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High stability of flexible perovskite transparent conductive oxide film via van der Waals heteroepitaxy
Transparent conducting oxides (TCOs), such as indium-tin oxide (ITO) and indium gallium zinc oxide (IGZO), are essential components in the design of flexible optoelectronic devices. However, it is still a challenge to prepare low-cost flexible transparent conductive film for achieving the flexible functionality together with high optical transparency and electrical conductivity. Herein, fluorphlogopite (mica) is selected as the bendable and transparent substrate for the fabrication of low-cost perovskite CaVO 3 TCO epitaxial film. Benefitting from thin mica substrate with high Young’s modulus of 200 GPa, flexible CaVO 3 TCO epitaxial film exhibits high mechanical stability in terms of electrical conductivity and optical transparency after bending up to 10 5 cycles. The change of resistance can reflect the bending curvature and strain state. Finally, our results provide a new pathway to design superior mechanical flexibility and stability CaVO 3 TCO films for application in flexible, light-weight, and portable smart optoelectronic devices.
Synthesis and energy band characterization of hybrid molecular materials based on organic–polyoxometalate charge-transfer salts
A cationic amphiphilic molecule was synthesized and employed to encapsulate Lindqvist ([M{sub 6}O{sub 19}]{sup 2−}) and Keggin polyoxometalates ([SiM{sub 12}O{sub 40}]{sup 4−}, M=Mo, W) to form hybrid molecules through electrostatic interaction. The X-ray diffraction results illustrate that the former hybrids possess lamellar nanostructures in their solid states, while the latter hybrids show a cubic Im3m packing model with low intensities and poor long-range order. These hybrids have clear charge-transfer characters as shown in their deeper colors and UV–vis diffuse reflectance spectra. According to the reported reduction potentials of the POM acceptors and the band gaps deduced from their diffuse reflectance spectra, we have calculated the theoretical values of the lowest unoccupied molecular orbital (LUMO) position similar to the electron affinity (E{sub A}) of solid materials. Such energy level parameters are comparable to those of electroluminescence and electron-transport materials commonly used in organic electroluminescence devices. These organic–polyoxometalate charge-transfer salts have more advantages, such as higher decomposition temperatures, easier film fabrication and better electron affinities, which presumably would be used for electron-transport materials in the area of the electroluminescence. - Graphical abstract: Hybrid molecular materials with charge-transfer characters formed by a positively charged donor L and acceptors of the Lindqvist-type and Keggin-type POMs have lamellar and cubic structures in their solid state. - Highlights: • Charge-transfer salts are obtained by self-assembling POMs with an anthracene cation. • Their energy parameters are comparable to those of optoelectronic materials in OLEDs. • These POM-based hybrids could be applied in the area of optoelectronic devices.
Synthesis, microstructure, and electronic band structure properties of nanocrystalline neodymium-doped bismuth titanate ferroelectric films fabricated by the sol–gel method
Graphical abstract: X-ray diffraction indicates that the films are polycrystalline with the pure perovskite phase. Ten Raman active modes and one silicon substrate mode can be observed. The A{sub 1} {sub g}[Bi] at about 59 cm{sup −1} is unchanged whereas the B{sub 1} {sub g} and A{sub 1} {sub g}[Ti] phonon modes shift towards higher frequencies. Photoluminescence shows that the intensities of the two peaks increase with Nd concentration except the Bi{sub 3}NdTi{sub 3}O{sub 12} film, due to the smallest grain size and oxygen vacancy defects. Good optical functions of the BNT films are achieved due to the SE suggesting potential applications in ferroelectric-based optoelectronic devices. - Abstract: Bi{sub 4−x}Nd{sub x}Ti{sub 3}O{sub 12} (BNT) films with different Nd contents (from 0 to 1 with 0.25 intervals) are prepared by the sol–gel process. The Nd substitution effects on the preferred orientation, surface morphology, phonon modes, emission bands, andelectronic band structures of the BNT films are investigated by microscopy, Raman scattering, photoluminescence, and spectroscopic ellipsometry (SE) at room temperature. X-ray diffraction indicates that the films are polycrystalline with the pure perovskite phase. Ten Raman active modes and one silicon substrate mode can be observed. The A{sub 1} {sub g}[Bi] at about 59 cm{sup −1} is unchanged whereas the B{sub 1} {sub g} and A{sub 1} {sub g}[Ti] phonon modes shift towards higher frequencies. Photoluminescence shows that the intensities of the two peaks increase with Nd concentration except the Bi{sub 3}NdTi{sub 3}O{sub 12} film, due to the smallest grain size and oxygen vacancy defects. Good optical functions of the BNT films are achieved due to the SE suggesting potential applications in ferroelectric-based optoelectronic devices.
