Multijunction photovoltaic device and fabrication method

Abstract: A multijunction photovoltaic device includes first and second amorphous silicon PIN photovoltaic cells in a stacked arrangement. An interface layer, composed of a doped silicon compound, is disposed between the two cells and has a lower bandgap than the respective n- and p-type adjacent layers of the first and second cells. The interface layer forms an ohmic contact with the one of the adjacent cell layers of the same conductivity type, and a tunnel junction with the other of the adjacent cell layers. The disclosed device is fabricated by a glow discharge process.

Description: BACKGROUND OF THE INVENTION Field of the Invention The present invention relates in general to multijunction photovoltaic devices and, more particularly, to improvements in the junctions between adjacent cells in a multijunction amorphous silicon photovoltaic device. Description of the Related Art Photovoltaic devices are used to convert solar radiation into electrical energy. This conversion is achieved as a result of what is known as the photovoltaic effect. When solar radiation strikes a photovoltaic cell and is absorbed by an active region of the cell, pairs of electrons and holes are generated. The electrons and holes are separated by an electric field built into the cell. In accordance with a known construction of solar cells using amorphous silicon, the built-in electric field is generated in a structure consisting of p-type, intrinsic and n-type layers (PIN) of hydrogenated amorphous silicon (a-Si:H). In solar cells having this construction, the electron-hole pairs are produced in the intrinsic layer of the cell when solar radiation of the appropriate wavelength is absorbed. The separation of the electrons and holes occurs under the influence of the built-in electric field, with the electrons flowing toward the region of n-type conductivity and the holes flowing toward the region of p-type conductivity. This flow of electrons and holes creates the photovoltage and photocurrent of the photovoltaic cell. Some of the incident light is absorbed by the doped layers (the p-layer and the n-layer) but the carriers generated in these layers have an extremely short carrier lifetime and recombine before they can be collected. Hence, absorption in the doped layers does not contribute to the photocurrent of the photovoltaic cell and a minimization of absorption in doped layers enhances the short-circuit current of PIN photovoltaic cells. Absorption loss in the p-layer is a function of the bandgap of the p-layer. Thus, by adjusting the bandgap of the p-layer, the absorption loss in the p-layer can be minimized by including in the p-layer a bandgap widening material such as carbon, nitrogen, oxygen or fluorine. For example, the p-layer can be provided as hydrogenated amorphous silicon carbide (a-SiC:H) with p-type doping. However, the addition of bandgap widening material to the p-layer increases its resistance. Therefore the amount of bandgap widening material that is added is usually limited by the amount of resistance considered tolerable in the device. The n-type layer functions to form a rectifying junction with the intrinsic layer. In order to enhance this function, it is desirable to provide the n-layer with a high conductivity. However, it is also desirable to provide the n-layer with a wide optical bandgap since, as described above, carriers generated therein do not contribute to the photocurrent of the cell. Unfortunately, as is the case of the p-layer, the addition to the n-layer layer of any of the bandgap widening elements described above results in an increase in the resistance of the n-layer. Therefore, the n-layer is typically provided with a concentration of a bandgap widening element that is limited by the amount of resistance considered tolerable in the device. It is desirable to increase the total number of photons of differing energy and wavelength which are absorbed in order to maximize the photocurrent output of a photovoltaic device. One technique for increasing photon absorption, and thereby increase device efficiency, is to provide a multijunction photovoltaic device with two or more photovoltaic cells arranged in a stacked configuration, i.e., one on top of the other. Such a multijunction photovoltaic device, also known in the art as a tandem junction solar cell, is disclosed in U.S. Patent No. 4,272,641 issued to Hanak (the '641 patent). In particular, that patent teaches the construction of tandem junction amorphous silicon solar cells, wherein each cell has the above described PIN structure. FR-A-2 602 913 also discloses a photovoltaic device in which a plurality of unit photovoltaic cells is stacked in series. According to one embodiment, there is described a device which comprises a first unit photovoltaic cell applied at its light receiving side onto a transparent electrode. A second unit photovoltaic cell is applied on the first unit photovoltaic cell and a back electrode is applied, thereto. The device further comprises one intermediate layer of, for example, amorphous