Two-step spin-coating of vacancy-ordered double perovskites enables growth of thin films for electronic devices

Vacancy-ordered double perovskites (VODPs), such as Cs2TeX6 (X = Cl, Br, I), are lead-free alternatives to conventional metal-halide perovskites (MHPs). One limitation of VODPs is the lack of processes to form thin films relevant for physical characterization and electronic devices. A two-step spin-coating method was developed for synthesizing high-quality films of Cs2TeBr6. Independently depositing CsBr and TeBr4 enables high precursor concentration and control over crystallization dynamics. By optimizing spin-coating parameters, conversion of precursors to phase pure films was observed using structural and surface characterization methods. The growth of mixed-halide systems was investigated using alternative salts including CsCl and CsI. Formation of halide alloys was found to depend on the existence of routes to byproducts. Lastly, single carrier diodes of Cs2TeBr6 were designed following valence band characterization with photoelectron spectroscopy. Temperature-dependent space-charge-limited current measurements revealed that transport occurs by hopping and the hole mobility is 3.2 x 10-5 cm2 V-1 s-1 near room temperature. The insights from the 2-step procedure provide a pathway towards making semiconducting devices from VODPs.

Methods
Powder X-Ray Diffraction
Powder X-ray diffraction patterns were collected with a Panalytical Empyrean Powder Diffractometer in reflection mode. Cu Kα1 was used as the X-ray radiation source with an accelerating voltage of 45 kV and beam current of 40 mA. Scans were performed from 2θ =  3° to 2θ = 60° with a step size of 0.04° and step time of 40 ms.

UV-Vis
A Shimadzu UV3600 UV-Vis-NIR Spectrometer was used to collect diffuse reflectance spectra. Scans were performed from 300 to 900 nm with a step size of 0.5 nm. Reflectance data was converted to absorbance with the Kubelka-Munk transform. Tauc plots for indirect band gap materials were generated to determine the band gap with a linear fit of the absorption edge.

Photoluminescence
Emission spectra was collected in reflectance mode with a 430 nm long pass filter, spectrometer, and visible CCD detector. Samples were excited at 405 nm with a continuous wave laser diode.  

Grazing incidence wide-angle X-ray scattering (GIWAXS)
GIWAXS experiments were performed at Stanford Synchrotron Radiation Lightsource (SSRL) on beam line 11-3, which has a fixed energy of 12.7 keV and is equipped with a two-dimensional Rayonix MX225 CCD area detector. Lanthanum hexaboride (LaB6) was used to refine the beam center and sample-to-detector distance. Data was collected with an incidence angle of 3° to access a large range in q. Geometric corrections to the raw images were made using Nika. Data was azimuthally integrated with WAXStools.

Scanning Electron Microscopy (SEM)
SEM images were collected with a Thermo Scientific Apreo C LoVac SEM. Films were mounted onto SEM stubs with double-sided copper tape and imaged with accelerating voltages of 5.00 kV and beam currents of 0.40 nA.

Ultraviolet and X-ray Photoelectron Spectroscopy
Ultraviolet photoelectron spectroscopy (UPS) and X-ray photoelectron spectroscopy (XPS) experiments were performed with a Thermo Scientific ESCALAB Xi+ XPS Microprobe. For UPS experiments, before film casting, 20 nm of chromium followed by 90 nm of gold were deposited by thermal evaporation onto quartz substrates so that films did not experience charging.  Cs2TeBr6 films were cast with an initial TeBr4 layer spun at 8000 rpm so that the film was sufficiently thin to prevent charging issues. Nickel tape was adhered to the surface of the film and wrapped around the edge of the substrate to further assist with charge dissipation. A helium I radiation source was used, performing 5 scans with a pass energy of 1.5 eV, dwell time of 150 ms, and energy step size of 0.05 eV. Additional low energy scans were conducted with electron charge compensation to ensure that sample charging didn't alter the onset, aligning with data collected without compensation. A corrected helium I satellite line background was subtracted using CasaXPS. For XPS experiments, X-rays are generated with a monochromated aluminum anode (1486.7 eV). Scans were performed with a pass energy of 100 eV, dwell time of 20 ms, and energy step size of 0.5 eV. Charge compensation was applied, and charge shift was accounted for by calibrating to trace oxygen photoemission at 531 eV. Thermo Scientific Avantage Data System was used to fit peaks with a Smart background and quantify atomic percentages.

Device Fabrication and Testing
Devices were tested with a LakeShore Cryogenic Vacuum Probe Station and Keithley 2400 SourceMeter. Current-voltage scans were performed with a step size of 0.05 V.