Light trapping structures for photovoltaics using silicon nanowires and silicon micro-pyramids

The current photovoltaic industry is dominated by crystalline or poly-crystalline Si in a planar pn-junction configuration. The use of silicon nanowire arrays (SiNWA) within this industry has shown great promise due to its application as an anti-reflective layer, as well as benefits in charge carrier extraction. In this work, we use a metal assisted chemical etch process to fabricate SiNWAs onto a dense periodic array of pyramids that are formed using an alkaline etch masked with an oxide layer. The hybrid micro-nano structure acts as an anti-reflective coating with experimental reflectivity below 1% over the visible and near-infrared spectral regions. This represents an improvement of up to 11 and 14 times compared to the pyramid array and SiNWAs on bulk, respectively. In addition to the experimental work, we optically simulate the hybrid structure using the commercial Lumerical FDTD package. The results of the optical simulations support our experimental work, illustrating a reduced reflectivity in the hybrid structure. The nanowire array increases the absorbed carrier density within the pyramid by providing a guided transition of the refractive index along the light path from air into the silicon. Furthermore, electrical simulations which take into account surface and Auger recombination show an effi ciency increase for the hybrid structure of 56% over bulk, 11% over pyramid array and 8.5% over SiNWAs. Opto-electronic modelling was performed by establishing a tool flow to integrate the eff ective optical simulator Lumerical FDTD with the excellent fabrication and electrical simulation capability of Sentaurus TCAD. Interfacing between the two packages is achieved through tool command language and Matlab, off ering fast and accurate electro-optical characteristics of nano-structured PV devices.Open Acces

Copper-based p-type semiconducting oxides: from materials to devices

IDS FunMa

Antireflection Patterning of Si for InAsSb Nanowire Infrared Photodetectors

The pattering of an optical surface with physical features like with very low reflectivity and high transmission of IR or visible light can be used e.g. for antireflective windows for solar cells and photodetectors. Here we present results on antireflective patterning of Si for back side illuminated InAsSb nanowire long wavelength IR (8-15 µm) photodetectors. Photolithography and dry etching techniques are used to pattern Si into uniform pillars and by using a Fourier transform infrared spectrometer we measure the transmittance. We use numerical simulations to aid the design of the dimensions of Si pillars. Results from transmittance measurements exhibit diameter dependent peaks that are in good agreement with the numerical simulations. The numerical results reveal that the transmission peaks are due to resonant wave guiding modes that enhance the coupling of light into the Si pillars. We have found that with the proper selection of pillar diameter a resonance enhanced transmission peak at specific wavelength can be achieved. We have found that the transmission of IR through bared Si is 53% which is independent of wavelength. Arrays of Si pillars with mean diameter 1.33 μm ± 36 nm show transmission peak at wavelength of 6.2 μm and array of Si pillars having mean diameter 1.42 μm ± 35 nm shifts the transmission peak at wavelength of 7.5 μm. Doughnut shaped Si hollow pillars are also observed as a result of photolithography. The transmission spectrum of Si hollow pillar arrays is also analyzed

Silicon Nanopillar Solar Cells Made by Near Field Phase-Shift Photolithography

A novel photolithographic technique which promises quick and easy large area patterning is explored to satisfy the nanowire applications requirements. Nanohole patterns for ordered nanowire growth and nanopillar arrays for PV applications are produced by means of Near Field Phase-Shift Photolithography. Furthermore, solar cells based on silicon nanopillars have been fabricated showing high e ciencies. The quality of di erent passivating materials was evaluated in this particular approach, concluding that a double layer of SiO2/SiNx is the most appropriate.Outgoin

Optimisation of coated nanowire solar cells by simulation

Radial junction nanowire solar cells are expected to give a higher performance than planar junction solar cells. This is due to the enhanced optical path length for absorption, and the shorter distance for excitons to travel to interfaces where they can dissociate. By means of simulation, we have optimised, with respect to geometrical parameters, solar cells consisting of light absorber coated metal- oxide nanowires. We have primarily studied nanowires with a TiO2 core and CdSe shell, as well as ZnO cores with PbS nanoparticles. Optical scattering and absorption is determined using Finite-Difference Time-Domain (FDTD) simulations. Charge transport is studied using finite-difference drift-diffusion modelling. The generation rate calculated from FDTD is inputted into the charge transport model. The aim is to determine which combination of nanowire parameters (such as coating thickness, height, radius and spacing) lead to the the highest power efficiencies. We found that core and shell thickness has more effect on absorption than nanowire arrangement.Open Acces

