Session G01: Understanding the Physics of Perovskites

Time-Resolved Lattice Dynamics of Metal Halide Perovskite Nanomaterials

Metal halide perovskite nanoparticles show promise for a variety of optoelectronics purposes, including use in photovoltaics, LEDs, photocatalysts, and lasing applications. Many of the intended applications present nanomaterials with high energy, high intensity photoexcitation, raising interest in the transient or permanent electronic and thermal effects that such exposure might have on the crystal lattice. Structural rearrangements of nanomaterials in response to photoexcitation, such as lattice distortions and phase transitions, are also of particular interest, as these engender long carrier lifetime. We have been investigating the sensitivity of the nanoparticle lattice to photon irradiation of varying intensity and monitor the time-evolution of the lattice distortions using time-resolved X-ray diffraction (TRXRD). We compare different forms of hybrid organic-inorganic and fully inorganic CsPbI₃ perovskite nanoparticles and observe unexpected transient behaviors. We find that lattice distortions develop rapidly and recover with a range of timescales depending on composition. These studies, paired with evaluations of thermalization and phonon evolution studies offer improved insights into the design of nanomaterials for applications. 

1. Center for Nanoscale Materials, Argonne National Laboratory, Lemont, IL 60439 

2. Department of Chemistry, Northwestern University, Evanston, IL 60208    

Investigation of the nature of the magnetic interactions and ordered structures of the MnCl4-based hybrid organic-inorganic perovskites.

In recent years, magnetic two-dimensional (2D) hybrid organic-inorganic perovskites (HOIPs) (magnetic 2D HOIPs) have gained significant attention due to their unique properties. Although previous studies have provided valuable insights into the properties of magnetic 2D HOIPs, the nature of the magnetic interactions and transitions remains fully understood. We have investigated MnCl₄-based 2D HOIPs. We conducted zero field cooling (ZFC) and field cooling (FC) bulk susceptibility measurements on powder samples of (C₆H₅CH₂NH₃)₂MnCl₄, 3-(C₇H₉ClN)₂MnCl₄ and x-(C₇H₉FN)₂MnCl₄, where x = 2 and 4 represent different relative attachments of NH₃ molecules and F/Cl atoms to the benzene ring. In this talk, we will present our results, including their Neel ordering temperatures (T_N), two-dimensional ordering temperatures (T_2D), Curie-Weiss temperatures (T_cw), and UV-vis data. We will also present our neutron diffraction data to understand the magnetic transitions.   

Surface Anchoring SnO2 with Fluorinated PCBM for an Efficient and Hysteresis–free N-I-P Configured Perovskite Solar Cells

One of the key challenges limiting the power conversion efficiency (PCE) of Perovskite solar cells (PSCs) is the mobility and charge separation mechanism between the electron transport layer (ETL) and perovskite [1]. Herein, we studied the effect of using PCBM in the n-i-p configuration PSC. Passivation of SnO2 ETL with fluorinated (FPCBM, F4PCEE) and non-fluorinated PCBM materials demonstrated the recorded improvement of the PCE up to 24.16%. This improvement in performance was mostly due to a significant increase in open circuit voltage (Voc) of 1.143 volts. Incorporating tetra fluorinated PCBM (F4PCEE), a relatively higher fill factor (83.4%) was recorded due to improved quality of the electron layer and hence better charge extraction capacity. For reference devices, the stable (2D PEAI cation incorporated perovskites) and n-i-p configurated PSCs were studied with 22.35% efficiency. Contact angle of the non-contact device layers was improved upto 60% as compared to reference device (45%). Time-resolved photoluminscene used to explore the dynamic properties of interfacial charge and observed the quicker PL quenching for the F4PCC which also exhibits the efficient electron transportation. Overall, high efficiency, reduced hysteresis loss with increased hydrophobicity and photoluminescence quenching of the device layers demonstrate the promising performance of the device in term of electron transportation.

Reference:[1] Shen, Wenjian, et al., Materials Reports: Energy 1.4 (2021): 100060.   

