The excellent catalytic activity of metallic MoS2 edges for the hydrogen evolution reaction (HER) has led to substantial efforts towards increasing the edge concentration. The 2H basal plane is less ...active for the HER because it is less conducting and therefore possesses less efficient charge transfer kinetics. Here we show that the activity of the 2H basal planes of monolayer MoS2 nanosheets can be made comparable to state-of-the-art catalytic properties of metallic edges and the 1T phase by improving the electrical coupling between the substrate and the catalyst so that electron injection from the electrode and transport to the catalyst active site is facilitated. Phase-engineered low-resistance contacts on monolayer 2H-phase MoS2 basal plane lead to higher efficiency of charge injection in the nanosheets so that its intrinsic activity towards the HER can be measured. We demonstrate that onset potentials and Tafel slopes of ∼-0.1 V and ∼50 mV per decade can be achieved from 2H-phase catalysts where only the basal plane is exposed. We show that efficient charge injection and the presence of naturally occurring sulfur vacancies are responsible for the observed increase in catalytic activity of the 2H basal plane. Our results provide new insights into the role of contact resistance and charge transport on the performance of two-dimensional MoS2 nanosheet catalysts for the HER.
Claudine Katan, Aditya D. Mohite and Jacky Even discuss the possible impact of various entropy contributions (stochastic structural fluctuations, anharmonicity and lattice softness) on the ...optoelectronic properties of halide perovskite materials and devices.
Achieving technologically relevant performance and stability for optoelectronics, energy conversion, photonics, spintronics and quantum devices requires creating atomically precise materials with ...tailored homo- and hetero-interfaces, which can form functional hierarchical assemblies. Nature employs tunable sequence chemistry to create complex architectures, which efficiently transform matter and energy, however, in contrast, the design of synthetic materials and their integration remains a long-standing challenge. Organic-inorganic two-dimensional halide perovskites (2DPKs) are organic and inorganic two-dimensional layers, which self-assemble in solution to form highly ordered periodic stacks. They exhibit a large compositional and structural phase space, which has led to novel and exciting physical properties. In this Review, we discuss the current understanding in the structure and physical properties of 2DPKs from the monolayers to assemblies, and present a comprehensive comparison with conventional semiconductors, thereby providing a broad understanding of low-dimensional semiconductors that feature complex organic-inorganic hetero-interfaces.
Ultrathin molybdenum disulphide (MoS2) has emerged as an interesting layered semiconductor because of its finite energy bandgap and the absence of dangling bonds. However, metals deposited on the ...semiconducting 2H phase usually form high-resistance (0.7 kΩ μm-10 kΩ μm) contacts, leading to Schottky-limited transport. In this study, we demonstrate that the metallic 1T phase of MoS2 can be locally induced on semiconducting 2H phase nanosheets, thus decreasing contact resistances to 200-300 Ω μm at zero gate bias. Field-effect transistors (FETs) with 1T phase electrodes fabricated and tested in air exhibit mobility values of ~50 cm(2) V(-1) s(-1), subthreshold swing values below 100 mV per decade, on/off ratios of >10(7), drive currents approaching ~100 μA μm(-1), and excellent current saturation. The deposition of different metals has limited influence on the FET performance, suggesting that the 1T/2H interface controls carrier injection into the channel. An increased reproducibility of the electrical characteristics is also obtained with our strategy based on phase engineering of MoS2.
State‐of‐the‐art light‐emitting diodes (LEDs) are made from high‐purity alloys of III–V semiconductors, but high fabrication cost has limited their widespread use for large area solid‐state lighting. ...Here, efficient and stable LEDs processed from solution with tunable color enabled by using phase‐pure 2D Ruddlesden–Popper (RP) halide perovskites with a formula (CH3(CH2)3NH3)2(CH3NH3)n−1PbnI3n+1 are reported. By using vertically oriented thin films that facilitate efficient charge injection and transport, efficient electroluminescence with a radiance of 35 W Sr−1 cm−2 at 744 nm with an ultralow turn‐on voltage of 1 V is obtained. Finally, operational stability tests suggest that phase purity is strongly correlated to stability. Phase‐pure 2D perovskites exhibit >14 h of stable operation at peak operating conditions with no droop at current densities of several Amperes cm−2 in comparison to mixtures of 2D/3D or 3D perovskites, which degrade within minutes.
