Semiconductor nanocrystal-based photovoltaics are an interesting new technology with the potential to achieve high efficiencies at low cost. CuInSe2 nanocrystals have been synthesized in solution using arrested precipitation and dispersed in solvent to form a “solar ink”. The inks have been deposited under ambient conditions to fabricate photovoltaic devices with efficiency up to 3%. Despite the low cost spray coating deposition technique, device efficiencies remain too low for commercialization. Higher efficiencies up to 7% have been achieved using a high temperature selenization process, but this process is too expensive. New nanocrystal film treatment processes are necessary which can improve the device efficiency at low cost. To this end, CuInSe2 nanocrystals were synthesized using a diphenyl phosphine:Se precursor which allows for precise control over the nanocrystal size. The size is controlled by changing the temperature of the reaction. The smallest size nanocrystals demonstrated extremely high device open circuit voltage. Ligand exchange procedures were used to replace the insulating oleylamine capping ligand used during synthesis with more conductive halide ions or inorganic chalcogenidometallate cluster (ChaM) ligands. These ligands led to improved charge transport in the nanocrystal films. A high-intensity pulsed light processing technique known as photonic curing was used which allows for high temperature sintering of nanocrystal films on temperature-sensitive substrates. High energy pulses cause the nanocrystals to sinter into large grains, primarily through melting and resolidification. The choice of metal back contact has a dramatic effect on the final film morphology, with Au and MoSe2 back contacts providing much better adhesion with the CuInSe2 than Mo back contacts. Nanocrystal sintering without melting can be achieved by replacing the oleylamine ligands with ChaM ligands prior to photonic curing. Low energy photonic curing pulses vaporize the oleylamine ligands without inducing sintering or grain growth. This greatly improved nanocrystal coupling and interparticle charge transport. Multiexcitons were successfully extracted from these nanocrystal films and external quantum efficiencies over 100% were observed. Transient absorption spectroscopy was used to study the multiexciton generation process in CuInSe2 nanocrystal films and colloidal suspensions. The multiexciton generation efficiency, threshold, and Auger lifetimes for CuInSe2 compare well with other nanocrystal materials. / text

We report on an efficient assembly approach to a variety of electrostatically stabilized all-inorganic semiconductor nanocrystals (NCs) via their linking with appropriate ions into multibranched gel networks. These all-inorganic non-ordered 3D assemblies can combine strong interparticle coupling which facilitates charge transport between the NCs with their diverse morphology, composition, size, and functional capping ligands. Moreover, the resulting dry gels (aerogels) are highly porous monolithic structures, which preserve the quantum confinement of their building blocks. The inorganic semiconductor aerogel made of 4.5 nm CdSe colloidal NCs, capped with iodide ions and bridged with Cd2+ ions, exhibited a surface area as high as 146 m2/g.

We report on an efficient assembly approach to a variety of electrostatically stabilized all-inorganic semiconductor nanocrystals (NCs) via their linking with appropriate ions into multibranched gel networks. These all-inorganic non-ordered 3D assemblies can combine strong interparticle coupling which facilitates charge transport between the NCs with their diverse morphology, composition, size, and functional capping ligands. Moreover, the resulting dry gels (aerogels) are highly porous monolithic structures, which preserve the quantum confinement of their building blocks. The inorganic semiconductor aerogel made of 4.5 nm CdSe colloidal NCs, capped with iodide ions and bridged with Cd2+ ions, exhibited a surface area as high as 146 m2/g.

