Gideon Segev

The electronic structure of metal oxide/organo metal halide perovskite junctions in perovskite based solar cells.

Cross-sections of a hole-conductor-free CH3NH3PbI3 perovskite solar cell were characterized with Kelvin probe force microscopy. A depletion region width of about 45 nm was determined from the measured potential profiles at the interface between CH3NH3PbI3 and nanocrystalline TiO2, whereas a negligible depletion was measured at the CH3NH3PbI3/Al2O3 interface. A complete solar cell can be realized with the CH3NH3PbI3 that functions both as light harvester and hole conductor in combination with a metal oxide. The band diagrams were estimated from the measured potential profile at the interfaces, and are critical findings for a better understanding and further improvement of perovskite based solar cells.

Loss mechanisms and back surface field effect in photon enhanced thermionic emission converters

Photon Enhanced Thermionic Emission (PETE) solar converters are based on emission of energetic electrons from a semiconductor cathode that is illuminated and heated with solar radiation. By using a semiconductor cathode, photo generated electrons enable high electron emission at temperatures much lower than the common range for thermionic emitters. Simple models show that PETE conversion can theoretically reach high efficiency, for example, above 40% at concentration of 1000 suns. In this work, we present a detailed one-dimensional model of PETE conversion, accounting for recombination mechanisms, surface effects, and spatial distribution of potential and carrier concentration. As in the previous PETE models, negative space charge effects, photon recycling, and temperature gradients are not considered. The conversion efficiency was calculated for Si and GaAs based cathodes under a wide range of operating conditions. The calculated efficiencies are lower than predictions of previous zero-dimensional models. We analyze the loss mechanisms and show that electron recombination at the cathode contact is a significant loss. An electron-blocking junction at the cathode back contact is therefore essential for achieving high efficiency. The predicted efficiencies for Si and GaAs cathodes with homo-junction back surface field layers are both around 31%, but with more favorable assumptions on the contact structure, it may be near 40%. The analysis leads to important conclusions regarding the selection of cathode material and back surface junction configuration.

Solar energy conversion with photon-enhanced thermionic emission

© 2016 IOP Publishing Ltd. Photon-enhanced thermionic emission (PETE) converts sunlight to electricity with the combined photonic and thermal excitation of charge carriers in a semiconductor, leading to electron emission over a vacuum gap. Theoretical analyses predict conversion efficiency that can match, or even exceed, the efficiency of traditional solar thermal and photovoltaic converters. Several materials have been examined as candidates for radiation absorbers and electron emitters, with no conclusion yet on the best set of materials to achieve high efficiency. Analyses have shown the complexity of the energy conversion and transport processes, and the significance of several loss mechanisms, requiring careful control of material properties and optimization of the device structure. Here we survey current research on PETE modeling, materials, and device configurations, outline the advances made, and stress the open issues and future research needed. Based on the substantial progress already made in this young topic, and the potential of high conversion efficiency based on theoretical performance limits, continued research in this direction is very promising and may yield a competitive technology for solar electricity generation.

Negative space charge effects in photon-enhanced thermionic emission solar converters

In thermionic energy converters, electrons in the gap between electrodes form a negative space charge and inhibit the emission of additional electrons, causing a significant reduction in conversion efficiency. However, in Photon Enhanced Thermionic Emission (PETE) solar energy converters, electrons that are reflected by the electric field in the gap return to the cathode with energy above the conduction band minimum. These electrons first occupy the conduction band from which they can be reemitted. This form of electron recycling makes PETE converters less susceptible to negative space charge loss. While the negative space charge effect was studied extensively in thermionic converters, modeling its effect in PETE converters does not account for important issues such as this form of electron recycling, nor the cathode thermal energy balance. Here, we investigate the space charge effect in PETE solar converters accounting for electron recycling, with full coupling of the cathode and gap models, and addressing conservation of both electric and thermal energy. The analysis shows that the negative space charge loss is lower than previously reported, allowing somewhat larger gaps compared to previous predictions. For a converter with a specific gap, there is an optimal solar flux concentration. The optimal solar flux concentration, the cathode temperature, and the efficiency all increase with smaller gaps. For example, for a gap of 3 μm the maximum efficiency is 38% and the optimal flux concentration is 628, while for a gap of 5 μm the maximum efficiency is 31% and optimal flux concentration is 163.