silicon of n- or p- type conductivity. Each unit cell is made of mainly amorphous silicon a characteristic of this device is the structure of the impurity doped layers at the n/p contact interface between the first and second adjacent unit photovoltaic cells. In order for such multifunction PIN photovoltaic devices to operate at maximum efficiency, current must. flow unimpeded from each photovoltaic cell to the next adjacent cell in the stack of cells. However, the nature of the multijunction PIN photovoltaic device, i.e., p-i-n-p-i-n..., results in an n-p junction occurring at each interface between adjacent PIN cells and therefore in series electrically with those adjacent cells. Disadvantageously, each of these n-p junctions represents a diode having a polarity opposite to that of the photovoltage generated by each of the adjacent photovoltaic cells. The n-p junctions are non-linear elements that oppose the flow of photocurrent and thereby impose a substantial power loss on the device. Figure 1 illustrates a plot of current vs. voltage (IV) of a multijunction PIN photovoltaic device. In particular, curve 100 (broken line) represents the IV characteristic for such a photovoltaic device in which no steps have been taken to overcome the adverse effect of the n-p junctions at the interfaces between adjacent cells. As illustrated by curve 100, an inflection occurs in the region where the photocurrent of the device changes direction. Such an inflection represents an undesirable increase in the series resistance of the device due to the n-p junction. This aspect of the IV curve, characteristic of the n-p junction, limits the amount of photocurrent that can be conducted by the photovoltaic device, and therefore lowers the fill factor and power generation capability of the device. As used herein, the fill factor of a photovoltaic device is the ratio V mp I mp /I L V OC , where V mp and I mp are respectively the voltage and current at maximum power delivery of the device, and V OC and I L are respectively the maximum voltage and current achievable in the device. A solution to the above described problem caused by the n-p junctions is to modify the structure of the multijunction device so that the junction occurring between each pair of adjacent cells performs like a tunnel junction. One known method for creating a tunnel junction between adjacent solar cells of a multijunction photovoltaic device constructed from crystalline semiconductor materials, such as silicon, is to heavily dope the respective n- and p-layers of the n-p junction formed by the adjacent cells. However, this method for creating a tunnel junction cannot readily be applied to the above described multijunction PIN devices because amorphous silicon is not easily doped to yield a highly conducting film. Such difficulty in achieving suitably high conductivity is particularly the case with wide bandgap alloys such as hydrogenated amorphous silicon carbide (a-SiC:H) and hydrogenated amorphous silicon nitride (a-SiN:H) which are preferred materials for constructing the p- and n-type layers of amorphous silicon PIN photovoltaic devices since, as described above, their use tends to maximize the optical transmissivity of each photovoltaic cell of the multijunction device. As a result, an attempt to highly dope the p- and n-layers of an amorphous silicon multijunction PIN device constructed with wide bandgap alloys does not achieve a desirable tunnel junction characteristic at the n-p junction between adjacent cells. A method for creating a tunnel junction between adjacent solar cells of an amorphous silicon multijunction PIN device is disclosed in the above-incorporated U.S. Patent No. 4,272,641. There, an additional tunnel junction layer is disposed between adjacent PIN cells, such layer being provided as a cermet incorporating a metal, or as a thin metal layer and a cermet, hereinafter the "metallic layer." While the metallic layer may function in conjunction with the adjacent cell layers to reduce the above described inflection in the IV curve 100 of the device (Fig. 1), the provision of the extra metallic layer substantially inhibits the manufacturing process. In order to manufacture an amorphous silicon multijunction photovoltaic device using such a metallic layer, a first photovoltaic cell is formed in a first material deposition system, for example a glow discharge chamber as described in U.S. Patent No. 4,272,641. Next, the device must be removed from the glow discharge chamber and placed in a second material deposition system where the metallic layer is deposited. For example, U.S. Patent No. 4,272,641 describes deposition of the metallic layer by a sputtering process. Then the device must be returned to the first deposition system where a second photovoltaic cell is formed on the metallic layer. Of course, if the device includes more than two cells, the process of transferring between the two deposition systems must be continued. The additional deposition system and the time required to manufacture a multijunction device in accordance with such a process results in an overall increased