Skip to main content

Mahshid Ahmadi

Followers

6

Following

1

Co-author

1

Public Views

University of Surrey

Colorado School of Mines

Victor Burlakov

Universidad Rey Juan Carlos

Vincenza Armenise

Jhonatan Rodriguez-Pereira

Universidad Industrial de Santander

Universidade Estadual de Londrina

Interests

Uploads

Papers by Mahshid Ahmadi

Photoinduced iodide repulsion and halides-demixing in layered perovskites

Materials Today Nano

Mixing halides in metal halide perovskites (MHPs) is an effective approach to adjust MHPs bandgap... more Mixing halides in metal halide perovskites (MHPs) is an effective approach to adjust MHPs bandgap for applications in tandem solar cells. However, mixed-halide (MH-) MHPs undergo light-induced-phase-segregation (LIPS) under continuous illumination. Therefore, understanding the mechanism of LIPS is necessary for developing stable MH-MHPs. In this work, we investigated LIPS in layered (L) MHPs and discovered a critical role of spacer cations in LIPS. Through probing chemical changes of LIPS, we unveil light-induced-iodide-repulsion and the formation of Br-rich-phase in illuminated regions during LIPS. This discovery also gives insight into LIPS process in three dimensional (3D) MHPs. By further investigating LIPS in 3D MHPs, we reveal that LIPS induces not only the formation of Br-rich and I-rich domains but also an overall change of halide distribution along the film thickness direction, which can affect the electronic energy alignment and consequently MHPs devices performance. Moreover, LIPS is more significant in the bulk due to larger population of photogenerated charge carriers. Overall, this study reveals the chemical mechanism of LIPS in MHPs and its potential effect on device performance, offering insight into understanding LIPS mechanism and improving the stability of MHPs.

Light–ferroelectric interaction in two-dimensional lead iodide perovskites

Journal of Materials Chemistry A

This study reveals that light–ferroelectricity interaction in 2D (4,4-DFPD)2PbI4 MHP originates f... more

Strain in Metal Halide Perovskites: The Critical Role of A-Site Cation

ACS Applied Energy Materials, 2021

We investigate the role of the A-site cation in the lattice strain of metal halide perovskites (M... more

Nanoscale Transport Imaging of Active Lateral Devices: Static and Frequency Dependent Modes

Kelvin Probe Force Microscopy, 2018

Electronic and ionic transport in semiconductors, ionic conductors, and dielectrics underpins mul... more Electronic and ionic transport in semiconductors, ionic conductors, and dielectrics underpins multiple applications from information technology devices to electroactive ceramics, batteries, fuel cells, and photovoltaics. In this chapter, we review the applications of Kelvin Probe Force Microscopy (or Scanning Surface Potential Microscopy) to map the charge transport in lateral devices. In these measurements, the SPM probe serves as a non-invasive probe of potentials created by the external electrodes, similar to the four-probe resistance measurements. We briefly discuss the invasiveness of such measurements, as exemplified by Scanning Gate Microscopy. We further discuss extensions of the KPFM based transport measurements to probe frequency dependent transport, an analog to impedance spectroscopy, and frequency mixing phenomena. Finally, implementations of lateral transport measurements in liquids are discussed.

Strain–Chemical Gradient and Polarization in Metal Halide Perovskites

Advanced Electronic Materials, 2020

Light‐Ferroic Interaction in Hybrid Organic–Inorganic Perovskites

Advanced Optical Materials, 2019

The phosphaaluminirenes HC[(CMe)(NDipp)] 2 Al[C(R) P] (Dipp = 2,6-i-Pr 2 C 6 H 3 , R = tBu or ad... more The phosphaaluminirenes HC[(CMe)(NDipp)] 2 Al[C(R) P] (Dipp = 2,6-i-Pr 2 C 6 H 3 , R = tBu or adamantyl) 2 and 3, featuring an unsaturated three-membered AlCP ring, have been synthesized as crystalline solids via a [1 + 2] cycloaddition reaction of the aluminum(I) complex HC[(CMe)(NDipp)] 2 Al (1) with phosphaalkynes. Computational investigations infer three-centered 2π-electron aromaticity of the AlCP rings. Compound 3 is readily protonated by tBuOH to induce a ring-opening σbond metathesis, giving an alumina-substituted P-hydrogeno phosphaalkene 4. Remarkably, the high strain of the AlCP ring of 3 allows for facile ring enlargement in reactions with CyNC, bis(diisopropylamino) cyclopropenylidene (BAC), elemental Se, Ph 2 CO, PhCHCHCOPh, and PhCN at room temperature. These furnish a series of unprecedented main group heterocycles 5−10 with the CP unsaturated bonds remaining intact. The mechanisms are considered in light of thorough density functional theory (DFT) calculations.

