Dahyun Nam

A band-gap-graded CZTSSe solar cell with 12.3% efficiency

Although Cu2ZnSn(S,Se)4 (CZTSSe) has attracted attention as an alternative to CuInGaSe2 (CIGS) as an absorber material in solar cells, its low efficiency is a serious shortcoming preventing its commercialization. To realize a high-efficiency CZTSSe solar cell, improved grain crystallinity, inhibited secondary-phase formation, controlled defect generation, adequate Na content, and band gap grading are required in the absorber layer. Few studies have focused specifically on band gap grading. In this study, a method of using SeS2, a new potential chalcogenization source material, to control the S and Se contents in a CZTSSe absorber and its effects were investigated. Using an appropriate SeS2/Se weight ratio, band gap grading was realized within the depletion region. By increasing the value of VOC through band gap grading in the depletion region, a record VOC deficit of 0.576 V was achieved. Furthermore, the possibility of enhancing JSC through the formation of a type-inverted n-type phase at the absorber surface in response to an appropriate alignment of the conduction-band minimum energy level and the Fermi energy pinning level is discussed. By introducing the chalcogenization source material SeS2 during the annealing process, CZTSSe solar cells with a maximum efficiency of 12.3% were obtained.

Excitation energy dependent Raman spectrum of MoSe2.

Raman investigation of MoSe2 was carried out with eight different excitation energies. Seven peaks, including E1g, A1g, E2g1, and A2u2 peaks are observed in the range of 100–400 cm−1. The phonon modes are assigned by comparing the peak positions with theoretical calculations. The intensities of the peaks are enhanced at different excitation energies through resonance with different optical transitions. The A1g mode is enhanced at 1.58 and 3.82 eV, which are near the A exciton energy and the band-to-band transition between higher energy bands, respectively. The E2g1 mode is strongly enhanced with respect to the A1g mode for the 2.71- and 2.81-eV excitations, which are close to the C exciton energy. The different enhancements of the A1g and E2g1 modes are explained in terms of the symmetries of the exciton states and the exciton-phonon coupling. Other smaller peaks including E1g and A2u2 are forbidden but appear due to the resonance effect near optical transition energies.

Davydov Splitting and Excitonic Resonance Effects in Raman Spectra of Few-Layer MoSe2

Raman spectra of few-layer MoSe2 were measured with eight excitation energies. New peaks that appear only near resonance with various exciton states are analyzed, and the modes are assigned. The resonance profiles of the Raman peaks reflect the joint density of states for optical transitions, but the symmetry of the exciton wave functions leads to selective enhancement of the A1g mode at the A exciton energy and the shear mode at the C exciton energy. We also find Davydov splitting of intralayer A1g, E1g, and A2u modes due to interlayer interaction for some excitation energies near resonances. Furthermore, by fitting the spectral positions of interlayer shear and breathing modes and Davydov splitting of intralayer modes to a linear chain model, we extract the strength of the interlayer interaction. We find that the second-nearest-neighbor interlayer interaction amounts to about 30% of the nearest-neighbor interaction for both in-plane and out-of-plane vibrations.

Polarized Raman spectroscopy of Cu-poor and Zn-rich single-crystal Cu2ZnSnSe4

Cu 2ZnSnSe4 (CZTSe) is a p-type semiconductor which has been developed as an absorber layer of polycrystalline thin film solar cells. Generally, Cu-poor and Zn-rich compositions tend to give the highest solar conversion efficiencies. Raman spectroscopy has been used to detect secondary phases such as ZnSe and Cu 2SnSe3 in CZTSe thin films. However, the fundamental phonon modes in single-crystal CZTSe with a composition matching that of high-efficiency thin film solar cells have not yet been fully understood. We performed polarized Raman measurements on Cu-poor and Zn-rich single-crystal CZTSe and identified 12 peaks, including two low-frequency peaks. By comparing the polarization dependence of the Raman peaks with a group theoretical analysis, we concluded that the crystal structure of CZTSe single-crystal is kesterite and made appropriate peak assignments.

Polarization-independent light emission enhancement of ZnO/Ag nanograting via surface plasmon polariton excitation and cavity resonance.

In this study, we observed that the photoluminescence (PL) intensity of ZnO/Ag nanogratings was significantly enhanced compared with that of a planar counterpart under illumination of both transverse magnetic (TM) and transverse electric (TE)-mode light. In the TM mode, angle-resolved reflectance spectra exhibited dispersive dips, indicating cavity resonance as well as grating-coupled surface plasmon polariton (SPP) excitation. In the TE mode, cavity resonance only was allowed, and broad dips appeared in the reflectance spectra. Strong optical field confinement in the ZnO layers, with the help of SPP and cavity modes, facilitated polarization-insensitive PL enhancement. Optical simulation results were in good agreement with the experimental results, supporting the suggested scenario.

Raman Spectroscopic Study on Alkyl Chain Conformation in 1‐Butyl‐3‐methylimidazolium Ionic Liquids and their Aqueous Mixtures

Ionic liquids of 1-butyl-3-methylimidazolium ([BMIM]) cation with different anions (Cl- , Br- , I- , and BF4- ), and their aqueous mixtures were investigated by using Raman spectroscopy and dispersion-included density functional theory (DFT). The characteristic Raman bands at 600 and 624 cm-1 for two isomers of the butyl chain in the imidazolium cation showed significant changes in intensity for different anions as well as in aqueous solutions. The area ratio of these two bands followed the order I- >Br- >Cl- >BF4- (in terms of the anion X in [BMIM]X), indicating that the butyl chain of [BMIM]I tends to adopt the trans conformation. The butyl chain was found to adopt the gauche conformation upon dilution, irrespective of the anion type. The Raman bands in the butyl C-H stretch region for [BMIM]X (X=Cl- , Br- , and I- ) blueshifted significantly with the increase in the water concentration, whereas that for [BMIM]BF4 changed very little upon dilution. The blueshift in the C-H stretch region upon dilution also followed the order: [BMIM]I>[BMIM]Br>[BMIM]Cl>[BMIM]BF4 , the same order as the above trans conformation preference of the butyl chain in pure imidazolium ionic liquids, which suggested that the cation-anion interaction plays a role in determining the conformation of the chain.

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.

Detection of secondary phases in co-evaporated Cu 2 ZnSnSe 4 thin films by Raman spectroscopy

We prepared CZTSe thin films by the co-evaporation method. In order to investigate the presence of various kinds of secondary phases, we also prepared reference ZnSe, SnSe, CuSnSe, and CuSe thin films in the same way. The depth dependence of the secondary phases was investigated by preparing samples that are sputtered to various depths. We used Raman spectroscopy measurement with several lasers to detect secondary phases. CZTSe films were compared with the reference films, and only ZnSe and MoSe2 phases were detected. The relative amount of ZnSe varied with the depth from the sample surface. The ZnSe Raman peak seems to originate from the top and the bottom of the sample since the signal decreased initially and then increased as deeper parts of the film were probed. Intensity of CZTSe peak did not change much until the bottom part of the sample was reached and the MoSe2 peak appeared.

Spectroscopic imaging study on CIGS thin film solar cells

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.

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

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.