Field-induced spin splitting and anomalous photoluminescence circular polarization in $\mathrm{C}{\mathrm{H}}_3\mathrm{N}{\mathrm{H}}_3\mathrm{Pb}{\mathrm{I}}_3$ films at high magnetic field

The organic-inorganic hybrid perovskites show excellent optical and electrical properties for photovoltaic and a myriad of other optoelectronics applications. Using high-field magneto-optical measurements up to 17.5 T at cryogenic temperatures, we have studied the spin-dependent optical transitions in the prototype $\mathrm{C}{\mathrm{H}}_3\mathrm{N}{\mathrm{H}}_3\mathrm{Pb}{\mathrm{I}}_3$, which are manifested in the field-induced circularly polarized photoluminescence emission. The energy splitting between left and right circularly polarized emission bands is measured to be ∼1.5 meV at 17.5 T, from which we obtained an exciton effective $g$ factor of ∼1.32. Also from the photoluminescence diamagnetic shift we estimate the exciton binding energy to be ∼17 meV at low temperature. Surprisingly, the corresponding field-induced circular polarization is “anomalous” in that the photoluminescence emission of the higher split energy band is stronger than that of the lower split band. This “reversed” intensity ratio originates from the combination of long electron spin relaxation time and hole negative $g$ factor in $\mathrm{C}{\mathrm{H}}_3\mathrm{N}{\mathrm{H}}_3\mathrm{Pb}{\mathrm{I}}_3$, which are in agreement with a model based on the k·p effective-mass approximation.

Figure 1

The PL emission measurements from the $\mathrm{C}{\mathrm{H}}_3\mathrm{N}{\mathrm{H}}_3\mathrm{Pb}{\mathrm{I}}_3$ film at low temperature. (a) Experimental apparatus for measuring the high-field circularly polarized PL emission. (b) The photoexcitation species in $\mathrm{C}{\mathrm{H}}_3\mathrm{N}{\mathrm{H}}_3\mathrm{Pb}{\mathrm{I}}_3$ that include photocarriers, free excitons (and their phonon replica), and excitons trapped in shallow traps. (c) The PL emission spectrum from $\mathrm{C}{\mathrm{H}}_3\mathrm{N}{\mathrm{H}}_3\mathrm{Pb}{\mathrm{I}}_3$ film at $T=4\mathrm{K}$, which is decomposed into three emission bands, namely, free exciton, phonon replica, and trapped exciton. The fitting is based on multipeak Lorentzian functions.

Figure 2

Zeeman-type spin splitting in $\mathrm{C}{\mathrm{H}}_3\mathrm{N}{\mathrm{H}}_3\mathrm{Pb}{\mathrm{I}}_3$ determined by the circularly polarized PL emission at high magnetic field. (a) Schematic illustration of the radiative recombination in $\mathrm{C}{\mathrm{H}}_3\mathrm{N}{\mathrm{H}}_3\mathrm{Pb}{\mathrm{I}}_3$ between spin sublevels of electrons in the CB and holes in the VB. (b) Normalized $\sigma +$ and $\sigma -$ circularly polarized PL emission spectra at 4 K measured at $B=0$ and $B=17.5\mathrm{T}$, respectively, which show a field-induced splitting between two transitions. (c) Plots of the central wavelength of the two free-exciton-polarized PL bands at various fields $B$, showing both Zeeman splitting and diamagnetic blueshift. (d) Energy splitting Δ$E$ between the $\sigma +$ and $\sigma -$ PL bands vs $B$ up to 17.5 T measured at 4 K.

Figure 3

Field-induced circular polarization in the PL emission of a $\mathrm{C}{\mathrm{H}}_3\mathrm{N}{\mathrm{H}}_3\mathrm{Pb}{\mathrm{I}}_3$ film. (a) Schematic illustration of the relaxation and recombination processes between spin sublevels of electrons in the CB and holes in the VB. The thermal spin distribution of holes in the VB introduces unequal PL intensities with $\sigma -$ and $\sigma +$ polarizations. Note that the hole $g$ factor is negative in $\mathrm{C}{\mathrm{H}}_3\mathrm{N}{\mathrm{H}}_3\mathrm{Pb}{\mathrm{I}}_3$. (b) Field-induced circularly polarized PL emission bands $(\sigma +$ and $\sigma -$, red and blue lines, respectively) from $\mathrm{C}{\mathrm{H}}_3\mathrm{N}{\mathrm{H}}_3\mathrm{Pb}{\mathrm{I}}_3$ film measured at 4 K and $B=17.5\mathrm{T}$. (c) False color plot of the difference, ΔPL, of the circularly polarized PL spectra $\left[\mathrm{\Delta }\mathrm{PL}=\mathrm{PL}\left(\sigma +\right)-\mathrm{PL}\left(\sigma -\right)\right]$ at various $B$ fields. (d) The circular polarization value, $P$ vs $B$, for the free-exciton band (black squares), and a fit using Eq. (3) without (black line) or with (red line) a decreasing factor, $D=0.3$.

Figure 4

The temperature dependence of the PL circular polarization in $\mathrm{C}{\mathrm{H}}_3\mathrm{N}{\mathrm{H}}_3\mathrm{Pb}{\mathrm{I}}_3$ film at high magnetic field. (a) False color plot of the PL circular polarization value $P$ at $B=17.5\mathrm{T}$ as a function of temperature $T$ between 4 and 150 K. (b) The $P$ value for the free-exciton PL band measured at $B=17.5\mathrm{T}$ at various temperatures (black squares), compared with a theoretical prediction based on  (3) with constant $D$ (red line). Inset: the ratio of the spin relaxation time to the recombination time, ${\tau }_s/\tau$ vs temperature extracted from fitting the $P$ value at each temperature, with suitable $D$ using  (3) (black squares). The red line is an exponential fit for $T<50$ K.