Chang Woo Hong

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.

A Simple Aqueous Precursor Solution Processing of Earth-Abundant Cu2SnS3 Absorbers for Thin-Film Solar Cells

A simple and eco-friendly method of solution processing of Cu2SnS3 (CTS) absorbers using an aqueous precursor solution is presented. The precursor solution was prepared by mixing metal salts into a mixture of water and ethanol (5:1) with monoethanolamine as an additive at room temperature. Nearly carbon-free CTS films were formed by multispin coating the precursor solution and heat treating in air followed by rapid thermal annealing in S vapor atmosphere at various temperatures. Exploring the role of the annealing temperature in the phase, composition, and morphological evolution is essential for obtaining highly efficient CTS-based thin film solar cells (TFSCs). Investigations of CTS absorber layers annealed at various temperatures revealed that the annealing temperature plays an important role in further improving device properties and efficiency. A substantial improvement in device efficiency occurred only at the critical annealing temperature, which produces a compact and void-free microstructure with large grains and high crystallinity as a pure-phase absorber layer. Finally, at an annealing temperature of 600 °C, the CTS thin film exhibited structural, compositional, and microstructural isotropy by yielding a reproducible power conversion efficiency of 1.80%. Interestingly, CTS TFSCs exhibited good stability when stored in an air atmosphere without encapsulation at room temperature for 3 months, whereas the performance degraded slightly when subjected to accelerated aging at 80 °C for 100 h under normal laboratory conditions.

A facile and green synthesis of colloidal Cu2ZnSnS4 nanocrystals and their application in highly efficient solar water splitting

The development of solution-processable routes as well as compounds consisting of earth abundant elements is highly desirable to reduce the fabrication cost. Recently, kesterite Cu2ZnSnS4 (CZTS) nanocrystals (NCs) have attracted great attention for photoelectrochemical (PEC) water splitting owing to their suitable low-cost, earth-abundancy and suitable band gap energy. However, the environmentally benign synthesis of high-quality CZTS NCs without toxic solvents remains elusive. Here, a green chemistry approach employing vegetable oil as a non-toxic solvent for the synthesis of monodisperse and size-tunable CZTS NCs is introduced for the first time. Additionally, the relationship between the abnormal size behavior of the CZTS NCs and the degree of decomposition in the vegetable oil using electrospray ionization mass spectrometry (ESI-MS) measurements is elucidated for the first time. As a conceptual strategy, a ternary abundant compound based heterojunction nanostructure for efficient solar water splitting by introducing CZTS NCs onto 5 nm Zn(O,S) passivated layer/hydrothermally grown TiO2 nanorod arrays (TNRs) is designed and developed. Remarkably, this ternary CZTS NCs/Zn(O,S)/TNR photoelectrode shows a photocurrent density as high as 15.05 mA cm−2 at 1.23 V (vs. the NHE), which is the highest ever for previously reported CZTS NC-based photoelectrodes. The reasons for the enhanced PEC performance are discussed in detail based on different PEC characterizations. More importantly, this work reflects the sophistication of eco-friendly solution phase synthesized CZTS NCs without using any toxic chemicals as an earth abundant sensitizer and constitute a new paradigm towards the enhanced PEC performance with quantum dot based hetero-nanostructures.

Facile linker free growth of CdS nanoshell on 1-D ZnO: Solar cell application

One dimensional type-II core/shell heterostructures are widely employed in solar cells because of their adventitious role in both light absorption and charge separation. Here we report a facile two step chemical approach to synthesizing ZnO/CdS core/shell nanorod arrays. ZnO nanorods (ZNR) with a high aspect ratio were grown using a hydrothermal technique where a uniform CdS shell was deposited using a facile, linker free, one pot, Hexamethylenetetramine (HMTA) based reflux technique for the first time. Though the reflux technique is quite similar to the chemical bath deposition technique (CBD), we obtained more uniform CdS coating and improved solar cell performance with the ZnO/CdS heterostructure compared to CBD-grown ZnO/CdS heterostructures. To obtain a conformal coating of CdS, we optimized the CdS deposition time. Formation of pure phase ZnO/CdS core/shell heterostructure was confirmed by high resolution transmission electron microscopy and X-ray photoelectron spectroscopy (XPS) depth analysis. Improved solar cell performance of 1.23% was obtained for ZnO/CdS core/shell structures with ZnS surface treatment.

