A CONDUCTIVE CONTACT PATTERN FOR A SOLAR CELL

FIELD OF THE PRESENT DISCLOSURE [0001] The present disclosure relates to a solar cell, particularly a solar cell including a conductive contact pattern on the surface of a semiconductor wafer. The present disclosure particularly relates to a method of producing a solar cell including a conductive contact pattern.

BACKGROUND OF THE PRESENT DISCLOSURE

[0002] Solar cells are photovoltaic devices that can convert sunlight into electrical power. Solar cells formed from semiconductor wafers may generate electricity by absorption of light to generate mobile charge carriers. The mobile charge carriers can generate electrical current which can be collected by metal contacts formed on the surface of the semiconductor wafer.

[0003] It is desirable to manufacture high efficiency solar cells at low cost. The output power of solar cells is enhanced by the surface area of the solar cell that is available for sunlight to generate charge carriers, as well as the efficiency of collecting and transmitting the photogenerated charge carriers. [0004] The present disclosure is particularly directed at the process for forming a conductive contact on a surface of a solar cell. The conductive contact may function as a collector of charge carriers from the surface of semiconductor wafer. Put another way, the conductive contact collects electrical current from the surface of the semiconductor wafer surface. [0005] Low resistance of the conductive contacts can keep ohmic losses low, and less harvested light energy is lost as heat. A large conductive contact such as a contact with a large cross-sectional area has lower resistance than a small contact.

[0006] However, a competing effect results from the footprint of the conductive contact on the surface of the solar cell, particularly the front surface. A large contact has a large footprint, and partially blocks sunlight from reaching the semiconductor. The effect of blocking sunlight by the footprint of the conductive contact can be referred to as shadowing. It is desirable to have a conductive contact which has a large cross-section, yet a small footprint.

SUMMARY

[0007] A method for producing a solar cell which includes forming a conductive contact pattern on a surface of a semiconductor wafer is disclosed. Forming the conductive contact pattern includes forming a patterned mask on the semiconductor wafer, depositing a conductive material through the mask, and removing the patterned mask. A solar cell which is produced by the method is also disclosed.

[0008] The disclosure is also directed to an apparatus for carrying out the disclosed methods and including apparatus parts for performing each described method steps. These method steps may be performed by way of hardware components, a computer programmed by appropriate software, by any combination of the two or in any other manner. Furthermore, the disclosure is also directed to methods by which the described apparatus operates. It includes method steps for carrying out every function of the apparatus. [0009] Furthermore, embodiments according to the invention are also directed at methods by which the described apparatus operates. BRIEF DESCRIPTION OF THE DRAWINGS

[0010] So that the manner in which the above recited features of the present disclosure can be understood in detail, a more particular description, briefly summarized above, may be had by reference to embodiments. The accompanying drawings relate to embodiments.

[0011] Fig. 1 shows a solar cell according to embodiments described herein;

[0012] Fig. 2 shows a patterned mask on a semiconductor wafer, according to embodiments described herein;

[0013] Fig. 3 shows a conductive material deposited through a patterned mask on a semiconductor wafer, according to embodiments described herein;

[0014] Fig. 4 shows a conductive material deposited on a semiconductor wafer, according to embodiments described herein;

[0015] Fig. 5 shows a mask on a semiconductor wafer, according to embodiments described herein; [0016] Fig. 6 shows a patterned mask on a semiconductor wafer, according to embodiments described herein;

[0017] Fig. 7 shows a conductive material deposited through a patterned mask on a semiconductor wafer, according to embodiments described herein;

[0018] Fig. 8 shows a conductive material deposited on a semiconductor wafer, according to embodiments described herein;

[0019] Fig. 9 shows a method for forming a conductive contact pattern on a surface of a semiconductor wafer, according to embodiments described herein. DETAILED DESCRIPTION

[0020] Herein, a "charge carrier" can be for example electrons and holes. Herein, "conductive contact pattern" and "contact pattern" are used interchangeably. Herein, "conductive contact" can refer to a contact for collecting current from a semiconductor wafer, commonly a metallic contact. Herein "contact" and "conductive contact" may be used interchangeably. Herein, a semiconductor wafer can be for example a silicon based wafer such as a doped silicon wafer and can include for example an antireflection surface which may be a semiconductor material, a dielectric material, or combination thereof. Herein, particularly of a feature of a conductive contact and/or contact pattern can be in a direction normal to a wafer surface; width can be tangent to the wafer surface, particularly along the narrower dimension of a feature.