Toward Electrochemical Studies on the Nanometer and Atomic Scales: Progress, Challenges, and Opportunities

ACS Nano, 2019

Electrochemical reactions and ionic transport underpin the operation of a broad range of devices ... more Electrochemical reactions and ionic transport underpin the operation of a broad range of devices and applications, from energy storage and conversion to information technologies, as well as biochemical processes, artificial muscles, and soft actuators. Understanding the mechanisms governing function of these applications requires probing local electrochemical phenomena on the relevant time and length scales. Here, we discuss the challenges and opportunities for extending electrochemical characterization probes to the nanometer and ultimately atomic scales, including challenges in down-scaling classical methods, the emergence of novel probes enabled by nanotechnology and based on emergent physics and chemistry of nanoscale systems, and the integration of local data into macroscopic models. Scanning probe microscopy (SPM) methods based on strain detection, potential detection, and hysteretic current measurements are discussed. We further compare SPM to electron beam probes and discuss the applicability of electron beam methods to probe local electrochemical behavior on the mesoscopic and atomic levels. Similar to a SPM tip, the electron beam can be used both for observing behavior and as an active electrode

Dynamic Control of Ferroionic States in Ferroelectric Nanoparticles

SSRN Electronic Journal

The polar states of uniaxial ferroelectric nanoparticles interacting with a surface system of ele... more The polar states of uniaxial ferroelectric nanoparticles interacting with a surface system of electronic and ionic charges with a broad distribution of mobilities is explored, which corresponds to the experimental case of nanoparticles in solution or ambient conditions. The nonlinear interactions between the ferroelectric dipoles and surface charges with slow relaxation dynamics in an external field lead to the emergence of a broad range of paraelectric-like, antiferroelectriclike ionic, and ferroelectric-like ferroionic states. The crossover between these states can be controlled not only by the static characteristics of the surface charges, but also by their relaxation

Giant isotope effect on phonon dispersion and thermal conductivity in methylammonium lead iodide

Science Advances, 2020

Lead halide perovskites are strong candidates for high-performance low-cost photovoltaics, light ... more Lead halide perovskites are strong candidates for high-performance low-cost photovoltaics, light emission, and detection applications. A hot-phonon bottleneck effect significantly extends the cooling time of hot charge carriers, which thermalize through carrier–optic phonon scattering, followed by optic phonon decay to acoustic phonons and finally thermal conduction. To understand these processes, we adjust the lattice dynamics independently of electronics by changing isotopes. We show that doubling the mass of hydrogen in methylammonium lead iodide by replacing protons with deuterons causes a large 20 to 50% softening of the longitudinal acoustic phonons near zone boundaries, reduces thermal conductivity by ~50%, and slows carrier relaxation kinetics. Phonon softening is attributed to anticrossing with the slowed libration modes of the deuterated molecules and the reduced thermal conductivity to lowered phonon velocities. Our results reveal how tuning the organic molecule dynamics ...

Evolution Pathway of CIGSe Nanocrystals for Solar Cell Applications

The Journal of Physical Chemistry C, 2012

CuIn x Ga 1−x Se 2 nanocrystals synthesized via the hot injection route have been used to make th... more CuIn x Ga 1−x Se 2 nanocrystals synthesized via the hot injection route have been used to make thin film solar cells with high power conversion efficiency. Thus, CuIn x Ga 1−x Se 2 nanocrystals have the potential to provide a low cost and high efficiency solution to harvest solar energy. Stoichiometry control of these nanocrystals offers the possibility of tuning the band gap of this material. It is important to understand how the composition of quaternary CuIn x Ga 1−x Se 2 nanocrystals evolves to control the stoichiometry of this compound. We report a systematic study of the growth and evolution pathways of quaternary CuIn 0.5 Ga 0.5 Se 2 nanocrystals in a hot coordination solvent. The reaction starts by the formation of a mixture of binary and ternary nanocrystals, which transforms subsequently to CuIn 0.5 Ga 0.5 Se 2 nanocrystals. These binary and ternary compounds dissolve in the course of the reaction, so as to provide the molecular precursor for monophasic CuIn 0.5 Ga 0.5 Se 2 nanocrystals to form. Here, we study the growth sequence of these spherical, monophasic CuIn 0.5 Ga 0.5 Se 2 nanocrystals as a function of time. Control experiments indicated that the phase changes of CuIn 0.5 Ga 0.5 Se 2 nanocrystals are temperature-and time-dependent. The change in the stoichiometry of CuIn 0.5 Ga 0.5 Se 2 during growth was estimated using Vegard's law.

$Figure 1. X-ray diffraction pattern of phase pure tetragonal Culny ;Gay Se, nanocrystals synthesized for 120 min at 230 °C. The difference between the calculated (red) and experimental (blue) intensities is shown in gray with the Bragg markers shown at the bottom. The low magnification TEM image of the Culng;Gay Se, nanocrystals synthesized at 230 °C exhibiting monodispersed size and shape is shown in the inset.$

