Defects and Halide Perovskites—Tautology, Oxymoron, or ... How Do They Get Along?

A range of optical and electrical experimental measurements experiments show low (≤~10¹⁶ cm^-3) to ultra-low (≤~10¹¹ cm^-3) densities of optoelectronically active defects in materials that can be prepared near room temperature, RT, under atmospheric pressure from organic solvents. In this respect even calling the behaviour of Lead Halide Perovskites, HaPs, that resembles “RT annealing”, unusual is an understatement. We have recently discussed the two leading rationalizations for these low defect densities, i.e., Self-Healing, SH, and Defect Tolerance, DT. Using simple <i>logic arguments,</i> we show that this must include an active reaction to damage, such as SH, which we earlier invoked to explain the behavior of CuInSe₂ and CIGS,¹ rather than be only tolerance to damage, DT.² For 3D HaPs we and others presented actual experimental evidence for SH, a process that, in inanimate matter, is explained by equilibrium thermodynamics.³ We will also discuss results on <i>single crystals</i> of 2D and 2D-3D HaPs, and compare these to those obtained on 3D single crystals, in terms of their recovery from photodamage.⁴ We argue that the main cause for such healing and the resulting low defect densities is the small free energy of reaction that just stabilizes the materials against decomposition.⁵ To follow these arguments and models it is important to distinguish between static defects, of the kind we are familiar with in materials science, incl. semiconductors, and dynamic ones with life times comparable to or less than the electron-lattice interaction times.<br/>* Work done with, among others, D R Ceratti, S Elboher-Aharon, G Hodes, L Kronik, S Kumar, I Lubomirsky, Y Rakita.<br/>¹Guillemoles et al.,<i> Adv. Mater</i>. (1999); ²Cahen, Hodes, Kronik, ACS En. Lett. (2021); ³Ceratti et al., <i>Mater. Hor</i>. (2021); <i>Adv. Mater</i>. (2018); ⁴Elboher Aharon et al., TBP; ⁵ Rakita et al., <i>Mater. Hor</i>. (2019); Kumar, Hodes, Cahen, <i>MRS Bull</i> (2020);