4PACz ((4-(9H-carbazol-9-yl)butyl)phosphonic acid, CAS No. 20999-36-4) is a self-assembly monolayer (SAM) hole transport material. It has a carbazole terminal that can be in direct contact with active layer materials and a phosphonic acid anchor that can be chemically attached to an electrode surface. It also has a n-butyl chain spacer that connects the terminal and anchor. 4PACz has a longer spacer than 2PACz which allows for more stabilizing van der Waals interactions.
When deposited as a monolayer on an electrode, PACzs exhibit lower resistance, higher optical transmittance. Thus it delivers a higher short-circuit current density and fill factor than the most frequently used hole transport layer PEDOT:PSS. The work functions of 2PACz, 3PACz, and 4PACz are 4.94 eV, 4.97 eV, and 4.81 eV respectively, and are slightly lower than that of PEDOT:PSS on ITO, 5.1 eV.
Serving as hole selective contact for organic solar cells and perovskite solar cells, 4PACz is an alternative to PEDOT:PSS with superior performance and the convenience of solution deposition at low concentration, i.e. 1 mM.
Typical processing solvents: Anhydrous ethanol (IPA, IPA/DMF are also superior solvents) Typical concentration: 0.30 - 1.0 mg/ml ( 1 mM - 1 mg/ml) Typical processing procedure: 4PACz is dissolved in IPA at a concentration of 1.0 mM. and stirred for 30 min at 80 °C before deposition. 40 uL of the SAM solution is dropped onto the substrate and spin-coated at 3000 rpm for 30 s. Then after being annealed at 100 ℃ for 15 min, the coated substrate is washed with IPA (60 uL) through spin-coating at 3000 rpm for 30 s, annealed at 100 ℃ for another 5 min (DOI: 10.1002/anie.202213560).