Fermi Level Shift Induced by Defect-Photocarrier Interaction at Mixed- Cation Halide Perovskite Surface

In recent years, organometal halide perovskites have garnered tremendous attention for photovoltaic and other optoelectronic applications. However, because these materials inherently have a high concentration of point defects, they usually are not stable under device operation conditions. We investigated the interaction between defects and photocarriers by purposely increasing the concentration of defects in MAPbI₃ films. In films with excess iodide vacancies (V_I), exposure to light induces a significant Fermi level shift (E_f ) of up to 0.7 eV. Moreover, halide vacancies can trap photocarriers for hours, even after light exposure ends. We attribute these observations to the capturing of photoexcited electrons by V_I at the perovskite surface resulting in lattice deformation that stabilizes the captured electron. This process is akin to polaron formation at a defect site. We further investigated the defect-photocarrier interaction in mixed cation halide perovskites with cations like formamidinium (FA) and cesium (Cs) that have different radii. We found that the light-induced shift in the Fermi level in FA_0.2MA_0.8PbI₃ is significantly larger than that in Cs_0.1MA_0.9PbI₃. The larger E_f shift by the incorporation of large ions (FA⁺) suggests that a larger lattice can facilitate the observed defect-photocarrier interaction.   

Oral: Trends of ion migration barriers in metal halide perovskites

The intrinsic instability caused by the ion migration in metal halide perovskites seriously limits the device stability and performance. While strategies, such as alloying, dimensionality reduction, and introduction of ionic additives, have been adopted to enhance stability in the metal halide perovskites, the roles of ion migration kinetics and the underlying controlling physical rules remain elusive. In this work, we provide a systematic, first principles study on trends in ion migration barriers concerning local structure environment and electronic structure factors. The migration barriers of Cs and halogen X in perovskites CsBX₃ (B = Pb, Sn; X = Cl, Br, I) were calculated using the climbing image nudged elastic band method. An analysis of the saddle image structure corresponding to Cs-site and X-site migration reveals that the structures are same, except for the compositional change. Interestingly, it is found that the migration barrier is highly correlated with changes in the integrated antibonding states below the Fermi level along the migration pathway.

 

Supported by NSF Award Number DMR-2127630   

Electric Field and Temperature Dependent Circular Photogalvanic Effect in Methylammonium Lead Iodide Microcrystals FETs

One of the most promising materials in the renewable energy industry is the halide perovskite CH3NH3PbI3 (MAPI). MAPI is known for its efficient conversion of light into long-lived charged carriers, which may be attributed to the formation of indirect spin-polarized sub-bands. Circular photogalvanic effect (CPGE) measurements sensitively probe these spin-polarized states, believed to originate in MAPI due to the Rashba effect. However, whether such an effect occurs at the surface or inside the bulk activated by thermal fluctuation is still under debate.

We analyze the CPGE in MAPI microcrystal FETs with energetic and spatial resolutions at various temperatures. Our results show a strong CPGE signal at 80 K, challenging the thermally induced Rashba effect explanation. Additionally, a sudden polarity change in the CPGE signal just below the bandgap energy could potentially involve exciton formation. Likewise, reversing the source-drain bias results in CPGE polarity shifts, which can be attributed to the Edelstein effect. Furthermore, our spatial-resolved CPGE indicates a long spin diffusion length in this material. We further show the impact of ion migration and gate tuning on CPGE, which can suppress the helicity-dependent photocurrent.   

Recent Advances in Carrier-Resolved Photo-Hall Technology and its Application for World-Record Perovksites and Kesterites

We report exciting advances in “carrier-resolved photo-Hall” technique that allow us to extract very rich information of both majority and minority carriers in electronics materials, such as carrier density, mobility, recombination lifetime and diffusion length. Our technique utilizes a new equation relating Hall mobility difference (Δμ_H), Hall coefficient (H), and conductivity (σ): Δμ_H = d(σ²H)/dσ, and employs a rotating parallel-dipole-line magnet system for a.c. field Hall measurement. We have successfully applied this technique to a range of solar absorbers, including CuZnSnSSe (CZTS) and lead-iodide-based perovskites, and added temperature-dependent measurement capabilities in the range of 20-340K to extract additional information about acceptor or donor levels and mobility scattering mechanisms. This progress brings a more complete understanding of how to probe charge carriers in electronic materials using the most common excitations in nature: electric field, magnetic fields, photons, and phonons or lattice vibration.   