Widespread opportunities for materials engineering of nanocrystals: Synthetically tailorable effects and methodologies
Since their initial development, semiconductor nanocrystals (NCs) and metal nanoparticles (NPs) have become widely utilized in a variety of applications, including solar energy harvesting, lighting, catalysis, biomedical imaging, and targeted cancer treatments. Because of their reduced dimensionality, NCs and NPs have vastly different optical and electronic properties than their bulk counterparts, and the properties often depend heavily on the NC or NP surface. Surface defects and ligands alter the optoelectronic properties of NCs and NPs and affect their interaction with the environment, allowing tailoring of their properties without changing the overall chemical composition. In this review, we discuss research progress focusing on optoelectronic properties of NPs and NCs and their respective applications. We highlight how different surface treatments and ligand functionalization of nanomaterials can influence these applications.
Thin-film semiconductor perspective of organometal trihalide perovskite materials for high-efficiency solar cells
Organolead trihalide perovskites (OTPs) are arising as a new generation of low-cost active materials for solar cells with efficiency rocketing from 3.5% to over 20% within only five years. From “dye” in dye sensitized solar cells to “hole conductors” and “electron conductors” in mesoscopic heterojunction solar cells, there has been a dramatic conceptual evolution on the function of OTPs in photovoltaic devices. OTPs were originally used as dyes in Gratzel cells, achieving a high efficiency above 15% which, however, did not manifest the excellent charge transport properties of OTPs. An analogy of OTPs to traditional semiconductors was drawn after the demonstration of highly efficient planar heterojunction structure OTP devices and the observation of their excellent bipolar transport properties with a large diffusion length exceeding 100 nm in CH 3 NH 3 PbI 3 (MAPbI 3 ) polycrystalline thin films. Here, this review aims to provide the most recent advances in the understanding of the origin of the high OTP device efficiency. Specifically we will focus on reviewing the progress in understanding 1) the characterization of fantastic optoelectronic property of OTPs, 2) the unusual defect physics that originate the optoelectronic property; 3) morphology control of the perovskite film from fabrication process and film post-treatment, and 4) device interface and charge transport layers that dramatically impact device efficiency in the OTP thin film devices; 5) photocurrent hysteresis; 6) tandem solar cells; 7) stability of the perovskite materials and solar cell devices.
Zinc-Stabilized Manganese Telluride with Wurtzite Crystal Structure
Alloying of semiconductors with similar crystal structures is often used to fine-tune materials properties for optoelectronic applications. However, examples of semiconductor alloys between compounds with two different crystal structures, where properties are changing dramatically as a function of composition, are much rarer. Even more unusual are such heterostructural alloys where the structure and properties can be changed with only a small amount of substitution (<10%). One exception is yttria-stabilized zirconia (YSZ) with cubic fluorite crystal structure, used for a wide range of applications. Here, we report on experimental observation of such unusual behavior in zinc-stabilized manganese telluride (ZSMT), and on theoretical explanation of its compositional stabilization in the wurtzite crystal structure. Thin films of Mn 1-x Zn x Te alloys synthesized at 400 degrees C change from the nickeline (NC) to the wurtzite (WZ) crystal structure at x as low as 0.08. Theoretical calculations indicate that this structural transition occurs due to small MnTe polymorph energy differences, and strong preference of Zn atoms to adopt tetrahedral vs octahedral coordination environment. The calculated 1 eV increase of the band gap from NC to WZ structure is consistent with the experimental measurements on ZSMT films. These measurements also indicate 1000-10,000 times lower conductivity than the nickeline MnTe, due to low ZSMT hole concentration. Overall, the wide band gap and low hole density make ZSMT a promising candidate for p-type transparent transistors and other optoelectronic applications.