Featuring size-tunable electrical and optical properties, semiconductor nanocrystals (NCs) attract intensive interest in developing promising functional materials for optoelectronic appli-cations. The surface ligands not only play an important role in the synthesis and colloidal sta-bility of NCs, but also significantly affect their photophysical and electrochemical properties. In this dissertation, I am dealing with the surface ligand engineering of NCs (including both perovskite and metal chalcogenide families) for optical and photocatalytic applications. Polymer ligands are regarded to enable better colloidal stability, durability and processability of fluorescent NCs, which is especially important for perovskite NCs. However, the current wide-used polymer ligands fail to provide sufficient surface passivation for the NCs, which is unfavorable for their luminescence. To address this issue, a dual-ligand system based on par-tially hydrolyzed poly(methyl methacrylate) (h-PMMA) and highly branched polyethyl-enimine (PEI) was designed to stabilize perovskite NCs. The hydrophobic polymer of h-PMMA imparts excellent film-forming properties and durability to the resulting NC-polymer composite. The PEI forms an amino-rich, strongly binding ligand layer on the surface of the NCs being responsible for the significant improvement of the photoluminescence quantum yield and the stability of the resulting material. These superior properties allowed us to fabri-cate a proof-of-concept thin film organic light-emitting diode (OLED) with h-PMMA/PEI-stabilized perovskite NCs. A further insight into the roles of double polymer ligands (h-PMMA and PEI) during the mechanosynthesis of perovskites nanoparticles (NPs) was pro-vided. The h-PMMA can form micelles in the grinding solvent of dichloromethane to act as size-regulating templates for the growth of NPs. The PEI with large amounts of amino groups induced enrichment of PbBr2 in the reaction mixture, which in turn caused the formation of heterostructured CsPbBr3-CsPb2Br5-mPbBr2 and CsPbBr3-Cs4PbBr6-nCsBr NPs. Not only polymer, but also inorganic ligands can be extremely attractive for capping of NCs. In the frame of this thesis, a two-step surface modification strategy was developed to control-lably destabilize the colloidal NCs, which in turn facilitated their 3D assembly into aerogels. Specifically, the long-chain oleic acid ligands were exchanged to the ultra-short-chain inorganic (NH4)2S ligands. These new ligands were further protonated by changing the dispersing solvent, which caused desired colloidal destabilization. The as-prepared CdSe NC aerogels with highly porous and self-supporting structure were found to be attractive for solid-state photocatalysis in a gas phase. Indeed, the (NH4)2S ligand is favourable for the adsorption and activation of substrate molecules (i.e., H2O and CO2) on the large open surface of NC gel, thereby promoting the progress of CO2 photoreduction. As a result, the photocatalytic activity for CO2 reduction of CdSe NC aerogels created in this work is 12-fold higher than that of the pristine non-assembled NC-precipitates.:Abstract 1 Contents 3 Abbreviations 6 List of Figures and Tables 8 1. Colloidal Semiconductor Nanocrystals and their Ligand Shell 13 1.1. Colloidal Semiconductor Nanocrystals 14 1.1.1. Inorganic Core of NCs 15 1.1.1.1. Metal Chalcogenide NCs 16 1.1.1.2. Metal Pnictide NCs 16 1.1.1.3. Halide Perovskites NCs 17 1.1.2. The Surface Ligands for NCs 18 1.1.2.1. The Classification of Surface Ligands based on Head-Groups 18 1.1.2.2. The Classification of Surface Ligands based on Tail-Groups 19 1.2. The Role of Ligands 20 1.2.1. The Role of Ligands in the Synthesis of NCs 20 1.2.2. The Role of Ligand in Colloidal NCs Dispersion and Stability 22 1.2.3. The Role of Ligand in the Light-Matter Interactions as Applied to NCs 24 1.3. The Surface Ligand Engineering of NCs 26 1.3.1. Introducing Ligands during the Synthesis 26 1.3.2. Introducing the Ligands during Post-Synthesis Process 27 1.4. Challenges to be Addressed in this Dissertation 29 2. Polymer Ligands Enhance the Stability and Fluorescence of Perovskite for Optical Application 31 2.1. Background and Motivation 32 2.2. Results and Discussion 34 2.2.1. Spectral Characterization 34 2.2.2. Morphological Characterization 40 2.2.3. Surface Composition 41 2.2.4. Processability, Stability and Durability 43 2.2.5. Green-LED 46 2.3. Conclusions 48 3. Polymer Ligands Assist Mechanosynthesis of Perovskite Nanoparticles 49 3.1. Background and Motivation 50 3.2. Results and Discussion 50 3.2.1 Morphology and Composition 51 3.2.2 Formation and Phase Conversion of the Nanoparticles 53 3.2.3. Spectral Characterization 58 3.3. Conclusions 60 4. Ligand Protonation Promote 3D Assembly of CdSe Nanocrystals for CO2 Photoreduction 62 4.1. Background and Motivation 63 4.2. Results and Discussion 64 4.2.1. The Gelation Method 64 4.2.2. Surface Composition of the NC Aerogels 67 4.2.3. Performance of CdSe-S Aerogels in Photoreduction of CO2 68 4.2.4. Photocatalytic Mechanism of the CdSe-S/Ni Aerogel 70 4.3. Conclusion 73 5. Conclusions and Perspectives 75 Appendix. Experimental Section 78 A.1. Reagents 78 A.2. NCs synthesis 78 A.2.1 Mechanosynthesis of Polymer-Coordinated Perovskite NCs 78 A.2.2 Oil Phase Synthesis of Colloidal CdSe NCs 79 A.2.3 Ligand Protonation-Promoted Assembly of CdSe-S NCs into Gel 79 A.3. Optical and Photocatalytic Applications of NCs 80 A.3.1 Optical Applications of Polymer-Stabilized Perovskite NCs 80 A.3.2 Photocatalytic Applications of CdSe-S Aerogels 80 A.4. Characterization Methods 81 A.4.1 Morphology Characterization 81 A.4.2.Element Characterization 81 A.4.3 Diffraction Characterization 82 A.4.4 Spectroscopy Characterization 82 A.4.5 Gas adsorption Measurement 82 A.4.6 Electrochemical Measurements 83 A.4.7 Other Characterizations 83 A.5. Additional Data 84 Bibliography 87 List of Publications 96 Acknowledgements 98 Erklärung 100