THE ROLE OF FILM MORPHOLOGY AND DEFECTS ON THE PHOTOSTABILITY AND CARRIER TRANSPORT OF METHYLAMMONIUM LEAD TRIIODIDE

Organic-inorganic hybrid perovskites are viewed as a cost-effective alternative for photovoltaic devices, among other applications. Solution and low-temperature processing have gathered much attention for this class of materials. However, the stability and variability of reported fundamental properties have limited their progress toward practical applications. They suffer from environmental instabilities and undergo photochemical processes under light illumination that ultimately cause material degradation. The photostability of methylammonium lead triiodide (MAPbI3) is probed by Raman spectroscopy, revealing that photodecomposition scales with surfaces and grain boundaries. Surface or defective regions are also shown to affect carrier transport properties and act as scattering centers for low-energy emitted photons. The disordered nature of MAPbI3 and the additional structural defects of polycrystalline films result in restricted carrier diffusion lengths on the order of a micron determined by photoluminescence imaging, despite relative emission yields being much higher than an inorganic semiconductor like GaAs. In the literature, carrier diffusion lengths for MAPbI3 have been significantly larger when determined by photocurrent measurements. However, conducting photoluminescence imaging on polycrystalline films under an applied bias illustrates that carrier diffusion is still relatively small. Carrier drift or the possible reabsorption of traveling low-energy photons reaching the perovskite/electrode interface can give a false impression of a much longer carrier diffusion within MAPbI3.