RELATED APPLICATIONS

This application claims the benefit under 35 U.S.C. § 119(e) from U.S. Provisional Patent Application No. 61/676,960 filed on July 29, 2012, the contents of which are incorporated by reference as if fully set forth herein.

FIELD AND BACKGROUND OF THE INVENTION

The present invention, in some embodiments thereof, relates to a photo- thermionic converter and, more particularly, but not exclusively, to solar converters, for example, side illuminated solar converters, illuminated from a side of a cathode generally perpendicular to an electron emission surface thereof.

Currently the highest conversion efficiencies from solar energy to electricity are achieved with two different technologies: multi-junction concentrator photovoltaic systems, offering cell-level efficiency of about 40%, and system-level efficiency (including optical and other losses) of 25-30%; and dish-Stirling concentrator thermal systems, with engine efficiency around 40% and system-level efficiency again in the range of 25-30%. The multi-junction cells used in the photovoltaic path may be further improved, but at the expense of increasing complexity and using materials that are difficult to find and process. Usage of waste heat from these cells to improve overall efficiency is difficult since increasing the temperature to improve usability of the heat will harm the efficiency of the cells. The thermal conversion path, on the other hand, relies on high-temperature Stirling engines that are complex, expensive, and considered to be unreliable. Usage of waste heat from a Stirling engine is again difficult, since increasing the lower temperature of the engine to improve the usability of the heat will decrease the engine efficiency.

Thermionic devices convert heat directly into electrical power using thermionic emission: when a cathode with metal or semiconductor surface at high temperature is exposed to vacuum or a rarefied vapor, electrons may be emitted from the surface, at a rate that depends strongly on the surface temperature. The device should further include an anode disposing waste heat to a heat sink, a vacuum gap between the two electrodes, and external electrical connections that allow closing the circuit through an external load. Thermionic converters typically require high cathode temperatures, over 1000°C, and even at these high temperatures the conversion efficiency from heat to electricity is usually less than 20%. Thermionic converter technology has therefore not been widely implemented. The heat source for heating the cathode can be any suitable source that can provide high temperatures, for example combustion of a fuel, a nuclear reaction, or concentrated solar energy.

A recently proposed conversion process combines both photonic and thermal conversion of light into electricity and is therefore suitable for conversion of solar energy (J. W. Schwede, I. Bargatin, D. C. Riley, B. E. Hardin, S. J. Rosenthal, Y. Sun, F. Schmitt, P. Pianetta, R. T. Howe, Z. Shen, and N. A. Melosh, "Photon enhanced thermionic emission for solar concentrator systems," Nat. Mater., vol. 9, p. 762-767, 2010). In Photon Enhanced Thermionic Emission (PETE), the cathode is illuminated with photons of energy above the band gap, increasing the cathode's conduction band electron population, and raising the conduction band quasi-Fermi level. As a result, the electrons' energy barrier to the vacuum is reduced, allowing electron emission at temperatures considerably lower than standard thermionic emission. The PETE conversion process can be divided into three steps. First, the cathode absorbs above band gap photons, adding electrons to the conduction band. The optically generated electrons then thermalize to the lattice temperature. Finally, electrons having energies higher than the vacuum level are emitted to the vacuum and are collected by the anode, providing the desired electrical output.

In all devices that convert sunlight to electricity, a large fraction of the incident sunlight is converted to heat, which heat usually needs to be removed from the device. Since the best converters offer an efficiency of 40% at the most, the amount of waste heat is around 60% or more of the incident radiation. This waste heat is an energy resource that might be usable, for example to produce additional electricity with a suitable heat engine, in order to increase the overall efficiency of the converter. When the converter is a PV cell, for example, this waste heat is available at a relatively low temperature of less than 100°C. Heat at such low temperature cannot be effectively converted to electricity and therefore is removed and lost. If the cell temperature were increased, to allow a better conversion of the waste heat to electricity, then the cell performance is reduced and thus the benefit is limited (A. Kribus and G. Mittelman, "Potential of polygeneration with solar thermal and photovoltaic system," Journal of Solar Energy Engineering, vol. 130, pp. 011001-011006, 2008).

In a standard thermionic converter, recovery of waste heat is possible from the back surface of the anode. In principle the anode temperature can be raised somewhat in order to improve the conversion of the waste heat to electricity. However, when it is raised too much, the back current of electrons emitted from the anode towards the cathode will be high and reduce the efficiency of the thermionic converter. Therefore, a significant temperature difference between the cathode and anode must be maintained, and the ability to raise the anode temperature is limited.

In the field of PV cells, various stacked arrangements have been described, for example, in B. Sater and N. Sater, "High voltage silicon vmj solar cells for up to 1000 suns intensities," in Conference Record of the Twenty-Ninth IEEE Photovoltaic Specialists Conference, 2002., may 2002, pp. 1019 - 1022; R. Pozner, G. Segev, R. Sarfaty, A. Kribus, and Y. Rosenwaks, "Vertical junction Si cells for concentrating photovoltaic s," Progress in Photovoltaics: Research and Applications, vol. 20, no. 2, pp. 197-208, 2012. [Online]. Available: www.dx.doi.org/10.1002/pip.1118; S. Keller, S. Scheibenstock, P. Fath, G. Willeke, and E. Bucher, "Theoretical and experimental behavior of monolithically integrated crystalline silicon solar cells," J. Appl. Phys., vol. 87, no. 3, pp. 1556 -1563, Feb 2000; and P. Ortega, S. Bermejo, and L. Castaer, "High voltage photovoltaic mini-modules," Prog. Photovoltaics Res. Appl., vol. 16, no. 5, pp. 369-377, 2008, available online at www.dx.doi.org/10.1002/pip.816.

Additional background art includes G. N. Hatsopoulos and E. P. Gyfopoulos, Thermionic Energy Conversion, Volume 1 (MIT Press, 1973) and Volume 2 (MIT Press, 1979); J. S. Edelson, "Thermionic generator," U.S. Patent 6229083, May 8, 2001; J. S. Edelson, "Method and apparatus for thermionic generator," U.S. Patent 5994638, November 30, 1999; L. L. Begg and H. H. Steckert, "Thermionic converter and method of making same," U.S. Patent 6037397, March 14, 2000; A. Kribus and G. Mittelman, "Potential of polygeneration with solar thermal and photovoltaic system," Journal of Solar Energy Engineering, vol. 130, pp. 011001-011006, 2008; F. A. Koeck, R. J. Nemanich, Y. Balasubramaniam, K. Haenen, and J. Sharp, "Enhanced thermionic energy conversion and thermionic emission from doped diamond films through methane exposure," Diamond Relat. Mater., vol. 20, no. 8, pp. 1229 - 1233, 2011, available online at wwwdotsciencedirectdotcorn/science/article/pii/S092596351100 2184; S. R. P. Silva, G. A. J. Amaratunga, and K. Okano, "Modeling of the electron field emission process in polycrystalline diamond and diamond-like carbon thin films," vol. 17, no. 2. AVS, 1999, pp. 557-561, available online at www.link.aip.org/link/?JVB/17/557/l; P. Lerner, P. H. Cutler, and N. M. Miskovsky, "Theoretical analysis of field emission from a metal diamond cold cathode emitter," Journal of Vacuum Science & Technology B: Microelectronics and Nanometer Structures, vol. 15, no. 2, pp. 337-342, 1997, available online at www.link.aip. org/link/?JVB/15/337/l; P. Lerner, N. M. Miskovsky, and P. H. Cutler, "Model calculations of internal field emission and j-v characteristics of a composite n-si and n-diamond cold cathode source," vol. 16, no. 2. AVS, 1998, pp. 900-905, available online at www.link.aip.org/link/?JVB/16/900/l; F. A. Koeck, R. J. Nemanich, A. Lazea, and K. Haenen, "Thermionic electron emission from low work- function phosphorus doped diamond films," Diamond Relat. Mater., vol. 18, no. 5-8, pp. 789 - 791, 2009, proceedings of Diamond 2008, the 19th European Conference on Diamond, Diamond-Like Materials, Carbon Nanotubes, Nitrides and Silicon Carbide, available online at wwwdotsciencedirectdotcom/science/article/pii/-

S0925963509000259.

SUMMARY OF THE INVENTION

There is provided in accordance with some embodiments of the invention, a photo-converter device which converts incident electromagnetic radiation on a radiation receiving surface into electrical current, comprising:

at least one cell, comprising:

a cathode separated by a gap from an anode, such that electrons emitted by said cathode travel on average along a first direction from said cathode to said anode through said gap, wherein the radiation receiving surface is a surface of the cathode, with an average direction of orientation which is not facing or opposite said gap.

In an exemplary embodiment of the invention, an average normal direction to the radiation receiving surface is at an angle of between 30 and 150 degrees relative to said first direction, or at an angle of between 10 and 170 degrees. In an exemplary embodiment of the invention, an average normal direction to the radiation receiving surface is at an angle of between 45 and 135 degrees relative to said first direction.

