Session B63: Spectroscopy of Halide Perovskites

Angle Resolved Photoelectron Spectroscopy Studies on the Surface Electronic Structure of Tetragonal CH3NH3PbI3

Organic-inorganic halide perovskites (OIHP) have been highlighted in optoelectronics research area for the last decade. OIHPs exhibit long carrier diffusion length (λ) despite of its moderate-to-low mobility (μ). In case of CH₃NH₃PbI₃, the highest value of reported λ reaches beyond μm while the μ is about only <100cm²s^-1V^-1. Several underlying mechanisms have been proposed in order to account for the unusual transport property. One of them is the suppression of charge-carrier recombination by formation of spin-polarized indirect bandgap due to the Rashba effect. However, experimental evidence of such effect is lacking so far and there are ongoing debates regarding its presence and effectiveness. In this presentation, we present our unprecedented high-quality ARPES results measured on single crystal CH₃NH₃PbI₃ (001) surface. We also show circular dichroism ARPES (CD-ARPES) from which we can exploit the information about spin-polarization of electronic structure and discuss the compatibility of the Rashba scenario. We believe our work provides important experimental evidence of one of the most debatable issues in OIHPs.   

Wavelength Dependent Photostability of Perovskite Thin Films

In the ten years since their first reported use in a photovoltaic device, hybrid organic-inorganic perovskite solar cell’s power conversion efficiencies have surpassed 25% and further research efforts have continued to improve their scalability and long-term operational stability. One stability that has to be thoroughly investigated is the long term photostability of the perovskite materials. Here we will present our recent in-situ wavelength-dependent photostability investigation on bare perovskite thin films. Pristine co-evaporated CH₃NH₃PbI₃ thin films were initially grown on pyrolytic graphite surfaces, and they were then illuminated for prolonged durations to approximately 1 sun (1mW/mm²) of either blue (408 nms) or red (650 nms) laser light. The films were maintained under constant vacuum conditions, and x-ray photoemission spectroscopy measurements were taken after each exposure. The dichotomy of the behavior of the films while under either the red or blue light, is suggestive of a wavelength dependence to the degradation mechanisms.
  

Excitonic behavior in hybrid halide perovskite for optoelectronic applications

Organic-inorganic lead halide perovskites, CH₃NH₃PbX₃ (X=Cl, Br, I), have gained great attentions for their attractive optoelectronic properties and excellent performances in various optoelectronic devices. To resolve the remained issues and advance the optoelectronic application devices, understanding intrinsic properties of this material is significant. In particular, free carrier and excitonic behaviors play a crucial role to support the charge transport process which aids the advanced device design. We verified structural properties using temperature dependence photoluminescence and transmission spectra. Under 160 K as orthorhombic phase, unusual excitonic behaviors were found near band edge. We also determined surface potential distribution with and without external light source using Kelvin probe force microscopy. We described electronic band structures of halide perovskite effected by photovoltage. Since the large band gap energy about 3 eV, different photovoltages characteristics were displayed depending on the wavelength of the light source, and it is affected by the defect states on the band edges and the bound excitons on the band edge. Those results suggest that the identified perovskite structure and excitonic properties will apply to many optoelectronic devices.   

Recombination Dynamics of Chlorine Doped Hybrid Perovskite Single Crystals

Controlling the photo excited properties and behavior of hybrid perovskites by halide doping has the potential to impact a wide range of emerging technologies, including solar cells and radiation detectors. Crystalline samples of methyl ammonium lead bromide (MAPbBr) were substituted with varying levels of Cl at the c site and probed with transient reflectivity spectroscopy. MAPbBr perovskites exhibit an increase in transient reflectivity following photoexcitation, with electron-phonon relaxation on a few picosecond timescale and a triple exponential recombination dynamic which is not complete after 1.5 nanoseconds. The recombination dynamics strongly depend on pump fluences, while the electron-phonon relaxation remains unchanged. We also observed that the doping level affected the electron-phonon coupling within the conduction band, becoming stronger with increased percentage of Cl.   

Ultrafast Carrier Dynamics of Dual Emissions from Methylammonium Lead Iodide Perovskites

Hybrid lead iodide perovskites have been attractive for its optical properties for many applications in different fields. One of the most promising applications is as a material for future generation of solar cells. The understanding of the photoexcitation dynamics is very important for improving their performance in solar cells and other applications. We implemented optical two-dimensional coherent spectroscopy to study the carrier dynamics and coupling of dual emissions in Methylammonium Lead Iodide film at 115 K. The sample was prepared by spin-coating perovskite precursor onto a glass substrate and successive annealing at 100 °C. From the 2D spectra we observed an ultrafast redistribution of the photoexcited carriers into the two emission resonances. The dynamics of carrier relaxation into the high energy and low energy resonance are different. The high-energy resonance is a short-lived transient state. The low-energy resonance has a longer lifetime and interacts with coherent phonons.   

