Kee-Jeong Yang

A band-gap-graded CZTSSe solar cell with 12.3% efficiency

Although Cu2ZnSn(S,Se)4 (CZTSSe) has attracted attention as an alternative to CuInGaSe2 (CIGS) as an absorber material in solar cells, its low efficiency is a serious shortcoming preventing its commercialization. To realize a high-efficiency CZTSSe solar cell, improved grain crystallinity, inhibited secondary-phase formation, controlled defect generation, adequate Na content, and band gap grading are required in the absorber layer. Few studies have focused specifically on band gap grading. In this study, a method of using SeS2, a new potential chalcogenization source material, to control the S and Se contents in a CZTSSe absorber and its effects were investigated. Using an appropriate SeS2/Se weight ratio, band gap grading was realized within the depletion region. By increasing the value of VOC through band gap grading in the depletion region, a record VOC deficit of 0.576 V was achieved. Furthermore, the possibility of enhancing JSC through the formation of a type-inverted n-type phase at the absorber surface in response to an appropriate alignment of the conduction-band minimum energy level and the Fermi energy pinning level is discussed. By introducing the chalcogenization source material SeS2 during the annealing process, CZTSSe solar cells with a maximum efficiency of 12.3% were obtained.

Abstraction of Blue Photoluminescence in Al-Doped ZnO Nanoparticles Prepared by Electron Beam Deposition

Al-doped ZnO nanoparticles with photoluminescence (PL) centered at blue were fabricated through electron beam deposition of Al2Ox and ZnO films on amorphous SiOx surface and annealing in oxygen at different temperatures. The luminescence at near-band-edge (NBE) and deep levels was suppressed by a homogeneous mix of ZnO and Al2Ox. The center peak of 420 nm was shifted to 441 nm in the sample with the nanoparticle size of 10 nm, the characteristics of which were not changed by the oxygen annealing temperature. The decay of PL in Al-doped nanoparticles obeyed a single exponential function and the estimated lifetime was 3.3 ms.

Luminance enhancement of electroluminescent devices using highly dielectric UV-curable polymer and oxide nanoparticle composite

A flexible hybrid structure electroluminescent (HSEL) device was fabricated from ZnS:Cu phosphor microparticles dispersed in a UV-curable polymer matrix. We observed a maximum luminance of 111 cd/m2 at 10 kHz and 170 V from a device wherein the mixing ratio between the phosphor particles and highly dielectric polymer binder was 70:30 wt%. Furthermore, by uniformly dispersing highly dielectric BaTiO3 nanoparticles within the polymer matrix, we were able to obtain a luminance of up to 211 cd/m2 in the HSEL device. Compared to the conventional thermal curing process, this UV process greatly simplifies the fabrication steps by combining phosphors and dielectric materials at room temperature. This process also demonstrates a promising pathway toward creating flexible and printed EL devices in the future.

Dye-Doped Polymer Dispersed Liquid Crystal Films for Flexible Displays

Red, green, and blue dyes were doped to polymer dispersed liquid crystal (PDLC) films for flexible display applications. Dichroic dye-doped liquid crystal droplets had a bipolar configuration. The E7–DG6071–dye composition showed better chromaticity data than other compositions. The small-particle-size red-dye-doped PDLC film showed good color differences. To improve the color difference, the dye particle size has to be small, and the bead milling process can make dye particles small. In this system, the bigger the liquid crystal droplet size, the higher the PDLC film driving voltage (Von), except in the LC–DG6071–red dye composition. This is the reason that the splay deformation increase is greater than the droplet size increase. In the electro-optic characteristics of dye-doped PDLC film, the TL205–DG6071–red dye composition had the lowest Von and the TL205–DG7052–red dye had the highest contrast ratio.

The Effects of Conditions for Polymerization Induced Phase Separation Processes on the Electro-optic Characteristics of Polymer Dispersed Liquid Crystals

The electro-optic properties of PDLC films were investigated by process conditions such as PDLC injection temperature, UV curing temperature and UV power. The mixture of liquid crystal (TL205) and the prepolymer substituted 1,6-hexanediol diacrylate (HDDA) for trimethylolpropane triacrylate (TMPTA), and the crosslinker in PN393 was used for PDLC formulation. The contrast ratio and driving voltage of PDLC films were mainly affected by UV curing temperature, rather than by the injection temperature. The film that was prepared at a relatively low UV process temperature revealed a good contrast ratio at a low driving voltage.

