Claims:WE CLAIM:
1. A portable device to apply conductor paste on thin film sample employed for devising high efficiency silicon solar cells comprising a screen bearing pattern, above which a refilling tank of conducting silver paste is placed, in the rail of which a squeegee flood bar assembly is mounted, which is actuated to release the conducting paste onto the screen.

2. A portable device to apply conductor paste on thin film sample employed for devising high efficiency silicon solar cells as claimed in claim 1, wherein the squeegee/flood bar assembly is connected to assembly of first and second arm joined by means of a hinge.

3. A portable device to apply conductor paste on thin film sample employed for devising high efficiency silicon solar cells as claimed in claim 1 or 2, wherein a push button and the assembly of arms activate the squeegee/flood bar assembly as the hinge joint travels along a rail positioned vertically at one end of the tank.

4. A portable device to apply conductor paste on thin film sample employed for devising high efficiency silicon solar cells as claimed in any of the preceding claims, wherein the vertical rail is integrally connected to a plate orthogonally constituting a part substantially having a L-section.
5. A portable device to apply conductor paste on thin film sample employed for devising high efficiency silicon solar cells as claimed in any of the preceding claims wherein said push button is mounted though the plate, under which a compressed spring is provided, wherein bottom tip of the button rests on the second arm.

6. A portable device to apply conductor paste on thin film sample employed for devising high efficiency silicon solar cells as claimed in any of the preceding claims wherein upon pressing the push button the squeegee gets activated and traverses a distance along the length of the refilling tank so as to release the paste in the required pattern on the screen.

7. A portable device to apply conductor paste on thin film sample employed for devising high efficiency silicon solar cells as claimed in any of the preceding claims wherein the flood bar connected on the opposite end of the assembly collects back the excess paste upon release of push button.

8. A portable device to apply conductor paste on thin film sample employed for devising high efficiency silicon solar cells as claimed in any of the preceding claims wherein a top cover is provided for protection of the internal assembly from accidental stresses.

9. A portable device to apply conductor paste on thin film sample employed for devising high efficiency silicon solar cells as claimed in any of the preceding claims wherein a bottom cover is provided for protection of the internal assembly from accidental stresses, which prevents spilling of the conductor paste.

10. A portable device to apply conductor paste on thin film sample employed for devising high efficiency silicon solar cells as claimed in any of the preceding claims is associated with the advantageous features such as herein described.
, Description:A portable device to apply conductor paste on thin film sample employed for devising high efficiency silicon solar cells
FIELD OF INVENTION
[001] This invention is directed to a portable device to apply conductor paste on thin film sample employed for devising high efficiency silicon solar cells. The conductor paste forming pad can be applied in a required pattern uniformly such that the electrical conductivity parameters of thin films can be studied accurately.

BACKGROUND/PRIOR ART

[002] Photovoltaic (PV) energy is on the edge of becoming one of the main global sources of energy, and crystalline silicon has been dominating the market constituting 90% of manufactured solar cells. Thin film solar cell technologies are also promising as they have very low temperature coefficient and their performance is not impeded by low intensity light. Silicon-based hetero junction solar cells (Si-HIT) combine the advantages of crystalline silicon solar cells and thin film technologies enabling development of solar cells with record efficiency energy conversion up to 26.3%. In order to produce the electrical structure of hetero junction solar cells, thin layers of doped and intrinsic amorphous silicon are deposited on both sides of n-type silicon wafers. A transparent conducting oxide (TCO) layer is also applied to absorb the generated power. While ITO layers are mostly deposited by sputtering technique, amorphous silicon (a-Si) thin films are generally deposited by plasma enhanced chemical vapour deposition (PECVD).

[003] The optimization of the p-type, n-type and the intrinsic amorphous Silicon (a-Si) layers play a very important role in the research of HIT solar cells. The optical band gap, conductivity and deposition rate of the amorphous layers are all tailored by the PECVD process parameters. Usually during research p-type and n-type layers are deposited on normal glass slides and intrinsic layer is deposited on alkali free borosilicate glass, so that the optical and electrical characteristics can be conveniently measured.