Designing and Evaluating the Growth and Doping of Group IV Nanowires for Photovoltaic Applications

Group IV nanowires (NWs) have become an important class of materials in the fields of photovoltaics, photonics, sensors, microelectromechanical systems, and thermoelectrics. Dopants within the NWs dramatically modify their conductivity and transport properties, whether the dopants were added intentionally or unintentionally during the growth process. Despite recent progress made in the development of nanowire photovoltaics and field effect transistors, the relationship between the ionized dopants, electrical mobility and device performance remains debated since the behavior of dopants in bulk materials may not be completely transferrable to the nanoscale. In this dissertation, we study several different aspects of group IV NWs, particularly Si and Ge. For metal-catalyzed GeNWs synthesized by a solution-based method, we investigated the unusually high chemical doping of the catalyst atoms near or even above the solubility limit within the GeNWs. We then examined the active doping level by manipulating the backgate potential of the fabricated metal oxide semiconductor field effect transistor (MOSFET) devices, extracting active doping levels and electrical mobility values from a fit to the conductivity data. Even though a high atomic percentage of dopants can be incorporated in the NWs, we found that in general only a small fraction of the dopants are electrically active. For example, only 10% of the dopants from both solution-grown GeNWs and CVD-synthesized SiNWs were active. With the help of finite-element modeling tools, backgate MOSFET conductivity fitting can be used as a general strategy for probing charge carrier types, electrical mobility, and active doping levels. Finally, we investigated radially-doped SiNW as photovoltaic (PV) devices. We compare the PV performance of different types of radially-structured SiNW shells and their photovoltaic performance, including crystalline, amorphous and dielectric shell morphologies created through various synthetic approaches, including thermal CVD, plasma enhanced CVD, and catalyst poisoning strategies. We found out that SiNWs with crystalline shell p/i/n junction has intense built-in electric fields of ~10^7 V/m within its well defined depletion region. The very high IQE values throughout the NW structure make it insensitive to surface recombination. By depositing additional dielectric layer on these SiNWs we could boost the short circuit current density by up to 80%. Finite-element analysis was also used to gauge the photovoltaic performances of the amorphous shell SiNWs, providing design guidelines on the optimization parameters of the growth strategy.Doctor of Philosoph

Semiconductor Nanowire Based Piezoelectric Energy Harvesters: Modeling, Fabrication, and Characterization

