Researchers at the Argonne-Northwestern Solar Energy Research Center (ANSER) have developed a new way to protect perovskite-based solar cells from water and stabilize them against heat. By carefully growing an ultrathin layer of metal oxide on a carbon coating, the researchers made a perovskite device that worked even after exposing it to a stream of water.
Solar cells are made up of layers, each with a specific duty. The perovskite layer absorbs sunlight, which can excite an electron. The electron could go back to where it started, unless it can be successfully extracted out of the absorbing layer quickly. For this device, the researchers placed a layer of PC61BM, a carbon-based material, on top of the perovskite, which has two roles.
First, PC61BM is good at extracting electrons once they are excited by sunlight. Second, the PC61BM layer protects the perovskite from water vapor, which is one of the reactants used for forming the final protective coating — titanium dioxide.
The titanium dioxide layer was grown using atomic layer deposition (ALD), a method that deposits alternating layers of titanium and oxygen atoms. The researchers demonstrated that depositing the titanium dioxide by ALD creates a barrier with no pinholes, effectively blocking moisture from entering the film. Only about 20 nanometers of titanium dioxide on the PC61BM were needed to protect the perovskite. This layer is around 1,000 times thinner than the thickness of a human hair.
On top of the titanium dioxide, aluminum electrodes were deposited and protected by a thin layer of gold.
On the opposite side of the perovskite, the team placed a nickel oxide layer that is good at extracting the positively charged holes left by the electrons. Glass, coated with a conductive film, is used as a support that allows light to pass through and a circuit to be formed.
The device held up to pure water and a temperature of 100 °C (around 200 °F) thanks to the titanium dioxide layer. “The key challenge to commercialization of any halide perovskite-based devices is the environmental stability,” says Soo Kim.the team.
This work is one of the first examples of protecting perovskite from liquid water with an ultrathin metal oxide layer. Alex Martinson, who directed the work, said, “It is surprising when something simple works so well.”