PTAA – poly[bis(4-phenyl)(2, 4, 6-trimethylphenyl)amine] – is a successful hole-transporting material for perovskite solar cells, but can only be applied using spin-coating, which is not suitable for wide areas, according to the research team. Blade-coating is compatible with large surfaces, but the physical properties of PTAA prevent it from forming even layers when blade coated, leaving voids within the finished solar cell.
Knowing this, the team went on a hunt for a hole transport that is compatible with blade coating.
“We screened various hole-transporting materials and found that self-assembled mono-layers are a class of promising materials for the up-scaling of perovskite devices,” said City University of Hong Kong professor Alex Jen.
The molecules selected contain a chemical group which can naturally bond to the substrate, which can anchor the molecule, plus a chemical group on the other end that is easily wetted by the perovskite layer when applied, minimising defect formation.
“Furthermore, since the self-assembled mono-layer is a mono-layer, charge carriers can be extracted from perovskite to substrate electrode efficiently through charge tunnelling, resulting in enhanced device performance,” added Jen.
The molecule can be solution-processed and is amenable to optimisation for: better perovskite growth, improved defect density at its interface with the perovskite, and better energy band alignment with the perovskite.
A small (18cm2) proof-of-concept solar module showed 14% efficiency and, in an inert atmosphere, individual cells retained 90% of initial efficiency after 500 hours at 40°C.
City University of Hong Kong worked with the Southern University of Science and Technology, Shenzhen on the project, which is covered in ‘Self-assembled monolayer enabling improved buried interfaces in blade-coated perovskite solar cells for high efficiency and stability’, published by Nano Research Energy.