Session C11: Organometal Halide Perovskites III

Photocurrent Mapping in Single-Crystal Methylammonium Lead Iodide Perovskite Nanostructures

We investigate solution-grown single-crystal methylammonium lead iodide (MAPbI₃) nanowires and nanoplates with spatially resolved photocurrent mapping. Sensitive perovskite photodetectors with Schottky contacts are fabricated by directly transferring the nanostructures on top of pre-patterned gold electrodes. Scanning photocurrent microscopy (SPCM) measurements on these single-crystal nanostructures reveal a minority charge carrier diffusion length up to 21 µm, which is significantly longer than the values observed in polycrystalline MAPbI₃ thin films. When the excitation energy is close to the bandgap, the photocurrent becomes substantially stronger at the edges of nanostructures, which can be understood by the enhancement of light coupling to the nanostructures. Temperature and gate dependent SPCM results will also be shown and discussed to illustrate the charge transport mechanisms. These perovskite nanostructures with long carrier diffusion lengths and strong photonic enhancement not only provide an excellent platform for studying their intrinsic properties but may also boost the performance of perovskite-based optoelectronic devices.   

Nanoscale imaging of photocurrent generation of perovskite solar cells

The efficiency of perovskite solar cells is steadily improving, however, the fundamental operational principles and the mechanisms behind the degradation are insufficiently known, causing the delay of commercialization. We study photocurrent generation of methylammonium lead iodide perovskite solar cells using a near-field scanning photocurrent microscopy (NSPM). We observe that the spatial pattern of photocurrent is dependent on the sample annealing temperature: photocurrent is high at grain boundaries in samples annealed at moderate temperature (100 °C), while the opposite pattern (photocurrent is high at grain interiors and low at grain boundaries) is observed in samples annealed at higher temperature (130 °C). Correlating NSPM results with other characterization techniques (electron microscopy, x-ray diffraction, current-voltage and external quantum efficiency), we show that the spatial pattern of photocurrent is caused by material inhomogeneity and dynamics of segregation of lead iodide. Next, we examined the nanoscale signatures of aging caused by light exposure under normal operation. It is found that the extended light exposure drives further structural and compositional changes of materials revealed by the nanoscale photocurrent imaging.   

Investigation of local nanoscale electronic variation and photo excited carrier dynamics in hybrid organic-inorganic mixed cation perovskite films

Hybrid organic-inorganic perovskite materials are promising materials for photovoltaic applications, due to high efficiencies of light collection along with simple and cost effective production methods. One challenge arising in prepared films of hybrid perovskites are the presence of micro- and nano-scale variations that may affect properties desirable for solar cell applications. For example, spatial variations observed in the photoluminescence yield will affect the overall efficiency of devices. In order to understand the electronic effects of such variations, we employ time resolved photoemission electron microscopy (TR-PEEM) to investigate the electronic states and photo excited carrier dynamics with spatial resolution of several tens of nanometers in mixed cation cesium formamidinium methylammonium lead triiodide (CsFAMAPbI3) films. Our initial findings show evidence for occupied mid-gap states that occur in localized spots in the films. Further, we observe the different photo excited behaviors at these local areas.   

Exciton Dissociation at Grain Boundaries in Lead Halide Ruddelsden Popper Perovskite Thin Films

Solution processable Ruddlesden-Popper phase lead halide perovskite (RPP) thin films show greatly enhanced moisture and air stability compared to their 3D lead halide perovskite counterparts. Photovoltaic cells based on RPPs appear to get a performance boost due to exciton dissociation at grain boundaries. Herein we report the direction observation of exciton dissociation in phase pure n=3 RPP polycrystalline thin films using transient absorption spectroscopy. We observe distinct spectral features corresponding to excitons in the bulk of the grains and ‘free carrier’ states which appear to be segregated on the surface of the grains. These ‘surface states’ are the lowest energy electronic excitation in the system, and thus comprise the optical gap of the material. The observed kinetics indicate that grain boundary states are populated both directly after photoexcitation, and via the decay of photo-excited excitons. The interconversion kinetics between excitons and grain boundary states are mediated by the average polycrystalline grain size which is determined by the identity of the long chain organic cation, and other aspects of film processing conditions.   

