SOLAR CELL HAVING A THIN FILM SILICON MULTIPLE LAYER STRUCTURE

Abstract: Solar cell, comprising at least three substantially thin film parallel silicon layers, stacked upon each other, and at least two conductors providing an electrical contact with at least two of said layers, said conductors extending in a direction substantially transverse with respect to said layers, wherein the thin film layers are provided by amorphous silicon of the p-type (p-Si), intrinsic amorphous silicon (i-Si) and amorphous silicon of the n-type (n-Si) respectively, in the order given by the formula (I): p-Si, (i-Si, n-Si, i-Si, p-Si)x, i-Si, n-Si, where preferably 0</=x</=5, the amorphous silicon is hydrogenated in a concentration in the range of about 1 at.% - about 10 at.% relative to Si, preferably in a concentration of about 1 at.% relative to Si, one of said conductors provides an electrical contact with each of said p-Si layers and the other of said conductors provides an electrical contact with each of said n-Si layers. A certain amount of crystalline silicon of the p-type is provided within a p-Si layer, and a certain amount of crystalline silicon of the n-type is provided within an n-Si layer.

Description: SOLAR CELL HAVING A THIN FILM SILICON MULTIPLE LAYER STRUCTURE The invention relates to a solar cell, comprising at least three substantially thin film parallel silicon layers, stacked upon each other, and at least two conductors providing an electrical contact with at least two of said layers, said conductors extending in a direction substantially transverse with respect to said layers. A solar cell of the above mentioned type has been disclosed in WO 93/12543, which teaches the deposition of thin film silicon layers from solution in molten metal or other known techniques onto supporting glass superstrate. These techniques imply the formation of thin films of crystalline silicon. It is a disadvantage of the known solar cell that its thickness is relatively large, in order to provide a sufficient volume to absorb incident light. Moreover, the materials forming the layers of the known solar cell have a poor mechanical strength, giving rise to additional requirements in construction and design and accompanying additional production costs. It is a purpose of the invention to provide a solar cell of the above mentioned type, in which the above and other disadvantages of the known thin film silicon solar cells are overcome. This purpose is reached in a solar cell of the type mentioned above, wherein the thin film layers comprise amorphous silicon (a-Si) . A solar cell according to the invention involves many advantages over the prior art crystalline silicon solar cell, in that its manufacturing by thin film depostion allows inter alia a wide area at a low material consumption, processing at low temperatures, p-n doping and alloying control during deposition, deposition on inexpensive substrates of different kind and shape, easy integrated manufacturing and mass production at low cost. The thickness of an amorphous silicon thin film solar cell according to the invention is an order of magnitude less than the thickness of a known device having a similar energy efficiency, but using crystalline silicon. Moreover, the mechanical strength of the solar cell according to the invention is superior over the mechanical strength of the known solar cell. In an embodiment of a solar cell according to the invention the thin film layers are provided by amorphous silicon of the p-type (p-Si) , intrinsic amorphous silicon (i-Si) and amorphous silicon of the n-type (n-Si) respectively, in the order given by the formula (I) : p-Si, (i-Si,n-Si,i-Si,p-Si) x ,i-Si,n-Si (I), where x is the number 0 or a natural number, preferably 0__x<_5, one of said conductors provides an electrical contact with each of said p-Si layers and the other of said conductors provides an electrical contact with each of said n-Si layers. If x is unequal to 0, the structure of this embodiment consists of multiple interleaved parallel layers, thus greatly increasing the collection probability for carriers generated by the absorption of light. If the spacings between the layer junctions are properly chosen, the collection probability for all generated carriers approaches unity. In a further embodiment, the thickness of each a-Si layer is less than carrier collection length in said each layer. In such an embodiment, carrier injection between the layers will advantageously then result in sharing of current between the layers, as the multiple interleaved layers of n-Si and p- Si provide parallel paths for current conduction between contacts, thus reducing resistive losses. In order to further improve the conductivity of the parallel paths between the contacts, a certain amount of crystalline silicon of the p-type may be provided within a p- Si layer, and/or a certain amount of crystalline silicon of the n-type is provided within an n-Si layer. The amount of p-type and/or n-type crystalline silicon may just be as little to provide small areas of finite dimensions, comprising micro-crystalline silicon, or as much as to provide an intermediate layer of crystalline silicon within the respective amorphous layer. Preferably, in a solar cell according to the invention the a-Si is hydrogenated. (Hydrogenated a-Si is denoted below by a-Si:H) . Addition of hydrogen to the a-Si, for example in a concentration in the range of about 1 at.% - about 10 at.% relative to Si, preferably in a concentration of about 1 at.% relative to Si, results in formation of SiH-bonds, thus rendering the dangling bonds in a-Si inactive. As a result, the spectral response in a-Si is superior over many other solar cell materials, whereas majority and minority carrier lifetimes have been found to be at least 10 ns. Therefore, a multiple layer a-Si solar cell yields a relatively high efficiency in comparison with prior art solar cells, without additional light trapping. In addition, the a-Si may be alloyed with a material selected from germanium (Ge) , carbon (C) and a combination of said materials. The a-Si layers may be intrinsic or doped with electrically or optically active impurities, chosen to optimize the response of the cell to the solar spectrum. An embodiment of a solar cell according to the invention comprises e.g. a substrate or a superstrate for the thin film silicon layers and a covering toplayer or bottomlayer respectively. Substrate or superstrate and toplayer or bottomlayer respectively may be provided in a way per se known, whereby at least the substrate or the superstrate, or respectively the toplayer or the bottomlayer is transparant, or whereby substrate and toplayer or superstrate and bottomlayer respectively are both transparant. By making an n-type busbar for one cell very close to a p- type busbar groove for an adjacent cell, two regions can be linked during metallization. This provides automatic series interconnection of adjacent a-Si cells, eliminating interconnects and subsequent soldering other than the output leads. Two adjacent grooves may actually overlap, forming one wider groove with side walls oppositely doped. In this case, filling the groove with metal again automatically provides the series connection. The invention will now be illustrated by means of reference to the accompanying drawings. Fig. 1 shows a schematic cross section of a first embodiment of an a-Si:H multilayer solar cell according to the invention, and Fig. 2 shows a schematic cross section of a second embodiment of an a-Si:H multilayer solar cell according to the invention. Fig.l shows a multiple solar cell 1, comprising interleaved parallel layers 2,3,4 of intrinsic hydrogenated amorphous silicon (i-Si), each said layer 2,3,4 being bound by a layer 5,6 of hydrogenated amorphous n-type silicon (n- Si) and a layer 7,8 of hydrogenated amorphous p-type silicon (p-Si) . Transverse with respect to the planes of the layers 2-8 grooves have been formed, the walls 10,12 and 9,11 of which have been doped to form amorphous n-Si 15 and amorphous p-Si 16 respectively. In the grooves metal contacts 13,14 have been provided in intimate contact with the amorphous n- Si 15 and p-Si 16 respectively. Carriers generated by incident light in the intrinsic layers 2-3 drift towards the the n-Si layers 5,6 or the p-Si layers 7,8 respectively, depending on their sign, and are transported via the respective layers 5-8 towards the metal contacts 13,14, which thus can provide a photo-current to a circuit connected (not shown) . The solar cell 1 has been extended in the direction of the layers in a repeating pattern, such as to provide upon a single substrate (not shown) a multiple solar cell, the contacts of which are electrically connected in series. The solar cell 1 may easily be extended in a direction transverse with respect to the layers by repeating the sequence of four consecutively stacked layers of amorphous silicon. Fig. 2 shows another embodiment of a multiple solar cell 21 according to the invention. This cell 21 has in general the the same configuration as the solar cell 1 shown in fig. 1. Corresponding members have been indicated by corresponding reference signs. The advantage of the configuration of cell 21 shown in fig. 2 is the formation within the current transporting amorphous n-Si and p-Si layers 5,6 and 7,8 of small layers of corresponding micro-cristalline silicon 25,26 and 27,28 respectively, thus considerably reducing the resistance of these layers. A further reduction of resistance in the cell 21 has been attained by the doping of the groove walls 10,12 and 9,11 with micro-crystalline n-type silicon 35 and p-type silicon 36 respectively.

