Junwoo Lee

Green-Solvent-Processable, Dopant-Free Hole-Transporting Materials for Robust and Efficient Perovskite Solar Cells

In addition to having proper energy levels and high hole mobility (μh) without the use of dopants, hole-transporting materials (HTMs) used in n-i-p-type perovskite solar cells (PSCs) should be processed using green solvents to enable environmentally friendly device fabrication. Although many HTMs have been assessed, due to the limited solubility of HTMs in green solvents, no green-solvent-processable HTM has been reported to date. Here, we report on a green-solvent-processable HTM, an asymmetric D-A polymer (asy-PBTBDT) that exhibits superior solubility even in the green solvent, 2-methylanisole, which is a known food additive. The new HTM is well matched with perovskites in terms of energy levels and attains a high μh (1.13 × 10-3 cm2/(V s)) even without the use of dopants. Using the HTM, we produced robust PSCs with 18.3% efficiency (91% retention after 30 days without encapsulation under 50%-75% relative humidity) without dopants; with dopants (bis(trifluoromethanesulfonyl) imide and tert-butylpyridine, a 20.0% efficiency was achieved. Therefore, it is a first report for a green-solvent-processable hole-transporting polymer, exhibiting the highest efficiencies reported so far for n-i-p devices with and without the dopants.

Control of the molecular geometry and nanoscale morphology in perylene diimide based bulk heterojunctions enables an efficient non-fullerene organic solar cell

Herein we present the design of three perylene diimide (PDI) derivatives with different molecular geometries; namely the monomeric PDI1, the bay-linked PDI2 dimer, and the bay-linked PDI4 tetramer with a 9,9′-spirobifluorene core that are utilized as electron acceptors in non-fullerene organic solar cells (OSCs). In all cases the PTB7-Th polymer is used as the electron donor. Among the three PTB7-Th:PDI systems, the highest power conversion efficiency (PCE) is obtained by the PDI4-based OSC device that exhibits a maximum PCE = 6.44% followed by the PDI2-based (PCE = 5.32%) and PDI1-based (PCE = 2.48%) devices. The detailed study of the photoluminescence quenching, morphology and temperature-dependent charge transport properties of the three systems reveal that the highest PCE of PTB7-Th:PDI4 is a consequence of the three-dimensional (3D) molecular architecture of PDI4 that tunes energetic disorder in the PDI phase and contributes to the improvement of electron transport. Transient photovoltage characterization experiments further identify that the actual effect coming from the 3D molecular geometry of the PDI4 acceptor on PCE is the minimization of non-geminate charge recombination losses. This study provides updated guidelines for optimizing further the molecular structure of 3D small molecular electron acceptors that can be used in highly efficient non-fullerene OSCs.

Portable microwave air plasma device for wound healing

A portable microwave air plasma has been developed for safe and effective wound healing. The device is operated by a fixed microwave power and two different air gas flows (main and cooling air flow). It was found that the speeds of the two air flows determine the stability of the plasma jet and gas temperature and thereby regulate the concentrations of the individual reactive species. Two different regimes, i.e. the NO abundant (0.1 slm main air flow) and ozone abundant regimes (4 slm main air flow), were identified as suitable for wound healing without thermal damage and toxicity. These regimes show similar plasma characteristics (e.g. less than 40 °C at the treatment point, less than 4 ppm of NO2) except for different NO and ozone amounts. Both regimes show more than twice as fast wound healing speed compared with the untreated case without any histological damages. Faster healing speed with intrinsic ozone safety make the NO abundant regime the best operation regime for wound healing. Finally, the stability of the developed device was demonstrated by a one-hour continuous operation test with a 24 V battery.

Comparison of measured 2D ELMs with synthetic images from BOUT++ simulation in KSTAR

A detailed study of edge-localized mode (ELM) dynamics in the KSTAR tokamak is performed using a two-dimensional (2D) electron cyclotron emission imaging (ECEI) diagnostic system. Highly coherent mode structures rotating in the poloidal view plane are routinely observed in the inter-ELM pedestal region where the optical thickness for ECE rapidly changes and the interpretation of emission intensity is complicated. To have confidence on the measurements, the observed images are compared with synthetic images of the ELM structure deduced from three-field BOUT++ simulations. The synthetic process considers instrumental effects of the ECEI diagnostic, intrinsic broadening of the ECE and background noise. The synthetic 2D images highly resemble the observed structure, providing confidence that the ELM dynamics can be visualized by ECEI.