$Figure 2. X-ray diffraction patterns and Rietveld refinement of nanocrystals grown for (a) 10 min at 130 °C, (b) 20 min at 160 °C, (c) 40 min at 200 °C, and (d) 60 min at 230 °C. Vertical lines show the positions of Bragg peaks, and the Bragg markers from top to bottom are: (a) and (b) CuSe, Se, and CuGaSe,; (c) CuSe, CIGSel, and CIGSe2; (d) CIGSel and CIGSe2. The gray line is the difference between the calculated and experimental intensities. The blue dots correspond to experimental, and the red line corresponds to calculated intensities. The peak marked “*” corresponds to a diffraction peak from hexadecylamine. Se See Nee NA ee En eee See ee ey Serene, Bee ee eee 3.1. Crystal Chemistry of Cu—In—Ga-—Se Phase Evolution. At the start of the reaction (10 min after injection of Se-HDA), a mixture of different phases (CuSe, Se, and CuGaSe,) was formed. The phase distribution can be seen in Table $1. The variation in the XRD background in Figure 2 may arise from the presence of an amorphous phase in the sample. From TEM (EDX), amorphous In,Se; was found (Figure S2a). Se nanorods (12 + 1 wt %) crystallized upon injection of Se into the reaction, leading to partial consumption of Se in the HDA (Figure S2b). The major phase was found to be binary CuSe nanocrystals (57 + 1 wt %) (Figure 2a). This is found to be the intermediate step in the growth of Culng;Gag;Se, nanocrystals. Lattice fringe spacings are obtained from the high-resolution transmission electron microscopy (HRTEM) images and SAED patterns (Figure S3a). The spacings are found to be d, = 3.32 A, d, = 3.33 A that correspond to the (010) and (100) lattice planes of hexagonal P6;/mmc CuSe, respectively. The Cu:Se ratio was close to 1:1 as determined by semi-quantitative EDX analysis, which is consistent with one of the phases determined using XRD. In addition, the XRD pattern also revealed the presence of crystalline ternary CuGaSe, nanocrystals (31 + 1 wt %) and was further confirmed by HRTEM and CBED. The interplanar spacings determined from HRTEM/CBED are d, = 1.93 A, formation of Culng;Gag;Se, nanocrystals and their phase evolution, controlled extractions were carried out at specific reaction intervals. Because of the complexity of the reaction, intermediate pathways for formation of the products need to be accounted for. As was previously studied using thermo- gravimetric analysis,” these precursors have different thermal decomposition temperatures. In general, the precursor reaction with HDA to form the precursor complexes occurs at a lower temperature than the decomposition temperature®’ so it would$

$TEM images and the proposed schematic growth mechanism (Figure 4), large CuSe particles continuously dissolve as the reaction progressed to 200 °C. Stage 3 involved full dissolution of CuSe followed by the formation of two different CIGSe phases (Figure 4 and Table $1). This is a strong indication that CIGSe phases are more stable at higher temperature than CuSe. Another 60 min at 230 °C resulted in the equilibrium single phase of Culng;Gag;Se, nanocrystals, and no obvious change in the shape of the particles was observed. Recently, Kar et al. showed that the formation of CulnSe, nanocrystals in oleylamine involved by the formation of CuSe and InSe nanocrystals as the primary intermediates.** Besides, the phase transformation of biphasic Cu,S—CulInS, to monophasic CulnS, nanocrystals turned out to be an essential intermediate step.°**? Figure 5 shows the evolution of the crystal structure Figure 5. X-ray diffraction patterns showing the evolution of the crystal structure of Culn,Ga,_,Se,.$

Origin of Defects and Positron Annihilation in Hybrid and All-Inorganic Perovskites

Chemistry of Materials

Role of Decomposition Product Ions in Hysteretic Behavior of Metal Halide Perovskite

ACS nano, 2021

Ion migration is one of the most debated mechanisms and credited with multiple observed phenomena... more Ion migration is one of the most debated mechanisms and credited with multiple observed phenomena and performance in metal halide perovskites (MHPs) semiconductor devices. However, to date, the migration of ions and their effects on MHPs are not still fully understood, largely due to a lack of direct observations of temporal ion migration. In this work, using direct observation of ion migration in-operando, we observe the hysteretic migration behavior of intrinsic ions (i.e., CH3NH3+ and I-) as well as reveal the migration behavior of CH3NH3+ decomposition ions. We find that CH3NH3+ decomposition products can be affected by light and accumulate at the interfaces under bias. These MHP decomposition products are tightly related to the device performance and stability. Complementary results of time-resolved Kelvin probe force microscopy (tr-KPFM) demonstrate a correlation between dynamics of these interfacial ions and charge carriers. Overall, we find that there are a number of mobile ...

Mapping fast evolution of transient surface photovoltage dynamics using G-Mode Kelvin probe force microscopy

arXiv: Applied Physics, 2018

Optoelectronic phenomena in materials such as organic/inorganic hybrid perovskites depend on a co... more Optoelectronic phenomena in materials such as organic/inorganic hybrid perovskites depend on a complex interplay between light induced carrier generation and fast (electronic) and slower (ionic) processes, all of which are known to be strongly affected by structural inhomogeneities such as interfaces and grain boundaries. Here, we develop a time resolved Kelvin probe force microscopy (KPFM) approach, based on the G-Mode SPM platform, allowing quantification of surface photovoltage (SPV) with microsecond temporal and nanoscale spatial resolution. We demonstrate the approach on methylammonium lead bromide (MAPbBr3) thin films and further highlight the usefulness of unsupervised clustering methods to quickly discern spatial variability in the information rich SPV dataset. Using this technique, we observe concurrent spatial and ultra-fast temporal variations in the SPV generated across the thin film, indicating that structure is likely responsible for the heterogenous behavior.

