Sachit Grover

Applicability of Metal/Insulator/Metal (MIM) Diodes to Solar Rectennas

The current-voltage (I-V) characteristics of metal/insulator/metal (MIM) diodes illuminated at optical frequencies are modeled using a semiclassical approach that accounts for the photon energy of the radiation. Instead of classical small-signal rectification, in which a continuous span of the dc I-V curve is sampled during rectification, at optical frequencies, the radiation samples the dc I-V curve at discrete voltage steps separated by the photon energy (divided by the electronic charge). As a result, the diode resistance and responsivity differ from their classical values. At optical frequencies, a diode with even a moderate forward-to-reverse current asymmetry exhibits high quantum efficiency. An analysis is carried out to determine the requirements imposed by the operating frequency on the circuit parameters of antenna-coupled diode rectifiers, which are also called rectennas. Diodes with low resistance and capacitance are required for the RC time constant of the rectenna to be smaller than the reciprocal of the operating frequency and to couple energy efficiently from the antenna. Existing MIM diodes do not meet the requirements to operate efficiently at visible-to-near-infrared wavelengths.

Traveling-Wave Metal/Insulator/Metal Diodes for Improved Infrared Bandwidth and Efficiency of Antenna-Coupled Rectifiers

We evaluate a technique to improve the performance of antenna-coupled diode rectifiers working in the IR. Efficient operation of conventional, lumped-element rectifiers is limited to the low terahertz. By using femtosecond-fast MIM diodes in a traveling-wave (TW) configuration, we obtain a distributed rectifier with improved bandwidth. This design gives higher detection efficiency due to a good match between the antenna impedance and the geometry-controlled impedance of the TW structure. We have developed a method for calculating the responsivity of the antenna-coupled TW detector. Three TW devices, made from different materials, are simulated to obtain their impedance and responsivity at 1.5, 3, 5, and 10 μm wavelengths. The characteristic impedance of a 100-nm-wide TW is in the range of 50 Ω and has a small variation with frequency. A peak responsivity of 0.086 A/W is obtained for the Nb-Nb2 O5 -Nb TW diode at 3-μm wavelength. This corresponds to a quantum efficiency of 3.6% and is a significant improvement over the antenna-coupled lumped-element diode rectifiers. For IR imaging, this results in a normalized detectivity of 4 × 106 Jones at 3 μm. We have identified several ways for improving the detectivity of the TW detector. Possible methods include decreasing the diode resistance, reducing the noise, and increasing the effective antenna area.

Graphene geometric diodes for terahertz rectennas

We demonstrate a new thin-film graphene diode called a geometric diode that relies on geometric asymmetry to provide rectification at 28 THz. The geometric diode is coupled to an optical antenna to form a rectenna that rectifies incoming radiation. This is the first reported graphene-based antenna-coupled diode working at 28 THz, and potentially at optical frequencies. The planar structure of the geometric diode provides a low RC time constant, on the order of 10−15 s, required for operation at optical frequencies, and a low impedance for efficient power transfer from the antenna. Fabricated geometric diodes show asymmetric current–voltage characteristics consistent with Monte Carlo simulations for the devices. Rectennas employing the geometric diode coupled to metal and graphene antennas rectify 10.6 µm radiation, corresponding to an operating frequency of 28 THz. The graphene bowtie antenna is the first demonstrated functional antenna made using graphene. Its response indicates that graphene is a suitable terahertz resonator material. Applications for this terahertz diode include terahertz-wave and optical detection, ultra-high-speed electronics and optical power conversion.

Rectenna solar cells

Description based on online resource; title from PDF title page (ebrary, viewed October 11, 2013).

Quantum theory of operation for rectenna solar cells

Optical rectennas, sub-micrometre antenna-coupled diodes, can directly rectify solar and thermal electromagnetic radiation, and have been proposed as an alternative to conventional semiconductor photovoltaics. We develop a comprehensive description of the operating principle of rectenna solar cells. In prior work classical concepts from microwave rectenna theory have been applied to the analysis of photovoltaic power generation using these ultra-high frequency rectifiers. Because of their high photon energy the interaction of petahertz frequency waves with fast-responding diodes requires a semiclassical analysis. We use the theory of photon-assisted transport to derive the current–voltage [I(V)] characteristics of metal/insulator/metal tunnel diodes under illumination. We show how power is generated in the second quadrant of the I(V) characteristic, derive solar cell parameters, and analyse the key variables that influence the performance under monochromatic radiation and to a first order approximation. The efficiency improves with reduced dark current under reverse bias and increasing incident electromagnetic power.

