Nanoconfinement to Reveal Structural Evolution of Solution-Processed MAPbI₃ Crystals

We demonstrate the use nanoconfinement as a strategy to reveal structural evolution in solution-processed methylammonium lead triiodide (MAPbI₃) systems. MAI and PbI₂ precursors were co-deposited from organic solvents into anodized aluminum oxide (AAO) templates comprising uniaxially aligned nanopores. 2D x-ray diffraction patterns revealed the formation of an intermediate crystalline phase in as-spun samples. Interestingly, this phase adopted a preferential orientation in the AAO templates, allowing for prediction of the unit cell dimensions based on the observed diffraction pattern. We identified the intermediate phase as a PbI₂:solvent complex with sheets of PbI₂ running parallel to the long axes of the pores. Upon thermal annealing, the PbI₂:solvent complex converted to MAPbI₃, accompanied by a complete loss of preferential crystal orientation. Critically, nanoconfined MAPbI₃ crystals displayed significantly longer stability compared to unconfined MAPbI₃ crystals during exposure to humidity. Such extended stability is critical to progressing solution-processed metal-halide perovskite solar cells towards commercialization.