Lang Shen

High Performance Ultrathin GaAs Solar Cells Enabled with Heterogeneously Integrated Dielectric Periodic Nanostructures

Due to their favorable materials properties including direct bandgap and high electron mobilities, epitaxially grown III-V compound semiconductors such as gallium arsenide (GaAs) provide unmatched performance over silicon in solar energy harvesting. Nonetheless, their large-scale deployment in terrestrial photovoltaics remains challenging mainly due to the high cost of growing device quality epitaxial materials. In this regard, reducing the thickness of constituent active materials under appropriate light management schemes is a conceptually viable option to lower the cost of GaAs solar cells. Here, we present a type of high efficiency, ultrathin GaAs solar cell that incorporates bifacial photon management enabled by techniques of transfer printing to maximize the absorption and photovoltaic performance without compromising the optimized electronic configuration of planar devices. Nanoimprint lithography and dry etching of titanium dioxide (TiO2) deposited directly on the window layer of GaAs solar cells formed hexagonal arrays of nanoscale posts that serve as lossless photonic nanostructures for antireflection, diffraction, and light trapping in conjunction with a co-integrated rear-surface reflector. Systematic studies on optical and electrical properties and photovoltaic performance in experiments, as well as numerical modeling, quantitatively describe the optimal design rules for ultrathin, nanostructured GaAs solar cells and their integrated modules.

Nanostructured Silicon Photocathodes for Solar Water Splitting Patterned by the Self-Assembly of Lamellar Block Copolymers

We studied a type of nanostructured silicon photocathode for solar water splitting, where one-dimensionally periodic lamellar nanopatterns derived from the self-assembly of symmetric poly(styrene-block-methyl methacrylate) block copolymers were incorporated on the surface of single-crystalline silicon in configurations with and without a buried metallurgical junction. The resulting nanostructured silicon photocathodes with the characteristic lamellar morphology provided suppressed front-surface reflection and increased surface area, which collectively contributed to the enhanced photocatalytic performance in the hydrogen evolution reaction. The augmented light absorption in the nanostructured silicon directly translated to the increase of the saturation current density, while the onset potential decreased with the etching depth because of the increased levels of surface recombination. The pp(+)-silicon photocathodes, compared to the n(+)pp(+)-silicon with a buried solid-state junction, exhibited a more pronounced shift of the current density-potential curves upon the introduction of the nanostructured surface owing to the corresponding increase in the liquid/silicon junction area. Systematic studies on the morphology, optical properties, and photoelectrochemical characteristics of nanostructured silicon photocathodes, in conjunction with optical modeling based on the finite-difference time-domain method, provide quantitative description and optimal design rules of lamellar-patterned silicon photocathodes for solar water splitting.

Cross-plane Thermoelectric and Thermionic Transport across Au/ h -BN/Graphene Heterostructures

The thermoelectric voltage generated at an atomically abrupt interface has not been studied exclusively because of the lack of established measurement tools and techniques. Atomically thin 2D materials provide an excellent platform for studying the thermoelectric transport at these interfaces. Here, we report a novel technique and device structure to probe the thermoelectric transport across Au/h-BN/graphene heterostructures. An indium tin oxide (ITO) transparent electrical heater is patterned on top of this heterostructure, enabling Raman spectroscopy and thermometry to be obtained from the graphene top electrode in situ under device operating conditions. Here, an AC voltage V(ω) is applied to the ITO heater and the thermoelectric voltage across the Au/h-BN/graphene heterostructure is measured at 2ω using a lock-in amplifier. We report the Seebeck coefficient for our thermoelectric structure to be −215 μV/K. The Au/graphene/h-BN heterostructures enable us to explore thermoelectric and thermal transport on nanometer length scales in a regime of extremely short length scales. The thermoelectric voltage generated at the graphene/h-BN interface is due to thermionic emission rather than bulk diffusive transport. As such, this should be thought of as an interfacial Seebeck coefficient rather than a Seebeck coefficient of the constituent materials.

