Materials for Manufacturing
The reduction in costs for photovoltaics will be driven in large part by materials development, which can be improved materials synthesis and new materials. Here at the Conn Center we are researching means to reduce the cost of solar energy by focusing on materials that can be adapted into roll-to-roll manufacturing. Nanomaterials are an ideal materials source since they are simply incorporated into inks and slurries for solution deposition. The current push in the coatings industry is for aqueous based inks and slurries which reduces the costs associated with manufacturing (materials and handling). We are investigating methods to make nanoparticulate inks for simple incorporation into traditional printing technologies.
Copper Inks for Conductive Patterns
Crystalline silicon solar cells utilize conductive silver traces on the front of the cell to efficiently pass the current from the silicon to an external circuit. As the volume of solar modules has increased and the prices plummeted in the past few years, the price of silver has become significant. Replacing the silver with a lower cost material such as copper has become a priority of the industry. We have taken this research one step further developing a copper dispersion that is synthesized in water in a single step process. Furthermore, the final formulation is immediately ready for deposition using traditional printing techniques. The result is a significantly reduced materials cost that is simply integrated into current production processes.
Figure A. Schematic of CuO ink formulations. A) micellular preparation using copper salts and capping agent in a water dispersion, B) addition of sodium boro hydroxide as a reductant affect particle size.
Titanium dioxide is an extremely inexpensive material that is produced in very large quantities with numerous industrial applications including paints, sunscreens, catalysis and optical coatings. In most cases the titanium dioxide is embedded within an organic structure; however, these are rarely used in applications requiring durable films. In order to achieve a robust film for extreme operating environments, it is advantageous to have a fully inorganic film. We have developed a unique formulation composed of an industrial source of titanium dioxide powder and an inorganic-organic binder dispersed into water. This ink can be deposited using a number of traditional printing techniques and the organics can be removed in downstream processes such as ultraviolet radiation, intense pulsed light, atmospheric plasma and thermal processes. The viscosity of the inks and the overall porosity of the thin films can be engineered to specific specifications.
Figure B. Schematic presentation of the IPL process (flow is left to right) left deposited nanomaterial film, middle IPL treatment and right is a bulk thin film.
Printed Electronics Using Intense Pulsed Light
The printed electronics industry is producing conductive lines for flexible electronics, RFID tags, displays and photovoltaics. The photovoltaic industry in particular is interested in replacing expensive silver patterns with much less expensive copper. The process to deposit the copper lines is well suited to traditional printing techniques (screen printing, pad printing, gravure and inkjet) provided that a suitable ink exists. The Cu inks that we have developed can be printed in a number of methods and more importantly can be sintered using IPL. This results in a very scalable, inexpensive manufacturing platform that can be established into current process workflow with little overhead requirements.
Figure C. Demonstration of an IPL sintered copper thin film on plastic. a) X-ray diffraction pattern indicating the lack of oxidized copper, b) Scanning electron micrograph showing connectivity between neighboring particles and c) demonstration of flexible copper film on a plastic substrate.
Intense Pulsed Light Processing of Solar Materials
The deposition of semiconductors for solar cells typically requires very high temperatures and inert or noxious gasses. These high heat, specialty ovens imposes a batch processing technique. This establishes extended costs associated with the high thermal energy requirements, material handling and disposal as well as safety issues. The batch process also must also bring the substrate to the desired temperatures. Using the high energy potential ultraviolet and intense pulsed light sources we can initiate chemical reactions within a thin film.
Figure D. Demonstration of the effect of IPL treatment on the nanoparticle size of semiconductors. i) scanning electron micrograph (SEM) of as synthesized CdS, ii) SEM of IPL treated CdS showing larger particles. iii) artistic rendering of synthesized nanoparticles and iv) artistic rendering of IPL treated particles.
Plasma Process as a Cleaning Step in a Roll-to-roll Application
The atmospheric plasma process (higher plasma density than a corona treater) is typically utilized by the plastic thin film industry to improve the adhesion between substrates and coatings. The process involves exciting oxygen ions within the plasma to etch the surface of a polymer film yielding an increase in reactive sites on the surface. The treatment also has the effect of reducing the surface energy of the plastic improving the wettability from solution phase inks and slurries. The technique can also be used to remove organic species from a metal oxide thin film or oxidize chemical compositions within a thin film.
Video B. Plasma process as a cleaning step in a roll-to-roll application.
Water Based Photoanode Ink for Dye Sensitized Solar Cells
Dye Sensitized Solar Cells (DSSC) and derivatives utilize a thin layer of titanium dioxide as a photoanode. The current state of the art deposition of the titanium dioxide for the photoanode of a DSSC utilizes a paste synthesized from a relatively extensive process. We have developed a very simple ink which includes a industrially available titanium dioxide powder and a low cost inorganic-organic binder dispersed in water. The resulting ink can be deposited using traditional printing techniques and we have produced a DSSC with an efficiency of 9.2%. Further optimization of the ink could conceivably result in even higher efficiencies. This ink also has applications in other solar cell designs as well as self cleaning and optical coatings.
Figure E. Water based ink demonstrated in a Dye Sensitized Solar Cell device. A) Current-voltage characteristic of a device with 9.2% efficiency, B) cartoon of the role of the inorganic-organic binder and C) schematic of the simplicity of formulating the ink.
Copper for Solar Fuels
Photoelectrochemical water splitting for solar hydrogen production represents one of the grand challenges towards carbon free energy generation. The splitting of water using CuO is well known; however, would require very large areas to be economically feasible. Nanoscaled coatings, such as nanowires, greatly enhances the surface area for hydrogen production. We have shown that it is possible to produce nanowires of CuO arrayed over a large area on a copper foil using a solution deposition followed by an oxygen plasma treatment.
Figure F. Production of CuO nanowires using a roll-to-roll continuous production method (flow is left to right). Dip coating deposition of copper foil in a solution leading to an oxidation and formation of copper hydroxide nanowires, followed by an oxygen plasma process creating copper oxide nanowires.
Optically Transparent Nanocomposite Coatings
Functional optically transparent films are becoming extremely important to a number of industries especially in applications where high flexibility is required. Applications in the consumer electronics, displays, photovoltaics and ophthalmic are driving the requirements to lower weight which often means coating plastics. Nanocomposite coatings are an ideal solution as the nanomaterials do not adversely interfere with the transmission of light, but can impart important properties such as conductivity, hydrophobicity/hydrophilicity, abrasion resistance, strength and optics. The nanoscaled elements can be spherical (nanoparticles), cylindrical (nanowires) or plates (graphene). We are currently working with several nanomaterials that can be deposited into a UV-curable polymer to impart the desired properties, without reducing the transparency. The nanocomposite coatings can be applied using traditional printing techniques and the UV-curing technique is extremely rapid.
Figure G. Cross-section of a UV-cured transparent multilayered nanocomposite composed of repeating layers of titanium dioxide / polymer and silicon dioxide / polymer. Inset is a closer look showing a high packing density of nanoparticles.