The effects of additives SnF2 (10%) and EDAI2 (1%) on the dynamics of carrier
relaxation of formamidinium tin triiodide
(FASnI3) perovskite were studied using femtosecond transient
absorption spectra (TAS) with excitation at 600 and 870 nm. The TAS
were analyzed according to a parallel sequential kinetic model with
a global fit through singular-value decomposition. For excitation
at 600 nm, two relaxation paths were found: one involved hot and cold
carriers in the bulk state undergoing shallow bulk-defect-mediated
charge recombination; the other involved trap carriers in the surface
state undergoing deep surface-defect-mediated charge recombination.
For excitation at 870 nm, only cold carriers were subjected to the
bulk-state relaxation channel. Our spectral results indicate significant
effects of the additives on retarding charge recombination in both
bulk and surface states as well as decreasing the bandgap renormalization
energy, the bandwidth of the photobleaching (PB) band, and the Stokes
shift between the PB and photoluminescence
bands, explaining how the device performance of FASnI3 solar
cells became enhanced in the presence of SnF2 and EDAI2.
The dynamics of exciton and free-carrier
relaxation of low-dimensional
tin iodide perovskites, BA2FA
n–1Sn
n
I3n+1,
where n = 1 (N1), 2 (N2), 5 (N5), and 10 (N10), were
investigated with femtosecond transient absorption spectra (TAS).
The absorption and photoluminescence spectra of N1 and N2 show exciton
characteristics due to quantum confinement, whereas N5 and N10 display
a free-carrier nature, the same as for bulk three-dimensional (3D)
films. The TAS profiles were fitted according to a global kinetic
model with three time coefficients representing the interactions of
biexcitons, trions, and excitons for N1 and N2 and hot carriers, cold
carriers, and shallow trap carriers for N5 and N10. The carrier relaxation
dynamics of N5 and N10 were similar to those of 3D FASnI3 except for the absence of surface recombination in the deep-trap
states due to passivation of the grain surfaces by the long alkyl
chain for these quasi-2D samples (N5/N10 vs 3D).
We have developed a simple optical method based on measurements of phase retardation[1] versus out-of-plane cell rotation angle and data analyzed by extended Jones matrix method to obtain pretilt angles of inverse twisted nematic (ITN) and tilted homeotropic liquid crystal (LC) cells. In our Jones-matrix calculations, we have assumed a constant pretilt angle as the LC-director tilt angles throughout the ITN cell as well as the tilted homeotropic cell. We have compared the measured pretilt angles of the ITN cell with those of an accompanying tilted homeotropic (ATH) cell. The results showed increasing discrepancies between them with increasing rubbing depth for LC alignments. However, these discrepancies could be explained by assuming that the surface-LC tilt angle of the ITN cell was equal to the measured pretilt angle of the ATH cell to generate a convex-type LC-director-deformation profile within the ITN cell based on Oseen-Frank elastic continuum theory with infinite anchoring energies. The average of the LC-director tilt angles over this profile was equivalent to the measured pretilt angle of the ITN cell.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.