Organometal Halide Perovskite Research Articles

Modified bismuth perovskite thin film morphology using greener solvent and electric-field assisted spray-coating

Eliminating lead (Pb) from the most promising perovskite active material (CH3NH3PbI3) based optoelectronic devices (OEDs) has led to exploring lead-free Organo-Metal Halide Perovskites (OMHPs). Bi-based OMHPs have captured significant attention owing to their optoelectronic properties. Bi-OMHPs are often non-conformal, hampering optical absorption and charge separation leading to the degrading device performance. To address this, conformal Bi-OMHP thin-films with improved absorption are deposited using a modified spray technique using engineered greener solvent system which greatly reduced toxicity. The developed protocol retards rapid crystallization of the material leading larger grains of ~4 µm. Further, the spray deposition under the influence of electric field (EF) facilitates fine atomization of the precursor, resulting in improved conformality with reduced pinholes. The developed spray technology combined with engineered solvent can be adaptable for eco-friendly large-scale fabrication of OEDs.

Underestimated effect of the polymer encapsulation process on the photoluminescence of perovskite revealed by in situ single-particle detection.

Photostability has always been an important issue that limits the performance of organo-metal halide perovskites in optoelectronic devices. Although the photostability can be partially improved by polymer coating/encapsulation, one rising question that needs to be considered is whether the improvement of photostability is accessed at the expense of intangible loss in photoluminescence (PL) properties. By in situ analyzing the evolution of PL properties of individual perovskite crystals during the polymer encapsulation procedure, we demonstrate here that poly(methyl methacrylate), a common polymeric encapsulant, would passivate the surface defects of perovskite crystals, leading to the suppress of PL blinking. However, somewhat counterintuitive, the toluene solvent will induce the PL decline of individual perovskite crystals via accumulation of the number of quenchers that, most probably, are related to the ion migration in perovskite. The findings at the single-particle level emphasize the often-neglected role of the polymer matrix and the solvent in the optical properties of perovskite material during the polymer encapsulation process, and will guide the further design of more stable and high-performance devices based on perovskite.

Deactivation/Activation of Quenching Defects in CH3NH3PbI3 Perovskite by Direct Electron Injection/Extraction.

Organometal halide perovskites (OMHPs) have emerged as advisible materials for application in optoelectronic devices over the past decade. However, a variety of complex slow responses in OMHPs under an external electric field have been observed, and the mechanisms for these responses remain a topic of intense debate. In this work, with an external voltage applied to the CH3NH3PbI3 crystal, reversible photoluminescence (PL) enhancement and quenching behaviors respectively near the anode and the cathode were observed under wide-field fluorescence microscopy. Further experiments attribute the reversible PL enhancing responses to the electron injection effect increasing the radiative recombination, while PL quenching was attributed to be due to the electron extraction effect increasing the nonradiative recombination. The control of PL by external applied voltage indicates brilliant carrier mobility in the CH3NH3PbI3 crystal and also reminds us to focus on the effect of hole/electron injection on the materials which may limit the performance of perovskite-based optoelectronic devices.

On the efficiency of perovskite solar cells with a back reflector: effect of a hole transport material.

Organometal halide perovskites are promising, high-performance absorbers in solar cells. However, the light-harvesting performance of these devices is still limited by excessive charge carrier recombination. Charge carrier management can be improved, taking into account the transport properties of layers surrounding the absorber. In particular, the choice of an appropriate hole-transport material (HTM) could provide a path towards increasing the device performance of perovskite solar cells (PSCs). The Lambertian reflection on the cell's back-surface reflector could increase the power conversion efficiency (PCE) of PSCs as well. Taking into account these facts, we analyse the absorptance and the PCE of a perovskite thin-film solar cell with the Lambertian reflection on the cell's back-surface reflector for various organic and inorganic HTMs. The analysis is done by means of the Monte-Carlo ray tracing simulations complemented by the transfer-matrix method to account for the interference phenomenon in the local generation rate G of carriers in a thin-film multilayer system. This function is employed further in the transport equations to calculate the current-voltage characteristics of the cell. We show that wide band gap HTMs, that possess negligible absorption, increase the photocurrent in the perovskite, passing reflected photons from the back reflector. In contrast, at the same perovskite thickness the PSC gains less photocurrent with narrow band gap HTMs, where an excessive non-radiative recombination takes place. Our analysis demonstrates that the optimal thickness of the solar cell with the typical absorber CH3NH3PbI3 is ∼300 nm, providing the maximal efficiency ∼18.8% for the wide band gap HTM (CuSCN) at the moderate absorber purity (the diffusion length D ∼ 1 μm).

