ZnO nanorod arrays were fabricated using a hydrothermal method. The nanorods were studied by scanning electron microscopy, photoluminescence (PL), time-resolved PL, X-ray photoelectron spectroscopy, and positron annihilation spectroscopy before and after annealing in different environments and at different temperatures. Annealing atmosphere and temperature had significant effects on the PL spectrum, while in all cases the positron diffusion length and PL decay times were increased. We found that, while the defect emission can be significantly reduced by annealing at 200 degrees C, the rods still have large defect concentrations as confirmed by their low positron diffusion length and short PL decay time constants.

We prepared ZnO nanostructures using chemical and thermal evaporation methods. The properties of the fabricated nanostructures were studied using scanning electron microscopy, x-ray diffraction, photoluminescence, and electron paramagnetic resonance (EPR) spectroscopy. It was found that the luminescence in the visible region has different peak positions in samples prepared by chemical and evaporation methods. The samples fabricated by evaporation exhibited green luminescence due to surface centers, while the samples fabricated by chemical methods exhibited yellow luminescence which was not affected by the surface modification. No relationship was found between green emission and g ϳ 1.96 EPR signal, while the sample with yellow emission exhibited strong EPR signal.

ZnO tetrapod nanostructures have been prepared by the evaporation of Zn in air (no flow), dry and humid argon flow, and dry and humid nitrogen flow. Their properties have been investigated using scanning electron microscopy (SEM), X‐ray diffraction (XRD), photoluminescence (PL) and photoluminescence excitation (PLE) spectroscopies (at different temperatures), and electron paramagnetic resonance (EPR) spectroscopy at –160 °C and room temperature. It is found that the fabrication conditions significantly influence the EPR and PL spectra obtained. While a g = 1.96 EPR signal is present in some of the samples, green PL emission can be observed from all the samples. Therefore, the green emission in our samples does not originate from the commonly assumed transition between a singly charged oxygen vacancy and a photoexcited hole [K. Vanheusden, C. H. Seager, W. L. Warren, D. R. Tallant, J. A. Voigt, Appl. Phys. Lett. 1996, 68, 403]. However, the green emission can be suppressed by coating the nanostructures with a surfactant for all fabrication conditions, which indicates that this emission originates from surface defects.

Unreacted lead iodide is commonly believed to be beneficial to the efficiency of methylammonium lead iodide perovskite based solar cells, since it has been proposed to passivate the defects in perovskite grain boundaries. However, it is shown here that the presence of unreacted PbI2 results in an intrinsic instability of the film under illumination, leading to the film degradation under inert atmosphere and faster degradation upon exposure to illumination and humidity. The perovskite films without lead iodide have improved stability, but lower efficiency due to inferior film morphology (smaller grain size, the presence of pinholes). Optimization of the deposition process resulted in PbI2‐free perovskite films giving comparable efficiency to those with excess PbI2 (14.2 ± 1.3% compared to 15.1 ± 0.9%) Thus, optimization of the deposition process for PbI2‐free films leads to dense, pinhole‐free, large grain size perovskite films which result in cells with high efficiency without detrimental effects on the film photostability caused by excess PbI2. However, it should be noted that for encapsulated devices illuminated through the substrate (fluorine‐doped tin oxide glass, TiO2 film), film photostability is not a key factor in the device degradation.