GoKawiilThin-film fabrication
CsI (99.9%) and PbCl 2 (99.99%) were purchased from Xi’an Polymer Light Technology Corp. SnI 2 (99.99%), CsAc (99.9%), KAc (99.0%), NaAc (99.995%) and Sn(Ac) 2 were purchased from Merck, and RbAc (99.8%) was purchased from Alfa Aesar. These powders were directly used as thermal-deposition sources. CsSnI 3 films were deposited following previously reported procedures and subjected to an initial annealing to complete film formation37. Subsequently, A-Ac (A = Cs+, K+, Rb+ or Na+) layers were deposited on the pre-annealed CsSnI 3 films, followed by a second annealing step at 100 °C for 10 min in an N 2 -filled glovebox. The thickness of each deposited source was measured with a quartz crystal microbalance.
TFT fabrication and characterization
The bottom-gate, bottom-contact TFTs were fabricated on highly doped Si substrates with a 100-nm thermally grown SiO 2 dielectric. The Ni/Au (3/30 nm) electrodes were thermally deposited with a shadow mask (width W = 200 µm and length L = 100 µm). The CsSnI 3 channel layer (W = 1,200 μm and L = 600 μm) was then deposited through the shadow mask, as previously reported, and annealed at 340 °C for 2 min. Finally, the KAc layer was deposited onto the perovskite surface and annealed at 100 °C for 10 min in an N 2 -filled glovebox. All TFTs were characterized using a Keithley 4200SCS at room temperature. Device stability was evaluated by periodically measuring the electrical characteristics after storage under specific conditions. To assess their thermal stability, the TFTs were stored on a hotplate at 100 °C in an N 2 -filled glovebox. Air stability was tested by storing the TFTs in the dark at room temperature and a relative humidity of 20–40%.
The values of saturation mobility (μ sat ) and V TH were extracted by linearly fitting I DS 1/2 versus V GS at the saturation regime, and linear mobility (μ lin ) was extracted at the linear regime from the I DS versus V GS curve following equation (1):
$${\mu }_{\mathrm{sat}}=\frac{2L}{{{WC}}_{{\rm{i}}}}\frac{|{I}_{\mathrm{DS}}|}{{{(V}_{\mathrm{GS}}-{V}_{\mathrm{TH}})}^{2}},\,\,\,\,\,{\mu }_{\mathrm{lin}}=\frac{L}{{{WC}}_{{\rm{i}}}{V}_{\mathrm{DS}}}\frac{{\partial I}_{\mathrm{DS}}}{{\partial V}_{\mathrm{GS}}},$$ (1)
where L, W and C i are the channel length and width and the insulating dielectric areal capacitance, respectively. The subthreshold swing is the inverse of the maximum slope of the I DS versus V GS plot.
Film characterization
The crystal structural properties of the films were investigated using XRD with a SmartLab 9 kW (Rigaku). The thermogravimetric analysis was conducted using a SDT Q-600 (TA Instruments). The samples for transmission electron microscopy were prepared by vapour deposition on a carbon grid and subsequently measured using a JEOL JEM-2200FS (with Image Cs-corrector). The XPS characterization was performed with a Nexsa and an Ulvac PHI Genesis. The surface morphology was examined using high-resolution field emisson-scanning electron microscopy (FE-SEM) (JSM 7800F Prime) with a 5-keV electron beam. The atomic force microscopy images were collected using an NX-10 (Park Systems). The time-of-flight secondary-ion mass spectrometry measurements were performed on an IONTOF M6 instrument in dual-beam mode using a Bi-based analysis beam with O 2 + sputtering for positive-ion depth profiling and two-dimensional mapping and a Bi3+ analysis beam with Ar1050+ sputtering for negative-ion depth profiling and two-dimensional mapping. The depth was calibrated from the sputter crater depth measured by a surface profiler. The Kelvin probe force microscopy measurements were performed using a Jupiter XR (Oxford Instruments). Fourier-transform infrared spectroscopy was performed using a Vertex 70v (Bruker) in transmittance mode. The Hall measurements of the films were carried out using the van der Pauw method with a 0.51-T magnet (HMS-3000, Ecopia). Ultraviolet–visible absorption spectroscopy was performed with a JASCO V-770 spectrophotometer. Time-resolved photoluminescence signals generated by 2.41 eV fs pulses from an OPO coupled Ti:sapphire laser were analysed with a time-correlated single photon counting (TCSPC) device (PicoQuant, PicoHarp 300). Start and stop signals for the device were obtained with a photodiode (PicoQuant, TDA200) and a single-photon avalanche diode (Micro Photon Devices, PD-100-CTC), respectively. The temporal resolution, defined as the full-width at half-maximum of the instrumental response function, was 50 ps at 2.41 eV. A 750-nm long-pass filter was placed in front of the single-photon avalanche diode to block the incident laser beam. The average power was maintained below 200 nW to prevent unwanted photo-induced degradation. The laser source for steady-state photoluminescence (SSPL) was the same as time-resolved optical measurement. The excitation beam was focused onto a spot approximately 1 μm in diameter using an objective lens (×40 and numerical aperture 0.60). Backscattered photoluminescence signals collected by the same objective lens were directed into a Czerny–Turner spectrometer (Andor, Shamrock 303i) equipped with a CCD camera (Andor, Newton).
First-principles calculations
... continue reading