Atomic Layer Deposition of 2D Metal Dichalcogenides for Electronics, Catalysis, Energy Storage, and Beyond

2D transition metal dichalcogenides (TMDCs) are among the most exciting materials of today. Their layered crystal structures result in unique and useful electronic, optical, catalytic, and quantum properties. To realize the technological potential of TMDCs, methods depositing uniform films of controlled thickness at low temperatures in a highly controllable, scalable, and repeatable manner are needed. Atomic layer deposition (ALD) is a chemical gas-phase thin film deposition method capable of meeting these challenges. In this review, the applications evaluated for ALD TMDCs are systematically examined, including electronics and optoelectonics, electrocatalysis and photocatalysis, energy storage, lubrication, plasmonics, solar cells, and photonics. This review focuses on understanding the interplay between ALD precursors and deposition conditions, the resulting film characteristics such as thickness, crystallinity, and morphology, and ultimately device performance. Through rational choice of precursors and conditions, ALD is observed to exhibit potential to meet the varying requirements of widely different applications. Beyond the current state of ALD TMDCs, the future prospects, opportunities, and challenges in different applications are discussed. The authors hope that the review aids in bringing together experts in the fields of ALD, TMDCs, and various applications to eventually realize industrial applications of ALD TMDCs.Peer reviewe

Atomic layer deposition of NiS and its application as cathode material in dye sensitized solar cell

Nickel sulfide (NiS) is grown by atomic layer deposition (ALD) using sequential exposures of bis(2,2,6,6-tetramethylheptane-3,5-dionate) nickel(II) [Ni(thd)(2)] and hydrogen sulfide (H2S) at 175 degrees C. Complementary combinations of in situ and ex situ characterization techniques are used to understand the deposition chemistry and the nature of film growth. The saturated growth rate of ca. 0.21 angstrom per ALD cycle is obtained, which is constant within the ALD temperature window (175-250 degrees C). As deposited films on glass substrates are found polycrystalline without any preferred orientation. Electrical transport measurement reveals degenerative/semimetallic characteristics with a carrier concentration of ca. 9 x 10(22) cm(-3) at room temperature. The ALD grown NiS thin film demonstrates high catalytic activity for the reduction of I-/I-3(-) electrolyte that opens its usage as cost-effective counter electrode in dye sensitized solar cells, replacing Pt. (C) 2015 American Vacuum Society

Atomic layer deposition of aluminum sulfide thin films using trimethylaluminum and hydrogen sulfide

Sequential exposures of trimethylaluminum and hydrogen sulfide are used to deposit aluminum sulfide thin films by atomic layer deposition (ALD) in the temperature ranging from 100 to 200 degrees C. Growth rate of 1.3 angstrom per ALD cycle is achieved by in-situ quartz crystal microbalance measurements. It is found that the growth rate per ALD cycle is highly dependent on the purging time between the two precursors. Increased purge time results in higher growth rate. Surface limited chemistry during each ALD half cycle is studied by in-situ Fourier transformed infrared vibration spectroscopy. Time of flight secondary ion-mass spectroscopy measurement is used to confirm elemental composition of the deposited films. (C) 2014 American Vacuum Society

Molecular layer deposition of alucone films using trimethylaluminum and hydroquinone

A hybrid organic-inorganic polymer film grown by molecular layer deposition (MLD) is demonstrated here. Sequential exposures of trimethylaluminum [Al(CH3)(3)] and hydroquinone [C6H4(OH)(2)] are used to deposit the polymeric films, which is a representative of a class of aluminum oxide polymers known as "alucones." In-situ quartz crystal microbalance (QCM) studies are employed to determine the growth characteristics. An average growth rate of 4.1 angstrom per cycle at 150 degrees C is obtained by QCM and subsequently verified with x-ray reflectivity measurements. Surface chemistry during each MLD-half cycle is studied in depth by in-situ Fourier transform infrared (FTIR) vibration spectroscopy. Self limiting nature of the reaction is confirmed from both QCM and FTIR measurements. The conformal nature of the deposit, typical for atomic layer deposition and MLD, is verified with transmission electron microscopy imaging. Secondary ion mass spectroscopy measurements confirm the uniform elemental distribution along the depth of the films. (C) 2014 American Vacuum Society

One-step Solution-Processed Formamidinium Lead Tribromide Formation for Better Reproducible Planar Perovskite Solar Cells

