Dae-Ho Son

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.

Solution-processed Cu2ZnSnS4 absorbers prepared by appropriate inclusion and removal of thiourea for thin film solar cells

We studied how to effectively add/remove organic chemicals to/from CZTS precursor thin films to prepare uniform CZTS thin films that show optimal properties. We used multi-functional thiourea, as both a stabiliser and a source of sulphur, to prepare the precursor solutions. This is because it forms complexes with metal chlorides, which stabilise the CZTS precursor solutions and enable CZTS thin films to be spin-coated onto substrates thereby enabling fabrication of CZTS absorbers. However, the excess thiourea, required to stabilise the CZTS precursor solutions, induced the formation of a ZnS secondary phase in the CZTS thin films, which deteriorated the photovoltaic properties of the CZTS solar cells. We therefore pre-annealed the thin films to inhibit ZnS formation. We used the thiourea-stabilized CZTS precursor solutions and simple solution processing techniques to prepare CZTS precursor thin films, and optimized the pre-annealing temperature to fabricate CZTS solar cells that showed 5.29% efficiency.

Low Voltage, High Performance Thin Film Transistor with HfInZnO Channel and HfO2 Gate Dielectric

We fabricated thin film transistors (TFTs) using HfInZnO thin films as active channel layers. The thin films of HfInZnO were deposited by co-sputtering from HfO 2 and InZnO targets. The HfInZnO TFTs were investigated according to the radio-frequency power applied to the HfO 2 target. The transistor on and off currents were greatly influenced by the composition of Hf atoms suppressing the formation of oxygen vacancies. The electrical characteristics of the TFTs show a field-effect mobility of 3.53 cm 2 V ―1 s ―1 , a threshold voltage of 1.28 V, an on/off ratio of 1.4 × 10 ―7 , and a subthreshold swing of 95 mV/dec.

Pixel-isolation liquid crystals formed by polarization-selective UV-curing of a prepolymer containing cinnamate oligomer

A pixel isolated liquid crystal display was fabricated by polarization-selective anisotropic photoreaction of a prepolymer containing a cinnamate oligomer. The cinnamate oligomer was mainly distributed on the surface region of a UV-cured polymer wall. Anisotropic photo-dimerization of cinnamate moiety was achieved by polarized UV exposure. It was found that the polymer walls containing cinnamate dimers formed by polarized UV exposure showed ordered orientation of LC molecules at the boundary of the polymer walls resulting in electro-optic performance improvement.

Effect of Zr Addition on Sol-Gel Processed InZrZnO Thin-Film Transistor

In this study, solution-processed InZrZnO thin films and a newly developed thin-film transistors (TFTs) were fabricated and characterized electrically. The InZrZnO TFTs were investigated according to the variation of the Zr-metal doping concentration. It was found that the off currents of InZrZnO TFTs were greatly influenced by the composition of Zr atoms suppressing formation of oxygen vacancies. The optimal transistor of InZrZnO channel layer shows good performance properties. The electrical characteristics of a 2.92 mol% Zr-doped InZnO TFT shows a field effect mobility of 0.05 cm2 V−1 s−1, a threshold voltage of 6.1 V, an on/off ratio of 1.4 × 107, and a subthreshold swing of 0.42 V/dec. The InZrZnO TFT also shows better bias stability than undoped InZnO TFT, suggesting Zr plays a key role in regards to stability of TFT.

The effect of bi-component acrylate prepolymers on the phase separation and electro-optical properties of pixel-isolated liquid crystals

Abstract In this paper, we propose a pixel isolated liquid crystal (PILC) display using prepolymers containing bi-component acrylate monomers. The phase separation behavior and polymer wall structure of PILC were closely related to the solubility parameter and surface free energy of acrylate monomers. The polymer wall structure of bi-component prepolymers was readily controlled by the composition of acrylate monomers. We also investigated the electro-optical performances of PILC devices prepared by using bi-component prepolymers and found that they were related with the surface anchoring effect of polymer walls. Anchoring energy between LC and polymer walls was decreased by introducing fluorinated acrylate monomer into prepolymers and the driving properties of LC molecules by an external electric field were improved. Thus, the control method of polymer wall structure by using bi-component prepolymers can be applied to various types of LC–polymer composite systems for obtaining improved flexible displays.

Effect of ZnO Layer Thickness on Efficiency of Cu(In,Ga)Se2 Thin-film Solar Cells

The effect of intrinsic ZnO(i-ZnO) layer thickness on the efficiency of Cu(In,Ga)Se2 (CIGS) thin film solar cells was investigated using ITO/ZnO/CdS/CIGS/Mo structures. CIGS thin films were deposited on Mo-coated soda-lime glass using co-evaporation. CdS buffer layers of about 50nm thickness were then grown by chemical bath deposition on the top of CIGS layer. Finally, the ZnO and ITO layers were deposited using rf-magnetron sputtering, resulting in solar cells with ITO/ZnO/CdS/CIGS/Mo structure. From the optical and electrical characteristics of the solar cells, we found a close relationship between the transmittance of the ZnO layer and the efficiency of the solar cells. Several characteristics improved for solar cells with a 50 nm thick ZnO layer relative to those with both 90 nm thick and no ZnO layer. Therefore, we conclude that the optimum ZnO thickness for CIGS-based solar cells is around 50 nm.

Lattice-patterned LC-polymer composites containing various nanoparticles as additives

In this study, we show the effect of various nanoparticle additives on phase separation behavior of a lattice-patterned liquid crystal [LC]-polymer composite system and on interfacial properties between the LC and polymer. Lattice-patterned LC-polymer composites were fabricated by exposing to UV light a mixture of a prepolymer, an LC, and SiO2 nanoparticles positioned under a patterned photomask. This resulted in the formation of an LC and prepolymer region through phase separation. We found that the incorporation of SiO2 nanoparticles significantly affected the electro-optical properties of the lattice-patterned LC-polymer composites. This effect is a fundamental characteristic of flexible displays. The electro-optical properties depend on the size and surface functional groups of the SiO2 nanoparticles. Compared with untreated pristine SiO2 nanoparticles, which adversely affect the performance of LC molecules surrounded by polymer walls, SiO2 nanoparticles with surface functional groups were found to improve the electro-optical properties of the lattice-patterned LC-polymer composites by increasing the quantity of SiO2 nanoparticles. The surface functional groups of the SiO2 nanoparticles were closely related to the distribution of SiO2 nanoparticles in the LC-polymer composites, and they influenced the electro-optical properties of the LC molecules. It is clear from our work that the introduction of nanoparticles into a lattice-patterned LC-polymer composite provides a method for controlling and improving the composites electro-optical properties. This technique can be used to produce flexible substrates for various flexible electronic devices.

Influence of the ZnS precursor thickness on high efficiency Cu2ZnSn(S,Se)4 thin-film solar cells grown by stacked-sputtering and selenization process

CZTSSe thin-films were deposited by stacked sputtering methods (ZnS/SnS/Cu) and annealed with selenization. We adjusted the thickness of the ZnS precursor layer in CZT precursors. A 337 nm thickness of ZnS precursor was shown an efficiency of up to 9.1%. We investigated the secondary phases by Raman spectroscopy and Kelvin probe force microscopy with depth profiles. The Cu2SnSe3, ZnSe, and MoSe2 secondary phases appeared near the back contact region. The phase distributions of the CZTSSe thin-films are different depending on ZnS precursor thickness with different depths. This phase characterization can describe the influences to the device performance of the CZTSSe thin-film solar cells.