The use of silicon nanostructures in solar cells offers a number of benefits, such as the fact they can be used on flexible substrates. A silicon wire-array structure, containing reflecting nanoparticles for enhanced absorption, is now shown to achieve 96% peak absorption efficiency, capturing 85% of light with only 1% of the silicon used in comparable commercial cells. Si wire arrays are a promising architecture for solar-energy-harvesting applications, and may offer a mechanically flexible alternative to Si wafers for photovoltaics1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17. To achieve competitive conversion efficiencies, the wires must absorb sunlight over a broad range of wavelengths and incidence angles, despite occupying only a modest fraction of the array’s volume. Here, we show that arrays having less than 5% areal fraction of wires can achieve up to 96% peak absorption, and that they can absorb up to 85% of day-integrated, above-bandgap direct sunlight. In fact, these arrays show enhanced near-infrared absorption, which allows their overall sunlight absorption to exceed the ray-optics light-trapping absorption limit18 for an equivalent volume of randomly textured planar Si, over a broad range of incidence angles. We furthermore demonstrate that the light absorbed by Si wire arrays can be collected with a peak external quantum efficiency of 0.89, and that they show broadband, near-unity internal quantum efficiency for carrier collection through a radial semiconductor/liquid junction at the surface of each wire. The observed absorption enhancement and collection efficiency enable a cell geometry that not only uses 1/100th the material of traditional wafer-based devices, but also may offer increased photovoltaic efficiency owing to an effective optical concentration of up to 20 times.

Enhanced absorption and carrier collection in Si wire arrays for photovoltaic applications

Semiconductor nanowires represent an important class of nanostructure building block for photovoltaics as well as direct solar-to-fuel application because of their high surface area, tunable bandgap and efficient charge transport and collection. In this talk, I will highlight several recent examples in this lab using semiconductor nanowires and their heterostructures for the purpose of solar energy harvesting. In addition, we have also discovered that the thermoconductivity of the silicon nanowires can be significantly reduced due to phonon scattering, pointing to a very promising approach to design better thermoelectrical materials. It is important to note that the engines that generate most of the world's power typically operate at only 30–40 per cent efficiency, releasing roughly 15 terawatts of heat to the environment. If this “wasted heat” could be recycled, the impact globally would be enormous. Our silicon nanowire thermoelectric technology could have a significant impact in alternative energy generation.

Semiconductor nanowires for energy conversion.

Solar energy represents one of the most abundant and yet least harvested sources of renewable energy. In recent years, tremendous progress has been made in developing photovoltaics that can be potentially mass deployed. Of particular interest to cost-effective solar cells is to use novel device structures and materials processing for enabling acceptable efficiencies. In this regard, here, we report the direct growth of highly regular, single-crystalline nanopillar arrays of optically active semiconductors on aluminium substrates that are then configured as solar-cell modules. As an example, we demonstrate a photovoltaic structure that incorporates three-dimensional, single-crystalline n-CdS nanopillars, embedded in polycrystalline thin films of p-CdTe, to enable high absorption of light and efficient collection of the carriers. Through experiments and modelling, we demonstrate the potency of this approach for enabling highly versatile solar modules on both rigid and flexible substrates with enhanced carrier collection efficiency arising from the geometric configuration of the nanopillars.

http://nano.eecs.berkeley.edu/publications/NatureMat_2009_SNOPcell.pdf

Three-dimensional nanopillar-array photovoltaics on low-cost and flexible substrates

Silicon nanowire-based solar cells on metal foil are described. The key benefits of such devices are discussed, followed by optical reflectance, current-voltage, and external quantum efficiency data for a cell design employing a thin amorphous silicon layer deposited on the nanowire array to form the p-n junction. A promising current density of ∼1.6mA∕cm2 for 1.8cm2 cells was obtained, and a broad external quantum efficiency was measured with a maximum value of ∼12% at 690nm. The optical reflectance of the silicon nanowire solar cells is reduced by one to two orders of magnitude compared to planar cells.

