The importance of relativistic effects on two-photon absorption spectra in metal halide perovskites

Abstract

Despite intense research into the optoelectronic properties of metal halide perovskites (MHPs), sub-bandgap absorption in MHPs remains largely unexplored. Here we recorded two-photon absorption spectra of MHPs using the time-resolved microwave conductivity technique. A two-step upward trend is observed in the two-photon absorption spectrum for methylammonium lead iodide, and some analogues, which implies that the commonly used scaling law is not applicable to MHPs. This aspect is further confirmed by temperature-dependent conductivity measurements. Using an empirical multiband tight binding model, spectra for methylammonium lead iodide were calculated by integration over the entire Brillouin zone, showing compelling similarity with experimental results. We conclude that the second upward trend in the two-photon absorption spectrum originates from additional optical transitions to the heavy and light electron bands formed by the strong spin-orbit coupling. Hence, valuable insight can be obtained in the opto-electronic properties of MHPs by sub-bandgap spectroscopy, complemented by modelling.

Water Triggered Synthesis of Highly Stable and Biocompatible 1D Nanowire, 2D Nanoplatelet, and 3D Nanocube CsPbBr3 Perovskites for Multicolor Two-Photon Cell Imaging

“…Because both H 3 O + and OH – can act as surface ligands with higher activity as compared to OA and OAm, in the presence of water, it changes the perovskite orientation growth to form nanowires, which could lower the surface energy. 30 − 44 The XRD spectra reported in Figure 3 B shows only two peaks which corresponding to (100) and (200). XRD result and TEM image reported in Figure 1 and Figure 5 evidently reveal that the final product is 1D CsPbBr 3 nanowires.…”

“…In the presence of minor amounts of water, H 2 O molecules can partly ionize into H 3 O + and OH – with the help of OA and OAm. 30 − 44 In the next step, OH – can partially replace OA or OAm on the surface. In this condition, the structure became loose, which allowed it to form decoupled [PbBr 6 ] 4– octahedrons.…”

“… 1 − 3 Using the reference, 2PA cross-section for PEG coated 1D CsPbBr 3 nanowires was determined using eq 2 . 44 − 54 where F is the observed fluorescence intensity from fluorescein and PEG coated 1D CsPbBr 3 nanowires. Similarly, Φ is the quantum yield, and C is the concentration of fluorescein and PEG coated 1D CsPbBr 3 nanowires.…”

“…Reported 2P cross-sections for 2D and 3D nanocrystals match very well with the reported data. 44 − 54 …”

“… 28 − 40 As a result, enhancing the long-time stability of CsPbX 3 perovskites is the most important parameter to promote commercial applications. 30 − 44 …”