In an exemplary embodiment of the invention, said device is configured for solar radiation conversion, including at least one of matching a band-gap property and an absorption depth of said cathode to wavelengths of solar radiation. Optionally or alternatively, said anode comprises or is coated with a low work function layer, facing said gap.

Optionally or alternatively, said cathode comprises or is coated with an electron blocking layer on a face away from said gap.

Optionally or alternatively, said cathode comprises or is coated with an emission enhancement layer facing said gap.

Optionally or alternatively, said cathode has an electron emitting surface of a cross-sectional area which is a factor of at least two of a cross-sectional area of said radiation receiving surface. Optionally, said factor is at least 6. Optionally, said factor is at least 10.

In an exemplary embodiment of the invention, said radiation receiving surface covers at least 70% of a surface of said cell facing said radiation.

In an exemplary embodiment of the invention, said radiation receiving surface covers at least 30% of a surface of said cell facing said radiation.

In an exemplary embodiment of the invention, said radiation receiving surface covers at least 90% of a surface of said cell facing said radiation.

In an exemplary embodiment of the invention, the device comprises a solar concentrator which concentrates light on said cell. Optionally, said cell is arranged as part of an array of cells and wherein said concentrator concentrates sunlight on a side of said array and a side of said cathodes.

In an exemplary embodiment of the invention, said device comprises a plurality of such cells, stacked anode to cathode and sharing a common radiation receiving surface. Optionally, said stacked cells are electrically connected in series. Optionally, an anode of one cell is mounted on a cathode of a next cell in the stack.

In an exemplary embodiment of the invention, the device comprises a plurality of such stacks of cells arranged side by side and electrically connected in parallel. In an exemplary embodiment of the invention, said device has a total emission area of a factor of at least 4 of said radiation receiving surface area and wherein said radiation receiving surface covers at least 90% of a side of said device.

In an exemplary embodiment of the invention, said device is isothermal, with substantially no temperature differences between said anode and said cathode. Optionally, the device comprises a heat removing mechanism common to a plurality of cells and receiving heat from both said cathode and said anode of a plurality of cells. Optionally, said heat removing mechanism comprises a waste heat converter.

In an exemplary embodiment of the invention, the device is configured to have an optimal energy generation temperature of between 370 and 450 degrees Kelvin.

In an exemplary embodiment of the invention, the device is to have an optimal energy generation temperature of between 400 and 900 degrees Kelvin.

There is provided in accordance with some embodiments of the invention, a method of generating electricity using a Photon Enhanced Thermionic Emission (PETE) device, comprising:

illuminating a radiation receiving surface of said device with concentrated solar radiation;

generating energetic electrons in a cathode of said device by said radiation; emitting said electrons across a gap to an anode, in an average emission direction not parallel to an average normal direction to the radiation receiving surface; and

closing a circuit on said device using a load. Optionally, said generated electrons are allowed to travel in said cathode a distance greater than a thickness of said cathode from its one surface to its emitting surface. Optionally or alternatively, said illuminating comprises illuminating said radiation into said gap. Optionally or alternatively, said illuminating comprises illuminating with at least 5 suns.

In an exemplary embodiment of the invention, said illuminating comprises illuminating with at least 200 suns.

In an exemplary embodiment of the invention, said illuminating comprises heating said device to between 300 and 600 degrees Kelvin.

In an exemplary embodiment of the invention, said illuminating comprises heating said device to between 400 and 900 degrees Kelvin. In an exemplary embodiment of the invention, the method comprises generating electrical energy with a conversion efficiency of at least 30% of illuminating energy.

In an exemplary embodiment of the invention, the method comprises generating electrical energy by said electrons with a conversion efficiency from said illumination of at least 20%.

In an exemplary embodiment of the invention, the method comprises generating electrical energy by collecting heat from said device and converting said heat to electricity with a conversion efficiency from said illumination of at least 10%.

There is provided in accordance with some embodiments of the invention, a method of forming a Photon Enhanced Thermionic Emission (PETE) device comprising stacking a plurality of cathode-gap-anode cells.

There is provided in accordance with some embodiments of the invention, a Photon Enhanced Thermionic Emission (PETE) converter device comprising an array of cells including at least 4 stacked cells that are stacked anode to cathode. Optionally, the device includes at least 30 such stacked cells.

There is provided in accordance with some embodiments of the invention, a method of manufacturing PETE converter devices according to claim 37, the method comprising:

etching a plurality of trenches on at least one side of a semiconductor wafer, for each of a number of wafers equal to the number of stacked cells;

depositing anodes and contacts on one side of each wafer, in the trenches or opposite the trenches, for at least all but an end one of the wafers, the contacts being deposited between the anodes and the wafer;

stacking the wafers together, such that each anode is separated from a surface of the next wafer by at least one trench, forming a gap, the wafer surface opposite each anode across the gap forming a cathode emission surface; and

dicing the stack of wafers into PETE converter devices, each cathode and anode with gap between them comprising one cell in the stack, and each contact joining the anode in one cell to the cathode in the next cell in the stack.

Optionally, stacking the wafers together comprises adding one wafer at a time to the stack in order, the method also comprising making each wafer thinner after adding it to the stack, before adding the next wafer to the stack. In an exemplary embodiment of the invention, said cell is configured to receive most of said radiation at an angle of between 30 and 150 degrees relative to said first direction, or at angle of between 10 and 170 degrees.

In an exemplary embodiment of the invention, said cell is configured to receive most of said radiation at an angle of between 45 and 135 degrees relative to said first direction.

Optionally, the average emission direction is at an angle between 30 and 150 degrees relative to the average normal direction to the radiation receiving surface, or an angle between 10 and 170 degrees.

Optionally, the average emission direction is at an angle between 45 and 135 degrees relative to the average normal direction to the radiation receiving surface.

There is provided in accordance with some embodiments of the invention, a photo-converter device which converts incident electromagnetic radiation on a radiation receiving surface into electrical current, comprising:

at least one cell, comprising:

a cathode separated by a gap from an anode, such that electrons emitted by said cathode over an emission surface travel to said anode through said gap, wherein the radiation receiving surface is a surface of the cathode, different from the emission surface, and the total area of the radiation receiving surface for all cells is less than half the total area of the emission surfaces for all cells.

There is provided in accordance with some embodiments of the invention, a method of generating electricity using a Photon Enhanced Thermionic Emission (PETE) device, comprising:

illuminating a radiation receiving surface of said device with concentrated solar radiation;

generating energetic electrons in a cathode of said device by said radiation; emitting said electrons across a gap to an anode; and

closing a circuit on said device using a load;

wherein an operating temperature of the cathode and an operating temperature of the anode are less than 200 K apart, or less than 100 K apart, or less than 50 K apart, or less than 20 K apart, or less than 10 K apart. There is provided in accordance with some embodiments of the invention, a photo-convertor device which converts incident electromagnetic radiation into electrical current, comprising:

at least one cell, comprising:

a cathode separated by a gap from an anode, such that electrons emitted by said cathode travel along a first direction from said cathode to said anode through said gap, wherein said cathode has a radiation receiving surface which is not facing or opposite said gap. Unless otherwise defined, all technical and/or scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which the invention pertains. Although methods and materials similar or equivalent to those described herein can be used in the practice or testing of embodiments of the invention, exemplary methods and/or materials are described below. In case of conflict, the patent specification, including definitions, will control. In addition, the materials, methods, and examples are illustrative only and are not intended to be necessarily limiting.

Implementation of the method and/or system of embodiments of the invention can involve performing or completing selected tasks manually, automatically, or a combination thereof. Moreover, according to actual instrumentation and equipment of embodiments of the method and/or system of the invention, several selected tasks could be implemented by hardware, by software or by firmware or by a combination thereof using an operating system.

For example, hardware for performing selected tasks according to embodiments of the invention could be implemented as a chip or a circuit. As software, selected tasks according to embodiments of the invention could be implemented as a plurality of software instructions being executed by a computer using any suitable operating system. In an exemplary embodiment of the invention, one or more tasks according to exemplary embodiments of method and/or system as described herein are performed by a data processor, such as a computing platform for executing a plurality of instructions. Optionally, the data processor includes a volatile memory for storing instructions and/or data and/or a non- volatile storage, for example, a magnetic hard-disk and/or removable media, for storing instructions and/or data. Optionally, a network connection is provided as well. A display and/or a user input device such as a keyboard or mouse are optionally provided as well.

BRIEF DESCRIPTION OF THE DRAWINGS

Some embodiments of the invention are herein described, by way of example only, with reference to the accompanying drawings. With specific reference now to the drawings in detail, it is stressed that the particulars shown are by way of example and for purposes of illustrative discussion of embodiments of the invention. In this regard, the description taken with the drawings makes apparent to those skilled in the art how embodiments of the invention may be practiced.

