Improvements in direct bandgap semiconductor solar devices.

Abstract

The photocell comprises a semiconductor element 20 formed from a direct bandgap semiconductor and a transparent bias material 28 overlapping a first portion of the semiconductor front surface 22, the bias material providing a first depletion region 30 to the semiconductor element. A collector 40 is in direct contact with a second portion of the front surface. The collector provides a second depletion region 44 to the semiconductor element. The collector 40 is in close proximity to the bias material 28 but does not make direct conductive contact with it. A continuous region, at least partially depleted of majority carriers, extends between the first and second depletion regions on the front surface of the semiconductor element. The continuous region may include the overlapping portion of the first depletion region 30 and the second depletion region 44, or it may include an additional depletion region 160 formed by a charged dielectric 147.

Description

[Technical field]

[0001] The present invention relates to photovoltaic cells, ie, semiconductor devices capable of converting light into electrical energy.

[0002] [CROSS REFERENCE TO RELATED APPLICATIONS] This application claims the benefit of U.S. Provisional Patent Application No. 62 / 909,424, filed October 2, 2019, the disclosure of which is incorporated herein by reference.

[0003] The disclosures of U.S. patent application Ser. Nos. 16 / 436,004, filed June 10, 2019, 14 / 531,037, and 10 / 355,157, filed November 3, 2014, each entitled "HIGH BAND GAP SOLAR CELLS WITHOUT SEMICONDUCTOR JUNCTIONS," and U.S. Provisional Patent Application Ser. No. 61 / 899,400, filed November 4, 2013, are hereby incorporated by reference. [Background technology]

[0004] Semiconductors have energy levels called the "valence band" and the "conduction band". Electricity is conducted by the movement of electrons in the conduction band and positively charged vacancies or "holes" in the valence band. In an "n-type" semiconductor, there are more conduction band electrons than holes, and therefore the conduction band electrons are called the "majority carriers". In contrast, in a "p-type" semiconductor, there are more holes than electrons, and therefore the holes are the majority carriers. The difference in energy between the valence band and the conduction band is called the band gap of the semiconductor. Many semiconductors can be made as either n-type or p-type by adding small amounts of different dopants.

[0005] One form of photovoltaic cell is a pn junction cell. Such a cell incorporates a layer of n-type semiconductor and a layer of p-type semiconductor in contact with each other. These layers cooperate to define a pn junction. Electrodes are provided in contact with the semiconductors on either side of the junction. The Fermi level is the energy level at which there is a 50% probability that the level is filled with electrons. P-type and n-type materials have different Fermi levels when isolated from each other. In this disclosure, the term "standard Fermi level" is used to refer to the Fermi level of a material that is not affected by contact with another element. In a pn junction cell, the Fermi levels are balanced with each other by the diffusion of electrons from the n-type semiconductor into the p-type semiconductor. This causes a portion of the n-type semiconductor close to the junction to be positively charged and a portion of the p-type semiconductor close to the junction to be negatively charged. These portions constitute a "space charge region" that creates an electric field in the vicinity of the junction. When light impinges on a semiconductor material, absorption of the incoming photons causes electrons to move from the valence band to the conduction band of the semiconductor material, thus increasing the number of charge carrier pairs, i.e., holes in the valence band and electrons in the conduction band.

[0006] The electric field of the space charge region accelerates the charge carriers across the p-n junction, driving additional electrons into the n-type material and additional holes into the p-type material. The holes and electrons move in opposite directions. The electrons go to the first electrode in contact with the n-type material, and the holes go to the second electrode in contact with the p-type material. This creates a potential difference between the electrodes, and therefore useful, usable electrical energy at the electrodes. If a load, such as a resistor, is connected to both electrodes by an external circuit, current will flow through the load.

[0007] A pn junction cell can be easily made from silicon, and silicon pn junction cells are widely used today to convert light energy into electricity, such as in rooftop power generation systems and power generation systems by electric power companies. However, silicon has a band gap of about 1.1 eV (electron volts). Photons of light with energy greater than or equal to the band gap of the semiconductor are absorbed, while photons with energy less than the band gap of the semiconductor pass through the semiconductor. However, when photons with energy greater than the band gap are absorbed, the excess energy is useless and is wasted. The 1.1 eV band gap of silicon corresponds to light in the near infrared part of the spectrum. All of the visible light and ultraviolet light in solar radiation is composed of photons with energy greater than the band gap of silicon. Thus, the theoretical maximum energy conversion efficiency of a silicon photovoltaic cell exposed to typical solar radiation at earth's surface, represented by the AMI 1.5 standard spectrum, is about 30%.

[0008] It is desirable to form photovoltaic cells from materials that have a wide band gap, for example, about 1.8 electron volts or greater. Wide band gap materials can more efficiently absorb light in the visible and ultraviolet portions of the spectrum. Cells formed from wide band gap materials can be used in conjunction with cells formed from narrow band gap materials, such as silicon. In such a structure, the wide band gap cell is placed in front of the narrow band gap cell. Long wavelength light is not absorbed by the wide band gap cell, but travels to the narrow band gap cell where it is absorbed. For example, a structure with a front cell formed from a semiconductor with a 1.8 eV band gap and a rear cell formed from silicon has a theoretical maximum efficiency of 42% for typical solar radiation.

