“Imagine having a notebook full of paper with solar cells on it. You use a paper a day to charge a clock, charge a cell phone, then you just throw it away.” This is the future that Trisha Andrew, assistant professor at UW-Madison, envisions. She and her colleagues in the Andrew Lab are working to develop a new and improved solar cell. This research earned Andrew a spot on the Forbes magazine “30 under 30 in Energy” list for 2012.
Andrew notes that “today, most solar cells that you buy are heavy, they’re ungainly, they’re cumbersome, and you need to have special contractors and technicians in order to incorporate them into various applications.” The primary material used in these cells is silicon, a naturally abundant element, and the wiring is made of silver, which is fairly rare and expensive. A mid-range solar cell has an average power conversion efficiency of 12 to 15 percent. Implementing a solar panel is quite expensive, with costs ranging from tens to hundreds of thousands of dollars. The need for a change in this technology is evident. Andrew’s team is taking a whole new approach to solar cells, deviating completely from the current technology.
Dyes traditionally used for commercial purposes like car paint have some qualities that are ideal for making a solar cellTrisha Andrew
The new solar cell that Andrew’s team is working on is completely different; it is small, affordable and disposable. The primary material that they are using might surprise you: organic dye.
The dyes that they are using to make their solar cells are scientifically known as chromophores. They have traditionally been used for commercial purposes like car paint and have some qualities that are ideal for making a solar cell. “To make a solar cell, you want two things: to absorb light and to convert it to electricity. Ideally, you want a material that does both. Dyes are colored because they absorb light incredibly well, so the goal is to make them conduct electricity,” Andrew says. Due to the high light absorbency of the dye, the amount of material needed is minimal. The average thickness of the material needed for Andrew’s solar cell is four times less than silicon solar cells, measuring 50 nanometers. For a point of reference, the thickness of a human hair is 90,000 nanometers. Since there is only a minimal amount of material needed, the cells are very economical but still effective.
Another advantage to using organic dyes over other synthetic materials is that, through a very simple process, they can be deposited on a variety of surfaces. “Just about any arbitrary substrate works. You can put the solar cells on saran wrap, paper, glass slides and many more surfaces,” Andrew says. Each of the substrates requires a slightly different method for printing. For example, printing on paper is accomplished through chemical vapor deposition, while polymers are spin coated.
The challenge in using these organic, nontraditional materials to create solar cells lies in how to regulate the conversion to electricity. To do this, Andrew’s team is manipulating several fundamental properties of electrons. Electrons are charged particles that also have a spin. The spin exists in two states, which Andrew explains to be “equivalent to having a spinning top with the axis either pointing up or pointing down.” By manipulating this spin property, they can control the life span of a particle called an axiton. This process allows them to either productively combine charges to produce a current or make them interfere and cancel out. Andrew talks about the spin property as “the knob we are playing with in order to increase the max power that we get out of our solar cells.”
Currently, each individual solar cell made from organic dye has a power conversion efficiency of two percent but when arranged in an array, the system as a whole increases to five percent efficiency. Although this figure is significantly lower than that of silicon solar cells, the team is not necessarily looking to increase the efficiency of their devices. They are more concerned with the maximum power point, or how much wattage they can get out of one cell. Each individual cell takes up approximately a millimeter squared. On a four-inch by four-inch piece of paper, you can make a “matrix of roughly 128 devices and can generate enough current density to power a little LED clock from radio shack,” according to Andrew.
These cells made of ink and aluminum and printed on letter paper could very well be the solar power of the future. One day, you might be able to keep your cell phone from dying simply with a sheet of notebook paper. As Andrew says, “Ultimately, it’s something ubiquitous that you use, dispose of, and pay very little money for. The goal of this is to try and open up various avenues with which you can start incorporating solar power.”