Method and an apparatus for treating a surface of a TCO material, in a semiconductor device

Abstract

The present disclosure provides a method for treating a surface portion of a TCO material in a semiconductor device that comprises a structure arranged to facilitate current flow in one direction. To perform the method the surface portion of the TCO is exposed to an electrolyte and a current is induced in the device. The current allows reducing the TCO material in a manner such that the adhesion of a metallic material to the exposed surface portion is improved over the adhesion of the metallic material to a non-exposed surface portion.

Claims

1. A method for treating a plurality of portions of a surface of a TCO layer in a semiconductor device, the semiconductor device comprising a structure arranged to facilitate current flow in one direction, the method comprising the steps of: exposing the plurality of surface portions of the TCO layer to an electrolyte, the exposed surface portions comprising finger portions for adherence with finger electrodes, there being a non-exposed surface region of the TCO layer between each pair of adjacent finger portions, the electrolyte being suitable for electrochemically reducing each exposed surface portion of the TCO layer when an electrical current is induced through the TCO layer; and inducing a current in the TCO layer by inducing a current in the semiconductor device, the induced current being directed transversally to the exposed surface portions of the TCO layer and being substantially uniform across all exposed surface portions of the surface of the TCO layer; wherein the substantially uniform induced current reduces the exposed surface portions of the TCO layer, such that the TCO layer includes TCO material at the exposed surface portions and the TCO material at the exposed surface portions has substantially uniformly reduced surfaces, and improves uniformity of adhesion of a metallic material to each exposed surface portion in comparison to adhesion of the metallic material to the non-exposed regions of the TCO layer; wherein the reduction at the plurality of exposed surface portions of the TCO layer is induced by exclusive contact of the exposed surface portions with the electrolyte.

2. The method of claim 1 wherein the structure arranged to facilitate current flow in one direction comprises a light absorbing layer and at least one carrier selective layer.

3. The method of claim 1 wherein the structure arranged to facilitate current flow in one direction comprises a p-n junction and the step of inducing a current in the TCO layer comprises the step of biasing the p-n junction.

4. The method of claim 3 wherein the method further comprises the step of electrically interconnecting an electrode element to the semiconductor device and electrically interconnecting the electrode element to a wet electrode positioned for contacting the electrolyte so that the induced current can flow through an electrical circuit comprising: the electrolyte, the semiconductor device, the TCO layer; the electrode element and the wet electrode.

5. The method of claim 4 wherein the TCO layer is arranged as a continuous layer on an n-type or p-type region of the semiconductor device and the induced current flows in a direction transverse to the layer.

6. The method of claim 4 wherein the TCO layer is arranged as a continuous layer on an n-type region of the semiconductor device and the step of inducing a current in the TCO layer comprises the step of exposing a portion of the semiconductor device to electromagnetic radiation to induce a photo-generated current.

7. The method of claim 6 wherein the method further comprises the step of controlling a structural or electrical property of the surface of the TCO layer by modulating a property of the induced current.

8. The method of claim 7 wherein the property of the induced current is the magnitude of the induced current and is modulated by modulating the intensity of the radiation.

9. The method of claim 6 wherein the method further comprises the step of applying a voltage between the semiconductor device and an electrode in the electrolyte, the applied voltage being such to decrease the voltage drop induced by the electromagnetic radiation on the p-n junction.

10. The method of claim 4 wherein the TCO layer is arranged as a continuous layer on a p-type region of the semiconductor device and the step of inducing a current in the TCO layer comprises the step of applying a voltage between the semiconductor device and an electrode in the electrolyte, the applied voltage being such to forward bias the p-n junction.

11. The method of claim 9 wherein the method further comprises the step of controlling a structural or electrical property of the surface of the TCO layer by modulating a property of the applied voltage.

12. The method of claim 9 wherein the semiconductor device comprises an electrode element that is at least semi-transparent to the electromagnetic radiation and wherein the voltage is applied via the electrode element.

13. The method of claim 1 wherein the TCO layer is etched while the method is performed.

14. The method of claim 1 wherein a concentration of metallic elements in the TCO layer at its surface is increased while the method is performed, and wherein a roughness of the exposed portions is increased after the method is performed.

