Scientists at the Okinawa Graduate Institute of Science and Technology (OIST) have solved a fundamental weakness in a promising solar technology known as perovskite solar cells (PSC). Their innovations seem to improve both the stability and scalability of the devices at one time and could be key to bringing the PSCs to market.
PSCs are a type of solar cell that includes a structured compound of perovskite, more commonly a hybrid material based on organic or inorganic tin halide, as the active layer of light capture.
Third-generation solar cells efficiently convert sunlight into usable electricity and their manufacture costs less energy than old-school silicon cells. PSCs, in particular, have attracted the attention of science and industry thanks to their low cost and high efficiency. Although their performance is promising in laboratory tests, the devices still suffer from low stability and can not be produced commercially until they are designed to last.
"We need solar modules that can last for at least 5 to 10 years. For now, the lifespan of the PSCs is much shorter, "said Dr. Longbin Qiu, first author of the article and a postdoctoral fellow in the Surface Sciences and Materials Unit of OIST Energy, led by Prof. Yabing Qi .
The study, published online in "Advanced Functional Materials" on December 13, 2018, supports previous evidence that a material commonly used in PSCs, called titanium dioxide, degrades devices and limits their lifespan. The researchers replaced this material with tin dioxide, a stronger conductor without these degrading properties. They optimized their method of applying tin dioxide to produce stable, efficient and scalable PSCs.
In experiments, the researchers discovered that tin-dioxide-based devices had a lifespan three times greater than PSC devices that use titanium dioxide. "Tin dioxide can give users the performance of the device they need," said Qiu.
An improved design
PSCs consist of layered materials, each with a specific function. The "active layer", made of perovskite materials, absorbs incoming sunlight in the form of particles called photons. When a photon hits a solar cell, it generates negatively charged electrons and holes positively charged in the active layer. Scientists control the flow of these electrons and holes by sandwiching the active layer between two "transport materials", thus creating a built-in electric field.
To help guide electrons in the right direction, many PSCs include an "electron transport layer." Most PSCs employ titanium dioxide as their electron transport layer, but when exposed to sunlight, the material reacts with the perovskite and ultimately degrades the device. Tin dioxide represents a viable replacement for titanium dioxide, but prior to this study, it had not been successfully incorporated into a large-scale device.
Using a common technique in the industry called sputtering deposition, the researchers learned how to make an effective electron transport layer from tin dioxide. The deposition by sputtering works by bombarding the target material, here the tin dioxide, with charged particles, which causes it to spray upwards on a waiting surface. By precisely controlling the power of the spray and the speed of deposition, the researchers produced smooth layers with a uniform thickness over a large area.
Their new solar cells achieved an efficiency of more than 20 percent. To demonstrate the scalability of this new method, the researchers then fabricated 5-by-5-centimeter solar modules with a designated area of 22.8 square centimeters, and found that the resulting devices showed an efficiency greater than 12 percent. This research, which was supported by the Concept Test Program of the OIST Technological Innovation and Development Center, represents a crucial step towards meeting the current industry standard for PSC efficiency.
Moving to the market
The researchers plan to continue to optimize their PSC design with the goal of producing large-scale solar modules with greater efficiency. The research unit experiments with flexible and transparent solar devices and aims to apply its optimized PSC design in solar windows, curtains, backpacks and folding loading units.
"We want to scale these devices to a large size, and although their efficiency is already reasonable, we want to push them further," said Professor Qi. "We are optimistic that in the coming years, this technology will be viable for commercialization."
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