cost of the device and reduction in production yield. Further, while the metallic layer disclosed in U.S. Patent No. 4,272,641 is optically transmissive, it has a lower index of refraction than that of the adjacent n- and p-type a-Si:H layers of the cells it is disposed between. As a result of the different indexes of refraction, light is undesirably reflected at the interfaces between the metallic layer and the adjacent a-Si:H layers. Summary of the Invention The present invention has been made in view of the above circumstances and has as an object to provide an amorphous silicon multijunction photovoltaic device including a tunnel junction between adjacent cells which exhibits characteristics that do not limit the power generation of the device. Another object of the invention is to provide an amorphous silicon multijunction photovoltaic device including a tunnel junction between adjacent cells which may be formed without the need for additional facilities and time to carry out the manufacturing process. Additional objects and advantages of the invention will be set forth in part in the description which follows and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims. To achieve the objects and in accordance with the purpose of the invention, as embodied and broadly described herein, the multijunction photovoltaic device of this invention comprises: a first photovoltaic cell including a plurality of first cell layers of hydrogenated amorphous silicon of respectively different conductivity types, the layers being arranged in a stacked configuration and including a bottom layer having a first bandgap, the bottom layer being doped with a dopant of a first conductivity type. The device also comprises a second photovoltaic cell including a plurality of second cell layers of hydrogenated amorphous silicon of respectively different conductivity types, the layers being arranged in a stacked configuration and including a top layer having a second bandgap, the top layer being doped with a dopant of a second conductivity type opposite the first conductivity type. The device further comprises an interface layer of a semiconductor compound including silicon disposed between and in contact with the bottom layer of the first cell and the top layer of the second cell. The interface layer has a third bandgap smaller than the first and second bandgaps. The invention is also directed to a method for forming a multijunction photovoltaic device on a substrate. The method comprises the steps of depositing in succession on the substrate, by a glow discharge process, p-type, intrinsic and n-type layers of hydrogenated amorphous silicon of a first photovoltaic cell, the n-type layer having a first bandgap; depositing on the n-type layer, by the glow discharge process, an interface layer of a silicon compound material having a second bandgap smaller than the first bandgap; and depositing in succession on the interface layer, by the glow discharge process, p-type, intrinsic and n-type layers of hydrogenated amorphous silicon of a second photovoltaic cell, the p-type layer of the second cell having a third bandgap larger than the second bandgap. Brief Description of the Drawings The accompanying drawings, which are incorporated in and constitute a part of this specification illustrate embodiments of the invention and, together with the description, serve to explain the objects, advantages, and principles of the invention. In the drawings, Fig. 1 is a graph of the current-voltage characteristic of a prior art photovoltaic device and the current-voltage characteristic of a preferred embodiment of the claimed invention; Fig. 2 is a cross-sectional view of an amorphous silicon multijunction photovoltaic device constructed in accordance with a first embodiment of the present invention; and Fig. 3 is a cross-sectional view of an amorphous silicon multijunction photovoltaic device constructed in accordance with a second embodiment of the present invention. Detailed Description of the Preferred Embodiments Referring now to Fig. 2, there is shown a multijunction photovoltaic device 200 constructed in accordance with a first embodiment of the invention. Device 200 includes a substrate 202 having an incident surface 204 for receiving solar radiation 206. Substrate 202 preferably comprises glass or another suitable material which is transparent to solar radiation. A front conductive contact layer 208 is formed on substrate 202. Front conductive layer 208 is preferably an optically transparent conductor such as a metal oxide, e.g., tin oxide. Substrate 202 and front conductive contact layer 208 support two photovoltaic cells 210 and 212 in a stacked configuration. Photovoltaic cell 210 is comprised of a p-type hydrogenated amorphous silicon carbide (a-SiC:H) layer 220 deposited on front conductive layer 208, an intrinsic hydrogenated amorphous silicon layer 222 formed on p-type layer 220, and an n-type a-SiC:H layer 224 formed on intrinsic layer 222. Photovoltaic cell 212 has substantially the same basic structure as that of photovoltaic cell 210, in that it is comprised