[0009] However, many wide bandgap semiconductors are difficult to fabricate as p-type materials, and therefore wide bandgap pn junction cells are too expensive or have too many defects to be practical in photovoltaic systems.

[0010] Another type of photovoltaic cell is known as a "Schottky cell." In a Schottky cell, a metal element forms an interface with a semiconductor called the "Schottky contact." The metal element also serves as one of the electrodes of the cell, and the other electrode is connected to the semiconductor remote from the Schottky contact. The semiconductor is usually fully n-type or fully p-type, most commonly fully n-type. The metal element is selected so that the metal has a standard Fermi level different from that of the semiconductor. If the semiconductor is n-type, the standard Fermi level of the metal is lower than the standard Fermi level of the semiconductor. The Fermi levels of the metal and semiconductor are equilibrated by the transfer of electrons in the conduction band from the semiconductor adjacent the interface to the metal. This causes the semiconductor in the "depletion region" adjacent the interface to be partially or totally depleted of electrons and therefore positively charged, and the metal element to be negatively charged. The potential difference across the depletion region is called the "built-in voltage." In the ideal case where the metal is in direct contact with the semiconductor and no surface states form at the interface, the built-in voltage is equal to the difference between the normal Fermi level of the semiconductor and the normal Fermi level of the metal. In operation, additional minority carriers (holes in n-type semiconductors) created in the semiconductor by absorption of light are accelerated by the electric field in the depletion region towards the interface, while majority carriers are directed towards the other electrode.

[0011] However, Schottky cells have significant drawbacks. When the cell is operated to supply power to an external load connected to the electrodes, a voltage appears across the load that opposes the current. This load voltage is applied to the electrodes that contain the metallic elements of the Schottky contacts. The load voltage drives the carriers in the opposite direction to that required for power generation. In addition, the load voltage reduces the built-in voltage and reduces the thickness of the depletion region. As will be further described below, the reduction in the thickness of the depletion region is particularly noticeable when the semiconductor is a direct semiconductor. In the direct semiconductors referred to in this disclosure, photons are absorbed by the transition of electrons from the valence band to the conduction band, which does not require interaction with or the creation of another particle or wave. Such a direct transition process should be contrasted with an indirect transition process, which usually involves interactions involving "phonons", i.e., vibrational waves in the semiconductor material, in addition to photons and electrons. Summary of the Invention [Problem to be solved by the invention]

[0012] Direct bandgap semiconductors absorb light efficiently. However, carrier recombination, which occurs when an electron drops from the conduction band to the valence band, occurs much more rapidly in direct bandgap semiconductors than in indirect bandgap semiconductors. If carrier pairs are formed by the absorption of a photon in the space charge region, the carriers are quickly separated from each other under the influence of the electric field. However, if carrier pairs are generated by the absorption of a photon outside the space charge region, the carrier pairs will recombine without ever reaching the electrodes, and all of the energy captured by the formation of the carrier pairs will be lost. Therefore, if the thickness of the space charge region is reduced by the applied voltage to less than the thickness of the region where absorption occurs, the efficiency of the cell will decrease. Most wide bandgap semiconductors are direct bandgap semiconductors. Therefore, Schottky cells using wide bandgap semiconductors do not provide a satisfactory solution. [Means for solving the problem]

[0013] One aspect of the invention provides a photovoltaic cell. The cell according to this aspect of the invention preferably includes a semiconductor element formed from a direct band gap semiconductor. The cell according to this aspect of the invention preferably also includes a transparent biasing agent overlying a first portion of the front surface of the semiconductor, the biasing agent providing a first depletion region in the semiconductor element. The cell preferably further includes a collector in direct contact with a second portion of the front surface separate from the first portion of the front surface, the second portion of the front surface being smaller than the first portion of the front surface, the collector having a standard Fermi level different from the standard Fermi level of the semiconductor element so as to provide a second depletion region in the semiconductor element. For example, the biasing agent may be a metal or semiconductor having a standard Fermi level different from the standard Fermi level of the semiconductor element, the collector being in direct conductive contact with the biasing agent but proximate to the biasing agent. The cell most preferably has a continuous minority carrier channel extending between the first and second depletion regions at the front surface. As further described below, the minority carrier channel is a region of the semiconductor that is at least partially depleted of majority carriers. The minority carrier channel may be entirely constituted by the first and second depletion regions, which overlap each other. Alternatively, a dielectric material having a fixed charge with the same polarity as the majority carriers of the semiconductor may be disposed on the front surface of the semiconductor element in the gap between the bias material and the collector to provide a third depletion region that merges with the first and second depletion regions to form a continuous minority carrier channel. In a further alternative, the bias material may be a dielectric material having a fixed charge with the same polarity as the majority carriers of the semiconductor. In this case, the bias material and the collector may be continuous or adjacent to each other, and the continuous minority carrier channel may be formed by the merger of the first and second depletion regions. The cell preferably further includes an electrode in contact with the semiconductor element at a location spaced apart from the depletion regions.