High performance ultrathin GaAs solar cells

Epitaxially grown III-V compound semiconductors, such as gallium arsenide (GaAs), can provide superior photovoltaic (PV) performance due to many attractive material properties. However, the high cost of growing device-quality epitaxial materials has prevented their widespread adoption in terrestrial applications. In this regard, decreasing thicknesses of constituent epitaxial materials without compromising their photovoltaic performance is one of conceptually viable means to lower the cost. Here we present a type of thin film GaAs PV system with drastically reduced active layer thickness (~200 nm), where dielectric periodic nanostructures and a metallic reflective element are heterogeneously integrated on the front- and back-surfaces of solar cells for nanophotonic light management to enhance the absorption and photovoltaic performance of ultrathin GaAs solar cells.

Plasmon Resonant Amplification of a Hot Electron-Driven Photodiode

We report plasmon resonant excitation of hot electrons in a photodetector based on a metal/oxide/metal (Au/Al2O3/graphene) heterostructure. In this device, hot electrons, excited optically in the gold layer, jump over the oxide barrier and are injected into the graphene layer, producing a photocurrent. To amplify this process, the bottom gold electrode is patterned into a plasmon resonant grating structure with a pitch of 500 nm. The photocurrent produced in this device is measured using 633-nm-wavelength light as a function of incident angle. We observe the maximum photocurrent at ±10° from normal incidence under irra-diation with light polarized parallel to the incident plane (p-polarization) and perpendicular to the lines on the grating, and a constant (angle-independent) photocurrent under irradiation with light polarized perpendicular to the incident plane (s-polarization) and parallel to the grating. These data show an amplification factor of 4.6× under resonant conditions. At the same angle (±10°), we also observe sharp dips in the photoreflectance corresponding to waveve-ctor matching between the incident light and the plasmon mode in the grating. In addition, finite-difference time-domain simulations predict sharp dips in the photoreflectance at ±10°, and the electric field intensity profiles show clear excitation of a plasmon resonant mode when illuminated with p-polarized light at this angle.

Enhanced Cross-plane Thermoelectric Transport of Rotationally-disordered SnSe2 via Se Vapor Annealing

We report cross-plane thermoelectric measurements of SnSe and SnSe2 films grown by the modulated element reactant (MER) approach. These materials exhibit ultralow cross-plane thermal conductivities, which are advantageous for thermoelectric energy conversion. The initially grown SnSe films have relatively low cross-plane Seebeck coefficients (-38.6 μV/K) due to significant unintentional doping originating from Se vacancies when annealed in nitrogen, as a result of the relatively high vapor pressure of Se. By performing postgrowth annealing at a fixed Se partial pressure (300 °C for 30 min using SnSe2 as the Se source in a sealed tube), a transition from SnSe to SnSe2 is induced, which is evidenced by clear changes in the X-ray diffraction patterns of the films. This results in a 16-fold increase in the cross-plane Seebeck coefficient (from -38.6 to -631 μV/K) after Se annealing due to both the SnSe-to-SnSe2 transition and the mitigation of unintentional doping by Se vacancies. We also observe a corresponding 6-fold drop in the electrical conductivity (from 3 to 0.5 S/m) after Se annealing, which is consistent with both a drop in the carrier concentration and an increase in band gap. The power factor S2σ increased by 44× (from 4.5 nW/m·K2 to 0.2 μW/m·K2) after Se annealing. We believe that these results demonstrate a robust method for mitigating unintentional doping in a promising class of materials for thermoelectric applications.

