Minho Joo

Microstructure and Light-Scattering Properties of ZnO:Al Films Prepared Using a Two-Step Process through the Control of Oxygen Pressure

ZnO:Al films were prepared using a two-step process involving the control of oxygen pressure. The seed layers were deposited under various Ar to oxygen pressure ratios, and the bulk layers were prepared under pure Ar. The growth mode was systematically examined and clearly different microstructures were shown according to the deposition condition of the seed layer. With increase of oxygen pressure, the crystallinity and the degree of (002) texturing increased. The haze values of etched films also increased with increasing oxygen pressure, which was explained by the grain-structure of as-deposited films.

Investigation of Thermally Induced Degradation in CH 3 NH 3 PbI 3 Perovskite Solar Cells using In-situ Synchrotron Radiation Analysis

In this study, we employ a combination of various in-situ surface analysis techniques to investigate the thermally induced degradation processes in MAPbI3 perovskite solar cells (PeSCs) as a function of temperature under air-free conditions (no moisture and oxygen). Through a comprehensive approach that combines in-situ grazing-incidence wide-angle X-ray diffraction (GIWAXD) and high-resolution X-ray photoelectron spectroscopy (HR-XPS) measurements, we confirm that the surface structure of MAPbI3 perovskite film changes to an intermediate phase and decomposes to CH3I, NH3, and PbI2 after both a short (20 min) exposure to heat stress at 100 °C and a long exposure (>1 hour) at 80 °C. Moreover, we observe clearly the changes in the orientation of CH3NH3+ organic cations with respect to the substrate in the intermediate phase, which might be linked directly to the thermal degradation processes in MAPbI3 perovskites. These results provide important progress towards improved understanding of the thermal degradation mechanisms in perovskite materials and will facilitate improvements in the design and fabrication of perovskite solar cells with better thermal stability.

Experimental observation of local electrical signature of suspended graphene grown via chemical vapour deposition method

We employ electrostatic force microscope (EFM) techniques to explore local electrical properties of suspended graphene on Cu foil, SiO2/Si and PET substrate. By using electrical modulation of amplitude in a tapping mode atomic force microscope tip, we can obtain distinguished electrostatic force amplitude mapping of graphene on various substrates. In particular, at nano-valley domains on Cu and a SiO2/Si surface, relatively weaker electrostatic attractive interaction is observed than at nano-peak domains. In SiO2/Si, we find that electrostatic force distribution of graphene still follows the substrate surface morphology. Furthermore, employing EFM to graphene on a PET system can be suggested as a facile tool to investigate electrical performance of graphene.

Investigation of shunt path evolution originated from transparent conductive oxides in Si-based thin film solar cells

The evolution of the shunt path on the performance of Si-based thin film solar cells with an glass/Al-doped ZnO (AZO)/amorphous (a)-Si:H/a-SiGe:H/Al was investigated by conductive atomic force microscopy, electroluminescence measurement, and transmission electron microscopy. AZO films were highly textured for the light management before the deposition of absorption layers. The cell performance was found to be strongly dependent on the existence of nanocracks formed in a-Si:H/a-SiGe:H layers. The defects by nanocracks are expected to attribute to the leakage current in the cells. The authors introduce two types of shunt path evolution modes: pinhole defects (type A) and highly textured groove of AZO film (type B). Both crack defects by types A and B induced high leakage current, leading to a relatively reduced fill factor on the performance.

In Situ Spectroscopic and Computational Studies on a MnO2–CuO Catalyst for Use in Volatile Organic Compound Decomposition

In situ near-edge X-ray absorption fine structure (NEXAFS) spectroscopy and density functional theory calculations were conducted to demonstrate the decomposition mechanism of propylene glycol methyl ether acetate (PGMEA) on a MnO2–CuO catalyst. The catalytic activity of MnO2–CuO was higher than that of MnO2 at low temperatures, although the pore properties of MnO2 were similar to those of MnO2–CuO. In addition, whereas the chemical state of MnO2 remained constant following PGMEA dosing at 150 °C, MnO2–CuO was reduced under identical conditions, as confirmed by in situ NEXAFS spectroscopy. These results indicate that the presence of Cu in the MnO2–CuO catalyst enables the release of oxygen at lower temperatures. More specifically, the released oxygen originated from the Mn–O–Cu moiety on the top layer of the MnO2–CuO structure, as confirmed by calculation of the oxygen release energies in various oxygen positions of MnO2–CuO. Furthermore, the spectral changes in the in situ NEXAFS spectrum of MnO2–CuO following the catalytic reaction at 150 °C corresponded well with those of the simulated NEXAFS spectrum following oxygen release from Mn–O–Cu. Finally, after the completion of the catalytic reaction, the quantities of lactone and ether functionalities in PGMEA decreased, whereas the formation of C=C bonds was observed.

Enhancement of crystallinity in ZnO:Al films using a two-step process involving the control of the oxygen pressure

The ZnO:Al films were prepared using a two-step process through the control of oxygen pressure by DC-pulsed magnetron sputtering. The seed layers were prepared with various Ar to oxygen pressure, and the bulk layers were deposited under pure Ar. At the seed-layer condition of Ar/O 2 = 24/1 (Ar rich), the seed layer showed grains that led to the formation of hillock, and the grains of seed layer were seem to act as nucleation sites for columnar growth of bulk layer. While, at the seed-layer condition of Ar/O 2 =4/1 (oxygen rich), grains show large lateral growth with a nonuniform distribution, and a number of dislocations are shown in grains. As oxygen pressure during the deposition of seed layer increased, the decrease of local strain (or the increase of crystallinity) was observed. The etched surface showed the crater-like structure and the abrupt morphology change appeared at high oxygen pressure, which was likely due to nonuniform grain structure and/or the decrease in the number of chemical attack sites by the drastic decrease of grain boundary. The haze values increased with increasing oxygen pressure, which was explained by the change of crater size, as shown in the AFM image.