Yong Su Kim

Deep level trapped defect analysis in CH3NH3PbI3 perovskite solar cells by deep level transient spectroscopy

We report the presence of defects in CH3NH3PbI3, which is one of the main factors that deteriorates the performance of perovskite solar cells. Although the efficiency of the perovskite solar cells has been improved by curing defects using various methods, deeply trapped defects in the perovskite layer have not been systematically studied, and their function is still unclear. The comparison and analysis of defects in differently prepared perovskite solar cells reveals that both solar cells have two kinds of deep level defects (E1 and E2). In the one-pot solution processed solar cell, the defect state E1 is dominant, while E2 is the major defect in the solar cell prepared using the cuboid method. Since the energy level of E1 is higher than that of E2, the cuboid solar cell shows higher open-circuit voltage and efficiency.

Study on the molecular distribution of organic composite films by combining photoemission spectroscopy with argon gas cluster ion beam sputtering

X-ray/ultraviolet photoelectron spectroscopy (XPS/UPS) and argon gas cluster ion beam (GCIB) sputtering were combined to directly study the molecular configurations of organic composite films consisting of more than two different materials. In contrast to Ar ion sputtering, Ar GCIB sputtering does not critically distort the chemical states and atomic compositions of organic materials, thereby enabling the chemical structures of uncontaminated bulk regions and air-exposed surface regions of organic materials to be precisely examined. Using this combination, the molecular configurations of single-wall and multiwall carbon nanotube (SWNT and MWCNT) films, composed of corresponding CNTs and surfactants, could be specifically characterized based on the chemical state transitions in the C 1s core level depth profiles. Further, the XPS/UPS spectra showed variations in the chemical states with increasing sputtering time, which were fully consistent with the surface morphologies observed in the high-resolution atomic force microscopy and high-resolution secondary electron microscopy images. Hence, we believe that combining XPS/UPS with Ar GCIB sputtering can be an excellent method for investigating the molecular distributions of organic composite films.

Direct comparative study on the energy level alignments in unoccupied/occupied states of organic semiconductor/electrode interface by constructing in-situ photoemission spectroscopy and Ar gas cluster ion beam sputtering integrated analysis system

Through the installation of electron gun and photon detector, an in-situ photoemission and damage-free sputtering integrated analysis system is completely constructed. Therefore, this system enables to accurately characterize the energy level alignments including unoccupied/occupied molecular orbital (LUMO/HOMO) levels at interface region of organic semiconductor/electrode according to depth position. Based on Ultraviolet Photoemission Spectroscopy (UPS), Inverse Photoemission Spectroscopy (IPES), and reflective electron energy loss spectroscopy, the occupied/unoccupied state of in-situ deposited Tris[4-(carbazol-9-yl)phenyl]amine (TCTA) organic semiconductors on Au (ELUMO: 2.51 eV and EHOMO: 1.35 eV) and Ti (ELUMO: 2.19 eV and EHOMO: 1.69 eV) electrodes are investigated, and the variation of energy level alignments according to work function of electrode (Au: 4.81 eV and Ti: 4.19 eV) is clearly verified. Subsequently, under the same analysis condition, the unoccupied/occupied states at bulk region of TCTA/Au structures are characterized using different Ar gas cluster ion beam (Ar GCIB) and Ar ion sputtering processes, respectively. While the Ar ion sputtering process critically distorts both occupied and unoccupied states in UPS/IPES spectra, the Ar GCIB sputtering process does not give rise to damage on them. Therefore, we clearly confirm that the in-situ photoemission spectroscopy in combination with Ar GCIB sputtering allows of investigating accurate energy level alignments at bulk/interface region as well as surface region of organic semiconductor/electrode structure.

Direct analytical method of contact position effects on the energy-level alignments at organic semiconductor/electrode interfaces using photoemission spectroscopy combined with Ar gas cluster ion beam sputtering

UNLABELLED Poly(3, 4-ethylenedioxythiophene) (PEDOT) polymerized with poly(4-styrenesulfonate) (PSS) is one of the most widely used conducting organic electrodes owing to its outstanding optical/electrical properties and high work function. Because its work function depends significantly on the molecular arrangements between PEDOT and PSS molecules on the surface, the contact position of PEDOT PSS films on organic semiconductors (OSCs) must also be an essential consideration. However, existing analysis methods based on in situ deposition/analysis are limited in their ability to accurately investigate the electronic structures of the buried interface regions under the solution-processed electrode or OSC layer in organic devices. Therefore, to overcome such limitations, we propose a top-down method based on photoemission spectroscopy analysis combined with Ar gas cluster ion beam (GCIB) sputtering. Through this method, both energy-level alignments and molecular distributions at various OSC/electrode interfaces can be successfully characterized without reference to any deposition process.

