2, Oak Ridge National Laboratory, Oak Ridge, Tennessee, United States
In this work, we demonstrate that the electronic heterogeneity in lead halide perovskite thin films is not intrinsic to the material, but depends on synthesis method, orientation, and grain size. These results have significant implications for the design of thin films with uniform electronic properties for high efficiency solar cells, and to the fundamental understanding of structure property relationships at the perovskite surface.
In the literature, photoluminescence (PL), time resolved photoluminescence (trPL), and charge transfer have been reported to vary for adjacent grains. Also, surface voltage and current have been reported to vary across different facets of the same crystal. This spatial variation implies that all bulk measurements correspond to ensemble averages, rather than intrinsic properties of the material. Furthermore, it is not currently known if the perovskite solar cells are limited by uniform loss or by spatially localized spots of poor efficiency, such as misoriented grains. In-depth examination of the causes of the spatially varying electronic properties, and their impact on macroscopically observable properties is necessary to accurately study the fundamental properties of this unique material.
However, many inconsistencies are currently present in the literature. Some optical studies conclude that perovskite thin films exhibit large spatial variations in trap site density on different grains while others conclude no variations in trap site density, or attributed PL quenching to other phenomena. Synthesizing these studies into one body of knowledge is difficult due to the different thin film morphologies and specific imaging techniques used by each group. Therefore, further work is needed for the field to reach a conclusion regarding the causes of the observed electronic heterogeneity.
Here we developed a methodology to separate the contributions the morphological and electronic contributions to PL and trPL heterogeneities in methylammonium lead iodide thin films, and reconcile the inconsistencies found in the literature. Spatially resolved PL and trPL are combined with light reflectance, transmission measurements and atomic force microscopy in order to quantitatively map the electronic properties and morphologies of perovskite thin films, and statistical techniques were used to separate the PL features caused by morphological variation from those due to true electronic heterogeneity. By comparing samples with different crystallographic orientations and grain sizes the factors causing electronic heterogeneities have been isolated. Our results provide a crucial step toward understanding the factors which control heterogeneity in trap site density at the surface, and toward bridging the gap between thin films and single crystals.