Morphology and surface analyses for CH3NH3PbI3 perovskite thin films treated with versatile solvent-antisolvent vapors.

Organometal halide perovskite (CH3NH3PbI3) semiconductors have been promising candidates as a photoactive layer for photovoltaics. Especially for high performance devices, the crystal structure and morphology of this perovskite layer should be optimized. In this experiment, by employing solvent–antisolvent vapor techniques during a modified sequential deposition of PbI2–CH3NH3I layers, the morphology engineering was carried out as a function of antisolvent species such as: chloroform, chlorobenzene, dichlorobenzene, toluene, and diethyl ether. Then, the optical, morphological, structural, and surface properties were characterized. When dimethyl sulfoxide (DMSO, solvent) and diethyl ether (antisolvent) vapors were employed, the CH3NH3PbI3 layer exhibited relatively desirable crystal structures and morphologies, resulting in an optical bandgap (Eg) of 1.61 eV, crystallite size (t) of 89.5 nm, and high photoluminescence (PL) intensity. Finally, the stability of perovskite films toward water was found to be dependent on the morphologies with defects such as grain boundaries, which was evaluated through contact angle measurement.

Photon-induced deactivations of multiple traps in CH3NH3PbI3 perovskite films by different photon energies.

Photon-induced trap deactivation is commonly observed in organometal halide perovskites. Trap deactivation is characterized by an obvious photoluminescence (PL) enhancement. In this work, the properties of traps in CH3NH3PbI3 perovskite films were studied based on the PL enhancement excited by lasers of different wavelengths (633 nm and 405 nm). Two types of electron traps were identified; one can be deactivated by both 633 nm and 405 nm illuminations, whereas the other one can only be deactivated by 405 nm illumination. The energy levels of both types of traps were beneath the conduction band minimum. The expressions of the PL enhancement kinetics due to the trap deactivations by lasers of different wavelengths were derived. The ratio of the constants of the radiative recombination rate and the initial capture rates for both traps was determined from the PL enhancement. The trap deactivation was a photon-related process rather than a photocarrier-related process, and the deactivation time was inversely proportional to the photon flux density.

An organometal halide perovskite photocathode integrated with a MoS2 catalyst for efficient and stable photoelectrochemical water splitting

An organometal halide perovskite (OHP) photocathode integrated with a MoS2 catalyst achieved long-term photoelectrochemical (PEC) stability and high half-cell solar-to-hydrogen conversion efficiency (HC-STH).

Surface passivation of organometal halide perovskites by atomic layer deposition: an investigation of the mechanism of efficient inverted planar solar cells.

Interface passivation plays a pivotal role in achieving high-efficiency organic metal halide perovskite solar cells (PSCs). It has been recently revealed that atomic layer deposition (ALD) of wide-band gap oxides shows great potential to effectively passivate defects at the interface, and ALD is also of great technological promise for industrial upscaling. However, the conflicting observations of ALD passivation are often reported in the literature, even with very similar ALD conditions. To unveil the involved crucial mechanism, this work carefully investigates the evolution of a representative MAPbI3 perovskite surface during the ALD of Al2O3, by employing the technique of in situ X-ray photoelectron spectroscopy. The ALD at 125 °C was found to cause significant degradation of the perovskite; lowering the deposition temperature can largely minimize the degradation, and 75 °C was found to be the best ALD temperature. Following this conclusion, inverted planar perovskite solar cells were prepared in ambient conditions with ALD Al2O3 interlayers. Indeed, cells with the interlayer deposited at 75 °C exhibited a significantly enhanced power conversion efficiency from 18.8% (champion 19.2%) to 20.0% (champion 20.4%). Photoluminescence measurements further evidence that the ALD layer can effectively passivate defect states at the perovskite surface. Considering the great representativeness and broad applicability of MAPbI3 and ALD Al2O3, the mechanism and strategy reported herein should be of significant value for the perovskite interface engineering in general.