Third-order optical nonlinearities of lead halide perovskite single crystals

Lead halide perovskite semiconductors, MAPbX₃ (MA = CH₃NH₃, X = I, Br, and Cl), have attracted much attention as a new class of photonic device materials because of their excellent optoelectronic properties. Recently, we found unique nonlinear optical responses such as large optical modulation [1] and efficient higher-order harmonic generation [2] in MAPbX₃ perovskites. However, the origin of large nonlinear optical responses remains unclear.
In this work, we determined the third-order nonlinear optical coefficients of the wide-gap semiconductor MAPbCl₃ single crystals by using the Z-scan method. We measured the excitation wavelength and polarization dependences of these coefficients. It is found that the absolute values of coefficients depended on the crystal orientation of perovskite single crystals. These excitation wavelength dependences can be quantitatively explained by a two-band model including the exciton effect and Kane’s kp theory. The Kane energy and the exciton reduced mass of MAPbCl₃ were determined [3].
[1] T. Handa et al., Sci. Adv. 5, eaax0786 (2019).
[2] H. Hirori et al., APL Mater. 7, 041107 (2019).
[3] K. Ohara et al., Phys. Rev. Mater. 3 (2019) in press.   

Fine Features of Free and Bound Excitons in the Photoluminescence of Single Crystal MAPbI3

Understanding the photoluminescence (PL) characteristics and exciton properties of hybrid organic-inorganic perovskites (HOIPs) plays a key role in optimizing their performance in optoelectronic devices. In particular, PL is an important technique for the investigation of carrier-phonon interactions and quantification of defects in a system. However, these properties are inaccessible when important signatures are masked by the wide inhomogeneous broadening seen in polycrystalline films, which are commonly used in devices. Here, we study PL on high quality MAPbI₃ single crystals in the orthorhombic phase. The high crystalline quality and well controlled defect density allow us to obtain PL spectra with narrow features and reveal new ones that were not resolvable before. Temperature dependence and excitation intensity dependence data clearly show features of the free exciton and several bound excitons, with consistent energy levels verified among several samples. Assignment of each peak will be discussed, as well as exciton binding energy and exciton-phonon couplings. These newly resolved fine features on high quality single crystals will allow further in-depth studies of the intrinsic photophysics and defect properties related to growth conditions in HOIPs.   

Spintronic Terahertz Emission by Ultrafast Spin-Charge Current Conversion in Reduced Dimensional Organic-Inorganic Hybrid Perovskites

Reduced Dimensional Hybrid Metal Halides (RD-HMHs) are a new class of synthetic semiconductor prepared via low-temperature solution processing with a wide array of possible chemical and structural arrangements enabled by the versatility of their molecular cations. RD-HMHs have already been shown to possess remarkable photovoltaic, excitonic, and optoelectronic properties, but their rich spintronic functionalities have yet to be utilized. We report the successful observation of the inverse Rashba-Edelstein Effect (IREE) in a 2D HMH (BA)₂(MA)_n-1Pb_nI_3n+1 driven by spin pumping from an adjacent ferromagnetic Ni₈₁Fe₁₉ layer using femtosecond laser pulses. We observed spin-dependent THz radiation at room temperature. The phase of the THz field can be controlled by means of an external magnetic field. Circular polarization dependence of the THz radiation confirms the helicity-dependent directional THz radiation and reveals the Rashba states in the RD-HMH materials. This work opens the door for next generation, low-cost THz emitters with tunable functionalities.   

Utrafast Spectroscopy Studies in Chiral Hybrid Perovskites

An organic–inorganic hybrid perovskite incorporating chiral organic molecules has been demonstrated as a new class of chiral semiconductors. Here we report our recent studies on the optical and spin properties in FAPbBr3 nanoparticles with different ratio of chiral ligands by using the broadband ultrafast spectroscopy.   

