THIN-FILM SOLAR CELL

Abstract: The invention relates to a thin-film solar cell provided with at least one p-i-n junction comprising at least one p-i junction which is at an angle alpha with that surface of the thin-film solar cell which collects light during operation and at least one i-n junction which is at an angle beta with the light-collecting surface. In this context, the relationships 45 < alpha < 135 degrees and 45 < beta < 135 degrees apply. The invention also relates to a panel provided with a plurality of such thin-film solar cells and to methods for fabricating such a thin-film solar cell.

Description: Thm-film solar cell The invention relates to a thin-film solar cell provided with-at least one p-i-n junction comprising at least one p-i junction which is at an angle a with that surface of the thm-film solar cell which collects light during operation and at least one i-n junction which is at an angle β with the light-collectmg surface Such a thm-film solar cell is disclosed by the international Patent Application W095/27314 The known th -film solar cell comprises p- and n-doped layers stacked alternately, with or without an interposed layer of intrinsic material. The active p-n junctions or p-i-n junctions, respectively, are positioned, in the case of the known thm-film solar cell, parallel to the light-collecting surface of the thin-film solar cell onto which the sunlight impinges during operation. In the case of the known thm-film solar cell, the doped layers determine both the optical and the electrical characteristics of the thm- film solar cell This makes it more difficult to optimize the performance of the thm-film solar cell. Increasing the dopant concentration of the p-doped layers, for example, on the one hand, leads to a higher field strength of the internal electric field This higher field strength leads to a higher collection probability of the charge carriers and therefore to a higher photocurrent. On the other hand, said increase in the dopant concentration of the p-doped layers does, however, lead to an increase in the absorption of sunlight by these zones Less sunlight therefore reaches the intrinsic layer, and as a result fewer charge carriers are generated. The photocurrent produced by the thin-film solar cell is reduced as a result. The negative effect of the increase in light absorption cancels the positive effect of the higher field strength, and consequently the resulting photocurrent of the thin-film solar cell remains virtually the same. It is an object of the present invention to provide a thin-film solar cell of the type mentioned at the outset which overcomes this drawback. To this end, the thin-film solar cell according to the invention is characterized in that 45 < a < 135 degrees and 45 < β < 135 degrees, and more in particular 80 < α < 100 degrees and 80 < β < 100 degrees. As a result of this orientation of the p-i junction and the i-n junction with respect to the light-collecting surface the sunlight impinges, at least in part, directly onto the intrinsic material without first passing through one of the doped zones, as in the case of the above- described known thin-film solar cell. In the context of the above-described example this means that an increase in the dopant concentration of the p-doped zones results in a higher electric field strength, without the concomitant increase in the absorption of sunlight cancelling the positive effect thereof. More generally this means that the optical and electrical characteristics of the th -film solar cell according to the invention can therefore be optimized separately. Using the thm-film solar cell according to the invention it is therefore possible to achieve a higher conversion efficiency in converting solar energy into electrical energy than by using the known thin-film solar cell. In a preferred embodiment of the thm-film solar cell, the p-i junction and/or the i-n junction is eandrous at least in cross-section. In a further preferred embodiment, the p-doped zone and/or the n-doped zone each comprise one or more fingers. In these preferred embodiments the area of the junction zone between the intrinsic material and the doped zones has been advantageously enlarged, which leads to a higher resulting photocurrent for the thin-film solar cell. The invention also relates to a panel provided w th a plurality of thin-film solar cells, wherein the thm-film solar cells are electrically connected to one another. The invention also relates to a method for fabricating a thin- film solar cell according to the invention, said method comprising the following steps: (a) depositing a layer of intrinsic material on a substrate; (b) defining the location of the p-doped zone in the layer of intrinsic material; (c) defining the desired angle α for the p-i junction with respect to the light-collecting surface of the thin-film solar cell, where 45 < α < 135 degrees; (d) establishing, with