Crystal growth control and analysis of organometal halide perovskite by vacuum deposition

Crystal growth control and analysis of organometal halide perovskite by vacuum deposition

Suppressed Mn2+ doping in organometal halide perovskite nanocrystals by formation of two-dimensional (CH3NH3)2MnCl4.

The additional (CH3NH3)2MnCl4 phase was observed upon increasing the Mn2+ dopant concentration, preventing the formation of Mn2+-doped CH3NH3PbCl3 NCs. We introduce an approach to avoid the formation of an impurity phase by switching the sequence for the addition of MnCl2 in the crystal growth process.

Novel design strategies for perovskite materials with improved stability and suitable band gaps.

The instability of organometallic halide perovskites is deemed a key hindrance hampering their commercial utilization in solar cell research. In the current work, we investigate and compare the dynamics properties of both free NH4+ and that immobilized in a NH4+-H2O-H2O-H2O-H2O-NH4+ network in a one-dimensional (1D) Pb-I skeleton. The simulations show that both the space occupancy and the hydrogen bonding formation ability of the A-site groups significantly influence the transition of the 1D NH4PbI3 perovskite materials to two-dimensional/three-dimensional (2D/3D) hybrid structures. Based on these observations, two possible pathways enhancing the structural stability of the perovskite materials are proposed. To narrow the big band gap introduced by the quantum confinement effect in the low-dimensional structures, large metal complexes are introduced as A-site groups considering that metal ions are involved in the formation of both conduction and valence bands of the perovskites. In this way, the band gap of the 1D perovskite materials is narrowed and the structural stability is enhanced accordingly. In addition, by optimizing the ratio of NH4+ to CH6N3+ (GUA+) groups, novel 2D/3D hybrid perovskite materials of (NH4)1-x(GUA)xPbI3 with improved stability and narrower band gaps are suggested as well. These structural design ideas hopefully illuminate the development of innovative and stable perovskite materials.

Review and perspective of materials for flexible solar cells

Abstract Thin-film flexible solar cells are lightweight and mechanically robust. Along with rapidly advancing battery technology, flexible solar panels are expected to create niche products that require lightweight, mechanical flexibility, and moldability into complex shapes, such as roof-panel for electric automobiles, foldable umbrellas, camping tents, etc. In this paper, we provide a comprehensive assessment of relevant materials suitable for making flexible solar cells. Substrate materials reviewed include metals, ceramics, glasses, and plastics. For active materials, we focus primarily on emerging new semiconductors including small organic donor/acceptor molecules, conjugated donor/acceptor polymers, and organometal halide perovskites. For electrode materials, transparent conducting oxides, thin metal films/nanowires, nanocarbons, and conducting polymers are reviewed. We also discuss the merits, weaknesses, and future perspectives of these materials for developing next-generation flexible photovoltaics.

Chloride gradient render carrier extraction of hole transport layer for high Voc and efficient inverted organometal halide perovskite solar cell

A carrier transport layer and its interfacial effects on an organometal halide perovskite light harvesting layer play an influential factor in either photovoltaic performance or long-term stability of a perovskite solar cell (OHPSC). Although the understandings of the carrier transport layers and interfacial effects on regular structured OHPSCs have been explored, knowledge of an interface between hole transport layer of NiOx and perovskite in an inverted OHPSC is still necessary to be developed. Here, we performed a universal NiOx film with the sequential passivation strategy of NiCl2 (SPS-NiCl2 treatment) for either wide bandgap or narrow bandgap of OHPSCs. The SPS-NiCl2 treated NiOx film not only implements the passivation at the perovskite layer/NiOx film interface but also confers itself a gradient energy level of valance band inducing by chloride. Comprehensive characterizations reveal that the SPS-NiCl2 treated NiOx film suppresses non-radiative recombination at the interface and enlarges the splitting of the quasi-Fermi level at the interfaces. The photoconversion efficiency (PCE) of the champion device comprised of the SPS-NiCl2 treated NiOx film can achieve 19.53% with a record Voc of 1.16 V, the lowest Voc deficit of 390 mV in NiOx based inverted OHPSCs. The corresponding devices with encapsulation also exhibit superior long-term stability, and over 80% of initial PCE can be maintained after 1500 h damp-heat test. This study sheds the light on managing the interfacial issues of an inverted OHPSC and offers a feasible path to develop a universal hole transport layer for perovskite layers with different energy bandgap.

