Solar engineers are racing to produce virtually impossible-thin, flexible solar panels. Engineers envision them being employed in a variety of mobile applications, ranging from self-powered wearable gadgets and sensors to lightweight airplanes and electric cars. Against this context, Stanford University researchers have achieved record efficiency in a promising class of solar materials.
One of the most significant advantages of these transition metal dichalcogenides – or TMDs – is that they absorb extremely high quantities of sunlight that reaches their surface as compared to other solar materials.
“Imagine an autonomous drone powered by a solar array atop its wing that is 15 times smaller than a piece of paper,” said Koosha Nassiri Nazif, a PhD fellow in electrical engineering at Stanford and co-lead author of a study published in Nature Communications on December 9. “That is what TMDs promise.”
The quest for new materials is required because silicon, the reigning monarch of solar materials, is far too heavy, bulky, and stiff for applications requiring flexibility, lightweight, and high power, such as wearable gadgets and sensors, aircraft, and electric vehicles.
“Today, silicon accounts for 95 percent of the solar business, but it is far from flawless.” “We need new materials that are light, flexible, and, honestly, more environmentally friendly,” said Krishna Saraswat, an electrical engineering professor and senior author of the work.
A competitive substitute
While TMDs offer immense potential, research attempts to yet have failed to convert more than 2% of the sunlight they absorb into energy. That figure is approaching 30% for silicon solar panels. TMDs will need to close that gap if they are to be extensively deployed.
The new Stanford prototype achieves 5.1 percent power conversion efficiency, but the scientists believe that with optical and electrical modifications, they may attain 27 percent efficiency. That amount would be comparable to the greatest solar panels on the market today, including silicon.
Furthermore, the prototype had a 100-fold higher power-to-weight ratio than any other TMD ever produced. This ratio is critical for mobile applications such as drones, electric cars, and the ability to power expeditionary equipment while traveling. When measured in terms of specific power – the amount of electrical power production per unit weight of the solar cell – the prototype produced 4.4 watts per gram, which is comparable to other current-day thin-film solar cells, including other experimental prototypes.
“We believe we can enhance this critical ratio tenfold via optimization,” Saraswat stated, adding that the practical limit of their TMD cells is a staggering 46 watts per gram.
Their greatest advantage, however, is their amazing thinness, which not only reduces material consumption and expense but also makes TMD solar cells lightweight and flexible, allowing them to be molded to irregular forms such as a vehicle roof, an airplane wing, or the human body. The Stanford team created an active array that is only a few hundred nanometers thick. The photovoltaic TMD tungsten diselenide array is made up of photovoltaic TMD tungsten diselenide and gold contacts that are separated by a single atom thick layer of conducting graphene. All of this is sandwiched between a skin-like flexible polymer and an anti-reflective layer that optimizes light absorption.
TMD cells are less than six microns thick when fully formed — roughly the thickness of a lightweight office trash bag. To achieve the thickness of a single sheet of paper, 15 layers would be required.
While thinness, lightweight, and flexibility are all desirable goals in and of themselves, TMDs provide additional technical benefits. They are long-term solid and dependable. TMDs, unlike other thin-film competitors, do not contain any hazardous compounds. They’re also biocompatible, which means they might be employed in wearable applications that need direct contact with human skin or tissue.
A bright future
TMDs have several advantages, but they also have drawbacks, most notably in the technical complexities of mass manufacture. Transferring an ultrathin layer of TMD to a flexible, supporting material frequently causes damage to the TMD layer.
The transfer procedure that attaches the thin TMD solar arrays to the flexible substrate was created by Alwin Daus, who co-authored the work with Nassiri Nazif. He described the technological obstacle as significant. According to Daus, who was a postdoctoral fellow in Eric Pop’s research group at Stanford at the time the research was undertaken, one stage entailed moving the layer of atomically thin graphene onto a flexible substrate only a few millimeters thick. He is presently a senior researcher at Germany’s RWTH Aachen University.
Because of this complicated procedure, the TMD is thoroughly incorporated in the flexible substrate, resulting in increased endurance. The researchers put their gadgets through their paces by bending them around a metal cylinder less than a third of an inch thick.
“Powerful, versatile, and long-lasting,” Nassiri Nazif stated, “TMDs represent a potential new route in solar technology.”