[0014] The semiconductor element, including the portion extending from the collector and bias material to the electrode, is desirably either entirely p-type or entirely n-type. For example, the entire semiconductor element may be formed entirely from p-type semiconductors or entirely from n-type semiconductors.

[0015] Most preferably, the built-in voltage of the first depletion region is greater than the built-in voltage of the second depletion region, and as described further below, the difference in the built-in voltages provides a lateral potential gradient that drives minority carriers from the first depletion region into the second depletion region through the minority carrier channel, thereby driving the minority carriers laterally towards the collector.

[0016] Further aspects of the invention include circuits including photovoltaic cells, methods of photovoltaic power conversion and tandem cell structures, as further described below. [Brief description of the drawings]

[0017] [Figure 1] FIG. 2 is a schematic plan view of a cell according to one embodiment of the present invention connected to an external circuit. [Diagram 2] 2 is a schematic cross-sectional view taken along line 2-2 in FIG. 1, showing the cell together with external circuit elements. [Diagram 3] FIG. 3 is an enlarged fragmentary view of the area indicated in FIG. 2. [Figure 4] FIG. 3 is a diagram similar to FIG. 2 but showing a cell according to another embodiment of the invention. [Diagram 5] FIG. 5 is an enlarged fragmentary view of the area indicated in FIG. [Figure 6] FIG. 3 is a diagram similar to FIG. 2 but showing a cell according to yet another embodiment of the invention. [Figure 7] FIG. 4 is a schematic plan view showing elements of a cell according to yet another embodiment of the present invention. [Figure 8] FIG. 4 is a schematic plan view showing elements of a cell according to yet another embodiment of the present invention. [Figure 9] 11 is a schematic cross-sectional view illustrating a multi-cell structure according to yet another embodiment of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0018] 1-3 show a cell according to one embodiment of the invention. The cell includes a semiconductor element 20 having a front surface 22, visible in FIG. 1, and a rear surface 24 (FIG. 2) spaced apart in a thickness direction (vertical direction in FIG. 2) from the front surface 22. In this embodiment, the entire semiconductor element is formed from a single type of semiconductor direct bandgap semiconductor, in this case an n-type direct bandgap semiconductor. In this disclosure, references to the type of semiconductor should be understood to describe whether the semiconductor is n-type or p-type when the semiconductor is not subject to an external element or magnetic field. The semiconductor element 22 has a recess on its front surface, and one or more electrodes 26 contact the semiconductor element within the recess. Although only a single electrode is shown, the electrodes may be formed as multiple elements that may optionally be conductively connected to each other. Desirably, the electrode 26 makes ohmic contact with the semiconductor.

[0019] Overlying the front surface 22 of the semiconductor device is a biasing material 28. The biasing material 28 desirably covers a majority of the first surface, referred to herein as the "first portion" of the front surface. The biasing material may be metallic or semiconducting. In this example, the biasing material 28 is applied as a layer thin enough to be transparent to the wavelengths of light absorbed by the semiconductor, but thick enough to form a depletion region as described below. If the biasing material is metallic, it is typically less than 10 nm thick, and most typically between 5 nm and 10 nm thick. The term "transparent" as used in this disclosure refers to an element through which a substantial portion of the light of the wavelengths of interest is transmitted. Complete transparency, i.e., 100% transmission, is not required. In Figures 2 and 3, the thickness of the biasing material 28 is greatly exaggerated for clarity of illustration. In the illustrated embodiment, the biasing material is in direct contact with the front surface of the semiconductor, with no intervening material between the biasing material and the semiconductor. In other embodiments, a thin layer of dielectric may be provided between the bias material and the semiconductor front surface, provided that the dielectric layer is thin enough to allow electrons to tunnel through the layer.

[0020] As will be further described below, in a circuit incorporating the cell, the bias material 28 is not directly conductively connected to any element of the circuit other than the semiconductor. In particular, the bias material is not conductively connected between the semiconductor element and a load in the circuit. Thus, the bias material does not act as an electrode of the cell and does not carry the photocurrent generated by the cell during operation. Preferably, the bias material does not have terminals adapted for conductive connection to external circuit elements. If a dielectric passivation layer (not shown) is provided on the front surface of the cell, this passivation layer desirably covers the entirety of the bias material, thereby making the bias material physically inaccessible for connection to any external circuit elements.

[0021] In this embodiment, the semiconductor device 20 is n-type and the bias material 28 has a normal Fermi potential EFM that is lower than the normal Fermi level EFS of the semiconductor. In other words, the work function Φm of the bias material is greater than the work function of the semiconductor under normal conditions. The work function of a material is the energy required to move an electron from the Fermi level of the material to vacuum. The work function of a metal is also referred to as the "electron affinity" of the metal.