Novel photoelectric material of perovskite-like (CH3)3SPbI3 nanorod arrays with high stability

Organometallic halide perovskite materials make great achievements in optoelectronic fields, especially in solar cells, in which the organic cations contain amine components. However, the amine with N−H bonds is easily hydrolyzed with moisture in the air, weakening the perovskite materials stability. It is desirable to develop other non-amine stable perovskite materials. In this work, sulfur-based perovskite-like (CH3)3SPbI3 nanorod arrays were fabricated by a solution-processed method, which can be indexed hexagonal crystal structure in the space group P63mc. The binding force is exceptionally strong between the non-amine (CH3)3S+ and [PbI6]4− octahedral, leading to high stability of (CH3)3SPbI3. The (CH3)3SPbI3 nanorod arrays can keep the morphology and crystal structure in an ambient atmosphere over 60 days. In addition, the (CH3)3SPbI3 nanorod arrays can offer direct charge transfer channels, which show excellent optoelectronic properties. The (CH3)3SPbI3 nanorod arrays-based solar cells with VOx hole transfer layers achieved a power conversion efficiency of 2.07% with negligible hysteresis. And the (CH3)3SPbI3 nanorod arrays were also effectively applied in photodetectors with interdigitated gold electrodes. This work demonstrates that sulfur-based perovskite-like (CH3)3SPbI3 is a novel promising stable compound with great potential for practical optoelectronic applications.

Potential replacement to lead: Alkali metal potassium and transition metal zinc in organo-metal halide perovskite materials

Following the plenty of research work on solving the issue of the popular and promising perovskite solar cells, this article is focused on methylammonium lead iodide (MAPbI3) and the alternative element which can perform as efficiently as lead does. Here, not only the different concentrations of the substitute element are investigated, but also higher stable perovskite is introduced as well. Several characterization techniques are used to assure the validity of the results. The large alkali metal potassium (K) and the transition metal zinc (Zn) are studied side by side. The results are interesting when the 5% concentration of the KI alloyed MAPbI3 shows considerable transport properties like 54 nm diffusion length, 245 × 10−9 Sm−1 photoconductivity, and significant majority and minority carrier mobility–lifetime product as compared to MAPbI3 but it becomes more interesting when after 60 days the 5% concentration of the aged ZnCl2 alloyed MAPbI3 turns into a leading element of this research by showing high stability among of all. The morphology investigations show both perovskite materials achieve large islands which are desired morphology in perovskite materials as well.

Electrode material and modification dependent performances of lateral photodetectors based on MAPbI3 single crystal

Abstract As new-type optoelectronic materials, organometallic halide perovskites (MAPbX3, X = I, Br, Cl) is an emerging solution-processable material with excellent optoelectronic performances that can be used for photodetectors and solar cells. Here, the effects of electrode materials (Au, Ag and Cu) on MAPbI3 single crystal photodetectors are investigated. Among the three MSCPs, the one with Au electrode exhibits the lowest dark current of 0.01 μA, which indicate that the electron drift current is the dominant component of dark current in MAPbI3 single crystals. While for the 650 nm wavelength illumination, the one with Ag electrode has largest R of 24.5 A/W and highest external quantum efficiency (EQE) of 4676% due to lower electron injection energy barrier at the contact. More importantly, the C60 layer and CuPc:C60 bulk heterojunction layer were used to modify the Au electrode respectively, a large R of 174 A/W and EQE of 33,354% is obtained by C60:CuPc-MSCP and C60-MSCP exhibits the largest PDR of 15,952 and D* of 4.5 × 1013 Jones. These results suggest that lateral photodetectors based on MAPbI3 single crystal modified by heterojunction buffer layer will obtain better optical performance.

