Organometal Halide Research Articles

Reducing Anomalous Hysteresis in Perovskite Solar Cells by Suppressing the Interfacial Ferroelectric Order.

Despite the booming research in organometal halide perovskite solar cells (PSCs) of recent years, considerable roadblocks remain for their large-scale deployment, ranging from undesirable current-voltage hysteresis to inferior device stability. Among various plausible origins of hysteresis, interfacial ferroelectricity is particularly intriguing and warrants a close scrutiny. Here, we examine interfacial ferroelectricity in MAPbI3 (FAPbI3)/TiO2 and MAPbI3/phenyl-C61-butyric-acid-methyl-ester (PCBM) heterostructures and explore the correlations between the interfacial ferroelectricity and the hysteresis from the perspective of nonadiabatic electronic dynamics. It is found that the ferroelectric order develops at the MAPbI3/TiO2 interface owing to the interaction between the polar MA ions and TiO2. The polarization switching of the MA ions under an applied gate field would drastically result in different rates in interfacial photoelectron injection and electron-hole recombination, contributing to the undesirable hysteresis. In sharp contrast, ferroelectricity is suppressed at the FAPbI3/TiO2 and MAPbI3/PCBM interfaces, thanks to elimination of the interfacial electric field between perovskite and TiO2 via substitution of strong polar MA (dipole moment: 2.29 debye) by weak polar FA ions (dipole moment: 0.29 debye) and interface passivation, leading to consistent interfacial electronic dynamics and the absence of hysteresis. The present work sheds light on the physical cause for hysteresis and points to the direction to which the hysteresis could be mitigated in PSCs.

Blue perovskite LEDs dependent on temperature optical properties

Using an excitonic luminescence model, researchers analyze temperature-dependent optical properties of chlorine-based organometallic halide perovskite film in relation to blue luminescence.

Temperature-dependent nonmonotonous evolution of excitonic blue luminescence and Stokes shift in chlorine-based organometallic halide perovskite film

The chlorine-based organometallic halide perovskite (Cl-OHP) film with a (001)-preferred orientation and good crystallization has been synthesized by a hybrid sequential deposition process. The photoluminescence and absorption spectra of the Cl-OHP film in the blue light region have been investigated at operating temperatures ranging from 10 to 350 K. The Cl-OHP film shows a strong exciton-related emission of which the exciton binding energies at low temperature and high temperature are 136 meV and 41 meV, respectively. It is found that the blueshift from excitonic luminescence is initially observed at temperature below 175 K, and then, the redshift occurs from 175 to 350 K. Meanwhile, the bandgap of the Cl-OHP film widens with the increase in operating temperature. The nonmonotonous shifts on the emission peak energy are attributed to the competition between the Stokes effect and bandgap widening. This should contribute to the understanding of photophysical processes in Cl-OHP materials and devices.

Influence of Charge Transport Layers on Capacitance Measured in Halide Perovskite Solar Cells

Summary Capacitance-based techniques have been used to measure the electrical properties of halide perovskite solar cells (PSCs) such as defect activation energy and density, carrier concentration, and dielectric constant, which provide key information for evaluating the device performance. Here, we show that capacitance-based techniques cannot be used to reliably analyze the properties of defects in the perovskite layer or at its interface, because the high-frequency capacitance signature is due to the response of charge carriers in the hole-transport layer (HTL). For HTL-free PSCs, high-frequency capacitance can be considered as the geometric capacitance for analyzing the dielectric constant of the perovskite layer because there is no trapping and de-trapping of charge carriers in the perovskite layer. We further find that the low-frequency capacitance signature can be used to calculate the activation energy of the ionic conductivity of the perovskite layer, but the overlapping effects with charge transport materials must be avoided.

Frequency- and Power-Dependent Photoresponse of a Perovskite Photodetector Down to the Single-Photon Level.

