Halide Semiconductors Research Articles

Hybrid Halide Perovskites: Discussions on Terminology and Materials.

Hybrid halide perovskites (HP) have emerged in the last decade as a new class of semiconductors with superior performances in photovoltaic and electronic devices. The literature about these halide semiconductors is abundant and a lot of names/expressions are used to define networks, structures, or materials. In this context, there is a need to offer some discussions about the relevance of using the word "perovskite" and the associated expressions ("RP" (Ruddlesden-Popper), "DJ" (Dion-Jacobson), "ACI" (alternating cations in the interlayer space), "hexagonal perovskites", "hollow perovskites", "d-HP" (deficient 3D HP),…). Moreover, the description of perovskite networks through elimination/substitution processes from the ABX3 structure will be compared to the known dimensional reduction concept.

Hybrid Halide Perovskites: Discussions on Terminology and Materials

AbstractHybrid halide perovskites (HP) have emerged in the last decade as a new class of semiconductors with superior performances in photovoltaic and electronic devices. The literature about these halide semiconductors is abundant and a lot of names/expressions are used to define networks, structures, or materials. In this context, there is a need to offer some discussions about the relevance of using the word “perovskite” and the associated expressions (“RP” (Ruddlesden–Popper), “DJ” (Dion–Jacobson), “ACI” (alternating cations in the interlayer space), “hexagonal perovskites”, “hollow perovskites”, “d‐HP” (deficient 3D HP),…). Moreover, the description of perovskite networks through elimination/substitution processes from the ABX3 structure will be compared to the known dimensional reduction concept.

Lead-Free Halide Perovskites and Perovskite Variants as Phosphors toward Light-Emitting Applications.

Lead halide perovskites have attracted tremendous research interests in the light-emitting field because of their high defect tolerance, solution processability, tunable spectrum, and efficient emission. In terms of luminescence types, both the narrowband emission derived from free-exciton (FE)and broadband white light emission from self-trapped exciton (STE) show great advantages in light-emitting applications. Despite the fascinating characteristics, their commercialization still suffers from the presence of toxic lead (Pb) and unsatisfactory stability. In this spotlight, we mainly focus on the lead-free candidates as phosphors for possible light-emitting applications. Thanks to the chemical diversity of metal halide perovskites and perovskite variants, many excellent lead-free light-emitting materials have recently been synthesized and characterized. We first classify these materials into three types according to material structures, including (1) double perovskites A2B(I)B(III)X6, (2) vacancy ordered perovskites A2B(IV)X6, (3) miscellaneous perovskite variants or halide semiconductors, which refer to halides without clear relation to the perovskite structure. We then highlight the importance of electronic dimensionality, defect passivation, and impurity doping in developing highly efficient perovskite-based emitters. We also discuss their applications in white light-emitting diodes (W-LED). Further challenges toward practical applications and potential applications are also included in a section on outlook and future challenges.

Novel Electrodes and Engineered Interfaces for Halide-Semiconductor Radiation Detectors

A new class of inorganic halide semiconductors are emerging as high-efficiency low-cost candidates for spectroscopic radiation detection. We report on solving one of the major challenges of these halide radiation detectors. At room temperature halide semiconductor detectors polarize under applied electric field, which not only degrades the charge collection efficiency of the detectors, but also promotes chemical reaction of the metal electrodes with the halide ions. This increases the metal-semiconductor interface noise and early failure of the spectroscopic detection capabilities of the device. We report on a solution to this challenge by application of novel electrodes on Thallium Bromide (TlBr) radiation detectors with virtually defect-free electrode-semiconductor interfaces, showing low noise and high detection stability for an extended period of time under accelerated ageing conditions. A number of TlBr detectors fabricated by this technique have demonstrated continuous stable detection performance (e.g. ±1% change in 662 keV gamma channel) for more than 4000 hours at room temperature. This report also shows continuously recorded 137Cs gamma radiation response of a unidirectionally-biased pixelated TlBr detector over more than 2 months (a total of 2880 data sets), which exhibit excellent stability. The developed approach has resulted in unprecedented low-noise stable performance of halide semiconductor detectors at room temperature, overcoming one of the major obstacles to the full consideration of TlBr (and other halide semiconductors) as a potentially low-cost replacement for Cadmium Zinc Telluride (CZT).

C6N2H18][SbI5]: A Lead-free Hybrid Halide Semiconductor with Exceptional Dielectric Relaxation.