Improved Radiation Sensing with Methylammonium Lead Bromide Perovskite Semiconductors

2019 IEEE Nuclear Science Symposium and Medical Imaging Conference (NSS/MIC), 2019

Methylammonium lead bromide (CH3NH3PbBr3 or MAPB) perovskite semiconductors are a potential low-c... more Methylammonium lead bromide (CH3NH3PbBr3 or MAPB) perovskite semiconductors are a potential low-cost option for high energy resolution semiconductor detectors for gamma and neutron sensing. MAPB has recently been demonstrated for its ability to sense alpha particles and neutrons, and pertinent electronic properties have previously been reported. In this presentation, we report on several techniques of improving detector performance to achieve better spectroscopic performance for gamma sensing. These methods include anion and partial cation substitutions of MAPB to improve carrier transport and limit ionic conductivity and several surface treatment and metallization techniques for improved carrier extraction and minimization of device polarization.

Dual Gamma/Neutron Sensing with Methylammonium Lead Tribromide Perovskite

2020 IEEE Nuclear Science Symposium and Medical Imaging Conference (NSS/MIC)

This report several techniques are under investigation to improve the spectroscopic capability of... more This report several techniques are under investigation to improve the spectroscopic capability of methylammonium lead tribromide perovskite semiconductors (CH3NH3PbBr3, or MAPB). These techniques include birefringence level measurements used to evaluate the quality of the single crystals and increase fabrication success rate of radiation responsive crystals. In addition, several characterization techniques are performed on a partial cation-substitution of MAPB with lithium-6 to analyze the structure and surface properties of the single crystal produced. Finally, we will present the thermal neutron sensing results from a MAPB crystal substituted with lithium-6.

Exploring spin-orbital coupling effects on photovoltaic actions in Sn and Pb based perovskite solar cells

Nano Energy

Abstract Organo-metal halide perovskites, as emerging photovoltaic materials, have demonstrated i... more Abstract Organo-metal halide perovskites, as emerging photovoltaic materials, have demonstrated interesting spin states due to spin-orbital coupling (SOC) effects. However, replacing the Pb with the Sn can inevitably affect the SOC and consequently changes the internal photovoltaic processes in the development of environmentally friendly perovskite devices. Here, by operating the spin states with circularly polarized photoexcitation we report that the spin-dependent photocurrent (Jsc) becomes much more prominent upon replacing Pb with Sn, increasing the spin dependence from 0.25% to 1.25% by switching the photoexcitation from linear to circular polarization. Essentially, the spin-dependent Jsc is determined by the spin relaxation time, changing with the SOC strength, as compared to the charge dissociation time. On the other hand, our magneto-photocurrent (magneto-Jsc) results show that the internal magnetic parameter decreases from 281 mT to 41 mT upon Sn-Pb replacement, providing an evidence that the SOC is indeed weakened from Pb to Sn based solar cells. Furthermore, the spin-dependent photoluminescence (PL) indicates that weakening the SOC upon the Sn-Pb replacement leads to more antiparallel spin states (singlets) available for PL but less parallel spin states (triplets) available for photovoltaic action. Therefore, SOC plays an important role in the development of photovoltaic actions in Sn-based perovskite solar cells.

High-Throughput Study of Antisolvents on the Stability of Multicomponent Metal Halide Perovskites through Robotics-Based Synthesis and Machine Learning Approaches

Antisolvent crystallization methods are frequently used to fabricate high-quality perovskite thin... more Antisolvent crystallization methods are frequently used to fabricate high-quality perovskite thin films, to produce sizable single crystals, and to synthesize nanoparticles at room temperature. However, a systematic exploration of the effect of specific antisolvents on the intrinsic stability of multicomponent metal halide perovskites has yet to be demonstrated. Here, we develop a highthroughput experimental workflow that incorporates chemical robotic synthesis, automated characterization, and machine learning techniques to explore how the choice of antisolvent affects the intrinsic stability of binary perovskite systems in ambient conditions over time. Different combinations of the endmembers, MAPbI3, MAPbBr3, FAPbI3, FAPbBr3, CsPbI3, and CsPbBr3, are used to synthesize 15 combinatorial libraries, each with 96 unique combinations. In total, roughly 1100 different compositions are synthesized. Each library is fabricated twice using two different antisolvents: toluene and chloroform....

Improved Radiation Sensing with Methylammonium Lead Tribromide Perovskite Semiconductors

Nuclear Instruments & Methods in Physics Research Section A-accelerators Spectrometers Detectors and Associated Equipment, 2021

Recently, organometallic halide perovskites (OMHPs) have attracted much interest as a potential m... more Recently, organometallic halide perovskites (OMHPs) have attracted much interest as a potential medium resolution detector for ionizing radiation sensing applications. Despite moderate success in the development of OMHP radiation detectors to date, efforts to optimize bulk carrier properties are often hindered by device degradation caused by surface recombination, ionic conductivity, environmental instability, and interface phenomena. In this study, methods of improving the interfacial and surface properties, detector stability, and responsivity of methylammonium lead tribromide (MAPB) semiconductor radiation detectors were investigated. We demonstrated that chemomechanical polishing with dimethylformamide (DMF) as a finishing step decreased surface roughness, removed surface trap states, and greatly enhanced device stability compared to mechanical polishing. Further, using a tin oxide (SnO2) interface layer as hole blocking/electron transporting layer greatly increased the device f...

