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Solute/Solvent Interactions And Excited State Photophysics Of 1,4-Diphenyl-1,3-Butadiene And 1,4-Diphenyl-1,3-CyclopentadieneDickson, Nicole M. 15 April 2008 (has links)
No description available.
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Ultrafast Protein Conformation DynamicsLink, Justin J. January 2008 (has links)
No description available.
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EXCITED STATE DYNAMICS AND CHARGE REDISTRIBUTION OF EXTREMOPHILE DNA PHOTOLYASE AND FLAVIN COFACTORSBarnard, David Thomas January 2018 (has links)
Repair mechanisms for damaged DNA are essential for the proliferation of nearly all forms of life. Although DNA is quite robust, the vital information-storing molecule can often be damaged from environmental exposures such as ultra-violet (UV) light. Exposure to UV light can result in various types of mutagens creating structural damages. One specific type of UV-induced damage is the creation of a cyclobutylpyrimidine dimer (CPD). This specific type of lesion can be efficiently repaired by the flavoenzyme DNA photolyase (PL). DNA photolyase is an ancient protein found across kingdoms and plays a crucial role in preventing mutagenesis and cell death. DNA photolyase is a monomeric flavoprotein that utilizes blue light to repair UV-induced CPD lesions in DNA via an electron transfer mechanism. All photolyases contain at least one flavin adenine dinucleotide (FAD) molecule as the catalytic cofactor responsible for initiating the electron transfer induced repair process. Flavin cofactors are intriguing because of their unique ability to donate one or two electrons. The conservation of FAD and the unique U-shaped configuration of FAD in PL led researchers to question if the adenine moiety of the FAD molecule was essential in the DNA repair mechanism and generated a spectral signature indicative of a radical adenine species. The importance of the adenine moiety could be linked to structural changes associated with environmental temperature. The rate of electron transfer is exponentially dependent on temperature and DNA photolyase is found in organisms which thrive in harsh environments that vary in temperature, pH, ionic strength etc. Photolyase presents a unique opportunity to study the adaptations that are required for proteins to function in extreme environments where temperature dependent processes should show dramatic differences. We have used ultrafast transient absorption spectroscopy to compare the similarities and differences in excited state dynamics of the FAD cofactor. Photolyase isolated from the hyperthermophilic archaea Sulfolobus solfataricus (SsPL) is compared to PL isolated from the mesophilic E. coli (EcPL). These results indicate differences in the dynamics of fully reduced flavin between enzymes as a function of temperature. We present evidence for charge separation in the FAD cofactor in the thermophilic enzyme previously seen in computation studies of photolyase. To investigate the excited state charge redistribution of flavin which is critical to its role in nature, the charge redistribution of the precursors to flavin biosynthesis were examined. Lumazine is a precursor in the biosynthetic pathway of flavins. As such, lumazine could have served as an enzymatic cofactor prior to flavins. Lumazine has been identified in biological processes, however it is not as prevalent as flavins. We utilize Stark spectroscopy to examine the charge redistribution in excited state lumazine to understand / Chemistry
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Estudo das características semicondutoras de filmes de óxido de zinco modificados com pontos quânticos de telureto de cádmio / Study of semiconductor features of zinc oxide films modified with cadmium telluride quantum dotsSantos, Vanessa Nascimento dos 25 February 2016 (has links)
