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While photovoltaics hold much promise as a sustainable electricity source, continued cost reduction is necessary to continue the current growth in deployment. A promising path to continuing to reduce total system cost is by increasing device efficiency. This thesis explores several silicon-based photovoltaic technologies with the potential to reach high power conversion efficiencies. Silicon microwire arrays, formed by joining millions of micron diameter wires together, were developed as a low cost, low efficiency solar technology. The feasibility of transitioning this to a high efficiency technology was explored. In order to achieve high efficiency, high quality silicon material must be used. Lifetimes and diffusion lengths in these wires were measured and the action of various surface passivation treatments studied. While long lifetimes were not achieved, strong inversion at the silicon / hydrofluoric acid interface was measured, which is important for understanding a common measurement used in solar materials characterization. Cryogenic deep reactive ion etching was then explored as a method for fabricating high quality wires and improved lifetimes were measured. As another way to reach high efficiency, growth of silicon-germanium alloy wires was explored as a substrate for a III-V on Si tandem device. Patterned arrays of wires with up to 12% germanium incorporation were grown. This alloy is more closely lattice matched to GaP than silicon and allows for improvements in III-V integration on silicon. Heterojunctions of silicon are another promising path towards achieving high efficiency devices. The GaP/Si heterointerface and properties of GaP grown on silicon were studied. Additionally, a substrate removal process was developed which allows the formation of high quality free standing GaP films and has wide applications in the field of optics. Finally, the effect of defects at the interface of the amorphous silicon heterojuction cell was st

We report on an study of the GaP/Si interface for application in silicon heterojunction solar cells. We analyzed the band alignment using X-ray photoelectron spectroscopy (XPS) and cross-sectional Kelvin probe force microscopy (x-KPFM). Our measurements show a high conduction band offset (0.9 eV) leading to a barrier in electron extraction which we microscopically resolved via x-KPFM. XPS reveals the presence of Si-Ga bonds which explains the observed interface dipole that leads to low open circuit voltage and low fill factor in GaP/Si heterojunction solar cells. Furthermore, we investigated the electronic and morphologic changes in GaP upon Si and Mg doping.

The world record efficiency and open circuit voltage for crystalline silicon solar cells are held by a-Si/Si heterojunction devices. While a-Si provides excellent surface passivation, these heterojunction devices are limited by non-ideal optical and electronic properties. Gallium phosphide is a candidate material for replacing a-Si in a heterojunction device, promising lower parasitic absorption and better carrier mobilities. In this work, we present our results in growing high quality GaP thin films directly on Si using a two-step nucleation and growth scheme with metalorganic chemical vapor deposition, characterization, and x-ray photoelectron spectroscopy band offset measurements toward realizing a GaP/Si heterojunction device.

Tandem Ga_(1-x)In_xP/Si microwire array solar cells are a route towards a high efficiency, low cost, flexible, wafer-free solar technology. Coupled full-field optical and device physics simulations of a Ga_(0.51)In_(0.49)P/Si wire array tandem are used to predict device performance. A 500 nm thick, highly doped “buffer” layer between the bottom cell and tunnel junction is assumed to harbor a high density of lattice mismatch and heteroepitaxial defects. Under simulated AM1.5G illumination, the device structure explored in this work has a simulated efficiency of 23.84% with realistic top cell SRH lifetimes and surface recombination velocities. The relative insensitivity to surface recombination is likely due to optical generation further away from the free surfaces and interfaces of the device structure. To move towards realizing these device structures, GaP and Ga_(1-x)In_xP layers were grown heteroepitaxially with metalorganic chemical vapor deposition on Si microwire array substrates. The layer morphology and crystalline quality have been studied with scanning electron microscopy and transmission electron microscopy, and they provide a baseline for the growth and characterization of a full device stack.

We performed two experiments to measure lateral flow of photoexcited charge carriers near the heterointerface in silicon heterojunction (SHJ) solar cells. Using light beam methods, we probed current extraction differences between areas of varying intrinsic layer thickness and the effective cross section of junction defects. Both measurements demonstrated a strong bias voltage dependence of lateral transport and transport lengths of tens to hundreds of microns as bias approached operating voltages. Lateral carrier flow near the heterointerface is proposed as one of the reasons that SHJ solar cells are extremely sensitive to interfacial defects.