
Open access
Author
Date
2020Type
- Doctoral Thesis
ETH Bibliography
yes
Altmetrics
Abstract
Modern light-emitting technologies require earth abundant materials, which exhibit
high photoluminescence quantum yield ( PL) and excellent color purity. Moreover, the
intrinsic ability to control the emission dipole orientation is highly desired, in order
to enhance the light outcoupling e ciency. At the same time, the low complexity of
processing is essential for ensuring the cost e ciency. Recently, colloidal lead halide
perovskite nanocrystals (NCs), composed of APbX3 (A = CH3NH3, CH(NH2)2, Cs
and X = Cl, Br, I), emerged as highly attractive alternative to the state-of-the-art
light-emitting materials. Although they meet most of the desired characteristics, their
accessibility still remains challenging in most of the cases. A complete understanding of
the mechanisms governing their synthesis processes and fundamental physical properties
is necessary, so that the potential of these outstanding class of materials can be fully
utilized on a commercial scale. The rst part of this thesis, focuses on the development
of synthetic protocols used to obtain quantum-con ned CH3NH3PbBr3 NCs. Speci c
reaction conditions allow to tune the size of the NCs and therefore their band gap
energies. Detailed optical and crystallographic analyses, revealed a two-dimensional
(2D) geometry of NCs with the reduced dimensionality. More speci cally, atomically- at
colloidal quantum wells (CQWs) are formed and their precise thickness is characterized
by a quantized nature, since it is determined by the stacking number of perovskite unit
cells n. Consequently CQWs with tunable thicknesses from n = 7 10 to n = 1 were
synthesized. With this approach, the emission wavelength tunability over the green-toblue
spectral range was achieved. Moreover, it was shown that the reduced thickness of
the CQW, results in an increase of exciton binding energy, which boosts the radiative
recombination. As a result, e cient electroluminescent (EL) devices were fabricated,
utilizing perovskite CQWs as emitters. Their performance could be further improved by
incorporating crystalline CQWs into a matrix of organic host molecules, owing to Förster
Resonance Energy Transfer (FRET) phenomenon. Subsequently, the same series of layercontrolled
CH3NH3PbBr3 CQWs was investigated towards their luminescent properties
in thin lms. Thorough morphological and crystallographic characterization was used to
probe these CQW solids. Upon spin coating perovskite-containing colloidal dispersions onto solid state substrate, formation of highly-oriented self-assembled superlattices was
observed. The resulting PL in these lamellar solids surprisingly exceeded the values
achieved for the respective colloidal suspensions. Together with an inverse correlation
between photoluminescence lifetime ( PL) and PL, these observations were clearly
distinct from typical quantum dot (QD) solid systems. Based on multiscale theoretical
analysis, it was found out that the collective motion of surface organic cations is more
restricted to orient along [100] direction, thereby inducing a more direct band gap,
which facilitates radiative recombination. Furthermore, ultrapure green emission was
demonstrated by downconverting a commercial blue GaN LED at room temperature,
which allowed to exceed luminous e cacy exhibited by green-emitting InGaN LEDs.
Following these ndings, the next chapter encompasses the investigation of transition
dipole moment orientation in 2D perovskite CQWs. By varying the length of the capping
ligand during CQW synthesis, formation of multiple quantum well (MQW) systems
with tunable quantum barrier (QB) thickness was achieved. As evidenced by k-space
spectroscopic analysis, TDM was found out to align predominantly parallel to the surface
plane, independent on the QB thickness. It directly implies, that even when separated
by as little as 6.5 Å, the adjacent QW layers remain decoupled and as a consequence
no interlayer exciton is formed. The observed localization of Wannier-Mott-like excitons
is due to the strong ionic dielectric response that screens the interlayer electrostatic
interactions. A signi cant PL decrease and an emergence of low-energy emission
in MQWs at low temperature, suggest the occurrence of charge-transfer mechanism
as phonon modes become frozen. The preferential orientation of TDM is retained
in the mixed-halide superlattices, covering the entire blue-to-orange visible spectrum.
Last part of the thesis demonstrates a practical application of hybrid perovskite
colloidal nanocrystals. Because the human eye is more sensitive to the green spectral
region, pure green LEDs are essential for realizing an ultrawide color gamut for nextgeneration
displays. Here, e cient ultrapure green EL based on colloidal nanocrystals of
formamidinium lead bromide (CH(NH2)2PbBr3) is demonstrated. Through dielectric
quantum well (DQW) engineering and LED device optimization, maximum current
e ciency of 13 cd/A was reached. Nevertheless, most importantly, this was achieved
with CIE color coordinates of (0.168, 0.773), which would allow to cover 97% of
recommendation 2020 standard. Show more
Permanent link
https://doi.org/10.3929/ethz-b-000406005Publication status
publishedExternal links
Search print copy at ETH Library
Publisher
ETH ZurichSubject
Photoluminescence; Light-Emitting Diodes (LEDs); Colloidal quantum dots; Lead halide perovskite; Anisotropic emitters; colloidal quantum wellsOrganisational unit
04323 - HCI Chemieneubau09502 - Shih, Chih-Jen / Shih, Chih-Jen
More
Show all metadata
ETH Bibliography
yes
Altmetrics