Self-Assembled Perovskite Nanoislands on CH3NH3PbI3 Cuboid Single Crystals by Energetic Surface Engineering

Organometal perovskite single crystals have been recognized as a promising platform for high-performance optoelectronic devices, featuring high crystallinity and stability. However, a high trap density and structural nonuniformity at the surface have been major barriers to the progress of single crystal-based optoelectronic devices. Here, the formation of a unique nanoisland structure is reported at the surface of the facet-controlled cuboid MAPbI₃ (MA = CH₃NH₃⁺) single crystals through a cation interdiffusion process enabled by energetically vaporized CsI. The interdiffusion of mobile ions between the bulk and the surface is triggered by thermally activated CsI vapor, which reconstructs the surface that is rich in MA and CsI with reduced dangling bonds. Simultaneously, an array of Cs-Pb-rich nanoislands is constructed on the surface of the MAPbI₃ single crystals. This newly reconstructed nanoisland surface enhances the light absorbance over 50% and increases the charge carrier mobility from 56 to 93 cm² V⁻¹ s⁻¹. As confirmed by Kelvin probe force microscopy, the nanoislands form a gradient band bending that prevents recombination of excess carriers, and thus, enhances lateral carrier transport properties. This unique engineering of the single crystal surface provides a pathway towards developing high-quality perovskite single-crystal surface for optoelectronic applications.     

A promising unisource thermal evaporation for in situ fabrication of organolead halide perovskite CH3NH3PbI3 thin film

In this work, a new method of fabricating organolead halide perovskite CH₃NH₃PbI₃ thin film by unisource thermal evaporation was proposed, which ensured high‐quality film with composition, crystal‐structure homogeneity, full surface coverage, well‐defined grain structure, high crystalline, and reproducibility, suggesting its promising applicability for significant optimization in efficient. Copyright © 2015 John Wiley & Sons, Ltd.     

Peptide‐Passivated Lead Halide Perovskite Nanocrystals Based on Synergistic Effect between Amino and Carboxylic Functional Groups

A new strategy has been developed using peptides with amino and carboxylic functional groups as passivating ligands to produce methyl ammonium lead bromide (CH₃NH₃PbBr₃) perovskite nanocrystals (PNCs) with excellent optical properties. The well‐passivated PNCs can only be obtained when both amino and carboxylic groups are involved, and this is attributed to the protonation reaction between ? NH₂ and ? COOH that is essential for successful passivation of the PNCs. To better understand this synergistic effect, peptides with different lengths have been studied and compared. Due to the polar nature of peptides, peptide‐passivated PNCs (denoted as PNCs_peptide) aggregate and precipitate from nonpolar toluene solvent, resulting in a high product yield (≈44%). Furthermore, the size of PNCs_peptide can be varied from ≈3.9 to 8.6 nm by adjusting the concentration of the peptide, resulting in tunable optical properties due to the quantum confinement effect. In addition, CsPbBr₃ PNCs are also synthesized with peptides as capping ligands, further demonstrating the generality and versatility of this strategy, which is important for generating high quality PNCs for photonics applications including light‐emitting diodes, optical sensing, and imaging.     

Centimeter‐Sized Cs4PbBr6 Crystals with Embedded CsPbBr3 Nanocrystals Showing Superior Photoluminescence: Nonstoichiometry Induced Transformation and Light‐Emitting Applications

An HBr‐assisted slow cooling method is developed for the growth of centimeter‐sized Cs₄PbBr₆ crystals. The obtained crystals show strong green photoluminescence with absolute photoluminescence quantum yields up to 97%. More importantly, the evolution process and structural characterizations support that the nonstoichiometry of initial Cs₄PbBr₆ crystals induce the formation of nanosized CsPbBr₃ nanocrystals in crystalline Cs₄PbBr₆ matrices. Furthermore, high efficiency and wide color gamut prototype white light‐emitting diode devices are also demonstrated by combining the highly luminescent Cs₄PbBr₆ crystals as green emitters and commercial K₂SiF₆:Mn⁴⁺ phosphor as red emitters with blue emitting GaN chips. The optimized devices generate high‐quality white light with luminous efficiency of ≈151 lm W⁻¹ and color gamut of 90.6% Rec. 2020 at 20 mA, which is much better than that based on conventional perovskite nanocrystals. The combination of improved efficiency and better stability with comparable color quality provides an alternative choice for liquid crystal display backlights.     