Electro optic characteristics of polymer-dispersed liquid-crystal films for flexible display

The electro optic properties of a polymer-dispersed liquid-crystal (PDLC) film were investigated by varying the driving conditions such as applied voltage and frequency. The PDLC was formed from a mixture of a liquid crystal (TL205) and prepolymer consisting of 2-ethylhexyl acrylate, Darocur 4265, 1,6-hexanediol diacrylate, and Ebecryl 810. When driven by applied voltage, the PDLC film showed contrast ratio of 17.59–23.64 at frequencies of 1 × 102–1 × 106 Hz, but the contrast ratio decreased to 4.01–6.46 within specific voltage ranges. With an increase in frequency, the dielectric anisotropy of the LC decreased, causing an increase in both the threshold voltage and driving voltage. Thus, the electro optic properties of the PDLC film could be modified by varying the voltage and frequency conditions.

Effects of Mo back-contact annealing on surface potential and carrier transport in Cu2ZnSnS4 thin film solar cells

The effects of Na on Cu2ZnSnS4 (CZTS)-based solar cells have been examined with respect to surface potential and carrier transport. The Mo back-contact was annealed in a furnace for 10 minutes under a nitrogen atmosphere at different temperatures and CZTS thin films were subsequently grown by sputtering and sulfurization. The thickness of MoS2, formed during the sulfurization process, decreased as the Mo annealing temperature increased. Interestingly, the Na contents diffused from soda lime glass has increased as well. The current and surface potential near CZTS grain boundaries were investigated by Kelvin probe force microscopy (KPFM) and conductive atomic force microscopy (C-AFM) were used. Surface current increased with increasing annealing temperature and surface potential increased up to approximately 50 mV near GBs, which led to inhibition of electron–hole recombination and an increase in minority carrier collection near GBs. This observation explains the improvement of solar cell open circuit voltage (VOC) and current density (JSC).

Light Scattering Characteristics of Newly Designed Polymer Dispersed Liquid Crystals Films

Polymer Dispersed Liquid Crystal (PDLC) films were prepared by the phase separation method using the liquid crystal (TL205) and a prepolymer. This work investigated the characteristics of scattering angle – light transmittance. In this work, it was found that at a narrow scattering angle incident lights was only weakly scattered and retained polarization, as the scattering angle increased the scattered light became increasingly depolarized.

Analysis of Electro-Optic Properties of a Polymer Network Liquid Crystal Display with Crossed Polarizers

An active-matrix polymer network liquid crystal display (AMPNLCD) of 1.79″ QCIF(176 × 144) was fabricated. The operating mode of normally black was reversed to normally white by using crossed polarizers. The contrast ratio, as a result, was drastically increased, which was comparable to a conventional liquid crystal display. The response time at a typical driving voltage, 2.8 V, was also fast enough to play still image smooth. The electro-optic properties of AMPNLCD with crossed polarizers were evaluated and discussed.

Fabrication and Optical Properties of Erbium-Doped Polymer Films

The crosslinked, fluorinated, patterned and erbium (Er3+) doped polymer films were fabricated by UV-curing after a simple spin-coating. Er3+-doped solutions containing Irgacure 819 used as a radical-type photoinitiator were prepared using erbium (III) trifluoromethane sulfonate as an erbium source, 2,2,3,3,4,4,5,5-octafluoropentyl acrylate as a fluorinated monomer, trimethylolpropane triacrylate and Ebecyl 810 resin as a crosslinking agent with various weight ratios. The fluorinated polymer films fabricated in this study showed the excellent transmission property more than 80% over visible and NIR region, and high thermal decomposition temperature about 310°C sufficient for optical device applications. From these results, the Er3+-doped polymer films developed in this study could be applied to optical telecommunication devices.