Overcoming the Photovoltage Plateau in Large Bandgap Perovskite Photovoltaics
Development of large bandgap (1.80-1.85 eV Eg) perovskite is crucial for perovskite-perovskite tandem solar cells. However, the performance of 1.80-1.85 eV Eg perovskite solar cells (PVKSCs) are significantly lagging their counterparts in the 1.60-1.75 eV Eg range. This is because the photovoltage (Voc) does not proportionally increase with Eg due to lower optoelectronic quality of conventional (MA,FA,Cs)Pb(I,Br)3 and results in a photovoltage plateau (Voc limited to 80% of the theoretical limit for ~1.8 eV Eg). Here, we incorporate phenyl- ethylammonium (PEA) in a mixed-halide perovskite composition to solve the inherent material-level challenges in 1.80-1.85 eV Eg perovskites. The amount of PEA incorporation governs the topography and optoelectronic properties of resultant films. Detailed structural and spectroscopic characterization reveal the characteristic trends in crystalline size, orientation, and charge carrier recombination dynamics and rationalize the origin of improved material quality with higher luminescence. With careful interface optimization, the improved material characteristics were translated to devices and Voc values of 1.30-1.35 V were achieved, which correspond to 85-87% of the theoretical limit. Using an optimal amount of PEA incorporation to balance the increase in Voc and the decrease in charge collection, a highest power conversion efficiency of 12.2% was realized. Our results clearly overcome the photovoltage plateau in the 1.80-1.85 eV Eg range and represent the highest Voc achieved for mixed-halide PVKSCs. This study provides widely translatable insights, an important breakthrough, and a promising platform for next- generation perovskite tandems.
Impact of Arsenic- and Indium-Terminated InGaAs Stressors on Carrier Confinement, Strain, Defects, and Transport Properties of Tensile-Strained Ge
Device-quality tensile-strained Ge (ε-Ge) grown on a large bandgap semiconductor with superior electrical and optical carrier confinement is essential for group-IV-based optoelectronics. Properties of ε-Ge active layers synthesized on In 0.24 Ga 0.76 As buffers with two different surface terminations─arsenic-rich and indium-rich─were experimentally demonstrated, highlighting the factors not considered in theoretical calculations. High-resolution X-ray diffraction and Raman spectroscopy analyses of these ε-Ge/In 0.24 Ga 0.76 As heterostructures confirmed the fully strained (1.6%) and partially relaxed (0.82%) nature of the ε-Ge bonded with arsenic-terminated (Ge As-terminated ) and indium-terminated (Ge In-terminated ) In 0.24 Ga 0.76 As stressors, respectively. High-resolution cross-sectional transmission electron microscopy showed a coherent, sharp, and fully strained ε-Ge/In 0.24 Ga 0.76 As heterointerface in the Ge As-terminated heterostructure, whereas microtwin defects were present in the Ge In-terminated heterostructure. These heterostructures were further characterized by evaluating the minority carrier lifetimes, high for Ge As-terminated (525 ns) and low for Ge In-terminated (69 ns), using the photoconductive decay technique. Moreover, band alignment was constructed using X-ray photoelectron spectroscopy, where the Ge As-terminated heterostructure revealed that both holes and electrons were confined within the ε-Ge active layer as a type-I band alignment with ΔE V, As-terminated = 0.22 eV and ΔE C,As-terminated = 0.38 eV. On the other hand, the Ge In-terminated heterostructure exhibited a type-II band alignment with ΔE V,In-terminated = – 0.02 eV and ΔE C,In-terminated = 0.53 eV. Furthermore, the magnetotransport properties revealed high mobility (321 cm 2 /(V s)) with single-electron transport in Ge As-terminated heterostructure and low mobility (3.34 cm 2 /(V s)) with multihole transport in the Ge In-terminated heterostructure. Therefore, preferring the ε-Ge on the arsenic-rich surface of In 0.24 Ga 0.76 As stressor over the indium-rich surface during material synthesis offers device-quality materials with high carrier lifetime and superior carrier confinement, which can provide an opportunity to fabricate efficient group-IV-based optoelectronic devices.
Orientation-Controlled Large-Area Epitaxial PbI 2 Thin Films with Tunable Optical Properties
Lead iodide (PbI 2 ) as a layered material has emerged as an excellent candidate for optoelectronics in the visible and ultraviolet regime. Micrometer-sized flakes synthesized by mechanical exfoliation from bulk crystals or by physical vapor deposition have shown a plethora of applications from low-threshold lasing at room temperature to high-performance photodetectors with large responsivity and faster response. However, large-area centimeter-sized growth of epitaxial thin films of PbI 2 with well-controlled orientation has been challenging. Additionally, the nature of grain boundaries in epitaxial thin films of PbI 2 remains elusive. Here, we use mica as a model substrate to unravel the growth mechanism of large-area epitaxial PbI 2 thin films. The partial growth leading to uncoalesced domains reveals the existence of inversion domain boundaries in epitaxial PbI 2 thin films on mica. Combining the experimental results with first-principles calculations, we also develop an understanding of the thermodynamic and kinetic factors that govern the growth mechanism, which paves the way for the synthesis of high-quality large-area PbI 2 on other substrates and heterostructures of PbI 2 on single-crystalline graphene. Here, the ability to reproducibly synthesize high-quality large-area thin films with precise control over orientation and tunable optical properties could open up unique and hitherto unavailable opportunities for the use of PbI 2 and its heterostructures in optoelectronics, twistronics, substrate engineering, and strain engineering.
