MPA-CPA (2-(4-(Bis(4-methoxyphenyl)amino)phenyl)-1-cyanovinyl)phosphonic acid, CAS No. 2212003-31-9) is a self-assembled monolayer material. It consists of a dimethoxy-triphenylamine terminal group, a cyano-vinyl linker, and phosphonic acid anchoring group. MPA-CPA coated substrates promote high quality perovskite films with minimal defects at the buried interface. It does this by providing a superwetting underlayer bedding for perovskite deposition, enabling highly efficient perovskite solar cells with great stability.

MPA-CPA is amphiphilic with hydrophilic cyano-vinyl group on the linker and phosphonic acid as the anchor, and also a hydrophobic methoxy-substituted triarylamine terminal group. The hydrophilic groups promote the hydrophilicity of self-assembled monolayer. This makes it highly wettable for the perovskite precursor solutions to form very uniform and large-area perovskite films on the MPA-CPA coated substrate without any pre-wetting treatment. The phosphonic acid anchoring group also gives rise of pinhole-free and coherent interface between the substrate electrode and the perovskite active layer.

The anchoring group acts as an excellent passivation agent to minimise the deep-level defects owing to the synergistic coordination of the cyano and phosphonic groups with lead ions through Pb–N and Pb–O bonds. As a result, the perovskite films deposited on top of and passivated by MPA-CPA self-assembled monolayer shows a high photoluminescence quantum yield (PLQY) of 17%, twice of that obtained by 2PACz-passivated perovskite.

Serving as hole selective contact for organic solar cells and perovskite solar cells, MPA-CPA is an alternative to PEDOT:PSS with superior performance with the convenience of solution deposition at low concentration, i.e. 1 mM.

Solution Processing Procedure

Typical processing solvents: Anhydrous ethanol (IPA, IPA/DMF are also superior solvents)
Typical concentration: 1.0 mg/ml
Typical processing procedure: MPA-CPA is dissolved in anhydrous ethanol at a concentration of 1.0 mg/ml. The solution is spun coated on the substrate for 30 s at 3000 rpm in a nitrogen filled glove box. The resulting MPA-CPA coated substrate is then cooled to room temperature after being annealed for 10 min at 100 ℃ (DOI: 10.1126/science.adg3755). The coated substrate can further be washed with a mixed solution of DMSO and DMF (volume ratio of 4:1) by spin-coating the mixture of the solvents for 30 s at 3000 rpm, then annealed for 5-10 min at 100 ℃.

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