Inserido no contexto de fontes de energia renováveis, este trabalho consiste na síntese e caracterização de filmes de bastões de ZnO modificados com quantum dots de CdTe a fim de serem aplicados em células fotoeletroquímicas. Bastões de ZnO são materiais interessantes, porque este tipo de estrutura facilita o transporte de portadores de carga, minimizando a perda destes nos contornos de grão, sua recombinação e aniquilação. A modificação do filme de ZnO com nanocristais de CdTe deve aumentar a eficiência da fotoconversão, facilitando a separação de carga e transferência de elétrons, e isso aumenta a estabilidade dos nanocristais, impedindo a corrosão anódica e a decomposição destes. O filme de ZnO foi eletrodepositado potenciostaticamente sobre a superfície de ITO. As análises de MEV e EDX indicaram que filme de ZnO obtido é homogêneo e consiste de bastões com razão atômica de Zn e O de acordo com a estequiometria 1:1. O resultado de DRX apresentou três planos característicos do ZnO na forma cristalina wurtzita. O plano (002) foi o predominante, indicando a orientação dos bastões no eixo c vertical ao substrato. O filme de ZnO tem espessura de 550 nm, bandgap 3,27 eV, potencial de banda plana de 0,4 V e densidade de portadores de carga de 8,9 x 1019 cm-3. O procedimento sintético dos pontos quânticos de CdTe ocorreu a partir da dissolução de óxido de cádmio em ácido tetradecilfosfônico e octadeceno (ODE) a 300 °C. Subsequentemente, a solução precursora de cádmio foi resfriada a 260 °C e então a solução precursora de telúrio, preparada pela dissolução de telúrio e tributilfosfina em ODE, foi injetada. Os nanocristais obtidos foram dispersos em hexano, precitados com etanol e finalmente os quantum dots foram armazenados em tolueno. A partir das análises de UV-Vis e TEM foi possível estimar o tamanho dos pontos quânticos de CdTe com aproximadamente 4 nm. O DRX dos nanocristais de CdTe apresentou os planos característicos principais da estrutura da blenda de zinco. O eletrodo de ZnO modificado com os quantum dots de CdTe (ZnO/CdTe) foi obtido após 24 h de imersão em uma solução de acetonitrila contendo ácido mercaptopropiônico e ácido propiônico. Subsequentemente, o filme de ZnO modificado com o ligante foi imerso por 48 h na dispersão de pontos quânticos de CdTe. O espectro de FTIR revelou a ausência do estiramento simétrico de C=O em 1700 cm-1. Por outro lado o espectro revelou a presença dos modos assimétrico e simétrico vas(CO2-) e vs(CO2-) que foram observados em 1631 e 1417 cm-1, respectivamente. A transformação de Kulbeka-Munk do espectro de reflectância do eletrodo ZnO/CdTe apresentou a banda relativa ao CdTe no mesmo comprimento de onda observado quando este encontrava-se na dispersão. O eletrodo ZnO/CdTe mostrou um valor de fotocorrente de 138 µA, um valor 10 vezes maior que o obtido para o ZnO. Nos experimentos de IPCE (eficiência de conversão do fóton incidente à corrente) um aumento de aproximadamente cinco vezes também foi observado para o eletrodo de ZnO/CdTe. A dinâmica dos portadores de carga foi investigada por TAS (Espectroscopia de Absorção Transiente) nas escalas de tempo fs e µs para os eletrodos de ZnO e de ZnO/CdTe. A análise TAS indicou um tempo de vida menor para o filme ZnO/CdTe em comparação com filme ZnO. A medidas com o eletrodo de Clark demonstraram uma produção de oxigênio pelo eletrodo de ZnO/CdTe. Assim, o filme de ZnO/CdTe proposto apresenta-se como um material promissor para aplicações fotoeletroquímicas. / Placed in the context of renewable energy sources, this work consists of the synthesis and characterization of ZnO films modified CdTe quantum dots to be applied in photoelectrochemical cells. ZnO rods are interesting materials because this kind of structure facilitates the charge carriers transport, minimizing the loss of these at grain boundaries and their recombination and annihilation. The ZnO film modification with CdTe nanocrystals should increase the photoconversion efficiency by facilitating charge separation and electron transfer, and it increases the nanocrystals stability, preventing it from anodic corrosion and decomposition. The ZnO film was electrodeposited potenciostatically on ITO surface. SEM and EDX analysis indicated that the ZnO film obtained is homogeneous and it consists of rods with atomic ratio of Zn and O according to 1:1 stoichiometry. XRD result showed three characteristic planes of ZnO in wurtzite crystalline form. The (002) plane is the predominant, indicating the rods orientation in the c-axis vertical to the substrate. The ZnO film also has a thickness of 550 nm, bandgap of 3.27 eV, flat band potential of 0.4 V and density of charge carriers 8,9 x 1019 cm-3. The synthetic procedure of CdTe quantum dots occurred from the dissolution cadmium oxide in tetradecylphosphonic acid and octadecene (ODE) to 300 °C. Subsequently, cadmium precursor solution of was cooled to 260 °C and then the tellurium precursor solution, prepared by dissolving tellurium in