of a p-type a-SiC:H layer 230, an intrinsic hydrogenated amorphous silicon layer 232, and an n-type a-SiC:H layer 234. The p- and n-type layers, of cells 210 and 212, include carbon concentrations of approximately 20% and 15 %, respectively, at least adjacent to the intrinsic layer in each cell. A grading of the carbon concentration in each of n-type layer 224 and p-type layer 230 is described more fully below with respect to a method for constructing device 200. It is noted that while both the p- and n- type layers of cells 210 and 212 include carbon, a multijunction device can be constructed in accordance with the invention without including a bandgap widening element in all doped layers. While the p- and n-type layers of cells 210 and 212 can include carbon as a bandgap widening element, intrinsic layers 222 and 232 are substantially free of such bandgap widening elements. However, narrow portions of the intrinsic layer, respectively in contact with the p- and n- type layers containing carbon, can include a small, graded concentration of carbon. Further, in accordance with a known practice in the art, intrinsic layers 222 and 232 can include light p-type doping, e.g., a concentration of boron of less than 10 17 cm -3 , which is known to improve carrier generation capability of the intrinsic layer. Exemplary thicknesses and doping concentrations of the layers constituting each cell 210 and 212 are disclosed in the above-incorporated U.S. Patent No. 4,272,641. Device 200 also includes a back contact layer 236 formed on n-type layer 234 and preferably provided as a metal such as aluminum. An interface layer 240 of relatively low bandgap, high conductivity semiconductor material is interposed between the n-type a-SiC:H layer 224 of photovoltaic cell 210 and the p-type a-SiC:H layer 230 of photovoltaic cell 212. In particular, layer 240 has a lower bandgap than either of adjacent layers 224 or 230 and can be provided as a p- or n-type semiconductor material that forms an ohmic contact with the adjacent one of layers 224 and 230 of the same conductivity type and that forms a tunnel junction with the one of layers 224 and 230 of the opposite conductivity type. Since layer 240 has a lower bandgap, it also has a higher electrical conductivity than either of adjacent layers 224 and 230. In general, while layers 224 and 230 can each have a bandgap on the order of 1.9 eV, the bandgap of layer 240 is preferably on the order of less than 1.7 eV. In accordance with a first variation of the first embodiment, interface layer 240 is provided as a p + -type hydrogenated amorphous silicon layer substantially free of carbon, nitrogen, fluorine or similar bandgap widening elements. Layer 240 so provided preferably has a thickness in the range of 1.0nm to 20.0nm and preferably approximately 2.0nm, and is preferably doped with an acceptor dopant, such as boron, to a concentration of approximately 1%. As a result, layer 240 provided as a p + -type layer forms a highly conductive, ohmic contact with p-type layer 230 of cell 212 and a tunnel junction with n-type layer 224 of cell 210. In accordance with a second variation of the first embodiment, interface layer 240 is provided as a microcrystalline layer of n + -type hydrogenated silicon that is substantially free of carbon, nitrogen, fluorine or other bandgap widening elements. As used herein, microcrystalline describes a material whose crystalline order is in the range of substantially 2.5nm to 50.0nm. The presence of microcrystallinity can be determined by one or more of electron diffraction, x-ray diffraction, or Raman scattering spectroscopy. Since doped microcrystalline silicon is an indirect bandgap material having a relatively low bandgap, e.g., approximately 1.1 ev, it provides the dual advantages of high conductivity and high optical transmissivity. Layer 240 provided as a microcrystalline n + -type layer preferably has a thickness in the range of 1.5nm to 5.0nm and preferably approximately 5.0 nm, and is preferably doped with a donor dopant, such as phosphorus, to a concentration of approximately 1%. As a result, layer 240 provided as an n + -type microcrystalline layer forms a highly conductive, ohmic contact with n-type layer 224 of cell 210 and a tunnel junction with p-type layer 230 of cell 212. Layer 240 provided as a microcrystalline layer has a relatively low optical bandgap but retains a high optical transmissivity because it is an indirect optical bandgap material. While the first embodiment of the present invention can be successfully constructed and practiced with n-type layer 224 of cell 210 provided with a uniform carbon concentration throughout, it is preferred herein that the carbon concentration of layer 224 be graded from a maximum value at the interface with adjacent intrinsic layer 222 to a minimum value, preferably a zero concentration, at the interface with layer 240. Such grading can be achieved by either a stepped or continuous grading of the carbon concentration. As discussed above, carbon is included in n-type layer 224, across all or part of the thickness of layer, in order