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Colloidally synthesized nanocrystals (NC) are a comparatively young field of research with the first NC-synthesis reported in 1986. Solar cells based on these colloidally synthesized NCs are an even younger research field. The first NC-based solar cells were made only in the 2000s. Since then, they have been continuously improved and the power efficiency has steadily increased. However, most progress has been trial-and-error based achievements. To further and systematically improve the device performance, it is crucial to develop an understanding of the underlying processes that limit the device efficiency. As many of the contributing parameters are complexly interlinked with each other, it is non-trivial to determine the factors that limit the overall solar cell performance. In this thesis, we leverage the recent deepened understanding and quantification of the underlying chemical and physical parameters of NC-based solar cells to establish our own simple model. Additionally, we use an off the shelf 1D drift-diffusion simulation tool to gain insight into device operation and therefore guide further device design decisions. In the first chapter, we list the improvements to the synthesis of lead sulfide NCs and how their composition, as well as size, shape and especially surface termination can be controlled. This means that we can select electronic, phononic und photonic properties of the NC-solid. Post- synthesis modification and thin-film deposition techniques influence the final crystallographic structure as well as electrical and optical properties of the resulting thin film. Combined, this allows us to control the NC- based semiconductor properties from atomistic- to macro scale, all while still being a completely solution-based technique not requiring any vacuum or high temperature steps. We highlight the key advances in NC synthesis as well as thin-film fabrication that result in significant performance improvements of the NC-based solar cells. The key takeaway is the importance of the NC surface and the ability to control and modify the surface composition, surface area and shape, and ligand coverage. We further discuss how these findings can be adapted to help new emerging NC materials. In the second chapter, we measure temperature dependent transient photovoltages on PbS NC-base solar cells. We show that the measured decay time cannot directly be interpreted as the carrier lifetime in NC-based solar cells. Instead, the lifetime can be modeled as an RC circuit. Where the resistive part is indicative of the degree of Shockley-Read-Hall (SRH) recombination and the capacitance is the space charge capacitance. We develop a model with which photovoltage decay on NC-devices can be analyzed and help guide further device improvements. In chapter 3, we implement a device level drift diffusion simulation to gain inside into device performance and guide further device design. We show how we apply the learnings about SRH recombination from chapter 2 together with the new understanding of interfaces, transport, and origin of trap states in NC-based semiconductors. Additionally, we demonstrate how we adapt traditional 1D drift diffusion for NC-based solar cells. We achieve high levels of agreement between our simulation and measurement without relying on any fitting. We then use the simulation to understand the impact of various parameters on the final device performance and envision improvements to the device design that could lead to better performing NC-based solar cells. In the last chapter we further expand the device level simulation and show that we can simulate NC-based solar cells independent of device architecture. We simulate both Schottky- and heterojunction-type solar cells and validate the parametrization by comparing the simulation to measurements. The simulations reveal that Schottky-type devices are limited by surface recombination between the PbS and aluminum contact while heterojunction devices are currently limited by nanocrystal dopants and electronic defects in the PbS layer. We found a number of opportunities for further performance enhancement, including the reduction of dopants in the nanocrystal active layer, the control over doping and electronic structure in electron- and hole-blocking layers (e.g., ZnO), and the optimization of the interfaces to improve the band alignment and reduce surface recombination. |