Unraveling the Energy Landscape and Energy Funneling Modulated by Hole Transport Layer for Highly Efficient Perovskite LEDs

Laser & Photonics Reviews

Unraveling the hysteretic behavior at double cations-double halides perovskite - electrode interfaces

Nano Energy

Photoinduced iodide repulsion and halides-demixing in layered perovskites

Materials Today Nano

Mixing halides in metal halide perovskites (MHPs) is an effective approach to adjust MHPs bandgap... more Mixing halides in metal halide perovskites (MHPs) is an effective approach to adjust MHPs bandgap for applications in tandem solar cells. However, mixed-halide (MH-) MHPs undergo light-induced-phase-segregation (LIPS) under continuous illumination. Therefore, understanding the mechanism of LIPS is necessary for developing stable MH-MHPs. In this work, we investigated LIPS in layered (L) MHPs and discovered a critical role of spacer cations in LIPS. Through probing chemical changes of LIPS, we unveil light-induced-iodide-repulsion and the formation of Br-rich-phase in illuminated regions during LIPS. This discovery also gives insight into LIPS process in three dimensional (3D) MHPs. By further investigating LIPS in 3D MHPs, we reveal that LIPS induces not only the formation of Br-rich and I-rich domains but also an overall change of halide distribution along the film thickness direction, which can affect the electronic energy alignment and consequently MHPs devices performance. Moreover, LIPS is more significant in the bulk due to larger population of photogenerated charge carriers. Overall, this study reveals the chemical mechanism of LIPS in MHPs and its potential effect on device performance, offering insight into understanding LIPS mechanism and improving the stability of MHPs.

Light–ferroelectric interaction in two-dimensional lead iodide perovskites

Journal of Materials Chemistry A

This study reveals that light–ferroelectricity interaction in 2D (4,4-DFPD)2PbI4 MHP originates f... more

Strain in Metal Halide Perovskites: The Critical Role of A-Site Cation

ACS Applied Energy Materials, 2021

We investigate the role of the A-site cation in the lattice strain of metal halide perovskites (M... more

Nanoscale Transport Imaging of Active Lateral Devices: Static and Frequency Dependent Modes

Kelvin Probe Force Microscopy, 2018

Electronic and ionic transport in semiconductors, ionic conductors, and dielectrics underpins mul... more Electronic and ionic transport in semiconductors, ionic conductors, and dielectrics underpins multiple applications from information technology devices to electroactive ceramics, batteries, fuel cells, and photovoltaics. In this chapter, we review the applications of Kelvin Probe Force Microscopy (or Scanning Surface Potential Microscopy) to map the charge transport in lateral devices. In these measurements, the SPM probe serves as a non-invasive probe of potentials created by the external electrodes, similar to the four-probe resistance measurements. We briefly discuss the invasiveness of such measurements, as exemplified by Scanning Gate Microscopy. We further discuss extensions of the KPFM based transport measurements to probe frequency dependent transport, an analog to impedance spectroscopy, and frequency mixing phenomena. Finally, implementations of lateral transport measurements in liquids are discussed.

Strain–Chemical Gradient and Polarization in Metal Halide Perovskites

Advanced Electronic Materials, 2020

Light‐Ferroic Interaction in Hybrid Organic–Inorganic Perovskites

Advanced Optical Materials, 2019

The phosphaaluminirenes HC[(CMe)(NDipp)] 2 Al[C(R) P] (Dipp = 2,6-i-Pr 2 C 6 H 3 , R = tBu or ad... more The phosphaaluminirenes HC[(CMe)(NDipp)] 2 Al[C(R) P] (Dipp = 2,6-i-Pr 2 C 6 H 3 , R = tBu or adamantyl) 2 and 3, featuring an unsaturated three-membered AlCP ring, have been synthesized as crystalline solids via a [1 + 2] cycloaddition reaction of the aluminum(I) complex HC[(CMe)(NDipp)] 2 Al (1) with phosphaalkynes. Computational investigations infer three-centered 2π-electron aromaticity of the AlCP rings. Compound 3 is readily protonated by tBuOH to induce a ring-opening σbond metathesis, giving an alumina-substituted P-hydrogeno phosphaalkene 4. Remarkably, the high strain of the AlCP ring of 3 allows for facile ring enlargement in reactions with CyNC, bis(diisopropylamino) cyclopropenylidene (BAC), elemental Se, Ph 2 CO, PhCHCHCOPh, and PhCN at room temperature. These furnish a series of unprecedented main group heterocycles 5−10 with the CP unsaturated bonds remaining intact. The mechanisms are considered in light of thorough density functional theory (DFT) calculations.

