Y. I. Kim

Relationship between initial and time-dependent cracks in charge transport of solution-processed organic thin-film transistor

ABSTRACT We report on the comprehensive effect of initial and time-dependent crack on the charge transport of 6, 13-bis (triisopropylsilyethynyl) pentacene (TIPS-PEN) thin-film transistors (TFTs) upon exposure to air. Among the two types of TIPS-PEN films, with and without initial thermal cracks, temporally generated cracks were found to be severely affected by the initial status of the thermal cracks upon air-exposure. After 12 months of air-exposure, the charge transport characteristics of TIPS-PEN TFTs with initial thermal cracks degraded up to 98%, while those without initial thermal cracks degraded only 88%. Based on our long-term experimental results, we conclude that the initial physical status of the cracks play a significant role in the growth of time-dependent cracks and the capability of charge transport in solution-processed TFTs.

Dependence of the hybrid solar cell efficiency on the thickness of ZnO nanoparticle optical spacer interlayer

ABSTRACT Optical spacer interlayers are commonly used in solar cells to improve the power conversion efficiency (PCE) of the device. This paper reports a study on the effect of zinc oxide (ZnO) optical spacer interlayer on PCPDTBT:ZnO hybrid solar cell. Optical spacers help to concentrate the spatial distribution of electric field due to light onto the active layer, and thus, improve the light absorption in the active layer. Finite-Difference Time-Domain (FDTD) calculations was carried out on stacked solar cell structure using a simulation software, Lumerical FDTD solutions. Maxwells equations were solved and the light absorbed by the active layer was computed. The design was optimized with regard to active layer thickness and ZnO optical spacer thickness.

Signal-to-noise ratio measurement and radiation damage study of silicon PIN diode with a proton beam

We designed and fabricated silicon PIN diodes having p+ active area of 1.0 cm × 1.0 cm on a 5-in. high resistivity (≫5 kΩ⋅cm) n-type FZ (float zone), 380 μm-thick silicon wafer. For signal-to-noise ratio (SNR) measurement of the diode, we required that the fabricated diode should be fully depleted at the operation voltage and the level of the leakage current of the sensor should be less than 10 nA/cm2 at the operation voltage. The SNR of the diode was measured by using a 45 MeV proton beam from the MC-50 cyclotron at Korea Institute of Radiological and Medical Sciences (KIRAMS) in Seoul, Korea. Experiment was performed at room temperature and 46 volts was biased to fully deplete the diode. The SNR of the diode was measured to be 20.8 after corrected for the minimum ionizing particle. The different PIN diode which has relatively large leakage current was used for radiation damage test with the same proton beam. The leakage currents as a function of the reverse bias voltages were measured before and after the proton beam irradiation. The prototype was irradiated with 1.4 ×107 fluence. We annealed the diode after irradiation of the proton beam and measured the leakage current dependence on times at the operation voltage. We present the understanding of radiation-induced detector deterioration and the SNR of the silicon diode in this paper.

Performance test of the silicon PIN diode with radioactive sources

Silicon PIN diodes with an active area of 1 cm × 1 cm were fabricated on a high resistivity, (100)-oriented, n-type, 380-μm thick and 5-inch silicon wafer. These diodes were originally fabricated on purpose to monitor strip sensor fabrication processes. To evaluate the performance of the silicon PIN diode, we measured the signal-to-noise ratio (SNR) and energy resolution using radioactive sources. We measured the radiation detection responses of the silicon PIN diodes and observed photopeaks from radioactive sources, Am-241, Sr-90 and Cs-137. The spectrum by simulation was compared with those of the source tests to understand the measurement results. The tests with Am-241 and Cs-137 sources used random trigger because γ fully deposits its energy in a silicon PIN diode. Another PIN diode was used for trigger purpose with Sr-90, β source. Am-241 test showed a sharp peak of 59.5 keV γ as we expected. The γ peak was used for calibration that converts ADC value into energy. We used Cs-137 source to compare a resolution at the lower energy level (32 keV γ) with 59.5 keV γ. Two different conditions were set for Cs-137; without a lead-block and with 3mm-lead-block to reduce β background. In the test of Sr-90, the SNR of the silicon PIN diode was measured to be 36.0. We present the energy resolution and the SNR of the diode measured by using the radioactive sources.

Prototype development of silicon array sensor for x-ray detector

A prototype of silicon array sensor with 256 pixels and a frontend electronics system for its readout was developed for X-ray imaging. The characteristics of the sensor and some test results of the electronics system are presented.