[004] Thus, to measure the electrical conductivity of thin films of a-Si; they are deposited on glass slides, and conducting silver pads are applied on them. Usually these pads are made manually with the help of external applicator, screens or stencil, which results in pads with a considerable degree of variation in profile and dimensions. This leads to inaccurate measurements and conclusions.

[005] For studying the electrical properties of thin film materials, conductivity is a commonly used parameter in solar cells. This property often offers a quick glimpse of the quality of the films, although other parameters, which are harder to measure, such as mobility and carrier lifetime are also important in characterizing the electrical properties of films. For measuring the conductivity of thin films samples, conducting pads are made on the a-Si thin film layers using Conducting Silver paste (S) in a fixed pattern, for which reference may be made to figure 1. These pads are then dried in a furnace at around 250°C. A potential difference of 50 V is applied across these pads and the current is measured using a picometer in light and in dark. For error free results the conducting pads should be of same shape and size without any smearing or smudging.

[006] However, when the conducting pads are applied using the paste there are chances of variation in size and shape of the pattern as illustrated in Fig. 2 owing to manual and cumbersome methods of applying the paste. This variation in size and shape results in inaccurate measurement of current (as it is of the order of nano ampere and pico ampere) leading to inaccurate calculation of conductivity.

[007] In order to provide a solution to the above, it was attempted to render patterns uniform by using stencil made of different materials. However, still there were problems of smudging of the paste.

[008] Now, reference may be made to the following patents.

[009] United States Patent US4254707 A by Daryl G. Lambert, Howard M. Sage, Lee W. Ester; May, 1979 relates to Screen Printing Apparatus with screen peel-off from work. The screen is pivotally mounted at one end, and the other end is upwardly liftable to peel-off the screen from the work, as the squeegee traverses, so that those portions of the screen through which ink has already been applied to the work may be physically removed from the work to prevent smearing of ink on the work, as well as to prevent distortion. The upward movement of the screen away from the work is synchronized with the traversal of the squeegee across the screen. The traversing movement of the squeegee is of the single sided rack-and-pinion drive type and a bar that carries the squeegee as it moves in its traversing movement also actuates a mechanism for pivoting a shaft that, in tum, effects the raising motion of the screen. This patent relates to the traditional way of screen printing and does not provide a portable solution for formation of small conducting pad patterns for thin film samples.

[0010] United States Patent US6539856B2 John W. Jones, David J. Foster February, 1998 relates to method of screen printing stencil production. A coated film product comprises a support base, a stencil-forming layer and an intermediate release layer. The stencil forming layer is a blend of two grades of polyvinyl alcohol having different degrees of hydrolysis. The stencil-forming layer is imaged by drop-wise application of cross-linking agent which hardens the stencil-forming layer to resist washing out with water. The hardened areas remaining after washing out are however sufficiently tacky for the washed-out film to adhere to the screen mesh by application of pressure and, after removal of the support base, from the stencil layer of screen-printing screen. This invention relates to method of stencil production using screens and is not related to printing of conducting pad patterns for thin film sample characterization.

[0011] United States Patent US6074893A, by Yoshikazu Nakata, Syozo Otomo, Kazunari Tanaka, Koichi Uno, Sept, 1993 relates to Process for forming fine thick-film conductor patterns. A process for forming fine thick-film conductor patterns on a ceramic substrate which comprises the steps of forming grooves in a positive-working photoresist layer on the substrate by the photolithographic technology, filling the grooves with a conductive paste by squeezing with a squeegee, removing the photoresist layer by a wet process, and firing the remaining conductive paste pattern is improved. By the improvements, the conductive paste is squeezed into the grooves by the screen printing technique using a mask, and/or the solvent in the conductive paste consists essentially of one or more hydrocarbons, and/or the squeegee is made of a material having a flexural modulus in a range of 30-200 kgf/mm2. This invention relates to the complex method of forming thick-film conductor patterns using photolithographic technology and in no way relates to the simple mechanical and portable device for making conductor patterns on thin film samples.