Semiconductor nanowire (NW) arrays’ unique advantages over bulk forms, including enhanced surface area, high mechanical flexibility, high sensitivity to small forces, better charge collection, and enhanced light absorption through trapping, make them ideal templates on which to build other structures. This research on the piezoelectric behavior of NWs used in high-performance energy harvesters is based on device modeling, fabrication, and characterization. These activities optimize the electrical properties of a NW device in response to a compression/release force applied to the NWs. The dissertation first discusses the piezoelectric and semiconductor properties of wurtzite compound nanomaterials, emphasizing III-nitride semiconducting InN and GaN NWs. Static analysis identifies the role of carrier density, temperature, force, length/diameter ratio, and Schottky barrier height. Piezoelectric nanogenerators (NGs) based on vertically aligned InN nanowires (NWs) are fabricated, characterized, and evaluated. In these NGs, arrays of exclusively either p-type or intrinsic InN NWs prepared by plasma-assisted molecular beam epitaxy (MBE) demonstrate similar average piezoelectric properties. The p-type NGs show 160% more output current and 70% more output power product than the intrinsic NGs. The features driving performance enhancement are reduced electrostatic losses due to a higher NW areal density and longer NWs, and improved electromechanical energy conversion efficiency due to smaller NW diameters. These findings highlight the potential of InN based NGs as a power source for self-powered systems and the importance of NW morphology in overall NG performance. The second part is devoted to demonstrate a series of flexible transparent ZnO p-n homojunction nanowire (NW)-based piezoelectric nanogenerators (NGs) with different p-doping concentrations. The lithium-doped segments are grown directly and consecutively on top of intrinsic nanowires (n-type). When characterized under cyclic compressive strains, the overall NG performance is enhanced by up to eleven-fold if the doping concentration is properly controlled. This improvement is attributable to reduction in the mobile charge screening effect and optimization of the NGs’ internal electrical characteristics. Experimental results also show that an interfacial MoO3 barrier layer, at an optimized thickness of 5-10 nm, reduces leakage current and substantially improves piezoelectric NG performance. The third part presents the first cascade-type compact hybrid energy cell (CHEC) that is capable of simultaneously or individually harvesting solar and strain energies. It is made of an n-p junction NW-based piezoelectric nanogenerator to harvest strain energy and an nc/a-Si:H single junction cell to harvest solar energy. The CHECs ability to harvest energy effectively simultaneously, and complementary is demonstrated by deploying six CHECs to power LEDs and a wireless strain gauge sensor node. Under ~10 mW/cm2 illumination and vibrations of 3 m/s2 at 3 Hz frequency, the output current and voltage from a single 1.0 cm2 CHEC are 280 μA and 3.0 V, respectively; enough to drive many low power commercial electronics. This dissertation aims to deepen understanding of the piezoelectric behavior of semiconductor NWs on hard and flexible substrates. Thus, this research in the field of nanopiezoelectrics could have a substantial impact on many areas, ranging from the fundamental study of new nanomaterial properties and mechanical effects in nanostructures to diverse applications like aerospace

Nanoscale Semiconductor Materials and Devices Employing Hybrid 1D and 2D Structures for Tunable Electronic and Photonic Applications

Das, Suprem R. Ph.D., Purdue University, December 2013. Nanoscale Semiconductor Materials and Devices employing Hybrid 1D and 2D structures for Tunable Electronic and Photonic Applications. Major Professor: Dr. David B. Janes. Continued miniaturization of microelectronic devices over past decades has brought the device feature size towards the physical limit. Likewise, enormous `waste energy\u27 in the form of self-heating in almost all of the electronic and optoelectronic devices needs an `energy-efficient low power\u27 and `high performance\u27 material as well as device with alternate geometry. III-V semiconductors are proven to be one of the alternate systems of materials for various applications including CMOS devices, low power and high performance transistor devices, power transistors, as well as thermoelectric applications. InSb, being the bulk semiconductor with lowest bandgap, highest mobility, low effective mass, and highest spin–orbit coupling has potential of providing numerous novel applications. Also, InSb in nanowire form has not been explored in many aspects. First part of this thesis explores the possibility of growing InSb nanowires using solution based electrodeposition technique followed by field effect transistor studies. InSb nanowires have recently shown very promising magneto-transport properties at low temperatures and with magnetic field due to its high spin orbit coupling. This thesis demonstrates initial low temperature device studies on hybrid devices with InSb channel and superconducting electrodes (aluminum). In the last section of InSb nanowire studies, the thesis explores hierarchial branched nanowires with different diameters that demonstrate near unity optical absorption in UV–VIS regime and wavelength dependent absorption in near infrared (NIR) regime. A photonic coupling model was developed to explain the phenomena. The unique photonic properties of the structurally tailored branched nanowire arrays could be used to devise new types of photonic, optoelectronics and/or photovoltaic devices. The second half of the thesis explores another class of hybrid material structure involving 2D semiconductor/semimetal ‘Graphene’ and 1D silver nanowires. While the ultimate goal was to push the limit of ‘transparent and flexible technology’ the thesis, also critically explores the physics of percolation doping to beat the conduction–transparency bottleneck. The thesis demonstrates theory of ‘co-percolation’ involving two individual networks in which the invidual\u27s weakness is circumvented by the other. This study not only applies to the particular system chosen but also could be readily applied to any large scale 2D–1D nanoscale systems such as layered semiconductors, topological insulators and nanowires