Organometal Halide Research Articles

Regulated perovskite crystallinity via green mixed antisolvent for efficient perovskite solar cells

Abstract Organometallic halide perovskite absorption layer with excellent surface morphology and large grain size is very important for fabricating efficient perovskite solar cells (PSCs). The antisolvent assisted crystallization (ASAC) has been proved to be an effective tactic to prepare compact and uniform polycrystalline perovskite film. However, most of the antisolvents currently used are highly toxic, and the perovskite crystallization process is difficult to control using pure antisolvent. Here, a green strategy is developed to improve the nucleation kinetics of perovskite precursor and the perovskite crystallization process. A mixed green antisolvent of ethyl acetate (EA)/petroleum ether (PE) is employed to regulate the crystallization of perovskite and replace traditional chlorobenzene (CB) and pure EA. With optimized proportion of EA and PE, the supersaturation degree is controlled via reducing the solvent evaporation rate and the nucleation rate, and it is good for crystal growth. Moreover, the grain growth rate and grain size are easily regulated, which contribute to the uniform, large grain and non-pinhole perovskite film. Correspondingly, the intrinsic perovskite solar cells based on CH3NH3PbI3 achieve a power conversion efficiency of 17.19% and an average efficiency of 16.25% (std 0.43).

Fast Charge Diffusion in MAPb(I1- xBr x)3 Films for High-Efficiency Solar Cells Revealed by Ultrafast Time-Resolved Reflectivity.

Organometallic halide perovskite solar cells such as MAPbI3 have shown great promise as a low-cost, high-efficiency photovoltaic candidate. Recent studies demonstrated that by substituting an appropriate amount I ions of MAPbI3 with Br ions, the device performance parameters, such as moisture stability or power conversion efficiency, can be further optimized. In this study, using time-resolved optical reflectivity to track the carrier dynamics in MAPb(I1- xBr x)3 films with different Br contents, we found that photocarriers in MAPb(I1- xBr x)3 films with x = 0.01 and 0.02 diffuse much faster than those in films with other Br contents. We suggest that the faster charge carrier diffusion benefits from larger crystal grain size. As a result, this suppresses electron and hole recombination and increases the carrier extraction efficiency, in agreement with the higher power conversion efficiency reported previously.

Hexagonal MASnI3 exhibiting strong absorption of ultraviolet photons

MASnI3, an organometallic halide, has great potential in the field of lead-free perovskite solar cells. Ultraviolet photons have been shown to generate deep trapping electronic defects in mesoporous TiO2-based perovskite, affecting its performance and stability. In this study, the structural, electronic, and optical properties of the cubic, tetragonal, and hexagonal phases of MASnI3 were studied using first-principles calculations. The results indicate that the hexagonal phase of MASnI3 possesses a larger indirect bandgap and a larger carrier effective mass along the c-axis compared to the cubic and tetragonal phases. These findings were attributed to enhanced electronic coupling and localization in the hexagonal phase. Moreover, the hexagonal phase exhibited high absorption of ultraviolet photons and high transmission of visible photons, particularly along the c-axis. These characteristics demonstrate the potential of hexagonal MASnI3 for application in multijunction perovskite tandem solar cells or as coatings in mesoporous TiO2-based perovskite solar cells to enhance ultraviolet stability and photon utilization.

Stable and scalable 3D-2D planar heterojunction perovskite solar cells via vapor deposition

The device stability and toxic-solvent-free processing are indispensable development to industrialize organometal halide perovskite solar cells. In this work, the conventional top surface of vapor prepared 3D-MAPbI3 was replaced by 2D-(BA)2(MA)n‐1PbnI3n+1 perovskite platelet via molecular substitution from MA (CH3NH) to BA (C4H9NH2). Our fully vapor process fabricated a 3D-2D perovskite heterojunction for solar cell application, realizing the power conversion efficiency as high as 16.50% and the average one as 15.40%. Furthermore, unencapsulated devices can sustain their 81% efficiency after 30 days under 55% RH, and also 74% efficiency after 30 days under 80 °C heat stress. Stability of humidity resistance arises from the reduced n value of 2D perovskite caused by the surface reaction of moisture, and the improvement of thermal stability comes from the buffering of MA molecules release. Meanwhile, the fabrication of large scale device with high efficiency is also demonstrated in our experiment.

