Adam W. Sood

Broadband nanostructured antireflection coating on glass for photovoltaic applications

Ultra-high, omnidirectional transmittance through a coated glass window is demonstrated over the entire accessible portion of the solar spectrum. The average broadband transmittance has been increased to greater than 98.5% at normal incidence, and exceeds 97.8% at all wavelengths between 440 nm and 1800 nm, significantly outperforming conventional MgF2 coated glass. The measured improvement in transmittance results from coating the window with a new class of materials consisting of porous SiO2 nanorods. The step-graded antireflection structure also exhibits excellent omnidirectional performance, enabling average broadband transmittance in excess of 96% at incident angles as high as 70°.

Ultra-high transmittance through nanostructure-coated glass for solar cell applications

Ultra-high, broadband transmittance through coated glass windows is demonstrated over a wide range of incident angles. Near perfect 100% transmittance through a glass substrate has been achieved over select spectral bands, and the average transmittance increased to over 97% for photons incident between 0° and 75° with wavelengths between 400 nm and 1600 nm. The measured improvements in transmittance result from coating the windows with a new class of materials consisting of porous SiO2 nanorods.

Characterization of SiGe-detector arrays for visible-NIR imaging sensor applications

SiGe based focal plane arrays offer a low cost alternative for developing visible- near-infrared focal plane arrays that will cover the spectral band from 0.4 to 1.6 microns. The attractive features of SiGe based foal plane arrays take advantage of silicon based technology that promises small feature size, low dark current and compatibility with the low power silicon CMOS circuits for signal processing. This paper discusses performance characteristics for the SiGe based VIS-NIR Sensors for a variety of defense and commercial applications using small unit cell size and compare performance with InGaAs, InSb, and HgCdTe IRFPAs. We present results on the approach and device design for reducing the dark current in SiGe detector arrays. The electrical and optical properties of SiGe arrays at room temperature are discussed. We also discuss future integration path for SiGe devices with Si-MEMS Bolometers.

Large-area nanostructured self-assembled antireflection coatings for photovoltaic devices

The scalability of nanostructured, self-assembled antireflection (AR) coatings has been demonstrated on 6-inch glass and silicon wafers. Ultra-high transmittance through these large-area coatings has been confirmed by measuring the short circuit current of a CIGS-based thin film photovoltaic (PV) device placed below the large-area AR-coated glass wafer. At normal light incidence, the light transmitted through the AR coated glass wafer yields 5% more short-circuit current compared to the uncoated glass wafer. At off-angle incidence, the light transmitted through the AR-coated wafer yields nearly 20% higher short-circuit current compared to light transmitted through an uncoated glass wafer. The large-area AR coating preserves ultra-high transmittance over a wide range of incident angles and has the potential to enhance PV device performance from dawn to dusk.

Development of large area nanostructured AR coatings for EO/IR sensor applications

Electro-optical/infrared nanosensors are being developed for a variety of defense and commercial systems applications. One of the critical technologies that will enhance EO/IR sensor performance is the development of advanced antireflection coatings with both broadband and omnidirectional characteristics. In this paper, we review our latest work on high quality nanostructure-based antireflection structures, including recent efforts to deposit nanostructured antireflection coatings on large area substrates. Nanostructured antireflection coatings fabricated via oblique angle deposition are shown to enhance the optical transmission through transparent windows by minimizing broadband reflection losses to less than one percent, a substantial improvement over conventional thin-film antireflection coating technologies. Step-graded antireflection structures also exhibit excellent omnidirectional performance, and have recently been demonstrated on 6-inch diameter substrates.

High-voltage quantum well waveguide solar cells

Photon absorption, and thus current generation, is hindered in conventional thin-film solar cell designs, including quantum well structures, by the limited path length of incident light passing vertically through the device structure. Optical scattering into lateral waveguide structures provides a physical mechanism to dramatically increase photocurrent generation through in-plane light trapping. However, the insertion of wells of high refractive index material with lower energy gap into the device structure often results in lower voltage operation, and hence lower photovoltaic power conversion efficiency. In this work, we demonstrate that the voltage output of an InGaAs quantum well waveguide photovoltaic device can be increased by employing a novel III-V material structure with an extended wide band gap emitter heterojunction. Analysis of the light IV characteristics from small area test devices reveals that nonradiative recombination components of the underlying dark diode current have been reduced, exposing the limiting radiative recombination component and providing a pathway for realizing solar-electric conversion efficiency of 30% or more in single-junction cells.

