One problem with traditional solar cells is making them. Because current solar cells are based on silicon, they are costly and can even be environmentally unfriendly to manufacture since they require several complicated and energy intensive steps including high temperatures.
With the PV panel market trending towards flexible thin-film PV cells, the challenge for scientists has been to find a way to manufacture large numbers of cells that are both efficient and cheap.
One concept to make PV cells more economically is to actually print them onto a flexible substrate such as plastic or a metal foilfrom large rolls similar to how newspapers are printed. In fact, using such low-cost technology such as ink jet printers could aid in manufacturing fast produced, low-cost, thin-film PVs while reducing raw material waste.
The Printable PV Concept
Traditional PV cells are comprised of several layers of different materials pressed together, the important layer being the one that captures sunlight and converts it into electricity. This semi-conducting material uses the sunlight’s photons to create negative and positive electrical charges. This material, generally silicon, must also be able to easily allow the electric charge to pass through the material to complete the electrical circuit that creates the electricity.
A cheaper approach being explored by researchers is to make the semi-conducting material as a solution that allows for easy electron mobility and use it as a colloidal 'ink.' This ink can then be printed or sprayed onto a flexible solar cell substrate surface in a continuous manner to increase manufacturing output resulting in generating PV cells much cheaper to mass produce. By choosing the right combination of ink and substrate surface it is possible to make efficient solar cells using very little material or energy.
This, however, requires a semi-conductor precursor that is soluble.
That is being done by manipulating the composition of inexpensive metal precursors at a molecular level. One method results in the creation of semi-conducting nano-crystals, also known as quantum dots. Because these tiny particles have diameters measured in millionths of a millimeter, they can easily be suspended in a solution and “inked on” to a variety of surfaces. The result is a very thin film with photovoltaic properties.
Current Research Advances
In June 2011, it was announced independently by different teams of researchers around the world that they each had demonstrated various methods and approaches that could produce cheaper “printed” layers of PV cells.
One U.S. team consisting of researchers from the University of Chicago and the U.S. Department of Energy's Argonne National Laboratory developed a soluble quantum dot precursor of semi-conductors joined together to create new molecules they call "molecular metal chalcogenide complexes." The material is heated at much lower temperature than required for manufacturing silicon solar cells, resulting in a layer of material with good “semi-conducting properties.”
On the other side of the U.S.,Oregon State University researchers have created a copper-indium-gallium-selenide (CIGS) ink and used basic inkjet printing technology to print a ultra thin PV film onto a substrate.
While CIGS, also called chalcopyrite, is known to have a good solar conversion efficiency, the researchers were only able to create a printed PV cell with a power conversion efficiency of about 5 percent. But this is the first working CIGS PV cell printed with such inkjet technology, according to Chih-hung Chang, OSU professor in the School of Chemical, Biological and Environmental Engineering. And despite the low efficiency, the researchers are excited about the method as a way to reduce wasted materials by nearly 90 percent. That’s because materials such as indium are expensive and any method that almost eliminates the waste is worth working on.
PhD student Brandon MacDonald and his colleagues from CSIRO and the University of Melbourne’s Bio21 Institute, announced in June 2011 they had also developed apatented ink technology based on semi-conducting nano-crystals, which can be printed directly onto a variety of surfaces. These researchers showed that by basically printing multiple layers of nano-crystals, they could fill in the gaps or defects formed during the normal drying process. The result is a much more densely packed and uniform ultra thin film PV surface.
Their nano-crystals used cadmium telluride, which is highly efficient as a photovoltaic material. That means that the PV cells can be made extremely thin. In fact, the total material used in these cells was about 1 percent of what would be used in a typical silicon PV cell, according to the researchers. Compared to other types of cadmium telluride PV cells, it was about one-tenth the material.
Production Obstacles
Since they use much less photovoltaic material, the advances of printed PV cells can mean a cheaper way to make flexible, thin-film solar cells just in material cost alone. This is important because the price of these raw materials, such as silicon and indium, are subject to market price forces since they have other uses outside the solar industry.
The manufacturing technology is also less expensive in terms of the energy needed to produce printed PV panels, which is a major factor in the time it takes to repay the cost of producing traditional PV cells.
The technology is still evolving and it appears it will be a while before it moves from the 3 to 5 percent conversion efficiency to the traditional solar cell efficiency in the 12 percent range that would make them commercially viable. The other issue for manufacturers is the lifespan of these ultra thin film PV materials, and the substrate material they are actually printed onto. Traditional, flat-plate cells have a lifespan of about 20 years before they show signs of declining conversion efficiencies.
But despite the current short comings of low efficiency, these very inexpensive, lightweight and flexible printed PVs are actually well suited for certain potential, if not immediate, applications.
Emerging PV Markets
Efficient, flat-plate silicon PV panels are best for modern applications that require be generation of enough power to pass through inverters for conversion into usable higher AC voltage. But for smaller, “off-grid” use that requires only the current directly from the panels, efficiency can be less an issue then having a cheap, lightweight and flexible source of power for the very basics.
According to Simon Bransfield-Garth, CEO of Eight19, one opening mass market are some of the remote, emerging economies in countries where whole areas lack the electric grid infrastructure that flat-plate PVs have the most use and are most cost-effective. Eight19 was formed in 2010 as a spin-off of Cambridge University’s work developing plastic solar technologies.
“For example, the peak energy gap in India is set to grow as average electricity consumption doubles over the next 5 years. Much of the gap is currently filled by expensive and polluting local diesel generation, with a significant impact both on carbon footprint and, because most is imported, future energy security,” said Bransfield-Garth in a written statement at the June 23rd Economist Conference's UK Energy Summit.
These emerging markets are also areas where a liquid “ink” PV material could be printed, stamped or even sprayed directly onto surfaces such as roofing material. Such applications would for this material could be functional even before researchers achieve the magic 12 percent efficiency range.