Nickel Oxide as Efficient Hole Transport Materials for Perovskite Solar Cells

Organic–inorganic halide perovskite solar cells (PSCs) have achieved great success in recent years with a demonstrated power conversion efficiency (PCE) increasing rapidly from 3.8% to 22.3% for single junction devices. Most high‐performance PSCs consist of a perovskite absorber sandwiched between an electron transport layer (ETL) and a hole transport layer (HTL), which extracts electrons (holes) and blocks holes (electrons) from the absorber efficiently. Inorganic hole transport materials have extracted extensive attention due to their higher mobility and better stability. Particularly, the excellent hole selective transport property of nickel oxide (NiOx) has been highlighted by its recent application in organometallic halide PSCs, due to the favorable band alignment formed between the halide perovskite absorber and NiOx HTL. This comprehensive review summarizes the recent progress in the fabrication of NiOx films and their application in PSCs. Special attention is paid to the optoelectronic properties of NiOx films, which strongly depend on the synthesis methods and post‐treatment conditions, as well as the resulting photovoltaic device performance. Surface modification and doping strategies that are used to improve the optoelectronic properties of NiOx films and the resulting device performance are discussed with emphasis. Finally, a short perspective of NiOx‐based PSCs is also provided.

Multifunctional energy devices caused by ionic behaviors in perovskite-polymer hybrid films

Ionic movement in organometallic halide perovskite is generally looked as a very negative factor to perovskite based opto-electronic devices. It is considered one of the key inducements to efficiency and device stability. Here, we report that the ionic behaviors can lead to multifunctional energy utilizations by a single device with perovskite-polymer hybrid films. By manipulating and utilizing the ionic movement in the device, it acts as a solar cell and/or an ionic storage battery. Besides high efficiency for a solar cell, it has large specific capacities and fast charge ability of battery. It can be charged by light, which shows a promising device that directly transfers the light to electric energy for storage. This work paves a desirable way to achieve multifunctional devices for energy utilizations. It also indicates that ionic behaviors in perovskite have potential applications in various energy devices such as solar cells, ion battery and capacitors.

High-Detectivity Perovskite Light Detectors Printed in Air from Benign Solvents

Summary Here, we propose a simple and low-temperature approach for the synthesis of methylammonium lead halide perovskite inks based on sub-micrometer-sized particles with tunable band gap. The particles allow the formulation of inks in benign solvents, such as propan-2-ol, and the printing of the photoactive layers of planar photoconductors with scalable, large-area coating techniques under ambient conditions. When surface traps are passivated with [6,6]-phenyl-C61-butyric acid methyl ester fullerene (PCBM), a high photoconductive gain (exceeding 200 in the blue) and reduced noise produce record-high specific detectivities exceeding 7 × 1013 cmHz0.5/W and gain-bandwidth product values of 7.5 × 106 Hz. Given the extreme simplicity of the presented device architecture and the straightforward processing, yielding printable light detectors rivalling established technologies, the present work promises a short-term deployment of printed perovskite detectors in a multitude of opto-electronic applications.

