Researchers from Chonnam National University, South Korea, have developed a method — called co-precipitation — to make a solar thin film comprising nanoporous nickel oxide (NiOx) as the hole transporting layer (HTL) for a perovskite solar cell with formamidinium lead iodide (FAPbI3) and/or methylammonium lead bromide (MAPbBr3) as the perovskite layer. In addition, they used organic air-stable inorganic zinc oxide nanoparticles compound as the electron transporting layer (ETL) to protect the perovskite layer from air.
A new approach to air-stable perovskite solar cells
“Our main goal is to protect the perovskite layer from air,” says study co-author Dr Sawanta S Mali from the Polymer Energy Materials Laboratory at Chonnam National University. Although PEDOT:PSS is widely used as HTL in p-i-n-inverted-type perovskite solar cells, it is highly sensitive to moisture, which results in fast degradation. “The low hole mobility and high moisture sensitivity of organic HTL hampers the stability and efficiency of the perovskite solar cells,” Mali explains. “Besides, the conventional n-type fullerene-based ETL — herein C60 or PCBM — suffered from uniform deposition, low electron mobility and diffusion of metal contacts in the perovskite layer.”
To protect the air-sensitive perovskite material, the South Korea team has added an air-stable, n-type ZnO nanoparticle electron transport layer (ETL) over the top. “This inorganic n-type ZnO ETL can protect the perovskite layer from air and avoid metal diffusion,” says Mali. He projects that the combined effect a highly stable p-type NiOx HTL and an n-type ZnO ETL capping layer will be useful in the development of highly efficient, air-stable perovskite solar cells.
The co-precipitation method
The researcher notes that due to the low hole mobility and unstable nature of conventional PEDOT:PSS, inorganic, stable HTLs are the best choice. While NiOx has been used in inverted p-i-n-type perovskite solar cells before, the NiOx nanoparticles suffered from pin-hole, high grain boundaries and transparency issues. A thicker NiOx layer also causes lower mobility and less transparency. Fullerene-based ETLs are not ideal either, due to their low electron mobility and metal contact diffusion to the perovskite layer. “In the present case, the novel nanoporous NiOx was synthesized by simple co-precipitation technique using NiSO4 and K2S2O8 in aqueous ammonia solvent at room temperature,” reports Mali. “The prepared nanoporous NiOx architecture facilitates high hole mobility with great stability. The nanoporous architecture provides easier hole transportation through the nanowalls, while the porous structure provides an excellent perovskite/NiOx interface.”
Stable perovskite solar cells with p-type NiOx
The p-type PEDOT:PSS previously used for inverted (p-i-n) type perovskite solar cells is unstable and shows low hole mobility in presence of trace moisture. And, as the expert mentioned, the low hole mobility and high moisture sensitivity of organic HTL hampers the stability and efficiency of the cells. P-type NiOx, on the other hand, is highly stable with an excellent hole mobility, which facilitates an additional stability to inverted p-i-n perovskite solar cells, according to Mali. “Furthermore, this novel nanoporous architecture provides easier hole extraction ability and great interfaces to the perovskite layer in order to extract holes effectively, compared with conventional HTLs.”
In the past, fullerene-based materials have typically been used for the ETLs. But the device suffered from low electron mobility and metal contact diffusion. Thus, the professor stresses the necessity of low-cost, highly stable inorganic materials with high charge mobility.
Benefits of the co-precipitation method
According to the researcher, the new electrodes his team has developed are more stable than conventional organic HLTs, and they are free from additives doping. Another advantage is that the HTLs and ETLs are synthesized via a simple chemical process. “The developed inorganic n-type non-fullerene ETL is air-stable, low-cost and could be easily deposited onto the perovskite layer,” Mali says. “Furthermore, this n-type ETL electrode terminates the metal contact diffusion. The performance of this device is promising good stability.”
Impact on the design of future perovskite solar cells
The study shows that the NiOx-based solar devices are much more stable in air than polymer-based alternatives: “Without additional encapsulation, the devices showed little deterioration in performance after more than five months,” Mali reports. “By contrast, polymer-based devices deteriorated over the first few days and were completely dead within five days.”
The novel method reported from South Korea is currently limited to lab scale. However, Mali shares that he and his fellow researchers strongly believe that if they use this device architecture with different coating methods, such as ultrasonic spray coating or roll-to-roll process, the pioneering method would be “great for commercialization.”
In addition, the professor reckons the charge mobility of HTL and ETLs can further be improved through suitable doping. “Currently, we are developing Li-doped NiOx and Mg-ZnO, in order to further improve the hole and electron mobility in perovskite solar cells. In addition, the unique composition of 3D and 2D perovskite will further improve the stability of perovskite solar cells.”
Mali sees large area perovskite devices based on this architecture being made using an ultrasonic spray process. “This all-metal-oxide sandwiched perovskite layer architecture would be the best choice towards air-stable PSCs,” he says. “However, large-area deposition using spin coating is still a big obstacle. Therefore, other coating techniques, such as ultrasonic spray or roll-to-roll processes, would be the best choice towards the commercialization of this type of perovskite solar cells.
Protecting the unstable perovskite layer
“To protect the unstable perovskite layer is the biggest challenge in perovskite solar cell technology,” says Mali, pointing out that previous devices that demonstrated world-record efficiencies were limited to highly expensive hole-transporting materials, such as spiro-MeOTAD or PTAA. He says these HTLs are not only expensive but unstable, too. That is why even more so stresses that the development of alternative inorganic metal-oxide hole transporting layers is the most promising approach to achieving air stability.
As the professor explained, the conventional PEDOT:PSS HTLs degrade fast and the conventional n-type fullerene-based ETL suffers from uniform deposition, low electron mobility and diffusion of metal contacts in the perovskite layer. “Keeping these problems in our mind, we have to develop low-cost, highly stable inorganic materials with high charge mobility,” Mali says. “The highly stable p-type NiOx HTL and n-type ZnO ETL capping layer would be the best choice towards highly efficient and air-stable PSCs.” This inorganic p-type NiOx nanoporous architecture facilitates fast hole transportation, while the n-type ZnO ETL can protect the perovskite layer from air and avoid metal diffusion.
The research is detailed in the article “Nanoporous p-type NiOx electrode for p-i-n inverted perovskite solar cell toward air stability,” published in the journal Materials Today. Dr Chang Kook Hong from Chonnam National University was the corresponding author.
Written by Sandra Henderson, Research Editor, Solar Novus Today