15. The method of claim 1 wherein the method further comprises the step of selecting a property of the electrolyte to influence a property of the surface of the TCO layer after treatment.

16. The method of claim 1 wherein the method further comprises the step of, prior to exposing the surface of the TCO layer to an electrolyte, forming a mask onto the TCO layer to define a patterned surface of the TCO layer to be exposed to the electrolyte.

17. The method of claim 1 wherein the semiconductor device is a silicon solar cell.

18. A method for plating a metallic material to a surface of a TCO layer in a semiconductor device, the semiconductor device comprising a p-n junction, the method comprising the steps of: exposing a plurality of portions of the surface of the TCO layer to an electrolyte that is suitable for electrochemically reducing the TCO layer when an electrical current is induced through the TCO layer, the surface portions comprising finger portions for adherence with finger electrodes, there being a non-exposed surface region of the TCO layer between each pair of adjacent finger portions; and inducing a current in the TCO layer by biasing the p-n junction, the induced current being directed transversally to the exposed surface portions of the TCO layer and being substantially uniform across all exposed surface portions of the surface of the TCO layer; allowing for the surface of the TCO layer to be reduced in a substantially uniform manner by the current and the electrolyte such that uniformity of adhesion of the metallic material to the exposed surface is improved; and plating the metallic material to the surface of the TCO layer; wherein the reduction at the plurality of exposed surface portions of the TCO layer is induced by exclusive contact with the electrolyte.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The embodiments of the invention will now be described, by way of example, with reference to the accompanying drawings in which:

(2) FIG. 1 is a flow diagram outlining steps for treating a surface portion of a TCO material in accordance with embodiments;

(3) FIG. 2 is a schematic diagram of an apparatus used to perform the method of FIG. 1;

(4) FIG. 3 is a flow diagram outlining steps for plating a metallic material to a TCO material in accordance with embodiments;

(5) FIG. 4 is a schematic diagram of a solar cell comprising a TCO layer treated in accordance with embodiments;

(6) FIG. 5 shows images of metal fingers after adhesion testing;

(7) FIG. 6 shows SEM images of a surface of a solar cell before (a) and after treatment (b); and

(8) FIG. 7 shows two diagrams of the chemical surface condition of an ITO layer treated surface.

DETAILED DESCRIPTION OF EMBODIMENTS

(9) Embodiments of the present invention relate to a method and an apparatus for treating a TCO material in a semiconductor device so that the adhesion of some metals to the TCO material is improved. Embodiments also relate to a method of plating a metal to a TCO material treated to improve metal adhesion. Some embodiments are directed to photovoltaic devices which comprise TCO layers, such as heterojunction solar cells, where the TCO layers are treated to improve metal adhesion.

(10) The TCO treatment is performed by taking advantage of the properties of a structure arranged to facilitate current flow in one direction in the semiconductor device.

(11) Unlike other methods of treating TCOs, which require a direct mechanical contact to the TCO material, in the method and apparatus disclosed herein the surface of the TCO being treated is exclusively contacted by an electrolyte. In addition, the induced current is not required to flow laterally through the surface layer being treated. Instead the induced current is directed transversally to the surface being treated, therefore enabling uniform adhesion properties.

(12) Referring now to FIG. 1, there is shown a flow diagram 100 outlining steps for treating a surface portion of a TCO material in accordance with embodiments. At step 105 a semiconductor device comprising a structure arranged to facilitate current flow in one direction is provided. In the case of a solar cell, the structure may comprise an absorbing layer disposed between an electron selective membrane and a hole selective membrane. Alternatively, the structure may comprise a p-n junction, such as in a heterojunction solar cell or a homojunction solar cell.

(13) At step 110, a surface portion of the TCO material is exposed to an electrolyte that is suitable for electrochemically reducing the TCO material when an electrical current is induced through a region of the TCO material.

(14) At step 115, a current is induced in the semiconductor device. The current may be induced by applying a biasing voltage to the semiconductor device or by exposing the semiconductor device to radiation. In some cases the biasing voltage and the radiation are applied simultaneously.