In the drawings:

FIG. 1 is a schematic cross-sectional view of a side-illuminated thermionic converter according to an exemplary embodiment of the invention;

FIG. 2 is a schematic plot of cathode emission current, reverse emission current, and net current, as a function of operating voltage, for a thermionic converter such as the one shown in FIG. 1;

FIG. 3 is a schematic view of a side-illuminated thermionic converter system with a controller regulating temperature, illuminating flux, and/or operating voltage, according to an exemplary embodiment of the invention;

FIG. 4 is a schematic cross-sectional view of a prior art front-illuminated thermionic converter system;

FIG. 5 is a schematic cross-sectional view of a side-illuminated high voltage thermionic converter, according to an exemplary embodiment of the invention;

FIG. 6 is a plot of maximum power conversion efficiency vs. flux concentration, for different ratios of emission area to illuminated area, for a side-illuminated thermionic converter according to an exemplary embodiment of the invention;

FIG. 7 is a plot of optimal operating temperature vs. flux concentration, corresponding to the maximum conversion efficiency curves shown in FIG. 6;

FIG. 8 is a contour plot of conversion efficiency as a function of operating temperature and flux concentration, for the same side-illuminated thermionic converter as the plots in FIGs. 6 and 7; FIG. 9 is a schematic cross-sectional view of a side-illuminated high voltage thermionic converter, according to an exemplary embodiment of the invention;

FIG. 10 is a schematic plot of electric potential and related quantities, such as the quasi-Fermi potential for electrons, as a functional of the horizontal coordinate, for a side-illuminated high voltage thermionic converter such as the one shown in FIG. 9, according to an exemplary embodiment of the invention;

FIG. 11 is a schematic cross-sectional view of a side-illuminated high voltage thermionic converter system, including a waste heat converter, according to an exemplary embodiment of the invention;

FIG. 12 is a plot of maximum power conversion efficiency as a function of flux concentration, for the thermionic converter, and waste heat converter, and the overall system, for a thermionic converter system such as that shown in FIG. 11, according to an exemplary embodiment of the invention;

FIG. 13 is a plot of optimal operating temperature as a function of flux concentration, corresponding to the maximum efficiency of the overall system plotted in FIG. 12;

FIG. 14 is a schematic cross-sectional view of a side-illuminated high voltage thermionic converter system with a micro-concentrator for concentrating flux, according to an exemplary embodiment of the invention;

FIG. 15 is a schematic illustration of different stages in a manufacturing process for a high voltage side-illuminated thermionic converter, similar to those shown in FIGs.

5 and 9, according to an exemplary embodiment of the invention; and

FIG. 16 is a schematic illustration of different stages in an alternative manufacturing process for a high voltage side-illuminated thermionic converter, similar to those shown in FIGs. 5 and 9, especially suitable for a converter comprising thin cells, according to an exemplary embodiment of the invention. DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

Overview

An aspect of some embodiments of the invention relates to PETE converters in which an electron emission area is substantially larger than a light receiving area. In an exemplary embodiment of the invention, in a generally rectangular cross-section geometry for a cathode, a short side of the rectangle acts as a radiation receiver and a long side of the rectangle acts as an electron emitter. One potential benefit is that emission area can be decoupled from the optical system design. Another potential benefit is that heat loss from the radiation receiver is reduced due to its decrease in size relative to the emitter. Another potential benefit is that such cells can be stacked without affecting optical performance. Optionally, this stacking is used to improve electrical characteristics of such a stack. Another potential benefit is that waste heat can be collected from both anodes and cathodes of all cells simultaneously. Optionally, the cell is an isothermal cell.

In an exemplary embodiment of the invention, a plurality of cells are stacked.

Optionally, the stacking allows an anode of one cell to be mounted directly on a cathode of the next cell, with gaps between the anode-cathode constructs. This may allow the anode to be very thin. Optionally, thin anodes allow most of the radiation-receiving surface of a cell array to be active, as very little is wasted on anodes and none need be wasted on electrical connections.

Additionally or alternatively, stacking is optionally used to easily connect such cells in series. Such connection may allow the voltage of the stack to be increased, thereby possibly alleviating an ohmic resistance problem which is caused in low voltage high current cells.

In an exemplary embodiment of the invention, a waste heat collector, for example a Stirling engine, is mounted on a stack of cells to capture and use waste heat. In another example of a waste heat collector, a thermoelectric converter is provided in contact with the stack. In another example of heat collection, a heat exchanger that transfers the heat to a fluid for external use is provided, for example, the fluid being water, oil or steam. In another example, the heat is used for cooling, for example, as part of an absorption cooling system. In some embodiments, heat provided, e.g., by fluid, from multiple converters is conveyed (e.g., in series or parallel) to a single thermal energy converter such as a Stirling engine or a Rankine engine or steam generator (e.g., for industrial use) or absorption chiller.

It should be noted that some saving in insulation may be realized by only the device itself being insulated, rather than insulating cathodes from anodes, as is required in some prior art.

In an exemplary embodiment of the invention, energy is received by the cathode in the form of light (e.g., matched to the band-gap properties of the cathode), optionally in addition to heat, rather than only heat as in some prior art devices.

A potential advantage of some embodiments of the invention is the provision of a solar energy converter with the simplicity and high reliability inherent in solid-state devices, with a significant potential for waste heat recovery and conversion at moderate temperatures, and high overall conversion efficiency competitive with the existing leading technologies.

In an exemplary embodiment of the invention, the PETE device is provided with concentration optics to provide, for example, a light concentration of over 50 or over 200 suns, or for example, between 2 and 3000 suns concentration or more, for example, between 2 and 5 suns, or, for example, between 5 and 15 suns, or, for example, between 15 and 150 suns, or, for example, between 150 and 500 suns, or, for example, between 500 and 1000 suns, or, for example between 1000 and 2000 suns, or, for example, more than 2000 suns, or more than 2500 suns.

An aspect of some embodiments of the invention relates to side-illuminated PETE cells and arrays. In an exemplary embodiment of the invention, the radiation which generates emitted electrons is provided from a direction which is at an angle of 30 degrees or more to a direction normal to the surfaces that emit electrons. For example, if the cell has a cross-sectional shape of a rectangle, light will be received from a side which is perpendicular to the emitting side.

In some embodiments of the invention, light is allowed to enter from opposite sides of the cell, for example, two facing sides if the cell is rectangular in cross-section. Heat is optionally removed from a third and/or fourth side of the cell, which is perpendicular to both emission side and radiation receiving sides.

In some embodiments, radiation is allowed to enter from three sides of the cathode, which are perpendicular to the emission direction. In an exemplary embodiment of the invention, a device has a side designed to face the illumination and at least 50%, 70%, 80%, 90% or more of that side (e.g., including one or more of structures, connectors, and gaps) is arranged to receive radiation that will reach the cathode.

In an exemplary embodiment of the invention, the radiation receiving area of the cell (or stack or device) is smaller than an electron emitting area. Specifically, considering the area of a cell or device through which illumination can reach the cathode and considering the cross-sectional emission area of the electron emission surface (when projected on a plane and thus canceling the effects of projections, nanostructure, etc.), the emission area can be a factor of, for example, 2, 4, 6, 10, 20 or more, or an intermediate number, larger than the receiving area.

An aspect of some embodiments of the invention relates to a geometry of PETE cells, in which a stack of cells is provided, with alternating cathodes and anodes. In an exemplary embodiment of the invention, a gap is provided between a cathode and an anode. Additionally or alternatively, an anode is optionally mounted on the back of a cathode of a next cell in the stack. Optionally, the cells are provided as a two dimensional array.

In an exemplary embodiment of the invention, the array is manufactured by solid state manufacturing, optionally in layers of one or more cathodes, gaps and anodes and then dicing such a manufactured item to form a one dimensional array with side illumination and electrical connections perpendicular to said side. Optionally, a heat collector is attached on an opposite side of such an array. Optionally, multiple such arrays are laid side by side to form a two dimensional array. In an exemplary embodiment of the invention, a stack will include between 50 and 500 electrode pairs (cells). In one example, the width of each pair is 20 to 100 microns in silicon including the gap; so a linear dimension of the ID array can be between 1 mm to 5 cm. In III-V semiconductors the width of each pair can typically be lower, for example 10 to 30 microns. Optionally, a stack size of less than 1 mm is not used, so as to make handling and/or optics simpler.