Ferroelastic Domain-Induced Electronic Modulation in Halide Perovskites
Lead halide perovskites have emerged as promising materials for optoelectronic applications due to their exceptional properties. In the all-inorganic CsPbBr 3 perovskites, ferroelastic domains formed during phase transitions enhance bulk transport and emissive efficiency. However, the microscopic mechanisms governing carrier dynamics remain poorly understood. In this study, we employ cathodoluminescence (CL) and micro-Raman spectroscopy to image and investigate the electronic properties of the ferroelastic domain walls in CsPbBr 3 single crystals. CL measurements reveal a reduced emissive yield and a slight redshift in emission at the domain walls. Further, micro-Raman studies provide spatially resolved mapping of vibrational modes, exhibiting second-order phonon modes localized at the domain boundaries. In conclusion, our findings suggest that electron–phonon coupling at the twin domain walls plays a critical role in facilitating efficient charge separation, thereby improving the optoelectronic performance of the CsPbBr 3 perovskites.
Long-Range Exciton Diffusion in Two-Dimensional Assemblies of Cesium Lead Bromide Perovskite Nanocrystals
Förster resonant energy transfer (FRET)-mediated exciton diffusion through artificial nanoscale building block assemblies could be used as an optoelectronic design element to transport energy. However, so far, nanocrystal (NC) systems supported only diffusion lengths of 30 nm, which are too small to be useful in devices. In this work, we demonstrate a FRET-mediated exciton diffusion length of 200 nm with 0.5 cm 2 /s diffusivity through an ordered, two-dimensional assembly of cesium lead bromide perovskite nanocrystals (CsPbBr 3 PNCs). Exciton diffusion was directly measured via steady-state and time-resolved photoluminescence (PL) microscopy, with physical modeling providing deeper insight into the transport process. This exceptionally efficient exciton transport is facilitated by PNCs' high PL quantum yield, large absorption cross section, and high polarizability, together with minimal energetic and geometric disorder of the assembly. This FRET-mediated exciton diffusion length matches perovskites' optical absorption depth, thus enabling the design of device architectures with improved performances and providing insight into the high conversion efficiencies of PNC-based optoelectronic devices.
Lateral Heterostructures of Multilayer GeS and SnS van der Waals Crystals
Engineered heterostructures derive distinct properties from materials integration and interface formation. Two-dimensional crystals have been combined to form vertical stacks and lateral heterostuctures with covalent line interfaces. While thicker vertical stacks have been realized, lateral heterostructures from multilayer van der Waals crystals, which could bring the benefits of high-quality interfaces to bulk-like layered materials, have remained much less explored. In this work, we demonstrate the integration of anisotropic layered Sn and Ge monosulfides into complex heterostructures with seamless lateral interfaces and tunable vertical design using a two-step growth process. The anisotropic lattice mismatch at the lateral interfaces between GeS and SnS is relaxed via dislocations and interfacial alloying. Nanoscale optoelectronic measurements by cathodoluminescence spectroscopy show the characteristic light emission of joined high-quality van der Waals crystals. Spectroscopy across the lateral interface indicates valley-selective luminescence in the bulk SnS component that arises due to anisotropic electron transfer across the interface. The results demonstrate the ability to realize high-quality lateral heterostructures of multilayer van der Waals crystals for diverse applications, e.g., in optoelectronics or valleytronics.