the aid of ion implantation, the p-doped zone at the location as defined step (b) and at the angle a as defined in step (c); (e) defining the location of the n-doped zone in the layer of in- trinsic material; (f) defining the desired angle β for the i-n junction with respect to the light-collecting surface of the thm-film solar cell, where 45 < β < 135 degrees; (g) establishing, with the aid of ion implantation, the n-doped zone at the location as defined in step (e) and at the angle β as defined in step (f ) . Using the method according to the invention it is possible to fabricate, rapidly and reliably, a thm-film solar cell according to the invention. This method further has the advantage that the intrinsic material is deposited directly on the substrate, without the interposition of a doped layer which would affect the fabrication conditions of the intrinsic material. Moreover, with this method many types of substrates can be used as a starting material, among which are both transparent and nontransparent substrates . The invention also relates to an alternative method for fabricating a thm-film solar cell according to the invention, said method comprising the following steps: (a) depositing a ayer of p-doped material and a layer of n-doped material, respectively, on a substrate; (b) defining the position of the n-doped zone and the p-doped zone, respectively, in the layer of p-doped material and n-doped material, respectively; (c) removing a portion of the deposited doped material at the position as defined in step (b), (d) depositing a layer of n-doped material and p-doped material, respectively, on the thm-film solar cell which was obtained in step (c); (e) defining the position of the intrinsic zone in the layer of p-doped and n-doped material; (f) removing a portion of the p-doped material and the n-doped material at the position as defined in step (e), (g) depositing a layer of intrinsic material on the thm-film solar cell which was obtained in step (f). The alternative method has the advantage that t is cheaper than the first-mentioned method. The invention will now be explained more detail with reference to the accompanying drawings, in which Figure 1 shows a schematic sectional view of a portion of a first preferred embodiment of a thm-film solar cell according to the invention; Figure 2 shows the first preferred embodiment in its entirety in a plan view from above, Figure 3 schematically shows a sectional view, seen from above, of a unit comprising a number of thin-film solar cells according to the preferred embodiment of Figure 2, which are connected in series; and Figure 4 shows a plan view from below of a panel provided with a plurality of units according to Figure 3, which are connected in parallel. Figure 1 shows a schematic sectional view of a portion of a first preferred embodiment of a th -film solar cell 1 according to the invention The section was taken on I-I in Figure 2, in which the first preferred embodiment is shown in its entirety in a plan view from above. Figure 1 and Figure 2 will largely be described in combination. Thin-film solar cell 1 , sometimes referred to as "Transverse Junction Solar Cell", comprises a substrate 2 on which a zone of intrinsic material I is disposed. The intrinsic material I in Figure 1 comprises two doped zones p and n (6 and 7, respectively), by means of which p-i junction 8 and i-n junction 9, respectively, are defined Thm-film solar cell 1 has a surface 5 which, during operation, collects the sunlight. Hereinafter, surface 5 will be referred to as "light-collecting surface" During operation, when thm-film solar cell 1 absorbs (sun)lιght (indicated by arrow A), charge carriers are liberated in the intrinsic material I with the aid of the absorbed light. Between the doped zones p and n there is an electric field which ensures that the positive charge carriers in the intrinsic material 1 move in the direction of arrow B. These charge carriers then arrive in the p-doped zone p As a result of then establishing external electric connections to the p-doped and the n-doped zone it is possible for a photocurrent (shown in Figure 2) to be drawn from the thm-film solar cell 1 According to the invention, at least one p-i junction, for example 8, is at an angle a with the light-collecting surface 5, and at least one i-n junction, for example 9, is at an angle β with the light- collecting surface 5 Both angles α and β are preferably between 45 and 135 degrees. More preferably, the angles a and β are between 80 and 100 degrees. Most preference is given to angles α and β of approximately 90 degrees, as shown in the figure Preferably the angles α and β are approximately equal, although this is not a necessary condition. This position of the junctions ensures that the sunlight is absorbed directly, at least in part, by the intrinsic material l without the sunlight first having to pass through one or more doped zones. In