Plasmon resonant amplification of hot electron-driven photocatalysis

We report plasmon resonant excitation of hot electrons in a metal based photocatalyst in the oxygen evolution half reaction in aqueous solution. Here, the photocatalyst consists of a 100-nm thick Au film deposited on a corrugated silicon substrate. In this configuration, hot electrons photoexcited in the metal are injected into the solution, ultimately reversing the water oxidation reaction (O2 + 4H+ + 4e− ⇋ 2H2O) and producing a photocurrent. In order to amplify this process, the gold electrode is patterned into a plasmon resonant grating structure with a pitch of 500 nm. The photocurrent (i.e., charge transfer rate) is measured as a function of incident angle using 633 nm wavelength light. We observe peaks in the photocurrent at incident angles of ±9° from normal when the light is polarized parallel to the incident plane (p-polarization) and perpendicular to the lines on the grating. Based on these peaks, we estimate an overall plasmonic gain (or amplification) factor of 2.1× in the charge transfer rate. At these same angles, we also observe sharp dips in the photoreflectance, corresponding to the condition when there is wavevector matching between the incident light and the plasmon mode in the grating. No angle dependence is observed in the photocurrent or photoreflectance when the incident light is polarized perpendicular to the incident plane (s-polarization) and parallel to the lines on the grating. Finite difference time domain simulations also predict sharp dips in the photoreflectance at ±9°, and the electric field intensity profiles show clear excitation of a plasmon-resonant mode when illuminated at those angles with p-polarized light.We report plasmon resonant excitation of hot electrons in a metal based photocatalyst in the oxygen evolution half reaction in aqueous solution. Here, the photocatalyst consists of a 100-nm thick Au film deposited on a corrugated silicon substrate. In this configuration, hot electrons photoexcited in the metal are injected into the solution, ultimately reversing the water oxidation reaction (O2 + 4H+ + 4e− ⇋ 2H2O) and producing a photocurrent. In order to amplify this process, the gold electrode is patterned into a plasmon resonant grating structure with a pitch of 500 nm. The photocurrent (i.e., charge transfer rate) is measured as a function of incident angle using 633 nm wavelength light. We observe peaks in the photocurrent at incident angles of ±9° from normal when the light is polarized parallel to the incident plane (p-polarization) and perpendicular to the lines on the grating. Based on these peaks, we estimate an overall plasmonic gain (or amplification) factor of 2.1× in the charge transfer rate...

Enhanced thermoelectric efficiency in topological insulator Bi2Te3 nanoplates via atomic layer deposition-based surface passivation

We report in-plane thermoelectric measurements of Bi2Te3 nanoplates, a typical topological insulator with Dirac-like metallic surface states, grown by chemical vapor deposition. The as-grown flakes exposed to ambient conditions exhibit relatively small thermopowers around −34 μV/K due to unintentional surface doping (e.g., gas adsorption and surface oxidation). After removal of the unintentional surface doping and surface passivation by deposition of 30 nm of Al2O3 using atomic layer deposition (ALD), the Seebeck coefficient of these flakes increases by a factor of 5× to −169 μV/K. Here, we believe that the ALD-based surface passivation can prevent the degradation of the thermoelectric properties caused by gas adsorption and surface oxidation processes, thus, reducing the unintentional doping in the Bi2Te3 and increasing the Seebeck coefficient. The high surface-to-volume ratio of these thin (∼10 nm thick) nanoplates make them especially sensitive to surface doping, which is a common problem among nanomaterials in general. An increase in the sample resistance is also observed after the ALD process, which is consistent with the decrease in doping.We report in-plane thermoelectric measurements of Bi2Te3 nanoplates, a typical topological insulator with Dirac-like metallic surface states, grown by chemical vapor deposition. The as-grown flakes exposed to ambient conditions exhibit relatively small thermopowers around −34 μV/K due to unintentional surface doping (e.g., gas adsorption and surface oxidation). After removal of the unintentional surface doping and surface passivation by deposition of 30 nm of Al2O3 using atomic layer deposition (ALD), the Seebeck coefficient of these flakes increases by a factor of 5× to −169 μV/K. Here, we believe that the ALD-based surface passivation can prevent the degradation of the thermoelectric properties caused by gas adsorption and surface oxidation processes, thus, reducing the unintentional doping in the Bi2Te3 and increasing the Seebeck coefficient. The high surface-to-volume ratio of these thin (∼10 nm thick) nanoplates make them especially sensitive to surface doping, which is a common problem among nanomat...

Ultra-Low Power Light Emission via Avalanche and Sub-avalanche Breakdown in Suspended Carbon Nanotubes

We explore near-infrared light emission in suspended carbon nanotube field effect transistors over a wide range of gate and bias voltages. An abrupt increase in both the electric current (90 μA/V) and electroluminescence intensity is observed at high bias voltages (∼3.5 V), when gated in the “off’ state (i.e., Vgate > 0 V). For bias voltages below the threshold for avalanche breakdown, we observe light emission due to the creation of exitons by impact ionization under these high electric fields. Here, we find that there is a relatively small region over which low power (∼nW) light emission is observed. By plotting the relative luminescence efficiency (i.e., light intensity/electrical power) as a function of the gate and bias voltages, we observe a very sharp feature corresponding to avalanche emission at which the electroluminescence efficiency is 2–3 orders of magnitude higher than that under all other conditions of gate and bias voltage. A steep increase in the current with bias voltage (i.e., large dI/...