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The successful and widely used two-step process of producing the hybrid organic-inorganic perovskite CH3NH3PbI3, consists of converting a solution deposited PbI2 film by reacting it with CH3NH3I. Here, we investigate the solidification of PbI2 films from a DMF solution by performing in situ grazing incidence wide angle X-ray scattering (GIWAXS) measurements. The measurements reveal an elaborate sol–gel process involving three PbI2⋅DMF solvate complexes—including disordered and ordered ones—prior to PbI2 formation. The ordered solvates appear to be metastable as they transform into the PbI2 phase in air within minutes without annealing. Morphological analysis of air-dried and annealed films reveals that the air-dried PbI2 is substantially more porous when the coating process produces one of the intermediate solvates, making this more suitable for subsequent conversion into the perovskite phase. The observation of metastable solvates on the pathway to PbI2 formation open up new opportunities for influencing the two-step conversion of metal halides into efficient light harvesting or emitting perovskite semiconductors.
Colloidal quantum dot photovoltaic devices have improved from initial, sub-1% solar power conversion efficiency to current record performance of over 7%. Rapid advances in materials processing and device physics have driven this impressive performance progress. The highest-efficiency approaches rely on a fabrication process that starts with nanocrystals in solution, initially capped with long organic molecules. This solution is deposited and the resultant film is treated using a solution containing a second, shorter capping ligand, leading to a cross-linked, non-redispersible, and dense layer. This procedure is repeated, leading to the widely employed layer-by-layer solid-state ligand exchange. We will review the properties and features of this process, and will also discuss innovative pathways to creating even higher-performing films and photovoltaic devices.
Homoepitaxial SrTiO3 thin films were grown on SrTiO3 (001) using Pulsed Laser Deposition (PLD). The deposition process was monitored in-situ, via both x-ray reflectivity and surface diffuse x-ray scattering measurements in the G3 experimental station at the Cornell High Energy Synchrotron Source (CHESS). Using a CCD detector in 1D, or streak-camera, mode with approximately 0.3-second time resolution, data were collected during growths performed at two substrate temperatures: 695°C and 1000°C. While the specular reflectivity oscillations for the two growths are very similar, the diffuse scattering clearly shows a distinct change in the peak position. Using Atomic Force Microscopy (AFM), we illustrate how the peak position for the diffuse lobes of scattered intensity is directly related to the distribution of single unit cell high islands on the growing surface. Thus, the peak shift of the diffuse scattering indicates an order of magnitude change in the island density.
This is a copy of the slides presented at the meeting but not formally written up for the volume.
Homoepitaxial SrTiO3 thin films were grown on SrTiO3 (001) via Pulsed Laser Deposition. The growth was monitored in real-time by in situ X-ray reflectivity measurements at the anti-Bragg point of the (00L) Crystal Truncation Rod. Due to the need for a large X-ray intensity to monitor the anti-Bragg position, these experiments were performed at the Cornell High Energy Synchrotron Source (CHESS). We investigated the role of laser repetition rate and substrate temperature for films deposited at an O2 background pressure of 10-6 Torr. We observe a transition in growth mode from layer-by-layer to step-flow with increasing temperature while keeping laser repetition rate constant. We observed a similar transition in the growth mode when the substrate temperature is held constant and the laser repetition rate is decreased. The surface miscut is also observed to play a similar role. We show that this transition can be described in terms of the deposition rate, diffusion length, and step spacing.
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