Terahertz Emission from Hybrid Perovskites Driven by Ultrafast Charge Separation and Strong Electron–Phonon Coupling

Abstract

Unusual photophysical properties of organic-inorganic hybrid perovskites have not only enabled exceptional performance in optoelectronic devices, but also led to debates on the nature of charge carriers in these materials. This study makes the first observation of intense terahertz (THz) emission from the hybrid perovskite methylammonium lead iodide (CH NH PbI ) following photoexcitation, enabling an ultrafast probe of charge separation, hot-carrier transport, and carrier-lattice coupling under 1-sun-equivalent illumination conditions. Using this approach, the initial charge separation/transport in the hybrid perovskites is shown to be driven by diffusion and not by surface fields or intrinsic ferroelectricity. Diffusivities of the hot and band-edge carriers along the surface normal direction are calculated by analyzing the emitted THz transients, with direct implications for hot-carrier device applications. Furthermore, photogenerated carriers are found to drive coherent terahertz-frequency lattice distortions, associated with reorganizations of the lead-iodide octahedra as well as coupled vibrations of the organic and inorganic sublattices. This strong and coherent carrier-lattice coupling is resolved on femtosecond timescales and found to be important both for resonant and far-above-gap photoexcitation. This study indicates that ultrafast lattice distortions play a key role in the initial processes associated with charge transport.

High‐Order Shift Current Induced Terahertz Emission from Inorganic Cesium Bromine Lead Perovskite Engendered by Two‐Photon Absorption

“…Compared to the other photocurrent electrical detection with electrodes, THz emission spectroscopy is a nondestructive way with more information such as polarization, amplitude, phase, and polarity . THz emission spectroscopy has been used for the photophysical understanding of some advanced materials such as layered materials MoS 2 , WS 2 , WSe 2 , CH 3 NH 3 PbI 3 , and topological insulator . However, these is no evidence yet of high‐order shift current by TPA process involved in THz emission possibly due to low TPA cross‐sections in these materials.…”

Coherent photo-induced phonon emission in the charge-density-wave state of K0.3MoO3

“…As mentioned in the introduction, coherent phonon emission from ultrafast photo-excited surfaces has been previously observed in a broad range of materials, with two main mechanisms invoked to account for the pump-induced phonon generation. The first involves the excitation of LO phonons perpendicular to the surface, which are driven by their coupling to the rapid field gradients built up by the PD effect [23][24][25][26]29]referred to hereon as the PD-LO mechanism. The transverse-longitudinal incompatibility which prevents coupling between LO phonons and a THz electromagnetic field in the bulk is relaxed due to the sub-THzwavelength excitation depth.…”

“…The transverse-longitudinal incompatibility which prevents coupling between LO phonons and a THz electromagnetic field in the bulk is relaxed due to the sub-THzwavelength excitation depth. Besides relatively simple crystals such as InSb [24] and Te [23], the PD-LO mechanism has been established for more complex solids such as hybrid perovskites [29]. While the bands in the GS conductivity spectrum (figure 1(c)) must be due to TO phonons, one could consider whether the phonon emission here is due to the corresponding LO phonons (with a small LO-TO splitting due to the weak oscillator strength of the bands).…”

“…The generation of coherent phonons (typically with frequencies in the THz range) by sub-picosecond optical excitation is a well-established technique [15][16][17][18][19], especially for Raman-active modes (probed by coherent oscillations in the inter-band permittivity), mostly in conventional semiconductors and semimetals [20], but also for more complex inorganic crystals [21,22]. In comparison, studies of phonon emission from IR-active modes are significantly fewer [23][24][25][26], and only recently have such studies been extended to more complex solids [27][28][29]. As discussed in detail below, different physical mechanisms have been invoked to account for the generation of both transverse and longitudinal optical (TO, LO) phonons in different scenarios.…”

Observation of long spin lifetime in MAPbBr3 single crystals at room temperature

“…These Rashba states have been studied in two-dimensional electron gas (2DEG) systems [21] and 3D topological insulators [22], showing great promise for next generation spin-to-charge transduction applications at room temperature. The recent debut of spintronic studies using hybrid metal halide (HMH) materials [23][24][25][26][27], such as the prototypical material methylammonium lead bromide (CH 3 NH 3 PbBr 3 , MAPbBr 3 ), have attracted an immense amount of research interest beyond their exciting optoelectronic properties for applications in solar cells [28][29][30][31][32], light-emitting diodes [33][34][35], lasers [36][37][38], and photodetectors [39][40][41][42] etc. They are remarkably defect tolerant, and the presence of heavy elements such as Pb in their inorganic framework leads to exceptionally strong SOC.…”