Perovskite solar cell with self-assembled organic electron transport layer achieves 21.5% efficiency

A group of researchers led by Saudi Arabia's King Abdullah University of Science and Technology (KAUST) has fabricated a perovskite solar cell based on an organic electron transport layer (ETL) using self-assembled monolayer (SAM) molecules containing phosphonic acid as an anchoring group.

“All self-assembled monolayers were designed to collect holes, which works well for perovskite solar cells in the p-i-n polarity architecture,” the research's lead author, Stefaan De Wolf, told pv magazine. “Here, we tested a range of electron-selective SAMs designed and synthesized by Kaunas University of Technology and found that these work well in p-i-n polarity perovskite solar cells. So essentially the concept of SAM decoration of metal oxide to tune the charge selectivity has now been proven to work well for both polarities. Overall, this has quite some advantages, as for example low-temperature processibility of the contacts.”

The scientists used non-fullerene semiconductors composed of molecules known as anthraquinone (AQ) and naphthalenediimide (NDI), which they said allow covalent binding with indium tin oxide (ITO) surfaces within the cell and energetic matching with the perovskite. They used, in particular, two modified versions of the molecules, which they called PANDI and PAAQ.

The research group built the cell with a substrate made of glass and ITO, the electron-selecting SAMs, a perovskite absorber, a hole transport layer (HTL) based on Spiro-OMeTAD, a molybdenum oxide (MoOx) layer, and a silver (Ag) metal contact.

The academics conducted thermogravimetric analysis (TGA) on these modified molecules and found they are thermally stable with only 5% weight loss at 356 C and 268 C, respectively.

“UV–vis transmittance results of SAM functionalization on the ITO surface show negligible optical losses compared to bare ITO and ITO/SnO₂ films,” they stated, noting that these SAMs also showed relatively suitable energetic alignment with the perovskite absorber and ITO contact. “In particular, PANDI-based SAMs demonstrate a higher surface homogeneity on the ITO surface than PAAQ SAMs. We found that the increasing surface homogeneity on the ITO/PANDI can effectively suppress nonradiative interfacial recombination through the field-effect passivation, as indicated by longer charge carrier lifetimes and higher QFLS values.”

Tested under standard illumination conditions, the cell achieved a power conversion efficiency of 21.5%, which the research team is the highest efficiency ever reported to date for a perovskite solar cell relying on an organic electron transporting layer. The device also achieved an open-circuit voltage of 1.13 V, a short-circuit current density of 24.7 mA cm², and a fill factor of 77%

“We also tested the operational stability of our SAM-based devices at 65 C with above 90% retention of their initial performance for 1000 h,” the researchers also explained.

They identified the PANDI SAMs are the most suitable candidates for future perovskite solar cells with p-i-n structures, as they can be more easily applied to flexible substrates. “The PANDI-based device also showed improved operational long-term thermal stability, confirming that the PANDI SAM has a potential future to be utilized as ETL materials,” they concluded.

The solar cell was introduced in the study “Nonfullerene Self-Assembled Monolayers As Electron-Selective Contacts for n-i-p Perovskite Solar Cells,” which was recently published in ACS Energy Letters.