Person:

Kempa, Thomas Jan

This thesis presents the design and synthesis of nanowires (NW) with targeted and tunable optical properties. Moreover, we show how single and assembled NW devices can enable new photovoltaic (PV) and photonic platforms. Beginning with an investigation of axially modulated p-i-n junction NWs, we established several fundamental parameters dictating solar cell performance at the nanoscale and demonstratred the first series integration of multiple solar cells on a single NW. Thereafter, implementation of the first silicon NW photovoltaic device with radially modulated p-n junctions showed that power conversion efficiencies of 3-4% are attainable from a nanoscale architecture, exceeding efficiencies for many organic and hybrid organic-inorganic solar cells. Despite these achievements, the poor electrical characteristics and insufficient control over absorption properties characterizing the aforementioned devices would limit the promise of silicon NWs for next generation solar cells. We overcome these limitations with a class of polymorphic core/multi-shell silicon NWs with highly-crystalline hexagonally-faceted shells and embedded coaxial p/i/n junctions. NW PV devices 200-300 nm in diameter exhibit open-circuit voltages of 0.5 V and fill-factors of 73% under one-sun solar illumination. Single-NW wavelength-dependent photocurrent measurements agree quantitatively with FDTD simulations. Synthetic manipulation of NW size and morphology drives tuning of optical resonances such that optimized structures can yield current densities double those for films of comparable thickness. Further optimized NW devices achieve current densities of 17 mA/cm2 and power conversion efficiencies of 6%. We also present steps toward rational assembly of larger-scale NW PV arrays. Parallel integration of NWs preserves PV metrics while assembly of vertically-stacked NWs yields current densities of (25 mA/cm^2) and projected efficiencies of ~15% for (1 \mu m) thick assemblies. Finally, we present the first ever NW material possessing 3 degrees of structural freedom, thus expanding the NW "structome." Such NWs were achieved through the first demonstration of facet selective growth of silicon and germanium in the gas phase. Photonic devices based on this new material present intriguing optical properties, including selective attenuation, enhancement, and wavelength tunability of resonant cavity modes.

The solar power received by Earth far exceeds global power demands. Despite this, infrastructure shortages and high capital costs prevent the wide-scale adoption of photovoltaics to displace conventional energy technologies relying on carbon-based fuels. In response, new concepts and materials have been explored to develop next-generation solar cells capable of operating more efficiently and cheaply. Over the past decade, single semiconductor nanowire (NW) and NW array devices have emerged as promising platforms with which to examine new concepts. Small distances in NWs allow for efficient charge separation while tunable photonic modes permit light absorption properties distinct from bulk materials. Furthermore, the synthesis and fabrication of NW devices presents new opportunities such as with incorporation of complex heterostructures or use of cheaper substrates. Here, we present a critical discussion of the benefits and remaining challenges related to utilization of NWs for solar energy conversion and emphasize the synthetic advances leading towards significant improvement in the electrical and optical performance of NW devices. We conclude by articulating the unique capabilities of solar cells assembled from multiple, distinct NWs.