Low-cost solar cells based on solution-processed organic inorganic hybrid perovskites with high photovoltage are highly sought after, especially for their use in tandem cells or for driving electrochemical reactions. Towards this end, we herein present a single-step method for the preparation of regular planar heterojunction solar cells based on formamidinium lead tribromide (FAPbBr(3)), which is fabricated through an antisolvent-assisted crystallization process. This results in the formation of improved film quality of the perovskite layer in terms of uniformity, surface coverage, crystallinity,and light absorption. Devices fabricated using such films utilizing 2,2',7,7'-tetrakis-(N,N-p-dimethoxyphenylamino)-9.9'spirobifluorene (spiro-OMeTAD) as the hole-transport layer exhibits an open-circuit voltage (V,) of 1.32 V with a power conversion efficiency (PCE) close to 6%. The de-vice performance is much higher than when using devices based on the conventional one-,step process (i.e., without using an anti solvent), where a Vo, of 1.04 V and a PCE of 1.1 % are obtained. Moreover, the devices,show better reproducibility with very little hysteresis

Towards All-Inorganic Transport Layers for Wide-Band Gap Formamidinium Lead Bromide-Based Planar Photovoltaics

Hybrid perovskite photovoltaic devices heavily rely on the use of organic (rather than inorganic) charge-transport layers on top of a perovskite absorber layer because of difficulties in depositing inorganic materials on top of these fragile absorber layers. However, in comparison to the unstable and expensive organic transport materials, inorganic charge transport layers provide improved charge transport and stability to the device architecture. Here, we report photovoltaic devices using all-inorganic transport layers in a planar p-i-n junction device configuration using formamidinium lead tribromide (FAPbBr(3)) as an absorber. Efficient planar devices are obtained through atomic layer deposition of nickel oxide and sputtered zinc oxide as hole- and electron-transport materials, respectively. Using only inorganic charge-transport layers resulted in planar FAPbBr3 devices with a power conversion efficiency of 6.75 % at an open-circuit voltage of 1.23 V. The transition of planar FAPbBr3 devices making from all-organic towards all inorganic charge-transport layers is studied in detail

Atomic Layer Deposition of Transparent and Conducting p-Type Cu(I) Incorporated ZnS Thin Films: Unravelling the Role of Compositional Heterogeneity on Optical and Carrier Transport Properties

Optically transparent and highly conducting p-type Cu(I) incorporated ZnS (Cu:ZnS) films are deposited by stacking individual layers of CuS and ZnS using atomic layer deposition. The deposition chemistry and growth mechanism are studied by in situ quartz crystal microbalance. Compositional disorder in atomic scale is observed with increasing Cu incorporation in the films that results in systematic decrease in the optical transmittance in the visible spectrum. Again the conductivity also emphatically depends on the volume fraction of phase-segregated conducting covellite phase. An illustrious correlation prevailing the interplay between the optical transparency and the charge transport mechanism is established. The hole transport mechanism that indicates insulator to-metal transition with increasing Cu incorporation in the composite is explained in terms of an inhomogeneously disordered system. Under optimized conditions, the material having moderately high optical transmission with degenerate carrier concentration lies exactly at the confluence between the metallic and insulating regime. The lowest resistivity that is obtained here (1.3 x 10(-3) Omega cm) with >90% (after reflection correction) transmission is highly comparable to the best reported in the field and probably analogous to the commercially available n-type transparent conductors

Inorganic Hole Conducting Layers for Perovskite-Based Solar Cells

Hybrid organic-inorganic semiconducting perovskite photovoltaic cells are usually coupled with organic hole conductors. Here, we report planar, inverse CH3NH3PbI3-xClx-based cells with inorganic hole conductors. Using electrodeposited NiO as hole conductor, we have achieved a power conversion efficiency of 7.3%. The maximum V-oc obtained was 935 mV with an average V-oc value being 785 mV. Preliminary results for similar cells using electrodeposited CuSCN as hole conductor resulted in devices up to 3.8% in efficiency. The ability to obtain promising cells using NiO and CuSCN expands the presently rather limited range of available hole conductors for perovskite cells

Stable p-i-n FAPbBr(3) Devices with Improved Efficiency Using Sputtered ZnO as Electron Transport Layer

Radio-frequency magnetron sputtering is demonstrated as an effective tool to deposit highly crystalline thin zinc oxide (ZnO) layer directly on perovskite absorber as an electron transport layer (ETL). As an absorber, formamidinium lead tribromide (FAPbBr(3)) is fabricated through a modified single-step solution process using hydrogen bromide (HBr) as an additive resulting in complete surface coverage and highly crystalline material. A planar p-i-n device architecture with spin-coated poly-(3,4-ethylenedioxythiophene): polystyrenesulfonic acid (PEDOT: PSS) as hole transport material (HTM) and sputtered ZnO as ETL results in a short circuit current density of 9.5 mA cm(-2) and an open circuit potential of 1.19 V. Numerical simulations are performed to validate the underlying loss mechanisms. The use of phenyl C60 butyric acid methyl ester (PCBM) interface layer between FAPbBr(3) and sputter-coated ZnO offers shielding from potential plasma-related interface damage. The modified interface results in a better device efficiency of 8.3% with an open circuit potential of 1.35 V. Such devices offer better stability under continuous illumination under ambient conditions in comparison with the conventional organic ETL (PCBM)-based devices