Organometallic halide perovskites attract strong interests for their high photoresponsivity and solar cell efficiency. However, there was no systematic study of their power- and frequency-dependent photoresponsivity. We identified two different power-dependent photoresponse types in methylammonium lead iodide perovskite (MAPbI3) photodetectors. In the first type, the photoresponse remains constant from 5 Hz to 800 MHz. In the second type, absorption of a single photon can generate a persistent photoconductivity of 30 pA under an applied electric field of 2.5 × 104 V/cm. Additional absorbed photons, up to 8, linearly increase the persistent photoconductivity, which saturates with the absorption of more than 10 photons. This is different than single-photon avalanche detectors (SPADs) because the single-photon response is persistent as long as the device is under bias, providing unique opportunities for novel electronic and photonic devices such as analogue memories for neuromorphic computing. We propose an avalanche-like process for iodine ions and estimate that absorption of a single 0.38 aJ photon triggers the motion of 108-9 ions, resulting in accumulations of ions and charged vacancies at the MAPbI3/electrode interfaces to cause the band bending and change of electric material properties. We have made the first observation that single-digit photon absorption can alter the macroscopic electric and optoelectronic properties of a perovskite thin film.

Cadmium Sulfide Quantum Dots–Organometallic Halide Perovskite Bilayer Electrode Structures for Supercapacitor Applications

AbstractSupercapacitors are attracting great attention because of their fast charging–discharging ability as well as their high power density. The current research in this area focuses mainly on exploring novel low‐cost electrode materials with higher energy and power densities. In this work, thin‐film electrochemical capacitors were fabricated using layers of self‐synthesized cadmium sulfide quantum dots and organometallic halide perovskite materials as active electrodes. Organometallic halide perovskites exhibit interesting ionic responses as well as extraordinary electronic properties. These properties are exploited in fabricating the electrochemical capacitors, and the devices showed excellent cycling ability with stable capacitance outputs beyond 4000 cycles. Impedance spectroscopy measurements revealed that perovskites do not only serve as active electrodes but also as solid electrolytes, thereby enhancing the capacitance of the devices and hence the energy densities. The layers provide high surface areas for electrolytes to access the electrode materials; reasonably low charge transfer resistance and small relaxation time were also observed. This work opens new opportunities for developing thin‐film supercapacitors using low‐cost electrode materials and employing a facile, inexpensive solution‐process coating.

Precise Tuning of the Thickness and Optical Properties of Highly Stable 2D Organometal Halide Perovskite Nanosheets through a Solvothermal Process and Their Applications as a White LED and a Fast Photodetector.

Precise control of the thickness of large-area two-dimensional (2D) organometal halide perovskite layers is extremely challenging owing to the inherent instability of the organic component. Herein, a novel, highly reproducible, and facile solvothermal route is reported to synthesize and tailor the thickness and optical band gap of the organic-inorganic halide perovskite nanosheets (NSs). Our study reveals that self-assembly of randomly oriented perovskite nanorods leads to the growth of multilayered perovskite NSs at ∼100 °C, while at higher temperature, large-area few-layer to bilayer 2D NSs (CH3NH3PbBr3) are obtained through lattice expansion and layer separation depending precisely on the temperature. Interestingly, the thickness of the 2D NSs shows a linear dependence on the reaction temperature and thus enables precise tuning of the thickness from 14 layers to 2 layers, giving rise to a systematic increase in the band gap and appearance of excitonic absorption bands. Quantitative analysis of the change in the band gap with thickness revealed a strong quantum confinement effect in the 2D layers. The perovskite 2D NSs exhibit tunable color and a high photoluminescence (PL) quantum yield (QY) up to 84%. Through a careful analysis of the steady-state and time-resolved PL spectra, the origin of the lower PL QY in thinner NSs is traced to surface defects in the 2D layers, for the first time. A white light converter was fabricated using the composition-tuned 2D CH3NH3PbBrI2 NS on a blue light-emitting diode chip. The 2D perovskite photodetector exhibits a stable and very fast rise/fall time (24 μs/103 μs) along with high responsivity and detectivity of ∼1.93 A/W and 1.04 × 1012 Jones, respectively. Storage, operational, and temperature-dependent stability studies reveal high stability of the 2D perovskite NSs under the ambient condition with high humidity. The reported method is highly promising for the development of large-area stable 2D perovskite layers for various cutting-edge optoelectronic applications.

Hybrid Al2O3-CH3NH3PbI3 Perovskites towards Avoiding Toxic Solvents.

