Transparent electrode enables flexible solar cells

A hybrid transparent electrode that overcomes the “electron-transport bottleneck” of polycrystalline thin films could pave the way for flexible solar cells and color monitors, head-up displays in car windshields and opto-electronic circuits for sensors and information processing.

Transparent electrodes used in today’s touch-screen monitors, cellphone displays and flat-screen TVs use indium tin oxide (InSnO₂), an expensive material that is limited in availability, inflexible and degrades over time, becoming brittle and hindering performance.

The Purdue University electrode, first proposed as a concept in 2011 in Nano Letters (doi: 10.1021/nl203041n) and now demonstrated experimentally in a recent article in Advanced Functional Materials (doi: 10.1002/adfm.201300124), is made of graphene draped over silver nanowires.

“It’s like putting a sheet of cellophane over a bowl of noodles,” said David Janes, a professor of electrical and computer engineering. “The graphene wraps around the silver nanowires and stretches around them.”

Such hybrid structures could enable researchers to overcome the electron-transport bottleneck of extremely thin films.

Combining graphene and silver nano-wires overcomes drawbacks of each individual material: On their own, the graphene and nanowires conduct electricity with too much resistance to be practical for transparent electrodes. A sheet of graphene is made of individual segments called grains, and resistance increases at the boundaries between these grains. When two grains meet, the adjacent grains could be almost similar in orientation, which would make the resistance between them not too high, said electrical and computer engineering professor Muhammad A. Alam, who co-authored the Nano Letters paper. But when the misorientation of neighboring grains is very high, the electrons find it difficult to go from one grain to the next.

Silver nanowires, on the other hand, have high resistance because they are randomly oriented, making for poor contact between nanowires, resulting in high resistance.

“So neither is good for conducting electricity, but when you combine them in a hybrid structure, they are,” Janes said.

Figure 1 shows that the transparent conducting electrode can be made in two ways: Hybrid 1 involves silver nanowires (AgNW) sprinkled on top of graphene; in Hybrid 2, graphene covers a random collection of silver nanowires. The intimate contact offered by Hybrid 2 is key to the remarkably high performance of the thin film, Alam said.

Figure 1. A transparent conducting electrode for solar cells can be made by using silver nanowires (AgNW) sprinkled on top of graphene (a) or by covering a random collection of silver nanowires with graphene (b). The crystalline quality of the nanowires is preserved during processing (c); (d) shows finer details of the morphology. NW-NW Contact = the contact between two silver nanowires. R_LGB = low-resistance grain boundary. R_HGB = high-resistance grain boundary. R_G = resistance of the grain itself (electron transport within the grain). R_Gr-NW = the resistance of the electron trying to climb from the grain to the nanowire to bypass the grain boundary. R_AgNW = the resistance of the silver nanowire itself.

The researchers found that the material has an exceptionally low “sheet resistance,” or the electrical resistance in very thin layers of material, which is measured in units called “squares.” At 22 ohms per square, it is nearly five times better than InSnO₂ (for the same optical transparency), which has a sheet resistance of 100 ohms per square.

The hybrid structure also was found to have little resistance change when bent, whereas InSnO₂ shows dramatic increases in resistance when bent. The hybrid also is highly stable against environmental degradation because the wires are covered and protected by graphene.

“The generality of the theoretical concept underlying this experimental demonstration, namely ‘percolation doping,’ suggests that it is likely to apply to a broad range of other 2-D nanocrystalline materials, including graphene,” Alam said. Figure 1e is a model depicting the “co-percolating” network of graphene and silver nanowires.

The concept represents a general approach that should apply to any other polycrystalline materials, Alam said. “This is a beautiful illustration of how theory enables a fundamental new way to engineer material at the nanoscale and tailor its properties,” he said.

A patent application has been filed by the university’s Office of Technology Commercialization.