Abstract:
Lead halide perovskite nanocrystals (NCs) are important for future optoelectronic devices since they have a high photoluminescence quantum yield, show defect tolerance, and color tunability by their composition. The optical characteristics can be used in light emitting diodes (LEDs). However, the NCs need to fit simultaneously electronic requirements for their application in LEDs: long term stability under electrical bias, charge carrier balance, and facilitated charge injection are a prerequisite for their implementation. All three factors can be influenced by the organic ligand shell surrounding the NCs. By e.g. tight binding of ligands, the stability of the NCs can be improved. Hole and electron transfer rates can be tuned by ligand exchange and insulating surface ligands can be removed from the surface for easy charge transfer. Post-synthetic NC modification can lead to enhanced LED performance with reduced onset potential of luminance and high external quantum e!ciencies for extended time periods. Still, for building e!cient devices the underlying processes need to be understood and the energy barrier between the device layers determined. Therefore, charge injection of only one carrier needs to be studied depending on the surface chemistry of the NCs. Spectroelectrochemistry (SEC) is a method that helps understanding the role of surface ligands on charge injection into NCs. The combination of electrochemistry with photoluminescence (PL) spectroscopy makes the determination of the absolute valence (VB) and conduction band (CB) position possible. In addition, the effect of charging on the optoelectronic properties can be followed up.
In this cumulative dissertation, hole injection into CsPbBr3 NCs is studied
by PL SEC. The influences of the chemical environment on hole injection are
investigated. Thereby, the focus of this work lies on the modification of the ligand
shell and the self-assembly into supercrystals. The fundamentals of the examined
NCs and the method used for studying charge injection are introduced in chapter 2.
Chapter 3 uses PL SEC for investigating improved LED performance of didodecyldimethylammonium bromide (DDABr) covered NCs compared to as-synthesized NCs covered with oleic acid (OA)/oleylamine (OAm). Single carrier devices show an increase in hole current for DDABr NCs. In the PL SEC study, the VB of both ligands with different ligand densities is determined. Beforehand, ligand stripping is performed to create free surface sites for facilitated charge injection. Here, PL SEC shows that the VB position shifts to a lower potential for DDABr than OA/OAm NCs, while ligand stripping leads to an increased onset potential of VB doping for both ligand systems. The chapter highlights the improved charge carrier balance, resulting from promoted hole injection in LEDs for DDABr covered NCs caused by a VB shift of approx. 0.3 eV compared to OA/OAm covered NCs.
In chapter 4, inductive effects mediated by cinnamic acid (CA) derivatives on the electrochemical p-doping of CsPbBr3 NCs are studied. Protocols for the ligand exchange in solution are established in order to determine the hole injection potential for each system by PL SEC. The PL SEC study is supported by the performance of the CA-derivative covered NCs in LEDs and hole-only devices, which are in accordance with the reduced electrochemical hole injection potential for electron withdrawing compared to electron donating ligands determined by PL SEC. As this observation is in contrast to literature on PbS nanoparticles, the result is attributed to a kinetic effect. The comparison of the inductive effects introduced by the ligands highlight that the charge injection potential in LEDs is not only determined by the electronic structure, but can also be modified by a kinetic barrier introduced by the surface ligands.
In the final chapter of this thesis, electrochemical hole injection into supercrystals (SCs) is investigated. Here, the NCs are pretreated by ligand stripping and self-assembled into SCs. The spectroelectrochemical cell is placed on top of a confocal high-resolution microscope, which allows for spatial mapping of the SCs. The SCs show a high stability against the electrolyte solution and hole injection. The performance under electrical bias emphasizes the future application of SCs in LEDs.
In summary, the thesis shows that the hole injection potential can be tuned by 0.3 eV depending on the chemical environment of the CsPbBr3 NCs. Either by introducing DDABr as surface ligand or with electron withdrawing ligands, the hole injection potential can be tuned for a better alignment of the VB of the NCs with the hole transport layer in the LED. In addition, the assembly of NCs shows an increased stability after charge injection. The insights gained in this thesis demonstrate how charge carrier balance, and the long term stability under electrical bias as the electronic requirements of CsPbBr3 NCs can be tackled in future LED technology.
