Accidental Impurities in Epitaxial Pb(Zr0.2Ti0.8)O3 Thin Films Grown by Pulsed Laser Deposition and Their Impact on the Macroscopic Electric Properties.

Structural and electrical properties of epitaxial Pb(Zr0.2Ti0.8)O3 films grown by pulsed laser deposition from targets with different purities are investigated in this study. One target was produced in-house by using high purity precursor oxides (at least 99.99%), and the other target was a commercial product (99.9% purity). It was found that the out-of-plane lattice constant is about 0.15% larger and the a domains amount is lower for the film grown from the commercial target. The polarization value is slightly lower, the dielectric constant is larger, and the height of the potential barrier at the electrode interfaces is larger for the film deposited from the pure target. The differences are attributed to the accidental impurities, with a larger amount in the commercial target as revealed by composition analysis using inductive coupling plasma-mass spectrometry. The heterovalent impurities can act as donors or acceptors, modifying the electronic characteristics. Thus, mastering impurities is a prerequisite for obtaining reliable and reproducible properties and advancing towards all ferroelectric devices.

Iodine Migration and Degradation of Perovskite Solar Cells Enhanced by Metallic Electrodes.

We monitored the evolution in time of pinhole-free structures based on FTO/TiO2/CH3NH3PbI2.6Cl0.4 layers, with and without spiro-OMeTAD and counter electrodes (Ag, Mo/Ag, and Au), aged at 24 °C in a dark nitrogen atmosphere. In the absence of electrodes, no degradation occurs. While devices with Au show only a 10% drop in power conversion efficiency, remaining stable after a further overheating at 70 °C, >90% is lost when using Ag, with the process being slower for Mo/Ag. We demonstrate that iodine is dislocated by the electric field between the electrodes, and this is an intrinsic cause for electromigration of I- from the perovskite until it reaches the anode. The iodine exhaustion in the perovskite layer is produced when using Ag electrodes, and AgI is formed. We hypothesize that in the presence of Au the iodine migration is limited due to the buildup of I- negative space charge accumulated at the perovskite-OMeTAD interface.