[0022] The bias material 28 introduces a first depletion region 30 in the semiconductor adjacent the front surface. This depletion region extends behind the first region of the front surface covered by the bias material. As shown diagrammatically in FIG. 3, the first depletion region also extends laterally beyond the edge of the bias material in a direction parallel to the front surface of the semiconductor. The mechanism of formation of the first depletion region is similar to that of the formation of a Schottky contact. For electrons to be able to move between the semiconductor and the bias material, the Fermi levels of the bias material and the semiconductor at the interface between them must come into equilibrium where both levels are the same. For this to happen, electrons move from the semiconductor to the bias material. This causes the semiconductor to become electron deficient through the region adjacent the front surface 22 and therefore positively charged, and the bias material to become negatively charged. If the bias material is a metal, the charge in the bias material is concentrated in an extremely thin region, typically a few angstroms thick, called the "delta charge region" (not shown) that abuts the front surface of the semiconductor. Electrons in the conduction band of the semiconductor adjacent to the front surface are repelled by the negative charge on the bias material. With progressively increasing distance from the front surface, this repulsion is reduced by the progressively larger amount of positively charged semiconductor interposed between the bias material and the electrons. In other words, an electric field exists in the first depletion region 30 of the semiconductor.

[0023] Electrons in the conduction band of the semiconductor within the depletion region have additional potential energy provided by the electric field and are therefore at a higher energy level than electrons in the conduction band outside the depletion region. This is represented in FIG. 3 by curve 32, which shows the energy levels of the conduction band. At the interface of the semiconductor with the bias material, the energy level EC of the electrons in the conduction band is the standard value ECN for a semiconductor not affected by the bias material. In the region of the semiconductor outside the depletion region, the energy level of the electrons in the conduction band is at a more positive value EC30. The energy level EC decreases progressively with increasing distance from the interface, reaching EC30 at the boundary of the depletion region. The rate of decrease with distance represents the strength of the electric field in the semiconductor in the depletion region 30, which value reaches zero at the boundary of the depletion region. Since the band gap is a fixed quantity, the energy levels of the valence band follow an identical curve (not shown) to curve 32, but at a lower energy level. The term "band bending" is commonly used to describe the distortion of energy levels within the depletion space, which is also called the "space charge region" or "depletion zone."

[0024] The magnitude of the bend in these curves is equal to the difference between EC30 and ECN and is referred to as the built-in voltage VBI30 of the first depletion region 30. In the "ideal" case, where the bias material is in direct contact with the semiconductor without an intervening tunneling dielectric layer and where surface states at the interface do not affect the barrier height, the built-in voltage VBI30 is equal to the standard Fermi level difference between the semiconductor 20 and the bias material 28. The size of the depletion region depends on the built-in voltage and the dielectric constant of the semiconductor, along with the carrier concentration of the semiconductor, and can be easily calculated by one skilled in the art.

[0025] The cell further includes a conductive collector 40. The collector 40 is in direct contact with the semiconductor at a second portion of the semiconductor front surface 22. The second portion of the front surface is separate from the first portion covered by the bias material 28. However, the two surface portions are in close proximity to each other such that there is a small gap distance DG (FIG. 3) between the bias material 28 and the collector 40 at the semiconductor front surface 22. In this embodiment, the collector 40 is disposed in an opening 42 in the bias material, although this configuration is not required. The collector 40 is preferably opaque. The collector 40 includes a metal disposed at an interface with the semiconductor, the metal having a Fermi level different from the normal Fermi level of the semiconductor, and forming a Schottky contact with the semiconductor. In the illustrated embodiment, the collector 40 is a unitary mass of a single metal, although the collector may include multiple layers of different metals. The collector 40 is adapted to serve as a terminal for connecting to an external circuit.

[0026] The collector 40 forms a second depletion region 44 in the semiconductor. This depletion region extends behind the second region of the front surface covered by the collector. As shown diagrammatically in FIG. 3, the second depletion region also extends laterally beyond the collector 40 in a direction parallel to the front surface of the semiconductor. In this embodiment, the lateral extent of the second depletion region 44 beyond the collector 40 plus the lateral extent of the first depletion region 30 beyond the bias material 30 is greater than the gap distance DG, so that the first and second depletion regions merge with each other. The merged depletion regions form a continuous minority carrier channel extending between the first and second depletion regions.

[0027] Curve 45 shows diagrammatically the energy level of the conduction band in the second depletion region 44. Desirably, the built-in voltage VBI 44 of the second depletion region 44 is lower than the built-in voltage VBI 30 of the first depletion region 30. Thus, adjacent to the semiconductor front surface 22, across the gap between the bias material 28 and the collector 40, there is a laterally directed electric field component. This laterally directed electric field component tends to accelerate minority carriers (holes in an n-type semiconductor) laterally from the first depletion region 30 to the second depletion region 44 towards the collector 40. The laterally directed electric field component directs majority carriers (electrons) in the opposite laterally direction, away from the collector.

[0028] The cell as described above is in an open circuit, dark state. In use, the cell is connected to an external circuit element 50, such as a load 52 in series with a switch 54, to form a circuit including the load 50 and the cell. When the circuit is in an active state, the load is conductively connected between the collector 40 and the electrode 26, with the switch 54 closed. Light passes through the bias material into the semiconductor body. When the light is absorbed by the semiconductor, it drives electrons from the valence band to the conduction band, thus creating additional carriers, i.e., electrons and holes, in the first depletion region 30. The electric field component in the thickness direction in the first depletion region drives the additional carriers in the opposite direction. The minority carriers (holes) are driven towards the front surface of the semiconductor, while the majority carriers (conduction band electrons) are driven away from the front surface into the bulk of the semiconductor outside the depletion region 30. At the same time, the holes are propelled laterally towards the second depletion region 44 and the collector 40 by the laterally directed electric field. Furthermore, because the collector 40 is opaque, very little or no light reaches the second depletion region 44. Thus, essentially no additional carriers are generated in the second depletion region 44. The concentration of holes in the second depletion region is much lower than the concentration of holes in the first depletion region 30. The hole concentration gradient between the first and second depletion regions creates a diffusion current of holes formed in the first depletion region into the second depletion region and towards the collector.

