Jae-Hong Lim

Sb-doped p-ZnO∕Ga-doped n-ZnO homojunction ultraviolet light emitting diodes

ZnO p-n homojunction light emitting diodes were fabricated based on p-type Sb-doped ZnO∕n-type Ga-doped ZnO thin films. Low resistivity Au∕NiO and Au∕Ti contacts were formed on top of p-type and n-type ZnO layers, respectively. Au∕NiO contacts on p-type ZnO exhibited a low specific resistivity of 7.4×10−4Ωcm2. The light emitting diodes yielded strong near-band-edge emissions in temperature-dependent and injection current-dependent electroluminescence measurements.

Size-controlled electrochemical synthesis and properties of SnO2 nanotubes

SnO(2) nanotubes with controlled diameter and length were synthesized using an electrochemical method at room temperature. The length and wall thickness of the nanotubes increased monotonically with the deposition time and the diameter of the nanotubes was altered by varying the pore size of the scaffolds. Post-annealing at 400 degrees C in dry air significantly improved the crystallinity while maintaining the nanotube structure. The temperature-dependent photoluminescence spectra indicated an activation energy of 58 meV for emission centered at 410 nm. The temperature-dependent electrical resistance revealed that the dominant electrical conduction mechanism alters from the ionization of the main donor centers to impurity scattering as the temperature decreases. The electrical conductance of 200 nm diameter nanotubes increased to 33 times the original value upon UV illumination at 254 nm.

Influence of surface roughness of aluminum-doped zinc oxide buffer layers on the performance of inverted organic solar cells

Aluminum-doped zinc oxide (AZO) films (70 nm thick) with dissimilar surface roughness were created on indium tin oxide coated glass and were used as electrodes for inverted organic solar cells. The photovoltaic performance of the devices depended strongly on the surface roughness of the AZO films. Increases in the surface root-mean-square roughness of AZO films from 2.5 to 10.9 nm enhanced power conversion efficiency from 0.5% to 1.4% due to increased contact area between electrode and active layer.

Surface Modification of a ZnO Electron-Collecting Layer Using Atomic Layer Deposition to Fabricate High-Performing Inverted Organic Photovoltaics

A ripple-structured ZnO film as the electron-collecting layer (ECL) of an inverted organic photovoltaic (OPV) was modified by atomic layer deposition (ALD) to add a ZnO thin layer. Depositing a thin ZnO layer by ALD on wet-chemically prepared ZnO significantly increased the short-circuit current (Jsc) of the OPV. The highest power conversion efficiency (PCE) of 7.96% with Jsc of 17.9 mA/cm2 was observed in the inverted OPV with a 2-nm-thick ALD-ZnO layer, which quenched electron-hole recombination at surface defects of ZnO ripples. Moreover, an ALD-ZnO layer thinner than 2 nm made the distribution of electrical conductivity on the ZnO surface more uniform, enhancing OPV performance. In contrast, a thicker ALD-ZnO layer (5 nm) made the two-dimensional distribution of electrical conductivity on the ZnO surface more heterogeneous, reducing the PCE. In addition, depositing an ALD-ZnO thin layer enhanced OPV stability and initial performance. We suggest that the ALD-ZnO layer thickness should be precisely controlled to fabricate high-performing OPVs.

Electrical and gas sensing properties of polyaniline functionalized single-walled carbon nanotubes

Electrical and gas sensing properties of single-walled carbon nanotube networks functionalized with polyaniline (PANI-SWNTs) were systematically investigated to understand the gas sensing mechanisms and optimize sensing performance. The temperature-dependent electrical resistance and field-effect transistor (FET) transfer characteristics indicated that the electrical properties of PANI-SWNTs are dominated by the PANI coating. The FET transfer characteristics of PANI-SWNTs exposed to different NH(3) concentrations indicated that the dominant sensing mechanism is the deprotonation of PANI by NH(3). Sensing experiments with different gas analytes revealed that PANI-SWNTs responded positively to NH(3), and negatively to NO(2) and H(2)S with sensitivities of 5.8% per ppm(v) of NH(3), 1.9% per ppm(v) of NO(2), and 3.6% per ppm(v) of H(2)S. The lower detection limits were 50, 500, and 500 ppb for NH(3), NO(2), and H(2)S, respectively.