Correlating Crystallographic Orientation and Ferroic Properties of Twin Domains in Metal Halide Perovskites
Metal halide perovskite (MHP) solar cells have attracted worldwide research interest. Although it has been well established that grain, grain boundary, and grain facet affect MHPs optoelectronic properties, less is known about subgrain structures. Recently, MHP twin stripes, a subgrain feature, have stimulated extensive discussion due to the potential for both beneficial and detrimental effects of ferroelectricity on optoelectronic properties. Furthermore, connecting the ferroic behavior of twin stripes in MHPs with crystal orientation will be a vital step to understand the ferroic nature and the effects of twin stripes. In this work, we studied the crystallographic orientation and ferroic properties of CH 3 NH 3 PbI 3 twin stripes, using electron backscatter diffraction (EBSD) and advanced piezoresponse force microscopy (PFM), respectively. Using EBSD, we discovered that the orientation relationship across the twin walls in CH 3 NH 3 PbI 3 is a 90° rotation about ($\bar{11}0$), with the (030) and (111) directions parallel to the direction normal to the surface. By careful inspection of CH 3 NH 3 PbI 3 PFM results including in-plane and out-of-plane PFM measurements, we demonstrate some nonferroelectric contributions to the PFM responses of this CH 3 NH 3 PbI 3 sample, suggesting that the PFM signal in this CH 3 NH 3 PbI 3 sample is affected by nonferroelectric and nonpiezoelectric forces. If there is piezoelectric response, it is below the detection sensitivity of our interferometric displacement sensor PFM (<0.615 pm/V). Overall, this work offers an integrated picture describing the crystallographic orientations and the origin of PFM signal of MHPs twin stripes, which is critical to understanding the ferroicity in MHPs.
Solution-Phase Synthesis of Cesium Lead Halide Perovskite Nanowires
Halide perovskites have attracted much attention over the past 5 years as a promising class of materials for optoelectronic applications. However, compared to hybrid organic-inorganic perovskites, the study of their pure inorganic counterparts, like cesium lead halides (CsPbX 3 ), lags far behind. Here, a catalyst-free, solution-phase synthesis of CsPbX 3 nanowires (NWs) is reported. These NWs are single-crystalline, with uniform growth direction, and crystallize in the orthorhombic phase. Both CsPbBr 3 and CsPbI 3 are photoluminescence active, with composition-dependent temperature and self-trapping behavior. These NWs with a well-defined morphology could serve as an ideal platform for the investigation of fundamental properties and the development of future applications in nanoscale optoelectronic devices based on all-inorganic perovskites.
Mechanism for rapid growth of organic–inorganic halide perovskite crystals
Optoelectronic devices based on hybrid halide perovskites have shown remarkable progress to high performance. However, despite their apparent success, there remain many open questions about their intrinsic properties. Single crystals are often seen as the ideal platform for understanding the limits of crystalline materials, and recent reports of rapid, high-temperature crystallization of single crystals should enable a variety of studies. Here we explore the mechanism of this crystallization and find that it is due to reversible changes in the solution where breaking up of colloids, and a change in the solvent strength, leads to supersaturation and subsequent crystallization. Here, we use this knowledge to demonstrate a broader range of processing parameters and show that these can lead to improved crystal quality. Lastly, our findings are therefore of central importance to enable the continued advancement of perovskite optoelectronics and to the improved reproducibility through a better understanding of factors influencing and controlling crystallization.
Chapter 15: Summary and Outlook
In the various chapters of this book, numerous characterization techniques are presented that can be applied to thin-film solar cells to determine (micro)structural, compositional, electrical, and optoelectronic properties. What has yet to be more fully elucidated is to what extent these characterization techniques can be combined, in a correlative way, to enhance the information gathered on materials and devices. Indeed, it is valuable to consider combining techniques to verify the relevance of the measured materials properties or to obtain them on different length scales-to compare surface with bulk properties, or to correlate structure and composition of materials with electrical and optoelectronic properties.
Crystallographically uniform arrays of ordered (In)GaN nanocolumns
In this work, through a comparative study of self-assembled (SA) and selective area grown (SAG) (In)GaN nanocolumn (NC) ensembles, we first give a detailed insight into improved crystallographic uniformity (homogeneity of crystallographic tilts and twists) of the latter ones. The study, performed making use of: reflective high energy electron diffraction, X-ray diffraction and scanning electron microscopy, reveals that unlike their SA counterparts, the ensembles of SAG NCs show single epitaxial relationship to both sapphire(0001) and Si(111) underlying substrates. In the second part of the article, making use of X-ray diffraction, we directly show that the selective area growth leads to improved compositional uniformity of InGaN NC ensembles. This further leads to improved spectral purity of their luminescence, as confirmed by comparative macro-photoluminescence measurements performed on SA and SAG InGaN NC ensembles. An improved crystallographic uniformity of NC ensembles facilitates their integration into optoelectronic devices, whereas their improved compositional uniformity allows for their employment in single-color optoelectronic applications.