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This work reports a materials and degradation study of the ubiquitous perovskite absorber methylammonium lead iodide (MAPbI3). MAPbI3 films were solution deposited on systematically varied glass/metal oxide (MO), transparent conductive oxide (TCO)/MO, and TCO/organofunctional silane stacks. Results show that the underlying layers affect the absorber film morphology, and stability under exposure to heat and humidity. Critically, the use of an organofunctional silane modifier can decrease the grain size, but increase the stability of MAPBI3 deposited on indium tin oxide (ITO). Further, non-adjacent layers can also affect the stability of the perovskite films.

We show that a MAPbI 3 film that was encapsulated has survived the space environment on the International Space Station for a total of approximately 10 months on orbit with little to no chemical degradation. This effort is part of our ongoing efforts to determine the feasibility of MAPbI 3 -bearing solar cells for space applications. This sample was part of the thirteenth flight of the Materials International Space Station Experiment (MISSE-13), which flew from mid-March, 2020 until mid-January, 2021. We determined the robustness of the material through the use of transmission spectrophotometry. To our knowledge this report represents the longest known flight in space of a MAPbI 3 film.

In support of a sustainable human-lunar presence, there is a need for very large (>100kW) and high-voltage-capable solar arrays, estimated to cost over $150M. Perovskite-based thin film photovoltaics offer substantial advantages over state of the art solar arrays from the perspective of manufacturing large arrays. Many of the challenges perovskite solar cells experience in terrestrial operations, e.g degradation caused by moisture and oxygen exposure, are not applicable in long-term space applications. The solar cells must present thermal and vacuum stability and achieve relatively high power conversion efficiencies to become the next generation space photovoltaic technology. Data suggest that judicious choice of material interlayers plays a strong role in thermal vacuum performance. Perovskites regularly achieve ~23% single junction efficiencies but efficiencies in excess of 30% are possible via tandem devices. Perovskites present many opportunities for integration into multi-junction devices with preliminary investigations and perovskite-silicon, perovskite-CIGS and perovskite-perovskite tandems may be suitable for operation in space. Here we provide an overview of NASA Glenn Research Center’s Space Technology Mission Directorate funded research program to develop novel materials to enable the implementation of perovskite photovoltaic devices long-duration operation in space.

Layered transition metal oxides remain a relatively unexplored front in the study of two-dimensional (2D) van der Waals materials, providing opportunities to further advance semiconductor physics a...

In the field of atomically thin 2D materials, oxides are relatively unexplored in spite of the large number of layered oxide structures amenable to exfoliation. There is an increasing interest in ultra-thin film oxide nanostructures from applied points of view. In this perspective paper, recent progress in understanding the fundamental properties of 2D oxides is discussed. Two families of 2D oxides are considered: (1) van der Waals bonded layered materials in which the transition metal is in its highest valence state (represented by V$_2$O$_5$ and MoO$_3$) and (2) layered materials with ionic bonding between positive alkali cation layers and negatively charged transition metal oxide layers (LiCoO$_2$). The chemical exfoliation process and its combinaton with mechanical exfoliation are presented for the latter. Structural phase stability of the resulting nanoflakes, the role of cation size and the importance of defects in oxides are discussed. Effects of two-dimensionality on phonons, electronic band structures and electronic screening are placed in the context of what is known on other 2D materials, such as transition metal dichalcogenides. Electronic structure is discussed at the level of many-body-perturbation theory using the quasiparticle self-consistent $GW$ method, the accuracy of which is critically evaluated including effects of electron-hole interactions on screening and electron-phonon coupling. The predicted occurence of a two-dimensional electron gas on Li covered surfaces of LiCoO$_2$ and its relation to topological aspects of the band structure and bonding is presented as an example of the essential role of the surface in ultrathin materials. Finally, some case studies of the electronic transport and the use of these oxides in nanoscale field effect transistors are presented., 28 pages, 15 figures

