Chia-Jung Hsu

CZTS nanocrystals: a promising approach for next generation thin film photovoltaics

Cu2ZnSn(S,Se)4 (CZTSSe) has received considerable attention as a material capable of driving the development of low-cost and high performance photovoltaics. Its high absorption coefficient, optimal band gap, and non-toxic, naturally abundant elemental constituents give it a number of advantages over most thin film absorber materials. In this manuscript, we discuss the current status of CZTSSe photovoltaics, and provide a comprehensive review of Cu2ZnSnS4 (CZTS) and Cu2ZnSnSe4 (CZTSe) nanocrystal (NCs)-based fabrication methods and solar cell characteristics. The focus will be on the relevant synthetic chemistry, film deposition, and the production of high efficiency photovoltaic devices. Various colloidal synthesis routes are currently used to form the highest quality CZTSSe film from the nanocrystals with controllable phase, size, shape, composition, and surface ligands. A variety of recipes are summarized for producing nanocrystal inks that are appropriate for forming CZTSSe absorber materials with a wide range of controllable optoelectronic properties. Deposition and post-processing, such as annealing and selenization treatments, play an important role in defining the phase and structure of the resulting material. Various film treatment strategies are outlined here, and their resulting material quality, device performance, and dominant photovoltaic loss mechanisms are discussed. Suggestions regarding needed improvements and future research directions are provided based on the current field of available literature.

Rational Defect Passivation of Cu2ZnSn(S,Se)4 Photovoltaics with Solution-Processed Cu2ZnSnS4:Na Nanocrystals

An effective defect passivation route has been demonstrated in the rapidly growing Cu2ZnSn(S,Se)4 (CZTSSe) solar cell device system by using Cu2ZnSnS4:Na (CZTS:Na) nanocrystals precursors. CZTS:Na nanocrystals are obtained by sequentially preparing CZTS nanocrystals and surface decorating of Na species, while retaining the kesterite CZTS phase. The exclusive surface presence of amorphous Na species is proved by X-ray photoluminescence spectrum and transmission electron microscopy. With Na-free glasses as the substrate, CZTS:Na nanocrystal-based solar cell device shows 50% enhancement of device performance (∼6%) than that of unpassivated CZTS nanocrystal-based device (∼4%). The enhanced electrical performance is closely related to the increased carrier concentration and elongated minority carrier lifetime, induced by defect passivation. Solution incorporation of extrinsic additives into the nanocrystals and the corresponding film enables a facile, quantitative, and versatile approach to tune the defect property of materials for future optoelectronic applications.

Spatial Element Distribution Control in a Fully Solution-Processed Nanocrystals-Based 8.6% Cu2ZnSn(S,Se)4 Device

A fully solution-processed high performance Cu2ZnSn(S,Se)4 (CZTSSe, kesterite) device has been demonstrated. It is based on the rational engineering of elemental spatial distributions in the bulk and particularly near the surface of the film from nanocrystal precursors. The nanocrystals are synthesized through a modified colloidal approach, with excellent solubility over a large compositional window, followed by a selenization process to form the absorber. The X-ray photoluminescence (XPS) depth profiling indicates an undesirable Sn-rich surface of the selenized film. An excessive Zn species was quantitatively introduced through nanocrystals precursor to correct the element distribution, and accordingly a positive correlation between the spatial composition in the bulk/surface film and the resulting device parameter is established. The enhanced device performance is associated with the reduced interfacial recombination. With a Zn content 1.6 times more than the stoichiometry; the optimized device, which is fabricated by employing a full solution process from the absorber to the transparent top electrode, demonstrates a performance of 8.6%. This composition-control approach through stoichiometric adjustments of nanocrystal precursors, and the developed correlation between the spatial composition and device performance may also benefit other multielement-based photovoltaics.

Molecular solution approach to synthesize electronic quality Cu2ZnSnS4 thin films.