Self-prevention of instability in a low-power microwave Ar plasma jet for biomedical applications

The behavior of a low-power microwave Ar plasma jet according to the target shape and distance is investigated. The plasma jet shows distinct behavior when it contacts a human finger or grounded metals. No plasma channel and no attraction of the jet to the human finger and metal plate are observed in contrast to low-frequency plasmas. Glow-to-arc transition does not occur even at a very small target distance (<1 mm) between a sharp metal tip and bare electrodes. It is a highly favorable property of the microwave plasma for biomedical applications. Reflection coefficient, current, electric field and electron density are investigated to find the mechanism. This unique phenomenon is caused by the characteristic of microwave frequency systems. A decrease of the target distance induces impedance mismatching leading to the reduction of net input power. It is found that the change in the geometry of the plasma jet is the dominant factor for impedance mismatching. This prevents changes in the discharge regime including glow-to-arc transition, similar to ballast. The mechanism is different from the instability prevention methods including the dielectric barrier in low-frequency systems. Insignificant electric field induced on the metal plate by the impedance mismatching can be the reason for the absence of the plasma channel. Emission intensities of reactive species of the plasma jet are almost uniform regardless of the target distance. Electrical safety and performance can be ensured by the low-power microwave plasma jet.

Green-solvent processable semiconducting polymers applicable in additive-free perovskite and polymer solar cells: molecular weights, photovoltaic performance, and thermal stability

In this study, we demonstrated the effects of the molecular weight (MW) of a green-solvent processable semiconducting polymer (asy-PBTBDT) on its photovoltaic performance and device thermal stability in green processed devices for the first time. The asy-PBTBDT with a high MW (132 kDa) had the highest μh values (4.91 × 10−3 cm2 V−1 s−1 without dopants and 5.77 × 10−3 cm2 V−1 s−1 with dopants) as a result of increase in the π–π stacking along with MW as compared to low-MW asy-PBTBDTs (27 and 8 kDa). The high-MW asy-PBTBDT with a high μh achieved the best power conversion efficiencies of 18.2% and 20.0% for the undoped and doped states in PerSCs, respectively, and 5.7% in PSCs in green processed devices. Furthermore, the glass transition temperature increased with an increase in MW; this indicated an effective decrease in heat-induced morphological degradation in the photovoltaic devices. In addition, an increase in the chain density along with MW led to good robustness against humidity and oxygen.

Large-Aperture Broadband Polarization Rotator for the KSTAR ECE Imaging System

The electron cyclotron emission (ECE) imaging system installed in the KSTAR tokamak has been designed for 2D electron temperature measurement based on the X-mode 2nd harmonic ECE radiation at a magnetic field ~ 2 T. For operation at higher magnetic fields ( ~ 3?3.5 T) in the future, the frequency of the X-mode 2nd harmonic ECE ( > 170 GHz) exceeds the detectable range of the system. Instead of extending the detector frequency range, large-aperture half-wave plates for polarization rotation have been developed to utilize the O-mode fundamental ECE, which is within the detectable range ( ~ 65?135 GHz) of the present system.

New compact and efficient local oscillator optic system for the KSTAR electron cyclotron emission imaging system

Electron cyclotron emission imaging (ECEI) diagnostic on Korean Superconducting Tokamak Advanced Research utilizes quasi-optical heterodyne-detection method to measure 2D (vertical and radial) Te fluctuations from two toroidally separated poloidal cross section of the plasma. A cylindrical lens local oscillator (LO) optics with optical path length (OPL) 2-2.5 m has been used in the current ECEI system to couple the LO source to the 24 vertically aligned array of ECE detectors. For efficient and compact LO optics employing the Powell lens is proposed so that the OPL of the LO source is significantly reduced from ∼2.0 m to 0.4 m with new optics. The coupling efficiency of the LO source is expected to be improved especially at the edge channels. Results from the optical simulation together with the laboratory test of the prototype optics will be discussed in this paper.

Multimode excitation during the inter-ELM-crash periods in KSTAR H-mode plasma

Temporal and spatial modulation of the edge localized mode (ELM) structure has been observed during the inter-ELM-crash period by toroidally-separated two electron cyclotron emission imaging systems. The observed modulation is interpreted as a beat wave of two modes with adjacent toroidal mode number. An additional assumption is that each mode has to have a different poloidal rotation speed. In nonlinear simulation, the low-n mode can be driven by locking between the dominant modes. The modulation is reconstructed using beat waves not the locking of modes.

Enhancement of microwave plasma characteristics for biomedical applications using pulse modulation method

Pulse modulation technique was applied for microwave excited atmospheric pressure air and Ar plasmas with fixed average power. Breakdown and sustain powers were measured and UV-Vis emission spectra were observed. As the duty ratio decreased, pulse modulated plasmas showed enhanced results as compared to continuous signal excited plasmas. Breakdown powers of both of air and Ar plasmas were reduced as duty ratio decreased. Reactive species including hydroxyl radicals, atomic oxygen and nitric oxide were increased with the decrease of duty ratio. The change of breakdown power and increase of reactive species give advantages for biomedical applications. Gas temperature was reduced also in both of air and Ar plasmas with the decrease of duty ratio. The results related to the high energy electrons which were generated during on-time of the microwave signal. Blood coagulation experiment of the air plasma was conducted and it showed ~7 times faster coagulation than natural coagulation.