[0029] The depletion region has a low concentration of majority carriers (electrons), which limits electron-hole recombination. The continuous minority carrier channel ensures that minority carriers formed in the first depletion region 30 can travel to the second depletion region 44 without passing through a region of the semiconductor with a high concentration of majority carriers. Furthermore, because the minority carriers (holes) are driven by the electric field and the concentration gradient, the time required for the minority carriers to reach this collector is minimized, thus further limiting recombination losses. The majority carriers (electrons) enter the bulk of the semiconductor and diffuse to the electrode 26. The flow of minority carriers to the collector 40 and majority carriers to the electrode 26 constitutes the desired photocurrent in the cell.

[0030] Essentially no electric field exists in the semiconductor outside the depletion region. Thus, if carrier pairs are formed in the bulk region of the semiconductor device 20 outside the depletion regions 30 and 44, the carriers move only by diffusion. Because the carrier recombination time of direct bandgap semiconductors is very short, and because the bulk region has a high concentration of majority carriers, essentially all of the minority carriers generated by absorption of light outside the depletion region are lost by recombination. Most preferably, most of the light passing through the bias material 28 is absorbed in the first depletion region 30. In other words, the thickness of the first depletion region is most preferably greater than the thickness of the absorption region of the semiconductor, and more preferably at least about twice the thickness of the absorption region. The light intensity IX at a depth X from the front surface 22 of the semiconductor is given by the following equation: IX=I0e-αX where I is the intensity of light at the front surface 22 and α is the absorption coefficient of the semiconductor for light impinging on it. Unless otherwise specified in this disclosure, the value of α should be interpreted as an average value for the portion of solar radiation that has energy greater than the band gap of the semiconductor.

[0031] The thickness tA of the absorbing region as used in this disclosure is taken to be equal to the depth x which is equal to α-1. At this depth, IX / I0 is equal to e-1 or approximately 0.37. In other words, tA is the depth x at which approximately 63% of the incoming photons are absorbed. At a depth x equal to twice tA, IX / I0 is approximately 0.14, so 86% of the incoming photons are absorbed. By way of example only, for a zinc oxide semiconductor, tA is approximately 100 nanometers.

[0032] When the switch 54 is closed and the cell is illuminated, current flows from the collector 40 through the load 52 of the external circuit 50 to the electrode 26. In other words, electrons flow from the electrode 26 to the collector 40. The voltage across the load 52 appears as an external bias voltage between the collector 40 and the electrode 26. This external bias applied by the load cancels the electric field in the second depletion region applied by the Schottky contact between the collector and the semiconductor. This effectively reduces the built-in voltage and reduces the size of the second depletion region 44, as shown diagrammatically by the curve 45' (FIG. 3). Desirably, despite the reduction in the size of the second depletion region under load, there is still some overlap with the first depletion region, so that a continuous minority carrier channel remains in the loaded state. However, because the bias material 28 is not conductively connected to the collector 40 other than through the semiconductor, the external bias applied by the load voltage is not applied directly to the bias material, so that the built-in voltage and the size of the first depletion region 30 remain substantially unaffected.

[0033] Furthermore, the external bias voltage tends to generate a current that opposes the photocurrent, as some majority carriers from the bulk region of the semiconductor move toward the front of the cell against the electric field in the depletion region. This reverse current reduces the photocurrent flowing in the cell. However, since the external bias voltage is applied to the collector 40 and not to the bias material 28, this effect occurs primarily in the regions of the semiconductor body aligned with the collector 40. These regions constitute a relatively small portion of the cell. Furthermore, the reverse current is reduced by the action of the laterally directed electric field driving the majority carriers from the second depletion region toward the first depletion region described above. Furthermore, since the collector 40 in this embodiment is opaque, these regions of the semiconductor are not substantially illuminated. Little or no carriers are formed in the regions of the semiconductor aligned with the collector, and therefore these regions have a low carrier concentration. Thus, the reverse current is smaller than it would be in a comparable cell with a transparent collector.

[0034] The cells considered herein are referred to as "isolated collection and biasing system" or "ICBS" cells in that the collector and biasing material are not directly conductively connected to each other. As mentioned above, the only connection between the collector and the biasing material is through the semiconductor. In an ICBS cell, the depth of the first depletion region is substantially unaffected by the bias voltage applied by the load during operation. Furthermore, the reverse current induced by the load is smaller than the reverse current in an otherwise equivalent Schottky cell. Furthermore, the separate biasing material and collector allow the use of different materials in these elements, so that the material of each element can be optimized for its particular function with the semiconductor. These advantages are further illustrated below by Example 1.