An aspect of some embodiments of the invention relates to an electrical connection scheme for PETE cells, in which the cells are connected in series so as to yield a high voltage, for example, at least 10V, at least 30V, at least 60V, at least 100V, or smaller or intermediate voltages. Optionally, the cells are stacked so that an anode of one cell is electrically connected to a cathode of a next cell and no separate electrical connectors are needed. In an exemplary embodiment of the invention, the cells are connected in a two dimensional array, with the stacked cells connected in series and arranged in a parallel electrical connection with adjacent stacks. In an exemplary embodiment of the invention, such an array includes between 1 and 1000, for example, 50, 150, 300, 800 or intermediate numbers of stacks of between 1 and 1000, for example, 10, 30, 50, 100, 300 or intermediate number of cells in a stack. In addition, in the stack direction, multiple sub-stacks (e.g., 2, 3, 5 or intermediate or greater numbers) may be provided and, for example, connected electrically in parallel, so as to prevent damage or shading of a single cell from reducing the output power substantially on an entire stack. A stack may have a width, for example, of between 10 microns and 10 mm, for example, between 100 microns and 2 mm.

The efficiency of thermionic converters improves with higher temperature, and in the case of heating by solar energy this generally requires higher concentration of the sunlight. The increase in temperature leads to a significant increase in current, while the voltage is relatively unchanged. The thermionic converter is then characterized by low voltage and high current, as described by Hatsopoulos and Gyfopoulos, cited above. Hence, ohmic losses, which are proportional to the square of the current, may impose a significant loss in such systems, both inside the converter and in the external circuit. In thermionic converters the effect of series resistance loss is expected to impose an upper bound on radiation flux concentration, above which the efficiency declines and this limits the potential for achieving high efficiency. The PETE converter as analyzed in Schwede et al, cited above, is subject to the same problem of high current and low voltage.

An aspect of some embodiments of the invention relates to a substantially isothermal PETE array. In an exemplary embodiment of the invention, such an array uses less of thermally insulating materials, for example only around the array and not within the array. Additionally or alternatively, a heat conductor optionally interconnects cells and/or anodes and cathodes. Additionally or alternatively, such an array optionally has a heat collector arranged to simultaneously collect heat from a plurality of cells and from both anodes and cathodes thereof. In an exemplary embodiment of the invention, a waste heat collector for cooling cools an entire array simultaneously, for example, between 20 and 100 cells or more. In an exemplary embodiment of the invention, a continuous heat collector with good heat conduction is shared for such a plurality of cells. Additionally or alternatively, a waste heat converter, for example, a Stirling engine, is optionally shared by such a plurality of cells, in one or more cell arrays.

In an exemplary embodiment of the invention, the waste heat recovery should be done at a temperature as high as possible, but without damaging the performance of the main conversion process. In an exemplary embodiment of the invention, by requiring no temperature difference between anode and cathode, lower useful operating temperatures are provided. In one example, the converter is configured to work at 10 suns and 400 degrees K or at 1000 suns and 750 K.

It is noted that some prior art thermionic converter designs (for example U.S. patents 6229083 and 5994638, to Edelson, and U.S. patent 6037397 to Begg and Steckert, cited above) may be unable to utilize such thermal collection and/or electrical connections as provided in some embodiments of the invention, due to one or more of reduced active area, additional series resistance, and also additional thermal shorts between the cathode and anode that produce thermal losses due to the multiple electrical connections. Furthermore, since a large temperature difference exists between each cathode and its anode in conventional thermionic conversion, this thermal loss can be significant.

An aspect of some embodiments of the invention relate to providing a thermionic converter in which temperature difference between an anode and a cathode is replaced by and/or enhanced by photo-excitation of electrons in the cathode, as a source of input power. Optionally, the temperature difference is less than 400 K, less than 300 K, less than 200K, less than 100K, less than 50K, less than 20K, or at intermediate values. Optionally, the converter is side illuminated. Alternatively or additionally, it is illuminated from a side of the cathode opposite a gap between the cathode and an anode. For example, a cell such as described in Schwede et al, may be used.

Some embodiments of the invention are herein described, by way of example only, with reference to the accompanying drawings. With specific reference now to the drawings in detail, it is stressed that the particulars shown are by way of example and for purposes of illustrative discussion of embodiments of the invention. In this regard, the description taken with the drawings makes apparent to those skilled in the art how embodiments of the invention may be practiced.

Before explaining at least one embodiment of the invention in detail, it is to be understood that the invention is not necessarily limited in its application to the details of construction and the arrangement of the components and/or methods set forth in the following description and/or illustrated in the drawings and/or the Examples. The invention is capable of other embodiments or of being practiced or carried out in various ways.

Also, various embodiments and aspects of the present invention as delineated hereinabove and as claimed in the claims section below find support in the simulated examples shown below.

Exemplary Isothermal Photo- Thermionic Converter

Figure 1 shows a thermionic converter 100, according to some embodiments of the invention. The converter is powered by light 102, for example sunlight optionally concentrated, illuminating a semiconductor cathode 104 on one of its sides, a radiation receiving surface, shown at the top of cathode 104 in FIG. 1. A different side 106, shown on the right side of cathode 104 in FIG. 1, emits electrons 108 that have been driven into the conduction band of the semiconductor by the light, and given enough energy to overcome the work function of the cathode. The electrons cross a gap 110 of vacuum or low pressure gas, and are collected by a semiconductor or metal anode 112. A reverse current of electrons 114 may travel from the anode to the cathode if there is an electric field in the gap driving electrons in that direction, and if the anode is hot enough and has low enough work function, but the flow of electrons from the cathode to the anode is generally greater than the reverse current, so there is a net current from cathode to the anode. External conductors allow closing the circuit through an external load 116.

As used herein, a radiation receiving surface of a cathode means a surface that is adapted to convey radiation into the cathode where a significant fraction of the radiation will excite electrons into the conduction band. This may mean, for example, that the surface is transparent to light of a wavelength range that can excite electrons into the conduction band, and that the cathode is of a shape and size such that a photon of that wavelength range, striking the radiation receiving surface normal to the surface, will have a long enough ray-tracing path through the cathode that there is a significant chance, for example more than a 30% chance or more than a 50% chance or more than a 70% chance, that it will excite an electron into the conduction band before it leaves the cathode or is absorbed by another process. Optionally, such a radiation receiving surface has a coating that reduces reflection of light of that wavelength range.

Although in FIG. 1, the top surface of cathode 104 receiving light is oriented at 90 degrees to emission surface 106, the radiation receiving surface need not be oriented at 90 degrees to the emission surface, but is optionally oriented at a different angle to it, for example between 10 and 170 degrees, or between 30 degrees and 150 degrees, or between 45 and 135 degrees. The radiation receiving surface and the emission surface need not be planar, but one or both of them is optionally curved, and/or comprises multiple planar areas oriented in different directions, and in these cases, an average orientation of the light receiving surface is at an angle to an average orientation of the emission surface, for example at an angle of 90 degrees, or between 45 and 135 degrees, or between 30 and 150 degrees, or between 10 and 170 degrees. The radiation receiving portion of the surface of cathode 104 need not be facing entirely in a generally upward direction as shown in FIG. 1, but optionally also includes one or more surfaces, not necessarily continuous with the upper surface shown in FIG. 1, facing in a direction out of the plane of the drawing, or facing generally downward. Having a radiation receiving surface or surfaces facing a wide range of directions may be useful, for example, if light is concentrated on the converter from a range of different directions, by a lens or concave mirror.

In an exemplary embodiment of the invention, converter 100 is operated with the cathode and anode at essentially the same temperature, in contrast with conventional thermionic converters that are operated with a large temperature difference between the two electrodes. It is believed that the driving force of the conversion is the illumination of the cathode by radiation with photon energy equal to or higher than the bandgap of the semiconductor, for example concentrated sunlight. The electron emission current from the cathode is higher than the reverse emission current from the anode even though their temperatures are the same, possibly due to the absorption of energetic photons in the cathode that increases the electron population in the conduction band of the cathode. These electrons obtain a net voltage due to the difference in work function between the cathode and anode. The converter then produces a net power output and converts a part of the incident radiation energy to electricity.

Without being limited by this hypothesis, an example of this conversion process was calculated for a 10 19 cm - ^" 3 doped p-type semiconductor cathode with bandgap of 1.4 eV and electron affinity of 0 eV, anode with work function of 0.9 eV, incident radiation of 10 W/cm and solar AM1.5D spectrum, and temperature of 600 K for both electrodes. Figure 2 shows a plot 200 of the emission current plotted on vertical axis 202 vs. operating voltage plotted on horizontal axis 204, for the separate cathode emission (forward) current 206, and anode emission (reverse) current 208, and the net device current 210. For voltages significantly lower than the difference in work functions (0.398 V) the reverse emission is negligible. As the voltage is increased, the reverse emission will be increased, and electrons absorbed at the cathode will contribute to the conduction band electron population, and as a result the cathode emission will be increased as well. Assuming a high quality cathode with high charge carrier lifetime before recombination, nearly all the reverse emitted electrons will be re-emitted forward from the cathode, the increase in the cathode emission will approximately cancel the reverse emission, and the net current will hardly change. When the voltage reaches the difference in work functions, the reverse emission will saturate. When the voltage is further increased, the electron's potential barrier for emission from the cathode will be increased, but so will the electron concentration in the conduction band of the cathode, keeping the emission current nearly constant. Further increase in electron concentration will also increase recombination, leading to saturation: added electrons will recombine rather than be emitted, and the net emission current will decrease with voltage. Each point in the current vs. voltage curve corresponds to a certain power output, and the curve can be scanned to find the maximum power point (MPP). In an exemplary embodiment of the invention, a controller or inverter is connected to the converter array, and the controller or inverter performs such scans during operation of the converter array, and adjusts dynamically the electrical operation point in order to obtain the maximum power output.