Optimized opto-electronic and mechanical properties of orthorhombic methylamunium lead halides (MAPbX3) (X = I, Br and Cl) for photovoltaic applications

Organometallic halide perovskites (OMHPs) are absorbent materials, and can thus be employed in solar cells with power conversion efficiency (PEC) of 22% or higher. Using calculations, this work confirms earlier experimental findings and determines optimal properties to achieve maximum conversion efficiency for OMHPs. Values of energy band gap, density of states, absorption coefficient, refractive index, dielectric constant and elastic constants of orthorhombic methylamunium lead halides (MAPbX3) (X = I, Br and Cl) family are all calculated using Density Functional Theory (DFT) method with generalized gradient approximation (GGA). The stiffness of (MAPbX3) (X = I, Br and Cl) is investigated by calculating Young’s moduli E constants. Among the series, MAPbI3 is the stiffest material with Ex = 57.24 GPa. The perovskite family members are characterized by their energy band gap variation as: Eg MAPbI3, MAPbBr3, MAPbCl3 = 1.626, 2.207 and 2.748 eV, respectively. They also exhibit a remarkable absorption coefficient (α MAPbX3 = 105 cm−1) over a wide energy range particularly the visible spectrum [1.65–3.26 eV: 380–750 nm]. The anisotropy of optical properties (MAPbX3) (X = I, Br) is proven in the near and middle ultraviolet [3.1–5 eV] energy band.

Passivation of Grain Boundary by Squaraine Zwitterions for Defect Passivation and Efficient Perovskite Solar Cells.

Unavoidable defects in grain boundaries (GBs) are detrimental and critically influence the organometal halide perovskite performance and stability. To address this issue, semiconducting molecules have been employed to passivate traps along perovskite GBs. Here, we designed and synthesized three squaraine molecules (SQ) with zwitterionic structure to interact with under-coordinated Pb2+ and passivate Pb-I antisite defects. Density functional theory calculation shows symmetric O atoms could coordinate with perovskite grains simultaneously, resulting in continuous charge distribution at the SQ-perovskite interface. The energetic traps distribution in CH3NH3PbI3 perovskite is influenced significantly by the interaction between SQ and perovskite as analyzed by thermally stimulated current, in which the deep-level defects are considerably reduced due to efficient SQ passivation. In addition, we explore how SQ molecules with different energy offset affect the charge extraction, which is suggested to facilitate exciton separation at the perovskite-SQ interface. These benefits lead to enhanced perovskite efficiency from 15.77 to 18.83% with the fill factor approaching 80%, which is among the highest efficiency reported for MAPbI3 solar cells fabricated in an ambient environment at 60% relative humidity (RH). Considerable retardation of perovskite device degradation was achieved, retaining 90% of initial efficiency when kept 600 h at 60 ± 5% RH.

Vacuum co-deposited CH3NH3PbI3 films by controlling vapor pressure for efficient planar perovskite solar cells

Physical vacuum deposition (PVD) of organo-metal halide thin film has advantages of uniform and large scale deposition for emerging perovskite solar cell technology. Evaporation of organic halides such as CH3NH3I (MAI) is crucial for obtaining phase pure perovskite thin films. However, controlling the evaporation rate of MAI remains difficult owing to its “vapor-gas” like nature upon sublimation. Here we report, highly crystalline CH3NH3PbI3 (MAPbI3) films with micrometer grains can be prepared by gradually controlling the MAI vapor pressure rather than evaporation rate in co-evaporation method. When integrating an optimal film into a planar heterojunction solar cell, the best device achieved a power conversion efficiency of 15.74% by deploying TiO2 as an electron transport layer. This strategy alleviates the strict calibration of MAI evaporation rate and led to obtain phase pure MAPbI3 films for high performance solar cells.