tributylphosphine and in ODE was injected. The obtained nanocrystals were dispersed in hexane, precipitated with ethanol and finally the quantum dots were stored in toluene. From UV-Vis and TEM analysis was possible to estimate the quantum dots size of CdTe as 4 nm. The XRD of CdTe nanocrystals presented the main characteristic planes of zinc blend structure. ZnO electrode modified with CdTe quantum dots (ZnO/CdTe) was obtained by 24 h immersion in a solution of acetonitrile containing mecaptopropionic acid and propionic acid. Subsequently, the ZnO film modified with the linker was immersed for 48 h in CdTe quantum dots dispersion. FTIR spectrum reveals the absence of a symmetrical C=O stretching mode at approximately 1700 cm-1. Instead, the spectrum shows the presence of the asymmetric and symmetric vas(CO2-) and vs(CO2-) modes were observed at 1631 and 1417 cm-1, respectively. Kulbeka-Munk transformation of the reflectance spectrum of the ZnO/CdTe electrode presented the band related to CdTe in the same wavelength observed when this was in the dispersion. The ZnO/CdTe electrode showed a photocurrent value of 138 µA, a value 10 times greater than that obtained for ZnO. At IPCE experiments (incident photon-to-current efficiency) an increase of approximately five times was also noticed to the electrode of ZnO/CdTe. Dynamics of charge carriers was investigated by fs and µs TAS (Transient Absorption Spectroscopy) for ZnO and ZnO/CdTe electrodes. TAS analyses indicate a short life time to ZnO/CdTe electrode compared to ZnO film. Clark electrode measurements showed oxygen production by ZnO/CdTe electrode. Thus, ZnO/CdTe proposed electrode is presented as promising material for photoelectrochemical applications.
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Time resolved laser spectroscopyEkvall, Karin January 2000 (has links)
No description available.
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Energy Transfer Dynamics and Dopant Luminescence in Mn-Doped CdS/ZnS Core/Shell NanocrystalsChen, Hsiang-Yun 14 March 2013 (has links)
Mn-doped II-VI semiconductor nanocrystals exhibit bright dopant photoluminescence that has potential usefulness for light emitting devices, temperature sensing, and biological imaging. The bright luminescence comes from the 4T1→6A1 transition of the Mn2+ d electrons after the exciton-dopant energy transfer, which reroutes the exciton relaxation through trapping processes. The driving force of the energy transfer is the strong exchange coupling between the exciton and Mn2+ due to the confinement of exciton in the nanocrystal. The exciton-Mn spatial overlap affecting the exchange coupling strength is an important parameter that varies the energy transfer rate and the quantum yield of Mn luminescence. In this dissertation, this correlation is studied in radial doping location-controlled Mn-doped CdS/ZnS nanocrystals. Energy transfer rate was found decreasing when increasing the doping radius in the nanocrystals at the same core size and shell thickness and when increasing the size of the nanocrystals at a fixed doping radius.
In addition to the exciton-Mn energy transfer discussed above, two consecutive exciton-Mn energy transfers can also occur if multiple excitons are generated before the relaxation of Mn (lifetime ~10^-4 - 10^-2 s). The consecutive exciton-Mn energy transfer can further excite the Mn2+ d electrons high in conduction band and results in the quenching of Mn luminescence. The highly excited electrons show higher photocatalytic efficiency than the electrons in undoped nanocrystals.
Finally, the effect of local lattice strain on the local vibrational frequency and local thermal expansion was observed via the temperature-dependent Mn luminescence spectral linewidth and peak position in Mn-doped CdS/ZnS nanocrystals. The local lattice strain on the Mn2+ ions is varied using the large core/shell lattice mismatch (~7%) that creates a gradient of lattice strain at various radial locations. When doping the Mn2+ closer to the core/shell interface, the stronger lattice strain softens the vibrational frequency coupled to the 4T1→6A1 transition of Mn2+ (Mn luminescence) by ~50%. In addition, the lattice strain also increases the anharmonicity, resulting in larger local thermal expansion observed from the nearly an order larger thermal shift of the Mn luminescence compared to the Mn-doped ZnS nanocrystals without the core/shell lattice mismatch.