to increase the optical bandgap of that layer. However, disadvantageously, the presence of carbon decreases the conductivity of each of layers 224 and 230. For this reason, the carbon concentration of layer 224 is preferably graded so that the bandgap and hence the conductivity of layer 224 has a maximum value at the interface with layer 240 and can better form a conductive contact therewith. Further, with respect to the first variation of the first embodiment, the enhanced conductive contact between a graded n-type layer 224 and layer 240 provided as a p +- type layer results in improved performance of the tunnel junction formed therebetween. It is noted that p-type layer 230 of cell 212 can also be provided with a carbon concentration that is graded, by steps or continuously, from a maximum value at its interface with intrinsic layer 232 to a minimum value, e.g., zero concentration, at the interface with layer 240. Such grading of layer 230 improves its conductive contact with layer 240. However, the present inventors have observed that the open circuit voltage of a cell decreases when the p-layer is graded, while the cell open circuit voltage is not particularly sensitive to grading of the n-layer. In accordance with a third variation of the first embodiment, n-type layer 224 has a graded carbon concentration. Further, layer 240 consists of a first sublayer of n +- type a-Si:H, adjacent to layer 224, and a second sublayer of microcrystalline n +- type layer. The second sublayer is disposed between the first sublayer and layer 230 of cell 212 and is otherwise as described above with respect to the second variation of the first embodiment. In accordance with this third variation, the a-Si:H n +- type sublayer is substantially free of bandgap widening elements, has a thickness in the range of 1.0nm to 10.0nm and preferably 1.5nm, and is preferably doped with phosphorus to a concentration of approximately 1% cm- 3 . In accordance with the first embodiment, the tunnel junction formed between interface layer 240 and one of layers 224 and 230 functions with a substantially resistive characteristic so that the IV characteristic of device 200 is represented by curve 150 in Fig. 1. Advantageously, as can be seen, curve 150 does not include the inflection found in curve 100. As a result, device 200 constructed to include a tunnel junction in accordance with the first embodiment of the invention, enjoys an improved fill factor and power generation capability. Also in accordance with the first embodiment, in contrast to the prior art metallic layer described in the above referenced U.S. Patent No. 4,272,641, interface layer 240 is comprised of a silicon compound having substantially the same index of refraction as adjacent layers 224 and 230. As a result, device 200 does not experience undesirable reflectivity at the interfaces with layer 240. Methods for constructing each of the first and second variations of the first embodiment are described next. In this regard, only the formation of layers 224, 230 and 240 are described in detail. A suitable method for constructing the remainder of device 200 is disclosed in the above-incorporated U.S. Patent No. 4,272,641. The method for constructing device 200 in accordance with the first variation of the first embodiment is described first. All semiconductor layers of device 200 are formed by a glow discharge deposition process. The described method includes a method for providing each of n-type layer 224 and p-type layer 230 with a graded carbon concentration. First, layer 224 is formed by depositing successively narrower optical bandgap a-SiC alloys by gradually removing carbon during the deposition process. In particular, using silane, 4.5% phosphine in silane and methane, deposition of the wide bandgap portion of n-layer 224, adjacent intrinsic layer 222, begins with a mixture of 125sccm PH 3 -SiH 4 +25sccm SiH 4 +25sccm methane. Layer 224 is deposited by either dc or rf glow discharge using a power of 0.4 watts/in 2 yielding a deposition rate of about 0.4nm/second. Two or more layers, or a continuous grading out of carbon, are deposited in succession with the carbon being gradually removed, producing a total thickness of layer 224 of nominally 10.0nm. For example, the stepped grading of carbon can be accomplished over three layers. The final gas mixture is 125sccm PH3-SiH 4 + 75sccm SiH 4 . Layer 240 provided as a p +- type layer is then deposited by glow discharge to a 2.0nm thickness from a mixture of 100sccm 0.5% diborane-SiH 4 + 100sccm SiH 4 . Last, p-type a-SiC:H layer 230 is deposited by glow discharge to a thickness of approximately 10.0nm. Assuming layer 240 is deposited with a graded optical bandgap, the layer is initially deposited from a mixture of 30sccm 0.5% diborane-SiH 4 + 90sccm SiH 4 . Two or more layers, or a continuous grading of carbon, are deposited in succession so that the final mixture from which the layer is deposited includes 80 sccm methane. If the bandgap of layer 240 is not graded, the mixture continuously includes 80 sccm methane. Suitable flushes are provided between