[0035] Example 1

[0036] Generally, a number of ICBS cells according to the embodiment described above with reference to Figures 1-3 were formed using an n-type gallium arsenide semiconductor, an opaque collector formed from gold, and a bias material formed from silver. The collectors of the various ICBS cells were of different shapes. Each ICBS cell had a gap of about 100 nm between the collector and the bias material, so that the collector and the bias material were not directly conductively connected to each other, but rather a continuous minority carrier channel extended between the first and second depletion regions, as described above. The ICBS cells were tested. Schottky cells were formed from the ICBS cells by connecting a metal shunt between the collector and the bias material of each cell, such that the collector and the bias material formed a composite Schottky contact. The shunt did not contact the semiconductor. Thus, the Schottky cells were identical to the ICBS cells, except for the presence of the shunt. The performance of the ICBS cells and the Schottky cells were measured using the same illumination and measurement equipment. The results are shown in the table below.

[0037] [Table 1]

[0038] In the table, RS is the series resistance, Rsh is the shunt resistance, ΦB is the barrier height, J0 is the reverse saturation current, VOC is the open circuit voltage, JSC is the short circuit current, and FF is the fill factor. The fill factor is the ratio of the maximum power obtainable to the product of the open circuit voltage and the short circuit current. Each value in the table is the average for the ICBS cell or the average for the Schottky cell labeled in the "Cell Construction" column.

[0039] The ICBS cell had a higher VOC, a higher JSC and a higher FF than the Schottky cell, and therefore the ICBS cell generated substantially more power than the Schottky cell.

[0040] A cell according to a further embodiment of the invention (FIGS. 4 and 5) incorporates a semiconductor element 120, an electrode 126, a bias material 128 and a collector 140 as described above with reference to FIGS. 1 to 3. Again, the bias material 128 forms a first depletion region 130 and the collector 140 forms a second depletion region 144. In the cell of FIGS. 4 and 5, a dielectric 147 having a fixed charge is provided in the gap between the bias material 128 and the collector. The charge of the dielectric 147 is such that it repels the majority carriers in the semiconductor. For example, if the semiconductor element is n-type, the fixed charge in the dielectric adjacent to the semiconductor is negative. The charged dielectric therefore induces a further depletion region 160 in a small area of ​​the semiconductor aligned with the gap. This additional depletion region overlaps with the first depletion region 130 and the second depletion region 144, so that even if the gap distance DG between the bias material 128 and the collector 140 is such that the first depletion region 130 does not overlap with the second depletion region, there is a continuous minority carrier channel including the first depletion region and the second depletion region. In other words, the gap distance DG between the bias material 128 and the collector 140 may be larger than the sum of the lateral extent of the first depletion region beyond the bias material 128 and the lateral extent of the second depletion region 144 beyond the collector 140. Because the gap distance DG may be relatively large, the precision required in the process used to form the collector and the bias material may be reduced. Furthermore, a continuous minority carrier channel may be maintained under severe load conditions.

[0041] The dielectric 147 may act as a passivation layer covering the bias material 128, as best seen in Figure 4. The charge in the dielectric inducing the additional depletion region 160 may form as a surface charge at the interface of the dielectric and the semiconductor. In a further variation, the dielectric may be an electret, i.e., a dielectric such as an organic dielectric with separate fixed positive and negative charges. If the semiconductor element is n-type, the negative charges are located adjacent to the semiconductor and the positive charges are located away from the semiconductor.

[0042] A cell according to a further embodiment of the present invention (FIG. 6) includes a semiconductor element 220, an electrode 226, and a collector 240 similar to the corresponding elements described above. In the cell of FIG. 6, the bias material is a dielectric having a fixed charge therein. The dielectric used in this embodiment may be similar to the dielectrics described above with reference to FIGS. 4 and 5. In this case, the dielectric induces the first depletion region 230 by the repulsion of minority carriers. Since the bias material is a dielectric, the bias material 230 may contact the collector 240 without establishing a conductive connection with the collector. In other words, the first region of the semiconductor front surface occupied by the bias material may be continuous with the second region of the semiconductor front surface occupied by the collector. The bias material may be formed adjacent to or continuous with the collector, so that the gap distance between the collector and the bias material may be small or non-existent. This facilitates the formation and maintenance of a continuous minority carrier channel including the first depletion region and the second depletion region. A cell according to this embodiment can be fabricated by forming a dielectric bias material to leave one or more spaces where the collector is to be deposited, and then depositing the collector into these one or more spaces to completely fill the spaces, such that precise control of the collector deposition process is not required.