Directed Texturing of Metal-Halide Perovskite Films via Vapor Transport Deposition

Metal-halide perovskites are a promising material for solar cell applications, with device power conversion efficiencies exceeding 25%. Thin films of perovskites can be grown via both solution and vapor-based processing techniques. As perovskites move towards commercialization, examination of the impact of different processing techniques on materials properties is critical. Here, we demonstrate the ability to tune the relative orientation of lead iodide precursor platelets in thin films via vapor transport deposition (VTD). VTD is a moderate vacuum thin film processing technique that relies on an inert carrier gas to carry sublimated precursor materials to a cooled substrate. This technique has a broad processing parameter space, allowing us to probe the impact of variations in carrier gas flow rate, substrate temperature, and substrate material on film texture. We further demonstrate that the orientation of the lead iodide precursor strongly impacts the orientation of the final perovskite film; highlighting both a likely growth mechanism for perovskite materials deposited via VTD and the capacity to tune perovskite film crystallinity using VTD. Differences in film texturing for lead iodide as well as perovskite films are found both via 2D X-ray diffraction measurements and electron microscopy images.   

Chemical Vapor Deposited 3D and 2D Hybrid Halide Perovskite Films: Insights into Phase Stability and Exciton Dynamics

Chemical vapor deposition (CVD), a low-cost and scalable deposition technique, allows the growth of halide perovskite thin films without the use of solvents. Using either a two or a three-step CVD process, we show the versatility of this process in the growth of phase stable 3D and 2D lead-halide perovskite films. We discuss temperature dependent X-ray diffraction and photoluminescence measurements from CVD grown 3D and 2D hybrid halide perovskite films, shedding light on phase stability, exciton binding energies, and exciton-phonon interactions in these systems. The luminescence properties of the prototypical 2D lead-iodide perovskite films with butylammonium (BA) versus phenylethylammonium (PEA) show large differences with the latter showing a single photoluminescence peak, highlighting the lack of defect states.  Detailed results of excitonic dynamics using ultrafast transient absorption  from 2D hybrid halide perovskites with BA and PEA will be discussed. Our results reveal the important potential utility of a relatively simple CVD growth method for hybrid halide perovskite films, which affords a significantly higher degree of controllability of the growth of these materials than that of other methods.   

Ultra-microtome effects on lead halide perovskite defects

Metal halide perovskites (MHPs) have demonstrated impressive properties for optoelectronics such as solar cells, photodetectors, and light-emitting diodes, with surprising defect tolerance. However, the impact from defect type on material properties and device performance is not well understood, nor is it understood how synthetic technique, device fabrication, and microstructure influence this defect landscape. Recent combined theory and experimental work has shown that n-type Bi³⁺ doping is a deep-level trap with its effects on defect states still outstanding. TEM has the potential to answer said questions if these materials can be prepared and imaged without causing additional damage. To investigate the effects caused by two competing sample preparation methods, electron transparent CsPbBr₃ specimens are fabricated with non-aqueous wet- and dry-cut ultramicrotome. EDS, diffraction, STEM, and HRTEM are used to experimentally investigate intrinsic and preparation induced defects from ultra-microtome prepared specimens and compared to similar FIB prepared specimens.   

Phonon dynamics of hybrid organic-inorganic perovskite MAPbBr3

Metal halide perovskites (MHPs) continue to be a focal point of research due to its wide range of applications in low cost photovoltaic and light emitting devices. These materials exhibit extended charge carrier lifetimes, long carrier diffusion lengths, and exceptional carrier protection from defects.  Large polarons were reported to be responsible for the extended charge carrier lifetime. However, the relevant underlying microscopic processes such as phonon melting are still under debate. A recent study of inelastic neutron scattering measurements on CsPbBr₃ confirms the overdamping of transversal acoustic phonons (TA) along the Brillouin zone direction T-M-R-T in both tetragonal and cubic phases. Studies on MAPbI₃ (MA=CH₃NH₃) using inelastic X-Ray scattering at 350K, revealed softening of TA modes along T-M and T-R directions. However, a detailed temperature dependence study to see how the phonons evolve through the structural phase transitions has not been performed. We will show our results from inelastic X-Ray scattering measured on MAPbBr₃. We observed in MAPbBr₃, a phonon mode at Q- [4.14 0.38 L] gets softened at low temperature orthorhombic phase and becomes relatively well defined at room temperature cubic phase. We will also present our results from DFT calculations comparing with evolution of phonon modes in inelastic X-Ray measurements.