[0021] Herein, the formation of a conductive contact pattern can possibly be performed under relaxed cleanliness environments such as in a ISO class 3 or higher, ISO class 4 or higher, ISO class 5 or higher, ISO class 6 or higher, ISO class 7 or higher, or ISO class 8 or higher environment (referring to ISO 14644-1 cleanroom standards). Deposition of a conductive material through a mask such as a mask on a semiconductor wafer can be performed under relaxed cleanliness environments such as those named above. For example, procedures carried out after deposition of a mask such as a resist or resist layer may be more tolerant to airborne particles, and can be carried out under relaxed cleanliness environments such as those named above.

[0022] Herein, a solar cell can be for example a solar cell of size from 100 mm x 100 mm to 200 mm x 200 mm such as 150 mm x 150 mm. Solar cells can be substantially larger than semiconductor substrates used in the electronics industry. Typical solar cells sizes are 156 x 156mm (6 inches) or 125 x 125mm (5 inches). Semiconductor wafers which are used in the electronics industry are typically larger than this and pseud-circular in shape. Herein, "semiconductor wafer" and "wafer" can refer to a semiconductor wafer sized for use as a solar cell, for example a square wafer.

[0023] Reference will now be made in detail to the various embodiments, one or more examples of which are illustrated in the figures. Each example is provided by way of explanation and is not meant as a limitation. Further, features illustrated or described as part of one embodiment can be used on or in conjunction with other embodiments to yield yet a further embodiment. It is intended that the description includes such modifications and variations.

[0024] Fig. 1 illustrates a solar cell, according to an embodiment described herein. A solar cell can include a conductive contact pattern 36 on a surface of a semiconductor wafer 14 such as a silicon wafer. The conductive contacts of the conductive contact pattern 36 can be used to collect current, a flow of electric charge carriers such as electrons and/or holes, generated by the semiconductor wafer 14 as light generates charge carriers. As illustrated in Fig. 1, the conductive contact pattern 36 can include fingers 34 which can collect current from the surface of the semiconductor wafer 14. The conductive contact pattern 36 can optionally include busbar 32 for collecting current from the fingers for example. Alternatively or additionally, the solar cell can be a "wrap-through" type which can have conductive channels connecting the front to the back of the semiconductor wafer. The conductive contact pattern 36 of Fig. 1, including the optional busbars, can be on the front side and/or back side of the solar cell.

[0025] The material(s) and/or structure of the conductive contact pattern 36 can impact the overall efficiency of the solar cell. For example, a conductive contact pattern 36 made of a conductive material with low resistivity is desired to reduce ohmic loss. Maintaining a high conductivity of the contacts can reduce the amount of power lost as heat. In an embodiment, the conductive contact pattern 36 can include a conductive material which can be formed from a powder, ink, slurry, or paste, such as a conductive paste such as a metal paste such as a silver paste. The resistance of the fingers 34 for example can be inversely related to the cross- sectional area of the fingers 34. Less resistance of the contacts such as the fingers can reduce the amount of photogenerated current that is lost as heat.

