Nanopillars — sometimes referred to as “nanograss” because of their resemblance to blades of grass — have offered a way to increase the light absorption of thin films of silicon.
Now nanograss has been used by researchers at the University of Massachusetts Amherst in cooperation with others from Stanford University and Dresden University of Technology in Germany to overcome the discontinuous pathways — or dead-ends — that compromise the ability of positive-negative (p-n) junctions to extract energy in organic solar cells.
“For decades scientists and engineers have placed great effort in trying to control the morphology of p-n junction interfaces in organic solar cells,” said Alejandro Briseno of the University of Massachusetts Amherst in a news release. “We report here that we have at last developed the ideal architecture composed of organic single-crystal vertical nanopillars.”
The research, which was published in the journal Nano Letters, found a simple crystallization technique for growing vertically oriented nanopillars. The technique essentially builds on thermal evaporation by using a fast deposition rate.
The new technique made it possible to stack the various compounds like coins. This stacked geometry improved charge transport by providing the largest charge transport anisotropy, which is defined as electrons flowing faster in one direction rather than another. In this case, the anisotropy, or direction, is along the nanopillar, which is situated perpendicular to the substrate.
In this architecture, the device absorbs photons and converts them to electrons. The photons are absorbed by the nanopillars and the photons generate excited states referred to as excitons. The excitons then migrate to p-n interfaces (the outer shell of the nanopillars). The charges are separated at interfaces: positive charges are collected at anodes and negative charges are collected at cathodes from which they flow to an external circuit and produce electricity.
While the technique can be generalized to work with various substrates, including the two-dimensional materials graphene and molybdenum disulfide, the biggest challenge in developing the technique was finding the best substrate. However, serendipity stepped in to ease the path.
“For over a week the student was growing vertical crystals and we didn’t even realize until we imaged the surface of the substrate with a scanning electron microscope,” explained Briseno. “We were shocked to see little crystals standing upright! We ultimately optimized the conditions and determined the mechanism of crystallization.”
The abstract to the research paper claims that the nanopillar architecture resulted in a 32-percent increase in the conversion efficiency over amorphous planar devices of the same material.
The aim of these photovoltaics will not be to take your home off the grid but to offer a way to power low-energy devices. Briseno adds: “We envision that our nanopillar solar cells will appeal to low-end energy applications such as gadgets, toys, sensors and short lifetime disposable devices.”
Dexter Johnson is a contributing editor at IEEE Spectrum, with a focus on nanotechnology.