Abstract
Halide vacancies and associated metallic lead (Pb°) observed at the surface and deep inside macroscopic organolead trihalide perovskite crystals is removed through a facile and noninvasive treatment. Indeed, Br2 vapor is shown to passivate Br-vacancies and associated Pb° in the bulk of macroscopic crystals. Controlling the exposure time can markedly improve the overall stoichiometry for moderate exposures or introduce excessive bromide for long exposures, resulting in p-doping of the crystals. In the low dose passivation regime, Hall effect measurements reveal a ca. 3-fold increase in carrier mobility to ca. 15 cm2V-1s-1, while the p-doping increases the electrical conductivity ca. 10000-fold, including a 50-fold increase in carrier mobility to ca. 150 cm2V-1s-1. The ease of diffusion of Br2 vapor into macroscopic crystals is ascribed to the porosity allowed in rapidly grown crystals through aggregative processes of the colloidal sol during growth of films and macroscopic crystals. This process is believed to form significant growth defects, including open voids, which may be remnants of the escaping solvent at the solidification front. These results suggest that due to the sol-gel-like nature of the growth process, macroscopic perovskite crystals reported in this study are far from perfect and point to possible pathways to improving the optoelectronic properties of these materials. Nevertheless, the ability of the vapor-phase approach to access and tune the bulk chemistry and properties of nominally macroscopic perovskite crystals provides interesting new opportunities to precisely manipulate and functionalize the bulk properties of hybrid perovskite crystals in a noninvasive manner.
Highlights
Macroscopic single crystals of methylammonium lead tribromide (MAPbBr3) have been shown to exhibit a low density of trap states,[12] while polycrystalline thin films of the same chemical compositions suffer from several extrinsic factors such as grain boundaries, which play a role in defining charge transport
This approximation has recently been used by Snaith and Koch groups which have utilized X-ray photoelectron spectroscopy (XPS) on cleaved MAPbBr3 and methylammonium triiodide (MAPbI3) crystals to probe dopant concentration and the impact of light illumination, respectively, in the crystal bulk.[25, 26]
Hard X-ray photoelectron spectroscopy (HAXPES) measurements can allow for a deeper sample probing due to larger inelastic mean free paths of the photoelectrons at higher kinetic energies, these need to be carried out at a synchrotron facility and, as such, are beyond the scope of this study.[13]
Summary
Facile and noninvasive passivation, doping and chemical tuning of macroscopic hybrid perovskite crystals and their ease of solution-processability.[1,2,3,4,5] A spate of breakthroughs in device designs and thin film preparation protocols in the last few years has helped the perovskite solar cells community to achieve >23% power conversion efficiency (PCE).[4] Additional, concurrent advances have been made in the successful utilization of hybrid perovskites as light emitting diodes and photodetectors.[6,7,8] It is, difficult to completely avoid residual chemical contamination and defects in perovskite thin films, given the realities of solution-processing and the sol-gel nature of hybrid perovskite solidification.[9, 10] The presence of defects and contamination on surfaces is very much expected, but these have been believed unlikely to be present within the bulk of macroscopic single crystals, as the latter are considered the most pristine embodiments of the material.[11] Macroscopic single crystals of methylammonium lead tribromide (MAPbBr3) have been shown to exhibit a low density of trap states,[12] while polycrystalline thin films of the same chemical compositions suffer from several extrinsic factors such as grain boundaries, which play a role in defining charge transport. Recent reports have questioned the pristine quality of macroscopic crystals, suggesting that the electronic properties of hybrid perovskites are only modest at best, compared to gallium arsenide and silicon.[15, 16] It has been suggested that the internal structure of the MAPbBr3 crystals is far from perfect due to light- and environment-induced macroscopic voids.[17]
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