show many outstanding properties, such as high absorption coefficients, ideal bandgap for solar light absorption (1.5 eV), long electron-hole recombination lengths and high charge carrier mobilities[1-3], that leads to a photo-conversion efficiency of hybrid-perovskite-containing devices exceeding 20%.[4] However, anomalous behaviors have been reported for these materials, such as high apparent dielectric constants at low AC frequencies or photocurrent hysteresis of solar cell devices during operation.[5] In this study[6] we measure the electrical transport properties of CH3NH3PbI3, by means of DC galvanostatic polarization, AC impedance spectroscopy and open circuit voltage measurements in electrochemical cells. By using ion-blocking electrodes, we detect a clear stoichiometric polarization behavior, from which we can separate electronic and ionic contributions to the total conductivity. We show that, under certain conditions, ionic conductivity can substantially exceed electronic conductivity and we assess the nature of the migrating ions (iodine ions). It is noteworthy that such ionic conductivity can naturally explain the above mentioned anomalies (Figure 1). Moreover, from the experimental data, a strong trapping of the electronic carriers due to ionic defects is ascertained. As a natural follow-up to better understand the defect chemistry of such materials, conductivity response to different atmospheres (I2, O2) has been measured and acceptor doping has been successfully applied. Please click Additional Files below to see the full abstract

Non-stoichiometry and ion transport in halide perovskites: Equilibrium situation and light effects

In recent years, hybrid halide perovskites have been attracting great attention due to their exceptional photo-electrochemical properties.[1-2] When used as light-harvesters in solar cells, device efficiencies exceeding 22% can be realized. We showed that a deeper understanding of (i) functionality, (ii) stability, as well as (iii) the possibility to improve the performance require a thorough insight into non-stoichiometry and ion transport.[3-5] In this contribution, we study the nature of the ionic conductivity in methylammonium lead iodide (MAPbI3), the archetypal halide perovskite, by means of a great number of electrochemical and nuclear magnetic techniques.[4] To aid the experimental investigation, we include detailed defect chemical modelling describing the effects of iodine partial pressure (Fig. 1a), doping and interaction with oxygen.[5] We also discuss results that show the significance of ion redistribution phenomena for relevant interfaces. By extending this study to the situation under illumination, we observe a striking enhancement of ionic conductivity by more than 2 orders of magnitude in MAPbI3, alongside the expected increase in electronic conductivity.[6] We provide a mechanistic explanation of this astonishing phenomenon and discuss its relevance for future light-triggered ionic devices (“opto-ionics”, see Fig. 1b). Please click Additional Files below to see the full abstract

Latency correction in sparse neuronal spike trains

Background: In neurophysiological data, latency refers to a global shift of spikes from one spike train to the next, either caused by response onset fluctuations or by finite propagation speed. Such systematic shifts in spike timing lead to a spurious decrease in synchrony which needs to be corrected. New Method: We propose a new algorithm of multivariate latency correction suitable for sparse data for which the relevant information is not primarily in the rate but in the timing of each individual spike. The algorithm is designed to correct systematic delays while maintaining all other kinds of noisy disturbances. It consists of two steps, spike matching and distance minimization between the matched spikes using simulated annealing. Results: We show its effectiveness on simulated and real data: cortical propagation patterns recorded via calcium imaging from mice before and after stroke. Using simulations of these data we also establish criteria that can be evaluated beforehand in order to anticipate whether our algorithm is likely to yield a considerable improvement for a given dataset. Comparison with Existing Method(s): Existing methods of latency correction rely on adjusting peaks in rate profiles, an approach that is not feasible for spike trains with low firing in which the timing of individual spikes contains essential information. Conclusions: For any given dataset the criterion for applicability of the algorithm can be evaluated quickly and in case of a positive outcome the latency correction can be applied easily since the source codes of the algorithm are publicly available.Comment: 15 pages, 9 figure

Eliminating Flooding-related Issues in Electrochemical CO₂-to-CO Converters: Two Lines of Defense

By using silver (Ag) in nanostructured (nanowire, nanosphere, etc.) or thin-layer form as a catalyst for electrochemical CO2 reduction, very high CO-forming selectivity of almost 100% can be achieved. Supported by gas diffusion layers (GDLs),  the reactant CO2 in the gas phase can approach and potentially access active Ag sites, which allows current densities in the range of a few hundred mA cm–2 to be reached. Yet, the stability of gas diffusion electrode (GDE) based electrochemical CO2-to-CO converters is far from perfect, and the activity of GDE cathodes, especially when operated at high current densities, often significantly decays during electrolyses after no more than a few hours. The primary reason of stability losses in GDE-based CO2-to-CO electrolysers is flooding: that is, the excess wetting of the GDE that prevents CO2 from reaching Ag catalytic sites. In the past years, the authors of this paper at Empa and at the University of Bern, cooperating with other partners of the National Competence Center for Research (NCCR) on Catalysis, took different approaches to overcome flooding. While opinions differ with regard to where the first line of defense in protecting GDEs from flooding should lie, a comparison of the recent results of the two groups gives unique insight into the nature of processes occurring in GDE cathodes used for CO2 electrolysis

Thermochemical Stability of Hybrid Halide Perovskites

This contribution discusses the chemical stability of methylammonium (MA) halide perovskites (MAPbI(3), MAPbBr(3), and MAPbCl(3)); it considers degradation processes relevant for devices (vs T, O-2, H2O, voltage, illumination) by outlining their thermodynamic constraints and linking them to experimental observations. Thermodynamic considerations indicate that degradation under O-2 is highly favored, albeit in principle preventable by encapsulation. The same is true for H2O exposure. Intrinsic degradation is unavoidable in devices, and under real conditions, it can be thermodynamically favored, as is the case for photodecomposition. The Gibbs energies of the decomposition reactions show strong dependences on the A-cation (decomposition vs T), on the halide (vs illumination), or on both (vs O-2, H2O). The stability trends vs composition change with degradation pathway, but MAPbI(3) often appears as the most unstable. These serious stability issues appear to have no master solution, though some approaches (e.g., encapsulation, exploiting lower dimensionality, and alternative contact phases) have so far shown promise