Poly(methyl methacrylate) embedded perovskite films for improving solar cell performance

Semi-transparent photovoltaics are promising technologies to be applied for building-integrated elements and alternative windows. Notably, organometallic halide perovskite solar cells that have shown unprecedented power conversion efficiencies (PCEs) are highly attractive for integration as solar windows in buildings because of its low-cost manufacturing and tunable transparency in the visible regime. Here, we demonstrate an approach to fabricate void-passivated transparent perovskite films by utilizing a poly(methyl methacrylate) (PMMA) as a framework for crystal growth. The semitransparent perovskite film (average visible transmittance, AVT of ˜ 35.2%) is prepared from PbICl layer assisting sequential inter-diffusion process in which PMMA (PMMA dissolved in chlorobenzene) is added into CH3NH3I (MAI) solution. The presence of PMMA in perovskite films have a negligible effect on transparency, and the coverage and crystallinity of perovskite films are improved. Intensity-modulated photocurrent spectroscopy (IMPS) and time-correlated single photon counting (TCSPC) characterization further demonstrate that the PMMA embedded perovskite films have better carrier transport and enhanced carrier extraction rate. The semitransparent perovskite film based photovoltaic device with PMMA presents 15.46% efficiency with less hysteresis, which is mainly attributed to increased Jsc.

Size Dependence of Charge Carrier Dynamics in Organometal Halide Perovskite Nanocrystals: Deciphering Radiative Versus Nonradiative Components

In this work, we have synthesized and characterized three differently sized (3.1, 5.7, and 9.3 nm) methylammonium lead bromide (CH3NH3PbBr3) perovskite nanocrystals (PNCs) and passivated using (3-a...

Nonlinear optical properties of benzylamine lead(II) bromide perovskite microdisks in femtosecond regime

Organometal halide perovskites, an emerging class of direct bandgap semiconductors, are attractive candidates for many optoelectronic device applications. Herein, we have reported the nonlinear optical (NLO) properties of layered benzylamine lead(II) bromide perovskite microdisks (MDs) having a lateral dimension of a few micrometers and an average thickness of 35 nm, featuring narrow deep blue emission using the Z-scan technique. The NLO behavior switches over from saturable absorption to reverse saturable absorption under femtosecond laser pulse excitation. Our NLO studies have demonstrated tunable nonlinear behavior, which is attributed to the interplay between single and two-photon absorption by the carriers in the conduction band. Perovskite MDs exhibit an optical limiting behavior originating from the two-photon absorption mechanism.

Room-temperature synthesized formamidinium lead halide perovskite quantum dots with bright luminescence and color-tunability for efficient light emitting

Although organometal halide perovskites have garnered enormous interest as solution-processed absorbing layers in solar cells with power conversion efficiencies breaking 20%, nonetheless seldom effort has been made to light emitting devices fabricated from formamidinium lead halide perovskite quantum dots (QDs). Herein we demonstrate a facile and low-cost room-temperature synthesis of highly luminescent and color-tunable formamidinium lead halide perovskite QDs (FAPbX3, X = Cl, Br, and I or mixed halide systems Cl/Br and Br/I) using inexpensive commercial precursors. Through compositional engineering, the photoluminescence emission spectra are readily tunable in the range of 430–690 nm, covering most of the visible spectral region. The photoluminescence (PL) spectra of FAPbX3 QDs are of the characteristics including narrow emission line-widths of 16–34 nm largely, high quantum yields (QYs) of as high as 88%, and radiative lifetimes in the range of 19.8–53.0 ns. Furthermore, FAPbBr3 QDs exhibit single decay dynamics and nearly non-blinking at single-dot level have been observed. We also demonstrate the green light emitting diodes (LEDs) based on the as fabricated highly efficient perovskite QDs, which shows a high efficient luminance of 403 cd m−2, and a moderately high max external quantum efficiency (EQE) of 2.8% after optimizing the thickness of the perovskite QD layer. Additionally, the LEDs based on FAPbBr3 QDs demonstrate good stability of emission peaks with increasing applied voltage. These results indicate that FAPbX3 QDs have great potential application in lighting devices.

