Self-Assembled Monolayers (SAMs)
Self-assembled monolayers (SAMs) are orderly assembly of ultra-thin monolayers formed on a surface of the substrate directed by either weak physical force or strong chemical covalent bonding, normally through the process of deposition and thermal annealing. SAMs act as an interface, i.e. hole injection monolayer (HIML) between the electrode and the active layer materials in optoelectronic devices.
As hole extraction or transport layers, SAMs improve the device performance of dye-sensitized solar cells, organic solar cells, and inverted perovskite or polymer solar cells. The following are examples of solar cells device performance engaging different SAMs as hole transport layers:
Our collection of self-assembled monolayers includes molecules with carbazole terminal functional groups, alkyl linkers, and phosphonic acid anchoring groups.
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Self-assembled monolayers consist of three parts:
- The anchor
- The linker
- The terminal groups
The anchor connects the SAM molecules to the electrode surface by covalent or hydrogen bonding to passivate the surface defects. The terminal groups, in direct contact with the active layer, can modify the surface and interface properties. The linker is there to establish the spacer length and build a barrier between the anchor and the terminal functional groups.
There have been different types of anchoring groups, mainly including carboxylic acid, phosphoric acid, silane, and thiols. These groups effectively establish the connection of the SAM molecules to the substrate surface and modify the coverage ratio, the contact resistance, and the work function of the substrate.
The terminal functional groups can change the interfacial dipole moment to enlarge the build-in potential and tune the energy levels, plus they can alter the recombination energy losses. Moreover, the terminal functional groups can help to enhance crystallinity and morphology of the cast film. Electron-rich conjugated aromatics such as carbazole, dibenzo[c,g]carbazole, and acridine are the most frequently used terminal functional groups in perovskite and organic solar cells.
The linker connecting the anchor and the end terminal functional group is either a nonconjugated aliphatic alkyl chain or conjugated phenylene unit, which in return could impact the geometry and packing of the molecules onto the substrates.