Ki Seok Kim

Atomically Thin-Layered Molybdenum Disulfide (MoS2) for Bulk-Heterojunction Solar Cells

Transition metal dichalcogenides (TMDs) are becoming significant because of their interesting semiconducting and photonic properties. In particular, TMDs such as molybdenum disulfide (MoS2), molybdenum diselenide (MoSe2), tungsten disulfide (WS2), tungsten diselenide (WSe2), titanium disulfide (TiS2), tantalum sulfide (TaS2), and niobium selenide (NbSe2) are increasingly attracting attention for their applications in solar cell devices. In this review, we give a brief introduction to TMDs with a focus on MoS2; and thereafter, emphasize the role of atomically thin MoS2 layers in fabricating solar cell devices, including bulk-heterojunction, organic, and perovskites-based solar cells. Layered MoS2 has been used as the hole-transport layer (HTL), electron-transport layer (ETL), interfacial layer, and protective layer in fabricating heterojunction solar cells. The trilayer graphene/MoS2/n-Si solar cell devices exhibit a power-conversion efficiency of 11.1%. The effects of plasma and chemical doping on the photovoltaic performance of MoS2 solar cells have been analyzed. After doping and electrical gating, a power-conversion efficiency (PCE) of 9.03% has been observed for the MoS2/h-BN/GaAs heterostructure solar cells. The MoS2-containing perovskites-based solar cells show a PCE as high as 13.3%. The PCE of MoS2-based organic solar cells exceeds 8.40%. The stability of MoS2 solar cells measured under ambient conditions and light illumination has been discussed. The MoS2-based materials show a great potential for solar cell devices along with high PCE; however, in this connection, their long-term environmental stability is also of equal importance for commercial applications.

Two-dimensional transition metal dichalcogenide-based counter electrodes for dye-sensitized solar cells

Dye-sensitized solar cells (DSSCs) are gaining considerable interest as alternatives to the semiconductor-based thin film solar cells. The noble metal platinum (Pt) is conventionally used as counter electrode (CE) material for fabricating DSSCs. Since Pt is expensive and scarce, new materials have been explored to develop cost-effective Pt-free counter electrodes for DSSCs. Two-dimensional (2D) graphene-based counter electrodes have achieved the highest known power conversion efficiency ({eta}) of 13%, which has stimulated research activities in 2D layered transition metal dichalcogenides (TMDs) for developing Pt-free DSSCs. In this review, progress made on alternative counter electrodes for fabricating low-cost Pt-free DSSCs, based on earth-abundant 2D TMDs including MoS2, WS2, TiS2, FeS2, CoS2, NiS2, SnS2, MoSe2, NbSe2, TaSe2, NiSe2, FeSe2, CoSe2, Bi2Se3 and their based composites, are discussed and summarized. Also, the considerable progress made on thin films of MoS2 and MoS2 based carbon, graphene, carbon nanotubes (CNTs), carbon nanofibers (CNFs), and poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) composites as efficient counter electrodes (CEs) for DSSCs are discussed, in terms of their electrochemical and photovoltaic properties. At present, PCE values higher than that of standard Pt CE have been recorded for a number of TMD-based CEs, which include MoS2 and MoSe2/thin films deposited on Mo foil, MoS2/CNTs, MoS2/graphene, MoS2/carbon, MoSe2/PEDOT:PSS, NbSe2, FeS2, FeSe2 nanosheets, TiS2/graphene, and NiS2/graphene hybrid systems in DSSCs, for the reduction of triiodide (I3-) to iodide (I-). The highest PCE ({eta}=10.46%) versus Pt CE ({eta}=8.25%) at 1 Sun (100 mW/cm2, AM 1.5G) was measured for DSSCs having a low cost and flexible CoSe2/carbon-nanoclimbing wall counter electrode deposited on a nickel foam.

