Cation exchange assisted dimensional down-conversion of perovskite thin films using vapor annealing: An interplay of tolerance factor

This work demonstrates a method to form 1-D ethylammonium lead iodide (EAPbI(3)) thin-films by dimensional down-conversion of 3-D methylammonium lead iodide (MAPbI(3)) perovskite. The process leverages cation-exchange that occurs during exposure to ethylamine vapor and subsequent annealing. Based on the extensive materials characterization, a detailed mechanism of the transformation is proposed. The transformation also improves the morphology of the films, yielding compact EAPbI(3) films with roughness of 50-100 nm, low-pin-hole density, and grain-size of 1-3 mu m. The process can be used to fabricate various EAPbI(3) optoelectronic devices. Since lower dimensional perovskites are more stable against moisture than MAPbI(3), the process can also be used to fabricate 3D-1D graded perovskite interfaces for materials study and to stabilize MAPbI(3) solar cells with a protective outer layer of EAPbI(3.

Atomically-smooth single-crystalline VO2 (101) thin films with sharp metal-insulator transition

Atomically-abrupt interfaces in transition metal oxide (TMO) heterostructures could host a variety of exotic condensed matter phases that may not be found in the bulk materials at equilibrium. A critical step in the development of such atomically-sharp interfaces is the deposition of atomically-smooth TMO thin films. Optimized deposition conditions exist for the growth of perovskite oxides. However, the deposition of rutile oxides, such as VO2, with atomic-layer precision has been challenging. In this work, we used pulsed laser deposition to grow atomically-smooth VO2 thin films on rutile TiO2 (101) substrates. We show that an optimal substrate preparation procedure followed by the deposition of VO2 films at a temperature conducive for step-flow growth mode is essential for achieving atomically-smooth VO2 films. The films deposited at optimal substrate temperatures show a step and terrace structure of the underlying TiO2 substrate. At lower deposition temperatures, there is a transition to a mixed growth mode comprised of island growth and layer-by-layer growth modes. VO2 films deposited at optimal substrate temperatures undergo a sharp metal to insulator transition, similar to that observed in bulk VO2, but at a transition temperature of similar to 325K with similar to 10(3) times increase in resistance

Highly Efficient Flexible Perovskite Solar Cells on Polyethylene Terephthalate Films via Dual Halide and Low‐Dimensional Interface Engineering for Indoor Photovoltaics

Flexible perovskite solar cells are lightweight, bendable, and applicable to curved surfaces. Polyethylene terephthalate (PET) has become the substrate of choice compared to other plastic substrates like polyethylene naphthalate. PET is not only stable but also much cheaper to manufacture, an important factor for photovoltaics (PV). Herein, highly efficient devices on PET are demonstrated using a dual low-temperature (& LE;100 & DEG;C) approach, first by anion mixing (replacing I with Br) of the lead-containing perovskite composition, increasing bandgap (42% improvement), and then by interfacial engineering with tetrabutylammonium bromide (TBAB) (a further 26% improvement), reaching efficiencies of 28.9% at 200 lx and a record 32.5% at 1000 lx. The TBA(+) cation intercalates into the structure, substituting formamidinium cations at the perovskite/TBAB interface, inducing the formation of large-sized, lower dimensional structures over the 3D perovskite matrix. The resulting PV cell has 1.4 times higher carrier lifetime, one order of magnitude lower leakage currents, and 3 times lower defect densities, suppressing recombination. Importantly, stability (ISOS-D1 protocol) improves by more than double with treatment. Highly efficient and stable cells on PET films enable seamless integration with wearable, portable, smart building, and Internet of Things electronic devices, expanding the reach of indoor applications