Toward Electrochemical Studies on the Nanometer and Atomic Scales: Progress, Challenges, and Opportunities

ACS Nano, 2019

Electrochemical reactions and ionic transport underpin the operation of a broad range of devices ... more Electrochemical reactions and ionic transport underpin the operation of a broad range of devices and applications, from energy storage and conversion to information technologies, as well as biochemical processes, artificial muscles, and soft actuators. Understanding the mechanisms governing function of these applications requires probing local electrochemical phenomena on the relevant time and length scales. Here, we discuss the challenges and opportunities for extending electrochemical characterization probes to the nanometer and ultimately atomic scales, including challenges in down-scaling classical methods, the emergence of novel probes enabled by nanotechnology and based on emergent physics and chemistry of nanoscale systems, and the integration of local data into macroscopic models. Scanning probe microscopy (SPM) methods based on strain detection, potential detection, and hysteretic current measurements are discussed. We further compare SPM to electron beam probes and discuss the applicability of electron beam methods to probe local electrochemical behavior on the mesoscopic and atomic levels. Similar to a SPM tip, the electron beam can be used both for observing behavior and as an active electrode

Dynamic Control of Ferroionic States in Ferroelectric Nanoparticles

SSRN Electronic Journal

The polar states of uniaxial ferroelectric nanoparticles interacting with a surface system of ele... more The polar states of uniaxial ferroelectric nanoparticles interacting with a surface system of electronic and ionic charges with a broad distribution of mobilities is explored, which corresponds to the experimental case of nanoparticles in solution or ambient conditions. The nonlinear interactions between the ferroelectric dipoles and surface charges with slow relaxation dynamics in an external field lead to the emergence of a broad range of paraelectric-like, antiferroelectriclike ionic, and ferroelectric-like ferroionic states. The crossover between these states can be controlled not only by the static characteristics of the surface charges, but also by their relaxation

Giant isotope effect on phonon dispersion and thermal conductivity in methylammonium lead iodide

Science Advances, 2020

Lead halide perovskites are strong candidates for high-performance low-cost photovoltaics, light ... more Lead halide perovskites are strong candidates for high-performance low-cost photovoltaics, light emission, and detection applications. A hot-phonon bottleneck effect significantly extends the cooling time of hot charge carriers, which thermalize through carrier–optic phonon scattering, followed by optic phonon decay to acoustic phonons and finally thermal conduction. To understand these processes, we adjust the lattice dynamics independently of electronics by changing isotopes. We show that doubling the mass of hydrogen in methylammonium lead iodide by replacing protons with deuterons causes a large 20 to 50% softening of the longitudinal acoustic phonons near zone boundaries, reduces thermal conductivity by ~50%, and slows carrier relaxation kinetics. Phonon softening is attributed to anticrossing with the slowed libration modes of the deuterated molecules and the reduced thermal conductivity to lowered phonon velocities. Our results reveal how tuning the organic molecule dynamics ...

Evolution Pathway of CIGSe Nanocrystals for Solar Cell Applications

The Journal of Physical Chemistry C, 2012

CuIn x Ga 1−x Se 2 nanocrystals synthesized via the hot injection route have been used to make th... more CuIn x Ga 1−x Se 2 nanocrystals synthesized via the hot injection route have been used to make thin film solar cells with high power conversion efficiency. Thus, CuIn x Ga 1−x Se 2 nanocrystals have the potential to provide a low cost and high efficiency solution to harvest solar energy. Stoichiometry control of these nanocrystals offers the possibility of tuning the band gap of this material. It is important to understand how the composition of quaternary CuIn x Ga 1−x Se 2 nanocrystals evolves to control the stoichiometry of this compound. We report a systematic study of the growth and evolution pathways of quaternary CuIn 0.5 Ga 0.5 Se 2 nanocrystals in a hot coordination solvent. The reaction starts by the formation of a mixture of binary and ternary nanocrystals, which transforms subsequently to CuIn 0.5 Ga 0.5 Se 2 nanocrystals. These binary and ternary compounds dissolve in the course of the reaction, so as to provide the molecular precursor for monophasic CuIn 0.5 Ga 0.5 Se 2 nanocrystals to form. Here, we study the growth sequence of these spherical, monophasic CuIn 0.5 Ga 0.5 Se 2 nanocrystals as a function of time. Control experiments indicated that the phase changes of CuIn 0.5 Ga 0.5 Se 2 nanocrystals are temperature-and time-dependent. The change in the stoichiometry of CuIn 0.5 Ga 0.5 Se 2 during growth was estimated using Vegard's law.

$Figure 1. X-ray diffraction pattern of phase pure tetragonal Culny ;Gay Se, nanocrystals synthesized for 120 min at 230 °C. The difference between the calculated (red) and experimental (blue) intensities is shown in gray with the Bragg markers shown at the bottom. The low magnification TEM image of the Culng;Gay Se, nanocrystals synthesized at 230 °C exhibiting monodispersed size and shape is shown in the inset.$

