Researchers Make Quantum Dots and Perovskite Solar Cells at only 150°C

May 24, 2019 by Scott McMahan

Researchers at the University of Toronto have developed a method of growing a more stable electron selective layer for Perovskite solar cells and tandem solar cells combining crystalline silicon with Perovskite.

Crystalline silicon solar cells, which make up the majority of solar cell installations, must be processed to a very high purity with temperatures above 1,000 degrees Celsius, and the process also requires large amounts of hazardous solvents.

"Economies of scale have greatly reduced the cost of silicon manufacturing," said University Professor Ted Sargent, an expert in emerging solar technologies and the Canada Research Chair in Nanotechnology, who lead the research team. "Perovskite solar cells can enable us to use techniques already established in the printing industry to produce solar cells at very low cost. Potentially, perovskites and silicon cells can be married to improve efficiency further but only with advances in low-temperature processes."

On the other hand, perovskite solar cells depend on a layer of tiny crystals known as quantum dots that are each about 1,000 times smaller than the width of a human hair. These quantum dots are made of low-cost, light-sensitive materials known as quantum dots.

So, Perovskite material and crystalline silicon solar cells can be combined to pick up separate parts of the solar spectrum.

Perovskite raw materials can be mixed into a liquid in a kind of ‘solar ink.' This solar ink could be printed onto glass, plastic or other materials with a relatively simple inkjet printing process. However, in order to generate electricity, electrons excited by solar energy from Perovskite cells must be extracted from a layer of quantum dots that is held together by a passivation layer.

Some types of quantum dots are known to change their 3D structure even at room temperature, making them transparent and ineffective.

This passivation layer is also known to break down at temperatures above 100°C.

The team's breakthrough made both quantum dots and perovskites more stable when combined than they are separated.

"The most effective materials for making ESLs start as a powder and have to be baked at high temperatures above 500 degrees Celsius," said Post-doctoral researcher Hairen Tan the university's Faculty of Applied Science & Engineering. "You can't put that on top of a sheet of flexible plastic or on a fully fabricated silicon cell - it will just melt."

The research team developed a new process than enables them to grow an ESL or passivation layer made of nanoparticles in solution. This solution can be grown directly on top of the electrode. While heat is still needed, the process always stays below 150°C, a much lower temperature than the melting point of many plastics.

The team detailed the method they developed to combine Perovskites and quantum dots in the journal Nature.

Mengxia Liu, the paper's lead author, attributes part of the discovery to the collaborative environment in the team, which included researchers from numerous disciplines, including physics, chemistry, and her field of materials science.

The solar cell combining of Perovskite material and quantum dots achieved 20.1% efficiency.

In the future, Liu hopes that solar cell makers will improve her ideas even further to create solution-processed solar cells that meet all the same criteria as traditional silicon.