Myeng Gil Gang

Sputtering processed highly efficient Cu2ZnSn(S,Se)4 solar cells by a low-cost, simple, environmentally friendly, and up-scalable strategy

Earth abundant copper–zinc–tin chalcogenide (CZTSSe) is an emerging material for the development of low cost and sustainable thin film solar cells (TFSCs). A low cost, green, and up-scalable approach to the fabrication of TFSCs through a sputtering process is the main challenge to achieve high efficiency for Cu2ZnSn(S,Se)4 (CZTSSe) solar cells for industrialization. Based on a closed isothermal chamber annealing system, we could precisely calculate and control the chalcogenide partial vapour pressure during the annealing process. We designed, developed, and optimized an environmentally friendly strategy to synthesise a high quality CZTSSe absorber thin film and to fabricate a solar cell without using toxic H2Se and H2S gases as the Se and S sources or any other volatile compounds (SnS and Sn), and a chalcogenide diffusion barrier layer. We fabricated a CZTSSe TFSC with 9.24% efficiency, which is the highest performance for sputtering processed CZTSSe TFSC prepared without using toxic gases and additional processes. Based on this green strategy, we also fabricated the integrated submodule using CZTSSe absorber layers with efficiencies as high as η = 2.76% with eight interconnected cells (active area of 22.4 cm2). Our studies on this green synthesis strategy for CZTSSe solar cells could introduce a possible pathway to green fabrication for the low cost and highly efficient TFSC industrialization field.

Monodispersed wurtzite Cu2SnS3 nanocrystals by phosphine and oleylamine free facile heat-up technique

Wurtzite Cu2SnS3 (CTS) nanocrystals with an average size 8.5 nm were prepared for the first time by a phosphine and oleylamine free facile one-step thermolysis method. The as-prepared nanocrystals were investigated for structural, morphological and compositional properties. The advantages of the heat-up technique for a swift synthesis of gram scale, phase pure CTS nanocrystals; applicable for various energy devices like solar cell, batteries etc., are discussed. Nanocrystals were synthesized at different reaction times viz. 10, 20 and 30 minutes. The possible growth mechanism for colloidal CTS nanocrystals has been investigated. Herein we observed that CTS nanocrystals growth starts with the formation of copper sulphide nanoparticles, acting as nuclei for the subsequent growth of CTS nanocrystals. X-ray diffraction analysis revealed the formation of phase pure wurtzite CTS nanocrystals which is further supported by high resolution transmission electron microscopy, selected area electron diffraction and energy dispersive X-ray analysis. To the best of our knowledge colloidal wurtzite CTS nanocrystals synthesis has been reported with the use of strongly co-ordinating, highly toxic oleylamine (OLA) solvent, herein stepping towards a green synthesis we report for the first time on the synthesis of sub 10 nm wurtzite CTS nanocrystals using neutral, less toxic and less expensive octadecene (ODE) solvent.

Cobalt Iron Hydroxide as a Precious Metal-Free Bifunctional Electrocatalyst for Efficient Overall Water Splitting

Highly efficient and stable electrocatalysts from inexpensive and earth-abundant elements are emerging materials in the overall water splitting process. Herein, cobalt iron hydroxide nanosheets are directly deposited on nickel foam by a simple and rapid electrodeposition method. The cobalt iron hydroxide (CoFe/NF) nanosheets not only allow good exposure of the highly active surface area but also facilitate the mass and charge transport capability. As an anode, the CoFe/NF electrocatalyst displays excellent oxygen evolution reaction catalytic activity with an overpotential of 220 mV at a current density of 10 mA cm-2 . As a cathode, it exhibits good performance in the hydrogen evolution reaction with an overpotential of 110 mV, reaching a current density of 10 mA cm-2 . When CoFe/NF electrodes are used as the anode and the cathode for water splitting, a low cell voltage of 1.64 V at 10 mA cm-2 and excellent stability for 50 h are observed. The present work demonstrates a possible pathway to develop a highly active and durable substitute for noble metal electrocatalysts for overall water splitting.