The efficiency of the conversion device is the ratio of the maximum power output (found at the MPP in the current-voltage curve), to the power of the incident radiation. For every given incident radiation flux concentration there is an optimal operation temperature that yields the maximum conversion efficiency. Lower temperatures than the optimum yield lower cathode emission, while higher temperatures lead to increased reverse emission from the anode. In an exemplary embodiment of the invention, operating temperature is selected during design. Optionally, however, a controller controls one or more characteristics of the system during operation or during calibration to adjust to a new working point, for example, due to changes in light intensity.

Figure 3 shows a thermionic converter system 300 which uses such a controller. A thermionic converter cell 302, comprising one or more thermionic converters similar to converter 100, is illuminated by light 304, driving current across a load 306. A pump 308 pumps a cooling fluid through a tube 310 which removes heat from a heat sink 312 in thermal contact with converter cell 302. Tube 310 is optionally in the form of a radiator, or runs through a radiator, which loses heat to the environment, and/or tube 310 supplies heat to a device such as a heat engine or thermoelectric converter which extracts useful energy from the heat, as will be described below in the description of FIG. 11. A controller 314 optionally receives signals from a sensor 316 which measures the power consumed by the load, for example by measuring the voltage across the load and the current passing through the load. Controller 314 also optionally receives signals from a light sensor 318 which measures the intensity of light illuminating the converter system, and/or signals from a temperature sensor 320 which measures the temperature of the converter cell directly, or indirectly by measuring the temperature of the heat sink, or of the fluid circulating through the heat sink. Controller 314 optionally controls one or both of the light intensity reaching converter cell 302, for example by controlling the amount of light passing through an optical system 322 and/or a degree of concentration the light; and the temperature of converter cell 302, for example by controlling the rate at which pump 308 removes heat from heat sink 312. Optionally, such controlling is provided to ensure device does not over heat. By using feedback between the temperature sensor and the pump, controller 314 can set the temperature at any desired value within a range, and by using feedback between the light sensor and the optical system, controller 314 can set the light flux reaching converter cell 302 at any desired value within a range. Optionally, controller 314 also controls an effective resistance of load 306, within a range, and raises or lowers the effective resistance, to keep the voltage across the load at the maximum power point (MPP) for that value of temperature and light flux. By controlling the temperature, the light flux, the effective load resistance, or two or all three of them, the temperature can be kept at an optimum value for the light flux, and the operating voltage can be kept near the MPP. For example, the light flux can be kept at the highest available value, depending on how much sunlight is available, while the load resistance is adjusted so that the operating voltage is at or near the MPP, and the temperature is adjusted to be near the optimum temperature for that light flux.

Generally, the optimum temperature goes up with increasing light flux. This optimum value is optionally chosen from a look-up table based on theoretical modeling of the converter cell and the load, in order to maximize the power available to the load, or to assure that the power reaches a specified value. An example of a suitable theoretical model is given by Segev et al, "High performance isothermal photo- thermionic solar converters," Solar Energy Materials and Solar Cells 113 (2013), 114- 123, written by the inventors of the present application and published after the filing date of provisional application 61/676,960, which the present application takes priority from. Another theoretical model, described in provisional application 61/676,960, keeps n/n _eq at a constant value, where n is the electron density in the conduction band of the cathode, at a given temperature and light flux, while n _eq in the equilibrium density of electrons in the conduction band of the cathode, at that temperature and with no light flux. In this model, the emission current is to good approximation a function of n/n _eq , for example a linear function of log(n/n _eq ). Keeping n/n _eq constant tends to keep the efficiency approximately near its maximum value as the light flux and temperature change, if it starts out at its maximum value for a given light flux. Alternatively, by measuring the power consumed by the load, and measuring how it changes when the temperature and light flux change, while keeping the operating voltage near the MPP, the temperature is optionally set to an optimum value for the light flux, which maximizes the power available to the load, or assures that the power available to the load reaches a specified value.

With higher flux concentration, the temperature is allowed to increase (or flux is increased to match temperature needs), while not reducing the efficiency, or not reducing the efficiency relative to the theoretical maximum efficiency, or not reducing one or both of these quantities by too great an amount, or not reducing a different control parameter, such as n/n _eq , by too great an amount. For example, the absolute or relative efficiency, or a different control parameter, decreases by less than 30%, 20%, 20%, 5% or intermediate percentages, based for example on a table of expected values for different conditions of temperature and/or flux, or based on measuring the light flux and the load power. Any control algorithms, known in the art, can be used to keep the temperature close to its optimum value when the light flux changes, and to keep the operating voltage close to the MPP. Optionally, one of these variables, for example the operating voltage, is optimized with an inner control loop for a given temperature, and the other variable, for example the temperature, is adjusted with an outer control loop, changing the temperature only after the operating voltage has been reached or almost reached the MPP. Alternatively, a two-variable control algorithm is used to optimize the temperature and the operating voltage simultaneously.

In an exemplary embodiment of the invention, the design is optimized for operation at maximum illumination and therefore variations will be towards non- optimal, lower, temperatures. In some embodiments, a different operating point is chosen, for example, at 80% of maximum expected illumination. In an exemplary embodiment of the invention, a controller is used which dynamically seeks the MPP for any given illumination.

Exemplary Material Selection

In some embodiments of the invention, the material chosen for the cathode may include any semiconductor with suitable bandgap to absorb solar energy, for example silicon (Si), silicon carbide (SiC), or silicon germanium carbide (Sii-x-yGexCy, where x > 0, y > 0, and x+y < 1), or other elements or compounds of the IV family; gallium arsenide (GaAs), gallium indium phosphide (GalnP), gallium aluminum arsenide (AlGaAs), aluminum gallium nitride (AlGaN), copper gallium selenide (CuGeSe), combinations thereof, or other compounds from the III-V family; or cadmium telluride (CdTe) or other compounds from the II- VI family. The electron emission surface of the cathode is optionally coated with a coating that modifies the electron affinity, such as cesium (Cs), barium (Ba), strontium (Sr), aluminum nitride (A1N) along with their respective oxides or alloy mixture thereof, polycrystalline diamond or other carbon- based films. The electron emission surface may be further or alternatively patterned and/or modified by structures that improve the emission of electrons from the surface such as nano- wires or nano-tubes.

In an exemplary embodiment of the invention, the radiation-receiving surface of the cathode is coated with an anti-reflective coating and/or is patterned using methods known in the field of photovoltaic cells to increase the absorption of incident sunlight and/or reduce reflectivity. Optionally, the surface is curved (or has an array of microlenses formed or mounted thereon), for example it is domed. Optionally, this allows the surface to be effectively perpendicular to incident light, when placed at a focus of a light concentrator.

In an exemplary embodiment of the invention, the material chosen for the anode may include various electricity-conducting materials that are stable at high temperatures such as tungsten, with an optional surface layer or coating that lowers the work function at the surface, such coating comprising for example a layer of BaO, or diamond or another carbon based film. Any other anode materials and structures capable of withstanding high temperature and providing a relatively low work function, for example less than 2 eV, or less than 1.5 eV, or less than 1 eV, can also be employed.

In an exemplary embodiment of the invention, the cathode and anode contacts to the external circuit are made from any conductive material suitable for the chosen operating temperatures.

In an exemplary embodiment of the invention, the gap between the cathode and anode is under high vacuum. Alternatively, the gap may contain a low-pressure gas, for example, as known in the art of thermionic converters, to facilitate the transport of electrical charges between the two electrodes, for example cesium (described in Hatsopoulos and Gyfopoulos, Vol. 1, cited above) or methane (described in Koeck et al, 2011, cited above).

Side-illuminated cathode

The converter cathode according to some embodiments of the invention may be illuminated through a side surface, which is essentially perpendicular (or at an angle of at least 10 degrees, at least 20 degrees, or at least 45 degrees) to the surface where electrons are emitted. This is in contrast to conventional thermionic converters, where the heating or illumination is provided on a surface parallel to the electron emission surface. Examples of these two configurations are shown in FIGS. 1 and 4. FIG. 4 shows a conventional thermionic converter 400, which is illuminated with light 102 on a front surface of cathode 104 (shown on the left side of the cathode) opposite a surface 106 (shown on the right side of the cathode) where electrons are emitted. The side illumination shown in FIG. 1 allows the area of the surface for electron emission to be higher (e.g., by a factor of 2, 4, 8, 10, 20 or more or intermediate factors) than the area of the surface exposed to incident radiation (aperture surface), in contrast to the prior art front-illuminated thermionic converter shown in FIG. 4, where the illuminated surface has the same area as the emission surface. It can be advantageous to have the aperture surface much smaller than the emission surface, for example, because it may reduce an energy loss from the converter to the environment by emission of radiation, which loss is generally proportional to the aperture surface. Another potential advantage is that increasing the surface area for electron emission to the vacuum may increase the electron emission current and improve the conversion efficiency. Another potential advantage is that a higher concentration of light on a smaller area can be used, for a same total number of cells. In general, as compared to at least some prior designs, for a given incident flux and radiation receiving area, the area for electron emission can be increased and/or for a given area for electron emission, the area for radiation can be decreased, e.g., by increasing the incident flux of radiation.