Stable Perovskite Solar Cells with Improved Hydrophobicity Based on BHT Additive

Remarkable progress has been achieved in organometallic halide perovskite solar cells (PSCs). However, poor environmental stability of PSCs has been proved to be challenging and needs further improvement. Here, a strategy of butylated hydroxytoluene (BHT) antioxidant additive with hydrophobic tert butyl and hydrophilic hydroxyl is developed to enhance the hydrophobicity, control nucleation rate and passivate the defect states of perovskite. The tert butyl group effectively prevents the penetration of moisture, as well as the formed hydrogen bond between hydroxyl group and iodine controls the assembly process of BHT and perovskite. By optimizing weight ratio of BHT and MAPbI3 in precursor solution, the power conversion efficiency of PSCs increased from 14.93% to 18.28% and greatly enhanced device stability compared with control device. We believe that our tactic of BHT additive can be applied to various perovskite films and will facilitate the realization of stable perovskite optoelectronics.

Intergrain Connection of Organometal Halide Perovskites: Formation Mechanism and Its Effects on Optoelectrical Properties.

Two-step processes are commonly used for the fabrication of organic-inorganic perovskite solar cells; they convert a PbI2 film to a perovskite film by dipping it in CH3NH3I (MAI) solution or spin-coating the MAI solution onto it. Dipping yields perovskite films with discrete and rough morphologies, whereas spinning yields films with smooth and connected morphologies. The residual MAI solution that remains after spinning is the key factor that governs the smoothness of the resulting morphology; centrifugal force has no influence. A perovskite layer forms as soon as the MAI solution is loaded onto the PbI2 film, then the MAI residues left after spinning dissolve this outermost perovskite layer. The subsequent recrystallization of the dissolved perovskites increases the connectivity and smoothness of the crystals. The final morphology is dependent on the degrees of dissolution and recrystallization, which can be controlled by varying the processing conditions. A post-thermal treatment can be applied to induce the additional dissolution of the perovskites, which results in an increase in the final grain size while maintaining good connectivity. Combining these results, we fabricated an optimal film morphology that gives rise to perovskite solar cells with improved efficiency. The optimal perovskite film has a smooth and connected morphology as well as better carrier transport than rough and discrete films. This article provides fundamental understanding of the mechanism of formation during two-step processes of connected perovskite morphologies that can guide the further development of two-step processes for the fabrication of optimal perovskite morphologies.

High-performance room-temperature NO2 sensors based on CH3NH3PbBr3 semiconducting films: Effect of surface capping by alkyl chain on sensor performance

High-performance CH3NH3PbBr3 (MAPbBr3) room-temperature NO2 sensors have been developed for the first time based on MAPbBr3 films prepared directly from perovskite precursor solutions (PSFs) and from surface-capped MAPbBr3 nanoparticles (NPFs). AFM images of these films revealed that the grain size of MAPbBr3 crystal in PSFs decreases with the increasing on the concentration of precursor solution, and XRD patterns demonstrate MAPbBr3 in PSFs packed along with (00l) crystal orientation. However, MAPbBr3 NPFs have no preferential crystal orientation despite the good crystallinity of MAPbBr3 NPs. I-V curves revealed different hysteresis loops for these two kinds of films. Sensors with PSF or NPF as semiconducting layer exhibited remarkable and reversible change when they are exposed to NO2 gas. Interestingly, NPF sensors show better sensing performance with a lower detection limit (0.1 ppm) and a larger sensitivity. Experiments also revealed that the selectivity of NPF is better than that of PSF sensors. More importantly, NPF sensors are much more stable than PSF sensors due to the protected surface of perovskite NPs. This study opens a new promising application for organometallic halide perovskite materials, especially for the protected perovskite NPs.

Enhanced Stability and Optical Absorption in the Perovskite-Based Compounds MA1-x Csx PbI3-y Bry.