Phase‐pure Ruddlesden–Popper layered perovskites (RPLPs) are investigated for light‐emitting diode application. This work demonstrates that RPLPs represent a promising candidate for optoeletronics and the crystal orientation of RPLPs plays a vital role in light‐emitting diode devices, resulting in external quantum efficiency ≈1% with ultralow turn‐on voltage.
Three-dimensional (3D) organic–inorganic lead halide perovskites have emerged in the past few years as a promising material for low-cost, high-efficiency optoelectronic devices. Spurred by this ...recent interest, several subclasses of halide perovskites such as two-dimensional (2D) halide perovskites have begun to play a significant role in advancing the fundamental understanding of the structural, chemical, and physical properties of halide perovskites, which are technologically relevant. While the chemistry of these 2D materials is similar to that of the 3D halide perovskites, their layered structure with a hybrid organic–inorganic interface induces new emergent properties that can significantly or sometimes subtly be important. Synergistic properties can be realized in systems that combine different materials exhibiting different dimensionalities by exploiting their intrinsic compatibility. In many cases, the weaknesses of each material can be alleviated in heteroarchitectures. For example, 3D-2D halide perovskites can demonstrate novel behavior that neither material would be capable of separately. This review describes how the structural differences between 3D halide perovskites and 2D halide perovskites give rise to their disparate materials properties, discusses strategies for realizing mixed-dimensional systems of various architectures through solution-processing techniques, and presents a comprehensive outlook for the use of 3D-2D systems in solar cells. Finally, we investigate applications of 3D-2D systems beyond photovoltaics and offer our perspective on mixed-dimensional perovskite systems as semiconductor materials with unrivaled tunability, efficiency, and technologically relevant durability.
In the fast-evolving field of halide perovskite semiconductors, the 2D perovskites (A′)₂(A)n−1M
n
X3n+1 where A = Cs⁺, CH₃NH₃⁺, HC(NH₂)₂⁺; A′ = ammonium cation acting as spacer; M = Ge2+, Sn2+, Pb2+; ...and X = Cl⁻, Br⁻, I⁻ have recently made a critical entry. The n value defines the thickness of the 2D layers, which controls the optical and electronic properties. The 2D perovskites have demonstrated preliminary optoelectronic device lifetime superior to their 3D counterparts. They have also attracted fundamental interest as solution-processed quantum wells with structural and physical properties tunable via chemical composition, notably by the n value defining the perovskite layer thickness. The higher members (n > 5) have not been documented, and there are important scientific questions underlying fundamental limits for n. To develop and utilize these materials in technology, it is imperative to understand their thermodynamic stability, fundamental synthetic limitations, and the derived structure–function relationships. We report the effective synthesis of the highest iodide n-members yet, namely (CH₃(CH₂)₂NH₃)₂(CH₃NH₃)₅Pb₆I19 (n = 6) and (CH₃(CH₂)₂NH₃)₂(CH₃NH₃)₆Pb₇I22 (n = 7), and confirm the crystal structure with single-crystal X-ray diffraction, and provide indirect evidence for “(CH₃(CH₂)₂NH₃)₂(CH₃NH₃)₈Pb₉I28” (“n = 9”). Direct HCl solution calorimetric measurements show the compounds with n > 7 have unfavorable enthalpies of formation (ΔH
f), suggesting the formation of higher homologs to be challenging. Finally, we report preliminary n-dependent solar cell efficiency in the range of 9–12.6% in these higher n-members, highlighting the strong promise of these materials for high-performance devices.
