Brion Bob

Novel Solution Processing of High‐Efficiency Earth‐Abundant Cu2ZnSn(S,Se)4 Solar Cells

A novel solution-based approach is presented to process earth-abundant Cu(2)ZnSn(S,Se)(4) absorbers using fully dissolved CZTS precursors in which each of the elemental constituents intermix on a molecular scale. This method enables the low-temperature processing of chemically clean kesterite films with excellent homogeneity. The high performance of resulting optoelectronic devices represents a chance to extend the impact of CZTS into the next chapter of thin-film solar cells.

Nanoscale Joule Heating and Electromigration Enhanced Ripening of Silver Nanowire Contacts

Solution-processed metallic nanowire thin film is a promising candidate to replace traditional indium tin oxide as the next-generation transparent and flexible electrode. To date however, the performance of these electrodes is limited by the high contact resistance between contacting nanowires; so improving the point contacts between these nanowires remains a major challenge. Existing methods for reducing the contact resistance require either a high processing power, long treatment time, or the addition of chemical reagents, which could lead to increased manufacturing cost and damage the underlying substrate or device. Here, a nanoscale point reaction process is introduced as a fast and low-power-consumption way to improve the electrical contact properties between metallic nanowires. This is achieved via current-assisted localized joule heating accompanied by electromigration. Localized joule heating effectively targets the high-resistance contact points between nanowires, leading to the automatic removal of surface ligands, welding of contacting nanowires, and the reshaping of the contact pathway between the nanowires to form a more desirable geometry of low resistance for interwire conduction. This result shows the interplay between thermal and electrical interactions at the highly reactive nanocontacts and highlights the control of the nanoscale reaction as a simple and effective way of turning individual metallic nanowires into a highly conductive interconnected nanowire network. The temperature of the adjacent device layers can be kept close to room temperature during the process, making this method especially suitable for use in devices containing thermally sensitive materials such as polymer solar cells.

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.

Silver Nanowire Composite Window Layers for Fully Solution‐Deposited Thin‐Film Photovoltaic Devices

A silver nanowire-indium tin oxide nanoparticle composite and its successful application to fully solution processed CuInSe(2) solar cells as a window layer are demonstrated, effectively replacing the traditionally sputtered both intrinsic zinc oxide and indium tin oxide layers. The devices utilizing the nanocomposite window layer demonstrate photovoltaic parameters equal to or even beyond those with sputtered intrinsic zinc oxide and indium tin oxide contacts.

Solution-processed flexible transparent conductors composed of silver nanowire networks embedded in indium tin oxide nanoparticle matrices

Although silver nanowire meshes have already demonstrated sheet resistance and optical transmittance comparable to those of sputter-deposited indium tin oxide thin films, other critical issues including surface morphology, mechanical adhesion and flexibility have to be addressed before widely employing silver nanowire networks as transparent conductors in optoelectronic devices. Here, we demonstrate the efficacy of low temperature solution-processed flexible metal nanowire networks embedded in a conductive metal oxide nanoparticle matrix as transparent conductors, and investigate their microstructural, optoelectronic, and mechanical properties in attempting to resolve nearly all of the technological issues imposed on silver nanowire networks. Surrounding silver nanowires by conductive indium tin oxide nanoparticles offers low wire to wire junction resistance, smooth surface morphology, and excellent mechanical adhesion and flexibility while maintaining the high transmittance and the low sheet resistance. In addition, we discuss the relationship between sheet resistance and transmittance in the silver nanowire composite transparent conductors and their maximum achievable transmittance. Although we have selected silver nanowires and indium tin oxide nanoparticle matrix as demonstration materials, we anticipate that various metal nanowire meshes embedded in various conductive metal oxide nanoparticle matrices can effectively serve as transparent conductors for a wide variety of optoelectronic devices owing to their superior performance, simple, cost-effective, and gentle processing.Graphical abstract

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.

Highly Robust Silver Nanowire Network for Transparent Electrode

Solution-processed silver nanowire networks are one of the promising candidates to replace a traditional indium tin oxide as next-generation transparent and flexible electrodes due to their ease of processing, moderate flexibility, high transparency, and low sheet resistance. To date, however, high stability of the nanowire networks remains a major challenge because the long-term usages of these electrodes are limited by their poor thermal and chemical stabilities. Existing methods for addressing this challenge mainly focus on protecting the nanowire network with additional layers that require vacuum processes, which can lead to an increment in manufacturing cost. Here, we report a straightforward strategy of a sol-gel processing as a fast and robust way to improve the stabilities of silver nanowires. Compared with reported nanoparticles embedded in nanowire networks, better thermal and chemical stabilities are achieved via sol-gel coating of TiO2 over the silver nanowire networks. The conformal surface coverage suppressed surface diffusion of silver atoms and prevented chemical corrosion from the environment. These results highlight the important role of the functional layer in providing better thermal and chemical stabilities along with improved electrical properties and mechanical robustness. The silver nanowire/TiO2 composite electrodes were applied as the source and drain electrodes for In2O3 thin-film transistors (TFTs) and the devices exhibited improved electrical performance annealed at 300 °C without the degradation of the electrodes. These key findings not only demonstrated a general and effective method to improve the thermal and chemical stabilities of metal nanowire networks but also provided a basic guideline toward rational design of highly efficient and robust composite electrodes.

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.

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.

Silver nanowires with semiconducting ligands for low-temperature transparent conductors

Metal nanowire networks represent a promising candidate for the rapid fabrication of transparent electrodes with high transmission and low sheet-resistance values at very low deposition temperatures. A commonly encountered challenge in the formation of conductive nanowire electrodes is establishing high-quality electronic contact between nanowires to facilitate long-range current transport through the network. A new system involving nanowire ligand removal and replacement with a semiconducting sol-gel tin oxide matrix has enabled the fabrication of high-performance transparent electrodes at dramatically reduced temperatures with minimal need for post-deposition treatment.