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Time resolved laser spectroscopyEkvall, Karin January 2000 (has links)
No description available.
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Estudo das características semicondutoras de filmes de óxido de zinco modificados com pontos quânticos de telureto de cádmio / Study of semiconductor features of zinc oxide films modified with cadmium telluride quantum dotsVanessa Nascimento dos Santos 25 February 2016 (has links)
Inserido no contexto de fontes de energia renováveis, este trabalho consiste na síntese e caracterização de filmes de bastões de ZnO modificados com quantum dots de CdTe a fim de serem aplicados em células fotoeletroquímicas. Bastões de ZnO são materiais interessantes, porque este tipo de estrutura facilita o transporte de portadores de carga, minimizando a perda destes nos contornos de grão, sua recombinação e aniquilação. A modificação do filme de ZnO com nanocristais de CdTe deve aumentar a eficiência da fotoconversão, facilitando a separação de carga e transferência de elétrons, e isso aumenta a estabilidade dos nanocristais, impedindo a corrosão anódica e a decomposição destes. O filme de ZnO foi eletrodepositado potenciostaticamente sobre a superfície de ITO. As análises de MEV e EDX indicaram que filme de ZnO obtido é homogêneo e consiste de bastões com razão atômica de Zn e O de acordo com a estequiometria 1:1. O resultado de DRX apresentou três planos característicos do ZnO na forma cristalina wurtzita. O plano (002) foi o predominante, indicando a orientação dos bastões no eixo c vertical ao substrato. O filme de ZnO tem espessura de 550 nm, bandgap 3,27 eV, potencial de banda plana de 0,4 V e densidade de portadores de carga de 8,9 x 1019 cm-3. O procedimento sintético dos pontos quânticos de CdTe ocorreu a partir da dissolução de óxido de cádmio em ácido tetradecilfosfônico e octadeceno (ODE) a 300 °C. Subsequentemente, a solução precursora de cádmio foi resfriada a 260 °C e então a solução precursora de telúrio, preparada pela dissolução de telúrio e tributilfosfina em ODE, foi injetada. Os nanocristais obtidos foram dispersos em hexano, precitados com etanol e finalmente os quantum dots foram armazenados em tolueno. A partir das análises de UV-Vis e TEM foi possível estimar o tamanho dos pontos quânticos de CdTe com aproximadamente 4 nm. O DRX dos nanocristais de CdTe apresentou os planos característicos principais da estrutura da blenda de zinco. O eletrodo de ZnO modificado com os quantum dots de CdTe (ZnO/CdTe) foi obtido após 24 h de imersão em uma solução de acetonitrila contendo ácido mercaptopropiônico e ácido propiônico. Subsequentemente, o filme de ZnO modificado com o ligante foi imerso por 48 h na dispersão de pontos quânticos de CdTe. O espectro de FTIR revelou a ausência do estiramento simétrico de C=O em 1700 cm-1. Por outro lado o espectro revelou a presença dos modos assimétrico e simétrico vas(CO2-) e vs(CO2-) que foram observados em 1631 e 1417 cm-1, respectivamente. A transformação de Kulbeka-Munk do espectro de reflectância do eletrodo ZnO/CdTe apresentou a banda relativa ao CdTe no mesmo comprimento de onda observado quando este encontrava-se na dispersão. O eletrodo ZnO/CdTe mostrou um valor de fotocorrente de 138 µA, um valor 10 vezes maior que o obtido para o ZnO. Nos experimentos de IPCE (eficiência de conversão do fóton incidente à corrente) um aumento de aproximadamente cinco vezes também foi observado para o eletrodo de ZnO/CdTe. A dinâmica dos portadores de carga foi investigada por TAS (Espectroscopia de Absorção Transiente) nas