Evolution of detrimental secondary phases in unstable Cu2ZnSnS4 films during annealing

The formation and phase evolution of Cu-S based compounds in kesterite Cu2ZnSnS4 (CZTS) absorbing thin films is a critical factor affecting the performance of these materials in thin film solar cells (TFSCs). However, to the best of our knowledge, few studies have investigated the segregation of Cu-S based compounds in kesterite thin films during the sulfurization process. In this study, stacked Cu/SnS2/ZnS precursor thin films were annealed to systematically study the segregation and phase evolution of Cu-S based compounds in kesterite thin films subjected to functional sulfurization times at 550°C. The stacked precursor thin films appeared to be fully transformed to the pure kesterite phase when the sulfurization times are over 30 min. when analyzed using X-ray diffraction and Raman spectroscopy. However, transmission electron microscopy (TEM) characterization revealed that Cu-S based compounds segregated in the kesterite CZTS thin films annealed for 120 min. at 550°C. Based on the experimental results obtained for functional sulfurization times, a mechanism for Cu-S based compounds segregation and the phase evolution process is proposed.

Studies on the Controlling of the Microstructural and Morphological Properties of Al Doped ZnO Thin Films Prepared by Hydrothermal Method

Al doped ZnO (AZO) thin films were prepared on ZnO coated glass substrates by hydrothermal synthesis technique using aqueous solutions containing zinc nitrate hexahydrate, ammonium hydroxide, and different sodium citrate concentrations at 60 °C for 6 h. The effects of different trisodium citrate concentrations on the microstructural, crystallinity, morphological, optical, and chemical properties of thin films were investigated. X-ray diffraction studies showed that the AZO thin films were grown as a polycrystalline wurtzite hexagonal phase with a c-axis preferred orientation and without an unwanted second phase regardless of trisodium citrate concentrations. The thickness and grain sizes of AZO thin films decreased with increasing trisodium citrate concentration. The microstructure of AZO thin films was changed from flat to needle shaped and the morphology was smoother with increasing trisodium citrate concentrations. The AZO thin films have a high transmittance in the visible region ranging from 75 to 85% and a sharp edge from 366 to 374 nm.

Chemically Deposited CdS Buffer/Kesterite Cu2ZnSnS4 Solar Cells: Relationship between CdS Thickness and Device Performance

Earth-abundant, copper-zinc-tin-sulfide (CZTS), kesterite, is an attractive absorber material for thin-film solar cells (TFSCs). However, the open-circuit voltage deficit (Voc-deficit) resulting from a high recombination rate at the buffer/absorber interface is one of the major challenges that must be overcome to improve the performance of kesterite-based TFSCs. In this paper, we demonstrate the relationship between device parameters and performances for chemically deposited CdS buffer/CZTS-based heterojunction TFSCs as a function of buffer layer thickness, which could change the CdS/CZTS interface conditions such as conduction band or valence band offsets, to gain deeper insight and understanding about the Voc-deficit behavior from a high recombination rate at the CdS buffer/kesterite interface. Experimental results show that device parameters and performances are strongly dependent on the CdS buffer thickness. We postulate two meaningful consequences: (i) Device parameters were improved up to a CdS buffer thickness of 70 nm, whereas they deteriorated at a thicker CdS buffer layer. The Voc-deficit in the solar cells improved up to a CdS buffer thickness of 92 nm and then deteriorated at a thicker CdS buffer layer. (ii) The minimum values of the device parameters were obtained at 70 nm CdS thickness in the CZTS TFSCs. Finally, the highest conversion efficiency of 8.77% (Voc: 494 mV, Jsc: 34.54 mA/cm2, and FF: 51%) is obtained by applying a 70 nm thick CdS buffer to the Cu2ZnSn(S,Se)4 absorber layer.