Origin of Mechanoluminescence from Cu-Doped ZnS Particles Embedded in an Elastomer Film and Its Application in Flexible Electro-mechanoluminescent Lighting Devices

Mechanically driven light emission from particles embedded in elastomer films has recently attracted interest as a strong candidate for next-generation light sources on display devices because it is nondestructive, reproducible, real-time, environmentally friendly, and reliable. The origin of mechanoluminescence (ML) obtained from particles embedded in elastomer films have been proposed as the trapping of drifting charge carriers in the presence of a piezoelectric field. However, in this study, we propose a new origin of ML through the study of the microstructure of a Cu-doped ZnS particles embedded in an elastomer composite film with high brightness using transmission electron microscopy (TEM) to clearly demonstrate the origin of ML with respect to the microstructure of ML composite films. The TEM characterization of the ML composite film demonstrated that the Cu-doped ZnS particles were fully encapsulated by a 500 nm thick Al layer, which acts as an electron source for ML emission. Furthermore, we fabricated a flexible electro-mechanoluminescence (EML) device using a Cu-doped ZnS particles embedded in a flexible elastomer composite film. Our research results on a new emission mechanism for ML and its application in flexible light generating elastomer films represent an important step toward environmentally benign and ecofriendly flexible electro-mechanoluminescent lighting devices.

Towards environmentally benign approaches for the synthesis of CZTSSe nanocrystals by a hot injection method

With the earths abundance of kesterite, recent progress in chalcogenide based Cu2ZnSn(Sx,Se1-x)4 (CZTSSe) thin films has drawn prime attention in thin film solar cells (TFSCs) research and development. This review is focused on the current developments in the synthesis of CZTS nanocrystals (NCs) using a hot injection (HI) technique and provides comprehensive discussions on the current status of CZTSSe TFSCs. This article begins with a description of the advantages of nanoparticulate based thin films, and then introduces the basics of this technique and the corresponding growth mechanism is also discussed. A brief overview further addresses a series of investigations on the developments in the HI based CZTSSe NCs using different solvents in terms of their high toxicity to environmentally benign materials. A variety of recipes and techniques for the NCs ink formulation and thereby the preparation of absorber layers using NC inks are outlined, respectively. The deposition of precursor thin films, post-deposition processes such as sulfurization or selenization treatments and the fabrication of CZTSSe NCs based solar cells and their performances are discussed. Finally, we discussed concluding remarks and the perspectives for further developments in the existing research on CZTSSe based nanoparticulate (NP) TFSCs towards future green technology.

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.

Size and shape controlled hydrothermal synthesis of kesterite Cu2ZnSnS4 nanocrystals

Quaternary kesterite Cu2ZnSnS4 (CZTS) nanocrystals (NCs) have been synthesized using a simple, size and shape tunable, and low-cost hydrothermal technique without using toxic chemicals. The size and shape of the kesterite CZTS NCs could be controlled by using different complexing agents including non-complexing agent, hydrazine hydrate, tri-sodium citrate (Na3-citrate), and tetra-acetate disodium salt (Na2EDTA). The hydrothermally synthesized CZTS nanocrystals showed a kesterite structure, high optical absorption, and suitable band gap energy characteristics, indicating potential for application to thin film solar cells.

Effect of Sulfurization Temperature on the Properties of Cu 2 ZnSn(S,Se) 4 Thin Films

(CZTSSe) thin films were prepared by sulfurization of evaporated precursor thin films. Precursor was prepared using evaporation method at room temperature. The sulfurization was carried out in a graphite box with S powder at different temperatures. The temperatures were varied in a four step process from to . The effects of the sulfurization temperature on the micro-structural, morphological, and compositional properties of the CZTSSe thin films were investigated using X-ray diffraction (XRD), Raman spectra, field emission scanning electron microscopy (FE-SEM), and transmission electron microscopy (TEM). The XRD and Raman results showed that the sulfurized thin films had a single kesterite crystal CZTSSe. From the FE-SEM and TEM results, the (MoSSe) interfacial layers of the sulfurized CZTS thin films were observed and their thickness was seen to increase with increasing sulfurization temperature. The microstructures of the CZTSSe thin films were strongly related to the sulfurization temperatures. The voids in the CZTSSe thin films increased with the increasing sulfurization temperature.