In some exemplary embodiments of the invention, the emission surface is not smooth and may include, for example, steps or projections (e.g., pyramid shape and/or cone shaped and/or rod shaped). Generally, however, the emission surface defines a geometrical plane or a plurality of such emission planes, to which the direction of the incident radiation is substantially parallel, or at least not perpendicular. Optionally, an axis of an optical concentrator used to provide the illumination radiation is considered to be the radiation direction. In some embodiments, light is provided to a cell geometry so that light is allowed to pass through more than one cathode before being absorbed.

In an exemplary embodiment of the invention, most of the radiation is received over a range of angles of less than 90 degrees, less than 60 degrees, less than 30 degrees or intermediate ranges. In some embodiments of the invention, the thickness of the cathode layer in the side-illuminated converter, t (as shown in FIG. 1), determines the active area of interception of the incident radiation. In some embodiments of the invention, the width of the cathode, which is also generally the (net) depth of the converter device D, can determine the area for emission of electrons into the vacuum gap.

In an exemplary embodiment of the invention, the cathode thickness t is smaller than the electrons' diffusion length. For example, if the cathode is made of heavily doped p type silicon then the thickness could range from 10 to 100 μιη, for example, 20, 40, 60, 80 or intermediate or smaller or larger thicknesses. In other materials the cathode thickness may be lower, for example in GaAs the cathodes thickness may be less than 10 μιη since the diffusion length in GaAs is significantly shorter compared to silicon. The device depth D is optionally chosen according to the radiation absorption depth of the cathode material, and this can be done independently of the selection of the cathode thickness. The considerations used may be similar to the design considerations for vertical junction PV cells, where the absorption of radiation and the charge separation depend on two orthogonal directions and can be optimized and selected independently (described in Posner et al, cited above).

In an exemplary embodiment of the invention, D and t are selected so that electrons formed near the radiation receiving side of the cathode (the top in FIG. 1) can travel down (i.e. toward the cathode bottom in FIG. 1), before being emitted. This may reduce the disparity in emission from the top relative to bottom of the emission area. In an exemplary embodiment of the invention, the cathode material is selected to have high carrier mobility and carrier lifetime, so the transport of the electrons can be over longer distances without too much loss along the way. Optionally, the quality is matched to cathode size and/or desired degree of emission uniformity and/or efficiency.

In an exemplary embodiment of the invention, the thickness of the anode can be very small, for example, 10%, 5%, 1%, 0.5%, or 0.1% of the cathode thickness, or smaller or intermediate thicknesses. Optionally or alternatively, the anode is 1 micron, less than 0.3 microns, less than 0.1 microns, less than 0.05 microns or smaller or intermediate thicknesses. Such a low thickness is possible because the anode does not carry a significant current in the lateral directions, only across its thickness. The anode can then be implemented as a thin coating over the surface comprising the other side of the vacuum gap. In the case that the anode is metallic, it may also constitute a part of the electrical contact of the converter. The small thickness of the anode implies that the fraction of the incident radiation that is intercepted and reflected by the anode is negligible and it does not introduce an additional loss. For example, 70%, 80%, 90%, 95% or larger or intermediate percentages of the side area of the cell or cell array may act as a receiver for radiation. Optionally, the anode is thicker on its side away from the radiation receiving side of the cathode. Optionally, the anode is made reflective of radiation so it reflects incoming radiation to be absorbed by a cathode. Alternatively, the anode at the receiving side is made radiation absorbing so as to be heated by incident radiation.

In an exemplary embodiment of the invention, the design is uniform in a direction perpendicular to the plane of FIG. 1 (length). Alternatively, the design varies in length and/or thickness as a function of depth.

In an exemplary embodiment of the invention, the gap between the cathode and anode has preferably a width lower than 10 micrometers in order to reduce the effect of space charge that may adversely affect the transport of electrons when the gap is wider. As noted above, the inter electrode gap space may be in vacuum, or it may be filled with a low pressure gas such as methane or cesium vapor, which assists in charge transport and further reduces the negative space charge effects.

In an exemplary embodiment of the invention, the part of the incident radiation in a side-illuminated converter that impinges on the gap, may enter the inter-electrode gap instead of entering the active area of the cathode. Optionally, however, most of this radiation is not lost, since it may enter the cathode though its electron emitting surface, or may be reflected at the surface of the anode, and eventually reach the cathode at the other side of the vacuum gap.

Figure 5 shows a high voltage side-illuminated thermionic converter 500 comprising three thermionic converter unit cells in series, with electrical contacts at the ends, each unit cell similar to thermionic converter 100 in FIG. 1. As in FIG. 1, light 102 illuminates a top surface of each cathode, with three cathodes 502, 504, and 506 in the case of converter 500. An emission surface 508 of each cathode emits electrons across a gap 509, and the electrons emitted by cathodes 502, 504 and 506 are collected respectively by anodes 510, 512, and 514. A positive electrode 516 at the back of cathode 502, and a negative electrode 518 at the back of anode 514, are connected to contacts which can be connected to a load. Anode 510 is mounted on the back of cathode 504 and is in electrical contact with it. Anode 512 is mounted on the back of cathode 506 and is in electrical contact with it, while anode 514 is mounted on and is in electrical contact with electrode 518.

In some embodiments of the invention it is desirable that only holes will leave the cathode through the contact. Optionally, an electron blocking layer 520, such as that described in Lerner et al, 1997, Lerner et al, 1998, and Silva et al, all cited above, is added close to the contact opposite the emission surface of each electrode, i.e. close to negative electrode 516 in the case of cathode 502, and close to anodes 510 and 512 respectively in the case of cathodes 504 and 506. This barrier impedes the photo- generated electrons from being injected from the semiconductor cathode back to the metal contact reducing the net current. The barrier can be formed for example by implementing a p ⁺ p junction at the vicinity of the contact, a hetero-structure, or by an accumulative metal semiconductor contact. Optionally, p ⁺ diffusion layer 520 also covers the top and bottom surfaces of each cathode. Such layers repel electrons from the surfaces, reducing their recombination probability and forming a more uniform electron distribution within the cathode.

In an exemplary embodiment of the invention, the anode and contacts are made of two layers. The first layer is a low work function material which is the collector for the emitted electrons and a second layer forms an ohmic contact to the cathode, or, in the case of anode 514, an ohmic contact to electrode 518.

Typically, the conversion efficiency of a photo-thermionic converter depends on several material properties such as the bandgap of the cathode material, and the work functions of the cathode and anode materials, and on operational parameters such as the temperature and the flux and spectral distribution of the incident radiation. For example, when the incident radiation is concentrated sunlight, the cathode bandgap should optionally be in the range of 1-1.5 eV in order to absorb and convert a relatively high fraction of the incident radiation. It is potentially advantageous for the anode work function to be low, for example lower than 1.3, 1.2, 1.0 eV, for example, in the range of 0.5-1.0 eV or less, to increase the flat band voltage and/or the net voltage that the converter can produce. Typically, the optimal operating voltage for a photo-thermionic converter, which maximizes efficiency, is approximately the flat band voltage, which is defined as the operating voltage at which the average electric field across the gap is zero. Higher operating voltage produces an electric field in the gap that reflects emitted electrons back to the cathode, reducing the emission current and the output power. The flat band voltage is typically approximately the difference between the cathode and anode work functions, so a lower anode work function generally allows a higher operating voltage and higher output power. In some cases, a low anode work function may increase the reverse emission current, but most of the reverse emission electrons may be absorbed into the conduction band of the cathode and therefore contribute to increased forward emission, so that lower anode work functions are favorable in most cases. Work functions as low as 0.9eV have been demonstrated in doped CVD diamond, as described in Koeck et al, cited above.

In an exemplary embodiment of the invention, the converter efficiency was calculated as a function of the operating temperature and incident radiation flux for anode work function of 0.5 eV. Figure 6 shows a plot 600 of the maximum efficiency (efficiency at optimal temperature) as a function of flux concentration (in suns) for three values of S, the ratio of area for electron emission to the area for absorption of radiation. Efficiency is plotted on vertical axis 602, and flux concentration is plotted on horizontal axis 604. Curve 606 shows the efficiency vs. flux concentration for S = 1, curve 608 shows the efficiency vs. flux concentration for S = 2, and curve 610 shows the efficiency vs. flux concentration for S = 10. The efficiency is higher for larger S, indicating that increasing the area for electron emission can be very beneficial. An exemplary preferred area ratio may then be for example in the range of 5-10, so that the thickness of each cathode is 5-10 times smaller than the depth of the device. For concentration of the incident sunlight approaching 1000 the efficiency approaches 40%.