[0026] Shadowing of the semiconductor wafer 14 by the contact pattern 36 is undesirable because it reduces the available area of the semiconductor wafer 14 to receive light. For example, the contact pattern 36 can possibly partially block the transmission of light, such that a portion of the semiconductor wafer 14 is shadowed. Thus the region below the contact pattern 36 may not receive light, particularly if the contact pattern 36 is disposed facing the light source such as on the front surface of the semiconductor wafer 14. It is desirable to reduce the footprint of the contact pattern 36 so that a greater area of the semiconductor wafer 14 is not shadowed and is able to receive light. However, in addition to shadowing and resistance of the contacts, there can be other effects of the design of the contact pattern which can be taken into account for optimizing efficiency of a solar cell. [0027] It is desirable to consider multiple factors in determining the design of the contact pattern for maximizing output power and/or minimizing losses. There can be a tension between the benefit of small footprint of the contact pattern 36 to decrease shadowing of the underlying semiconductor wafer 14, and the benefit of a high cross-section of each of the fingers 34 to decrease electrical resistance. Other effects may also impact the overall output power and/or efficiency of the solar cell.

[0028] Herein is disclosed a solar cell including a conductive contact pattern 36 which can include a plurality of features, such as fingers 34 and optionally at least one busbar 32. Alternatively or additionally, the solar cell can be a "wrap- through" type which can have conductive channels connecting the front to the back of the semiconductor wafer. The plurality of features can have a high aspect ratio such as an aspect ratio of at least 0.5, particularly the fingers. The aspect ratio can be the ratio of the height of a feature divided by the width of the feature. A feature such as a finger 34 and/or busbar 32 of the contact pattern 36 can have a high aspect ratio in order to maintain a low footprint yet maintain a large cross- section. The high aspect ratio can make it possible to have a low resistance because of the high cross-section. The high aspect ratio can have a reduced shadowing effect because of the small footprint of a high aspect ratio feature, particularly in comparison to a symmetric feature of the same areal cross-section.

[0029] The width and/or thickness of a feature can be substantially greater than feature widths and/or thicknesses used in the electronics industry such as in the formation of transistors.

[0030] The width of a feature such as a finger can be for example from 10 micrometers or greater such as from 10 micrometers to 200 micrometers. The thickness of a feature such as a finger can be 5 micrometers or greater, and can be provided such that the aspect ratio of the finger is at least 0.5. The width of a busbar can be from 10 micrometers to 4 mm, and/or from the finger width to 4 mm. [0031] The thickness of a busbar can be in the same range as the fingers, and/or can be provided such that the aspect ratio is in the same range as the fingers.

[0032] The lengths of busbars can be for example on the scale of the solar cells, such as 100 mm or longer, 150 mm or longer, 200 mm or longer, and the like. For example the length of a busbar can match the length of the solar cell. [0033] Herein is disclosed a method for producing a solar cell which includes forming a conductive contact pattern 36 on a surface of a semiconductor wafer 14.

[0034] Figs. 2-4 illustrates a method of forming a conductive contact pattern according to an embodiment described herein. As in Fig. 2, a patterned mask 12 has been formed on a surface of the semiconductor wafer 14. As in Fig. 3, a conductive material 30 has been deposited through the patterned mask 12. As in Fig. 4, the patterned mask 12 is removed. The conductive material 30 that remains on the surface of the semiconductor wafer 14, after the patterned mask 12 is removed, can form the conductive contact pattern 36 as shown in Fig. 1.

[0035] Fig. 2 shows a patterned mask 12 on a semiconductor wafer, according to an embodiment described herein. The patterned mask 12 can be formed from a resist. For example, the patterned mask 12 is formed by screen printing a resist such as a photoresist. The screen printing can be done for example under ambient pressure such as atmospheric pressure. The screen printing can also be done at increased pressure such as above atmospheric pressure. An increased pressure may be used to pressurize the paste in order to force it through the screen. The patterned mask 12 can have openings. The openings can extend to the surface of the semiconductor wafer 14. For example, the patterned mask 12 can have openings such as openings of 10 micrometers width or greater, such as from 10 micrometers to 200 micrometers.