$Figure 2. X-ray diffraction patterns and Rietveld refinement of nanocrystals grown for (a) 10 min at 130 °C, (b) 20 min at 160 °C, (c) 40 min at 200 °C, and (d) 60 min at 230 °C. Vertical lines show the positions of Bragg peaks, and the Bragg markers from top to bottom are: (a) and (b) CuSe, Se, and CuGaSe,; (c) CuSe, CIGSel, and CIGSe2; (d) CIGSel and CIGSe2. The gray line is the difference between the calculated and experimental intensities. The blue dots correspond to experimental, and the red line corresponds to calculated intensities. The peak marked “*” corresponds to a diffraction peak from hexadecylamine. Se See Nee NA ee En eee See ee ey Serene, Bee ee eee 3.1. Crystal Chemistry of Cu—In—Ga-—Se Phase Evolution. At the start of the reaction (10 min after injection of Se-HDA), a mixture of different phases (CuSe, Se, and CuGaSe,) was formed. The phase distribution can be seen in Table $1. The variation in the XRD background in Figure 2 may arise from the presence of an amorphous phase in the sample. From TEM (EDX), amorphous In,Se; was found (Figure S2a). Se nanorods (12 + 1 wt %) crystallized upon injection of Se into the reaction, leading to partial consumption of Se in the HDA (Figure S2b). The major phase was found to be binary CuSe nanocrystals (57 + 1 wt %) (Figure 2a). This is found to be the intermediate step in the growth of Culng;Gag;Se, nanocrystals. Lattice fringe spacings are obtained from the high-resolution transmission electron microscopy (HRTEM) images and SAED patterns (Figure S3a). The spacings are found to be d, = 3.32 A, d, = 3.33 A that correspond to the (010) and (100) lattice planes of hexagonal P6;/mmc CuSe, respectively. The Cu:Se ratio was close to 1:1 as determined by semi-quantitative EDX analysis, which is consistent with one of the phases determined using XRD. In addition, the XRD pattern also revealed the presence of crystalline ternary CuGaSe, nanocrystals (31 + 1 wt %) and was further confirmed by HRTEM and CBED. The interplanar spacings determined from HRTEM/CBED are d, = 1.93 A, formation of Culng;Gag;Se, nanocrystals and their phase evolution, controlled extractions were carried out at specific reaction intervals. Because of the complexity of the reaction, intermediate pathways for formation of the products need to be accounted for. As was previously studied using thermo- gravimetric analysis,” these precursors have different thermal decomposition temperatures. In general, the precursor reaction with HDA to form the precursor complexes occurs at a lower temperature than the decomposition temperature®’ so it would$

$TEM images and the proposed schematic growth mechanism (Figure 4), large CuSe particles continuously dissolve as the reaction progressed to 200 °C. Stage 3 involved full dissolution of CuSe followed by the formation of two different CIGSe phases (Figure 4 and Table $1). This is a strong indication that CIGSe phases are more stable at higher temperature than CuSe. Another 60 min at 230 °C resulted in the equilibrium single phase of Culng;Gag;Se, nanocrystals, and no obvious change in the shape of the particles was observed. Recently, Kar et al. showed that the formation of CulnSe, nanocrystals in oleylamine involved by the formation of CuSe and InSe nanocrystals as the primary intermediates.** Besides, the phase transformation of biphasic Cu,S—CulInS, to monophasic CulnS, nanocrystals turned out to be an essential intermediate step.°**? Figure 5 shows the evolution of the crystal structure Figure 5. X-ray diffraction patterns showing the evolution of the crystal structure of Culn,Ga,_,Se,.$

Origin of Defects and Positron Annihilation in Hybrid and All-Inorganic Perovskites

Chemistry of Materials

Role of Decomposition Product Ions in Hysteretic Behavior of Metal Halide Perovskite

ACS nano, 2021

Ion migration is one of the most debated mechanisms and credited with multiple observed phenomena... more Ion migration is one of the most debated mechanisms and credited with multiple observed phenomena and performance in metal halide perovskites (MHPs) semiconductor devices. However, to date, the migration of ions and their effects on MHPs are not still fully understood, largely due to a lack of direct observations of temporal ion migration. In this work, using direct observation of ion migration in-operando, we observe the hysteretic migration behavior of intrinsic ions (i.e., CH3NH3+ and I-) as well as reveal the migration behavior of CH3NH3+ decomposition ions. We find that CH3NH3+ decomposition products can be affected by light and accumulate at the interfaces under bias. These MHP decomposition products are tightly related to the device performance and stability. Complementary results of time-resolved Kelvin probe force microscopy (tr-KPFM) demonstrate a correlation between dynamics of these interfacial ions and charge carriers. Overall, we find that there are a number of mobile ...

Mapping fast evolution of transient surface photovoltage dynamics using G-Mode Kelvin probe force microscopy

arXiv: Applied Physics, 2018

Optoelectronic phenomena in materials such as organic/inorganic hybrid perovskites depend on a co... more Optoelectronic phenomena in materials such as organic/inorganic hybrid perovskites depend on a complex interplay between light induced carrier generation and fast (electronic) and slower (ionic) processes, all of which are known to be strongly affected by structural inhomogeneities such as interfaces and grain boundaries. Here, we develop a time resolved Kelvin probe force microscopy (KPFM) approach, based on the G-Mode SPM platform, allowing quantification of surface photovoltage (SPV) with microsecond temporal and nanoscale spatial resolution. We demonstrate the approach on methylammonium lead bromide (MAPbBr3) thin films and further highlight the usefulness of unsupervised clustering methods to quickly discern spatial variability in the information rich SPV dataset. Using this technique, we observe concurrent spatial and ultra-fast temporal variations in the SPV generated across the thin film, indicating that structure is likely responsible for the heterogenous behavior.