Hybrid halide 2D perovskites deserve special attention because they exhibit superior environmental stability compared with their 3D analogs. The closer interlayer distance discovered in 2D ...Dion–Jacobson (DJ) type of halide perovskites relative to 2D Ruddlesden–Popper (RP) perovskites implies better carrier charge transport and superior performance in solar cells. Here, the structure and properties of 2D DJ perovskites employing 3‐(aminomethyl)piperidinium (3AMP2+) as the spacing cation and a mixture of methylammonium (MA+) and formamidinium (FA+) cations in the perovskite cages are presented. Using single‐crystal X‐ray crystallography, it is found that the mixed‐cation (3AMP)(MA0.75FA0.25)3Pb4I13 perovskite has a narrower bandgap, less distorted inorganic framework, and larger PbIPb angles than the single‐cation (3AMP)(MA)3Pb4I13. Furthermore, the (3AMP)(MA0.75FA0.25)3Pb4I13 films made by a solvent‐engineering method with a small amount of hydriodic acid have a much better film morphology and crystalline quality and more preferred perpendicular orientation. As a result, the (3AMP)(MA0.75FA0.25)3Pb4I13‐based solar cells exhibit a champion power conversion efficiency of 12.04% with a high fill factor of 81.04% and a 50% average efficiency improvement compared to the pristine (3AMP)(MA)3Pb4I13 cells. Most importantly, the 2D DJ 3AMP‐based perovskite films and devices show better air and light stability than the 2D RP butylammonium‐based perovskites and their 3D analogs.
2D Dion–Jacobson perovskites have better carrier charge transport because of the closer interlayer distance. Solar cells based on Dion–Jacobson perovskites having mixed organic cations and using solvent‐engineering methods and hydriodic acid additive achieve higher efficiencies with high fill factors. Most importantly, the Dion–Jacobson perovskite solar cells exhibit better environmental stability compared with butylammonium‐based perovskites and 3D analogs.
Ruddlesden–Popper reduced‐dimensional hybrid perovskite (RDP) semiconductors have attracted significant attention recently due to their promising stability and excellent optoelectronic properties. ...Here, the RDP crystallization mechanism in real time from liquid precursors to the solid film is investigated, and how the phase transition kinetics influences phase purity, quantum well orientation, and photovoltaic performance is revealed. An important template‐induced nucleation and growth of the desired (BA)2(MA)3Pb4I13 phase, which is achieved only via direct crystallization without formation of intermediate phases, is observed. As such, the thermodynamically preferred perpendicular crystal orientation and high phase purity are obtained. At low temperature, the formation of intermediate phases, including PbI2 crystals and solvate complexes, slows down intercalation of ions and increases nucleation barrier, leading to formation of multiple RDP phases and orientation randomness. These insights enable to obtain high quality (BA)2(MA)3Pb4I13 films with preferentially perpendicular quantum well orientation, high phase purity, smooth film surface, and improved optoelectronic properties. The resulting devices exhibit high power conversion efficiency of 12.17%. This work should help guide the perovskite community to better control Ruddlesden–Popper perovskite structure and further improve optoelectronic and solar cell devices.
The crystallization mechanism for Ruddlesden–Popper perovskite in real time from liquid precursors to the solid film is presented. The template‐induced nucleation and growth of the (BA)2(MA)3Pb4I13 phase, which is achieved only via direct crystallization without formation of intermediate phases, helps to realize superior films with high phase purity, perpendicular quantum‐well orientation, improved optoelectronic properties, and excellent photovoltaic performance.
Atomic layers of two-dimensional (2D) materials have recently been the focus of extensive research. This follows from the footsteps of graphene, which has shown great potential for ultrathin ...optoelectronic devices. In this paper, we present a comprehensive study on the synthesis, characterization, and thin film photodetector application of atomic layers of InSe. Correlation between resonance Raman spectroscopy and photoconductivity measurements allows us to systematically track the evolution of the electronic band structure of 2D InSe as its thickness approaches few atomic layers. Analysis of photoconductivity spectra suggests that few-layered InSe has an indirect band gap of 1.4 eV, which is 200 meV higher than bulk InSe due to the suppressed interlayer electron orbital coupling. Temperature-dependent photocurrent measurements reveal that the suppressed interlayer interaction also results in more localized p z -like orbitals, and these orbitals couple strongly with the in-plane E′ and E″ phonons. Finally, we measured a strong photoresponse of 34.7 mA/W and fast response time of 488 μs for a few layered InSe, suggesting that it is a good material for thin film optoelectronic applications.
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