depositions to avoid cross contamination. However, the entire fabrication process is carried out with a single glow discharge deposition system. The method for constructing device 200 in accordance with the second variation of the first embodiment is described next. Again, all layers of device 200 are formed by a glow discharge deposition process. In accordance with the method described next, both layers 224 and 230 are provided with a uniform carbon concentration. First, n-type a-SiC:H layer 224 is deposited by dc or rf discharge to a thickness of about 10.0nm using 125sccm 4.5%PH 3 -SiH 4 mixture + 25sccm CH 4 + 75 sccm SiH 4 using approximately 0.4 watts/in 2 at about 0.67 hPa (0.5 torr). Layer 240 as a microcrystalline n-type layer is deposited next. The formation of a microcrystalline doped layer has in the past been difficult to achieve for the extremely thin (1.0-10.0nm) layers required to maintain high optical transmission. This limitation has been overcome by using a high intensity hydrogen glow discharge process to crystallize an amorphous layer which then acts as the "seed" nuclei for the further growth of the crystalline phase. In particular, a 2.0nm thick non-carbon containing n-type layer of hydrogenated p-type silicon is deposited using gas flow rates of 125sccm of 4.5% PH 3 -SiH 4 + 100sccm SiH 4 . A 4 second glow discharge deposition at 0.67 hPa (0.5 torr) and 0.4 watts/in 2 is used to form the layer. Next, the amorphous deposited layer is recrystallized by using a hydrogen plasma created at 0.67 hPa (1.5 torr) (320sccm flow rate) for 5 minutes at an rf or dc power level of approximately 2 watts/in 2 . The microcrystalline n-layer is deposited at 2 hPa (1.5 torr) using a 2 watt/in 2 glow discharge with a flow rate of 5sccm of 4.5% PH 3 in SiH 4 diluted in a 320sccm flow of pure hydrogen. The deposition is conducted for 50 seconds yielding an approximate thickness of 5.0nm. Last, p-type a-SiC layer 230 of cell 212 is deposited for 17 seconds at 0.67 hPa (0.5 torr) from a gas mixture of 25sccm 0.5% B 2 H 6 in SiH 4 + 80sccm methane + 100 sccm SiH 4 at an rf or dc power level of approximately 0.4 watt/in 2 . With respect to the construction of the third variation of the first embodiment, the carbon concentration of layer 224 is graded out in the manner described above. Then, the n +- type a-Si:H sublayer is deposited by a glow discharge to a thickness of approximately 10.0nm from a mixture of 125 sccm PH 3 -SiH 4 + 75 sccm SiH 4 . Next, the n +- type microcrystalline layer and p +- type layer 230 are deposited as described above for the second variation of the first embodiment. Fig. 3 illustrates a multijunction photovoltaic device 300 constructed in accordance with a second embodiment of the invention. Device 300 includes substrate 202, front conductive layer 208 and photovoltaic cells 210 and 212 as described above for device 200. Device 300 also includes an interface layer 302 comprised of layers 304 and 306 sandwiched between n-type layer 224 of device 210 and p-type layer 230 of device 212 such that layer 304 is in contact with layer 224 and layer 306 is in contact with layer 230. In accordance with several variations of the second embodiment, layers 304 and 306 are respectively provided as n-type and p-type hydrogenated silicon layers each having a relatively low bandgap. In particular, each of layers 304 and 306 has a lower bandgap, and therefore a higher conductivity, than layers 224 and 230. As a result, in accordance with these variations, n-layer 304 forms a highly conductive, ohmic contact with n-type layer 224, p-type layer 306 forms a highly conductive, ohmic contact with p-type layer 230 and a tunnel junction is formed between layers 304 and 306. The bandgap of each of layers 304 and 306 is on the order described above for layer 240 of the first embodiment. In accordance with a first variation of the second embodiment, layer 304 is provided as a microcrystalline layer of n +- type hydrogenated silicon that is substantially free of carbon, nitrogen, fluorine or other bandgap widening elements. Layer 304 has a thickness in the range of 1.0nm to 10.0nm and preferably approximately 2.0nm, and a dopant concentration of approximately 1%. Layer 306 is provided as a layer of p +- type hydrogenated amorphous silicon that is substantially free of the above noted bandgap widening elements. Layer 306 has a thickness in the range of 0.5nm to 10.0nm and preferably approximately 1.5nm, and a dopant concentration of approximately 1%. In accordance with a second variation of the second embodiment, layer 304 is provided as a layer of n +- type hydrogenated amorphous silicon that is substantially free of the above-noted bandgap widening elements. Layer 304 so provided has a thickness in the range of 1.0nm to 10.0nm and preferably approximately 1.5nm, and a dopant concentration of approximately 1%. Layer 306 of this variation is the same as described above for the first variation. In accordance with a third variation of the second embodiment, layer 304 is provided as a microcrystalline layer of n +- type hydrogenated silicon as described