High‐Order Shift Current Induced Terahertz Emission from Inorganic Cesium Bromine Lead Perovskite Engendered by Two‐Photon Absorption

High‐order nonlinear optical phenomena are interesting from a fundamental point of view as they reveal intrinsic symmetries of the materials. Potentially they can also be used for practical optoelectronic applications. High‐order shift current is one of these phenomena, and yet it has never been detected in experiments, primarily due to the difficulty in conventional contact detection. In this work, the shift current due to the two‐photon absorption (TPA) from all‐inorganic perovskite CsPbBr₃ is first observed by a contactless and nondestructive terahertz (THz) emission method. The results reveal that the THz emission is dominated by a high‐order shift current (fourth‐order nonlinear optical effect) with the below‐bandgap photon energy of femtosecond laser excitation. The high‐order shift current origins from the broken inversion symmetry induced by the dynamic stereochemical activity of the Pb²⁺ lone pair. A microscopic TPA‐assisted nonlinear optical model is presented to describe the photophysical process of the THz emission. The model matches well with the quadratic pump fluence and angular dependence of the THz emission. This work can not only open a new venue for the all‐inorganic perovskite‐based nonlinear optoelectronics and THz devices, but also afford a THz technology for the high‐order nonlinear effect analysis.     

PbI2掺杂对CH3NH3PbI3光电性能的影响

采用溶液生成法制备了有机铅卤化钙钛矿(CH3 NH3PbI3)晶体粉末,并以过量的PbI2对其进行掺杂,采用X射线衍射谱(XRD)技术研究了掺杂前后样品的晶体结构变化.表面光电压谱(SPS)和相位谱(PS)显示掺杂前后的CH3 NH3 PbI3均为p型半导体,但后者有更强的光伏响应.场诱导表面光电压谱(FISPS)表明:当加正电场时,掺杂前后的CH3NH3PbI3均表现为p型半导体的载流子特性,当加负偏压时掺杂后的CH3NH3PbI3易形成反型层,出现光伏反转,且外加负偏压越大,光伏反转区域越大,表现出双极导电特性.     

几种新型非线性光学晶体的性能和应用

本文介绍LBO、BBO、KTP、LAP晶体的物理、化学、机械和抗潮等方面的性质,给出了它们折射率的Sellmeier方程、非线性光学系数、几种常见波长的位相匹配的计算及测量结果。并对它们的性能进行了比较和讨论。还介绍了用这些晶体制成的非线性光学器件的性能和指标,最后介绍了LBO、BBO、KTP晶体在光学参量振荡和在“飞秒’脉宽激光测量上的应用。     

三光子吸收情况下的激光相移研究

对三光子吸收情况下的非线性相移进行研究 ,由它的耦合方程研究非线性相移在介质出射面处的横向分布。由此比较与双光子吸收情况下的非线性相移的不同。     

CH3CSNH2/NH4OH钝化GaInAsSb/GaSb PIN红外探测器

引入一种新的低毒化合物CH3CSNH2/NH4OH对 GaInAsSb化合物探测器的表面进行了钝化处理，可使其暗电流降低一个数量级，动态电阻增大25倍多，且钝化83天后保持良好的钝化效果，取得了与（NH4)2S溶液一样理想的钝化效果. 并采用AES和XPS对钝化前后的GaInAsSb材料进行了分析.     

Enhancing the Optoelectronic Performance of Perovskite Solar Cells via a Textured CH3NH3PbI3 Morphology

Perovskite‐based solar cells are generally assembled as planar structures comprising a flat organoammonium metal halide perovskite layer, or mesoscopic structures employing a mesoporous metal‐oxide scaffold into which the perovskite material is infiltrated. To present, little attention has been directed toward the texturing of the perovskite material itself. Herein, a textured CH₃NH₃PbI₃ morphology formed through a thin mesoporous TiO₂ seeding layer and a gas‐assisted crystallization method is reported. The textured morphology comprises a multitiered nanostructure, which allows for significant improvements in the light harvesting and charge extraction performance of the solar cells. Due to these improvements, average short‐circuit current densities for a batch of 28 devices are in excess of 22 mA cm^?2, and the maximum recorded power conversion efficiency is 16.3%. The performance gains concomitant with this textured CH₃NH₃PbI₃ morphology provide further insights into how control of the perovskite microstructure can be used to enhance the cell performance.     

Layered (C6H5CH2NH3)2CuBr4 Perovskite for Multilevel Storage Resistive Switching Memory

Although there have been attempts to use non‐lead based halide perovskite materials as insulating layers for resistive switching memory, the ratio of low resistance state (LRS) to high resistance state (HRS) ( = ON/OFF ratio) and/or endurance is reported to be mostly lower than 10³. Resistive switching memory characteristics of layered (BzA)₂CuBr₄ (BzA = C₆H₅CH₂NH₃) perovskite with high ON/OFF ratio and long endurance are reported here. The X‐ray diffraction (XRD) pattern of the deposited (BzA)₂CuBr₄ layer shows highly oriented (00l) planes perpendicular to a Pt substrate. An Ag/PMMA/(BzA)₂CuBr₄/Pt device shows bipolar switching behavior. A forming step at around +0.5 V is observed before the repeated bipolar switching at the SET voltage of +0.2 V and RESET voltage of ‐0.3 V. The ON/OFF ratio as high as =10⁸ is monitored along with an endurance of ≈2000 cycles and retention time over 1000 s. The high ON/OFF ratio enables multilevel storage characteristics as confirmed by changing the compliance currents. Ohmic conduction at the LRS and Schottky emission at HRS are involved in electrochemical metallization process. The bipolar resistive switching property is retained after storing the device at ambient condition under relative humidity of about 50% for 2 weeks, which indicates that (BzA)₂CuBr₄ is stable memory material.     

