Article: Redox-active ions unlock substitutional doping in halide perovskites

Our Image Science & Computer Vision team has contributed to a paper reporting the first ever effective doping of halide perovskites. Published in the journal Materials Horizons from the Royal Society of Chemistry, the experiment results are paving the way for low-cost, highly-efficient solar cells and LEDs.

Semiconductors, crucial to electronic components, derive their performance from a process called doping, wherein specific atoms in their crystalline structure are replaced with elements that either release (N) or capture (P) electrons. While silicon, the most widely used semiconductor, has well-understood doping mechanisms, the process remains more intricate for other materials. This is the case for Halide perovskites for instance, which high energy conversion efficiency make them a predilection material for the development of solar cells. In perovskites, the crystalline structure is held together by ionic bonds, unlike silicon's covalent bonds. This poses challenges for traditional doping methods since replacing one ion with another capable of releasing (N) or capturing (P) an electron could disrupt the delicate balance with its opposite ion (either anion or cation). To overcome this issue, the authors introduced a solution involving metastable ions. Specifically, samarium (Sm) was injected into the crystalline structure as Sm²⁺, replacing lead ions (Pb²⁺). Upon insertion, Sm²⁺ oxidizes to Sm³⁺, serving as an N-type dopant without compromising the crystalline structure.
In this collaborative effort with the Integration from Material to System (IMS) Laboratory and the Institut des Sciences Moléculaires (ISM), our Image Science & Computer team has successfully achieved the first effective N doping of halide perovskites. This breakthrough resulted in an increase in the material's conductivity by more than three orders of magnitude. 

Figure below:
Measurement of perovskite conductivity as a function of SmI₂ dopant concentration in solution, demonstrating a three-order-of-magnitude increase at 10 mg/ml.

Abstract

Electrical doping of metal halide perovskites (MPHs) is a key step towards the use of this efficient and cost-effective semiconductor class in modern electronics. In this work, we demonstrate n-type doping of methylammonium lead iodide (CH₃NH₃PbI₃) by the post-fabrication introduction of Sm²⁺. The ionic radius of the latter is similar to that of Pb²⁺ and can replace it without altering the perovskite crystal lattice. It is demonstrated that once incorporated, Sm²⁺ can act as a dopant by undergoing oxidation to Sm³⁺. This results in the release of a negative charge that n-dopes the material, resulting in an increase of conductivity of almost 3 orders of magnitude. Unlike substitution doping with heterovalent ions, furtive dopants do not require counterions to maintain charge neutrality with respect to the ions they replace and are thus more likely to be incorporated into the crystalline structure. The incorporation of the dopant throughout the material is evidenced by XPS and ToF-SIMS, while the XRD pattern shows no phase separation at low and medium doping concentrations. A shift of the Fermi level towards a conduction energy of 0.52 eV confirms the doping to be n-type with a charge carrier density, calculated using the Mott–Schottky method, estimated to be nearly 10¹⁷ cm^-3 for the most conductive samples. Variable-temperature conductivity experiments show that the dopant is only partially ionized at room temperature due to dopant freeze-out.

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Further information on this CNRS article