escalas de tempo fs e µs para os eletrodos de ZnO e de ZnO/CdTe. A análise TAS indicou um tempo de vida menor para o filme ZnO/CdTe em comparação com filme ZnO. A medidas com o eletrodo de Clark demonstraram uma produção de oxigênio pelo eletrodo de ZnO/CdTe. Assim, o filme de ZnO/CdTe proposto apresenta-se como um material promissor para aplicações fotoeletroquímicas. / Placed in the context of renewable energy sources, this work consists of the synthesis and characterization of ZnO films modified CdTe quantum dots to be applied in photoelectrochemical cells. ZnO rods are interesting materials because this kind of structure facilitates the charge carriers transport, minimizing the loss of these at grain boundaries and their recombination and annihilation. The ZnO film modification with CdTe nanocrystals should increase the photoconversion efficiency by facilitating charge separation and electron transfer, and it increases the nanocrystals stability, preventing it from anodic corrosion and decomposition. The ZnO film was electrodeposited potenciostatically on ITO surface. SEM and EDX analysis indicated that the ZnO film obtained is homogeneous and it consists of rods with atomic ratio of Zn and O according to 1:1 stoichiometry. XRD result showed three characteristic planes of ZnO in wurtzite crystalline form. The (002) plane is the predominant, indicating the rods orientation in the c-axis vertical to the substrate. The ZnO film also has a thickness of 550 nm, bandgap of 3.27 eV, flat band potential of 0.4 V and density of charge carriers 8,9 x 1019 cm-3. The synthetic procedure of CdTe quantum dots occurred from the dissolution cadmium oxide in tetradecylphosphonic acid and octadecene (ODE) to 300 °C. Subsequently, cadmium precursor solution of was cooled to 260 °C and then the tellurium precursor solution, prepared by dissolving tellurium in tributylphosphine and in ODE was injected. The obtained nanocrystals were dispersed in hexane, precipitated with ethanol and finally the quantum dots were stored in toluene. From UV-Vis and TEM analysis was possible to estimate the quantum dots size of CdTe as 4 nm. The XRD of CdTe nanocrystals presented the main characteristic planes of zinc blend structure. ZnO electrode modified with CdTe quantum dots (ZnO/CdTe) was obtained by 24 h immersion in a solution of acetonitrile containing mecaptopropionic acid and propionic acid. Subsequently, the ZnO film modified with the linker was immersed for 48 h in CdTe quantum dots dispersion. FTIR spectrum reveals the absence of a symmetrical C=O stretching mode at approximately 1700 cm-1. Instead, the spectrum shows the presence of the asymmetric and symmetric vas(CO2-) and vs(CO2-) modes were observed at 1631 and 1417 cm-1, respectively. Kulbeka-Munk transformation of the reflectance spectrum of the ZnO/CdTe electrode presented the band related to CdTe in the same wavelength observed when this was in the dispersion. The ZnO/CdTe electrode showed a photocurrent value of 138 µA, a value 10 times greater than that obtained for ZnO. At IPCE experiments (incident photon-to-current efficiency) an increase of approximately five times was also noticed to the electrode of ZnO/CdTe. Dynamics of charge carriers was investigated by fs and µs TAS (Transient Absorption Spectroscopy) for ZnO and ZnO/CdTe electrodes. TAS analyses indicate a short life time to ZnO/CdTe electrode compared to ZnO film. Clark electrode measurements showed oxygen production by ZnO/CdTe electrode. Thus, ZnO/CdTe proposed electrode is presented as promising material for photoelectrochemical applications.
