Magneto-optical studies of hybrid organic/inorganic perovskite: The case of methyl-ammonium lead bromide

Picosecond time-resolved and cw magneto-optical methods have been used for studying the spin-related properties of excitons and photocarriers in methyl-ammonium lead bromide (${\mathrm{MAPbBr}}_3$) thin film, single crystal, and light-emitting diodes (LED), focusing on the Landé $g$ values of these species. Using the transient circularly polarized photoinduced quantum beatings (QB) under an applied magnetic field, B in ${\mathrm{MAPbBr}}_3$ single crystal, we obtained the anisotropic $g$ values of electrons for B field along [010] and [001]: $|g_{\left[001\right]}^{\mathrm{e}}|=2.15$ and $|g_{\left[010\right]}^{\mathrm{e}}|=1.75$, and for holes $|g_{\left[001\right]}^{\mathrm{h}}|=0.42$ and $|g_{\left[010\right]}^{\mathrm{h}}|=0.60$. We also used the magnetic circular dichroism method for measuring the bright excitons’ $g$ value, $g_{\mathrm{ex}}=g_{\mathrm{e}}+g_{\mathrm{h}}=2.5$. From these two types of measurements we conclude that $g_{\mathrm{h}}>0$ in ${\mathrm{MAPbBr}}_3$. This conclusion was corroborated by measuring the magnetoelectroluminescence response of LED based on ${\mathrm{MAPbBr}}_3$ active layer. The $g$ values in single crystal and their average in films are in excellent agreement with a k·p model that shows similarity and difference to those of ${\mathrm{MAPbI}}_3$. We also observed the influence of the Overhauser field on the QB frequencies that is induced by the dynamic nuclear polarization generated by the spin-aligned electrons using circularly polarized pump or probe beams.

Figure 1

(a) ${\mathrm{MAPbBr}}_3$ single crystal and its x-ray-diffraction spectra measured on three different facets at room temperature. (b) Crystal-phase transition from cubic cell at room temperature to orthorhombic cell at 4 K. (c) Sketch of the experimental apparatus for nondegenerate pump-probe ps transient Kerr rotation measurement using two lock-in amplifiers. The PEM is a photoelastic modulator for modulating the pump beam circular polarization between left $\left({\delta }^{+}\right)$ and right $\left({\delta }^{-}\right)$; $\lambda /2$ is a “half-wavelength plate;” LP is a linear polarizer; and BS is a beam splitter. The ${\mathrm{MAPbBr}}_3$ crystal was mounted in a cryostat and cooled down to 4 K. An electromagnet generates a magnetic field $B$ up to 700 mT in the direction parallel to the crystal surface (i.e., Voigt geometry).

Figure 2

Photoinduced quantum beatings in ${\mathrm{MAPbBr}}_3$ single crystal excited at 405 nm measured at pump fluence of $1.5\mathrm{W}/\mathrm{c}{\mathrm{m}}^2$ and at various magnetic field strengths at 4 K. Magnetic field dependence of the transient Kerr rotation dynamics measured on (010) facet with $B$ directed along [001] (a)–(d); and on (001) facet with $B$ along [010] (e)–(h), using probe beam at 552 nm. The insets show the corresponding FFT spectra with two FFT peaks for the fast and slow oscillatory frequencies, respectively. (d), (h) The fast and slow QB frequencies vs $B$ and the anisotropy of the obtained $g$ factors.

Figure 3

Magnetic field and excitation power dependencies of the spin lifetime extracted from the fit of the QB dynamics (Fig. 2) using Eq. (1) for electrons (a) and holes (b) at $T=4\mathrm{K}$.

Figure 4

The MCD spectra of ${\mathrm{MAPbBr}}_3$ at 10 K. (a) The MCD spectra obtained at two magnetic field strengths in the Faraday configuration. (b) The MCD amplitude dependence on the magnetic field and its linear fit. The MCD magnitude is proportional to the Zeeman energy splitting and the $g$ factor is obtained using Eqs. (3) and (4).

Figure 5

Magnetoelectroluminescence response measured in a ${\mathrm{MAPbBr}}_3$-based LED device at 10 K. (a) The LED device structure. (b) The I-V and EL-$V$ characteristic responses of the LED device. (c) The MEL($B$) response of the integrated EL spectrum measured at 4-V forward bias.

Figure 6

Dynamic nuclear-spin polarization in ${\mathrm{MAPbBr}}_3$ crystal controlled by the circularly polarized probe beam. (a) Transient Kerr rotation (t-KR) response measured at $B=0$ (black) and $B=30\mathrm{mT}$ (red) using a circularly polarized pump that is modulated at 41 kHz between LCP and RCP and a linearly polarized probe beam. No zero-field QB oscillation is observed. (b) Transient circularly polarized photoinduced reflection (c-PPR) dynamics measured at $B=0$ and 30 mT, respectively generated using a modulated LCP/RCP circular polarized pump beam and LCP (red) and RCP (blue) probe-beam polarizations, respectively. The c-PPR at $B=0$ dynamics (black) show a half-period QB oscillation of $\sim 7\mathrm{ns}$ (i.e., $f_1<80\mathrm{MHz}$). (c) The estimated Overhauser field $B_{\mathrm{N}}=5\mathrm{mT}$ that is induced by the circularly polarized probe beam through dynamic nuclear-spin polarization. In the vector diagram, ${\mathbf{B}}_{\mathrm{RCP}/\mathrm{LCP}}={\mathbf{B}}_{\mathrm{N}}+\mathbf{B}$ is the total field that acts on the carrier spin dynamics.

Figure 7

Dynamic nuclear-spin polarization in ${\mathrm{MAPbBr}}_3$ crystal governed by the circularly polarized pump beam. (a) t-KR response measured at $B=0$ with LCP pump beam (black) compared with that of pump polarization (green) that is modulated between LCP and RCP at 41 kHz using a photoelastic modulator (PEM). (b) t-KR dynamics measured with a LCP pump beam at $B=0$ and $B=5\mathrm{mT}$. (c) t-KR dynamics at $B=30\mathrm{mT}$ excited by LCP (brown) and RCP (red) pump beams, respectively. (d) t-KR dynamics at $B=50\mathrm{mT}$ excited, respectively, by LCP (brown) and RCP (red) pump beams and a circularly modulated pump beam (black). (e) The extracted Overhauser field $B_{\mathrm{N}}=1.4\mathrm{mT}$ induced by the circularly polarized pump beam through dynamic nuclear-spin polarization. In the vector diagram, ${\mathbf{B}}_{\mathrm{RCP}/\mathrm{LCP}}={\mathbf{B}}_{\mathrm{N}}+\mathbf{B}$ is the overall field that acts on the photocarriers’ spin dynamics in the sample.