Ultrasmall Blueshift of Near-Infrared Fluorescence in Phase-Stable ${\mathrm{Cs}}_2{\mathrm{Sn}\mathrm{I}}_6$ Thin Films

Cubic ${\mathrm{Cs}}_2{\mathrm{Sn}\mathrm{I}}_6$ perovskite is considered to be an ideal lead-free and air-stable alternative to traditional $A\mathrm{Pb}{X}_3$ [A = $\mathrm{Cs}$, $\mathrm{K}$, ${\mathrm{CH}}_3{\mathrm{NH}}_3$, $\mathrm{CH}({\mathrm{NH}}_2{)}_2$] perovskites for optoelectronic applications, such as solar cells and photodetectors. Herein, both experiments and theoretical calculations are carried out to study the lattice dynamics and temperature dependences of band gaps. Using Raman spectroscopy, we show that the obtained ${\mathrm{Cs}}_2{\mathrm{Sn}\mathrm{I}}_6$ film maintains a long-range crystalline structure of the cubic phase within 77–300 K, which is further confirmed via lattice dynamics calculations based on density functional theory (DFT). An ultrasmall net blueshift of 0.035 eV deduced from its temperature-dependent photoluminescence (PL) spectrum is observed as the temperature increases from 103 to 300 K, making it an ideal material for optoelectronic devices. Notably, this blueshift of the ${\mathrm{Cs}}_2{\mathrm{Sn}\mathrm{I}}_6$ film is significantly smaller than that of the DFT-calculated band-gap shift (∼0.085 eV) based on thermal expansion theory. This deviation is attributed to the strong phonon-electron (ph-e) interaction inherently occurring in ${\mathrm{Cs}}_2{\mathrm{Sn}\mathrm{I}}_6$ material, which counterbalances the thermal expansion effects. We further evaluate the magnitude of the ph-e interaction by fitting the full width at half maximum (FWHM) of the PL spectrum. The fitted Huang-Rhys factor (S) of 4.9 is consistent with reduced electronic dimensionality and increased vibrational degree of freedom, indicating stronger ph-e coupling strength in the ${\mathrm{Cs}}_2{\mathrm{Sn}\mathrm{I}}_6$ film compared with regular ${A\mathrm{Pb}X}_3$-type perovskites (S < 3.3). Adopting both thermal expansion and ph-e coupling effects, the ultrasmall band-gap shift is theoretically calculated and is in good agreement with the experimental blueshift of the band gap. In summary, our work illustrates the solid structural stability of ${\mathrm{Cs}}_2{\mathrm{Sn}\mathrm{I}}_6$ in both lattice and electronic structures, paving the way for the next generation of optoelectronic devices.

Figure 1

(a) Schematic representation of ${\mathrm{Cs}}_2{\mathrm{Sn}\mathrm{I}}_6$ vacancy-ordered metal-halide double perovskite in cubic phase. Green, purple, and gray atoms represent $\mathrm{Cs}$,I, and $\mathrm{Sn}$ elements, respectively. Blue parallel planes are the (1 1 1) lattice planes, aligning discrete $\mathrm{Sn}$-centered octahedra. (b) XRD spectrum of the as-synthesized, two-week-deposited, and six-week-deposited ${\mathrm{Cs}}_2{\mathrm{Sn}\mathrm{I}}_6$ samples. XPS patterns of fresh (c) and two-week-deposited (d) ${\mathrm{Cs}}_2{\mathrm{Sn}\mathrm{I}}_6$ thin film in ambient environment. (e) Nonresonant Raman scattering spectrum with Raman-active modes [δ(F_2g), ν(E_g), ν(A_1g)] and second and third modes of A_1g. Inset presents the total phonon DOS computed using DFT. (f) PL and absorption spectra of ${\mathrm{Cs}}_2{\mathrm{Sn}\mathrm{I}}_6$ thin film recorded at 300 K.

Figure 2

(a),(b) SEM images of ${\mathrm{Cs}}_2{\mathrm{Sn}\mathrm{I}}_6$ thin film grown on silicon-silica substrate. (c),(d) SEM images of ${\mathrm{Cs}}_2{\mathrm{Sn}\mathrm{I}}_6$ thin film grown on nanoporous AAO substrate.

Figure 3

Temperature dependence of crystal structure and phase stability of ${\mathrm{Cs}}_2{\mathrm{Sn}\mathrm{I}}_6$. (a) Temperature-dependent steady-state Raman scattering spectrum recorded from 77 to 300 K at 532 nm excitation. (b) Wave numbers and intensity of the three main Raman-active vibration modes [ν(A_1g), ν(E_g), δ(F_2g)] plotted against temperature, extracted from the steady-state Raman spectrum. Error bars represent fitting standard errors. (c) DFT-computed phonon band structure of cubic-phase ${\mathrm{Cs}}_2{\mathrm{Sn}\mathrm{I}}_6$. (d) Computed free energy profiles at different temperatures; black curve illustrates the volume thermal expansion.

Figure 4

Temperature dependence of the electronic structure and fluorescence of ${\mathrm{Cs}}_2{\mathrm{Sn}\mathrm{I}}_6$. (a) Fitted time-integrated photoluminescence of ${\mathrm{Cs}}_2{\mathrm{Sn}\mathrm{I}}_6$ recorded from 103 to 300 K at 532 nm excitation. (b) Calculated electronic band structure and DOS at different temperatures. (c) Partial DOS of ${\mathrm{Cs}}_2{\mathrm{Sn}\mathrm{I}}_6$ projected to different elements and different angular momentum channels. (d) Band-gap shift predictions with different density functionals.

Figure 5

(a) Temperature dependence of ${\mathrm{Cs}}_2{\mathrm{Sn}\mathrm{I}}_6$ emission line width. Data of PL FWHM are treated as functions of temperature, using Eq. (3) to fit data. Huang-Rhys factor is 4.9 and the phonon energy is 16 meV. (b) Huang-Rhys factor fitted by PL FWHM versus band-gap shift ratio for different perovskites [21, 22, 44, 45, 46]. (c) Frequency and relative intensity of harmonic Raman modes. (d) Temperature dependence of experimental and theoretical band gaps. Redshift by ph-e coupling is combined with calculated band-gap shift for comparison with experimental results.