Low-Temperature Energy Transfer via Self-Trapped Excitons in Mn2+-Doped 2D Organometal Halide Perovskites

We investigate the mechanisms of energy transfer in Mn2+-doped ethylammonium lead bromide (EA2PbBr4:Mn2+), a two-dimensional layered perovskite (2DLP), using cryogenic optical spectroscopy. At temperature T > 120 K, photoluminescence (PL) is dominated by emission from Mn2+, with complete suppression of band edge (BE) emission and self-trapped exciton (STE) emission. However, for T < 120 K, in addition to Mn2+ emission, PL is observed from BE and STEs. Data further reveal that for 20 K < T < 120 K, STEs form the most dominant routes in assisting energy transfer (ET) from 2DLP to Mn2+ dopants. However, at higher Mn2+ concentration, higher activation energies indicate defect states come into play, successfully competing with STEs for ET both from BE to STE states and from STE to Mn2+. Finally, using polarization-resolved spectroscopy, we demonstrate optical spin orientation of the Mn2+ ions via ET from 2DLP excitons at zero magnetic field. Our results reveal fundamental insights on the interactions between quantum confined charge carriers and dopants in organometal halide perovskites.

Rapid Annealing of Perovskite Solar Cells through Intense Pulsed Light

The perovskite solar technology has managed a fair amount of excitement since it was first introduced nearly ten years ago by Miyasaka’s group. The efficiency of small devices has increased rapidly from the 3.8% reported by Miyasaka to well over 20%. The efficiency now stands near that of the market dom,inated crystalline silicon solar cell, and the durability has been demonstrated for upwards of 1,000 hours. The organo-metal halide perovskite that is the core layer of the perovskite solar cell can be deposited using simple solution phase processes at low temperatures. The materials tend to have very high defect tolerances and hence have long excited charge carrier lifetimes. The excitement in the research and manufacturing communities’ centers around the simplicity of the device fabrication. Other layers within the stack also lead to simple solution phase deposition and low temperature processing.The deposition of the perovskite material generally involves a precursor chemistry that undergoes evaporation and crystallization. The crystallization typically involves thermal energy often at temperatures less than 100 oC. The activation energy of the organo-metal halide perovskite crystallization is very low especially when compared to that of amorphous silicon. Thus annealing is typically accomplished using low temperatures conduction (hot plates); however, several studies using radiative processes inducing much higher temperatures have also been established.These radiative processes include the intense pulsed light process (IPL). The IPL process involves the absorption of light energy into a thin film from an intense rapid pulse with a broad spectrum that can cover large areas. A thin film with absorptivity within the spectrum of light will absorb the energy, which is then rapidly transformed into thermal energy. The technology has been utilized in processing polymers, metals, metal oxides and semiconductors since the 1970’s and was recently reviewed specifically for materials in the photovoltaic technologies. The advantage of this technique for plastic substrates is that it can initiate very high temperatures within a thin film without damage to the underlying substrate.In this work we will describe the intense pulsed light process as it relates to the annealing of perovskite solar cells on plastic substrates on a roll-to-roll manufacturing line. The work will include the description of careful control of the IPL process parameters and the impact on the crystallization of the perovskite layer. The talk will concentrate on the tuning the IPL parameters to fit into high speed roll-to-roll applications. The films are characterized using UV-Vis spectroscopy, x-ray diffraction and scanning electron microscopy. Finally, devices built on the roll-to-roll platform will be discussed.

