04 Juin – Thesis defense - Simon Sandrez
09 h30 Amphi 1 - Bordeaux INP
Quasi-2D hybrid perovskite light-emitting diodes.
This thesis focuses on the study of perovskite-based light-emitting diodes (PeLEDs). This recent technology (first room temperature results in 2014) seems promising with conversion efficiencies already approaching those of more mature technologies (especially OLEDs). Hence, this project aims at identifying the main barriers for further development of efficient PeLEDs. Research rapidly headed towards a structure with reduced dimension, the quasi-2D PEA2(FAPbBr3)n−1PbBr4 perovskite, enabling charge carrier confinement and radiative recombination probabilities improvement compared to 3D structures. The first part was dedicated to perovskite active layer optimization, adjusting both composition and deposition conditions, and a maximum current efficiency of 42.0 cd/A (external quantum efficiency of 9.6 %) was obtained. However reproducibility issues appeared on these devices. Then, several parameters affecting the perovskite layer crystallization were identified. The underlying layer for perovskite deposition has a strong impact on its crystallinity, implying distinct behaviors in appearance and evolution of quasi-2D phases with various dimension. It is also shown that anti-solvent process, consisting in perovskite crystallization acceleration by injecting a solvent during the layer spreading, is crucial in this quasi-2D system. An automated injection equipment was developed in order to help controlling this process and to study finely the anti-solvent addition time effect on devices morphology and performances. It revealed a narrow optimal process window for efficient PeLEDs of around 0.5 s, highlighting the necessity to automate this process. In addition, the observation of a singular emission pattern on PEDOT:PSS/perovskite bilayer-based PeLEDs revealed a polymer electrical conductivity doping by the active layer, its conductivity substantially rising from 0.2 to 20 S/cm following the diffusion of some of the perovskite precursors into the PEDOT:PSS. This diffusion was confirmed by in-depth elemental analysis (ToF-SIMS), with an estimation of precursors diffusion depth in the PEDOT:PSS in the order of 150 nm. This doping phenomenon was extended to several perovskites, various PEDOT:PSS grades and other hole transport layers (HTLs), PTAA and poly-TPD. The implications on optoelectronic devices (PeLEDs and perovskite solar cells) were assessed, unveiling a risk of overestimation of their performances. Lastly, the impact of HTL on PeLED devices was discussed. Depending on the HTL, reproducibility issues during perovskite layer deposition, due to variable wettability, as well as fluorescence dynamics fluctuation were observed. The replacement of PEDOT:PSS by a poly-TPD layer finally enabled a substantial enhancement of PeLEDs performances with a maximum current efficiency of 43.6 cd/A.