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Photo-physics and applications of colloidal quantum dotsStubbs, Stuart Kenneth January 2010 (has links)
The work presented in this thesis was submitted to The University of Manchester for the degree of Doctor of Philosophy in June 2010 by Stuart K Stubbs and is entitled “Photo-physics and applications of colloidal quantum dots”. In this thesis the results of spectroscopic studies on various colloidal quantum dots, particularly related to the measurement and characterisation of multiple exciton generation are presented. Research conducted with Nanoco Technologies Ltd. that involved the design and development of hybrid quantum dot organic light emitting diodes for use in flat panel display technology is also presented. Cadmium selenide (CdSe), indium phosphide (InP), and lead sulphide (PbS) type I and cadmium selenide/cadmium telluride type II colloidal quantum dots were characterised using steady state photoluminescence and absorption spectroscopy. The fluorescence lifetimes of the decay of single excitons was measured in these quantum dots using time correlated single photon counting. An ultrafast transient absorption spectrometer was designed, built, and used to observe the picosecond dynamics of the decay of multiexcitons. These absorption transients were analysed in order to extract the quantum efficiency of producing multiple excitons per absorbed photon. The characteristic signature for multiple exciton generation was first found in CdSe using a time correlated single photon counting set-up. Results from the transient absorption spectrometer demonstrated efficient multiple exciton generation in InP for the first time as well as in PbS, where the efficiency was found to agree with values obtained by other research groups. The absorption transients taken for the type II CdSe/CdTe type II quantum dots demonstrated some novel decay dynamics that could not be attributed to the generation of multiple excitons. Quantum dot organic light emitting diodes were fabricated using Nanoco Technologies high quality cadmium based quantum dots and were shown to demonstrate bright, colour saturated emission originating from the quantum dot layer only. Using quantum dots of different sizes and structures red, green and blue devices were made and shown to be appropriate both in terms of brightness and chromaticity for the use as the red, green and blue pixels of a flat panel display. Because heavy metals like cadmium are restricted or banned from commercial products in many countries, Nanoco Technologies heavy metal free quantum dots, made from InP, were also incorporated in devices. Devices are demonstrated that emit from the quantum dot layer only, albeit at a lower luminance and efficiency than that found in the cadmium containing devices. This was the first demonstration of a heavy metal free, hybrid quantum dot organic light emitting diode.
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CARRIER TRANSPORT IN HYBRID LEAD HALIDE PEROVSKITES STUDIED BY ULTRAFAST PUMP-PROBE MICROSCOPYJordan M Snaider (6318551) 15 May 2019 (has links)
Insight into the nanoscale carrier transport in the rapidly developing class of solutionprocessed semiconductors known as metal halide perovskites is the focal point for these studies.
Further advancement in fundamentally understanding photophysical processes associated with
charge carrier transport is needed to realize the true potential of perovskites for photovoltaic
applications. In this work, we study photogenerated carrier transport to understand the underlying
transport behavior of the material on the 10s to 100s nanometer lengthscales. To study these
processes, we employ a temporally-resolved and spatially-resolved technique, known as transient
absorption microscopy, to elucidate the charge carrier dynamics and propagation associated with
metal halide perovskites. This technique provides a simultaneous high temporal resolution (200
fs) and spatial resolution (50 nm) to allow for direct visualization of charge carrier migration on
the nanometer length scale. There are many obstacles these carriers encounter between
photogeneration and charge collection such as morphological effects (grain boundaries) and carrier
interactions (scattering processes). We investigate carrier transport on the nanoscale to understand
how morphological effects influence the materials transport behavior. Morphological defects such
as voids and grain boundaries are inherently small and traditionally difficult to study directly.
Further, because carrier cooling takes place on an ultrafast time scale (fs to ps), the combined
spatial and temporal resolution is necessary for direct probing of hot (non-equilibrium) carrier
transport. Here we investigate a variety of ways to enhance carrier transport lengthscales by studying how non-equilibrium carriers propagate throughout the material, as well as, carrier
cooling mechanisms to extend the non-equilibrium regime.
For optoelectronic devices based on polycrystalline semiconducting thin films, grain
boundaries are important to consider since solution-based processing results in the formation of
well-defined grains. In Chapter 3, we investigate equilibrium carrier transport in metal halide
perovskite thin films that are created via the highly desired solution processing method. Carrier
transport across grain boundaries is an important process in defining efficiency due to the literary
discrepancies on whether the grains limit carrier transport or not. In this work, we employ transient
absorption microscopy to directly measure carrier transport within and across the boundaries. By
selectively imaging sub-bandgap states, our results show that lateral carrier transport is slowed
down by these states at the grain boundaries. However, the long carrier lifetimes allow for efficient
transport across the grain boundaries. The carrier diffusion constant is reduced by about a factor
of 2 for micron-sized grain samples by the grain boundaries. For grain sizes on the order of ∼200
nm, carrier transport over multiple grains has been observed within a time window of 5 ns. These
observations explain both the shortened photoluminescence lifetimes at the boundaries as well as
the seemingly benign nature of the grain boundaries in carrier generation. The results of this work
provide insight into why this defect tolerant material performs so well.
