Over the last decade, impressive development in lead halide perovskites (LHPs) have made them leading candidate materials for photovoltaics (PVs), X-ray scintillators, and light-emitting diodes (LEDs). The success of LHPs NCs in lighting and display applications is mainly originated from their high photoluminescence quantum yield (PLQY), narrow emission, sizable bandgap, and cost-effective fabrication. Consequently, a comprehensive understanding of the design principles of LHP NCs will fuel further innovations in their optoelectronic applications. This dissertation centers on the synthesis and self-assembly of LHP NCs. At first, we investigate the capability of colloidal synthetic routine to engineer the shape, size, and dimensionality of the resulting LHPs NCs (chapter 2), including 0D nanospheres, 2D nanoplates, and 3D nanocubes. Starting from the LHPs NCs, nanoplates (chapter 3), nanowires (chapter 4), and superstructures (chapter 5) are successfully achieved via various self-assembly strategies. In chapter 3, we present a liquid-air interfaces-assisted self-assembly technique to obtain micro-scale CsPbBr3 nanoplates from as-synthesized nanoscale NCs. The AC-HRTEM offered an atomic-level observation during the structural evolution and revealed an oriented attachment-mediated assembly mechanism. The assembled CsPbBr3 nanoplates exhibited ultrahigh stability under X-ray energy dispersive spectroscopy (EDS) mapping conditions (300-kV electron beam), and the first atomic-resolution EDS elemental mapping data of LHP NCs were acquired. In chapter 4, we demonstrate an efficient green-chemistry approach for the self-assembly of CsPbBr3 NCs into 1D nanowires and nanobelts via the light induction. As an elegant and promising green-chemistry approach, light-induced self-assembly represents a rational method for designing perovskites. In chapter 5, we will explore the self-assembly of CsPbBr3 NCs into superstructures to overcome the ‘green gap’ to achieve a pure green emission with high PLQY for realizing next-generation vivid displays. In summary, we systematically investigated the mechanisms of LHP NC self-assembly, the kinetics of their morphological evolution and phase transitions, and driving forces that govern the self-assembly process. The assembled LHP NCs manifest desirable properties (e.g., superfluorescence, improved photoluminescence lifetime, enhanced stability against moisture, light, electron-beam irradiation, and thermal-degradation) that translate into dramatic improvements in device performance.