(15) In the case where the semiconductor device is a solar cell, the TCO may be arranged as a continuous layer on an n-type or p-type region of the solar cell. If the method is used to treat a TCO on an n-type region, the current can be induced by exposing the solar cell to electromagnetic radiation. In some instances, the photo-generated current can be sufficient to perform the TCO treatment. In other instances the current flow is facilitated by applying a biasing voltage to the solar cell to compensate for the self-biasing of the p-n junction created by the radiation.

(16) When the method is used to treat a TCO on a p-type region, a forward biasing voltage is necessary to induce the current in the solar cell.

(17) One advantage of the method is that the induced current flows in a direction transverse to the layer, providing uniform reduction of the exposed portion of the TCO and therefore resulting in improved adhesion uniformity. In particular, the current flows through an electrical circuit comprising: the electrolyte, the solar cell and the TCO material. This is discussed in more detail in the next section with reference to FIG. 2.

(18) The current flowing through the electrolyte, the solar cell and the TCO material allows reduction of the TCO material and improved adhesion of some metallic materials, such as copper, to the surface portion of the TCO exposed to the reducing electrolyte. The adhesion is improved relative to the adhesion of the same metal to the same TCO where the TCO has not been treated with the process described above.

(19) Referring now to FIG. 2, there is shown a schematic diagram of an apparatus 200 used to perform the method of FIG. 1. Apparatus 200 comprises a chemical bath 202 suitable to contain an electrolyte 204. A portion of a TCO layer 207 of a solar cell 205 is exposed to the electrolyte 204. Solar cell 205 can be kept in position using a support in the chemical bath (not shown) or a vacuum holder (not shown). Alternatively, solar cell 205 may float on the surface of the electrolyte 204.

(20) Further, apparatus 200 comprises a wet electrode 206 immersed in the electrolyte and a power supplier 208 for applying a voltage between wet electrode 206 and an electrode element 210 which is in electrical contact with solar cell 205. Apparatus 200 comprises a radiation source 212 which is arranged to expose solar cell 205 to radiation. TCO layer 207 is exposed to the electrolyte, while the remaining portions of solar cell 205, are kept dry while the method is performed.

(21) In the embodiment of FIG. 2, the radiation source 212 is disposed at the bottom of chemical bath 202 and photons 213 travel towards the bottom surface of solar cell 205.

(22) In alternative embodiments, the top side of the device may be contacted using a semi-transparent electrode and the radiation source 212 may be disposed above solar cell 205.

(23) A photo-generated current flows in the circuit comprising: electrolyte 204, solar cell 205, TCO layer 207, electrode 210 and wet electrode 206. The current flows transversally through TCO layer 207 providing a better uniformity of the metal adhesion properties of the TCO across the layer.

(24) A patterned mask 215 is used to select specific areas of TCO layer 205 for treatment. Mask 215 can be formed, for example, by using a photoresist material. A metal can be plated through mask 215 or, alternatively mask 215 can be removed prior to the plating step.

(25) A portion of the TCO layer may be etched while the treatment is performed. The concentration of metallic elements in the treated portions 214 increases during treatment. For example, for ITO layers, a portion of SnO.sub.2 in the layer is reduced to SnO in the first instance and to Sn if the method is performed for a sufficient amount of time. The increased concentration of the metallic element, tin in the case of ITO, promotes the adhesion of other metallic materials, such as copper, to the TCO layer.

(26) Electrolyte 204 comprises H.sub.2SO.sub.4 with a weight concentration of 1% and Na.sub.2SO.sub.4 with a weight concentration of 0.14%. The chemical composition of electrolyte 204 can be varied to affect the final adhesion properties of TCO layer 207.

(27) The adhesion properties and the structural properties of the surface of the treated areas 207 are related to the overall amount of electrical charge transferred to the TCO material. For plating of copper to ITO, an overall charge of 22 mC/cm.sup.2 was found to provide good adhesion properties. However, depending on the nature of the TCO and the material being plated, a charge comprised between 1 mC/cm.sup.2 and 50 mC/cm.sup.2 can be used.