We report the synthesis of organometal halide perovskites by milling CH3NH3I and PbI2 directly with an Al2O3 scaffold to create hybrid Al2O3-CH3NH3PbI3 perovskites, without the use of organic capping ligands that otherwise limit the growth of the material in the three dimensions. Not only does this improve the ambient stability of perovskites in air (100 min versus 5 min for dimethylformamide (DMF)-processed material), the method also uses much fewer toxic solvents (terpineol versus dimethylformamide). This has been achieved by solid-state reaction of the perovskite precursors to produce larger perovskite nanoparticles. The resulting hybrid perovskite–alumina particles effectively improve the hydrophobicity of the perovskite phase whilst the increased thermal mass of the Al2O3 increases the thermal stability of the organic cation. Raman data show the incorporation of Al2O3 shifts the perovskite spectrum, suggesting the formation of a hybrid 3D mesoporous stack. Laser-induced current mapping (LBIC) and superoxide generation measurements, coupled to thermogravimetric analysis, show that these hybrid perovskites demonstrate slightly improved oxygen and thermal stability, whilst ultra-fast X-ray diffraction studies using synchrotron radiation show substantial (20×) increase in humidity stability. Overall, these data show considerably improved ambient stability of the hybrid perovskites compared to the solution-processed material.

Stability study of organometal halide perovskite and its enhanced X-ray scintillation from the incorporation of anodic TiO2 nanotubes.

Organometal halide perovskite-based optoelectronic devices are currently a hot research area owing to their unique properties, but widespread commercialization is plagued by their poor long-term stability. So far, the degradation mechanism of organometal halide perovskites is still indistinct due to limited real time systematic study. In this work, we in situ study the crystal evolution of an organometal halide perovskite CH3NH3PbI3, which is prepared on different kinds of framework substrates. Based on the in situ grazing incidence X-ray diffraction and X-ray near absorption edge spectrum, we observe the formation of some 2D networks of [PbI6]4− octahedra intermediates during CH3NH3PbI3 degradation in a moist environment at the early step of the degradation mechanism. We also show that the structural stability of CH3NH3PbI3 deposited anodic TiO2 nanotube substrates is relatively better than that of prepared perovskite on TiO2 nanoparticles in moisture. The confinement of the 3D [PbI6]4− octahedral crystal network probability reduces the ion migration by regular pores of crystalline TiO2 nanotubes, improving the stability of the organometal halide perovskite. Furthermore, the X-ray excited luminescence intensity of CH3NH3PbI3 fabricated on TiO2 nanotubes is boosted 88% compared with that of conventional TiO2 nanoparticle substrates, which demonstrates its potential application in scintillation detectors.

Good or evil: what is the role of water in crystallization of organometal halide perovskites?

Perovskite solar cells (PSCs) have the potential to become one of the most cost-efficient photovoltaic devices. However, current fabrication methods of PSCs still require strict environment control and ultrahigh purity chemicals, which could prevent their large-scale commercialization. To tackle this challenge, the role of water is the first to be thoroughly understood in a perovskite formation process. Until now, there is still controversy about whether water is harmful or beneficial for perovskite formation, not to mention exactly what role water plays therein. In this Focus article, we review recent studies on water involved chemical reactions, solvent interaction, intermediates, and crystal growth in the perovskite film formation process, in order to bring out a full picture about what water does in the perovskite formation process. As our current understanding stands, a suitable amount of water could be of help for growing high quality perovskite films due to the resultant formation of intermediates, such as MAPbI3·H2O, which facilitates the conversion from precursors to perovskites. However, too much water would induce the formation of relatively stable components, such as (MA)4PbI6·2H2O, which are left in the product-films as impurities resulting in degraded device performance. Continual efforts should be made to further understand and develop water-involved strategies for consistent PSC fabrication under ambient conditions.

Perovskite-Enhanced Silicon-Nanocrystal Optoelectronic Synaptic Devices for the Simulation of Biased and Correlated Random-Walk Learning.

Silicon- (Si-) based optoelectronic synaptic devices mimicking biological synaptic functionalities may be critical to the development of large-scale integrated optoelectronic artificial neural networks. As a type of important Si materials, Si nanocrystals (NCs) have been successfully employed to fabricate optoelectronic synaptic devices. In this work, organometal halide perovskite with excellent optical asborption is employed to improve the performance of optically stimulated Si-NC-based optoelectronic synaptic devices. The improvement is evidenced by the increased optical sensitivity and decreased electrical energy consumption of the devices. It is found that the current simulation of biological synaptic plasticity is essentially enabled by photogating, which is based on the heterojuction between Si NCs and organometal halide perovskite. By using the synaptic plasticity, we have simulated the well-known biased and correlated random-walk (BCRW) learning.