[0036] Fig. 3 shows a conductive material 30 which has been deposited through the patterned mask 12, according to an embodiment described herein. The conductive material 30 can include silver. For example the conductive material 30 can be formed from a powder, an ink, a slurry, and/or a conductive paste such as a metal paste such as a silver paste. The conductive material can be deposited by dispensing on the patterned mask a powder, an ink, a slurry, and/or a conductive paste such as a metal paste, for example a silver paste. The conductive material 30 can be deposited by screen printing on a patterned mask 12 such as a patterned mask formed from a resist.

[0037] Screen printing the conductive material 30 on the patterned mask 12 can be referred to as double printing, which can for example be done under ambient pressure such as atmospheric pressure. The screen printing can also be done at increased pressure such as above atmospheric pressure. An increased pressure may be used to pressurize the paste in order to force it through the screen. [0038] The conductive material 30 can be deposited through the patterned mask 12, such as through openings of the patterned mask 12. For example, the conductive material 30 can make contact with the semiconductor wafer 14 and/or an antireflection surface such as that of the semiconductor wafer. The conductive material can for example be deposited under ambient pressure such as atmospheric pressure. The screen printing can also be done at increased pressure such as above atmospheric pressure. An increased pressure may be used to pressurize the paste in order to force it through the screen.

[0039] It is possible, in an embodiment which may be combined with any other embodiment disclosed herein, that the thickness of the conductive material 30 is greater than the thickness of the patterned mask. When so, the aspect ratio of the resulting feature, such as a finger, may be advantageously large.

[0040] Fig. 4 shows a conductive material deposited on a semiconductor wafer after the patterned mask is removed, according to an embodiment described herein. Removal of the patterned mask 12 can be by the use of light, an electron beam, and/or exposure to a chemical, and/or heat, for example. The remaining conductive material 30 can form the conductive contact pattern 36, including for example a plurality of features, such as fingers 34 and optionally at least one busbar 32. A feature can have an aspect ratio of at least 0.5, the aspect ratio being the height of the feature divided by the width of the feature. The width of a finger can be for example from 10 micrometers to 200 micrometers.

[0041] Figs. 5-8 illustrates a method of forming a conductive contact pattern according to an embodiment described herein. As in Fig. 5, a mask 40 such as a resist layer, such as a continuous resist layer, is formed over a surface of a semiconductor wafer 14. The resist layer can for example be deposited under ambient pressure such as atmospheric pressure. The screen printing can also be done at increased pressure such as above atmospheric pressure. An increased pressure may be used to pressurize the paste in order to force it through the screen. As in Fig. 6, the mask 40 has been patterned, for example using a laser and a mask made of a photoresist. For example, a resist layer such as a continuous resist layer can be selectively removed, such as for example to form openings, and/or such that a patterned resist layer is formed which can be regarded as a patterned mask 12. As in Fig. 7, a conductive material 30 is deposited through the patterned mask 12. As in Fig. 8, the patterned mask 12 is removed, for example by the use of light, an electron beam, exposure to a chemical, and/or heat. The remaining conductive material 30 can form the conductive contact pattern 36.

[0042] In an embodiment which can be combined with any other embodiment described herein, the patterned mask 12 can be disposed directly on the surface of the semiconductor wafer 14. Openings of the patterned mask 12 can extend completely through the material of the mask, for example a resist such as a photoresist, and the openings can reach to and expose the surface of the semiconductor wafer 14 such as before the conductive material 30 is deposited. The conductive material 30 can be deposited through the openings, and can make direct contact with the semiconductor wafer 14. Alternatively or additionally, the the patterned mask 12 can be disposed directly on the surface of an antireflection surface such as that of the semiconductor wafer 14.

[0043] In an embodiment which can be combined with any other embodiment described herein, the conductive material can be deposited by dispensing a powder, an ink, a paste, and/or a slurry on the patterned mask. The deposition of the conductive material (30) can be such that at least some of the powder, ink, paste, or slurry is deposited through the mask, such as through openings of the mask.