Deciphering the NH4PbI3 Intermediate Phase for Simultaneous Improvement on Nucleation and Crystal Growth of Perovskite

The NH₄PbI₃‐based phase transformation is realized by simply adding NH₄I additive, in order to simultaneously control perovskite nucleation and crystal growth. Regarding the nucleation process, the NH₄⁺ with small ionic radius preferentially diffuses into the [PbI₆]^4? octahedral layer to form NH₄PbI₃, which compensates the lack of CH₃NH₃I (MAI) precipitation. The generation of NH₄PbI₃ intermediate phase results in extra heterogeneous nucleation sites and reduces the defects derived from the absence of MA⁺. Regarding the crystal growth process, the cation exchange process between MA⁺ and NH₄⁺, instead of the MAs directly entering, successfully retards the crystal growth. Such NH₄PbI₃ consumption process slows down the crystal growth, which effectively improves the perovskite quality with lowered defect density. The cooperation of these two effects eventually leads to the high‐quality perovskite with enlarged grain size, prolonged photoluminescence lifetime, lowered defect density, and increased carrier concentration, as well as the finally enhanced photovoltaic performance. Moreover, NH₃ as a byproduct further facilitates the proposed transformation process and no external residue remains even without any post‐treatment. Such methodology of introducing a novel phase transformation to simultaneously control nucleation and crystal growth processes is of universal significance for further devotion in the foreseeable perovskite solar cells (PSCs) evolution.     

Origin of Hysteresis in CH3NH3PbI3 Perovskite Thin Films

Organic–inorganic hybrid perovskite solar cells are attracting the attention of researchers owing to the high level of performance they exhibit in photovoltaic device applications. However, the attainment of an even higher level of performance is hindered by their anomalous current–voltage (I–V) hysteresis behavior. Even though experimental and theoretical studies have suggested that the perovskite materials may have a ferroelectric nature, it is still far from being fully understood. In this study, the origin of the hysteresis behavior in CH₃NH₃PbI₃ perovskite thin films is investigated. The behavior of ferroelectricity using piezoresponse force microscopy is first examined. Then, by comparing the scan‐rate‐dependent nano/macroscopic I–V curves, it is found that ion migration assisted by the grain boundaries is a dominant origin of I–V hysteresis from a macroscopic viewpoint. Consequently, the observations suggest that, even though ferroelectricity exists in the CH₃NH₃PbI₃ perovskite materials, ion migration primarily contributes to the macroscopic I–V hysteresis. The presented results can provide fundamental guidelines to the resolution of hysteresis issues in organic–inorganic hybrid perovskite materials.     

一种新型材料CH_3NH_3Pb_(0.5)Sn_(0.5)Br_3的制备与表征

采用溶液法制备一种新型有机无机杂化钙钛矿结构CH_3NH_3Pb_(0.5)Sn_(0.5)Br_3材料,采用X线衍射、热重分析、紫外可见光吸收、霍尔测试对其结构、稳定性、光电性能进行表征。结果表明,这种新型材料具有部分层状结构,在251.6℃下分解,同时光吸收范围在紫外和可见光范围,是一种p型半导体材料,载流子迁移率为63.1cm/V·s。     

High‐Quality Cuboid CH3NH3PbI3 Single Crystals for High Performance X‐Ray and Photon Detectors

Organic–inorganic halide hybrid perovskite materials are promising materials for X‐ray and photon detection due to their superior optoelectronic properties. Single‐crystal (SGC) perovskites have increasingly attracted attention due to their substantially low crystal defects, which contribute to improving the figures of merit of the devices. Cuboid CH₃NH₃PbI₃ SGC with the naturally favorable geometry for device fabrication is rarely reported in X‐ray and photon detection application. The concept of seed dissolution‐regrowth to improve crystal quality of cuboid CH₃NH₃PbI₃ SGC is proposed and a fundamental understanding of the nucleation and growth is provided thermodynamically. The X‐ray detector fabricated from cuboid CH₃NH₃PbI₃ SGC demonstrates the firstly reported high sensitivity of 968.9 µC^?1 Gy^?1 cm^?2 under ?1 V bias. The results also show that the favorable crystal orientation and high quality of cuboid CH₃NH₃PbI₃ leads to better responsivity and faster response speed than the more common dodecahedral CH₃NH₃PbI₃ in photodetection. Consequently, the work paves a way to synthesize high‐quality perovskite SGCs and benefits the application of MAPbI₃ SGCs with preferred crystal orientation and favorable crystal geometry for emerging device applications.     

