Brendan M. Kayes

Comparison of the device physics principles of planar and radial p-n junction nanorod solar cells

A device physics model has been developed for radial p-n junction nanorod solar cells, in which densely packed nanorods, each having a p-n junction in the radial direction, are oriented with the rod axis parallel to the incident light direction. High-aspect-ratio (length/diameter) nanorods allow the use of a sufficient thickness of material to obtain good optical absorption while simultaneously providing short collection lengths for excited carriers in a direction normal to the light absorption. The short collection lengths facilitate the efficient collection of photogenerated carriers in materials with low minority-carrier diffusion lengths. The modeling indicates that the design of the radial p-n junction nanorod device should provide large improvements in efficiency relative to a conventional planar geometry p-n junction solar cell, provided that two conditions are satisfied: (1) In a planar solar cell made from the same absorber material, the diffusion length of minority carriers must be too low to allow for extraction of most of the light-generated carriers in the absorber thickness needed to obtain full light absorption. (2) The rate of carrier recombination in the depletion region must not be too large (for silicon this means that the carrier lifetimes in the depletion region must be longer than ~10 ns). If only condition (1) is satisfied, the modeling indicates that the radial cell design will offer only modest improvements in efficiency relative to a conventional planar cell design. Application to Si and GaAs nanorod solar cells is also discussed in detail.

Photovoltaic Measurements in Single-Nanowire Silicon Solar Cells

Single-nanowire solar cells were created by forming rectifying junctions in electrically contacted vapor-liquid-solid-grown Si nanowires. The nanowires had diameters in the range of 200 nm to 1.5 microm. Dark and light current-voltage measurements were made under simulated Air Mass 1.5 global illumination. Photovoltaic spectral response measurements were also performed. Scanning photocurrent microscopy indicated that the Si nanowire devices had minority carrier diffusion lengths of approximately 2 microm. Assuming bulk-dominated recombination, this value corresponds to a minimum carrier lifetime of approximately 15 ns, or assuming surface-dominated recombination, to a maximum surface recombination velocity of approximately 1350 cm s(-1). The methods described herein comprise a valuable platform for measuring the properties of semiconductor nanowires, and are expected to be instrumental when designing an efficient macroscopic solar cell based on arrays of such nanostructures.

Growth of vertically aligned Si wire arrays over large areas (>1cm2) with Au and Cu catalysts

Arrays of vertically oriented Si wires with diameters of 1.5 µm and lengths of up to 75 µm were grown over areas >1 cm^2 by photolithographically patterning an oxide buffer layer, followed by vapor-liquid-solid growth with either Au or Cu as the growth catalyst. The pattern fidelity depended critically on the presence of the oxide layer, which prevented migration of the catalyst on the surface during annealing and in the early stages of wire growth. These arrays can be used as the absorber material in novel photovoltaic architectures and potentially in photonic crystals in which large areas are needed.

27.6% Conversion efficiency, a new record for single-junction solar cells under 1 sun illumination

Alta Devices, Inc. has fabricated a thin-film GaAs device on a flexible substrate with an independently-confirmed solar energy conversion efficiency of 27.6%, under AM1.5G solar illumination at 1 sun intensity. This represents a new record for single-junction devices under non-concentrated sunlight. This surpasses the previous record, for conversion efficiency of a single-junction device under non-concentrated light, by more than 1%. This is due largely to the high open-circuit voltage (Voc) of this device. The high Voc results from precise control of the dark current. The fact that this record result has been achieved with a thin-film shows that, for GaAs material systems, the majority of the growth substrate is not needed for device performance. This allows one to consider amortizing the potentially high cost of a GaAs growth substrate by growing a thin-film, lifting it off, and reusing the same substrate multiple times. This technology therefore has the potential to be a novel high-performance, thin-film option for terrestrial photovoltaics.

1.5-kV and 2.2-m \(\Omega \) -cm \(^{2}\) Vertical GaN Transistors on Bulk-GaN Substrates

In this letter, vertical GaN transistors fabricated on bulk GaN substrates are discussed. A threshold voltage of 0.5 V and saturation current >2.3 A are demonstrated. The measured devices show breakdown voltages of 1.5 kV and specific ON-resistance of 2.2 mΩ-cm 2 , which translates to a figure-of-merit of V BR 2 /R ON ~1 × 10 9 V 2 Ω -1 · cm -2 .