Abstract Methylammonium lead (II) iodide has recently attracted considerable interest which may lead to substantial developments of efficient and inexpensive industrial photovoltaics. The application of this material as a light-absorbing layer in solid-state solar cells leads to impressive efficiency of over 22% in laboratory devices. However, for industrial applications, fundamental issues regarding their thermal and moisture stability need to be addressed. MAPbI3 belongs to the perovskite family of materials with the general formula ABX3 ,where is the organic cation (methylammonium) which is reported to be a major source of instability. In this work, a variety of alkyammonium lead (II) iodide materials have been synthesized by changing the organic cation, to study the relationship between the structural and physical properties of these materials. [(A)PbI3] and (A)PbI4 series were studied. Three dimensional (3D) networks (MAPbI3,MAPbBr3), two dimensional (2D) layered systems (BdAPbI4, HdAPbI4, OdAPbI4), and one dimensional (1D) columns (EAPbI3, PAPbI3, EAPb2I6) were found for the materials. [PbI6] octahedral structural units were repeated through the material network depending on the dimensionality and connectivity of the materials. Where a bulkier cation was introduced, the crystallographic unit cell increased in size which resulted in lower symmetry crystals. The connectivity of the unit cells along the material networks was found to be based on corner-sharing and face-sharing. Lower dimensionality resulted in larger bandgaps and lower photoconductivity, and hence a lower light conversion efficiency for the related solar cells. The thermal and moisture stability was greater in the 1D and 2D materials with bulkier organic cations than with methylammonium. In total, an overview is provided of the relationship between the chemical dimensionality and physical properties of the organic-inorganic lead halide materials with focus on the solar cell application. / Svenska sammandrag: Metylammoniumbly(II)jodid har under de senaste åren genererat ett stort intresse som ett möjligt material for utveckling av effektiva och på industriell skala billiga solceller. Detta material har använts som ljusabsorberande skikt i fasta solceller med imponerande omvandlingseffektiviteter på över 22% för solceller i laboratorieskala. För att denna nya typ av solceller ska bli intressanta för produktion på industriell skala, så behöver grundläggande frågeställningar kring materialens stabilitet avseende högre temperaturer och fukt klargöras. MAPbI3 har formellt perovskitstruktur med den allmänna formel ABX3, där A utgörs av den organiska katjonen (metyammoniumjonen) och som kan kopplas till materialets instabilitet. I denna avhandling har olika alkylammoniumbly(II)jodidmaterial syntetiserats där den organiska katjonen modifierats med syftet att studera växelverkan mellan struktur och fysikaliska egenskaper hos de resulterande materialen. Material av olika dimensionalitet erhölls; tredimensionella (3D) nätverk (MAPbI3, MAPbBr3), tvådimensionella (2D) skiktade strukturer (BdAPbI4, HdAPbI4, OdAPbI4), och endimensionella (1D) kedjestrukturer (EAPbI3, PAPbI3, EAPb2I6). Flera nya lågdimensionella material (2D och 1D) tillverkats och karaktäriserats för första gången. Enkristalldiffraktometri har använts för att erhålla materialens atomära struktur. Strukturen hos material tillverkade i större mängder konfirmerades genom jämförelse mellan resultat från pulverdiffraktion och enkristalldiffraktion. Den oktaedriska strukturenheten [PbI6] utgör ett återkommande tema i materialen sammankopplade till olika dimensioner. Då större organiska katjoner används karaktäriseras i regel strukturerna av större enhetsceller och lägre symmetri. De lågdimensionella materialen ger typiskt störe elektroniskt bandgap, lägre fotoinducerad ledningsförmåga och därför sämre omvandlingseffektiviteter då de används i solceller. De lågdimensionella materialen (1D och 2D) som baseras på de större organiska katjonerna uppvisar bättre stabilitet med avseende på högre tempereratur och fukt. De tvådimensionella materialens elektroniska struktur har karaktäriserats med hjälp av röntegenfotoelektronspektroskopi, liksom röntgenabsorptions- och emissionsspektroskopi. Resultat från teoretiska beräkningar stämmer väl överens med de experimentella resultaten, och de visar att materialens valensband huvudsakligen består av bidrag från atomorbitaler hos jod, medan atomorbitaler från bly främst bidrar till edningsbandet. Sammantaget erbjuder avhandlingen en översikt av sambandet mellan kemisk dimensionalitet och fysikaliska egenskaper hos ett antal organiska/oorganiska blyhalogenidmaterial med fokus på tillämpning i solceller. / <p>QC 20170123</p>

In this thesis, we investigate the nature of charge carrier generation, relaxation and recombination in a range of lead halide perovskites. We focus on understanding whether the photophysical behaviour of these perovskite materials is like that of highly-ordered inorganic crystalline semiconductors (exhibiting ballistic charge transport) or disordered molecular semiconductors (exhibiting strong electron-phonon coupling and highly localised excited states) and how we can tune these photophysical properties with inorganic and organic additives. We find that the fundamental photophysical properties of lead halide perovskites, such as charge carrier relaxation and recombination, arise from the lead halide lattice rather than the choice of A-site cation. We show that while the choice of A-site cation does not affect these photophysical properties directly, it can have a significant impact on the structure of the lead halide lattice and therefore affect these photophysical properties indirectly. We demonstrate that lead halide perovskites fabricated from particular inorganic and organic A-site cation combinations exhibit low parasitic trap densities and enhanced carrier interactions. Furthering our understanding of how the photophysical properties of these materials can be controlled through chemical composition is extremely important for the future design of highly efficient solar cells and light emitting diodes.

No description available.

No description available.

No description available.