Improved Radiation Sensing with Methylammonium Lead Bromide Perovskite Semiconductors

2019 IEEE Nuclear Science Symposium and Medical Imaging Conference (NSS/MIC), 2019

Methylammonium lead bromide (CH3NH3PbBr3 or MAPB) perovskite semiconductors are a potential low-c... more Methylammonium lead bromide (CH3NH3PbBr3 or MAPB) perovskite semiconductors are a potential low-cost option for high energy resolution semiconductor detectors for gamma and neutron sensing. MAPB has recently been demonstrated for its ability to sense alpha particles and neutrons, and pertinent electronic properties have previously been reported. In this presentation, we report on several techniques of improving detector performance to achieve better spectroscopic performance for gamma sensing. These methods include anion and partial cation substitutions of MAPB to improve carrier transport and limit ionic conductivity and several surface treatment and metallization techniques for improved carrier extraction and minimization of device polarization.

Dual Gamma/Neutron Sensing with Methylammonium Lead Tribromide Perovskite

2020 IEEE Nuclear Science Symposium and Medical Imaging Conference (NSS/MIC)

This report several techniques are under investigation to improve the spectroscopic capability of... more This report several techniques are under investigation to improve the spectroscopic capability of methylammonium lead tribromide perovskite semiconductors (CH3NH3PbBr3, or MAPB). These techniques include birefringence level measurements used to evaluate the quality of the single crystals and increase fabrication success rate of radiation responsive crystals. In addition, several characterization techniques are performed on a partial cation-substitution of MAPB with lithium-6 to analyze the structure and surface properties of the single crystal produced. Finally, we will present the thermal neutron sensing results from a MAPB crystal substituted with lithium-6.

Exploring spin-orbital coupling effects on photovoltaic actions in Sn and Pb based perovskite solar cells

Nano Energy

Abstract Organo-metal halide perovskites, as emerging photovoltaic materials, have demonstrated i... more Abstract Organo-metal halide perovskites, as emerging photovoltaic materials, have demonstrated interesting spin states due to spin-orbital coupling (SOC) effects. However, replacing the Pb with the Sn can inevitably affect the SOC and consequently changes the internal photovoltaic processes in the development of environmentally friendly perovskite devices. Here, by operating the spin states with circularly polarized photoexcitation we report that the spin-dependent photocurrent (Jsc) becomes much more prominent upon replacing Pb with Sn, increasing the spin dependence from 0.25% to 1.25% by switching the photoexcitation from linear to circular polarization. Essentially, the spin-dependent Jsc is determined by the spin relaxation time, changing with the SOC strength, as compared to the charge dissociation time. On the other hand, our magneto-photocurrent (magneto-Jsc) results show that the internal magnetic parameter decreases from 281 mT to 41 mT upon Sn-Pb replacement, providing an evidence that the SOC is indeed weakened from Pb to Sn based solar cells. Furthermore, the spin-dependent photoluminescence (PL) indicates that weakening the SOC upon the Sn-Pb replacement leads to more antiparallel spin states (singlets) available for PL but less parallel spin states (triplets) available for photovoltaic action. Therefore, SOC plays an important role in the development of photovoltaic actions in Sn-based perovskite solar cells.

High-Throughput Study of Antisolvents on the Stability of Multicomponent Metal Halide Perovskites through Robotics-Based Synthesis and Machine Learning Approaches

Antisolvent crystallization methods are frequently used to fabricate high-quality perovskite thin... more Antisolvent crystallization methods are frequently used to fabricate high-quality perovskite thin films, to produce sizable single crystals, and to synthesize nanoparticles at room temperature. However, a systematic exploration of the effect of specific antisolvents on the intrinsic stability of multicomponent metal halide perovskites has yet to be demonstrated. Here, we develop a highthroughput experimental workflow that incorporates chemical robotic synthesis, automated characterization, and machine learning techniques to explore how the choice of antisolvent affects the intrinsic stability of binary perovskite systems in ambient conditions over time. Different combinations of the endmembers, MAPbI3, MAPbBr3, FAPbI3, FAPbBr3, CsPbI3, and CsPbBr3, are used to synthesize 15 combinatorial libraries, each with 96 unique combinations. In total, roughly 1100 different compositions are synthesized. Each library is fabricated twice using two different antisolvents: toluene and chloroform....

Improved Radiation Sensing with Methylammonium Lead Tribromide Perovskite Semiconductors

Nuclear Instruments & Methods in Physics Research Section A-accelerators Spectrometers Detectors and Associated Equipment, 2021

Recently, organometallic halide perovskites (OMHPs) have attracted much interest as a potential m... more Recently, organometallic halide perovskites (OMHPs) have attracted much interest as a potential medium resolution detector for ionizing radiation sensing applications. Despite moderate success in the development of OMHP radiation detectors to date, efforts to optimize bulk carrier properties are often hindered by device degradation caused by surface recombination, ionic conductivity, environmental instability, and interface phenomena. In this study, methods of improving the interfacial and surface properties, detector stability, and responsivity of methylammonium lead tribromide (MAPB) semiconductor radiation detectors were investigated. We demonstrated that chemomechanical polishing with dimethylformamide (DMF) as a finishing step decreased surface roughness, removed surface trap states, and greatly enhanced device stability compared to mechanical polishing. Further, using a tin oxide (SnO2) interface layer as hole blocking/electron transporting layer greatly increased the device f...

Unraveling the Energy Landscape and Energy Funneling Modulated by Hole Transport Layer for Highly Efficient Perovskite LEDs

Laser & Photonics Reviews

Unraveling the hysteretic behavior at double cations-double halides perovskite - electrode interfaces

Nano Energy