[0043] In any of the embodiments discussed herein, the collector may be formed in a configuration that includes multiple elements distributed over the front surface of the cell. For example, the cell of FIG. 7 includes a collector formed as multiple separate collector elements 341, each of which contacts the front surface of the semiconductor device through an opening in the bias material 328. The collector elements 341 are conductively connected to each other by conductors such as wires or traces 301. In a further variation (FIG. 8), the collector elements 441 include elongated strips that extend into slots formed in the bias material 428, and the collector elements are conductively connected to each other. In these and other embodiments using a multi-element collector, the above-mentioned conditions necessary to maintain a continuous minority carrier channel between a first depletion region formed by the bias material and a second depletion region formed by the collector without forming a conductive connection between the collector and bias material elements are provided along each boundary between the collector and bias material elements. The collector, whether it includes a single element or multiple elements, desirably occupies only a small portion of the front surface of the semiconductor device. Preferably, the bias material occupies at least about 75% of the front surface. The ratio of the area of ​​the first region covered by the bias material to the area of ​​the second region covered by the collector is preferably at least about 5:1, more preferably about 7:1 or greater. The embodiments of Figures 7 and 8 include an electrode (not shown) similar to electrode 26 for receiving majority carriers, and the electrode in these embodiments may be located on any surface of the semiconductor, such as the rear surface of the cell.

[0044] In the embodiments of Figures 1-6, the electrodes are disposed in recesses in the front surface of the semiconductor element, however, this is merely by way of example, and the front surface of the semiconductor element may be planar and the electrodes may be disposed on the front surface not occupied by the bias material or collector, on the rear surface of the semiconductor element, or anywhere else away from the bias material, collector and depletion region.

[0045] In the above-described embodiments, the entire semiconductor element is formed entirely from n-type semiconductors. If suitable direct transition semiconductors are available as p-type, the entire element can be formed from p-type semiconductors. The collector and bias elements in such embodiments have a higher Fermi level (lower work function) than the semiconductor that forms the depletion region. If a dielectric with a fixed charge is used to form the depletion region, the dielectric has a positive charge.

[0046] In the embodiments described above, the entire semiconductor device is formed entirely of one type of semiconductor, however the semiconductor device may also include regions of the opposite type outside the regions forming the cells.

[0047] The above-described cells can provide efficient photovoltaic conversion and can be fabricated using readily available direct transition semiconductors, including wide bandgap semiconductors. Suitable semiconductors include III-V and II-VI semiconductors. Examples of III-V semiconductors include those that contain one or more elements selected from the group consisting of gallium, indium, and aluminum, and one or more elements selected from the group consisting of nitrogen, phosphorus, arsenic, and antimony. Examples of II-VI semiconductors include those that contain metals selected from the group consisting of cadmium, zinc, mercury, and cobalt, and one or more elements selected from the group consisting of oxygen, sulfur, selenium, and tellurium. Other elements, such as transition metal elements and rare earth elements, can be alloyed with III-V and II-VI semiconductors to tune the bandgap. Thus, the terms "III-V semiconductor" and "II-VI semiconductor" as used in this disclosure refer to such semiconductors with or without alloying elements, unless otherwise specified.

[0048] The cells described above can be used for a variety of purposes. However, for applications such as photovoltaics, any of the cells described above can be used in a multi-cell structure incorporating two or more cells with different bandgaps arranged in order of decreasing bandgap with the cell with the widest bandgap at the front of the structure and the cell with the narrowest bandgap at the rear. The cell with the widest bandgap is placed in front of one or more narrower bandgap cells. The cells can be formed separately and then attached to each other. In other arrangements, the semiconductor with the wider bandgap is grown on the semiconductor with the lower bandgap with or without an intervening element such as a lattice matching layer. One example of a multi-cell structure is shown in FIG. 9. The structure includes a first cell 501 similar to the cells described above, having a direct bandgap semiconductor element 520, a bias material 528, and a collector 540 located on the front surface 522 of the semiconductor element. Again, as described above, the bias material forms a first depletion region 530 and a collector 540 that forms a second depletion region (not shown). Again, there is a continuous minority carrier channel that includes the first and second depletion regions. An electrode 526 is provided in contact with the wide bandgap semiconductor. The structure further includes a second cell 503 having a semiconductor element 505 disposed behind the rear surface 524 of the semiconductor element 520 of the first cell 501. The semiconductor element 505 of the second cell has a narrower bandgap than the semiconductor element 520 of the first cell. The second cell, cell 503, includes electrodes 507 and 509 in contact with the semiconductor 505. For example, cell 503 may be a conventional silicon Schottky cell or a conventional silicon pn junction cell. In a two cell structure, when the semiconductor element of the second, narrower bandgap cell is formed from silicon, the first, wider bandgap cell preferably has a bandgap of 1.4 eV or greater, more preferably 1.6 eV or greater, and most preferably 1.8 eV or greater.

[0049] If the wide bandgap cell 501 has a first depletion region 530 that is substantially thicker than the thickness of the absorption region, e.g., at least twice the thickness of the absorption region of the semiconductor 520, essentially all of the short wavelength photons having energy greater than the bandgap of the semiconductor 520 are absorbed in the first depletion region and converted to useful electrical energy. In this case, the thickness (from the front surface 522 to the rear surface 524) of the wide bandgap semiconductor element 520 can be much greater than the thickness of the first depletion region without compromising efficiency. However, if the thickness of the first depletion region is less than about twice the thickness of the absorption region, a significant portion of the short wavelength photons will not be absorbed in the first depletion region. In this case, the thickness of the wide bandgap semiconductor element 520 is desirably less than twice the thickness of the first depletion region 530, more desirably less than 1.5 times the thickness of the first depletion region, and most desirably equal to the thickness of the first depletion region. This minimizes absorption of short wavelength photons in the wide band gap semiconductor 520 outside the first depletion region, thereby allowing short wavelength photons that are not absorbed in the first depletion region 530 to enter the narrow band gap cell 503 where at least a portion of the energy of these photons is converted into useful electrical energy.