Self-Chargeable Lithium Ion Batteries with Perovskite Solar Cells

The necessity of developing new types of energy conversion and storage systems is evident by the rapidly decreasing fossil fuels and the continuously growing environmental issues. Coupling of lithium ion secondary batteries and photovoltaic cells is a reasonable candidate of new types of energy transform and storage system to reduce environmental concerns. Solar cells can ensure sustainable access to electrical power for charging LIBs anywhere around the world with no air pollution, hazardous waste or noise. Accordingly, the solar cell technology that generates electricity from the sunlight [1], could offer a viable approach to ‘self-charging’ of LIBs. However, most current solar cells, especially polymer solar cells, generally show low current densities and power conversion efficiencies, and to improve this we used perovskite solar cell for power supply of self-chargeable batteries. The recent availability of high-performance perovskite solar cells (PSCs) could not only facilitate the development of highly efficient (up to ~ 20 %) [2] and low cost solar cells for practical applications but also allow for the integration of PSCs into various energy systems. One of the important problems of PSC-LIB coupling is that when charging, the voltage of the PSCs must be higher than the operating voltage of the LIBs. If the above conditions are not established, PSCs may cause discharge of the LIBs. The most structurally stable LiFePO4(LFP)-LTO(Li4Ti5O12) batteries were used for coupling. In the case of LFP-LTO batteries, the OCV(open circuit voltage) value has a low voltage of 1.85V. It was confirmed that stable self-charging is achieved by connecting it to a manufactured PSCs pack with a charging voltage of 2.1 V. This work clearly indicates that the PSC-LIB units developed in this study hold great promise for potential applications as self-chargeable batteries to various portable electronics.In this study, we have fabricated lithium-ion pouch cell and coin cell based on LFP and (LTO) as a cathode and an anode, respectively. The cathode was fabricated by blending LFP powder with carbon black (Super P) and polyvinylidene difluoride (PVDF) at a weight ratio of 8:1:1. The anode was also prepared in the same way as the cathode using LTO powder and N-Methyl-2-pyrrolidone (NMP) was used as the solvent, respectively. The Electrolyte was used 1.2 M LiPF6 in a 1:1(v/v, %) mixture of ethylene carbonate and dimethyl carbonate. Pouch cell was fabricated with size of 3 cm x 5 cm. Ni and Al were used as cathode and anode lead tabs, respectively. The LIBs were assembled as the CR 2032 coin-type cells and pouch cells in an Ar-filled glove box. Structural properties and chemical compositions of LFP and LTO powders were investigated by x-ray diffraction (XRD) and x-ray photoelectron spectroscopy (XPS). The XRD pattern shows that the LiFePO4 cathode sheets are crystalline. The powder and surface morphologies of electrode sheet were characterized by FE-SEM. The primary particles size was identified as approximately 50 to 150 nm and the secondary particles size approximately 10mm. The electrochemical performance of LFP and LTO half cells and the LFP-LTO full cell were conducted by cyclic voltammetry and charge/discharge cycle tests at various current densities in the voltage ranges. Electrochemical impedance spectroscopy (EIS) was performed in frequency range from 0.1 Hz to 1 MHz, using 10 mV ac signals at room temperatures. LFP-LTO pouch cells showed good cycling stability a wide range of C-rates from 0.1 to 1.0 C with charge/discharge capacities of 142/138 mAhg-1 at 0.1 C and 152/144 mAhg-1 at 1.0 C. As a result, LiB-PSC coupling enabled the implementation of self-chargeable batteries. Finally, Perovskite solar cells and the Li-ion battery coupling were tested for a self-chargeable device.References Green, M. A. Solar cells: Operating Principles, Technology, and System Applications (Prentice-Hall, 1982). Lee, M. M., Teuscher, J ., Miyasaka, T., Murakami, T. N. & Snaith, H. J. Efficient hybrid solar cells based on meso-superstructured organometal halide perovskites. Science 338, 643-647 (2012)

Real-time observation of ion migration in halide perovskite by photoluminescence imaging microscopy

The stability issue of organometal halide perovskite optoelectronic devices, e.g. the current–voltage –hysteresis effect and the degradation of the device performance under external light/electric field, are closely related to the ionic movement inside perovskite materials. However, our understanding of ionic properties is still at a relatively preliminary stage, and the detailed dynamic process of ionic movement in perovskite materials is controversial. Photoluminescence (PL) imaging microscopy, as a versatile real-time observation and characterization method, has become a powerful tool to explore the ionic movement in different material systems. This review firstly summarizes the development of PL imaging microscopy for ion migration characterization in perovskite materials. Secondly, combined with the wide-field PL imaging microscope, the dynamic process of ion migration in halide perovskite materials is further discussed. Thirdly, the observations of the degradation process and the ion migration suppression in different perovskite optoelectronic devices characterized by PL imaging microscopy are also introduced. This methodology is of significance for the deep understanding of the interaction between carriers/defect ions in perovskite materials, and provides guidance to further improve the stability and efficiency of perovskite optoelectronic devices.