Phase-engineering compact and flexible CsPbBr3 microcrystal films for robust X-ray detection

All-inorganic CsPbBr3 perovskites have gained significant attention due to their potential in direct X-ray detection. The fabrication of stable, pinhole-free thick films remains challenging, hindering their integration in durable, large-area high-resolution devices. In this study, we propose a facile strategy using a non-conductive polymer to create a flexible, compact thick film under ambient conditions. Furthermore, we investigate the effect of introducing the 2D CsPb2Br5 phase into CsPbBr3 perovskite crystals on their photophysical properties and charge transport. Upon X-ray exposure, the devices consisting of the dual phase exhibit improved stability and more effective operation at higher voltages. Rietveld refinement shows that, due to the presence of the second phase, local distortions and Pb-vacancies are introduced within the CsPbBr3 lattice. This in turn presumably increases the ion migration energy barrier, resulting in a very low dark current and hence, enhanced stability. This feature might benefit local charge extraction and, ultimately, the X-ray image resolution. These findings also suggest that introducing a second phase in the perovskite structure can be advantageous for efficient photon-to-charge carrier conversion, as applied in medical imaging.

Thin films for THz measurements were prepared by spin-coating a 0.5 M DMSO solution of both the dual-phase and phase-pure MCs.The solutions were prepared by dissolving 0.290 g of the MCs in 1 mL of dry DMSO at around 70 °C.The solution was then filtered with a 0.20 μL pore PTFE hydrophilic filter.For obtaining the thin film, 70 μL of the perovskite solution was spin-coated on the fused silica glass, in a glovebox.Subsequent spin-coating at 1000 rpm for 10 s, followed by 60 s at 3000 rpm, with the addition of 125 μL of chlorobenzene 35 s before the end of the program, resulted in the desired thin film after solvent evaporation (for 10 min on a hot plate at 60 °C).Both of the films were annealed outside the glovebox for 30 min at 150 °C.The films were stored in the dark under an inert atmosphere until further measurements.