Photovoltaic performance (power conversion efficiency) is governed by the ShockleyQueisser limit which can be overcame if hot carriers can be harvested before they thermalize. To
convert sunlight to usable electricity, the photogenerated charge carriers need to migrate long
distances and or live long enough to be collected. It is unclear whether these hot carriers can
migrate a long enough distance for efficient collection. In Chapter 4, we report direct visualization
of hot-carrier migration in methylammonium lead iodide (CH3NH3PbI3) thin films by ultrafast transient absorption microscopy. This work demonstrates three distinct transport regimes. (i)
Quasiballistic transport, (ii) nonequilibrium transport, and (iii) diffusive transport. Quasiballistic
transport was observed to correlate with excess kinetic energy, resulting in up to 230 nanometers
of transport distance that could overcome grain boundaries. The nonequilibrium transport
persisted over tens of picoseconds and ~600 nanometers before reaching the diffusive transport
limit. These results suggest potential applications of hot-carrier devices based on hybrid
perovskites to ultimately overcome the Shockley-Queisser limit.
In the next work, we investigated a way to extend non-equilibrium carrier lifetime, which
ultimately corresponds to an accelerated carrier transport. From the knowledge of the hot carrier
transport work, we showed a proof of concept that the excess kinetic energy corresponds to long
range carrier transport. To further develop the idea of harvesting hot carriers, one must investigate
a way to make the carriers stay hot for a longer period (i.e. cool down slower). In Chapter 5, we
slow down the cooling of hot carriers via a phonon bottleneck, which points toward the potential
to overcome the Shockley-Queisser limit. Open questions remain on whether the high optical
phonon density from the bottleneck impedes the transport of these hot carriers. We show a direct
visualization of hot carrier transport in the phonon bottleneck regime in both single crystalline and
polycrystalline lead halide perovskites, more specifically, a relatively new class of alkali metal
doped perovskites (RbCsMAFA), which has one of the highest power conversion efficiencies.
Remarkably, hot carrier diffusion is enhanced by the presence of a phonon bottleneck, the exact
opposite from what is observed in conventional semiconductors such as GaAs. These results
showcase the unique aspects of hot carrier transport in hybrid perovskites and suggest even larger
potential for hot carrier devices than previously envisioned by the initial results presented in
Chapter 4. The final chapter will be divided into two sections, as we summarize and highlight our
collaborative efforts towards homogenization of carrier dynamics via doping perovskites with
alkali metals and our work on two-dimensional hybrid quantum well perovskites. Further studies
on the champion solar cell (RbCsMAFA) were performed to elucidate the role inorganic cations
play in this material. By employing transient absorption microscopy, we show that alkali metals
Rb+
and Cs+
are responsible for inducing a more homogenous halide (Iand Br-
) distribution,
despite the partial incorporation into the perovskite lattice. This translates into improved electronic
dynamics, including fluorescence lifetimes above 3 µs and homogenous carrier dynamics, which
was visualized by ultrafast microscopy. Additionally, there is an improvement in photovoltaic
device performance. We find that while Cs cations tend to distribute homogenously across the
perovskite grain, Rb and K cations tend to phase segregate at precursor concentrations as low as
1%. These precipitates have a counter-productive effect on the solar cell, acting as recombination
centers in the device, as argued from electron beam-induced current measurements. Remarkably,
the high concentration of Rb and Cs agglomerations do not affect the open-circuit voltage, average
lifetimes, and photoluminescence distribution, further indicating the perovskite’s notorious defect
tolerance.
A new class of high-quality two dimensional organic-inorganic hybrid perovskite quantum
wells with tunable structures and band alignments was studied. By tuning the functionality of the
material, the strong self-aggregation of the conjugated organic molecules can be suppressed, and
2D organic-halide perovskite superlattice crystals and thin films can be easily obtained via onestep solution-processing. We observe energy transfer and charge transfer between adjacent
organic and inorganic layers, which is extremely fast and efficient (as revealed by ultrafast
spectroscopy characterizations). Remarkably, these 2D hybrid perovskite superlattices are stable, due to the protection of the bulky hydrophobic organic groups. This is a huge step towards the
practicality of using perovskites for optoelectronics, since stability is always a huge concern with
water-sensitive materials. The molecularly engineered 2D semiconductors are on par with III-V
quantum wells and are promising for next-generation electronics, optoelectronics, and photonics.
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