Photovoltaic energy is a clean and renewable source of electricity; however, it faces resistance to widespread use due to cost. Nanostructuring decouples constraints related to light absorption and charge separation, potentially reducing cost by allowing a wider variety of processing techniques and materials to be used. However, the large interfacial areas also cause an increased dark current which negatively affects cell efficiency. This work focuses on extremely thin absorber (ETA) solar cells that used a ZnO nanowire array as a scaffold for an extremely thin CdSe absorber layer. Photoexcited electrons generated in the CdSe absorber are transferred to the ZnO layer, while photogenerated holes are transferred to the liquid electrolyte. The transfer of photoexcited carriers to their transport layer competes with bulk recombination in the absorber layer. After charge separation, transport of charge carriers to their respective contacts must occur faster than interfacial recombination for efficient collection. Charge separation and collection depend sensitively on the dimensions of the materials as well as their interfaces. We demonstrated that an optimal absorber thickness can balance light absorption and charge separation. By treating the ZnO/CdSe interface with a CdS buffer layer, we were able to improve the Voc and fill factor, increasing the ETA cell's efficiency from 0.53% to 1.34%, which is higher than that achievable using planar films of the same material. We have gained additional insight into designing ETA cells through the use of dynamic measurements. Ultrafast transient absorption spectroscopy revealed that characteristic times for electron injection from CdSe to ZnO are less than 1 ps. Electron injection is rapid compared to the 2 ns bulk lifetime in CdSe. Optoelectronic measurements such as transient photocurrent/photovoltage and electrochemical impedance spectroscopy were applied to study the processes of charge transport and interfacial recombination. With these techniques, the extension of the depletion layer from CdSe into ZnO was determined to be vital to suppression of interfacial recombination. However, depletion of the ZnO also restricted the effective diffusion core for electrons and slowed their transport. Thus, materials and geometries should be chosen to allow for a depletion layer that suppresses interfacial recombination without impeding electron transport to the point that it is detrimental to cell performance. Thin film solar cells are another promising technology that can reduce costs by relaxing material processing requirements. CuInxGa(1-x)Se (CIGS) is a well studied thin film solar cell material that has achieved good efficiencies of 22.6%. However, use of rare elements raise concerns over the use of CIGS for global power production. CuSbS2 shares chemistry with CuInSe2 and also presents desirable properties for thin film absorbers such as optimal band gap (1.5 eV), high absorption coefficient, and Earth-abundant and non-toxic elements. Despite the promise of CuSbS2, direct characterization of the material for solar cell application is scarce in the literature. CuSbS2 nanoplates were synthesized by a colloidal hot-injection method at 220 ℗ʻC in oleylamine. The CuSbS2 platelets synthesized for 30 minutes had dimensions of 300 nm by 400 nm with a thickness of 50 nm and were capped with the insulating oleylamine synthesis ligand. The oleylamine synthesis ligand provides control over nanocrystal growth but is detrimental to intercrystal charge transport that is necessary for optoelectronic device applications. Solid-state and solution phase ligand exchange of oleylamine with S2- were used to fabricate mesoporous films of CuSbS2 nanoplates for application in solar cells. Exchange of the synthesis ligand with S2- resulted in a two order of magnitude increase in 4-point probe conductivity. Photoexcited carrier lifetimes of 1.4 ns were measured by time-resolved terahertz spectroscopy, indicating potential for CuSbS2 as a solar cell absorber material.

Solar cells with thin Cu(In,Ga)(S,Se)2 absorber films are well established in the photovoltaics market. They offer an advantage over other thin film technologies thanks to their lower content of elements with high toxicity or low earth abundance like Cd and Te. One approach to further improve the quality of production of these cells is to develop a method of material quality assessment during production that is fast, contactless and non-destructive. Time-resolved photoluminescence (TRPL) measurements offer all these characteristics. This work aims to establish the requirements to extract meaningful information about charge carrier recombination dynamics and solar cell performance parameters from TRPL measurements. To achieve this goal experiments and simulations are carried out. The material parameters are extracted from experiments and then built into the simulation model. Results from experiments also serve as the basis to verify the validity of this model. Parameter variations within the simulations function as one of the main methods in this work to gain deeper physical insight into the processes taking place during TRPL measurements. engl.