above for the first variation. Also in accordance with the third variation, layer 306 is provided as a microcrystalline layer of p +- type hydrogenated silicon that is substantially free of the above-noted bandgap widening elements. In accordance with this variation, layer 306 has a thickness in the range of 1.0nm to 10.0nm and preferably approximately 1.5nm, and a dopant concentration of approximately 1%. In accordance with a fourth variation of the second embodiment, layer 306 is provided as described for the third variation and layer 304 is provided as described for the second variation. As in the case of the first embodiment, while each variation of the second embodiment can be successfully constructed and practiced with n-type layer 224 and p-type layer 230 each provided with a uniform carbon concentration throughout. However, it is preferred that the carbon concentration of layer 224 be graded as described above with respect to the first embodiment. The carbon concentration of p-layer 230 can also be graded as previously described. In accordance with the second embodiment of the invention, the tunnel-junction formed between layers 304 and 306 functions with a substantially resistive characteristic so that the IV characteristic of device 300, like device 200, is also represented by curve 150 in Fig. 1. As a result, device 300 constructed in accordance with the second embodiment of the invention enjoys an improved fill factor and power generation capability. Also, as in the case of the first embodiment, layers 304 and 306 are each comprised of a silicon compound having substantially the same index of refraction as layers 224 and 230. As a result, device 300 does not experience undesirable reflectivity at the interfaces with layer 302. Methods for constructing device 300 are described next. As in the case of the first embodiment, all layers of device 300 are formed by a glow discharge deposition process. With the exception of the microcrystalline p +- type layer included in the third and fourth variations of the second embodiment, the methods described above for constructing layer 240 of the first embodiment provided as p +- type a-Si:H, n +- type microcrystalline silicon, or n +- type a-Si:H, are fully applicable to layers 304 and 306. The p +- type microcrystalline layer 306 of the third and fourth variations can be formed in a manner analogous to that described above for forming the n +- type microcrystalline layer. For example, layer 306 can be deposited from a mixture of 0.5% diborane in silane diluted with 320 sccm hydrogen, by glow discharge with a power of 2 watts/in 2 . While devices 200 and 300 constructed in accordance with embodiments of the invention have been described as including carbon in their respective n-type and p-type layers as a bandgap widening element, the invention is not so limited. The invention can be practiced with equal effectiveness using a bandgap widening element other than carbon, e.g., fluorine, nitrogen, or oxygen. While devices 200 and 300 each include only two photovoltaic cells, the invention can be practiced with multijunction photovoltaic devices that include more than two cells. In such devices, an interface layer constructed in accordance with the invention would be interposed between each pair of adjacent photovoltaic cells in the multijunction device. The following example illustrates the advantages realized from constructing a multijunction photovoltaic device to include an interface layer that forms a tunnel junction with an adjacent cell layer. In the example, two multijunction devices, designated device #1 and device #2, were constructed. Both devices #1 and #2 included two PIN type photovoltaic cells having characteristics as described above for the illustrated embodiments. Device #1 did not include an interface layer. Device #2 included an interface layer such as described above for the first variation of the first embodiment of the invention. Thus, the interface layer was comprised of a boron doped p + -type hydrogenated amorphous silicon layer substantially free of carbon, nitrogen, fluorine or other bandgap widening elements. In particular, in device #2, the interface layer had a thickness of approximately 2.0 nm and a boron concentration of approximately 0.3%. Thus, devices #1 and #2 were identical except for the inclusion of the interface layer in device #2. Table 1 lists performance characteristics of devices #1 and #2 measured under identical irradiation conditions. Device #1 Device #2 Voc 2.141 volts 2.225 volts Jsc -6.4 mA/cm 2 -6.5 mA/cm 2 Fill Factor 58.1% 63.1% Efficiency 7.9% 9.2% Rs 129.16 ohm-cm 2 54.89 ohm-cm 2 In Table 1, Voc and Jsc are respectively the open circuit voltage and short circuit current density of the device. As seen in Table 1, the inclusion of the interface layer in device #2 results in an 8.6% increase in fill factor which is reflected by the corresponding increase in device efficiency. As also seen in Table 1, the inclusion of the interface layer in device #2 results in a substantial reduction in the series resistance of the multijunction device.