the case of the thin-film solar cell 1 according to the invention, the incident sunlight will therefore, for the greater part, lead directly to the liberation of charge carriers the intrinsic material 1, only a relatively small portion being absorbed by the doped zones, this being a, function, inter alia, of the angles α and β. Consequently, optimization of the intrinsic material I and the doped zones p, n can, to a large extent, be carried out separately, making it possible to improve considerably the performance of the thin-film solar cell 1 compared with the known thin-film solar cell. It is clearly discernible in Figure 2 that the p-doped zone 3 and the n-doped zone 4 are both preferably provided with fingers 6 and 7, respectively. The p-doped fingers 6 are positioned approximately transverse to the p-i junction between zone 3 and zone I. The n-doped fingers 7 are positioned approximately transverse to the i-n junction between zone I and zone 4. The p-doped fingers 6 and the n-doped fingers 7 are preferably placed alternately in a plane which runs approximately parallel to the light-collecting surface 5 shown in Figure 1. Thm-film solar cell 1 in this plane then has at least further p-i junctions 8 and further i-n junctions 9. In other words, with the aid of the fingers 6 and 7, respectively, in the preferred embodiment shown in Figure 2, the total area of the p-i junction and the i-n junction, respectively, has been advantageously enlarged. At the same time, nevertheless, enough intrinsic material l is present which is able to collect sunlight directly. In the shown preferred embodiment the thin-film solar cell 1 therefore delivers a larger resulting photocurrent. More generally the advantage just mentioned according to the invention is achieved by the use of one or more p-i/i-n junctions having at least a meandrous cross-section. Hereinafter the zone in which the fingers 6, 7 and the intrinsic material l are present will be denoted by l ' . In the following a number of optional measures are described which are particularly advantageous in the context of the thm-film solar cell according to the invention. The measures are not illustrated but will be understood most effectively with reference to Figure 1. In a further preferred embodiment, the thin-film solar cell according to the invention is provided with a supplementary layer of intrinsic material which is primarily disposed (not shown) between the p-i-n junction and the light-collecting surface 5. This supplementary layer advantageously increases the total photocurrent of the thm-film solar cell, which leads to higher efficiency. The supplementary layer preferably has a thickness of from 50 nm to 5 μm and more preferably a thickness of 1 50 nm . The th -film solar cell is optionally provided with a layer of electrically conductive material making contact with the p-doped zone and/or is provided with a layer of electrically conductive material making contact with the n-doped zone. The use of the electrically conductive material ensures that the loss in electrical output as a result of the lower series resistance of the fingers is restricted as much as possible. Likewise optionally, the thm-film solar cell according to the invention is provided with a textured surface on the light-collecting side. The use of such a textured surface has the effect of increasing efficiency. It should be noted that the substrate 2 in Figure 1 is made of an optically transparent material, for example glass. Alternatively, however, the substrate 2 may be made of optically nontransparent material, in which case, of course, the zones p and n and the intrinsic material l will be disposed on the substrate. Furthermore, it will be clear to those skilled in the art, that in practice the p-i junctions and i-n junctions will usually not have as ideal a profile as shown in the figures Instead they will tend to be subject to a spatial distribution which is determined, inter alia, by the doping technique used. In the context of the present patent application the term "junction" is therefore understood as that plane which best represents the junction zone. This is important, in particular, in defining the angle of a specific junction with the ligh -collecting surface. With reference to the above definition of the term "junction" it should be noted that the doping of the zones in question by no means need be distributed homogeneously in terms of the depth If required, a doping gradient can be established. This may, for example, increase with depth. Figure 3 schematically shows a sectional view, seen from above, of a unit 10 which comprises a number k of thin-film solar cells according to Figures 1 and 2, which are connected in series The p-doped zones are denoted by p J , whereas the n-doped zones in the unit are denoted by n-J , where j = 1 , ... , k The thin-fllm solar cells 1 to k inclusive are connected in series by the n-doped zone n