Successful implementation of molecular solution processing from a homogeneous and stable precursor would provide an alternative, robust approach to process multinary compounds compared with physical vapor deposition. Targeting deposition of chemically clear, high quality crystalline films requires specific molecular structure design and solvent selection. Hydrazine (N2H4) serves as a unique and powerful medium, particularly to incorporate selected metallic elements and chalcogens into a stable solution as metal chalcogenide complexes (MCC). However, not all the elements and compounds can be easily dissolved. In this manuscript, we demonstrate a paradigm to incorporate previously insoluble transitional-metal elements into molecular solution as metal-atom hydrazine/hydrazine derivative complexes (MHHD), as exemplified by dissolving of the zinc constituent as Zn(NH2NHCOO)2(N2H4)2. Investigation into the evolution of molecular structure reveals the hidden roadmap to significantly enrich the variety of building blocks for soluble molecule design. The new category of molecular structures not only set up a prototype to incorporate other elements of interest but also points the direction for other compatible solvent selection. As demonstrated from the molecular precursor combining Sn-/Cu-MCC and Zn-MHHD, an ultrathin film of copper zinc tin sulfide (CZTS) was deposited. Characterization of a transistor based on the CZTS channel layer shows electronic properties comparable to CuInSe2, confirming the robustness of this molecular solution processing and the prospect of earth abundant CZTS for next generation photovoltaic materials. This paradigm potentially outlines a universal pathway, from individual molecular design using selected chelated ligands and combination of building blocks in a simple and stable solution to fundamentally change the way multinary compounds are processed.

Facile single-component precursor for Cu2ZnSnS4 with enhanced phase and composition controllability

A simple and effective method has been demonstrated using soluble single-component precursors to achieve high quality Cu2ZnSnS4 (CZTS) thin films with enhanced phase and composition controllability. The soluble single-component precursors were composed of Cu2−xS colloidal nanocrystals (NC) and the homogeneously distributed Zn–Sn species on the surface. Zn species is presented as the surface ligand for the first time, in the form of Zn–Sn complex, providing both the colloidal stabilization and the essential components for CZTS. CZTS film composition is governed by the prescribed stoichiometry between Cu2−xS nanoparticles and the Zn–Sn complex in solution. This novel reaction process enables a favorable CZTS phase progression, with less than 175 °C formation temperature, and a minimized secondary phase growth mode, due to its local composition uniformity in both liquid and film state. Compared with the approaches based on molecular precursor or quarternary CZTS nanocrystals, the single-component precursor allows fine tuning of the kesterite materials for future advanced optoelectronics.

Benign solution processed Cu 2 ZnSn(Se, S) 4 photovoltaic

Solution-based approach to process earth-abundant Cu2ZnSn(Se, S)4 is proved to be a promising route for thin-film photovoltaic fabrication. Combining fully dissolved zinc/tin (Zn/Sn) and copper (Cu) hydrazinuim constituents in the ethanolamine (EA) and its mixture with dimethylsulfoxide (DMSO) forms the CZTS precursor, which facilitates the composition adjustment. All solutes in this precursor solution are in molecular scale intermixed with excellent homogeneity. Innovated 2-step annealing using sulfur and selenium vapor sequentially assists continuous grain growth, and prevents Mo(S, Se)2 formation. Resulting device achieves 7.5% power conversion efficiency.

Non-hydrazine solution processed CuIn(Se,S) 2 photovoltaic device

Solution processing of CuIn(Se,S)2 or Cu(InGa)S2 has proven to be one of the most promising strategies for low-cost, high-efficiency photovoltaic devices. With hydrazine as the sole solvent for dissolving Cu2S and In2Se3, power conversion efficiencies can achieve up to 12.2%, which is dramatically better than those through using Cu or In chloride or nitrate precursor dissolved in organic solvents, such as butylamine. Although the hydrazine-based process produces the CISS absorber layer with less impurity, the reactivity and toxicity of hydrazine limit the further investigation and application in industry. Here, we demonstrate an alternative for hydrazine-based process by using suitable mixture of ethanolamine (EA) and dimethyl sulfoxide (DMSO) to dissolve Cu and In hydrazinium precursor. Control experiments suggest that sulfur in DMSO coordinates with Cu or In in precursors, while EA stabilizes the dissolved Cu-In complex. XRD and Raman characterization indicate the formation of the CISS phase after annealing. Amorphous carbon in the as-formed film, which comes from the decomposition of solvents, can be removed by selenization of the film. Optimized devices exhibit a power conversion efficiency of 3.83% with only ~300 nm-thick CISS absorber layer, which is comparable to that in N2H4-based device with similar thickness.