What is the Mechanism of MAPbI₃ p-Doping by I₂? Insights from Optoelectronic Properties

Obtaining insight into, and ultimately control over, electronic doping of halide perovskites may improve tuning of their remarkable optoelectronic properties, reflected in what appear to be low defect densities and as expressed in various charge transport and optical parameters. Doping is important for charge transport because it determines the electrical field within the semiconducting photoabsorber, which strongly affects collection efficiency of photogenerated charges. Here we report on intrinsic doping of methylammonium lead tri-iodide, MAPbI₃, as thin films of the types used for solar cells and LEDs, by I₂ vapor at a level that does not affect the optical absorption and leads to a small (<20 meV, ∼9 nm) red shift in the photoluminescence peak. This I₂ vapor treatment makes the films 10× more electronically conductive in the dark. We show that this change is due to p-type doping because we find their work function to increase by 150 mV with respect to the ionization energy (valence band maximum), which does not change upon I₂ exposure. The majority carrier (hole) diffusion length increases upon doping, making the material less ambipolar. Our results are well-explained by I₂ exposure decreasing the density of donor defects, likely iodide vacancies (V_I) or defect complexes, containing V_I. Invoking iodide interstitials, which are acceptor defects, seems less likely based on calculations of the formation energies of such defects and is in agreement with a recent report on pressed pellets.