of the preceding thin-film solar cell j being electrically connected, m each case, to the p-doped zone p , of the adjacent thm-film solar cell + 1 For illustrative purposes, in Figure 3, such an electrical connection between zone n. of the first solar cell from unit 10 and zone ρ 2 of the second solar cell from unit 10 is denoted by reference numeral 11. Connection 11 is an n-p junction. If dop- g of the zones n- and p 2 is sufficiently effective, junction 11 acts as a low-resistance contact. Such an n-p junction is known in the art and will therefore not be discussed in any further detail withm the context of the present patent application. Owing to the series connection of the thm-film solar cells 1 to k inclusive, the resulting photocurrent J of unit 10 passes through p-doped zone p 1 and n-doped zone n k , as indicated with the aid of arrows in Figure 3. For the sake of simplicity, the region between p 1 and n, of unit 10 is hereinafter denoted as I". During operation, unit 10 can deliver a voltage of approximately 20 V, which is sufficient for normal use. Figure 4 shows a partial plan view from below of a panel 12 provided with a plurality of units 10 according to Figure 3, which are connected in parallel. The parallel connection of the various units 10 takes place by all the p-doped zones p 1 of the units 10 being electrically connected to contact 13. All the n-doped zones n k of all the units 10 are electrically connected to contact 14. Preferably, the units 10 witnin panel 12 are alternately placed in mirror image positions with respect to one another. Furthermore, on panel 12 portions of the substrate 2 have preferably been applied to prevent short-circuiting between the contacts 13 and 14. The resulting photocurrent of panel 12 can during operation be drawn at contacts 13 and 14. For the sake of simplification, any protective layers which may have to be applied at the location of the p-type, i-type and n- type zones are not shown. It should further be noted that the figures are not drawn to scale. So as to clarify the invention, but without in any way limiting the scope thereof, some dimensions of portions of a thm-film solar cell according to the invention are given below. The fingers 6, 7 preferably have a length of from 10 to 100 μm, preferably about 20 μm, and a width of from approximately 0.01 to 2 μm. The width of the i-type zone is approximately between 0.2 and 10 μ . The thm- film solar cell according to the invention preferably has a thickness of from 0.02 to 100 μm. Below, by way of example, two methods are discussed for fabricating a thm-film solar cell according to the invention, with reference to Figures 1 and 2. The first method starts with a first step of depositing a layer of intrinsic material I on a substrate 2 All the materials known in the art can be used for the intrinsic material I and the substrate 2. Preferab- ly the intrinsic material is crystalline silicon, polycrystalline silicon, amorphous silicon and its alloys, cadmium telluride, cadmium sulphide, copper indium diselenide, an alloy of the form CulnxGa..i —xSySe,i —y .or some other suitable semiconductor material, with or without a gradated band gap. The intrinsic material can optionally be composed of a combination of the said materials, the materials possibly having band gaps which differ from one another. The substrate is preferably made of glass, ceramic material or plastic. In step 2 the desired location for the p-doped zone p in the layer of intrinsic material I is defined. This definition can take place in a known manner, for example with the aid of a suitable mask and/or lithographic techniques. In step 3 the desired angle α for the p-i junction 3 with respect to the light-collecting surface 5 of the thin-film solar cell 1 is then defined. Preferably, α is between 45 and 135 degrees, more preferably « is between 80 and 100 degrees. Most preference is given to an angle o of approximately 90 degrees. In step 4, the p-doped zone p is then established, with the aid of the ion implantation technique, known per εe, m the intrinsic layer i at the position as defined in step 2 and at the angle a as defined in step 3. Steps 2 to 4 inclusive are then repeated for the n-doped zone n, the desired angle for the i-n junction 4 being referred to as β. Preferably, the relationship 45 < β < 135 degrees applies, more preferably the relationship 80 < β < 100 degrees. Most preference is given to an angle β of 90 degrees. It should be noted that p-type doping is preferably effected using boron. n-Type doping preferably makes use of phosphorus. All the other suitable substances known in the art can be used as an alternative, however . Using the method according to the invention it is possible for thin-film solar cells according to the invention to be fabricated reliably and rapidly. Given that the intrinsic layer has, in accordance with the method in accordance with the invention, been