Exciton versus Free Carrier Photogeneration in Organometal Trihalide Perovskites Probed by Broadband Ultrafast Polarization Memory Dynamics

We studied the ultrafast transient response of photoexcitations in two hybrid organic-inorganic perovskite films used for high efficiency photovoltaic cells, namely, ${\mathrm{CH}}_3{\mathrm{NH}}_3{\mathrm{PbI}}_3$ and ${\mathrm{CH}}_3{\mathrm{NH}}_3{\mathrm{PbI}}_{1.1}{\mathrm{Br}}_{1.9}$ using polarized broadband pump-probe spectroscopy in the spectral range of 0.3–2.7 eV with 300 fs time resolution. For ${\mathrm{CH}}_3{\mathrm{NH}}_3{\mathrm{PbI}}_3$ with above-gap excitation we found both photogenerated carriers and excitons, but only carriers are photogenerated with below-gap excitation. In contrast, mainly excitons are photogenerated in ${\mathrm{CH}}_3{\mathrm{NH}}_3{\mathrm{PbI}}_{1.1}{\mathrm{Br}}_{1.9}$. Surprisingly, we also discovered in ${\mathrm{CH}}_3{\mathrm{NH}}_3{\mathrm{PbI}}_3$, but not in ${\mathrm{CH}}_3{\mathrm{NH}}_3{\mathrm{PbI}}_{1.1}{\mathrm{Br}}_{1.9}$, transient photoinduced polarization memory for both excitons and photocarriers, which is also reflected in the steady state photoluminescence. From the polarization memory dynamics we obtained the excitons diffusion constant in ${\mathrm{CH}}_3{\mathrm{NH}}_3{\mathrm{PbI}}_3$, $D\approx 0.01{\mathrm{cm}}^2{\mathrm{s}}^{-1}$.

Figure 1

PM spectroscopy of ${\mathrm{CH}}_3{\mathrm{NH}}_3{\mathrm{PbI}}_3$ film excited at 400 nm at ambient. (a) Transient PM spectrum measured at $t=0$; various bands are assigned. The inset shows the film absorption spectrum, where two excitons, namely, $E_1$ and $E_2$, and the ${\mathrm{PA}}_1$ transition are assigned. (b) Decay dynamics of PB and ${\mathrm{PA}}_1$ bands up 500 and 10 ps (inset), respectively. The red line through the data points is a fit using a power law decay ${(t/t_0)}^{-\alpha }$, with $\alpha =0.21$.

Figure 2

POM dynamics of ${\mathrm{CH}}_3{\mathrm{NH}}_3{\mathrm{PbI}}_3$ film excited at 400 nm. (a),(b) Transient polarized response of ${\mathrm{PA}}_1$ (a) and PB (b) bands, where $\mathrm{\Delta }{T}_{\parallel }$, $\mathrm{\Delta }{T}_{\perp }$, and $\mathrm{POM}(t)$ decays are shown. The red and blue lines through the data points in (a) are fittings using power low decay ${(t/t_0)}^{-\alpha }$ with $\alpha =0.207$ and $\alpha =0.186$ for $\mathrm{\Delta }{T}_{\parallel }(t)$ (red) and $\mathrm{\Delta }{T}_{\perp }(t)$ (blue), respectively. The $\mathrm{POM}(t)$ response was calculated by using the fits (see the text). The inset in (b) shows the parallel (${\mathrm{PL}}_{\parallel }$) and perpendicular (${\mathrm{PL}}_{\perp }$) components of the PL emission spectrum with respect to the pump polarization spectrum in ${\mathrm{CH}}_3{\mathrm{NH}}_3{\mathrm{PbI}}_3$. The spectrum of the PL polarization degree ${\mathrm{P}}_{\mathrm{PL}}$ (see the text) is also shown.

Figure 3

(a) The photomodulation spectrum of ${\mathrm{CH}}_3{\mathrm{NH}}_3{\mathrm{PbI}}_3$ film at $t=0$ excited below gap (800 nm), where various bands are assigned. (b) The photomodulation spectrum of ${\mathrm{MAPbI}}_{1.1}{\mathrm{Br}}_{1.9}$ film at $t=0$ excited at 400 nm, where various bands are assigned. The inset in (b) shows the film absorption spectrum, where two excitons $E_1$ and $E_2$ and the ${\mathrm{PA}}_1$ transition are assigned.