Atomic understanding of the coherent interface between lead iodide perovskite and lead iodide

Metal halide perovskite semiconductors have shown great performance in solar cells.  Including an excess of lead iodide (Pbl2) in thin films -either as mesoscopic particles or embedded domains - often leads to improved solar cell performance.  

Atomic resolution scanning transmission electron microscope micrographs of formamidinium lead iodide (FAPbl3) perovskite films reveal that the FAPbl3:Pbl2 interface to be remarkably coherent.  

In the paper 'Atomistic understanding of the coherent interface between lead iodide perovskite and lead iodide' by the Department of Physics (Oxford) and Professor Pete Nellist, the researchers demonstrate that such interface coherence is achieved by the Pbl2 deviating from its common 2H hexagonal phase to form a trigonal 3R polytype through minor shift in the stacking of the weakly van-der-Waals-bonded layers containing the near-octahedral units.

The exact crystallographic interfacial relationship and lattice misfit were revealed, and the researchers further show that this 3R polytype of Pbl2 has similar X-ray diffraction (XRD) peaks to that of the perovskite, making XRD-based quantification of the presence of Pbl2 unreliable.  

Density functional theory demonstrates that this interface does not introduce additional electronic states in the bandgap, making it electronically benign.  These findings explain why a slight Pbl2 excess during perovskite film growth can help template perovskite crystal growth and passivate interfacial defects, improving solar cell performance.