deposited directly on the substrate, said intrinsic layer can be optimized in many ways. This optimization is limited only by the physical properties of the substrate. There follows an example of a thm-film solar cell which has been fabricated by means of the above-described method and has been tested in practice. EXAMPLE In this example the substrate is a monocryεtallme 4" wafer covered with SiO-,. A deposition method used for the i-type layer was 13.56 MHz PECVD. The positions for the doped zones were defined with the aid of photolithography. p-Type doping was carried out with the aid of boron ion implantation, and n-type doping was effected with the aid of phosphorus ion implantation, both employing an initial estimate of the dose and the energy. For the purpose of making contact and bridging n-p interfaces for series connection, use was made of aluminium. Finally, the entire thm-film solar cell was baked for 1 hour at a temperature of 200°C. The thin-film solar cell in this example has the following dimensions: thickness of i-type layer: 400 nm width of p-, l- and n-type zones * 1 μm finger length: 40 μ number of fingers.100 It should be noted that the width of the i-type zone is preferably greater than the width of the p-type and n-type zones. 30 of these thm-film solar cells were connected in series, which gave the following results: Isc = 40 nA V„o ^ c = 23 V η = 0.12% Additionally, an alternative method for fabricating a th:n-fιlm solar cell according to the invention is discussed below. What was said about the materials mentioned within the context of the first-mentioned method also applies to the materials mentioned within the context of the alternative method. The alternative method involves the deposition, in a first step, of optionally a layer of p-doped material or a layer of n-doped material on a substrate 2. To simplify the description it is assumed in the following that a layer of p-doped material is deposited on the substrate. It will be evident that in the case of a layer of n-doped material being deposited, "p-doped" in the following description will have to be replaced throughout by "n-doped" and vice versa In the second step of the method the position is then defined at which the n-doped material has to be established in the deposited p-doped layer of material. Said position definition can take place in a known manner, for example with the aid of a suitable mask and/or lithographic tech- niques. Then, in a third step, a portion of the p-doped material deposited in step one is removed at the position as defined in step two. In step four the n-doped material is then deposited on the thin-film solar- cell obtained in step three. Subsequently, in step five, the position of the intrinsic zone i in the p-doped and n-doped material is defined in a manner known per se. Step six comprises the removal of a portion of the p-doped material and the n-doped material at the position as defined in step five. This purpose can be served by using any of the techniques known in the art, for example etching. Finally, in step seven, a layer of intrinsic material i is deposited on the thin-film solar cell obtained in step six. The major advantage of the alternative method is that it is much cheaper, owing to deposition being used instead of ion implantation. It will be evident to those skilled in the art that in the case of the alternative method the angles between the light-collecting surface and the unc- tions between the doped material and the intrinsic material can likewise be chosen between 45 and 135 degrees, precisely the same as with the first- mentioned method. This can be done, for example, in steps five and six. The term "position" in the above therefore should be understood as referring to "location and/or angle". The step of establishing electrical contacts on the resulting p- and n-doped zones in question can be carried out with the aid of techniques known per se. The contacts are preferably established by vapour phase deposition of metal. With the aid of masks and litho-graphic techniques it is possible for the specific metal zone to be defined and positioned with respect to the doped zones. The thm-film solar cells fabricated according to one of the above-described methods can be interconnected, for example as shown in Figures 3 and 4, to form a panel. The panel may optionally be provided with reinforcing elements and can be framed. The panel is preferably treated in a known manner to prevent corrosion. The methods according to the invention are in the field of microelectronics technology. This means that the thin-film solar cell can be fabricated using relatively small amounts of material. The present invention is of course not limited to the described and illustrated preferred embodiment, but comprises any embodiment which is consistent with the preceding description and the accompanying drawings and is within the scope of the appended claims.