Optimized performance III-nitride-perovskite-based heterojunction photodetector via asymmetric electrode configuration.

Organometal halide perovskite photodetectors have recently drawn significant attention due to their excellent potential to perform as broadband photodetectors. However, the photoresponse in the ultraviolet (UV) spectrum can be improved by introducing wide bandgap semiconductors. In this work, we report on a methylammonium lead iodide/p-type gallium nitride (MAPI/p-GaN) heterojunction photodetector. We demonstrate that the device is capable of detecting in the UV region by p-GaN being hybridized with MAPI. We further investigate different symmetric and asymmetric metal-electrode contacts to enhance the device performance including the response time. The asymmetric electrode configuration is found to be the most optimal configuration which results in high photoresponse (photo-responsivity is 55 mA W−1 and fall time < 80 ms). As the light illumination occurs through the GaN side, its presence ultimately reduces MAPI degradation due to efficient absorption of the UV photons by GaN film.

Lead-free cesium tin halide nanocrystals for light-emitting diodes and color down conversion.

Organometal halide perovskites are attracting a great deal of attention because of their long carrier diffusion lengths, wide wavelength tunability, and narrow-band emission. However, the toxicity of lead has caused considerable environmental and health concerns. In this work, lead-free cesium tin halide nanocrystals are synthesized and investigated. CsSnBr3 and CsSnI3 nanocrystals, 25 and 7 nm in size, are synthesized by a facile hot injection method. Absorption spectroscopy, photoluminescence spectroscopy, and X-ray diffraction were used to understand their structural and optical properties. CsSnBr3 and CsSnI3 nanocrystals show emission peaks at 683 and 938 nm, respectively. These nanocrystals show shelf stability for a few months. Temperature-dependent photoluminescence is utilized to know more about fundamental physical parameters, such as exciton binding energy, charge carrier–phonon interactions and band gap. Light-emitting diodes and color down-conversion films are also demonstrated using these lead free perovskite nanocrystals.

Long-term stable and highly efficient perovskite solar cells with a formamidinium chloride (FACl) additive

Although the power conversion efficiency (PCE) of organometal halide perovskite solar cells (PSCs) has reached 25.2%, control of the crystallization process and its impact on film quality is still one of the main challenges.

A hysteresis-free perovskite transistor with exceptional stability through molecular cross-linking and amine-based surface passivation.

Organo-metal halide perovskite field-effect transistors present serious challenges in terms of device stability and hysteresis in the current-voltage characteristics. Migration of ions located at grain boundaries and surface defects in the perovskite film are the main reasons for instability and hysteresis issues. Here, we introduce a perovskite grain molecular cross-linking approach combined with amine-based surface passivation to address these issues. Molecular cross-linking was achieved through hydrogen bond interactions between perovskite halogens and dangling bonds present at grain boundaries and a hydrophobic cross-linker, namely diethyl-(12-phosphonododecyl)phosphonate, added to the precursor solution. With our approach, we obtained smooth and compact perovskite layers composed of tightly bound grains hence significantly suppressing the generation and migration of ions. Moreover, we achieved efficient surface passivation of the perovskite films upon surface treatment with an amine-bearing polymer, namely polyethylenimine ethoxylated. With our synergistic grain and surface passivation approach, we were able to demonstrate the first perovskite transistor with a complete lack of hysteresis and unprecedented stability upon continuous operation under ambient conditions. Added to the merits are its ambipolar transport of opposite carriers with balanced hole and electron mobilities of 4.02 and 3.35 cm2 V-1 s-1, respectively, its high Ion/Ioff ratio >104 and the lowest sub-threshold swing of 267 mV dec-1 reported to date for any perovskite transistor. These remarkable achievements obtained through a cost-effective molecular cross-linking of grains combined with amine-based surface passivation of the perovskite films open a new era and pave the way for the practical application of perovskite transistors in low-cost electronic circuits.

Properties and growth of large single crystals of one-dimensional organic lead iodine perovskite

Here, we demonstrate for the first time the growth of 2 mm × 4 mm × 8 mm sized single crystal one dimensional organic lead iodine perovskite – DMAPbI3 ((CH3)2NH2PbI3).