Giant Nonlinear Optical Absorption of Ion-Intercalated Tin Disulfide Associated with Abundant In-Gap Defects

Herein it is reported that electrochemical ion-intercalation is a convenient and effective strategy toward materials with giant nonlinear optical (NLO) absorption. Alkali-metal ions (i.e., Li⁺, Na⁺, K⁺) are electrochemically intercalated into SnS₂ nanosheets. All ion-intercalated samples exhibit remarkably enhanced optical nonlinearity compared with an untreated sample, and Li-intercalated SnS₂ (Li_0.952Sn^II_0.398Sn^IV_0.563S₂) possesses optimized strong NLO performance. Li_0.952Sn^II_0.398Sn^IV_0.563S₂ exhibits strong saturable absorption, and the corresponding nonlinear absorption coefficient (β_eff) is -1.7 × 10⁴ cm GW^–1 for the laser excitation at 515 nm. Li_0.952Sn^II_0.398Sn^IV_0.563S₂ shows prominent reverse saturable absorption with the laser excitation at 800 nm (β_eff: 2.8 × 10⁴ cm GW^–1) and 1030 nm (β_eff: 1.4 × 10⁴ cm GW^–1). All β_eff values are larger than most of the reported inorganic NLO materials at corresponding wavelengths. The optical limiting threshold of Li_0.952Sn^II_0.398Sn^IV_0.563S₂ is 8 × 10^–4 J cm^–2, two orders of magnitude smaller (better) than the bench-mark composite (e.g., SWNT-NH-TPP). Ion intercalation introduces abundant in-gap defects. The excitation of electrons in in-gap states to conduction band intensifies the Pauli-blocking effect and therefore promotes the saturable absorption under the 515 nm laser excitation, while the in-gap defect states acting as effective excitation pathway facilitate excited-state absorption for 800 and 1030 nm laser.     

In Situ Fabrication of Highly Luminescent Bifunctional Amino Acid Crosslinked 2D/3D NH3C4H9COO(CH3NH3PbBr3)n Perovskite Films

The perovskite quantum dots are usually synthesized by solution chemistry and then fabricated into film for device application with some extra process. Here it is reported for the first time to in situ formation of a crosslinked 2D/3D NH₃C₄H₉COO(CH₃NH₃) _n Pb_n Br_3n perovskite planar films with controllable quantum confine via bifunctional amino acid crosslinkage, which is comparable to the solution chemistry synthesized CH₃NH₃PbBr₃ quantum dots. These atomic layer controllable perovskite films are facilely fabricated and tuned by addition of bi‐functional 5‐aminovaleric acid (Ava) of NH₂C₄H₉COOH into regular (CH₃NH₃)PbBr₃ (MAPbBr₃) perovskite precursor solutions. Both the NH₃⁺ and the COO^? groups of the zwitterionic amino acid are proposed to crosslink the atomic layer MAPbBr₃ units via Pb? COO bond and ion bond between NH₃⁺ and [PbX₆] unit. The characterizations by atomic force microscopy, scanning electron microscopy, Raman, and photoluminescence spectroscopy confirm a successful fabrication of ultrasmooth and stable film with tunable optical properties. The bifunctional crosslinked 2D/3D Ava(MAPbBr₃)_n perovskite films with controllable quantum confine would serve as distinct and promising materials for optical and optoelectronic applications.     

溶剂热处理法制备的钙钛矿CH3NH3PbI3薄膜的微观形貌与电池性能的研究

近年来研究表明,通过增大晶粒尺寸和减少晶界数量可以有效减小钙钛矿太阳能电池的漏电流和增大并联电阻,极大地增加其能量转化效率。溶剂热处理工艺是一种利用溶解再结晶的原理增大薄膜晶粒的实用工艺,可用于制备大晶粒高质量的多晶薄膜。本文制备了不同溶剂热处理时长的旋涂制备的钙钛矿CH₃NH₃PbI₃薄膜,利用SEM和XRD分析了其形貌和晶体结构的变化,探索了薄膜晶粒形貌与电池性能的对应关系,应用优化后的溶剂热处理工艺成功制备出大晶粒、高性能的钙钛矿薄膜。实验表明,溶剂热处理法制备的钙钛矿CH₃NH₃PbI₃薄膜平均晶粒尺寸接近3μm,较普通热处理方法制备的薄膜晶粒尺寸(约300 nm)有显著增大。