Claims: CLAIMS 1. Thin-film solar cell provided with at least one p-j.-n junction comprising at least one p-i junction which is at an angle a with that sur- face of the thin-film solar cell which collects light during operation and at least one i-n junction which is at an angle β with the light-collecting surface, characterized in that 45 < o < 135 degrees and 45 < β < 135 degrees. 2. Thin-film solar cell according to Claim 1, wherein 80 < α < 100 degrees and 80 < β < 100 degrees. 3. Thm-film solar cell according to Claim 1 or 2, wherein the p-i junction (8) and/or the i-n junction (9) is meandrouε at least in cross- section. 4. Thin-film solar cell according to Claim 1, 2 or 3, wherein the p-doped zone (p) and the n-doped zone (n) of the p-i-n junction each comprise one or more fingers (6, 7). 5. Thm-film solar cell according to Claim 4, wherein the p-doped fingers (6) and the n-doped fingers (7) at least alternate in a plane which runs approximately parallel to the light-collecting surface (5). 6. Thm-film solar cell according to any one of the preceding claims, which is provided with a supplementary layer of intrinsic material which is primarily disposed between the p-i-n junction and the light-collecting surface (5). 7. Panel provided with a plurality of thm-film solar cells accord- mg to any one cf the preceding claims, wherein the thm-film solar cells are electrically connected to one another. 8. Panel according to Claim 7, comprising a number of units (10) which each comprise a number of thin-fllm solar cells ( 1 ) connected in series, the series connection being effected by electrical connection of the n-doped zone (n.,) of a first thin-film solar cell to the p-doped zone (p 2 ) of a further thin-film solar cell, which units (10) are connected parallel to one another by the outermost p-doped zones (p.,) of the units being electrically connected to one another and by the outermost n-doped zones (n k ) of the units being electrically connected to one another. 9. Method for fabricating a thm-film solar cell according to any one of the preceding claims 1 to 6 inclusive, characterized in that the method comprises the following steps * (a) depositing a layer of intrinsic material (I) on a substrate (2); (b) defining the location of the p-doped zone (p) in the layer of intrinsic material; (c) defining the desired angle α for the p- junction (3) with respect to the light-collecting surface (5) of the thm-film solar cell (1), where 45 < o < 135 degrees; (d) establishing, with the aid of ion implantation, the p-doped zone at the location as defined in step (b) and at the angle as defined in step ( c ) ; (e) defining the location of the n-doped zone (n) in the layer of intrinsic material; (f) defining the desired angle β for the i-n junction (4) with respect to the light-collecting surface (5) of the thm-film solar cell, where 45 < β < 135 degrees; (g) establishing, with the aid of ion implantation, the n-doped zone at the location as defined in step (e) and at the angle β as defined in step (f ) . 10. Method for fabricating a thm-film solar cell according to any one of the preceding claims 1 to 6 inclusive, characterized in that the method comprises the following steps: (a) depositing a layer of p-doped material and a layer of n-doped material, respectively, on a substrate (2); (b) defining the position of the n-doped zone (n) and the p-doped zone (p), respectively, in the layer of p-doped material and n-doped material, respectively; (c) removing a portion of the deposited doped material at the position as defined in step (b); (d) depositing a layer of n-doped material and p-doped material, respectively, on the th -film solar cell which was obtained in step (c); (e) defining the position of the intrinsic zone d) in the layer of p-doped and n-doped material; (f) removing a portion of the p-doped material and the n-doped material at the position as defined in step (e); (g) depositing a layer of intrinsic material d) on the thin-film solar cell which was obtained in step (f).

Inventors: KUZNETSOV VLADIMIR IWANOVICH, METSELAAR JAN WILLEM, KUZNETSOV, VLADIMIR, IWANOVICH, METSELAAR, JAN, WILLEM
Applicants: KUZNETSOV VLADIMIR IWANOVICH, METSELAAR JAN WILLEM, UNIV DELFT TECH, KUZNETSOV, VLADIMIR, IWANOVICH, METSELAAR, JAN, WILLEM, TECHNISCHE UNIVERSITEIT DELFT
Application year: 1997
Main classification: H01L31
IPC codes: H01L31