Chemically exfoliated nanoscale few-layer thin ${\mathrm{Li}}_{x}{\mathrm{CoO}}_{2}$ samples are studied as a function of annealing at various temperatures, using transmission electron microscopy and electron energy-loss spectroscopy (EELS) in various energy ranges, probing the O-$K$ and Co-${L}_{2,3}$ spectra as well as low-energy interband transitions. These spectra are compared with first-principles density functional theory (DFT) calculations. A gradual disordering of the Li and Co cations in the lattice is observed starting from a slight distortion of the pure ${\mathrm{LiCoO}}_{2}\phantom{\rule{4pt}{0ex}}R\overline{3}m$ to $C2/m$ due to the lower Li content, followed by a $P2/m$ phase forming at $\ensuremath{\approx}200{\phantom{\rule{0.16em}{0ex}}}^{\ensuremath{\circ}}\mathrm{C}$ indicative of Li-vacancy ordering, formation of a spinel-type $Fd\overline{3}m$ phase around $250{\phantom{\rule{0.16em}{0ex}}}^{\ensuremath{\circ}}\mathrm{C}$, and ultimately a rocksalt-type $Fm\overline{3}m$ phase above $350{\phantom{\rule{0.16em}{0ex}}}^{\ensuremath{\circ}}\mathrm{C}$. This disordering leads to a lowering of the band gap as established by low-energy EELS. The Co-${L}_{2,3}$ spectra indicate a change of average Co valence from an initial value of about 3.5 consistent with Li-deficiency related ${\mathrm{Co}}^{4+}$, down to 2.8 and 2.4 in the $Fd\overline{3}m$ and $Fm\overline{3}m$, indicative of the increasing presence of ${\mathrm{Co}}^{2+}$ in the higher-temperature phases. The O-$K$ spectra of the rocksalt phase are only reproduced by a calculation for pure CoO and not for a model with random distribution of Li and Co. This indicates that there may be a loss of Li from the rocksalt regions of the sample at these higher temperatures. The conductivity measurements indicate a gradual drop in conductivity above $200{\phantom{\rule{0.16em}{0ex}}}^{\ensuremath{\circ}}\mathrm{C}$. This loss in conductivity is clearly related to the more Li-Co interdiffused phases, in which a low-spin electronic structure is no longer valid and stronger correlation effects are expected. Calculations for these phases are based on $\text{DFT}+U$ with Hubbard-$U$ terms with a random distribution of magnetic moment orientations, which lead to a gap even in the paramagnetic phase of CoO.

Transition metal oxides often possess complex interactions between charge, spin, lattice, and orbital degrees of freedom, resulting in correlated electronic and magnetic phases. LixCoO2, one of the...

Phonon polaritons—light coupled to lattice vibrations—in polar van der Waals crystals are promising candidates for controlling the flow of energy on the nanoscale due to their strong field confinement, anisotropic propagation and ultra-long lifetime in the picosecond range1,2,3,4,5. However, the lack of tunability of their narrow and material-specific spectral range—the Reststrahlen band—severely limits their technological implementation. Here, we demonstrate that intercalation of Na atoms in the van der Waals semiconductor α-V2O5 enables a broad spectral shift of Reststrahlen bands, and that the phonon polaritons excited show ultra-low losses (lifetime of 4 ± 1 ps), similar to phonon polaritons in a non-intercalated crystal (lifetime of 6 ± 1 ps). We expect our intercalation method to be applicable to other van der Waals crystals, opening the door for the use of phonon polaritons in broad spectral bands in the mid-infrared domain., J.T.-G. and G.Á.-P. acknowledge support through the Severo Ochoa Program from the Government of the Principality of Asturias (nos. PA-18-PF-BP17-126 and PA-20-PF-BP19-053, respectively). J.M.-S. acknowledges finantial support from the Clarín Programme from the Government of the Principality of Asturias and a Marie Curie-COFUND grant (PA-18-ACB17-29) and the Ramón y Cajal Program from the Government of Spain (RYC2018-026196-I). K.C., X.P.A.G., H.V. and M.H.B. acknowledge the Air Force Office of Scientific Research (AFOSR) grant no. FA 9550-18-1-0030 for funding support. I.E. acknowledges financial support from the Spanish Ministry of Economy and Competitiveness (grant no. FIS2016-76617-P). A.Y.N. acknowledges the Spanish Ministry of Science, Innovation and Universities (national project no. MAT2017-88358-C3-3-R) and the Basque Government (grant no. IT1164-19). Q.B. acknowledges the support from Australian Research Council (grant nos. FT150100450, IH150100006 and CE170100039). R.H. acknowledges support from the Spanish Ministry of Economy, Industry, and Competitiveness (national project RTI2018-094830-B-100 and the Project MDM-2016-0618 of the María de Maeztu Units of Excellence Program) and the Basque Goverment (grant no. IT1164-19). P.A.-G. acknowledges support from the European Research Council under starting grant no. 715496, 2DNANOPTICA.

Phonon polaritons-light coupled to lattice vibrations-in polar van der Waals crystals are promising candidates for controlling the flow of energy on the nanoscale due to their strong field confinement, anisotropic propagation and ultra-long lifetime in the picosecond range