Atomic Layer Etching Mechanism of MoS2 for Nanodevices

Among the layered transition metal dichalcogenides (TMDs) that can form stable two-dimensional crystal structures, molybdenum disulfide (MoS2) has been intensively investigated because of its unique properties in various electronic and optoelectronic applications with different band gap energies from 1.29 to 1.9 eV as the number of layers decreases. To control the MoS2 layers, atomic layer etching (ALE) (which is a cyclic etching consisting of a radical-adsorption step such as Cl adsorption and a reacted-compound-desorption step via a low-energy Ar+-ion exposure) can be a highly effective technique to avoid inducing damage and contamination that occur during the reactive steps. Whereas graphene is composed of one-atom-thick layers, MoS2 is composed of three-atom-thick S(top)-Mo(mid)-S(bottom) layers; therefore, the ALE mechanisms of the two structures are significantly different. In this study, for MoS2 ALE, the Cl radical is used as the adsorption species and a low-energy Ar+ ion is used as the desorption species. A MoS2 ALE mechanism (by which the S(top), Mo(mid), and S(bottom) atoms are sequentially removed from the MoS2 crystal structure due to the trapped Cl atoms between the S(top) layer and the Mo(mid) layer) is reported according to the results of an experiment and a simulation. In addition, the ALE technique shows that a monolayer MoS2 field effect transistor (FET) fabricated after one cycle of ALE is undamaged and exhibits electrical characteristics similar to those of a pristine monolayer MoS2 FET. This technique is also applicable to all layered TMD materials, such as tungsten disulfide (WS2), molybdenum diselenide (MoSe2), and tungsten diselenide (WSe2).

Atomic layer etching of graphene through controlled ion beam for graphene-based electronics

The electronic and optical properties of graphene are greatly dependent on the the number of layers. For the precise control of the graphene layers, atomic layer etching (ALE), a cyclic etching method achieved through chemical adsorption and physical desorption, can be the most powerful technique due to barely no damage and no contamination. In this study, we demonstrated the ALE process of graphene layers without noticeably damaging the graphene by using a controlled low energy oxygen (O2+/O+)-ion for chemical adsorption and a low energy Ar+-ion (11.2u2009eV) for physical desorption. In addition, using a trilayer graphene, mono- and bi-layer graphene could be successfully fabricated after one- and two-cycle ALE of the trilayer graphene, respectively. We believe that the ALE technique presented herein can be applicable to all layered materials such as graphene, black phosphorous and transition metal dichalcogenides which are important for next generation electronic devices.

Characteristics of silicon nitride deposited by VHF (162 MHz)-plasma enhanced chemical vapor deposition using a multi-tile push–pull plasma source

To prevent moisture and oxygen permeation into flexible organic electronic devices formed on substrates, the deposition of an inorganic diffusion barrier material such as SiN x is important for thin film encapsulation. In this study, by a very high frequency (162 MHz) plasma-enhanced chemical vapor deposition (VHF-PECVD) using a multi-tile push–pull plasma source, SiN x layers were deposited with a gas mixture of NH3/SiH4 with/without N2 and the characteristics of the plasma and the deposited SiN x film as the thin film barrier were investigated. Compared to a lower frequency (60 MHz) plasma, the VHF (162 MHz) multi-tile push–pull plasma showed a lower electron temperature, a higher vibrational temperature, and higher N2 dissociation for an N2 plasma. When a SiN x layer was deposited with a mixture of NH3/SiH4 with N2 at a low temperature of 100 °C, a stoichiometric amorphous Si3N4 layer with very low Si–H bonding could be deposited. The 300 nm thick SiN x film exhibited a low water vapor transmission rate of 1.18 × 10−4 g (m2 d)−1, in addition to an optical transmittance of higher than 90%.

Atomic layer deposition of HfO2 on graphene through controlled ion beam treatment

The polymer residue generated during the graphene transfer process to the substrate tends to cause problems (e.g., a decrease in electron mobility, unwanted doping, and non-uniform deposition of the dielectric material). In this study, by using a controllable low-energy Ar+ ion beam, we cleaned the polymer residue without damaging the graphene network. HfO2 grown by atomic layer deposition on graphene cleaned using an Ar+ ion beam showed a dense uniform structure, whereas that grown on the transferred graphene (before Ar+ ion cleaning) showed a non-uniform structure. A graphene–HfO2–metal capacitor fabricated by growing 20-nm thick HfO2 on graphene exhibited a very low leakage current (<10−11 A/cm2) for Ar+ ion-cleaned graphene, whereas a similar capacitor grown using the transferred graphene showed high leakage current.