[0044] The methods disclosed herein of forming a conductive contact pattern can be utilized to produce conductive features (for example fingers 34 as shown in Fig. 1) which have a high aspect ratio such as at least 0.5. Also, the methods are such that feature widths of the conductive material 30 can be better controlled, such as having widths from 10 micrometers to 200 micrometers. The method for example reduces spreading of the deposited conductive material 30 on the surface of the semiconductor wafer 14. This may advantageously maintain a high aspect ratio as well as reduce shadowing effects.

[0045] Spreading, particularly lateral spreading, of the conductive material 30 can be mitigated by depositing the conductive material 30 through the patterned mask 12, particularly a mask in direct contact with the semiconductor wafer surface, Higher possible aspect ratios and/or smaller footprints of features of the contact pattern can result. Lateral spreading of the conductive material 30 can be mitigated. Lateral spreading may lead to finger enlargement. For example, when a mask such as a mask in contact with the semiconductor wafer 14 is not used, lateral spreading may be as much as 10 to 15 micrometers. The patterned mask 12 can serve to confine a conductive material 30 such as a conductive paste to reduce lateral spreading.

[0046] Fig. 9 illustrates a method of producing a solar cell including forming a conductive contact pattern according to an embodiment described herein. A conductive contact pattern can be formed by: forming a patterned mask on a semiconductor wafer 100, depositing a conductive material through the patterned mask 200, and removing the patterned mask 300. The patterned mask can be formed directly on a surface of a semiconductor wafer. Alternatively or additionally, the patterned mask can be formed directly an antireflection surface for example of the semiconductor wafer. The formation of the patterned mask can for example begin with deposition of a continuous resist layer which is subsequently patterned. For example a resist layer made of a photoresist can be used as a continuous resist layer which his subsequently patterned with a laser. The patterned mask can for example be formed by via screen printing a resist onto the semiconductor wafer.

[0047] In an embodiment which can be combined with any other embodiment described herein, the method for producing a solar cell includes generating an antireflection surface. The antireflection surface can be a coating and/or a physical and/or chemical modification of the semiconductor wafer surface. For example the semiconductor wafer is textured for example by etching, chemical treatment, and/or roughening to generate an antireflection surface. Alternatively or additionally, the generation of the antireflection surface can include chemical vapor deposition (CVD) such as plasma-enhanced CVD. The antireflection surface generation process can for example generate, such as by deposition, chemical treatment, and/or physical treatment: an amorphous layer, a dielectric layer, a Ti0 ₂ layer, a TiO _x layer, a Si0 ₂ layer, a SiO _x layer, a SiO layer, a S1 ₃ N ₄ layer, a SiN _x layer, a SiN _x :H layer, a nanoparticle and/or nanostructured layer, combinations thereof, or the like. The generation of the antireflection surface can include chemical and/or physical processes such that the surface of the semiconductor wafer is modified to form an antireflection surface such as a textured and/or rough surface. The generation of the antireflection surface can include roughening processes.

[0048] In an embodiment which can be combined with any other embodiment described herein, the solar cell can include an antireflection surface. For example the semiconductor wafer includes a textured surface for reducing reflection such as an amorphous layer, a dielectric layer, a Ti0 ₂ layer, a TiO _x layer, a Si0 ₂ layer, a SiO _x layer, a SiO layer, a S1 ₃ N ₄ layer a SiNx layer, a SiN _x :H layer, a nanoparticle and/or nanostructured layer, combinations thereof, or the like. The textured surface can be rough in order to reduce reflection. Alternatively or additionally, an antireflection coating may be applied to the semiconductor wafer, particularly the front surface of the semiconductor wafer. It is contemplated that in the formation of the conductive contact pattern, the patterned resist may be formed on the antireflection surface. For example, the resist may be deposited directly onto a SiN _x layer

[0049] While the foregoing is directed to embodiments of the invention, other and further embodiments of the invention may be devised without departing from the basic scope thereof, and the scope thereof is determined by the claims that follow.