[C6N2H18][SbI5] (1), a novel metal halide semiconductor with dielectric relaxation behavior, has been successfully synthesized, in which the cavities between the one-dimensional [SbI5] n2- polyanions are occupied by 2-methyl-1,5-pentanediammonium (2-MPDA) cations. 1 undergoes a reversible solid-state phase transition at TC = 192.7 K and shows a step-like dielectric anomaly. Interestingly, above TC, distinct dielectric dispersion in a wide temperature range is also witnessed. Remarkably, the solid state UV-vis diffuse reflectance spectrum of 1 exhibits a slightly gentler absorption edge at about 650 nm; that is, 1 adopts an indirect band gap with 1.92 electron volts. The combined narrow band gap, strong photoconductivity effect, and excellent dielectric relaxation might shed light on the exploitation of lead-free hybrid metal halide molecular materials with promising application prospects in thermoresponsive relaxation-type dielectric materials and photovoltaic conversion devices.

Rb4Ag2BiBr9: A Lead-Free Visible Light Absorbing Halide Semiconductor with Improved Stability.

Replacement of the toxic heavy element lead in metal halide perovskites has been attracting a great interest because the high toxicity and poor air stability are two of the major barriers for their widespread utilization. Recently, mixed-cation double perovskite halides, also known as elpasolites, were proposed as an alternative lead-free candidate for the design of nontoxic perovskite solar cells. Herein, we report a new nontoxic and air stable lead-free all-inorganic semiconductor Rb4Ag2BiBr9 prepared using the mixed-cation approach; however, Rb4Ag2BiBr9 adopts a new structure type (Pearson's code oP32) featuring BiBr6 octahedra and AgBr5 square pyramids that share common edges and corners to form a unique 2D layered non-perovskite structure. Rb4Ag2BiBr9 is also demonstrated to be thermally stable with the measured onset decomposition temperature of To = 520 °C. Optical absorption measurements and density functional theory calculations suggest a nearly direct band gap for Rb4Ag2BiBr9. Room temperature photoluminescence (PL) measurements show a broadband weak emission. Further, temperature-dependent and power-dependent PL measurements show a strong competition between multiple emission centers and suggest the coexistence of defect-bound excitons and self-trapped excitons in Rb4Ag2BiBr9.

Perspectives and Design Principles of Vacancy-Ordered Double Perovskite Halide Semiconductors

Halide perovskite semiconductors such as methylammonium lead iodide (CH3NH3PbI3) have achieved great success in photovoltaic devices; however, concerns surrounding toxicity of lead and material stability have motivated the field to pursue alternative perovskite compositions and structures. Vacancy-ordered double perovskites are a defect-ordered variant of the perovskite structure characterized by an antifluorite arrangement of isolated octahedral units bridged by A-site cations. In this Review, we focus upon the structure–dynamics–property relationships in vacancy-ordered double perovskite semiconductors as they pertain to applications in photovoltaics, and we propose avenues of future study within the context of the broader perovskite halide literature. We describe the compositional and structural motifs that dictate the optical gaps and charge transport behavior and discuss the implications of charge ordering, lattice dynamics, and organic–inorganic coupling upon the properties of these materials. The d...

Photovoltaic Effect in Indium(I) Iodide Thin Films

Halide semiconductors are undergoing a period of intense interest, buoyed by the outstanding optoelectronic properties of hybrid perovskites. It is worthwhile to consider whether related materials sharing unique aspects of the perovskite chemistry, such as the incorporation of metals that provide lone pair s electrons when integrated within the halide lattice, might yield semiconductors with comparable properties. One such metal that has not been widely studied in the context of photovoltaics is monovalent indium. In this work, we investigate a method of depositing films of indium(I) iodide, coupled with optoelectronic characterization and incorporation of these films into thin-film solar cells. We find that, although indium(I) iodide exhibits a photovoltaic effect, it is likely to be compromised by difficult-to-remove defects, e.g., iodine vacancies, which are expected to introduce recombination centers deep within the band gap.

What Defines a Perovskite?

What Defines a Perovskite?

Opportunities and Challenges in Perovskite Light-Emitting Devices

Lead halide perovskite semiconductors first gained prominence in photovoltaic technology, where they demonstrated impressive photon-to-electron conversion efficiencies in solution-processed devices. The benefits of perovskites are, however, more vividly expressed in light-emitting applications. This new class of halide semiconductors displays intense fluorescence, and possesses narrow emission line-width that could be easily tuned across the entire visible spectrum. These qualities allow excellent color reproduction that matches or exceeds the best commercial display devices. Here, we provide a brief review on the history and the new developments in perovskite light-emission and highlight some of the opportunities and challenges in this fledging technology. In particular, we study the advances in light-emitting diodes and photoluminescent devices and discuss the device lifespan, ion migration and spectral instability issues that would need to be circumvented in order for perovskites to be successful in ne...

