Two-Dimensional Colloidal Quantum Wells for Future Photonic Sources

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.