Understanding the Mechanisms of Plasmon-Enhanced Phenomena in Mesoscopic Solar Cells
Zarick, Holly Frances
Mesoscopic solar cells, including dye-sensitized (DSSCs) and perovskite solar cells (PSCs), have recently emerged as a global contender to silicon photovoltaics (PVs), due to their inexpensive and non-toxic constituents, and potential for scalable manufacturing enabled by the low cost solution processing. However, despite this promise, their low power conversion efficiencies (PCEs) have hindered their broad commercial adoption. Inefficient light harvesting remains a major challenge in these solar cells and, in this thesis, we have leveraged plasmonic nanostructures to boost the light absorption of DSSCs and PSCs. Plasmonic enhancement of solar cells provides a universal approach that can be applied to a wide variety of photovoltaic systems, as the optical properties of the nanostructures can be tuned by altering their morphology and composition. Further, the nanostructures can be incorporated into the mesoporous titania active layer of the solar cells by straightforward solution-processing. In this thesis, we have not only investigated the impact of nanostructures on the device performance of DSSCs and PSCs, but also explored the fundamental mechanisms that give rise to plasmon-enhanced phenomena. We first decoupled the impact of shape and composition by comparing the enhancements enabled by Au nanocubes (Au NCs) and Au/Ag core/shell nanostructures (Au/Ag NSs) embedded in the mesoporous active layer of DSSCs. Our results demonstrate a systematic dependence of device performance on nanoparticle density and ~30% increase in PCE for both Au NC- and Au/Ag NS-incorporated devices compared to reference at optimized concentrations. Compared to their monometallic counterparts, devices with bimetallic Au/Ag NSs require 4× less nanoparticle density to achieve optimal enhancements. We further examined the exciton generation, electron injection, and charge recombination in reference and Au/Ag NS-incorporated DSSCs with ultrafast transient absorption spectroscopy (TAS). The trends in carrier lifetimes obtained from TAS provide strong evidence of plasmon resonant energy transfer and enhanced photoinduced carrier generation, promoting efficient electron injection into semiconducting scaffolds prior to bulk recombination. In PSCs, a 26% increase in PCE is observed via incorporation of Au/Ag NSs into methylammonium lead bromide (MAPbBr3) perovskite-infiltrated mesoscopic active layer. TAS of reference and Au/Ag NSs-incorporated PSCs demonstrated the first evidence that plasmonic nanostructures accelerate carrier cooling, resulting in the relief of the hot phonon bottleneck observed in organolead halide perovskites. We have also observed a similar effect of rapid carrier cooling in PSCs when the cryptographically stable halide, Br, is substituted in MAPbI3 perovskites, resulting in mixed halide PSCs for highly stable and efficient devices.