Quantifying quantum dot ligand exchange

The intense colors emitted by lead halide semiconductor nanocrystals such as cesium lead bromide, CsPbBr3, make these materials promising candidates for application in television displays, solar cells, and light-emitting diodes. Standard synthesis procedures leave these nanocrystals, or quantum dots (QDs), covered with oleic acid and oleylamine ligands, which chemically stabilize the particles. But these so-called native ligands don’t stay put. They adsorb and desorb from these QDs more readily than they do from less ionic QDs such as cadmium selenide. Because the ligands affect QD optoelectronic properties—and because they often need to be replaced or altered after synthesis for applications—researchers need to quantify the ligand-exchange chemistry on these lead halide QDs. That’s exactly what Sara R. Smock, Travis J. Williams, and Richard L. Brutchey of the University of Southern California have done. Using NMR spectroscopy and other methods, the team showed that the native ligands undergo reversible e...

Tolerance Factor and Cooperative Tilting Effects in Vacancy-Ordered Double Perovskite Halides

Lattice dynamics and structural instabilities are strongly implicated in dictating the electronic properties of perovskite halide semiconductors. We present a study of the vacancy-ordered double pe...

A Versatile Thin-Film Deposition Method for Multidimensional Semiconducting Bismuth Halides

Despite the significant progress in fabricating hybrid organic–inorganic lead halide perovskite solar cells, their toxicity and low stability remain as major drawbacks, thereby hindering large-scale commercialization. Given the isoelectronic nature of lead(II) and bismuth(III) ions, potentially stable and nontoxic alternatives for efficient light absorption in thin-film photovoltaic (PV) devices may be found among bismuth-based halide semiconductors. However, high-quality polycrystalline films of many of these systems have not been demonstrated. Here we present a versatile and facile two-step coevaporation approach to fabricate A3Bi2I9 (A = Cs, Rb) and AgBi2I7 polycrystalline films with smooth, pinhole-free morphology and average grain size of >200 nm. The process involves an initial two-source evaporation step (involving CsI, RbI or AgI, and BiI3 sources), followed by an annealing step under BiI3 vapor. The structural, optical, and electrical characteristics of the resulting thin films are studied by X-r...

Ultrafast selective extraction of hot holes from cesium lead iodide perovskite films

Lead halide perovskites have some unique properties which are very promising for optoelectronic applications such as solar cells, LEDs and lasers. One important and expected application of perovskite halide semiconductors is solar cell operation including hot carriers. This advanced solar cell concept allows overcoming the Shockley–Queisser efficiency limit, thereby achieving energy conversion efficiency as high as 66% by extracting hot carriers. Understanding ultrafast photoexcited carrier dynamics and extraction in lead halide perovskites is crucial for these applications. Here, we clarify the hot carrier cooling and transfer dynamics in all-inorganic cesium lead iodide (CsPbI3) perovskite using transient absorption spectroscopy and Al2O3, poly(3-hexylthiophene-2,5-diyl) (P3HT) and TiO2 as selective contacts. We find that slow hot carrier cooling occurs on a timescale longer than 10 ps in the cases of CsPbI3/Al2O3 and CsPbI3/ TiO2, which is attributed to hot phonon bottleneck for the high photoexcited carrier density. An efficient ultrafast hole transfer from CsPbI3 to the P3HT hole extracting layer is observed. These results suggest that hot holes can be extracted by appropriate selective contacts before energy dissipation into the halide perovskite lattice and that CsPbI3 has a potential for hot carrier solar cell applications.

A class of Pb-free double perovskite halide semiconductors with intrinsic ferromagnetism, large spin splitting and high Curie temperature

We introduce perovskite halides into spintronics, and these materials are ferromagnetic semiconductors with large spin splitting and high Curie temperatures.

Anharmonicity and Octahedral Tilting in Hybrid Vacancy-Ordered Double Perovskites

The advantageous performance of hybrid organic–inorganic perovskite halide semiconductors in optoelectronic applications motivates studies of their fundamental crystal chemistry. In particular, recent studies have sought to understand how dipolar, dynamic, and organic cations such as methylammonium (CH3NH3+) and formamidinium (CH(NH2)2+) affect physical properties such as light absorption and charge transport. To probe the influence of organic–inorganic coupling on charge transport, we prepared the series of vacancy-ordered double perovskite derivatives A2SnI6, where A = Cs+, CH3NH3+, and CH(NH2)2+. Despite nearly identical cubic structures by powder X-ray diffraction, replacement of Cs+ with CH3NH3+ or CH(NH2)2+ reduces conductivity through a reduction in both carrier concentration and carrier mobility. We attribute the trends in electronic behavior to anharmonic lattice dynamics from the formation of hydrogen bonds that yield coupled organic–inorganic dynamics. This anharmonicity manifests as asymmetry ...