Claims: A multijunction photovoltaic device, comprising: a first photovoltaic cell including a plurality of first cell layers of hydrogenated amorphous silicon of respectively different conductivity types, said layers being arranged in a stacked configuration and including a bottom layer having a first bandgap, said bottom layer being doped with a dopant of a first conductivity type and a second photovoltaic cell including a plurality of second cell layers of hydrogenated amorphous silicon of respectively different conductivity types, said layers being arranged in a stacked configuration and including a top layer having a second bandgap, said top layer being doped with a dopant of a second conductivity type opposite the first conductivity type, characterised in that said multijunction photovoltaic device further comprises an interface layer of a semiconductor compound including silicon disposed between and in contact with said bottom layer of said first cell and said top layer of said second cell, said interface layer comprising a third bandgap smaller than said first and second bandgaps. The multijunction device of claim 1 wherein said interface layer is doped with a dopant of the first conductivity type to form a tunnel junction with said top layer and an ohmic contact with said bottom layer. The multijunction device of claim 1 wherein said interface layer is doped with a dopant of the second conductivity type to form a tunnel junction with said bottom layer and an ohmic contact with said top layer. A multijunction device according to claim 1 wherein said interface layer further comprising: a) a first sublayer of an amorphous hydrogenated silicon compound which forms an ohmic contact with the bottom layer of the first photovoltaic cell, and b) a second sublayer of a microcrystalline hydrogenated silicon compound which forms a tunnel junction with the top layer of the second photovoltaic cell, and wherein the first and second sublayers are doped with a dopant of the first conductivity type. A multijunction device according to any of claims 2,3 and 4 wherein said first and second conductivity types are n-type and p-type, respectively. A multijunction device according to any of claims 2,3 and 4 wherein each of said top and bottom layers further includes an element selected from the group consisting of carbon, nitrogen, oxygen and fluorine; and said interface layer being substantially free of carbon, nitrogen, oxygen and fluorine. A multijunction device according to claim 1 wherein said interface layer further comprises: a) a first sublayer doped with a dopant of said first conductivity type forming an ohmic contact with the bottom layer of the first photovoltaic cell, and b) a second sublayer doped with a dopant of said second conductivity type and forming an ohmic contact with the top layer of the second photovoltaic cell, wherein a tunnel junction is formed between said first and second sublayers. The multijunction device of claim 7 wherein said first layer comprises a microcrystalline hydrogenated silicon compound and said second layer comprises an amorphous hydrogenated silicon compound. The multijunction device of claim 7 wherein said first and second layers each comprise an amorphous hydrogenated silicon compound. The multijunction device of claim 7 wherein said first and second layers each comprise a microcrystalline hydrogenated silicon compound. The multijunction device of claim 7 wherein said first layer comprises an amorphous hydrogenated silicon compound and said second layer comprises a microcrystalline hydrogenated silicon compound. The multijunction device of claim 7 wherein said first and second conductivity types are n-type and p-type, respectively. The multijunction photovoltaic device of claim 1, wherein said first cell layers include: a first cell top p-type layer, a first cell middle intrinsic layer, and a first cell bottom n-type layer; and wherein said second cell layers include a second cell top p-type layer, a second cell middle intrinsic layer, and a second cell bottom n-type layer, said first cell bottom layer and second cell top layer respectively having said first and second bandgaps. A method of forming a multijunction photovoltaic device of claim 1 wherein said second photovoltaic cell is formed by: depositing in succession on the substrate, by a glow discharge process, p-type, intrinsic and n-type layers of hydrogenated amorphous silicon said n-type layer having a first bandgap; wherein said interface layer is formed by, depositing on said n-type layer, by the glow discharge process, an interface layer of silicon compound material having a second bandgap smaller than the first bandgap; and wherein said first photovoltaic cell is formed by depositing in succession on said interface layer, by the glow discharge process, p-type, intrinsic and n-type layers of hydrogenated amorphous silicon, said p-type layer of said first photovoltaic cell having a third bandgap larger than said second bandgap.

Inventors: ARYA RAJEEWA RASHMIKANT, CATALANO ANTHONY WILLIAM, ARYA, RAJEEWA RASHMIKANT, CATALANO, ANTHONY WILLIAM
Applicants: AMOCO ENRON SOLAR, AMOCO/ENRON SOLAR
Application year: 1992
Main classification: H01L31
IPC codes: H01L31