(28) Structural and/or electrical properties of the regions treated 214 can also be controlled by modulating the magnitude of the current flowing through the circuit by changing the intensity of light 213 and/or the magnitude of voltage 208.

(29) Referring now to FIG. 3, there is shown a flow diagram 300 outlining steps for treating a TCO layer and plating a metallic material to the treated TCO layer. At steps 105, 110 and 115, the TCO material is treated as discussed above with reference to FIG. 1. At 305, a sufficient amount of time is allowed for the TCO to be reduced. At step 310 a metal is plated to the treated surfaces by field induced plating, light induced plating or electroplating.

(30) Referring now to FIG. 4, there is shown a heterojunction solar cell device 400 manufactured using the method of FIG. 3. Solar cell 400 is formed using an n-type crystalline silicon substrate 402 disposed between two hydrogenated intrinsic amorphous silicon layers 404. The device 400 has a p-type hydrogenated amorphous silicon layer 406 on one side and an n-type hydrogenated amorphous silicon layer 408 on the other side. ITO layers 410 and 412 are disposed on the p-type and an n-type hydrogenated amorphous silicon layers 406 and 408. The ITO layers have a thickness comprised between 50 nm to 100 nm and are treated to improve metal adhesion in accordance with the method of FIG. 1.

(31) In addition to their contacting function, the ITO layers are also used as anti-reflective-coatings (ARCs). The control of the current used during the treatment step is crucial as the ITO layers etch during treatment. The applicant found that a low-magnitude current allows a better control of the ITO properties during treatment. Devices for regulating small currents, such as current limiting diodes (CLD) can be used in the treating apparatus. Copper fingers 414 are formed on cell 400 using field induced plating, light induced plating or electroplating.

(32) The adhesion of metallic fingers to TCO materials can be measured using a number of methods such as busbar pull test or a tape test. Currently, an industry standard has not been established to measure adhesion.

(33) The Applicants have engineered a new method to measure adhesion of metal finger to the ITO layers. The method comprises the steps of moving a stylus along the surface of the ITO in a direction transverse to the finger until it encounters a side wall of a metal finger and pushing the finger sideways until it lifts-off.

(34) The adhesion of fingers 414 is improved in comparison to copper fingers plated on an untreated ITO layer. By using the stylus measurement, the force required to lift-off a 30 μm wide metal finger with a height of 8 μm from an untreated ITO layer is less than 0.2N. Whereas in accordance with the method of FIG. 3 the force measured is at least 1N.

(35) Referring now to FIG. 5(a), there is shown a microscope image of a finger after adhesion testing. The finger was cut off from the surface rather than becoming dislodged. FIG. 5(b) is a FIB cross-sectional image of a copper plated finger with a width of approximately 25 μm and a height of approximately 10 μm.

(36) Referring now to FIG. 6, there are shown SEM images of the surface before (6a) and after treatment with a method in accordance with embodiments (6b).

(37) Referring now to FIG. 7, there are shown two diagrams of the chemical surface condition of an ITO layer treated in accordance with embodiments. FIG. 7(a) shows the atomic percentage of SnO.sub.2, SnO and Sn on the surface of an ITO layer treated in accordance with embodiments for different charge density levels. The percentage of Sn.sup.4+ (SnO.sub.2) drops when the treatment charge density is increased. On the other hand, an increase of Sn.sup.2+ (SnO) can be observed when the charge is increased and a small concentration of elemental Sn is shown when a charge density of 29 mC/cm.sup.2 is used.

(38) FIG. 7(b) confirms the result in FIG. 7(a) by showing a drop of metal-oxygen compounds concentration with the increase of the treatment charge density.

(39) Any discussion of documents, acts, materials, devices, articles or the like which has been included in the present specification is not to be taken as an admission that any or all of these matters form part of the prior art base or were common general knowledge in the field relevant to the present invention as it existed before the priority date of each claim of this application.

(40) Throughout this specification the word “comprise”, or variations such as “comprises” or “comprising”, will be understood to imply the inclusion of a stated element, integer or step, or group of elements, integers or steps, but not the exclusion of any other element, integer or step, or group of elements, integers or steps.