Exciton localization in solution-processed organolead trihalide perovskites

Organolead trihalide perovskites have attracted great attention due to the stunning advances in both photovoltaic and light-emitting devices. However, the photophysical properties, especially the recombination dynamics of photogenerated carriers, of this class of materials are controversial. Here we report that under an excitation level close to the working regime of solar cells, the recombination of photogenerated carriers in solution-processed methylammonium–lead–halide films is dominated by excitons weakly localized in band tail states. This scenario is evidenced by experiments of spectral-dependent luminescence decay, excitation density-dependent luminescence and frequency-dependent terahertz photoconductivity. The exciton localization effect is found to be general for several solution-processed hybrid perovskite films prepared by different methods. Our results provide insights into the charge transport and recombination mechanism in perovskite films and help to unravel their potential for high-performance optoelectronic devices.

repeating frequency). The complex photoconductivity is extracted from the transmitted THz electric field and photo-induced change in THz electric field. The data show a slight but distinguishable decrease of Δσ 1 with decreasing frequency, and negative Δσ 2 at low frequency. The Drude quality factor 3 is calculated to be ~0.96, apparently deviating from the ideal value of 1. The results indicate that our data do not support the free carrier scenario described by the Drude model. In the Drude model for free carriers, the complex photoconductivity is expressed as ∆σ(ω) =

Supplementary Notes
Supplementary Note 1 Analysis of temperature-dependent PL intensity. The temperature dependence of PL intensity, also known as the thermal quenching of PL, is usually described by 5 ) where a E stands for the thermal activation energy, and k is the Boltzmann constant.
It is noteworthy that equation (1) without the item 2 / 3 T is also frequently used in the literature. However, such simplified form does not take into account the temperature dependence of the radiative lifetime. 6 In equation (1), I 0 is the PL intensity at low temperature limit (0 K). In the case of our CH 3 NH 3 PbBr 3 films, however, the low-temperature data cannot be used directly due to the phase transition at 236, 155 and 145 K. By considering the cubic to tetragonal phase transition at 236 K, we first fit the data for cubic phase in Fig. 3 to equation (1). The fitting result is then extrapolated to temperatures lower than 236 K.
One can find from Fig. 3 that the expected PL intensity at temperature lower than ~200 K is constant. This assumption is reasonable and supported by the data for the tetragonal phase.
The internal quantum efficiency of PL at certain temperature can then be estimated as IQE (T) = I PL (T)/I PL (0 K). Here the PL intensity at 200 K can be taken as the I PL (0 K). In Fig. 3, the estimated relative IQE at 237 K is ~80 %.

Supplementary Note 2 Lineshape analysis of PL containing free carrier
recombination. The PL spectra are firstly corrected for self-absorption using 7 where α is the absorption coefficient, d is the sample thickness. The corrected spectrum is then fitted as the sum of exciton recombination and free carrier (FC) recombination. The lineshape of band-to-band or FC recombination in semiconductors is known as 8,9 ) where I FC (E) is the PL intensity, E g is the band gap, T is the carrier temperature. Aside from direct transitions described by the joint density of states in the bands, there will be Gaussian broadening from phonon interactions that allow indirect transitions. For exciton recombination at RT, the PL lineshape can be well approximated by Gauss function due to the large inhomogeneous broadening. Therefore, equation (3) should be convoluted with a Gaussian to account for such broadening. The entire lineshape is then given by where I ex (E) is a Gaussian to describe the exciton recombination, and G(Γ) is a Gaussian of width Γ to account for the broadening from phonon interactions. The best fit is shown in Supplementary Fig. 8b. The intensity ratio of FC to exciton recombination is determined as ~0.095, indicating a small contribution of FC to the overall PL spectra.