Molecular materials as interfacial layers and additives in perovskite solar cells.

Solar cells based on organo-metal halide perovskites have gained unprecedented research interest over the last few years due to their low-cost solution processability, high power conversion efficiency, which has recently reached a certified value of 25.2%, and abundance of raw materials. Nevertheless, the best efficiencies remain below the Shockley-Queisser theoretical limit of 32.5% due to several losses arising from either defect traps present in the bulk of the perovskite absorber or at the device heterointerfaces. While bulk defects are detrimental for the device performance by mainly limiting the open circuit voltage, interfacial layers are also crucial. They dictate the charge transfer/transport from the perovskite layer to the collecting electrodes, hence influencing the device photocurrent, but also act as protective barriers against oxygen and moisture penetration. Molecular materials and additives are widely used to improve the bulk properties of perovskite absorbers through the formation of high-quality perovskite films with superior optoelectronic properties, and improved crystallinity, and also of electronically clean interfaces with minimum losses during charge transfer/transport. In this review, we analyze the predominant pathways that contribute to voltage and current losses due to poor interfaces and also due to non-radiative recombination losses arising from inferior perovskite morphology and its inherent polycrystalline and highly defective nature. We then discuss strategies for achieving interfacial organic and inorganic molecular materials for application as electron and hole transport layers in perovskite solar cells with ideal energy levels, high charge mobilities and improved thermal, photo, and structural stability. Moreover, the prerequisites for molecular additives to achieve dimensionality engineering, defect passivation, molecular cross-linking, interfacial energy alignment and electronic doping are thoroughly discussed. Finally, we examine prospects for future research directions and commercialization.

Elimination of Light-Soaking Effect in Hysteresis-Free Perovskite Solar Cells by Interfacial Modification

The hysteresis and light-soaking effect have been observed in organo–metal halide perovskite solar cells (PSCs) under operating conditions, which inhibit the precise evaluation of power output. The mechanisms leading to these effects are little understood. Here, our studies have demonstrated that the light-soaking effect is related to the electron selective layer/perovskite interface in hysteresis-free PSCs. The introduction of [6,6]-phenyl C61-butyric acid methyl ester (PC61BM) doped with CH3NH3I molecules as the interfacial layer can effectively release or eliminate this effect due to the efficient charge transfer, accompanied with the best stable output efficiency of 19% and an ultrahigh fill factor over 84%. Capacitance–voltage and capacitance–frequency curves derived from electrochemical impedance spectroscopy demonstrate that the light-soaking effect in PSCs mainly originates from the charge accumulation at the PC61BM/perovskite interface. The CH3NH3I doped in PC61BM forestalls the ion movement amon...

The synthesis of MAPbBr3 nanoplatelets with long-term stability and high photoluminescence quantum yield through controlling the synthetic temperature and dilution treatment

Organometal halide perovskite nanocrystals are very promising for optoelectronic applications due to their unique optical properties. However, the OHP nanocrystals generally suffer from problems such as nonuniform morphology and poor stability. This work reports the synthesis of MAPbBr3 nanoplatelets through dilution of the irregular colloidal MAPbBr3 nanocrystals prepared at 50 °C. The dilution followed by self-organization of the nanocrystals result in the production of uniform square nanoplatelets with long term stability up to 6 months and high photoluminescence quantum yield about 90%. Furthermore, by using bidentate coordination molecule pyrazole as co-ligand, regular 2D structural Nanocrystals with round edge and aggregation were produced. The aggregated nanoplatelets prepared at 50 °C show the same self-assembly process after dilution to form uniform square nanoplatelets.

Study on the Movements of Organometallic Halide Perovskite Crystals on their Films

AbstractRecently, organometallic halide perovskites (OMHPs) have attracted significant attention due to their high light absorption coefficient, high charge carrier mobility, and long carrier lifetime. One of the key shortcomings of the OMHP device application is its poor stability under humid conditions. Thus, it is important to study the relationship between humidity and OMHP crystals in their thin films. In this work, the large‐scale movement behavior of CH3NH3PbI3‐xClx crystals in their films is discovered. It is concluded that the number of moving CH3NH3PbI3‐xClx crystals can be controlled by the humidity of the external environment. The movement behavior of the perovskite grains is attributed to the asymmetric decomposition of perovskite under high relative humidity (RH) conditions.