written by Kevin Max | photo by Vernon T. Williams
Wei Wang sat down in his lab at Oregon State University with an inkjet printer and an interesting question. Why can’t solar cells be made simply by printing them with an inkjet printer filled with a solution of solar-transmitting compound? His answer was the subject of a four-page research paper published in the industry journal Solar Energy Materials & Solar Cells and the focus of an article from the BBC. The 28-year-old Ph.D. candidate from Shanxi Province, China had discovered an easy process for making solar cells with a printer, a substrate and a combination of metal salts called CIGS. Wang’s discovery could revolutionize the solar industry as a cheap and environmentally friendly alternative to the traditional siliconbased solar cell, and, as a by-product, bring inkjets back into the avant garde.
Wang grew up in Datong, a mid-sized city in northern China known for coal mining, the manufacture of locomotives and cold winters. He came to Oregon State University in 2006 for his master’s in chemical engineering and stayed on to pursue a doctoral degree in the engineering school under professor Chih-hung Chang, his adviser. For the past three years, Wang has been researching his solar cell printing project.
When did you become interested in solar energy research?
During my master’s program at Oregon State, I focused on the application of bio-sensing materials through inkjet printers. That was the starting point for me. After I finished my master’s degree, my adviser and I decided to explore new techniques making solar cells through inkjet printers. There are several reasons pushing us in that direction. First, solar energy is the most abundant and cheapest “green” resource on Earth. Second, we already had extensive experience with inkjet printing and achieved success in electronic material synthesis and electronic device fabrication such as thin film transistor. More importantly, nobody had used an inkjet printer to fabricate cheap metal salts that make up a thin film solar cell called CIGS (an acronym for copper, indium, gallium and selenide). After an initial test of this process, which proved the feasibility of the chemistry, we became more confident in this solar energy research.
How did you end up at Oregon State University?
I knew my adviser, Chih-hung Chang, before I decided to come to Oregon State and was very interested in his research area at that point. Besides, I dislike hot and humid weather back home, so I chose Oregon State to continue my graduate study. Right now, I love the nice weather and great people in Oregon very much. It is my best decision ever.
Printing solar film from an inkjet seems far-fetched. How did the idea arise?
The idea arose from our personal interest and experience and economic demand. Honestly, printing solar cells by inkjet was not a totally new idea even three years ago. At that time, people had already inkjet printed organic solar cells. No one, however, was using this technique to fabricate inorganic CIGS thin film photovoltaic devices, which perform much better compared to organic solar cells.
What should we take away from your research?
For the general audience, it is easy to understand the benefit. Using an inkjet printer is a cheap technique that gives you the ability to control the printing pattern. That means this process has a low-cost advantage over other technologies such as the industry standard vacuum-based deposition, which wastes around 30 percent to 70 percent of starting materials. From a scientific perspective, my research has proven that it is possible to make use of cheap and simple metal salts to make thin film solar cells compared to other techniques, which usually use more expensive metal or ceramic starting materials. Our unique ink formula makes this process more stable and robust in ambient condition and more environmentally friendly. This could lower the cost more.
Tell us about chalcopyrite: What is it and how does its solar efficiency compare to that of the traditional silicon solar cell?
Chalcopyrite is a certain mineral containing copper, iron and sulfide. In my research, a certain compound, CIGS, is made for absorbing and converting sunlight to electric current. Solar cells made from silicon have reached up to 25 percent efficiency for monocrystalline cells and 20.4 percent
efficiency for multicrystalline cells. CIGS solar cells are comparable with multicrystalline silicon cells but less efficient than monocrystalline ones. So far, the best research of CIGS cells has achieved 20.3 percent efficiency with a theoretical efficiency limit at 30 percent. Due to a high absorption coefficiency of CIGS, only a one- or two-micrometer thick CIGS layer is needed to absorb 99 percent of light compared to more than 100 micrometer-thick silicon. This makes CIGS solar cells cheaper and more affordable than traditional silicon cells.
What we know is that the non-vacuum inkjet process is much cheaper and has a much higher rate of material utilization than vacuum-based techniques. However, the deposited films by non-vacuum processes are less uniform and less efficient than their vacuum counterparts.
Tell us about the process from chalcopyrite to printing on inkjet.
Through an inkjet, metal salts dissolved in solvents were printed on substrates and converted to solar absorbing CIGS material after annealing. Photovoltaic devices were finished by depositing several subsequent layers where photo current was generated. The printing process lasts about ten to twenty minutes for a one-square-inch area. The printing duration could decrease dramatically if more printing heads were used. However, it took two days to finish a whole cell in our research lab on campus.
The solar material you’ve produced though the inkjet-secreted process is still a fraction of the commercial photovoltaic conversion process. Can that be improved?
Of course. The results presented in the paper are just our initial step to verify the capability of inkjet printing CIGS solar cells. The reported 5 percent efficiency is far lower than the record value 20.3 percent. So there is plenty of room for us to improve our process to make devices work better with intense physical and chemical analyses. Also making the inkjet printing process more stable and faster is crucial for future commercialization purposes.
What are your biggest obstacles in bringing this to market?
Energy conversion efficiency is the most important measure of the quality of solar cells. In order to make solar cells affordable and acceptable in the commercial market, we have to reach a certain efficiency level. Besides, there are other factors such as stability and uniformity of solar cells. Currently, we have pushed the performance to 7-8 percent efficiency level. We hope we could improve our process to make 12 percent efficient solar cells within the next two years.
What are the commercial possibilities for inkjet-secreted solar cells?
When talking about solar cells, people most likely think about solar panels on roofs and in power stations. Of course, that is the largest field for solar panels. There are also lots of applications of solar cells in electric devices. Since we use an inkjet printer, which can print and pattern solar cells directly on substrates without further processing steps such as lithography, for example, it is possible to print solar cells directly into electronic circuits such as remote/wireless sensors or controllers. This is very difficult for other coating techniques such as traditional vacuum-based deposition techniques.
Could this also revitalize the market for inkjet printers?
Yes, it is possible. Considering the different properties between our inks for solar cells and traditional printing inks, we will have to redesign the printer head for inks used in the printing of solar cells. Actually, there are several inkjet manufacturers that have already started to develop different inkjet printers for various purposes.