Shallow vs. Deep Charge Localization in Hybrid Lead Halide Perovskites

Fascinating properties and startling photovoltaic performance of hybrid organic-inorganic pervoskites have brought quite a bit of attention to this class of halide semiconductors. Their all-inorganic analogs are also under considerable scrutiny for potential optoelectronic applications. Here, we shall discuss the electronic structure of a hybrid perovskite CH₃NH₃PbI₃, its connection to preferential orientation of the organic cations CH₃NH₃⁺ and tilting of the PbI₆ octahedra. We also compare its charge localization behavior at shallow levels and associated lattice relaxation versus those at deep polaronic states. The shallow level originates from screened Coulomb interaction between the perturbed or distorted host and an excited electron or hole. The host lattice has a tendency towards forming these shallow traps while deep polarons remain energetically unfavorable. Hence, active shallow traps are expected to shape the carrier dynamics in CH₃NH₃PbI₃,working as temporary trapping-detrapping centers which may be instrumental in reduced quenching of electron-hole pairs, adding to its defect tolerant behavior.   

First-principles Investigation of The Role of Organic Molecules Inside The α-phase of Hybrid Halide Perovskite CH3NH3BX3 (B= Pb, Sn; X= I, Br, Cl)

The hybrid halide perovskite materials, have attracted significant research attention because of their potential application in low cost highly efficient photovoltaics. While the device efficiencies have rapidly improved, the microscopic origin behind the success of these materials is still unclear. Especially, the presence of mobile cations in the CH₃NH₃BX₃, (MABX₃) materials is expected to play an important role in their fundamental properties. Here we performed a van der Waals-corrected density functional theory investigation of the MA⁺ cation structural and electronic properties in the α-phase of CH₃NH₃BX₃ with B=Pb and Sn. Our results reveal that the MA⁺ cation orientation influences the structural properties of the inorganic network and consequently effects the electronic properties of the materials. For example, if the MA⁺ cation is oriented along a (101)-like direction, the BX₆ octahedral cage will be distorted and the band gap becomes indirect.   

Inorganic Lattice Fluctuation Induces Charge Separation in Lead Iodide Perovskites: Theoretical Insights

The high photoconversion efficiency of lead halide perovskites-based photovoltaic devices may be attributed to the remarkable carrier properties in this class of materials, e.g., long carrier lifetime and long carrier diffusion length. These properties should be ascribed to the efficient charge separation; electrons and holes are well separated so that the recombination is suppressed.
However, the mechanism of such efficient charge separation is still under debate. Especially, role of the molecular cation component of the perovskites is a controversial problem in this research field.
In this work, through first-principles molecular dynamics simulations, we demonstrated that the structural fluctuation of the inorganic (lead iodide) part of the perovskites enhances the charge separation.
Also, the mechanism of the structural fluctuation-induced charge separation is discussed; it is shown that the charge separation is caused by the electrostatic potential fluctuation coupled to the inorganic lattice dynamics, based on both simple tight-binding model-based analyses and first-principles calculations.
These results suggest that the molecular cations are unlikely to be essential for the excellent carrier properties in these materials.   

Unexpectedly strong Auger recombination in halide perovskites

The emergence of halide perovskites in solar cell applications has triggered the exploration of electron-hole recombination processes. Some experimental measurements have shown unexpectedly high Auger recombination coefficients in halide perovskites as compared to III-V semiconductors with similar band gaps, but the underlying microscopic mechanism remains unclear. In the present work, we compute the Auger recombination coefficients in the prototype halide perovskite, CH₃NH₃PbI₃ (MAPbI₃), using first-principles calculations. We demonstrate that the unexpectedly high Auger coefficient observed in MAPbI₃ is due to an accidental resonance between the band gap and a complex of bands at energies approximately equal to the gap above the conduction-band edge. Furthermore, we show that the high Auger coefficient can be suppressed by eliminating the octahedral distortions of the halide lattice.   

Radiative Recombination in Hybrid Perovskites

Great progress has been made in hybrid-perovskite-based solar cells. However, the origin of the long carrier lifetimes that account for the high photovoltaic efficiency is still unclear. Various groups have proposed that the indirect band gap of hybrid perovskites, caused by spin mismatch or momentum mismatch at the band extrema, significantly suppresses radiative recombination and thus extends the carrier lifetime. In this work, we employ first-principles approaches to explicitly compute the spin texture and momentum mismatch for the prototypical hybrid perovskite, CH₃NH₃PbI₃, and assess their impact on the radiative recombination rate. We demonstrate that 1) the band extrema have similar spin orientation and 2) their slight momentum mismatch has only a minor impact on the radiative recombination rate. Our calculated radiative recombination coefficients are large, in agreement with experiments showing that radiative recombination coefficients in hybrid perovskites are of similar order of magnitude as in typical direct-gap semiconductors. These insights will guide future studies on hybrid perovskites.   