Figure 7 shows a plot 700 of the corresponding optimal operating temperatures that yield the maximum efficiency as a function of the incident radiation flux. The temperature is plotted on vertical axis 702, and the flux concentration on horizontal axis 704. Curves 706, 708 and 710 show the optimum temperature vs. flux concentration for the cases S = 1, S = 2, and S = 10, respectively. One preferred operating temperature range for the high-voltage converter is in the range of 320-450 K or 650 K or 700 K or 750 K or 800K or 900 K for the range of concentration of up to 1000. The optimal operating temperatures can be lower compared to the common temperatures in conventional thermionic converters (usually >1000°C), and lower than the cathode temperature of 500°C or higher as proposed for PETE converters with conventional geometry, by Schwede et al, cited above. Some additional desired temperatures and ranges are described below.

In some embodiments of the invention, the property that the anode and cathode are at about the same temperatures provides an upper bound for the desired operating temperature. This may be because the anode emission (reverse) current increases with temperature.

Some exemplary parameter value sets based on estimated results follow. In a practical implementation these results can vary, for example, by 10%, 20%, 30% or intermediate percentages. Parameter values can vary by a factor, for example, a factor of 0.1, 0.2, 0.5, 2, 5, 10 or intermediate or smaller or greater factors.

(i) Anode work function of 0.5eV electron affinity of OeV, band gap of 1.38, S=l:

for X (flux concentration^ 10, T=325K, efficiency= 0.34;

for X=100 T=370K, efficiency=0.348.

(ii) For the same set with S=10:

for X=10, T=300K, efficiency= 0.35;

for X=100 T=330K, efficiency=0.364.

(iii) Anode work function of 0.9eV electron affinity of OeV, band gap of 1.38,

S=l:

for X=10, T=545K, efficiency= 0.25;

for X=100 T=615K, efficiency=0.265.

(iv) For the same set with S=10:

for X=10, T=495K, efficiency= 0.27;

for X=100 T=555K, efficiency=0.285.

In some embodiments, the optimal efficiency increases with the flux concentration. Yet, for a given value of flux concentration there may be an optimum for the operating temperature. This is demonstrated in FIG. 8, which shows a contour plot 800 of efficiency as a function of operating temperature, plotted on vertical axis 802, and flux concentration, plotted on horizontal axis 804. Labeled contours 806 of constant efficiency are plotted, for efficiencies of 0.10, 0.12, 0.14, 0.16, 0.18, 0.20, 0.22, 0.24, and 0.26. Exemplary High Voltage Structure

Figure 9 shows a high-voltage isothermal side-illuminated converter device 900 according to some embodiments of the invention, comprising a plurality of adjacent unit cells, for example three cells 902, 904, and 906, each cell comprising a semiconductor cathode, a metal or semiconductor anode, and a vacuum gap between the cathode and anode, similar to converter 500 in FIG. 5. The cathodes are side-illuminated, and the anode of one pair is electrically connected in series to the cathode of the adjacent unit cell. All unit cells are then electrically connected in series, and external electrical contacts are provided at the electrodes of the first and last unit cells. The converter device may then produce high voltage corresponding to the sum of the voltages of the series connected unit cells, and low current corresponding to the current of a single unit cell. Each cell provides between 0.7 and 1.2 volts, for example, depending on materials used and/or specific design parameters. Optionally, since all adjacent cells in the converter are in contact with each other, the entire converter is isothermal.

Optionally, the thickness of the cells in a high-voltage converter such as device 900, in the horizontal direction in FIG. 9, is small compared to their dimensions in the other two directions, vertical and out of the plane of the drawing. By stacking many such cells together, it is then possible to produce a compact high-voltage converter. This can be done even with cells that are not flat, as the cells in FIG. 9 are, as long as the cells are thin and fit together compactly. For example, the cells could have curved surfaces, or sharply bent surfaces, but all the same shape, so that they fit together in a stack. The cells need not be the same size, but could form concentric C- shaped surfaces or hemispherical surfaces or conical surfaces or V-shaped surfaces that fit together, for example.

Figure 10 shows a schematic band diagram 1000 of such a device. It should be understood that, as is conventional in the field of thermionic converters, potentials that are more negative, i.e. have higher potential energy for electrons, are shown as higher in FIG. 10 than potentials that are more positive. In this example, the electron barrier at the contact was formed by a p ⁺ p junction. The overall voltage 1102 across the device is the sum of the voltages V _1; V ₂ , and V ₃ of the unit cells. In cell 902, on the left, the quasi- Fermi potential of the electrons in the cathode is close to the potential of the positive electrode in contact with the cathode. The potential 1008 immediately outside the emitting surface of this cathode is raised above the quasi-Fermi potential by the cathode work function 1006. Potential 1010 in the gap is nearly constant across the gap, when the operating voltage of the cell is close to the flat band voltage, which is generally close to the optimum operating voltage. As a result, potential 1012 right outside the surface of the anode is close to potential 1008, and the potential inside the anode is lower than potential 1012 by the anode work function 1014. The potential Vi across cell 902, then, is close to the difference between the cathode and anode work functions. If all the cells have the same design parameters and operating conditions, then the potential across each cell will be the same, and the potential 1002 across the whole device will be the potential across each cell, times the number of calls. For example, if the cathodes are based on p type silicon with 0 eV electron affinity, the anode work function is 0.5 eV and the vacuum gap width is 5-10 micrometers, then the output voltage of the converter device may be in the range of 40-100 Volts per centimeter in the direction of the series connection. For example, an array may be 1 cm in each direction.

Such a high-voltage converter device can have one or more of several potential advantages when compared to standard thermionic converters and to PETE converters as proposed in the literature. First, the high-voltage low-current configuration reduces the series resistance losses, which are a significant problem in the previous converters. The configuration of side-by- side unit cells provides the high voltage without need for complex interconnections according to a MIM design, which lead to thermal loss and active area loss. Second, the active area loss with respect to the incident radiation can be made even negligible, since the anode thickness can be made negligible compared to the cathode thickness, and radiation entering the inter-electrode gap may be reflected from the anode and then reach the cathode, so it is not lost. Third, the side illumination allows the surface area for electron emission to be larger than the surface area for absorption of incident solar radiation, potentially resulting in increased electron emission without increasing the radiative losses. Fourth, the entire converter can be isothermal, so that thermal isolation between anode and cathode is not required. Potentially a need to maintain a temperature difference between the two electrodes is eliminated, with the input power coming instead, for example, from electron excitation by light. In an exemplary embodiment of the invention, high conversion efficiency can be achieved at moderate temperatures, due, for example, to photo-excitation of the electrodes, compared to very high temperatures of over 1000°C that are needed in conventional thermionic converters.

Optional Waste heat Conversion

Figure 11 shows a thermionic converter system 1100 that extracts additional energy from waste heat, according to an exemplary embodiment of the invention. A high-voltage side-illuminate isothermal converter device, similar to device 900 in FIG. 9, transfers heat to an infrared absorber and thermal coupling 1101, which is mounted on a heat exchanger 1102, to control its temperature and/or to transfer the waste heat to a secondary device 1106 for converting heat to electricity, or for using the heat in another way. Heat exchanger 1102 transfers heat from the converter to a working fluid 1104 that is pumped through the heat exchanger and brings the heat to secondary device 1106. The secondary device may be, for example, a thermoelectric converter, or a heat engine such as a Stirling engine or a Rankine engine. Output electrical contacts 1108 can be used as a source of electric power. Alternatively, some of all of the waste heat is used for another purpose, for example for space heating, for supplying heat for an industrial process, or to power a refrigerator or air conditioner, with secondary device 1106 being an absorption chiller. In an exemplary embodiment of the invention, the waste heat is collected from both the anode and cathode and therefore it is available to the secondary device at the highest possible temperature, in contrast with a conventional converter where heat may be easily collected only from the anode and generally at a lower temperature (e.g., due to need to maintain also a temperature difference between cathode and anode).

In some embodiments, the usage of a higher temperature can lead to higher conversion efficiency (and/or lower cost) of the secondary converter and, potentially, higher overall power output and efficiency of the combined system. In some embodiments, higher efficiency is provided by ease of heat extraction. In an alternative design, without fluid circulation, a thermal converter, for example, a thermoelectric converter, is mounted directly at infrared absorber and thermal coupling 1101, and receives heat directly by conduction without the need for a mediating heat exchanger.

