Inkjet Printing Gone Solar

 

Fri, 08/14/2015 - 8:02am Lindsay Hock, Editor Image: PV Nano Cell With today’s demand to seek energy independence from the Middle East and other such regions, solar energy is a touted field. In this field, many photovoltaic products have come to market in hopes to help our nation reach this goal. Overall, the total photovoltaic (PV) market is valued at approximately $90 billion, and is expected to double by 2010. In the PV sector, silver inks and pastes represent an approximately $4.9 billion market. However, there is also another trend rearing its head, that of printed electronics. Printed electronics can add intelligence to any object, bringing everyday surfaces and objects to life. The printed electronics market is currently valued at approximately $8 billion, and is expected to reach $55 billion by 2020, and $340 billion by 2030. Conductive inks represent a $2.8 billion market and are expected to increase to $3.36 billion by 2018, with $735 million captured by new silver and copper nanostructure inks. To help these markets reach their full potential inkjet printing technology is commonly sought to advance applications. And markets that have developed from inkjet printing technology include customized, flexible 2-D and 3-D printed electronics and antennas. Inkjet printing in solar energy development Solar PV has entered the mainstream. The cumulative operating PV capacity in the U.S. is over 20 GW and solar PV accounts for more than half of new electric capacity added in the first half of 2014, according to SEIA. “Still, solar comprised only 0.4% of electricity generation in the U.S. in 2014,” says Fernando de la Vega, Founder and CEO of PV Nano Cell (PVN) in an interview with R&D Magazine. And, although the cost of PV technology has declined, it still needs to decrease more if solar energy will ever make up a significant share of the U.S. energy supply. Currently, a majority of solar PV panels use crystalline silicon, “which accounts for 75% of the total cost of solar cell production,” says de la Vega. Therefore, improvements in crystalline silicon cell production technologies can speed adoption by further driving down costs. “Silicon cell metallization—or the printing of conductive grids with conductive pastes to draw off the current for the production of electricity—is a major efficiency-limiting and cost-determining step in solar cell production,” says de la Vega. “Adopting a digital inkjet printing process based on PVN’s Sicrys inks has the potential to lower costs substantially.” PVN focuses on the mass production of conductive inks based on nanoparticles of an average size around 70 to 80 nm. And, according to de la Vega, the company’s conductive inks will accelerate the adoption of solar PV by achieving significant cost reductions in the production of silicon solar cells through inkjet printing with inks made of nanometric materials. PVN reduces the usage of silver, a more expensive metal, and increases efficiency. “PVN’s Sicrys single-crystal nanometric silver conductive ink promises to bring energy-generation prices closer to grid parity by enabling potential cost reductions of up to $0.2 to $0.3 per Watt in silicon solar cell manufacturing, or 10 to 20%,” says de la Vega. “Our silver conductive inks enable non-contact digital inkjet printing, which eliminates the mechanical forces applied to solar cells by traditional screen or stencil printing which will allow in the near future to reduce the wafer thickness leading to further cost reductions.” Overall, non-contact printing can prevent cell breakage, reduce wafer thickness and the cost of silicon required and advance applications for ultra-thin cells. As the conductive inks are single-crystal particles, they have no grain boundaries or weak points, which translates into inks with better performance, higher stability and higher metal content. Beyond solar In addition to solar PV, other applications for the technology include printed circuit boards, antennas, radio frequency identification, security, touchscreens and other printed electronics (PE). The major applications for mass production PE are PCB’s, smartphone antennas and 3-D printing. “The manufacturing and assembling of antennas is a costly and time-consuming process that adds considerably to the cost of products like cell phones,” says de la Vega. The use of inkjet technologies with conductive inks can save on costs, make design modification easier, reduce weight and size and enhance connectivity. “PVN is working with several OEMs to develop printed antenna prototypes using inkjet technology,” says de la Vega. “And preliminary results show that costs can be reduced by as much as half.” Similar advantages can be found with using conductive inks in 3-D printing, as electronics are embedded into the structural material. What’s next While most electronic devices have evolved into digital products, the processes used to create them are still stuck in analog ways. Implementing digital inkjet printing will close this gap. “However, the development of conductive inks for use in inkjet printing technologies faces technological challenges in terms of cost, conductivity, stability, substrate adhesion, the need for lower temperature and faster sintering and dependence on expensive precious metals, such as silver,” says de la Vega. Another challenge is the need to develop new processes and materials that support emerging applications. “In other words, we need to develop technologies that keep up with next-generation device manufacture processes suitable and compatible with mass-production requirements,” says de la Vega. It just may be that PVN’s Sicrys technology is on the forefront of solving the challenges faced by inkjet printing technology. And the company anticipates additional applications and enhanced capabilities for digital printing, including narrower patterns, additional inks with different metals and functionalities and inkjet printing of embedded passive component in PCBs.   http://www.rdmag.com/articles/2015/08/inkjet-printing-gone-solar

A new technique for subsurface characterization of thin-film photovoltaic materials

Although the performance of many materials used in devices is determined by microscale and nanoscale structures and buried interfaces hidden up to a few micrometers below the material surface, most commonly used nanoscale characterization tools are only sensitive to phenomena either at the surface or just a few nanometers below it. As a step towards addressing the increasing need for subsurface characterization with nanoscale spatial resolution, researchers from the CNST and the University of Maryland have demonstrated a new approach for probing the interior of photovoltaic devices. The technique uses a scanning probe microscope to control a tapered optical fiber with a nanoscale aperture that transmits laser light in order to locally excite a subsurface volume of a photovoltaic device. The photocurrent generated by the cell is then measured using a low-noise amplifier. By varying the wavelength of the laser light in the optical fiber, the penetration depth of the light can be readily changed from approximately 100 nm to more than 3 μm, allowing different volumes of the device to be excited. However, deeper penetration results in lower spatial resolution. Therefore, for comparison, the researchers used a focused ion beam available to users in the CNST NanoFab to prepare a wedge-shaped device sample by carefully thinning down the absorber thickness at a small angle while ensuring that the device still functioned as a solar cell. This shape effectively brings some of the buried interfaces closer to the exposed surface so that these interfaces can be interrogated with higher spatial resolution using light with shallower penetration. The characterization technique was demonstrated on cadmium telluride (CdTe) solar cells, a common photovoltaic technology. These solar cells are based on 1 μm to 3 μm thick multilayer polycrystalline films with the size of the crystal grains comparable to or smaller than the device thickness. The cells have both intentional and unintentional variation in composition throughout the absorber layer and device interfaces, making them ideal for testing this technique. Currently, solar cells of this type operate at efficiencies well below the theoretical limit, and their inefficiency is believed to be due to the uncontrolled and often unknown effects of the microscale structure buried beneath the surfaces of the devices. According to CNST researcher Nikolai Zhitenev, the new technique is just an early step towards the development of a full set of quantitative nanoscale sub-surface measurement tools. While patterning the photovoltaic device into a wedge shape inevitably modifies its structure and performance, the measurements are highly reproducible for a variety of processing conditions. Through the development of new theoretical models explicitly incorporating the effects of the device modification, the new measurement approach can be developed into a powerful quantitative technique.