Stanford Researchers Develop Material That Could Rival Silicon in the Solar Market
While the new material is significantly more lightweight and flexible than silicon, it boasts the same efficiency when used in solar panels.
A group of researchers from Stanford University has developed a flexible solar cell made of ultra-thin tungsten diselenide transition metal dichalcogenides (TMD)—and it may be set to dethrone silicon as the king of the photovoltaic market.
Silicon currently forms almost 95% of the current solar industry due to its low manufacturing cost and its nearly 30% power conversion efficiency. While TMD has been in the solar market for quite some time, it wasn’t until recently that it is being considered as a suitable way of harnessing solar energy.
TMD Rivals Silicon in Solar Panel Efficiency
In the past, the power conversion efficiency (PCE) of TMD was limited to only 0.7%. This was low compared to the 30% PCE of silicon. Now, however, Stanford researchers have found that tungsten diselenide TMD yields a power conversion efficiency of 5.1%, which can increase to 27% with certain optical and electrical enhancements. This figure is almost equivalent to the traditional solar panels available in the market today.
Professor Krishna Saraswat (left) and Dr. Koosha Nassiri Nazif (right) from Stanford University along with a picture of a Tungsten diselenide compound held by a pair of tweezers. Image used courtesy of Stanford University
"In electrical circuits, you can think of TMD solar panels as energy harvesters," co-lead author Dr. Koosha Nassiri Nazif told All About Circuits. "They can convert indoor and outdoor light into electricity to automatically recharge the batteries in the system or to directly provide power to the circuit, whether it’s a wearable smartwatch or the electrical engine of an autonomous drone."
Challenges of Current Photovoltaic Research
This research speaks to the challenges of silicon solar panel efficiency—one of the major causes of concern in the solar industry. These panels must improve in PCE (currently 30%) drastically if solar energy is to be considered a primary energy source. Silicon is a highly-rigid, heavy, and bulky material. It can be a relatively inefficient material for applications like space technology or wearable devices, which need a lightweight, flexible substrate. Many higher-efficiency materials are shelved because they can't easily be produced profitably on a large scale.
At the same time, silicon solar panels themselves need to be constantly monitored for certain physical characteristics like moisture resistance, making them a very unreliable source of energy.
What are Transition Metal Dichalcogenides?
According to the co-authors, Dr. Koosha Nassiri and Dr. Alwin Daus, finding a suitable contact material was one of the biggest challenges in this research.
Transition metal dichalcogenide or TMD occurs when a transition metal forms a covalent bond with chalcogens, resulting in a compound. TMDs are two-dimensional materials capable of forming planar structures spreading inches while being less than 1 nanometer thick. These properties make them a useful candidate for flexible ultra-thin solar cells.
Cross-sectional diagram of the device. Image used courtesy of Stanford University
The Stanford researchers created the tungsten diselenide TMD prototype by mechanically exfoliating the Si/SiO2 substrate. Graphene, a two-dimensional form of carbon, was layered with tungsten diselenide, which was further spin-coated by a polyimide substrate and an anti-reflective layer that significantly improved the absorption of light.
Because of its extraordinary thinness, the tungsten diselenide TMD minimizes the amount of required solar cell materials. It can also be molded into various asymmetrical shapes.
Electrically speaking, the tungsten diselenide TMD has improvements of about 10x in PCE and 100x in specific power (Ps) than any other TMDs that have been manufactured until now. Ps is a key figure of merit for solar PVs. It is the measure of power per gram (or kilogram) of a solar array.
Traditional TMDs have a PCE of less than 0.7% and a Ps of less than 0.04 Wg-1 while the tungsten diselenide TMD has a PCE of 5.1% and a Ps of 4.4 Wg-1.
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Power conversion efficiency (PCE) and specific power (power per weight) of lightweight and flexible thin-film solar technologies. Image used courtesy of Nature
The specific power can be further increased to 8.6 Wg-1 by reducing the polyimide substrate thickness to 1µm. The optical simulation showed that the absorption within the WSe2 layer could be improved to 80% by increasing the thickness of MoOx to 70nm.
The Research Ahead of TMD Solar Arrays
One of the major sectors using solar arrays is the space industry. Solar arrays currently used in spacecraft must be broken down to fit into the geometric constraints of the launch vehicle. But with the introduction of flexible TMD solar cells, this task could become very simple.
The high power per weight can decrease the overall mass of the aircraft, causing a reduction in the overall cost of launching a space vehicle. TMDs solar cells could also serve as a primary energy source in wearable devices—medical and non-medical—because of the non-toxic, environmentally-friendly materials used to manufacture them.
One of the major challenges to TMD, according to the researchers, is to synthesize the material on a large scale while maintaining uniformity and quality. More research is also required to ascertain the absorption percentage when the solar rays interact with the compound at different angles.