Flexible, high-efficiency solar cells: approaches and advanced design concepts

Flexible photovoltaic cells with a specific power of more than 275 W/kg have been demonstrated by depositing copper indium gallium diselenide (CIGS) absorber layers on ultra lightweight and highly durable titanium foil. Advanced device designs employing nanostructured optical coatings and exploiting optical cavity effects provide a pathway to further increase power-generating capability. Single-junction CIGS devices can potentially outperform multi-junction III-V structures in some environments, including under high air mass terrestrial spectrums.

Development of nanostructure based antireflection coatings for EO/IR sensor applications

EO/IR Nanosensors are being developed for a variety of Defense and Commercial Systems Applications. These include UV, Visible, NIR, MWIR and LWIR Nanotechnology based Sensors. The conventional SWIR Sensors use InGaAs based IR Focal Plane Array (FPA) that operate in 1.0-1.8 micron region. Similarly, MWIR Sensors use InSb or HgCdTe based FPA that is sensitive in 3-5 micron region. More recently, there is effort underway to evaluate low cost SiGe visible and near infrared band that covers from 0.4 to 1.6 micron and beyond to 1.8 microns. One of the critical technologies that will enhance the EO/IR sensor performance is the development of high quality nanostructure based antireflection coating. In this paper, we will discuss our modeling approach and experimental results for using oblique angle nanowires growth technique for extending the application for UV, Visible and NIR sensors and their utility for longer wavelength application. The AR coating is designed by using a genetic algorithm and fabricated by using oblique angle deposition. The AR coating is designed for the wavelength range of 250 nm to 2500 nm and 0° to 40° angle of incidence. These nanostructure AR coatings have shown to enhance the optical transmission in the band of interest and minimize the reflection loss to less than 3 percent substantial improvement from the thin film AR coatings technology.

Nanostructure based EO/IR sensor development for homeland security applications

Next Generation EO/IR focal plane arrays using nanostructure materials are being developed for a variety of Defense and Homeland Security Sensor Applications. Several different nanomaterials are being evaluated for these applications. These include ZnO nanowires, GaN Nanowires and II-VI nanowires, which have demonstrated large signal to noise ratio as a wide band gap nanostructure material in the UV band. Similarly, the work is under way using Carbon Nanotubes (CNT) for a high speed detector and focal plane array as two-dimensional array as bolometer for IR bands of interest, which can be implemented for the sensors for homeland security applications. In this paper, we will discuss the sensor design and model predicting performance of an EO/IR focal plane array and Sensor that can cover the UV to IR bands of interest. The model can provide a robust means for comparing performance of the EO/IR FPAs and Sensors that can operate in the UV, Visible-NIR (0.4- 1.8μ), SWIR (2.0-2.5μ), MWIR (3-5μ), and LWIR bands (8-14μ). This model can be used as a tool for predicting performance of nanostructure arrays under development. We will also discuss our results on growth and characterization of ZnO nanowires and CNTs for the next generation sensor applications. We also present several approaches for integrated energy harvesting using nanostructure based solar cells and Nanogenerators that can be used to supplement the energy required for nanostructure based sensors.

NANOSTRUCTRE BASED ANTIREFLECTION COATINGS FOR EO/IR SENSOR APPLICATIONS

EO/IR Nanosensors are being developed for a variety of Defense and Commercial Systems Applications. These include UV, Visible, NIR, MWIR and LWIR Nanotechnology based Sensors. The conventional SWIR Sensors use InGaAs based IR Focal Plane Array (FPA) that operate in 1.0-1.8 micron region. Similarly, MWIR Sensors use InSb or HgCdTe based FPA that is sensitive in 3-5 micron region. More recently, there is effort underway to evaluate low cost SiGe visible and near infrared band that covers from 0.4 to 1.6 micron. One of the critical technologies that will enhance the EO/IR sensor performance is the development of high quality nanostructure based antireflection coating. Prof. Fred Schubert and his group have used the TiO2 and SiO2 graded-index nanowires / nanorods deposited by oblique-angle deposition, and, for the first time, demonstrated their potential for antireflection coatings by virtually eliminating Fresnel reflection from an AlN-air interface over the UV band. This was achieved by controlling the refractive index of the TiO2 and SiO2 nanorod layers, down to a minimum value of n = 1.05, the lowest value so far reported. In this paper, we will discuss our modeling approach and experimental results for using oblique angle nanowires growth technique for extending the application for UV, Visible and NIR sensors and their utility for longer wavelength application.