Hot Carrier Plasmonics

Abstract

Despite the fact that 1 month of solar illumination contains vastly more energy than is stored in all of the earth's coal, oil, and natural gas reserves, solar power makes up much less than 1% of our power supply. The main reason for this discrepancy is the Cost/Watt of solar electricity. Traditional single-junction semiconductor solar cells are limited to a power conversion efficiency of approximately 30%, known as the Shockley-Queisser (SQ) limit. When photons with energy significantly greater than the bandgap energy of the semiconductor are absorbed, electrons and holes are generated with excess kinetic energy, so-called hot carriers. This extra energy is dissipated, e.g. by phonon emission (interaction with lattice). Further, for photons with energy below the bandgap energy, the absence of absorption results in no power generation. Attempts to indirectly surpass the efficiency limit have been suggested using multiple junctions, multi-exciton generation, or the addition of an intermediate band within the semiconductor bandgap; however, many challenges remain for these concepts. In the thesis, we will describe the methods and the underlying physics of photon detection and power conversion of both high and low energy photons using hot carrier effects before they lose their excess energy to heat.

For the absorption of high-energy photons, devices utilizing plasmonic nanostructures or three-layer stacks (transparent conductor-insulator-metal) can be used to generate and collect the hot carriers. We show experimental photocurrent generation from both monochromatic and broadband light sources, and uniform absorption for normal and oblique incident illumination. Power conversion efficiencies >10% are predicted with optimized structures. Excitation of the surface plasmon resonances further improves the device performance. In addition, we present a route to beating the SQ limit based on sub-bandgap photon absorption in a nanostructured metal contact followed by hot carrier injection. Our results provide a new pathway for high-efficiency photovoltaics that can be implemented using standard fabrication processes.

From a materials point-of-view, noble metals (gold and silver) are almost exclusively used in hot carrier plasmonic devices; however, many other materials may offer advantages for collecting hot carriers. We present results for several materials and show their potential applicability for hot carrier excitation and extraction. By considering the hot carrier distributions based on the electron density of states for the materials, we predict the preferred hot carrier type for collection and their expected performance under different illumination conditions.

By combining these concepts, hot carrier generation and collection can be exploited over a large range of incident wavelengths spanning the UV, visible, and IR. Further work is also suggested to more fully explore the potential of this phenomenon and create a long-lasting impact on renewable energy generation.