Doyoung Park

Determination of band gap energy (Eg) of Cu2ZnSnSe4 thin films: On the discrepancies of reported band gap values

We demonstrate experimental data to elucidate the reason for the discrepancies of reported band gap energy (Eg) of Cu2ZnSnSe4 (CZTSe) thin films, i.e., 1.0 or 1.5 eV. Eg of the coevaporated CZTSe film synthesized at substrate temperature (Tsub) of 370 °C, which was apparently phase pure CZTSe confirmed by x-ray diffraction (XRD) and Raman spectroscopy, is found to be around 1 eV regardless of the measurement techniques. However, depth profile of the same sample reveals the formation of ZnSe at CZTSe/Mo interface. On the other hand, Eg of the coevaporated films increases with Tsub due to the ZnSe formation, from which we suggest that the existence of ZnSe, which is hardly distinguishable from CZTSe by XRD, is the possible reason for the overestimation of overall Eg.

Optical and microstructural studies of atomically flat ultrathin In-rich InGaN∕GaN multiple quantum wells

Optical and microstructural properties of atomically flat ultrathin In-rich (UTIR) InGaN∕GaN multiple quantum well were investigated by means of photoluminescence (PL), time-resolved PL (TRPL), and cathodoluminescence (CL) experiments. The sample exhibits efficient trapping of the photoexcited carriers into quantum wells (QWs) and the effect of internal electric field in the QWs was found negligible by excitation power-dependent PL and TRPL. These phenomena were attributed to the nature of UTIR InGaN QWs, indicating the potential of this system for application in optoelectronic devices. Variation of TRPL lifetime across the PL band and spatially resolved monochromatic CL mapping images strongly suggest that there is micrometer-scale inhomogeneity in effective band gap in UTIR InGaN∕GaN QWs, which is originated from two types of localized areas.

Temperature-controlled synthesis of In2Ge2O7 nanowires and their photoluminescence properties

By controlling the heating temperature of a mixture of In and Ge powders, we have obtained monoclinic In2Ge2O7 nanowires at 600?700??C, whereas we have produced cubic In2O3 nanowires at 900??C. The In2Ge2O7 nanowires grown at 600??C were terminated by Au-containing nanoparticles, giving evidence that the vapour?liquid?solid model is the major growth mechanism. With the growth process at 700?900??C being dominated by a vapour?solid process, we have discussed the temperature-induced change in growth mechanisms. Photoluminescence measurements at 10?300?K revealed a broad visible emission centred at around 2.5?eV.

Characterization of Co-Evaporated

SmBa2Cu3O7 (SmBCO)-coated conductors were grown on IBAD-MgO templates by in-situ co-evaporation using a drum-in-dual-chambers system. The conductors, which had a coating thickness of up to 3 μm, showed a critical temperature of 92 ~ 94 K and a critical current of 150 ~ 625 A/cm-w at 77 K in a self-field. The phonon modes and cation disorder of the SmBCO-coated conductors were examined by polarized Raman scattering spectroscopy, X-ray diffraction (XRD), and scanning electron microscopy. XRD showed (00l) peaks for the SmBCO phase. The Raman spectra revealed common vibration modes, including Ba-Ba stretching, Cu2-Cu2 stretching, O(2)+/O(3)-, and apical O(4) modes. In addition, polarized-Raman spectra were obtained to examine the crystal orientation and grain growth behavior. Structural information from the Raman spectra turned out to be useful to determine the optimal thickness of SmBCO-coated conductors with high critical currents.

{\rm SmBa}_{2}{\rm Cu}_{3}{\rm O}_{7}

Cu(In1−xGax)Se (CIGS) based thin film solar cells are usually built by depositing CdS as a buffer layer and ZnO as a window layer on top of the CIGS absorber layer. In order to optimize their performances, it is essential to understand the interactions between the layers. In this study, we have investigated the interactions between the layers by examining the optical properties of the CIGS solar cell structure at each step of the fabrication process―CIGS, CIGS/CdS, and CIGS/CdS/ZnO― using photoluminescence and Raman spectroscopic imaging techniques. The images of the intensity of the Raman peak at 175 cm−1 due to the A1 vibration mode show that the homogeneity improves after CdS deposition. Micro-PL intensity imaging also confirmed this observation. Furthermore, the PL peak intensity and the energy position vary after each layer deposition.

Coated Conductors by Polarized-Raman Scattering Spectroscopy

Understanding of grain boundary (GB) is critical for photovoltaic applications since electron-hole recombination at GBs determines the conversion efficiency. However, our local electrical and optical analysis shows positive potential at GBs in Cu(In,Ga)Se2 (CIGS), which suppresses the recombination at GBs. We report on a direct measurement of potential distribution and local electrical transport on the surface of photovoltaic CIGS using a nano-scale electrical characterization of Kelvin probe microscopy and conductive atomic force microscopy. This reveals that the positively charged surface potential at GB is expected to increase the minority carrier collection and the enhanced current at GB leads to large carrier mobility and electron-hole separation at the GBs. Micro-Raman scattering results helps to analyze electrical behavior from defect analysis.

Spectroscopic imaging study on CIGS thin film solar cells

Raman scattering and photoluminescence on Cu 2 ZnSnSe 4 thin films grown by thermal co‐evaporation were performed. The photoluminescence spectrum shows a peak below 1.0 eV. The Raman spectra of Cu 2 ZnSnSe 4 show two main peaks at 170, 192–195 cm −1 and additional Raman mode at 260 cm −1 , which is ascribed to the Cu 2− x Se phase. The lateral distribution of the Cu 2− x Se phase in Cu 2 ZnSnSe 4 thin films is investigated by scanning micro‐Raman scattering. In Cu‐rich samples, the distribution of the Cu 2− x Se phase is found to be uneven.

Nano-scale observation of charge transport and potential distribution of photovoltaic Cu(In,Ga)Se 2 thin-films

Raman scattering and photoluminescence on Cu 2 ZnSnSe 4 thin films grown by thermal co‐evaporation were performed. The photoluminescence spectrum shows a peak below 1.0 eV. The Raman spectra of Cu 2 ZnSnSe 4 show two main peaks at 170, 192–195 cm −1 and additional Raman mode at 260 cm −1 , which is ascribed to the Cu 2− x Se phase. The lateral distribution of the Cu 2− x Se phase in Cu 2 ZnSnSe 4 thin films is investigated by scanning micro‐Raman scattering. In Cu‐rich samples, the distribution of the Cu 2− x Se phase is found to be uneven.