Multiple State Electrostatically Formed Nanowire Transistors

Electrostatically formed nanowire (EFN)-based transistors have been suggested in the past as gas sensing devices. These transistors are multiple gate transistors in which the source to drain conduction path is determined by the bias applied to the back gate, and two junction-side gates. If a specific bias is applied to the side gates, the conduction band electrons between them are confined to a well-defined area forming a narrow channel-the EFN. By applying a nonsymmetric bias on the side gates, the lateral position of the EFN can be controlled. We propose a novel multiple state EFN transistor (MSET) that utilizes this degree of freedom for the implementation of complete multiplexer functionality in a single device. The basic device functionality was verified through simulation of MSETs with three and four well defined conduction states. The multiplexer functionality allows a very simple implementation of binary and multiple valued logic functions.

Single bandgap solar converters unbounded by the Shockley Queisser limit

The Shockley-Queisser limit is known to set the uppermost limit on conversion efficiency for conventional single-bandgap solar cells. In past years different approaches were suggested in order to overcome this limit. Yet, apart for multi-junction cells that are inherently complicated devices, none were realized successfully. Photon-enhanced thermionic emission (PETE) conversion is similar to photovoltaic conversion, but relies on emission of photo-generated electrons across a vacuum gap between two surfaces having different work functions. Heat produced in PV cells is a loss of energy, and an increase in temperature decreases the cell efficiency. On the contrary, in PETE converters, heat produced by thermalization and non-radiative recombination increases the cathode temperature and with it the rate of electron emission. As a result, the conversion efficiency of PETE devices is not restricted by the Shockley Queisser limit that corresponds to the bandgap of the absorbing material. In this work we analyze ...

Three-dimensional object recognition using a quasi-correlator invariant to imaging distances

We present a new method for performing electro-optical three-dimensional (3-D) object recognition under incoherent white-light illumination. Perspective projections of the 3-D scene are acquired from multiple points of view and then processed into a single complex two-dimensional modified Fresnel hologram of the scene. This hologram is processed with a single filter which is matched to a single object, so that all identical objects in the scene yield similar correlation peaks in the 3-D space with almost no dependency on the distances of the objects from the acquisition plane. The new method is demonstrated by experiments.

High performance photo-thermionic solar converters

A new class of solar energy converters is described, based on photon-enhanced thermionic emission of electrons into a vacuum region between two electrodes. In contrast to conventional thermionic converters, the two electrodes are at the same temperature, and the cathode is side-illuminated rather than front-illuminated. This configuration leads to several advantages: close coupling of multiple units in a series connection high-voltage configuration; higher area for electron emission; and recovery of waste heat at a higher temperature compared to heat recovery from the anode of a conventional thermionic converter. The conversion efficiency from solar radiation to electricity of the new photo-thermionic device may be in the range of 30-40% at moderate operation temperatures and moderate concentration of the incident sunlight.

Vertical junction high-efficiency concentrator photovoltaic cells

High concentration PV systems usually prefer tandem III-V cells to Si cells, due to the much lower conversion efficiency of the latter. We re-examine the efficiency achievable with Si Vertical Multi-Junction (VMJ) cells consisting of series-connected vertical p-n junctions within a single cell. A comprehensive 2D numerical analysis of a Si vertical junction has been performed, over a wide range of design parameters and concentration levels. The results show outstanding performance potential under high concentration of 1,000 suns and higher, with efficiencies above 29% under realistic (non-ideal) assumptions. This compares to reported efficiencies of about 20% for both real cells and previous simulations using realistic assumptions. This difference may be attributed to two effects: a better representation of the active layer photoconductivity, which lowers drastically the cells series resistance under high concentration; and optimization of the junction dimensions without restriction due to the accepted fabrication process.

Vertical Junction Si Photovoltaic Cells for Concentrating PV

High concentration PV systems usually prefer tandem III–V cells to Si cells, due to the much lower conversion efficiency of the latter. We re‐examine the efficiency achievable with Si Vertical Multi‐Junction (VMJ) cells consisting of series‐connected vertical p‐n junctions within a single cell. A comprehensive 2D numerical analysis of a Si vertical junction has been performed, over a wide range of design parameters and concentration levels. The results show outstanding performance potential under high concentration of 1,000 suns and higher, with efficiencies above 29% under realistic (non‐ideal) assumptions. This compares to reported efficiencies of about 20% for both real cells and previous simulations using realistic assumptions. This difference may be attributed to two effects: a better representation of the active layer photoconductivity, which lowers drastically the cell’s series resistance under high concentration; and optimization of the junction dimensions without restriction due to the accepted fabrication process.