2D Lead Dihalides for High-Performance Ultraviolet Photodetectors and their Detection Mechanism Investigation.

2D halide semiconductors, a new family of 2D materials in addition to transition metal dichalcogenides, present ultralow dark current and high light conversion yield, which hold great potential in photoconductive detectors. Herein, a facile aqueous solution method is developed for the preparation of large-scale 2D lead dihalide nanosheets (PbF2-x Ix ). High-performance UV photodetectors are successfully implemented based on 2D PbF2-x Ix nanosheets. By modulating the components of halogens, the bandgap of PbF2-x Ix nanosheets can be tuned to meet varied detection spectra. The photoresponse dependence on incident power density, wavelength, detection environment, and temperature are systematically studied to investigate their detection mechanism. For PbI2 photodetectors, they are dominantly driven by a photoconduction mechanism and show a fast response speed and a low noise current density. A high normalized detectivity of 1.5 × 1012 Jones and an ION /IOFF ratio up to 103 are reached. On the other hand, PbFI photodetectors demonstrate a photogating mechanism mediated by trap states showing high responsivity. The novel 2D halide materials with wide bandgaps, superior detection performance, and facile synthesis process can enrich the Van der Waals solids family and hold great potential for a wide variety of applications in advanced optoelectronics.

Towards lead-free perovskite photovoltaics and optoelectronics by ab-initio simulations

Lead (Pb) free non-toxic perovskite solar cells have become more important in the commercialization of the photovoltaic devices. In this study the structural, electronic, optical and mechanical properties of Pb-free inorganic metal halide cubic perovskites CsBX3 (B = Sn, Ge; X = I, Br, Cl) for perovskite solar cells are simulated using first-principles Density Functional Theory (DFT). These compounds are semiconductors with direct band gap energy and mechanically stable. Results suggest that the materials have high absorption coefficient, low reflectivity and high optical conductivity with potential application in solar cells and other optoelectronic energy devices. On the basis of the optical properties, one can expect that the Germanium (Ge) would be a better replacement of Pb as Ge containing compounds have higher optical absorption and optical conductivity than that of Pb containing compounds. A combinational analysis of the electronic, optical and mechanical properties of the compounds suggests that CsGeI3 based perovskite is the best Pb-free inorganic metal halide semiconductor for the solar cell application. However, the compound with solid solution of CsGe(I0.7Br0.3)3 is found to be mechanically more ductile than CsGeI3. This study will also guide to obtain Pb-free organic perovskites for optoelectronic devices.

Main-Group Halide Semiconductors Derived from Perovskite: Distinguishing Chemical, Structural, and Electronic Aspects.

Main-group halide perovskites have generated much excitement of late because of their remarkable optoelectronic properties, ease of preparation, and abundant constituent elements, but these curious and promising materials differ in important respects from traditional semiconductors. The distinguishing chemical, structural, and electronic features of these materials present the key to understanding the origins of the optoelectronic performance of the well-studied hybrid organic-inorganic lead halides and provide a starting point for the design and preparation of new functional materials. Here we review and discuss these distinguishing features, among them a defect-tolerant electronic structure, proximal lattice instabilities, labile defect migration, and, in the case of hybrid perovskites, disordered molecular cations. Additionally, we discuss the preparation and characterization of some alternatives to the lead halide perovskites, including lead-free bismuth halides and hybrid materials with optically and electronically active organic constituents.

Photoinduced Emissive Trap States in Lead Halide Perovskite Semiconductors

The recent success of lead halide perovskites is given by their optimal primary optoelectronic properties relevant for photovoltaic and, more in general, for optoelectronic applications. However, a lack of knowledge about the nature of instabilities currently represents a major challenge for the development of such materials. Here we investigate the luminescence properties of polycrystalline thin films of lead halide perovskites as a function of the excitation density and the environment. First we demonstrate that in an inert environment photoinduced formation of emissive sub-band gap defect states happens, independently of the chemical composition of the lead halide semiconductor, which quenches the band-to-band radiative emission. Carrier trapping occurs in the subnanosecond time regime, while trapped carriers recombine in a few microseconds. Then, we show that the presence of oxygen, even in a very small amount, is able to compensate such an effect.