Lead halide perovskites have shown remarkable device performance in both solar cells and LEDs. Whilst the research efforts so far have been mainly focussed on device optimisation, little is known about the photophysical properties. For example, the nature of the bandgap is still debated and an indirect bandgap due to a Rashba splitting has been suggested. In this thesis, we study the early-time carrier relaxation and its impact on photoluminescence emission. We first study ultrafast carrier thermalization processes using 2D electronic spectroscopy and extract characteristic carrier thermalization times from below 10 fs to 85 fs. We then investigate the early-time photoluminescence emission during carrier cooling. We observe that the luminescence signal shows a rise over 2 picoseconds in CH3NH3PbI3 while carriers cool to the band edge. This shows that luminescence of hot carriers is slower than that of cold carriers, as is found in direct gap semiconductors. We conclude that electrons and holes show strong overlap in momentum space, despite the potential presence of a small band offset arising from a Rashba effect. Recombination and device performance of perovskites are thus better described within a direct bandgap model. We finally study carrier recombination in perovskites and the impact of photon recycling. We show that, for an internal photoluminescence quantum yield of 70%, we measure external yields as low as 15% in planar films, where light out-coupling is inefficient, but observe values as high as 57% in films on textured substrates that enhance out-coupling. We study the photo-excited carrier dynamics and use a rate equation to relate radiative and non-radiative recombination events to measured photoluminescence efficiencies. We conclude that the use of textured active layers has the ability to improve power conversion efficiencies for both LEDs and solar cells.

This dissertation describes our study on different physical properties of passivated and chemically modified hybrid metal halide perovskite materials and development of highly efficient charge transport layers for perovskite solar cells. We first developed an efficient electron transport layer via modification of titanium dioxide nanostructure followed by a unique chemical treatment in order to have clean interface with fast electron injection form the absorber layer in the perovskite solar cells. We then explored monovalent cation doping of lead halide perovskites using sodium, copper and silver with similar ionic radii to lead to enhance structural and optoelectronic properties leading to higher photovoltaic performance of the resulting perovskite solar cells. We also performed thorough experimental characterizations together with modeling to further understand the chemical distribution and local structure of perovskite films upon monovalent cation doping. Then, we demonstrate a novel passivation approach in alloyed perovskite films to inhibit the ion segregation and parasitic non-radiative losses, which are key barriers against the continuous bandgap tunability and potential for high-performance of metal halide perovskites in device applications, by decorating the surfaces and grain boundaries with potassium halides. This leads to luminescence quantum yields approaching unity while maintaining high charge mobilities along with the inhibition of transient photo-induced ion migration processes even in mixed halide perovskites that otherwise show bandgap instabilities. We demonstrate a wide range of bandgaps stabilized against photo-induced ion migration, leading to solar cell power conversion efficiencies of 21.6% for a 1.56 eV absorber and 18.3% for a 1.78 eV absorber ideally suited for tandem solar cells. We then systematically compare the optoelectronic properties and moisture stability of the two developed passivation routes for alloyed perovskites with rubidium and potassium where the latter passivation route showed higher stability and loading capacity leading to achieve substantially higher photoluminescence quantum yield. Finally, we explored the possibility of singlet exciton fission between low bandgap perovskites and tetracene as the triplet sensitizer finding no significant energy transfer between the two. We then used tetracene as an efficient dopant-free hole transport layer providing clean interfaces with perovskite layer leading to high photoluminescence yield (e.g. ~18%). To enhance the poor ohmic contact between tetracene and the metal electrode, we added capping layer of a second hole transport layer which is extrinsically doped leading to 21.5% power conversion efficiency for the subsequent solar cells and stabilised power output over 550 hours continuous illumination.

Advanced nanomaterials show a significant improvement in certain physical or functional properties compared to conventional materials. Such advanced materials are, for example, lead halide perovskites (LHP). It is a group of hybrid organic-inorganic materials with a direct bandgap exhibiting unique optical properties. The high quantum efficiency of photoluminescence makes nanocrystals or thin films of LHP suitable candidates for the production of light-emitting diodes, solar cells and LCD displays. Their inexpensive and simple fabrication together with their unique optical properties makes LHP one of the most developed materials of the last decade. This diploma thesis aims to study the optical properties of CsPbBr3 perovskite nanocrystals using high lateral resolution methods. CsPbBr3 perovskite nanocrystals show intense anti-Stokes photoluminescence. These nanocrystals can emit light with a lower wavelength than that of the light that causes their photoluminescence. The nanocrystals are prepared in two different ways: by evaporation or by crystallization of the precursor in a solution of dimethylformamide. The morphology, photoluminescence properties and chemical composition of individual nanocrystals are studied. Each nanocrystal is studied individually and its size, shape, photoluminescence properties and chemical compounds are determined, which leads to a deeper understanding of the anti-Stokes photoluminescence of perovskite nanocrystals.

No description available.