Exciton decoherence probed by magneto-photoluminescence in halide perovskite bulk single crystals

Understanding the physics behind the high performance of lead halide perovskite compounds is an urgent objective of photovoltaics research. Due to the existence of large spin-orbit interactions, the investigation of spin properties in these materials has emerged as the primary consideration for using halide perovskites as building blocks in spin-electronic devices. Herein we report on a combined theoretical and experimental effort to probe spin dynamics in halide perovskites using magneto-optical measurements. We perform magneto-photoluminescence measurements of MAPbBr₃ high quality single crystals, observing luminescence intensity suppression and recovery at magnetic fields corresponding to cyclotron frequencies where triplet exciton thermalization is in competition with exciton spin precession into the singlet exciton state. Theoretical modeling of exciton dynamics provides evidence for the existence of an Overhauser effect, arising from spin exchange between the excitonic and nuclear spin systems.   

Investigating the Optical and Electrical Properties of Two-dimensional Organic-inorganic Hybrid Perovskite Multiple Quantum Wells via Electroabsorption Spectroscopy Studies

Despite the tremendous progress observed in the development of high-efficient 3D organic-inorganic halide perovskites (OIHPs) with power conversion efficiencies (PCEs) above 24%, there is still a significant challenge for the field related to addressing their poor stability towards extrinsic (i.e., moisture and oxygen) and intrinsic factors (ion migration, water intake, thermal instability). To ameliorate these notorious pitfalls that deleteriously affect PCEs and the long-term stability of OIHP solar cells, the photovoltaics field has shifted towards developing and understanding the properties of 2D OIHPs as the organic cation spacers comprising these layered systems do provide superb environmental stability to the structure. However, this increase in stability is often accompanied by a decrease in PCEs. Hence, the task posed by the field deals with increasing the PCEs of 2D OIHPs without compromising their stability. In this talk, I will discuss the role of excitons/free carries, sample heterogeneity, orientation, and bias stress effects on the photovoltaic performance of OIHPs. I will then discuss the exciton dynamics of 2D OIHPs and how phase and quantum well purity may affect exciton transitions and binding energies.
  

Visible and near-infrared emission properties of melt-grown Dy doped CsPbCl3 perovskite crystals

Doping of lead halides perovskites with rare-earth metals ions has become of significant current interest for possible applications in optoelectronics. In this research, we studied the material preparation and the visible to near IR optical properties of Dy³⁺ doped CsPbCl₃. The crystal was synthesized from purified starting materials and melt-grown by vertical Bridgman technique. Visible emission studies under resonant-intra-4f excitation at ~450nm revealed bright yellow emission centered at 575nm arising from the ⁴F_9/2 to ⁴H_13/2 transition of Dy³⁺ ions. Additional emission bands arising from the ⁴F_9/2 excited states were centered at 485nm, 664nm and 752nm. The decay time of the ⁴F_9/2 level was non-exponential with an average lifetime of ~320 µs. Near-infrared emission studies were performed using a 808 nm diode lasers and revealed several IR bands located at 1.14 µm (⁶H_7/2+⁶F_9/2 ->⁶H_15/2), 1.33 µm(⁶H_9/2+⁶F_11/2 ->⁶H_15/2) , 1.55 µm (⁶F_5/2 ->⁶H_11/2), and ~1.7 µm (⁶H_11/2 ->⁶H_15/2) arising from lower excited states of Dy³⁺ ions. More details of the emission spectra and dynamics of Dy: CsPbCl₃ as well as a comparison to Dy: KPbCl₂ will be presented at the conference.   

Improvement of luminescence properties of surface-passivated mixed-halide perovskites with CdSe/ZnS quantum dots

CdSe/ZnS core-shell quantum dots (QDs) have been employed to enhance the optical properties of perovskite (PS). We investigated the luminescence properties of surface-passivated mixed halide perovskite (CH₃NH₃PbI₂Br) using photoluminescence (PL) and time-resolved PL spectroscopy. The integrated PL intensity and PL decay time of the QD/PS hybrid structure increased compared to those of the bare PS films, owing to charge transfer (CT) from the CdSe/ZnS QDs to the PS and reduced charge traps. The CT efficiency increased from ~7 to 63% for the QD/PS hybrid structure when the core diameter of the QDs decreased from 6.5 to 2.7 nm, respectively, which can be explained by the enhancement in the CT rate due to the control of energy level alignment of QDs. A shorter radiative lifetime was obtained in the QD/PS hybrid structures as compared to the bare PS film, which is attributed to the screening effects of the Rashba splitting and internal electric field due to the CT. These results allow us to understand fundamental mechanisms such as CT from QDs to PS as a function of the size of the QDs