Thermoelectric characteristics of Sb2Te3 thin films formed via surfactant-assisted electrodeposition

In this study, Sb2Te3 films were electrodeposited potentiostatically at room temperature from acidic nitric baths in the presence of a surfactant, cetyltrimethylammonium bromide (CTAB). This resulted in improvements in the surface morphologies of the films. The thus-deposited films also exhibited better adherence to their substrates. In addition, the carrier transport properties of the Sb2Te3 films, including their electrical conductivities and Seebeck coefficient values, could be improved by annealing them at 200 °C. This was due to the formation of Te nanodots in the Sb2Te3 matrix. The resulting Sb2Te3 films with Te nanodots 10–20 nm in size and a dispersion density of 11.4 vol% exhibited a high power factor of 716.0 μW m−1 K−2, which was about two times higher than that of Sb2Te3 films that did not contain Te nanodots. The results of the study suggested that the improvements in the thermoelectric characteristics of the films were due to carrier energy filtering at the Te–Sb2Te3 interface.

Self-aligned Ni-P ohmic contact scheme for silicon solar cells by electroless deposition

We report a Ni-P metallization scheme for low resistance ohmic contacts to n-type Si for silicon solar cells. As-deposited Ni-P contacts to n-type Si showed a specific contact resistance of 6.42 × 10−4 Ω·cm2. The specific contact resistance decreased with increasing thermal annealing temperature. When the Ni-P contact was annealed at 600°C for 30 min in ambient air, the specific contact resistance was greatly decreased, to 6.37 × 10−5Ω·cm2. The improved ohmic property was attributed to the decrease in the work function due to the formation of Ni-silicides from Ni in-diffusion during the thermal annealing process. Effects of the annealing process on the electrical and crystal properties of the contacts were investigated by means of various resistivity measurements (circular transmission line method (c-TLM), 4-point probe), glancing angle x-ray diffraction (GAXRD), and x-ray photoelectron spectroscopy (XPS).

Ultra-long bismuth telluride nanoribbons synthesis by lithographically patterned galvanic displacement

We demonstrated the wafer level batch synthesis and fabrication of single semiconducting thermoelectric nanoribbon based devices by Lithographically Patterned Galvanic Displacement (LPGD). The shape, composition, and dimension of nanoribbons were tailored by adjusting deposition conditions. High resolution TEM images with fast Fourier transform (FFT)-converted selected area electron diffraction (SAED) patterns confirmed the formation of polycrystalline Bi2Te3 intermetallic compound with a rhombohedral structure without elemental Te and Bi. The thickness dependent electrical resistivity of BixTey nanoribbons shows a classic size effect due to the increase in surface boundary scattering. The as-synthesized nanoribbons were n-type semiconductors with no clear trend between field effect carrier mobility and composition, which might be attributed to the trapped charges at the interface between the channel and dielectric layer. The preliminary results on thermoelectric properties (i.e. Seebeck coefficient and power factor) show that the Seebeck coefficient of as-synthesized 0.1 µm thick Bi30Te70 nanoribbon is comparable with bulk counterparts, however, the power factor was lower because of poor crystallinity which leads to higher resistivity.

Manipulation of cuprous oxide surfaces for improving their photocatalytic activity

Manipulating the surface characteristics of metal oxide electrodes allows the properties of the interface between electrodes and the electrolyte to be controlled and can lead to improvements in both efficiency and reliability of the electrodes. In this study, the facets exposed on the surfaces of Cu2O photoelectrodes were manipulated by controlling the pH of the bath during Cu2O film electrodeposition. The Cu2O film with (100)-type facets, deposited at a bath pH of 12, produced a photocurrent 19 times higher than that of the film deposited at pH 8.3 and possessing (111) facets. In addition, inverse-opal-structured Cu2O films were electrodeposited in alkali solutions using templates of polystyrene beads; these films exhibited even higher photocatalytic activities than the planar ones. The templated, three-dimensional (3D) Cu2O film deposited at pH 12 produced a photocurrent 2.14 times higher than that generated by the planar Cu2O film deposited at the same pH; this was a result of the greater surface area and higher light absorption of the 3D film.

Thermochemical hydrogen sensor based on chalcogenide nanowire arrays.

The hydrogen gas-sensing properties have been investigated of two types of thermochemical hydrogen (TCH) sensors composed of thermoelectric layers based on chalcogenide nanowire arrays and anodic aluminum oxide (AAO) templates. The monomorphic-type TCH sensor, which had only Bi2Te3 nanowire arrays, showed an output signal of 23.7 μV in response to 5 vol% hydrogen gas at room temperature, whereas an output signal of 215 μV was obtained from an n-p junction-type TCH sensor made of connected Bi2Te3 and Sb2Te3 nanowire arrays in an AAO template. Despite its small deposition area, the output signal of the n-p sensor was more than nine times that of the monomorphic sensor. This observation can be explained by the difference in electrical connections (parallel and serial conversions) in the TCH sensor between each type of nanowire array. Also, our n-p sensor had a wide detection range for hydrogen gas (from 400 ppm to 45 vol%) and a fast response time of 1.3 s at room temperature without requiring external power.