Reformulation of solar cell physics to facilitate experimental separation of recombination pathways

Experimentally identifying the spatial distribution of recombination in a solar cell is challenging, with only semi-quantitative information available from conventional characterization techniques. We develop a formulation of solar cell physics, based upon well-justified analytic approximations, to quantitatively extract information about recombination in different cell regions. We derive the dependence of VOC on light-intensity, temperature, and strength of recombination in the space-charge, quasi-neutral, and interface regions. Expanding the scope and utility of commonly used characterization techniques, we apply this formulation to evaluate the spatial distribution of recombination in exemplary crystalline silicon heterojunction and polycrystalline Cu(In,Ga)Se2 solar cells.

Pyramidal light trapping and hydrogen passivation for high-efficiency heteroepitaxial (100) crystal silicon solar cells

We report growth and characterization of heteroepitaxial silicon solar cells on sapphire to demonstrate the promise of heteroepitaxial crystal silicon (c-Si) film photovoltaics on inexpensive substrates coated with chemically inert crystalline buffer layers such as Al2O3. Our work isolates and addresses critical material and light-trapping issues that must be solved to develop film c-Si solar cells. Microscopy reveals high dislocation densities and other crystalline defects in the silicon layers, and these defects limit the unhydrogenated devices with a 1.5 μm absorber layer to below 1% sunlight-to-electricity conversion efficiency. By exposing an identical device to atomic H from a remote plasma, we demonstrate a 5.2% efficient device with dramatically improved quantum efficiency (QE) and open circuit voltage, as the minority carrier diffusion length increases from ∼1 μm to ∼4.5 μm. When we incorporate both hydrogen passivation and top surface pyramidal light trapping we further improve the QE and achieve 6.8% efficiency.

Carrier-selective, passivated contacts for high efficiency silicon solar cells based on transparent conducting oxides

We describe the design, fabrication and results of passivated contacts to n-type silicon utilizing thin SiO 2 and indium tin oxide. High-temperature silicon dioxide is grown on both surfaces on an n-type Si wafer to a thickness 0,contact , and a non-ohmic, high contact resistance. However, after a forming gas anneal, the passivation quality and the contact resistivity improve significantly. The contacts are characterized by measuring the recombination parameter current density of the contact (J 0,contact ) and the specific contact resistivity (ρ contact ) using a transmission line method (TLM) pattern. The best ITO/SiO 2 passivated contact in this study has J 0,contact = 93.5 fA/cm2 and ρ contact = 11.5 mOhm-cm2. These values are placed in context with other passivating contacts using an analysis that determines the ultimate efficiency and the optimal area fraction for contacts for a given set of (J 0,contact , ρ contact ) values. The ITO/SiO 2 contacts are found to have a higher J 0,contact , but a similar ρ contact compared to the best reported passivated contacts.

Optical rectenna solar cells using graphene geometric diodes

A solar cell using micro-antennas to convert radiation to alternating current and ultrahigh-speed diodes to rectify the AC can in principle provide extremely high conversion efficiencies. Currently investigated rectennas using metal/insulator/metal (MIM) diodes are limited in their RC response time and have poor impedance matching to the antenna. We have investigated a new rectifier, referred to as a geometric diode, which can overcome these limitations. The geometric diode consists of a conducting thin-film, such as graphene, patterned in a geometry that leads to diode behavior. We have experimentally demonstrated geometric diodes made from graphene and simulated their characteristics using the Drude model for charge transport. Here we compare the characteristics of rectennas using MIM diodes with those based on geometric diodes and show the improved performance of the latter.

Device Physics of Heteroepitaxial Film c-Si Heterojunction Solar Cells

We characterize heterojunction solar cells made from single-crystal silicon films grown heteroepitaxially using hot-wire chemical vapor deposition (HWCVD). Heteroepitaxy-induced dislocations limit the cell performance, providing a unique platform to study the device physics of thin crystal Si heterojunction solar cells. Hydrogen passivation of these dislocations enables an open-circuit voltage VOC close to 580 mV. However, dislocations are partially active, even after passivation. Using standard characterization methods, we compare the performance of heteroepitaxial absorbers with homoepitaxial absorbers that are free of dislocations. Heteroepitaxial cells have a smaller diffusion length and a larger ideality factor, indicating stronger recombination, which leads to inefficient current collection and a lower VOC than homoepitaxial cells. Modeling indicates that the recombination in the inversion layer of heterojunction cells made from defective absorbers is comparable with the overall recombination in the bulk. Temperature-dependent VOC measurements point to significant recombination at the interface that is attributable to the presence of dislocations.