A long-known “secret”: how to dope “undopable” Semiconductors, such as Halide Perovskites
David Cahen a, Antoine Kahn b
a Weizmann Institute of Science, Herzl St. 234, Rehovot 7610001, Israel
b Department of Electrical and Computer Engineering, Princeton University, Princeton, NJ 08544, United States
Proceedings of Device Physics Characterization and Interpretation in Perovskite and Organic Materials (DEPERO)
València, Spain, 2023 October 3rd - 5th
Organizers: Sandheep Ravishankar, Juan Bisquert and Evelyne Knapp
Invited Speaker, David Cahen, presentation 006
Publication date: 14th September 2023

Metal halide perovskite (HaP) materials, which are the heart of devices, such as solar cells and LEDs, challenge our understanding of semiconductors. We show that for HaPs control of the doping type and density, and properties derived from these, is to a first approximation, via their surfaces. While not unique to HaPs (the effect applies to ALL semiconductors with LOW electronically active bulk and surface defect densities), it is amplified for HaPs, because of their intrinsically low bulk and surface defect densities. Most polycrystalline (<< 1 um grain diameter) thin HaP films have carrier densities below the VOLUME, i.e., BULK densities that will result if even less than 0.1% of the surface sites function as electrically active defects. From single crystal data, we know that other bulk (electrically active) defect densities are orders of magnitude lower.

Because HaP interfaces result from contacts to HaP surfaces, the direct implication of this phenomenon is that interface defects will control HaP-based solar cells and LEDs, which involve multi-layered polycrystalline thin-film structures with two interfaces with the HaP layer.

While surface passivation effects on bulk electrical properties are relevant to all semiconductors and have been a crucial step in enabling the use of each of those in today’s electronic devices, they take on greater importance for HaPs, because achieving bulk doping at densities, relevant for electronics, has turned out to be difficult (to say the least), certainly by established doping methods. We show that only by realizing the role of surface/interface defects, will it become possible to control bulk HaP electronic properties.

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