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

Professor Pete Nellist collaborated with physicists from Oxford's Clarendon Laboratory and the University of York to investigate interface coherence within solar cells.

Metal halide perovskite semiconductors have shown great performance in solar cells, including an excess of lead iodide (Pbl₂) in the thin films, either as mesoscopic particles or embedded domains, which often lead to improved solar cell performance.  Atomic Resolution Scanning Transmission electron microscope micrographs of formamidinium lead iodide (FAPbl₃) perovskite films reveal the FAPbl₃:Pbl₂ interface to be remarkably coherent.

In the paper 'Atomistic understanding of the coherent interface between lead iodide perovskite and lead iodide' published in Advanced Materials Interfaces, Professor Nellist and the team demonstrate that such interface coherence was achieved by the Pbl₂ deviating from its common 2H hexagonal phase to form a trigonal 3R polytype through minor shifts in the stacking of the weak van-der-Waals-bonded layers containing the near-octahedral units, and that the exact crystallographic interfacial relationship and lattice misfit were revealed.

This paper further demonstrates that the 3R polytype of Pbl₂ has similar X-ray diffraction (XRD) peaks to that of the perovskite, making the 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.