Sunlight provides ample energy to meet our global needs, yet we currently harness only a small fraction of its potential. Moreover, optimization of solar cell device performance is of great interest to access a renewable energy source that gives off no carbon by-products. One avenue to improve device efficiency is to use semiconducting nanowires, which improve absorption of solar energy if the diameter of the structure is similar to the wavelength of light. The large-scale synthesis of these materials has been hampered by costly, time-consuming techniques − until now. As reported in Nano Letters, scientists at the Center for Energy Nanoscience (CEN) recently developed a new method that allows for the large-scale, cost-effective synthesis of vertically aligned semiconductor nanowires, akin to a densely packed forest of nano-tree trunks, which efficiently absorb incoming light.
To harvest sunlight efficiently, a material should absorb solar energy while minimizing energy reflection. The similar size of the nanowires and wavelength of the incident energy help that happen. "The light will go through multiple rounds of bouncing, being trapped in the nanowire arrays and have a better chance of getting absorbed," said author Maoqing Yao.This minimizes the amount of energy reflected and optimizes absorption, yielding more power output from the same amount of input energy. While such a system can give smaller, lighter devices to use in portable charging stations, they must also be cost-effective and scalable over a large area to compete commercially with coal and natural gas.
The CEN team prepared towers of gallium arsenide nanowires of controlled size and spacing rising from an underlying surface of the same material. To create this forest of nanowires, scientists first deposited tiny polymer spheres (100-890 nanometers) on a wafer, and then used oxygen plasma to uniformly shrink the spheres with space between them, giving what resembles a page of candy buttons. A layer of metallic aluminum or iron was put down to cover the entire surface, both the spheres and spaces between. Removal of the spherical nanoparticles by washing with an organic solvent revealed a regular pattern of gallium arsenide holes from which the nanowires grew by crystallization using a chemical vapor deposition method. This process gave a forest of evenly spaced tree trunks of semiconducting gallium arsenide nanowires, ideal for use in photovoltaic devices.
While some defects were present in the fields, such as missing nanowires, the arrays of pillared semiconductors had excellent absorption and reflected less than 10 percent of incoming light. Importantly, these values are similar to those observed for organized nanowires prepared from more expensive lithographic techniques. This novel method for the large-scale preparation of semiconducting gallium arsenide nanowires represents a great stride forward in the optimized harvesting of sunlight by photovoltaic devices and is currently being used to prepare patterned nanowire devices.