Mechanism of photonic crystal and waveguide effects in ZnO nanorods

Two different shaped ZnO nanorods were grown on ZnO buffered Al2O3 substrate by laser interference lithography and hydrothermal method. The light waveguide within ZnO nano rod and photonic crystal effects in arrayed ZnO nanorods was calculated by 3D-finite dimension time domain(3D-FDTD) programs. The ZnO photonic crystal effect and number of modes of ZnO nanorod was governed by arrangement and shape of ZnO nanorod, respectively.

In과 Ga가 미포함 된 Kesterite Cu2ZnSn(S1-x,Sex)4 (CZTSS) 박막형 태양전지 개발 현황

Chalcogenide-based semiconductors, such as CuInSe2, CuGaSe2, Cu(In,Ga)Se2 (CIGS), and CdTe have attracted considerable interest as efficient materials in thin film solar cells (TFSCs). Currently, CIGS and CdTe TFSCs have demonstrated the highest power conversion efficiency (PCE) of over 11% in module production. However, commercialized CIGS and CdTe TFSCs have some limitations due to the scarcity of In, Ga, and Te and the environmental issues associated with Cd and Se. Recently, kesterite CZTS, which is one of the In- and Ga- free absorber materials, has been attracted considerable attention as a new candidate for use as an absorber material in thin film solar cells. The CZTS-based absorber material has outstanding characteristics such as band gap energy of 1.0 eV to 1.5 eV, high absorption coefficient on the order of 104cm-1, and high theoretical conversion efficiency of 32.2% in thin film solar cells. Despite these promising characteristics, research into CZTS-based thin film solar cells is still incomprehensive and related reports are quite few compared to those for CIGS thin film solar cells, which show high efficiency of over 20%. The recent development of kesterite-based CZTS thin film solar cells is summarized in this work. The new challenges for enhanced performance in CZTS thin films are examined and prospective issues are addressed as well.

SILAR deposited iron phosphate as a bifunctional electrocatalyst for efficient water splitting

The development of efficient and earth-abundant electrocatalysts for overall water splitting is important but still challenging. Herein, iron phosphate (FePi) electrode is synthesized using a successive ionic layer deposition and reaction (SILAR) method on a nickel foam substrate at room temperature and is used as a bifunctional electrocatalyst for water splitting. The prepared FePi electrodes show excellent electrocatalytic activity and stability for the oxygen evolution reaction (OER) and hydrogen evolution reaction (HER). The FePi electrode exhibits low overpotential of 230 mV and 157 mV towards the OER and HER, respectively, with superior long-term stability. As a result, an electrolyzer that exploits FePi as both the anode and the cathode is constructed, which requires a cell potential of 1.67 V to deliver a 10 mA cm-2 current density in 1 M KOH solution. The exceptional features of the catalyst lie in its structure and active metal sites, increasing surface area, accelerated electron transport and promoted reaction kinetics. This study may provide a facile and scalable approach to design a high-efficiency, earth-abundant electrocatalyst for water splitting.

Band Tail Engineering in Kesterite Cu2ZnSn(S,Se)4 Thin-Film Solar Cells with 11.8% Efficiency

Herein, we report a facile process, i.e., controlling the initial chamber pressure during the postdeposition annealing, to effectively lower the band tail states in the synthesized CZTSSe thin films. Through detailed analysis of the external quantum efficiency derivative ( dEQE/ dλ) and low-temperature photoluminescence (LTPL) data, we find that the band tail states are significantly influenced by the initial annealing pressure. After carefully optimizing the deposition processes and device design, we are able to synthesize kesterite CZTSSe thin films with energy differences between inflection of d(EQE)/dλ and LTPL as small as 10 meV. These kesterite CZTSSe thin films enable the fabrication of solar cells with a champion efficiency of 11.8% with a low Voc deficit of 582 mV. The results suggest that controlling the annealing process is an effective approach to reduce the band tail in kesterite CZTSSe thin films.