As an example of the two-stage energy conversion, the performance of the photo-thermionic converter was calculated as a function of the incident radiation flux and of operation temperature, assuming that the secondary converter is a heat engine with efficiency which is 60% of the ideal Carnot efficiency corresponding to the operation temperature. The PETE cathode doping was 10 ¹⁹ cm ^"3 , the anode work function was 0.5eV, the band gap is 1.36eV and the electron affinity is OeV. Figure 12 shows a plot 1200 of the conversion efficiency, plotted on vertical axis 1202 as a function of incident radiation flux, plotted on horizon axis 1204 for each of the two conversion stages (the thermionic conversion efficiency shown by curve 1206 and heat engine conversion efficiency shown by curve 1208) and the efficiency for the entire device, shown by curve 1210. For each flux level, the temperature was optimized separately. The optimum temperature as a function of the incident flux is shown in FIG. 13, in a plot 1300. The optimum temperature is plotted on vertical axis 1302 and the flux concentration is plotted on horizontal axis 1304. For higher concentration, the optimization of the combined device leads to higher temperatures compared to the photo-thermionic converter alone, and producing overall conversion efficiency approaching 50%.

Solid Optics

In an exemplary embodiment of the invention, the optics used are free-space optics, for example, optics which track the sun using lenses or dish reflectors.

Figure 14 shows a side-illuminated high- voltage thermionic converter system 1400, according to an exemplary embodiment of the invention, in which light illuminates a window 1402, and solid concentrators are used, for example, converging light guides 1404, to concentrate light on the cathodes. While the concentrators in FIG. 14 are shown as being wider than a cathode in the stack direction, optionally or alternatively, the concentrators are wider than a cathode in the transverse direction, for example, wider by a factor of 1.5, 2, 3, 4, 5, 6 or more. Similar values for factors may be provided in the stack direction, optionally using a concentrator array that is significantly wider than the device. Optionally, the concentrators are formed of processed optical fibers. In an alternative design, the concentrators are solid state optical elements manufactured using solid state techniques.

Example fabrication method

The above designs may be manufactured in various ways using techniques known in the art for semi-conductor manufacturing. One approach for the fabrication of the photo-thermionic converter device, in accordance with an exemplary embodiment of the invention, is wafer stacking. Figure 15 shows an exemplary fabrication process 1500 with basic stages 1 through 8:

1. A plane p-type wafer 1502 is shown in cross-section.

2. Trenches 1504 of the inter electrode space of wafer 1502 are etched out, on both sides of the wafer.

3. Anodes and contacts 1506 are deposited in the etched trenches on one side.

4. An isolating layer of insulator 1508 is deposited on the unetched surfaces of wafer 1502, at least on one side.

5. An optional electron affinity lowering coating 1510 is deposited in the etched trenches on the other side of the wafer;

6. Wafers are stacked and/or bonded. Although only three wafers are shown stacked in stage 6 of FIG. 15, it should be understood that in practice many more wafers are typically used, corresponding to the number of cells in series in the converter. An additional anode/contact layer 1512 is added to the last wafer on the right.

7. Stacked wafers are diced.

8. Each converter is enclosed in vacuum encapsulation 1514.

Additional steps may be introduced, or some steps modified, or the order of some steps may be changed, for example, as known in the art of semiconductor processing, for example, to reduce processing costs, to improve the compatibility of materials and processes, and/or to introduce additional features such as reduced reflectivity to radiation. For example, steps 3, 4, and 5 are optionally done in any different order, the etching of the two sides of the wafer is optionally done at different times, and depositing the insulator is optionally done before etching the wafer, with portions of the insulator afterwards etched away. Optionally, the insulator is not deposited at all if the unetched surface of the wafer is already coated with a layer of insulator, for example a layer of silicon oxide that may coat a silicon wafer.

In some embodiments of the invention, trenches are etched on only one side of the wafer, for example only on the right side of the wafer in step 2 in FIG. 15, while on the left side of the wafer, any oxide coating is stripped away to expose the surface of the semiconductor, but no trenches are etched, and the anodes and contacts are deposited directly on the stripped surface of the semiconductor in step 3. Alternatively, trenches are etched only on the left side of the wafer in step 2, and any oxide coating is stripped from the right side of the wafer without etching trenches, and the electron affinity lowering coating is deposited directly on the stripped surface in step 5. Etching trenches on only one side has the potential advantage that it may be possible to make the gap narrower.

Typically, the depth of etched trenches 1504 and the thickness of insulating layer 1508 determine the inter-electrode gap width. In some embodiments, it is desirable to make this gap small, for example less than 20, 10 or 5 micrometers, in order to minimize the impact of the negative space charge that may accumulate in this gap. Achieving this desired gap width is well within the current capabilities of the semiconductor processing industry. Other processes such as coating, wafer bonding, dicing, and vacuum encapsulation are also standard processes that can be done on large scale by the semiconductor industry.

In the process described above the cathode thickness is determined by the wafer thickness. In order to fabricate thinner cathodes, when deemed beneficial for example to improve the conversion efficiency (or for suitable materials), a combination of lift off and wafer bonding procedures can be provided in accordance with an exemplary embodiment of the invention. For example, bonding a SOI wafer with a thin active layer and removing the handle wafer will produce thin silicon cathodes. Figure 16 illustrates an iterative process 1600 in which thin layers are stacked together to form a high voltage device. Process steps 1 through 8 are, for example:

1. Deposition of metal contacts/anode 1602 on a p-type SOI wafer.

2. Bonding to a handle wafer 1604.

3. Lift-off, leaving a thin layer 1606 for the cathode.

4. Etching out trenches 1608. 5. Growth of insulators 1610 on the unetched parts of the surface of thin layer 1606.

6. Optional deposition of electron affinity lowering coating 1612.

7. Bonding to an SOI wafer 1614 as in step 1.

8. Lift off of wafer 1614, leaving a thin layer 1616 which forms the cathode of the next cell.

Steps 4-8 may be repeated in order to fabricate several cells in series. Last, the wafers are diced and each device is encapsulated, optionally separately, as in stages 7 and 8 of FIG. 15. As in process 1500, additional steps may be introduced, or some steps modified, or the order of some steps may be changed, in process 1600.

General

It is expected that during the life of a patent maturing from this application many relevant bandgap materials and coating materials will be developed and the scope of the terms used are intended to include all such new technologies which create electrons when heated or illuminated, a priori.

As used herein the term "about" refers to ± 10 %.

The terms "comprises", "comprising", "includes", "including", "having" and their conjugates mean "including but not limited to".

The term "consisting of means "including and limited to".

The term "consisting essentially of" means that the composition, method or structure may include additional ingredients, steps and/or parts, but only if the additional ingredients, steps and/or parts do not materially alter the basic and novel characteristics of the claimed composition, method or structure.

As used herein, the singular form "a", "an" and "the" include plural references unless the context clearly dictates otherwise. For example, the term "a compound" or

"at least one compound" may include a plurality of compounds, including mixtures thereof.

Throughout this application, various embodiments of this invention may be presented in a range format. It should be understood that the description in range format is merely for convenience and brevity and should not be construed as an inflexible limitation on the scope of the invention. Accordingly, the description of a range should be considered to have specifically disclosed all the possible subranges as well as individual numerical values within that range. For example, description of a range such as from 1 to 6 should be considered to have specifically disclosed subranges such as from 1 to 3, from 1 to 4, from 1 to 5, from 2 to 4, from 2 to 6, from 3 to 6 etc., as well as individual numbers within that range, for example, 1, 2, 3, 4, 5, and 6. This applies regardless of the breadth of the range.

Whenever a numerical range is indicated herein, it is meant to include any cited numeral (fractional or integral) within the indicated range. The phrases "ranging/ranges between" a first indicate number and a second indicate number and "ranging/ranges from" a first indicate number "to" a second indicate number are used herein interchangeably and are meant to include the first and second indicated numbers and all the fractional and integral numerals therebetween.

It is appreciated that certain features of the invention, which are, for clarity, described in the context of separate embodiments, may also be provided in combination in a single embodiment. Conversely, various features of the invention, which are, for brevity, described in the context of a single embodiment, may also be provided separately or in any suitable subcombination or as suitable in any other described embodiment of the invention. Certain features described in the context of various embodiments are not to be considered essential features of those embodiments, unless the embodiment is inoperative without those elements.

Although the invention has been described in conjunction with specific embodiments thereof, it is evident that many alternatives, modifications and variations will be apparent to those skilled in the art. Accordingly, it is intended to embrace all such alternatives, modifications and variations that fall within the spirit and broad scope of the appended claims.

All publications, patents and patent applications mentioned in this specification are herein incorporated in their entirety by reference into the specification, to the same extent as if each individual publication, patent or patent application was specifically and individually indicated to be incorporated herein by reference. In addition, citation or identification of any reference in this application shall not be construed as an admission that such reference is available as prior art to the present invention. To the extent that section headings are used, they should not be construed as necessarily limiting.