[0050] Although the invention has been described herein with reference to particular embodiments, it is to be understood that these embodiments are merely illustrative of the principles and applications of the present invention. It is therefore to be understood that numerous modifications can be made to the illustrative embodiments and that other arrangements can be devised without departing from the spirit and scope of the invention as defined by the appended claims.

Claims

1. (a) a semiconductor device formed from a direct band gap semiconductor having a front surface and a rear surface; (b) a transparent bias material in direct contact with a first portion of the front surface, the bias material having a normal Fermi level different from a normal Fermi level of the semiconductor element so as to produce a first depletion region in the semiconductor element; (c) a collector in direct contact with a second portion of the front surface distinct from the first portion, wherein the second portion of the front surface is smaller than the first portion of the front surface, the collector having a standard Fermi level different from the standard Fermi level of the semiconductor element so as to provide a second depletion region in the semiconductor element, the collector being in direct conductive contact with but proximate to the bias material, the bias material not being conductively connected to the collector except through the semiconductor element, and there being a continuous minority carrier channel at the front surface extending between the first depletion region and the second depletion region; (d) an electrode contacting the semiconductor element at a location spaced from the depletion region; A photovoltaic cell comprising:

2. 10. The photovoltaic cell of claim 1, wherein the semiconductor element has a first region extending from the front surface to the electrode, the first region being entirely p-type or entirely n-type.

3. 3. The photovoltaic cell of claim 2, wherein a magnitude of a built-in voltage of the first depletion region is greater than a magnitude of a built-in voltage of the second depletion region.

4. 4. The photovoltaic cell of claim 3, wherein said first region is n-type and said collector has a lower work function than said bias material.

5. 4. The photovoltaic cell of claim 3, wherein said semiconductor element is selected from the group consisting of gallium arsenide and aluminum gallium arsenide, said bias material is silver, and said collector is gold.

6. 2. The photovoltaic cell of claim 1, wherein said semiconductor element is selected from the group consisting of III-V semiconductors and II-VI semiconductors.

7. 3. The photovoltaic cell of claim 2, wherein the electrode overlies and contacts at least a portion of the rear surface of the semiconductor device.

8. 2. The photovoltaic cell of claim 1, wherein the collector includes a plurality of spaced apart collector elements on the front surface of the semiconductor device, the collector elements being conductively connected to one another, and the bias material extending between the collector elements.

9. 9. The photovoltaic cell of claim 8, wherein each of said collector elements is opaque.

10. The photovoltaic cell of claim 1, wherein the electrode is in ohmic contact with the semiconductor element.

11. 10. The photovoltaic cell of claim 1, wherein said biasing material has no terminals in direct conductive connection with any element external to said cell.

12. 10. The photovoltaic cell of claim 1, wherein the bias material is a metal or a semiconductor, and there is a gap between the bias material and the collector.

13. 13. The photovoltaic cell of claim 12, wherein said first depletion region and said second depletion region merge with one another to form said continuous minority carrier channel.

14. 13. The photovoltaic cell of claim 12, further comprising a dielectric having a fixed charge disposed in the gap, the dielectric forming an additional depletion region, the additional depletion region merging with the first depletion region and the second depletion region to form the continuous minority carrier channel.

15. 10. The photovoltaic cell of claim 1, wherein the biasing material is a dielectric material having a fixed charge.

16. A circuit comprising a photovoltaic cell according to any one of claims 1 to 15, and a load electrically connected between the collector and the electrode of the photovoltaic cell, wherein the bias material is not conductively connected between the semiconductor element and the load.

17. 16. A circuit comprising a photovoltaic cell according to any one of claims 1 to 15, and circuit elements including a load electrically connected between the collector and the electrode of the photovoltaic cell, wherein the bias material is not directly conductively connected to any of the circuit elements other than the semiconductor element of the cell.

18. A multi-cell structure comprising a first photovoltaic cell according to any one of claims 1 to 15, and a second photovoltaic cell including a semiconductor element having a band gap narrower than the band gap of the direct transition semiconductor of the first photovoltaic cell, the semiconductor element of the second photovoltaic cell being disposed behind the rear surface of the first photovoltaic cell.

19. 20. The multi-cell structure of claim 18, wherein said first depletion region of said first photovoltaic cell has a thickness greater than a thickness of said direct bandgap semiconductor absorption region.

20. 20. The multi-cell structure of claim 19, wherein the first depletion region of the first photovoltaic cell has a thickness less than twice the thickness of the absorption region of the direct band gap semiconductor, and the semiconductor element of the first photovoltaic cell has a thickness less than twice the thickness of the first depletion region.

21. 16. A method of photovoltaic power generation comprising applying light to a photovoltaic cell according to any one of claims 1 to 15 with a load connected between the collector and an electrode of the cell, and diffusing electrical energy generated by the cell to the load, wherein the continuous minority carrier channel is maintained during the steps of applying light and diffusing.

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