Solar cells comprising methylammonium lead iodide perovskite (MAPI) are notorious for their sensitivity to moisture. We show that hydrated crystal phases are formed when MAPI is exposed to water vapour at room temperature and that these phase changes are fully reversed when the material is subsequently dried. The reversible formation of CH 3 NH 3 PbI 3 •H 2 O followed by (CH 3 NH 3 ) 4 PbI 6 •2H 2 O (upon long exposure times) was observed using time resolved XRD and ellipsometry of thin films prepared using 'solvent engineering', single crystals, and state of the art solar cells. In contrast to water vapour, the presence of liquid water results in the irreversible decomposition of MAPI to form PbI 2 . MAPI changes from dark brown to transparent on hydration; the precise optical constants of CH 3 NH 3 PbI 3 •H 2 O formed on single crystals were determined, with a bandgap at 3.1 eV. Using the single crystal optical constants and thin film ellipsometry measurements, the time dependent changes to MAPI films exposed to moisture were modelled. The results suggest that the mono-hydrate phase forms independently of the depth in the film suggesting rapid transport of water molecules along grain boundaries. Vapour phase hydration of an unencapsulated solar cell (initially J sc ≈ 19 mA cm -2 and V oc ≈ 1.05 V at 1 sun) resulted in more than a 90 % drop in short circuit photocurrent and around 200 mV loss in open circuit potential, but these losses were fully reversed after the cell was exposed to dry nitrogen for 6 hours. Hysteresis in the current-voltage characteristics was significantly increased after this dehydration, which may be related to changes in the defect density and morphology of MAPI following recrystallization from the hydrate. Based on our observations we suggest that irreversible decomposition of MAPI in the presence of water vapour only occurs significantly once a grain has been fully converted to the hydrate phase.

We present Raman and terahertz absorbance spectra of methylammonium lead halide single crystals (MAPbX, X = I, Br, Cl) at temperatures between 80 and 370 K. These results show good agreement with density-functional-theory phonon calculations. Comparison of experimental spectra and calculated vibrational modes enables confident assignment of most of the vibrational features between 50 and 3500 cm. Reorientation of the methylammonium cations, unlocked in their cavities at the orthorhombic-to-tetragonal phase transition, plays a key role in shaping the vibrational spectra of the different compounds. Calculations show that these dynamic effects split Raman peaks and create more structure than predicted from the independent harmonic modes. This explains the presence of extra peaks in the experimental spectra that have been a source of confusion in earlier studies. We discuss singular features, in particular the torsional vibration of the C-N axis, which is the only molecular mode that is strongly influenced by the size of the lattice. From analysis of the spectral linewidths, we find that MAPbI shows exceptionally short phonon lifetimes, which can be linked to low lattice thermal conductivity. We show that optical rather than acoustic phonon scattering is likely to prevail at room temperature in these materials.

The optical constants of methylammonium lead halide single crystals CH3NH3PbX3 (X = I, Br, Cl) are interpreted with high level ab initio calculations using the relativistic quasiparticle self-consistent GW approximation (QSGW). Good agreement between the optical constants derived from QSGW and those obtained from spectroscopic ellipsometry enables the assignment of the spectral features to their respective inter-band transitions. We show that the transition from the highest valence band (VB) to the lowest conduction band (CB) is responsible for almost all the optical response of MAPbI3 between 1.2 and 5.5 eV (with minor contributions from the second highest VB and the second lowest CB). The calculations indicate that the orientation of [CH3NH3](+) cations has a significant influence on the position of the bandgap suggesting that collective orientation of the organic moieties could result in significant local variations of the optical properties. The optical constants and energy band diagram of CH3NH3PbI3 are then used to simulate the contributions from different optical transitions to a typical transient absorption spectrum (TAS).

Self-assembly of particles into long-range, three-dimensional, ordered superstructures is crucial for the design of a variety of materials, including plasmonic sensing materials, energy or gas storage systems, catalysts and photonic crystals. Here, we have combined experimental and simulation data to show that truncated rhombic dodecahedral particles of the metal-organic framework (MOF) ZIF-8 can self-assemble into millimetre-sized superstructures with an underlying three-dimensional rhombohedral lattice that behave as photonic crystals. Those superstructures feature a photonic bandgap that can be tuned by controlling the size of the ZIF-8 particles and is also responsive to the adsorption of guest substances in the micropores of the ZIF-8 particles. In addition, superstructures with different lattices can also be assembled by tuning the truncation of ZIF-8 particles, or by using octahedral UiO-66 MOF particles instead. These well-ordered, sub-micrometre-sized superstructures might ultimately facilitate the design of three-dimensional photonic materials for applications in sensing.

No abstract

No abstract

No abstract

As contamination and environmental degradation increase nowadays, there is a huge demand for new eco-friendly materials. Despite its use for thousands of years, cellulose and its derivatives have gained renewed interest as favourable alternatives to conventional plastics, due to their abundance and lower environmental impact. We report the fabrication of photonic and plasmonic structures by moulding hydroxypropyl cellulose into sub-micrometric periodic lattices, using soft lithography. This is an alternative way to achieve structural colour in this material which is usually obtained exploiting its chiral nematic phase. Cellulose based photonic crystals are biocompatible and can be dissolved in water or not depending on the derivative employed. Patterned cellulose membranes exhibit tuneable colours and may be used to boost the photoluminescence of a host organic dye. Furthermore, we show how metal coating these cellulose photonic architectures leads to plasmonic crystals with excellent optical properties acting as disposable surface enhanced Raman spectroscopy substrates.