Silicon nanowire solar cells

We have demonstrated a low-temperature wafer-scale etching and thin film deposition method for fabricating silicon n−p core−shell nanowire solar cells. Our devices showed efficiencies up to nearly 0.5%, limited primarily by interfacial recombination and high series resistance. Surface passivation and contact optimization will be critical to improve device performance in the future.

/pdf/silicon-nanowire-radial-p-n-junction-solar-cells-vub1m1o03t.pdf

Silicon nanowire radial p-n junction solar cells.

The time resolved and DC photoconduction characteristics of Si nanowire devices are described. Si nanowires with diameters ranging from 20-100 nm were grown using the vapor-liquid-solid (VLS) growth mechanism under standard conditions and devices were fabricated in a back-gate field effect transistor (FET) configuration using simple photolithography. It is shown that under certain biasing conditions, illumination with light from light emitting diodes with wavelengths ranging from 480 nm to 625 nm causes changes in current as high as 4%. On the other hand, illumination by a broadband incandescent source causes a ∼4.1% percent change in current. Photoconductive decay curves show bi- and tri-exponential behavior, indicative of multiple potential recombination mechanisms occurring within the Si nanowire devices. p-n doped Si nanowires show similar behavior. Studies under various drain and gate voltages provides insight into the proposed mechanism. It is argued that the Shottky barrier plays a strong role in the observed photoconduction process in these wires, as do transitions involving surface and deep level trap states.

https://www.cambridge.org/core/services/aop-cambridge-core/content/view/S1946427400052672

Time Resolved Photoconduction Studies of Uniformly Doped and p-n Junction Si Nanowires

A sensor system, and an associated method for detecting harsh environmental conditions, is provided. The sensor system includes at least one sensor having an electrical sensing element. The electrical sensing element is based on certain classes of composite materials: (a) silicon carbide (SiC); (Mo,W)5Si3C; (Mo,W)Si2; or (b) (Mo,W)5Si3C; (Mo,W)Si2; (Mo,W)5Si3. The sensor system is useful for determining harsh environmental conditions. Gasification systems, which include at least one of the sensor systems are also described.

Sensor system and methods for environmental sensing

Silicon Nanowire Solar Cells: Device Physics, Fabrication, and Optoelectronic Properties

The optical properties of silicon nanowire films fabricated on bulk Si and glass substrates are reported. The total reflectance of aligned nanowire arrays formed by a wet etch process on bulk Si is lower than the control over all wavelengths below the bandgap, varying from ~1% at 300 nm to les 10% at 1,000 nm. Similar results are observed for nanowire thin films. The observed reflectance is related to the so-called "moth-eye" effect. Modeling work shows strong resonance of light within and between nanowires

Interaction of Light with Si Nanowire Films

The gate oxide reliability and channel mobility of carbon face (000-1) 4H Silicon Carbide (SiC) MOSFETs are investigated. Several gate oxidation processes including dry oxygen, pyrogenic steam, and nitrided oxides were investigated utilizing MOS capacitors for time dependent dielectric breakdown (TDDB), dielectric field strength, and MOSFETs for inversion layer mobility measurements. The results show the C-face can achieve reliability similar to the Si-face, however this is highly dependent on the gate oxide process. The reliability is inversely related to the field effect mobility where other research groups report that pyrogenic steam yields the highest electron mobility while this work shows it has weakest oxide in terms of dielectric strength and shortest time to failure.

Jody Fronheiser

Papers

Time Resolved Photoconduction Studies of Uniformly Doped and p-n Junction Si Nanowires

Sensor system and methods for environmental sensing

Silicon Nanowire Solar Cells: Device Physics, Fabrication, and Optoelectronic Properties

Interaction of Light with Si Nanowire Films

Evaluation of 4H-SiC Carbon Face Gate Oxide Reliability