Organometal halide perovskites have been outstanding from enormous amount of functional materials thanks to their highly cost-effective processability and prominent light harvesting capacity. Unfortunately, poor long-term stability seriously hinders their further development. The recent experimental observations suggest that Cesium is a promising candidate to enhance the stability of MAPbI3 . To explore the inherent mechanism, a first-principles investigation based on density functional theory, including hybrid functional, has been performed to analyze the electronic and optical properties of perovskite series MA0.75 Cs0.25 PbI3-y Bry . The results indicate that perovskite compound MA0.75 Cs0.25 PbI2 Br is significantly superior to the other doped series in terms of optical absorption within the visible-light range. In the meanwhile, both Bader charge analysis and charge density distribution show that the compound of MA0.75 Cs0.25 PbI2 Br is the most stable among all the doped perovskite series. Moreover, it is clearly manifested that the impact of cesium is mainly embodied in the enhancement of the stability rather than in the improvement of optical absorption. Our study sheds a new light on screening new-type light harvesting materials, and provides theoretical insight into the rationale design of highly efficient and stable photovoltaic devices based on these functional materials.

Blue Electrogenerated Chemiluminescence from Halide Perovskite Nanocrystals

Electrogenerated chemiluminescence (ECL) has been extensively used in ultrasensitive electroanalysis because it can be generated electrochemically without using expensive optics and light sources. Visible ECL emission can be obtained with a reasonable quantum yield and stability. Blue ECL is rare and often suffers from stability and poor quantum efficiency. Blue ECL emission at 473 nm from organometallic halide perovskite nanocrystals (PNCs), CH3NH3PbCl1.08Br1.92, is reported here for the first time using tripropylamine (TPrA) as co-reactant. The blue ECL emission peak resembles its photoluminescence peak position. In addition to this blue emission peak, the ECL spectra of CH3NH3PbCl1.08Br1.92 PNCs also showed a broad ECL peak at 745 nm. Generation of the second ECL peak at 745 nm from CH3NH3PbCl1.08Br1.92 PNCs was can be explained by the existence of surface trap states on as-synthesized PNC due to incomplete surface passivation. Halide anion tunability of ECL emission from CH3NH3PbX3 (X: Cl, Br, I) PNCs is also demonstrated. The fluorescence microscopy image of single PNC and stability of selected single PNCs are presented in this with simultaneous acquisition of fluorescence spectra using 405-nm laser excitation. The photoluminescence (PL) decay was described by PL lifetime (τ) of 1.2 ns. The effect of the addition of surfactants (oleic acid and n-octylamine) on the fluorescence intensity and stability of CH3NH3PbCl1.08Br1.92 PNCs is also discussed.

Semimetallicity and Negative Differential Resistance from Hybrid Halide Perovskite Nanowires

AbstractIn the rapidly progressing field of organometal halide perovskites, the dimensional reduction can open up new opportunities for device applications. Herein, taking the recently synthesized trimethylsulfonium lead triiodide (CH3)3SPbI3 perovskite as a representative example, first‐principles calculations are carried out and the nanostructuring and device application of halide perovskite nanowires are studied. It is found that the 1D (CH3)3SPbI3 structure is structurally stable, and the electronic structures of higher‐dimensional forms are robustly determined at the 1D level. Remarkably, due to the face‐sharing [PbI6] octahedral atomic structure, the organic ligand‐removed 1D PbI3 frameworks are also found to be stable. Moreover, the PbI3 columns avoid the Peierls distortion and assume a semimetallic character, contradicting the conventional assumption of semiconducting metal‐halogen inorganic frameworks. Adopting the bundled nanowire junctions consisting of (CH3)3SPbI3 channels with sub‐5 nm dimensions sandwiched between PbI3 electrodes, high current densities and large room‐temperature negative differential resistance (NDR) are finally obtained. It will be emphasized that the NDR originates from the combination of the near‐Ohmic character of PbI3‐(CH3)3SPbI3 contacts and a novel NDR mechanism that involves the quantum‐mechanical hybridization between channel and electrode states. This work demonstrates the great potential of low‐dimensional hybrid perovskites toward advanced electronic devices beyond actively pursued photonic applications.