Effective work function engineering for a TiN/XO(X = La, Zr, Al)/SiO2 stack structures

In this study, we demonstrated that work function engineering is possible over a wide range (+200 mV to −430 mV) in a TiN/XO (X = La, Zr, or Al)/SiO2 stack structures. From ab initio simulations, we selected the optimal material for the work function engineering. The work function engineering mechanism was described by metal diffusion into the TiN film and silicate formation in the TiN/SiO2 interface. The metal doping and the silicate formation were confirmed by transmission electron microscopy and energy dispersive spectroscopy line profiling, respectively. In addition, the amount of doped metal in the TiN film depended on the thickness of the insertion layer XO. From the work function engineering technique, which can control a variety of threshold voltages (Vth), an improvement in transistors with different Vth values in the TiN/XO/SiO2 stack structures is expected.

Direct evidence of flat band voltage shift for TiN/LaO or ZrO/SiO 2 stack structure via work function depth profiling

We demonstrated that a flat band voltage (VFB) shift could be controlled in TiN/(LaO or ZrO)/SiO2 stack structures. The VFB shift described in term of metal diffusion into the TiN film and silicate formation in the inserted (LaO or ZrO)/SiO2 interface layer. The metal doping and silicate formation confirmed by using transmission electron microscopy (TEM) and energy dispersive spectroscopy (EDS) line profiling, respectively. The direct work function measurement technique allowed us to make direct estimate of a variety of flat band voltages (VFB). As a function of composition ratio of La or Zr to Ti in the region of a TiN/(LaO or ZrO)/SiO2/Si stack, direct work function modulation driven by La and Zr doping was confirmed with the work functions obtained from the cutoff value of secondary electron emission by auger electron spectroscopy (AES). We also suggested an analytical method to determine the interface dipole via work function depth profiling.

Study on the disparate transition behaviors of the electrical/physical properties in PEDOT:PSS film depending on solvent species under a follow-up solution-treatment process

UNLABELLED In most solution-processed organic devices, a poly(3,4-ethylenedioxythiophene) (PEDOT) polymerized with poly(4-styrenesulfonate) (PSS) film is inevitably affected by various conditions during the subsequent solution-coating processes. To investigate the effects of direct solvent exposure on the properties of PEDOT polymerized with PSS (PEDOT:PSS) films, photoemission spectroscopy-based analytical methods were used before and after solvent-coating processes. Our results clearly indicate that PEDOT PSS films undergo a different transition mechanism depending on the solubility of the solvent in water. The water-miscible solvents induce the solvation of hydrophilic PSS chains. As a result, this process allows the solvent to diffuse into the PEDOT PSS film, and a conformational change between PEDOT and PSS occurs. On the other hand, the water-immiscible organic solvents cause the partial adsorption of solvent molecules at the PE surface, which leads to changes in the surface properties, including work function. Based on our finding, we demonstrate that the energy-level alignments at the organic semiconductor/electrode interface for the PEDOT PSS films can be controlled by simple solvent treatments.

Probing the persistence of energy-level control effects at organic semiconductor/electrode interfaces based on photoemission spectroscopy combined with Ar gas cluster ion beam sputtering

Oxygen (O2) plasma treatment is one of the most widely applied methods for modifying the electrode work function. However, owing to the instability of O2-plasma treatment effects under air-exposed conditions, it is necessary to confirm whether the O2-plasma treatment effects can be continuously maintained at organic semiconductor/electrode interfaces in realistic devices. In the present study, the electronic structures of organic semiconductor/O2-plasma treated electrode interfaces were characterized by using in situ deposition and ultraviolet photoemission spectroscopy analysis. The structures of the corresponding samples were re-analyzed after a 1-week-long exposure to air to confirm the energy-level changes. To achieve this, we inceptively designed the studies of the energy level alignments of air-exposed samples based on the photoemission spectroscopy combined with Ar gas cluster ion beam sputtering process. The results of our studies clearly confirm the consistency of O2-plasma treatment effects at organic semiconductor/electrode interfaces. In addition, we confirmed the preservation of controlled energy-level structures at C60/Au interfaces by examining the relative rates of electron transfer at the C60/Au interfaces, obtained from photoluminescence (PL) measurements.

Nanoscale Electrical Degradation of Silicon–Carbon Composite Anode Materials for Lithium-Ion Batteries

High-performance lithium-ion batteries (LIBs) are in increasing demand for a variety of applications in rapidly growing energy-related fields including electric vehicles. To develop high-performance LIBs, it is necessary to comprehensively understand the degradation mechanism of the LIB electrodes. From this viewpoint, it is crucial to investigate how the electrical properties of LIB electrodes change under charging and discharging. Here, we probe the local electrical properties of LIB electrodes with nanoscale resolution by scanning spreading resistance microscopy (SSRM). Via quantitative and comparative SSRM measurements on pristine and degraded LIB anodes of Si-C composites blended with graphite (Gr) particles, the electrical degradation of the LIB anodes is visualized. The electrical conductivity of the Si-C composite particles considerably degraded over 300 cycles of charging and discharging, whereas the Gr particles maintained their conductivity.