Repeated epitaxial growth and transfer of arrays of patterned, vertically aligned, crystalline Si wires from a single Si(111) substrate

Multiple arrays of Si wires were sequentially grown and transferred into a flexible polymer film from a single Si(111) wafer. After growth from a patterned, oxide-coated substrate, the wires were embedded in a polymer and then mechanically separated from the substrate, preserving the array structure in the film. The wire stubs that remained were selectively etched from the Si(111) surface to regenerate the patterned substrate. Then the growth catalyst was electrodeposited into the holes in the patterned oxide. Cycling through this set of steps allowed regrowth and polymer film transfer of several wire arrays from a single Si wafer.

Secondary ion mass spectrometry of vapor−liquid−solid grown, Au-catalyzed, Si wires

Knowledge of the catalyst concentration within vapor-liquid-solid (VLS) grown semiconductor wires is needed in order to assess potential limits to electrical and optical device performance imposed by the VLS growth mechanism. We report herein the use of secondary ion mass spectrometry to characterize the Au catalyst concentration within individual, VLS-grown, Si wires. For Si wires grown by chemical vapor deposition from SiCl 4 at 1000 degrees C, an upper limit on the bulk Au concentration was observed to be 1.7 x 10(16) atoms/cm(3), similar to the thermodynamic equilibrium concentration at the growth temperature. However, a higher concentration of Au was observed on the sidewalls of the wires.

Flexible Thin-Film Tandem Solar Cells With >30% Efficiency

Alta Devices, Inc. has previously reported on single-junction thin-film GaAs photovoltaic devices on flexible substrates with efficiencies up to 28.8% under AM1.5G solar illumination at 1-sun intensity. Here, we show that the same technology platform can be extended to tandem devices that are capable of even higher efficiencies: so far up to 30.8%. Specifically, here, we report on a lattice-matched, series-connected, two-junction device with InGaP as the light-absorbing material of the top cell and GaAs as the absorber in the bottom cell. The material is grown by metallorganic chemical vapor deposition, and then, the device is lifted off by the epitaxial liftoff (ELO) process, as previously reported. This demonstrates that ELO is not only capable of record-setting single-junction performance but capable of achieving world-class efficiency with a multijunction architecture as well.

Radial pn junction nanorod solar cells: device physics principles and routes to fabrication in silicon

We have developed quantitative device-physics models for a radial pn junction nanorod solar cell, that is, a cell which consists of densely packed nanorods attached to a conducting substrate, each nanorod with a pn junction in the radial direction. It is found that this novel design shows large improvements over the planar geometry so long as two conditions are satisfied: a) a planar solar cell made from the same material is collection limited, i.e. the diffusion length of minority carriers is too low to allow for collection of most or all of the light-generated carriers in the conventional planar geometry, and b) recombination in the depletion region is not too high, or, equivalently, the lifetime of carriers in the depletion region is not too short. In order to experimentally validate this concept, the vapor-liquid-solid (VLS) growth of silicon (Si) nanorods has been explored using metal catalyst particles that are not as deleterious to the minority carrier lifetime of Si as gold (Au), the most commonly used wire growth catalyst.

Single-nanowire Si solar cells

Solar cells based on arrays of CVD-grown Si nano- or micro-wires are being considered as a potentially low-cost route to implementing a vertical multijunction cell design via radial p-n junctions. This geometry has been predicted to enable efficiencies competitive with planar multicrystalline Si designs, while reducing the materials and processing costs of solar cell fabrication [1]. To further assess the potential efficiency of cells based on this design, we present here experimental measurements of minority carrier diffusion lengths and surface recombination rates within nanowires via fabrication and characterization of single-wire solar cell devices. Furthermore, we consider a potential Si wire array-based solar cell design, and present device physics modeling of single-wire photovoltaic efficiency. Based on experimentally observed diffusion lengths within our wires, we model a radial junction wire solar cell capable of 17% photovoltaic energy conversion efficiency.