Structural phase transitions, mechanical deformations, and the embryonic stages of melting and crystallization are examples of phenomena that can now be imaged in unprecedented structural detail with high spatial resolution, and ten orders of magnitude as fast as hitherto. No monograph in existence attempts to cover the revolutionary dimensions that EM in its various modes of operation nowadays makes possible. The authors of this book chart these developments, and also compare the merits of coherent electron waves with those of synchrotron radiation. They judge it prudent to recall some important basic procedural and theoretical aspects of imaging and diffraction so that the reader may better comprehend the significance of the new vistas and applications now afoot. This book is not a vade mecum - numerous other texts are available for the practitioner for that purpose.

Understanding photoexcited carrier dynamics is crucial for designing high-performance optoelectronic devices. Carrier cooling in semiconductors, charge transfer across interfaces, and recombination mechanisms are critical processes in photophysical systems that typically occur on the time scale of less than a picosecond to several nanoseconds. Ultrafast techniques, including ultraviolet-visible-infrared transient absorption (TA), time-resolved terahertz spectroscopy (TRTS), and time-resolved photoluminescence (TRPL), are ideal tools for studying charge carrier dynamics at such timescales. This thesis will focus on the application of complementary spectroscopy techniques and modeling to investigate carrier dynamics within CdSe/CdS core/shell colloidal quantum dots (QDs) and Cu3AsS4 and CdTe thin films.CdTe solar technology has attracted the photovoltaic (PV) community for the past three decades owing to its low production cost and record efficiency of 22.1%. However, some challenges must be overcome to further improve its efficiency to the 25% range. Cu3AsS4 thin film is a promising emerging candidate as a PV absorber material due to its earth-abundant and nontoxic constituent elements, but its optoelectronic properties are not well known. Carrier dynamics reveal important details about the recombination processes that limit PV performance. Improvements in the PV device efficiency require a full understanding of the routes for carrier recombination processes.TRPL, which measures emission, has conventionally been used to evaluate recombination mechanisms in thin film PVs, but carrier redistribution often dominates the response at short times. Here we report on the quantification of carrier dynamics and recombination mechanisms by complementary use of both TRTS, which measures photoconductivity, and TRPL combined with numerical modeling of the continuity equations and Poisson's equation. We were able to distinguish and quantify bulk and surface recombination in CdTe and Cu3AsS4 thin films, which is critical for the development of thin film PVs with higher efficiency.We also investigated the carrier dynamics in functionalized CdSe/CdS core/shell QDs using complementary ultrafast TA and TRPL spectroscopies and kinetic modeling. Cd-chalcogenide QDs have been widely studied because of their excellent optical properties and their facile tunability. The Cd-chalcogenide QDs have been studied for more than 20 years, but the ambiguities in the interpretation of the TA spectra are still under debate. For one thing, the photoexcited TA signal in Cd-chalcogenide QDs has been fully attributed to conduction band electrons, neglecting any contributions from valence band holes. In this work, we present a comprehensive picture of the electronic processes in photoexcited CdSe/CdS core/shell QDs. We have demonstrated through complementary spectroscopic experiments and kinetic modeling that holes affect the TA results and can contribute ~ 30% to the visible range and ~ 72% to the mid-IR range. The comprehensive picture of photophysical processes provided by the complementary ultrafast techniques and kinetic modeling in this work can accelerate both the fundamental science and application development of nanostructured and molecular systems.This thesis will focus on the application of spectroscopy techniques and modeling to investigate carrier dynamics in optoelectronic systems including thin film PVs and colloidal CdSe based QDs. The methodologies presented in this thesis can serve as a guideline for the accurate interpretation of spectroscopic measurements not only for the cases studied here but also for other optoelectronic systems.