Material characterization:
Scanning electron microscopy (SEM) was used to obtain images of the crystals.They were recorded on a FEI-Q FEG250 instrument at an acceleration voltage of 10 and 15 kV.
Electronic Supplementary Material (ESI) for Journal of Materials Chemistry C.This journal is © The Royal Society of Chemistry 2023 X-ray diffraction (XRD) patterns were obtained on a Malvern PANalytical Empyrean diffractometer, equipped with a PIXcel3D solid-state detector using a Cu anode (Cu K α1 : 1.5406 Å; Cu K α2 : 1.5444 Å).The powder samples were loaded onto a 96-well sample holder and X-ray diffractograms were recorded at room temperature within a 1.3 -45° 2θ range using a step size of 0.013°.The X-raydiffraction (XRD) data were analyzed by using the Rietveld technique and refinements were done using the FULLPROF program. 1 We refine lattice parameters and crystallographic positions for all elements using the "Thompson-Cox-Hastings pseudo-Voigt * Axial divergence asymmetry" peak shape, ensuring an exquisite match between experimental data and the refined peak shape.In order to achieve a precise alignment between the observed XRD peaks and the calculated data, we carefully adjust the full width at half maximum (FWHM) by refining the U, V, W parameters, along with the shape parameters.For dual phase sample, during XRD analysis, we consider two phases and determine their respective fractions through Rietveld refinement.Radiation damage of phase pure and mixed phase films has been tested at the ESRF ID09 beamline (X-ray energy 36.75 keV/mm 2 , spot size 40 μm×10 mm). 2 X-ray detection measurements were performed by exposing the devices for 30 seconds to a Pantak-Seifert X-ray source with tungsten anode (maximum X-ray photon energy 70 keV, peak intensity 30 keV) in the configuration of PEN/ITO/perovskite/conductive silicone.The X-ray spot size was about 3 mm and the X-ray dose rate was carefully calibrated with a RaySafe Model Solo R/F dosimeter.A Keithley 2000 was used to apply the bias voltage and to record both the dark-and response current.All the X-ray response characterization was conducted in the dark.
UV-Vis Diffuse Reflectance data were recorded between 400 and 800 nm with a step of 1 nm via a PerkinElmer Lambda 950 UV-vis spectrophotometer equipped with an integrating sphere at ambient temperature.BaSO 4 powder was used as reference.The diffused reflectance (R) was converted to F(R) using the Kubelka−Munk function: Steady-state photoluminescence (PL) excitation-emission maps were measured with an excitation range between 250 nm and 600 nm with a step of 5 nm and an emission range between 300 and 800 nm with a step of 5 nm via an Edinburgh FLS980 fluorimeter.
Time-correlated single photon counting (TCSPC) was used to determine the fluorescence decay times at the nanosecond time scale.The frequency-tripled output (266 nm, 8.18 MHz, 2 ps FWHM) of a mode-locked Ti:Sapphire laser (Tsunami, Spectra Physics) was used as excitation source.The linearly polarized excitation light was rotated to a vertical direction using a Berek compensator (New Focus) in combination with a polarization filter and directed onto the samples.The samples in powder form were placed in a quartz cuvette (1 mm path length), sealed by a Teflon stopper, and then mounted on the device.The emission was collected under 90° with respect to the incident light and guided through a polarization filter that was set at the magic angle (54.7°) with respect to the polarization of the excitation beam.The fluorescence was spectrally resolved by a monochromator (Sciencetech 9030, 100 nm focal length, wave-length accuracy 0.3 nm), and detected by a microchannel plate photomultiplier tube (MCP-PMT, R3809U-51, Hamamatsu).A time-correlated single photon timing PC module (SPC 830, Becker & Hickl) was used to obtain the fluorescence decay histogram in 4096 channels.The decays were recorded with 10000 counts in the peak channel, in time windows of 12 ns corresponding to 2.92 ps per channel and analyzed individually with a time-resolved fluorescence analysis (TRFA) software based on iterative reconvolution of the data with the instrumental response function (IRF).The full width at half-maximum (FWHM) of the IRF was typically in the order of 42 ps.
Terahertz photoconductivity measurements provide a contact-free means for photoconductivity measurements.Using an ultrashort visible laser pulse (400 nm, FWHM ≈ 50 fs), charge carriers were optically injected into the conduction bands.Subsequently, the conductivity of the photogenerated charge carrier was probed by freely propagating single-cycle THz electromagnetic transients with 1-2 ps duration.The quickly oscillating terahertz field E(t) accelerated the charges, which, in turn, modifies the terahertz pulse.The relative modification −ΔE/E(t) of the THz field can be directly measured and related to the photoconductivity σ.The analysis was typically performed in the frequency domain by Fourier transformation, that is, σ(ω) ≈ −ΔE/E(ω).The photoconductivity as a function of time delay t d between excitation and THz pulse σ(ω;t d ) was a complex value, and its real part, Re[σ(ω;t d )], corresponding to the changes in the peak intensity of THz pulse following photoexcitation, was directly related to the macroscopic transport parameters in electrical measurements, by Re[σ(ω ; t d )] = N pump × e × μ(ω ; t d ) where N pump and μ(ω; t d ) were the time-dependent carrier density and mobility following photoexcitation, respectively.Terahertz spectroscopy thus provides information on charge carrier dynamics on a timescale complementary to that obtained using time-resolved PL measurements (subps versus ns).The thin films were exposed to a 14.7 keV X-ray beam for 30 min with continuous GIWAXS measurements at every 1 min intervals.In the dual-phase CsPbBr 3 -CsPb 2 Br 5 film, the primary phase remains consistent throughout the entire 30 min X-ray exposure.However, in the phase-pure orthorhombic CsPbBr 3 film, a broad peak around 2.1 Å -1 splits into two peaks presumably due to beam damage, while the main phase remains unaffected.

Results and discussion
Table S1.Time components of the luminescence decay (λ exc = 460 nm, λ em = 530 nm) of the phase-pure and dual-phase MCs as fitted by a global analysis.It should be noted that the value of τ 1 (0.030 ns) is lower than the resolution of the instrument (0.042 ns) and is thus disregarded during further examination of the decays.

Figure S2 .
Figure S2.XRD pattern revealing the crystal structure of the phase-pure orthorhombic CsPbBr 3 and dual-phase CsPbBr 3 -CsPb 2 Br 5 of the powder sample (a, c) and the thick-film (b, d) respectively.

Figure S6 .
Figure S6.Photograph of a compact thick film fabricated in the lab, showing its flexibility.

Figure S8 .
Figure S8.X-ray beam damage test: 2D waterfall plot of synchrotron-based GIWAXS of a) the phasepure orthorhombic CsPbBr 3 and b) dual-phase CsPbBr 3 -CsPb 2 Br 5 films.The thin films were exposed to a 14.7 keV X-ray beam for 30 min with continuous GIWAXS measurements at every 1 min intervals.In the dual-phase CsPbBr 3 -CsPb 2 Br 5 film, the primary phase remains consistent throughout the entire 30 min X-ray exposure.However, in the phase-pure orthorhombic CsPbBr 3 film, a broad peak around 2.1 Å -1 splits into two peaks presumably due to beam damage, while the main phase remains unaffected.