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Abstract: High-end organic–inorganic lead halide perovskite semitransparent p–i–n solar cells for tandem applications use a phenyl-C61-butyric acid methyl ester (PCBM)/atomic layer deposition (ALD)-SnOx electron transport layer stack. Omitting the PCBM would be preferred for manufacturing, but has in previous studies on (FA,MA)Pb(Br,I)3 and (Cs,FA)Pb(Br,I)3 and in this study on Cs0.05FA0.79MA0.16PbBr0.51I2.49 (perovskite) led to poor solar cell performance because of a bias-dependent light-generated current. A direct ALD-SnOx exposure was therefore suggested to form a nonideal perovskite/SnOx interface that acts as a transport barrier for the light-generated current. To further investigate the interface formation during the initial ALD SnOx growth on the perovskite, the mass dynamics of monitor crystals coated by partial p–i–n solar cell stacks were recorded in situ prior to and during the ALD using a quartz crystal microbalance. Two major finds were made. A mass loss was observed prior to ALD for growth temperatures above 60 °C, suggesting the decomposition of the perovskite. In addition, a mostly irreversible mass gain was observed during the first exposure to the Sn precursor tetrakis(dimethylamino)tin(IV) that is independent of growth temperature and that disrupts the mass gain of the following 20–50 ALD cycles. The chemical environments of the buried interface were analyzed by soft and hard X-ray photoelectron spectroscopy for a sample with 50 ALD cycles of SnOx on the perovskite. Although measurements on the perovskite bulk below and the SnOx film above did not show chemical changes, additional chemical states for Pb, Br, and N as well as a decrease in the amount of I were observed in the interfacial region. From the analysis, these states and not the heating of the perovskite were concluded to be the cause of the barrier. This strongly suggests that the detrimental effects can be avoided by controlling the interfacial design.

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Abstract: Hard X-ray Photoelectron Spectroscopy (HAXPES) provides minimally destructive depth profiling into the bulk, extending the photoelectron sampling depth. Detection of deeply buried layers beyond the elastic limit is enabled through inelastic background analysis. To test the robustness of this technique, we present results on a thin (18 nm) layer of buried metal-organic complex buried below up to 200 nm of organic material. Overlayers with thicknesses 25-140 nm were measured using photon energies ranging 6-10 keV at the I09 end station at Diamond Light Source, and a new fixed energy Ga Kα (9.25 keV) laboratory-based HAXPES spectrometer was also used to measure samples with overlayers up to 200 nm thick. The sampling depth was varied: at Diamond Light Source by changing the photon energy, and in the lab system by performing angle-resolved measurements. For all the different overlayers and sampling depths, inelastic background modelling consistently provided thicknesses which agreed, within reasonable error, with the ellipsometric thickness. Relative sensitivity factors were calculated, and these factors consistently provided reasonable agreement with the expected nominal stoichiometry, suggesting the calculation method can be extended to any element. These results demonstrate the potential for the characterisation of deeply buried layers using synchrotron and laboratory-based HAXPES.

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Abstract: Halide perovskites have attracted substantial interest for their potential as disruptive display and lighting technologies. However, perovskite light‐emitting diodes (PeLEDs) are still hindered by poor operational stability. A fundamental understanding of the degradation processes is lacking but will be key to mitigating these pathways. Here, a combination of in operando and ex situ measurements to monitor the performance degradation of (Cs0.06FA0.79MA0.15)Pb(I0.85Br0.15)3 PeLEDs over time is used. Through device, nanoscale cross‐sectional chemical mapping, and optical spectroscopy measurements, it is revealed that the degraded performance arises from an irreversible accumulation of bromide content at one interface, which leads to barriers to injection of charge carriers and thus increased nonradiative recombination. This ionic segregation is impeded by passivating the perovskite films with potassium halides, which immobilizes the excess halide species. The passivated PeLEDs show enhanced external quantum efficiency (EQE) from 0.5% to 4.5% and, importantly, show significantly enhanced stability, with minimal performance roll‐off even at high current densities (>200 mA cm−2). The decay half‐life for the devices under continuous operation at peak EQE increases from <1 to ≈15 h through passivation, and ≈200 h under pulsed operation. The results provide generalized insight into degradation pathways in PeLEDs and highlight routes to overcome these challenges.