Exciton States in MAPbI3 Probed by Spin-dependent Pump-probe Spectroscopy

The solution-processed hybrid organic-inorganic perovskites (HOIPs) have shown remarkable optoelectronic properties, because deep defects have high formation energies and shallow defects are close to band edges. This also implies easy formation of bound excitons and complicated exciton physics. Therefore, it is crucial to understand the exciton states in HOIPs, as they affect light absorption, exciton dissociation, and light emission. We study the exciton states with ultrafast pump-probe spectroscopy based on Faraday rotation, and utilize differences in spin dynamics and optical spectra of Faraday rotation to distinguish different photoexcited species. In polycrystalline MAPbI₃ film samples with sufficiently long spin lifetimes, four quantum beating frequencies among exciton states show up as oscillations in time-resolved Faraday rotation in Voigt geometry. In magnetic field range of 0 to 350 mT, all four frequencies are linearly proportional to the field, implying four different g-factors, which can be easily grouped into a fast pair and a slow pair. We will also present the spin dynamics and optical spectra associated with four frequencies, and their correlation with absorption and photoluminescence, and infer useful information about the exciton states.
  

Enhanced Photoresponse in Hybrid Perovskite Thin Film via Interfacing with MoS2

In this work, we demonstrated up to two orders of magnitude enhancement in the photoresponse of hybrid perovskite CH₃NH₃PbI₃ (MAPbI₃) polycrystalline thin film via interfacing with few-layer MoS₂. We mechanically exfoliated few-layer MoS₂ flake and transferred it onto the SiO₂/Si substrate. The sample was fabricated into 2 point device, on top of which we spincoated uniformly 500 nm MAPbI₃ film. Between the two parallel Au electrodes, half of the area contains the MoS₂-MAPbI₃ hybrid structure, while the other half contains only single layer MAPbI₃. We performed high-resolution photocurrent mapping within those two channels, and observed that the MoS₂-MAPbI₃ region had up to two orders of magnitude higher photocurrent than single layer MAPbI₃. The enhancement is consistent with a much higher absorption from the MoS₂-MAPbI₃ hybrid structure due to the band alignment between these two materials, which facilitated photo-carrier separation. The MoS₂-MAPbI₃ hybrid device showed faster transient photoresponse of 200-300 μs, which makes it promising for constructing high performance photo-detectors.   

Mechanical Properties of Halide Perovskites: First Principles Calculations and Nanoindentation Measurements

Halide perovskites AM^IIX₃ (A = amine or alkali metal cation; M^II = divalent cation; X = Cl, Br and I) have emerged as potentially useful light-absorbing materials for solar cell applications. [1,2] One interesting and important feature is their mechanical flexibility and softness, which is crucial for the design and fabrication of devices. We have previously explored the mechanical properties of hybrid double perovskites using density functional theory combined with nanoindentation. [3–5] In this paper we review these studies and also present very recent results [6] which combine ab initio molecular dynamics and nano-indentation to investigate the mechanical properties of FAPbX₃ (FA = formamidinium). We show that the inorganic framework is the primary factor determining mechanical behavior but that hydrogen bonding can also have a significant influence.

Reference
[1] J. Am. Chem. Soc. 131, 6050 (2009).
[2] Science 338, 643 (2012).
[3] J. Mater. Chem. A 4, 12025 (2016).
[4] Chem. Mater. 29, 1089 (2017).
[5] J. Phys. Chem. Lett. 8, 5015 (2017).
[6] ChemSusChem 10, 3740 (2017).   

Elastic properties of hybrid organic-inorganic perovskites by Brillouin microspectroscopy and density functional theory

Hybrid organic-inorganic perovskites (HOIPs) have shown remarkable photovoltaic properties, including long carrier lifetimes. Furthermore, HOIPs can be fabricated from solution at room temperature, which makes them very attractive for efficient renewable energy applications. Despite the great amount of attention HOIPs have received for solar cell applications, their intrinsic properties are not fully known. Here, we present a joint experimental-theoretical study on the elastic properties of HOIPs, which are relevant for understanding the material stability and charge transport. For the experimental part, we chose single crystals of methylammonium lead chloride CH₃NH₃PbCl₃, methylammonium lead bromide CH₃NH₃PbBr₃ and their mixture CH₃NH₃PbBrxCl_1-x and used Brillouin microspectroscopy for their mechanical characterization. We compare our experimental results to elastic constants calculated using density functional theory (DFT), in which we take dispersion-corrections into account. We find good quantitative agreement between the experimental and theoretical data: chloride crystals are the stiffest, bromide crystals the softest, and the mixed crystals are mid-range. Furthermore, we present data on the directional acoustic speeds and discuss elastic anisotropy.