| 17 October 2012
Researchers at the University of Kansas (KU), Lawrence, Kansas (US), broke the all-carbon PV efficiency world record with a 1.3% efficient solar cell built from nanocarbons, materials that could help drastically reduce the cost of PV technologies in the future.
“The PV cell in this work is made entirely out of solution-processable carbon nanomaterials, a unique combination of single-wall carbon nanotubes, fullerenes and graphene derivatives,” explains Shenqiang Ren, assistant professor in KU's Department of Chemistry, who spearheaded the research in collaboration with Professor Jeffrey Grossman’s theory group at the Massachusetts Institute of Technology (MIT).
Carbon materials are inexpensive, readily available and environmentally sustainable. “The nanocarbon materials hold great promise for higher absorption and transport, in particular photostability, a key issue with polymer solar cells and other solution-processed thin-film PV cells,” Ren says. “In the other words, these nanocarbon solar cells don't degrade when exposed to light and air, which could make the encapsulation requirements less stringent.” What is more, the ultrathin nanocarbon active layer of only 100 nm retains the manufacturing advantages of polymer-based PV technologies.
According to Ren, nanocarbons will play a major role for future solar technologies: “Making thin, flexible and inexpensive PV devices that are durable under harsh conditions.” Not without this latest record venture: “The remarkable achievement is a synergistic merit between our experimental work here at KU and the theoretical predictions by Professor Grossman's group at MIT,” Ren lauds. “We were inspired by the simple idea that we need a metal and a semiconductor to construct a solar cell, and nanocarbon allotropes can be either metallic or semiconducting,” he says, further elaborating that they “started from ultrapure and durable nanocarbon allotropes to build a well-defined heterojunction interface and percolation network, leading to efficient charge separation and collection for a record-efficiency.”
Solar cells made from carbon nanotubes, though a promising concept, achieved energy conversion efficiencies below 1% in the past. KU’s world record of 1.3% thus is an exciting advance. In its infancy today, the technology has a long road ahead to commercialisation. “The beauty of single-wall carbon nanotubes in this nanocarbon system is their absorption spectrum, which is matched quite well with the sunlight spectrum, including the 48% infrared radiation,” Ren says. “A broad and tunable absorption, as well as a pathway for efficient exciton dissociation, charge transport and collection within nanocarbon are essential to cross a 10% efficiency.” To get there, solar researchers have to figure out how to most efficiently convert the absorbed photons to charge carriers. “We start to see progress by controlling chirality, heterojunction interface and morphology,” reports Ren, who also is one of the authors of the paper “Nanocarbon-Based Photovoltaics”, published in ACS Nano.
Materials cost is another concern, which his interdisciplinary team is addressing by expanding the search to other widely available carbon materials. “We want to expand the nanocarbon family to better understand their role in the device, with a goal to develop efficient and durable carbon optoelectronics. Nanocarbon PV is one of example.” Ren projects nanocarbon-based technologies will have a much broader impact, from green energy generation and storage, to information and environmental technology. “The superior mechanical and thermal properties, chemical stability and transport properties of nanocarbon materials will make them very suitable for emerging applications under harsh conditions,” he sums up his outlook.
Written by Sandra Henderson, Research Editor, Solar Novus Today