Open Access

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Abstract: With emerging interest in sodium‐ion battery (SIB) technology due to its cost effectiveness and high earth abundance of sodium, the search for an optimised SIB system becomes more and more crucial. It is well known that the formation of a stable solid electrolyte interphase (SEI) at the anode in Na‐based systems is a challenge due to the higher solubility of the SEI components compared to in the lithium‐ion battery case. Our knowledge about the formation and dissolution of the SEI in SIBs is so far based on limited experimental data. The aim of the present research is therefore to enhance the understanding of the behaviour of the SEI in SIBs and shed more light on the influence of the electrolyte chemistry on the dissolution of the SEI. By conducting electrochemical tests including extended open circuit pauses, we study the SEI dissolution in different time domains. With this, it is possible to determine the extent of self‐discharge due to SEI dissolution during a specific time. Instead of using a conventional separator, β‐alumina is employed as a Na‐conductive membrane to avoid crosstalk between the working electrode and Na‐metal counter electrode. The SEI composition is monitored using synchrotron‐based soft X‐ray photoelectron spectroscopy (SOXPES). The electrochemical and XPS results show that the SEIs formed in the studied electrolyte systems have different stabilities. The relative capacity loss after a 50‐hour pause in the tested electrolyte systems can be up to 30%. Among three carbonate‐based solvent systems, NaPF 6 in ethylene carbonate: diethyl carbonate exhibits the most stable SEI. The addition of electrolyte additives improves the SEI stability in PC. The electrolyte is saturated with typical SEI species, NaF and Na 2 CO 3 , in order to oppose the dissolution of the SEI. Furthermore, the solubilities of the latter additives in the different solvent systems are determined by inductively coupled plasma – optical emission spectrometry.

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Abstract: A major limit of electric vehicle performance is the energy density of available mobile energy storage systems. Lithium ion (Li-ion) batteries are now well-known and have enabled many portable electronics as well as electronic vehicles. However, for greater market penetration and range competitiveness, a battery with greater practical energy density must be designed. This dissertation focuses on three methods of improving energy density in lithium based batteries: lithium oxygen (Li-O2) batteries, Li-rich cathodes for Li-ion batteries, and high voltage operation of simple transition metal oxide cathodes for Li-ion batteries. To evaluate each of these technologies, the reactivity of each system is monitored. Li-O2 batteries, a “beyond Li-ion" battery technology, have a very high theoretical energy density that is difficult to realize. After initial excitement surrounding the novel chemistry, many challenges associated with Li-O2 batteries have been highlighted in the past decade. Among these challenges, the reactivity of oxygen in the system is one of the most pressing. In this dissertation, the possible stability of LiO2, an advantageous discharge product to the typical Li2O2, is examined alongside binder degradation in the system. As the workings of a Li-ion battery require the removal and intercalation of Li, the theoretical capacity is determined by the amount of Li available for extraction from the cathode. Consequently, a method of increasing energy density in Li-ion batteries is to increase the stoichiometric ratio of Li in the cathode material. These "Li-rich" cathode materials demonstrate large capacities, but must compensate the additional Li with cation double redox or anion redox. The electrochemical cells must be operated to > 4.5 V vs Li/Li+ to reach these additional capacities, and this operation results in greater reactivity and instabilities. This dissertation examines the trends of high voltage instability, especially as it relates to oxygen redox, in these Li-rich cathode materials. Typical Li-ion batteries utilize only a fraction of the theoretical capacity available, only extracting around half of the available lithium from the layered transition metal oxide cathode material during charge. This is due to enhanced degradation mechanisms and reduced cyclability when additional Li is extracted at the necessitated higher voltages. Enabling operation of layered transition metal oxides at high voltages would result in increased capacity without the need for “beyond Li-ion" technologies. But first, the instabilities associated with Li extraction at voltages beyond the typical cut-off must be well-studied. Currently, the stability and degradation mechanisms of cathode materials even as common as LiCoO2 remain unclear. In studies presented in this dissertation, high voltage reactivity for layered transition metal oxide cathode materials is investigated. The main conclusions of the studies presented here are drawn from measurements monitoring the reactivity of each of the aforementioned technologies. Among the measurements presented in these studies, outgassing and gas evolution measurements by differential electrochemical mass spectrometry have proved paramount in utility. As cell reactivity and instability at high voltages is often accompanied by outgassing, these measurements have assisted in the elucidation of instability origins. Practical application of Li-O2 batteries remains elusive as additional instabilities are discovered, Li-rich cathode materials show promise as various methods of mitigating high voltage instabilities are discussed, and the major sources of high voltage reactivity of layered transition metal oxide cathode materials are evaluated here.