Characteristics of pulsed dual frequency inductively coupled plasma

To control the plasma characteristics more efficiently, a dual antenna inductively coupled plasma (DF-ICP) source composed of a 12-turn inner antenna operated at 2 MHz and a 3-turn outer antenna at 13.56 MHz was pulsed. The effects of pulsing to each antenna on the change of plasma characteristics and SiO2 etch characteristics using Ar/C4F8 gas mixtures were investigated. When the duty percentage was decreased from continuous wave (CW) mode to 30% for the inner or outer ICP antenna, decrease of the average electron temperature was observed for the pulsing of each antenna. Increase of the CF2/F ratio was also observed with decreasing duty percentage of each antenna, indicating decreased dissociation of the C4F8 gas due to the decreased average electron temperature. When SiO2 etching was investigated as a function of pulse duty percentage, increase of the etch selectivity of SiO2 over amorphous carbon layer (ACL) was observed while decreasing the SiO2 etch rate. The increase of etch selectivity was related to the change of gas dissociation characteristics, as observed by the decrease of average electron temperature and consequent increase of the CF2/F ratio. The decrease of the SiO2 etch rate could be compensated for by using the rf power compensated mode, that is, by maintaining the same time-average rf power during pulsing, instead of using the conventional pulsing mode. Through use of the power compensated mode, increased etch selectivity of SiO2/ACL similar to the conventional pulsing mode could be observed without significant decrease of the SiO2 etch rate. Finally, by using the rf power compensated mode while pulsing rf powers to both antennas, the plasma uniformity over the 300 mm diameter substrate could be improved from 7% for the CW conditions to about around 3.3% with the duty percentage of 30%.

Silicon Nitride Deposition for Flexible Organic Electronic Devices by VHF (162 MHz)-PECVD Using a Multi-Tile Push-Pull Plasma Source

Depositing a barrier film for moisture protection without damage at a low temperature is one of the most important steps for organic-based electronic devices. In this study, the authors investigated depositing thin, high-quality SiNx film on organic-based electronic devices, specifically, very high-frequency (162u2009MHz) plasma-enhanced chemical vapor deposition (VHF-PECVD) using a multi-tile push-pull plasma source with a gas mixture of NH3/SiH4 at a low temperature of 80u2009°C. The thin deposited SiNx film exhibited excellent properties in the stoichiometry, chemical bonding, stress, and step coverage. Thin film quality and plasma damage were investigated by the water vapor transmission rate (WVTR) and by electrical characteristics of organic light-emitting diode (OLED) devices deposited with SiNx, respectively. The thin deposited SiNx film exhibited a low WVTR of 4.39u2009×u200910−4u2009g (m2 · day)−1 for a single thin (430u2009nm thick) film SiNx and the electrical characteristics of OLED devices before and after the thin SiNx film deposition on the devices did not change, which indicated no electrical damage during the deposition of SiNx on the OLED device.

Layer-controlled thinning of black phosphorus by an Ar ion beam

Black phosphorus (BP) is one of the most interesting two-dimensional (2D) layered materials due to its unique properties, including a band gap energy change from 0.3 eV (bulk) to 2.0 eV (monolayer) depending on the number of BP layers, for application in nanoelectronic devices. In general, 2D layered materials including BP have limitations in terms of synthesis due to the process factors such as time, temperature, etc., and thus, a thinning technique from the bulk material to a 2D material needs to be used while controlling the removed layer thickness. In this study, layer-controlled thinning of BP was performed by using a controlled Ar+ ion beam method and the BP thinning characteristics were investigated. By using the near monoenergetic ion energy in the range of 45–48 eV, BP could be thinned with the thinning rate of ∼0.55 nm min−1 down to bilayer BP without increasing the surface roughness and without changing the chemical binding states. The BP oxide on the pristine BP could also be successfully removed using the same Ar+ ion beam. 2D BP field-effect transistors (FETs) fabricated with the thinned bilayer–10-layer BPs exhibited electrical characteristics similar to those of pristine BP FETs suggesting no electrical damage on the BP layers thinned by the controlled monoenergetic Ar+ ion beam.

Highly uniform wafer-scale synthesis of α-MoO3 by plasma enhanced chemical vapor deposition

Molybdenum oxide (MoO3) has gained immense attention because of its high electron mobility, wide band gap, and excellent optical and catalytic properties. However, the synthesis of uniform and large-area MoO3 is challenging. Here, we report the synthesis of wafer-scale α-MoO3 by plasma oxidation of Mo deposited on Si/SiO2. Mo was oxidized by O2 plasma in a plasma enhanced chemical vapor deposition (PECVD) system at 150 °C. It was found that the synthesized α-MoO3 had a highly uniform crystalline structure. For the as-synthesized α-MoO3 sensor, we observed a current change when the relative humidity was increased from 11% to 95%. The sensor was exposed to different humidity levels with fast recovery time of about 8 s. Hence this feasibility study shows that MoO3 synthesized at low temperature can be utilized for gas sensing applications by adopting flexible device technology.