DAEJEON — A South Korean research team has succeeded in eliminating the main culprit behind the shortened lifespan of next-generation solar cells.
The Korea Institute of Energy Research said on the 15th that its photovoltaic research division has developed a new catalytic process technology that can extend the lifespan of perovskite solar cells.
The technology removes lithium ions, the primary cause of shortened cell life, at the source, a step expected to accelerate the commercialization of tandem solar cells based on perovskite.
Perovskite is mainly used as the top cell in tandem solar cells, which boost efficiency by stacking different types of cells. Depending on the order in which the multiple layers are deposited, the structures are classified as conventional or inverted. Conventional structures deliver high efficiency thanks to years of accumulated expertise in single-junction devices, but their lifespan declines in hot and humid conditions.
Perovskite solar cells contain a hole transport layer that allows the generated electricity to flow out of the device. In conventional structures, that layer uses a lithium salt along with an additive that helps dissolve the salt, in order to improve the efficiency of charge transport.
The problem is that the additive evaporates under the hot, humid conditions of actual operation, creating unnecessary gaps between the layers that make up the cell. Those gaps produce defects at the layer interfaces and shorten the cell's lifespan.
Heat and humidity also cause lithium ions to separate from the lithium salt and migrate indiscriminately inside the cell, further shortening its life.
The researchers developed what they call a filtering extraction process following a catalytic reaction, which induces only the necessary chemical reaction and then filters out the lithium ions together with the catalyst once the reaction is complete.
At the heart of the process is a nickel oxide (NiOx) catalyst. When nickel oxide powder is added to the solution used to make the hole transport layer, the reaction between the two materials turns the nickel oxide surface into a state that strongly attracts lithium. That makes it possible to separate lithium from the lithium salt and bind it to the nickel oxide surface without the additive that was a drawback of the existing process.
The team also applied a physical extraction step that filters out the nickel oxide powder after the reaction. Because the nickel oxide catalyst particles are large, they are caught by the filter along with the lithium ions attached to their surfaces. Using the resulting solution makes it possible to produce a defect-free hole transport layer with no residual additive, lithium or nickel oxide catalyst to degrade performance.
The researchers built perovskite solar cells based on the process and tested their durability at 85 degrees Celsius and 85% relative humidity. The cells retained 95% of their initial efficiency after 1,000 hours, demonstrating world-leading durability.
The cells also achieved a power conversion efficiency of 25.24%, higher than the 25.21% of the existing process using the additive, proving that efficiency gains are possible without it.
"By solving a chronic lifespan problem, this will serve as an important turning point in accelerating the commercialization of perovskite and tandem solar cells," said An Se-jin, a principal researcher at the institute.
The findings were published as the cover paper in the May issue of Advanced Science, an international materials science journal with an impact factor of 14.1.