Chemical Passivation with Phosphonic Acid Derivatives of ZnO Deposited by Atomic Layer Deposition and Its Influence on the Halide Perovskite Interface

We report on the modification of zinc oxide thin films deposited by atomic layer deposition (ALD-ZnO) with various phosphonic acid derivatives. Particularly, three molecules differing by their spacer and functionalizing groups were tested: 2-aminoethylphosphonic acid (2-AEPA), 4-aminobenzylphosphonic acid, and 4-fluorobenzylphosphonic acid (4-FBzPA). The resulting surfaces were investigated with surface-sensitive characterization techniques such as X-ray photoelectron spectroscopy and attenuated total reflection IR spectroscopy. We find differences in the phosphonic acid film growth, mostly driven by the nature of the functionalizing group: the amine-based molecules tend to cover the surface with disordered layers or multilayers, whereas the 4-FBzPA layer rather exhibits features of a monolayer. Finally, 2-AEPA and 4-FBzPA have been used as a mean to passivate the reactive interface between ALD-ZnO and a hybrid organic inorganic metal halide perovskite. Morphological and structural studies were carried out with scanning electron microscopy and X-ray diffraction, and solar cells using these layers as electron transport layers were synthesized. With the highest power conversion efficiency of 4.1%, the direct application of these surface modifications into complete devices is shown not to be enough to achieve high-efficiency solar cells with ALD-ZnO.