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Halide Perovskites

perovskites general

Halide perovskites were first discovered in 1893, but it took more than 100 years for their real potential to be discovered. Their fantastic optical and electronic properties make them interesting for a wide range of applications, from LEDs and lasers, over solar cells and photodetectors to more exotic applications such as X-ray detectors and thermal sensors. In nanocrystalline form, they show exceptionally bright and efficient photoluminescence with quantum yields up to 100%. The wavelength of the emission can be tuned throughout the entire visible spectrum either by changing the composition or by shrinking the nanocrystals down into the quantum confinement regime. Despite their fascinating potential, there is still very much we don't know yet about halide perovskites. Also, they still exhibit several issues which currently impede a full-scale commercialization. Here, we synthesize different halide perovskite nanocrystals to learn more about their fundamental properties and optimize them for integration into optoelectronic devices.

Background information on halide perovskites, uses in optoelectronics and more can be found in any of our recent reviews:

Metal Halide Perovskite Nanocrystals: Synthesis, Post-Synthesis Modifications, and Their Optical Properties
J. Shamsi, A.S. Urban, M. Imran, L. De Trizio, L. Manna
Chemical Reviews 119, 5, 3296-3348 (2019)

Advances in quantum-confined perovskite nanocrystals for optoelectronics
L. Polavarapu, B. Nickel, J. Feldmann, A. S. Urban
Advanced Energy Materials 7, 1700267 (2017)

Colloidal lead halide perovskite nanocrystals: synthesis, optical properties and applications
H. Huang, L. Polavarapu, J.A. Sichert, A.S. Susha, A.S. Urban, A.L. Rogach
NPG Asia Materials 8 (11), e328-e328

In case you have questions or would like more information on any of our materials,
feel free to contact Carola Lampe or Juri Crimmann.

Two-Dimensional Nanoplatelets

nanoplatelets_tuning

While green and red perovskite emitters are well established in the community, blue emitters still face some challenges due to their lack of stability owing to Cl-anions or poor optical performance showing in low photoluminescence quantum yields. Our group works with a recently developed synthesis to obtain perovskite nanoplatelets with a distinct thickness of 1.2 to 3.6 nm. Here, different emission wavelengths from green to deep blue can be achieved by taking advantage of strong quantum confinement effects. With postsynthetic treatments with ions and organic molecules we can increase both, the stability and efficiency of the nanoplatelets. Additionally, we are constantly improving the synthesis procedure to obtain better nanocrystals and to incorporate them into optoelectronic devices. The synthesis itself is a ligand-assisted reprecipitation process (LARP) and can be conducted at room temperature and ambient conditions.

The use of quantum confined nanoplatelets offers new possibilities for optoelectronic devices. Their narrow and bright emission is favorable for application in light emitting diodes (LEDs). In addition, by combining different nanoplatelet thicknesses it is possible to build perovskite-heterostructures. As recently shown by our group, energy transfer between nanoplatelets is highly efficient and could lead to the realization of energy transfer cascades.

Boosting tunable blue luminescence of halide perovskite nanoplatelets
through post-synthetic surface trap repair
B. J. Bohn, Y. Tong, M. Gramlich, M. L. Lai, M. Döblinger, K. Wang, R. L. Z. Hoye,
P. M. Buschbaum, S. D. Stranks, A. S. Urban, L. Polavarapu, and J. Feldmann
Nano Lett. 2018, 18, 8, 5231-5238

Elucidating the performance limits of perovskite nanocrystal light-emitting diodes
T. Morgenstern, C. Lampe, T. Naujoks, M. Jurow, Y. Liu, A. S. Urban, and W. Brütting
Journal of Luminescence 220, 116939

Nonradiative energy transfer between thickness-controlled halide perovskite nanoplatelets
A. Singldinger, M. Gramlich, C. Gruber, C. Lampe, and A. S. Urban
ACS Energy Letters 5, 1380-1385

Micellar-Encapsulated Nanocrystals

micelle_center

Halide perovskite nanocrystal can be synthesized through many different approaches. Many of which stop crystal growth by adding ligands at a precise temperature and/or specific timing. These ligands coordinate themselves towards the surface atoms and therefor stop the ongoing reaction. Controlling critical parameters like the temperature of solvents is mandatory during these methods. Slight deviations can lead to unusable products.

We developed a facile synthesis technique that yields to polymer-encapsulated perovskite nanocrystals, without the need of ligands limiting the crystal size. First, micelles are formed by adding polymers to the solvent. This happens, when the polymer concentration is higher than the critical micelle concentration. Secondly, precursor salts are added separately. Ions from these salts diffuse into the micelles where the crystal is formed. Crystal size is determined by the size of the polymer micelle. Thirdly, the solution is purified. This is needed because crystals grow not only inside micelles but also in all different sizes and shapes outside of micelles. All steps lead to a chemical equilibrium. That is why parameters like timing or temperature are not so crucial anymore and a high reproducibility can be ensured.

Micelle data


With this technique, we were able to cover the whole visible range with narrow emission spectra. Furthermore, micelle encapsulated nanocrystals are protected from environmentally induced degradation, such that even after 75 days of complete submersion in water photoluminescence can be measured. Additionally, halide ion migration is suppressed and nanocrystal layers of MAPI and MAPBr show Förster resonance energy transfer (FRET). All of these characteristics are very promising for integration into optoelectronic devices.

For more information, you can lookup

Polymer Nanoreactors Shield Perovskite Nanocrystals from Degradation
V. A. Hintermayr, C. Lampe, M. Löw, J. Roemer, W. Vanderlinden, M. Gramlich, A. X. Böhm, C. Sattler,
B. Nickel, T. Lohmueller, and A. S. Urban
Nano Lett., 2019, 19, 4928-4933