Wan-Ching 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.

Single-Crystal Linear Polymers Through Visible Light–Triggered Topochemical Quantitative Polymerization

Topochemical Polymerization In a topochemical reaction, chemical changes start at active sites in the solid and then proceed autocatalytically to neighboring regions. If one starts with a monomer that can form ordered structures similar to the final polymer, it is possible to polymerize chains in a fully ordered state and thus make very long single chains. Dou et al. (p. 272; see the Perspective by Goroff) describe an unexpected visible-light–induced polymerization of derivatives of a dye. Two of the derivatives underwent photoinduced single-crystal-to-single-crystal topochemical polymerization. Conjugated dye molecules can be polymerized through a topochemical reaction to produce exceptionally long, ordered chains. [Also see Perspective by Goroff] One of the challenges in polymer science has been to prepare large-polymer single crystals. We demonstrate a visible light–triggered quantitative topochemical polymerization reaction based on a conjugated dye molecule. Macroscopic-size, high-quality polymer single crystals are obtained. Polymerization is not limited to single crystals, but can also be achieved in highly concentrated solution or semicrystalline thin films. In addition, we show that the polymer decomposes to monomer upon thermolysis, which indicates that the polymerization-depolymerization process is reversible. The physical properties of the polymer crystals enable us to isolate single-polymer strands via mechanical exfoliation, which makes it possible to study individual, long polymer chains.

Reaction pathways for the formation of Cu2ZnSn(Se,S)4 absorber materials from liquid-phase hydrazine-based precursor inks

Kesterite Cu2ZnSn(Se,S)4 (CZTSSe) is rapidly becoming an important photovoltaic material due to the abundance and industrial compatibility of its constituent elements. Hydrazine-based slurry deposition has taken a leading role in producing high efficiency devices from this material system, outperforming even high vacuum deposition methods. In this paper, we study the reaction mechanisms involved in the overall transformation from the precursor ink to the solid-state framework and finally to the CZTSSe phase during deposition and subsequent thermal treatment. X-ray diffraction and Raman spectroscopy have been employed to track the various stages of the reaction pathway, and to mark the formation and consumption of precursor phases as they interact to form the final material. It was found that drying the precursor ink at room temperature results in the integration of copper and tin chalcogenide complexes to form a bimetallic framework, with hydrazine and hydrazinium molecules as spacers. After mild thermal annealing, the spacers are removed and the Cu2Sn(Se,S)3 + Zn(Se,S) → Cu2ZnSn(Se,S)4 reaction is triggered. This reaction pathway contains far fewer steps than most deposition processes, which typically start with elemental or binary chalcogenides. As the formation of secondary phases such as Cu2−xS, SnSe, and SnSe2 is no longer necessary to produce the final Cu2ZnSn(Se,S)4 phase, the relative simplicity of this formation mechanism is likely beneficial for the performance of the resulting solar cells.

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.

Studies of carrier recombination in solution-processed CuIn(S,Se)2 through photoluminescence spectroscopy

We investigated the effects of the cadmium sulfide (CdS) layer on defect passivation in hydrazine-based CuIn(S,Se)2 (CISSe) samples through photoluminescence measurements. Significant changes in the emission profile of the CISSe film are observed after a CdS layer is deposited on CISSe. It is likely that Cd diffusion into the CISSe film becomes more severe as a result of the fine grain size of our solution-processed films. Enhanced emission yields and longer carrier lifetimes are, thus, observed in Cd-treated (CdS-coated or Cd-soaked) CISSe films, indicating the action of Cd ions on Cu vacancies sites and a decrease in non-radiative recombination.

Growth kinetics during kesterite coevaporation

Kesterite solar cells have been considered as earth-abundant substitute to chalcopyrites. NRELs 9.2% co-evaporated kesterite solar cell is inspired by the copper-rich growth of co-evaporated chalcopyrites. The excess Cu_xSe_y is believed to conduct liquid-phase assisted grain growth and hence improves the device performance. The effect of the deposition sequence on film growth, morphology, and device performance, are explored in this study. At high deposition temperature, the expected binary precursors of Cu₂ZnSnSe₄ include Cu_xSe_y and ZnSe but not SnSe_x. Because SnSe_x is volatile, the sticking of Sn occurs only if the adsorbed Sn encounters Cu_xSe_y and ZnSe and the formation of kesterite takes place. Otherwise, SnSe_x will be re-evaporated. Here we designed deposition recipes to create precursor films with different ratio between ZnSe/Cu_xSe_y/as-formed kesterite in the first stage, and end the deposition with the same end-point composition. First, it is of interest if the existence of Cu_xSe_y phase provides the opportunity of grain growth analogous to co-evaporating CIGS. Second, by observing the evolution of the substrate temperature during deposition, the reaction progression may be better realized. Third, this series of depositions with different sequence also tells us the saturated Zn level relative to Cu and Sn, and where the excess portion of Zn stays in the film. Finally, the device performance above 9% is briefly presented.

Grain growth of CZTSSe via nanocrystal selenization

Achieving micro-sized and closed packed grains is an essential requirement for high performance chalcogenide thin film solar cells. The grain growth mechanism of CZTSSe starting from nanocrystal films appears to be affected by the capping ligands composed of long hydrocarbon chains. The growth of grains relies heavily on the selenium vapor supplied in the thermal process. This study decouples the selenium evaporation temperature apart from the substrate temperature to allow the investigations of their individual effects. We found that the two temperatures have distinctive effects on grain morphology, and the combination of a lower substrate temperature and a higher selenium evaporation temperature is favorable to produce flat and closely-packed grains and also a thinner MoSe2 layer. Cell efficiency of 8.0% has been achieved by the film with favorable morphology. Also we show that the removal of carbon from the as-deposit films effectively improves the grain size of the selenized films. The understanding of the roles of selenium vapor and carbon content may further improve the device performance.