Claims: CLAIMS 1. Solar cell, comprising at least three substantially thin film parallel silicon layers, stacked upon each other, and at least two conductors providing an electrical contact with at least two of said layers, said conductors extending in a direction substantially transverse with respect to said layers, characterised in that the thin film layers comprise amorphous silicon (a-Si) . 2. Solar cell according to claim 1, characterised in that the thin film layers are provided by amorphous silicon of the p-type (p-Si) , intrinsic amorphous silicon (i-Si) and amorphous silicon of the n-type (n-Si) respectively, in the order given by the formula (I) : p-Si, (i-Si,n-Si, i-Si,p-Si) x , i-Si,n-Si (I) , where x is the number 0 or a natural number, one of said conductors provides an electrical contact with each of said p-Si layers and the other of said conductors provides an electrical contact withe each of said n-Si layers. 3. Solar cell according to claim 2, characterised in that 0__x<_5. 4. Solar cell according to claim 2 or 3, characterised in that the thickness of each thin film layer is less than the carrier collection length in said layers . 5. Solar cell according to any of the claims 2-4, characterised in that a certain amount of crystalline silicon of the p-type is provided within a p-Si layer. 6. Solar cell according to any of the claims 2-4, characterised in that a certain amount of crystalline silicon of the n-type is provided within an n-Si layer. 7. Solar cell according to any of the preceding claims, characterised in that the a-Si is hydrogenated. 8. Solar cell according to claim 7, characterised in that the concentration of hydrogen (H) in the a-Si is in the range of about 1 at.% - about 10 at.% relative to Si. 9. Solar cell according to claim 8, characterised in that the concentration of hydrogen (H) in the a-Si is about 1 at.% relative to Si. 10. Solar cell according to any of the preceding claims, characterised in that the a-Si is alloyed with a material selected from germanium (Ge) , carbon (C) and a combination of said materials. 11. Solar cell according to any of the preceding claims, characterised in that the a-Si is doped with an optically active impurity.

Inventors: SARIS FRANS WILLEM, SARIS, FRANS, WILLEM
Applicants: SARIS FRANS WILLEM, STICHTING ENERGIE, SARIS, FRANS, WILLEM, STICHTING ENERGIEONDERZOEK CENTRUM NEDERLAND
Application year: 1996
Main classification: H01L27
IPC codes: H01L27, H01L31