[0012] US patent no US5178685A, by Jeffrey T. Borenstein, Ronald C. Gonsiorawski, June 1991 relates to method for forming solar cell contacts and interconnecting solar cells. Photovoltaic cells with silver-rich thick film electrical contacts having superior thermal aging properties are disclosed. The silver-rich contacts are formed by firing a silver ink or paste wherein substantially all of the silver in the ink or paste before firing is in the form of spherical particles. Preferably inter-cell connections to the contacts are accomplished using a tin and silver solder paste comprising between about 96% tin/4% silver and 98% tin/2% silver. Solar cells having contacts incorporating the present invention exhibit the capability of withstanding temperatures in the range of 150°C. with little or no thermal degradation of contact peel strength in the region of the solder bonds. This invention relates to the method of forming solar cell contacts by using particles and conducting pastes by depositing the same on surface and does not relate to application of conducting paste for measurement of electrical parameters of thin film samples.

[0013] However, none of the above can fulfill the requirements of present invention, for which it is designed.

[0014] In view of the above, it is required to introduce a simple portable mechanical device to form conducting pads on thin film samples serving as contacts for measuring the electrical properties of the thin layer, which can address the issues of the prior art(s) and serve the purpose effectively.

OBJECTS OF THE INVENTION

[0015] An object of the invention is to provide a portable device to apply conductor paste on thin film sample employed for devising high efficiency silicon solar cells which obviates shortcomings of the prior art(s).

[0016] Another object of the present invention is to provide a portable device to apply conductor paste on thin film sample employed for devising high efficiency silicon solar cells which is simple in construction.

[0017] Still another object of the present invention is to provide a portable device to apply conductor paste on thin film sample employed for devising high efficiency silicon solar cells which enables accurate study of the electrical conductivity parameters of thin films.
[0018] Yet another object of the present invention is to provide a portable device to apply conductor paste on thin film sample employed for devising high efficiency silicon solar cells which effectively serves the purpose.

[0019] Further object of the present invention is to provide a portable device to apply conductor paste on thin film sample employed for devising high efficiency silicon solar cells which is compact.

SUMMARY OF THE INVENTION

[0020] The invention proposes a portable device that can be implemented to apply conductor paste in the form of pads on thin film samples in a required pattern uniformly such that the electrical conductivity parameters of thin films can be studied accurately. This forms a simple mechanical portable device that can provide the required pattern precisely without any smearing of conducting pad.

[0021] According to the invention, there is provided a portable device to apply conductor paste on thin film sample employed for devising high efficiency silicon solar cells comprising a screen bearing pattern, above which a refilling tank of conducting silver paste is placed, in the rail of which a squeegee flood bar assembly is mounted, which is actuated to release the conducting paste onto the screen.

[0022] The squeegee/flood bar assembly is connected to assembly of first and second arm joined by means of a hinge.

[0023] A push button and the assembly of arms activate the squeegee/flood bar assembly as the hinge joint travels along a rail positioned vertically at one end of the tank.

[0024] The vertical rail is integrally connected to a plate orthogonally constituting a part substantially having a L-section.

[0025] Said push button is mounted though the plate, under which a compressed spring is provided, wherein bottom tip of the button rests on the second arm.

[0026] Upon pressing the push button the squeegee gets activated and traverses a distance along the length of the refilling tank so as to release the paste in the required pattern on the screen.

[0027] The flood bar connected on the opposite end of the assembly collects back the excess paste upon release of push button.

[0028] A top cover is provided for protection of the internal assembly from accidental stresses.

[0029] A bottom cover is provided for protection of the internal assembly from accidental stresses, which prevents spilling of the conductor paste.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
[0030] Further objects and advantages of this invention will be more apparent from the ensuing description when read in conjunction with the accompanying drawings of the exemplary embodiments and wherein:
Fig. 1 shows: Pattern for conducting pads in prior art.
Fig. 2 shows: Variation in size and shape of patterns according to prior art.
Fig. 3A shows: Exploded view of portable device for applying conducting paste on thin films of present invention.
Fig. 3B shows: Isometric view of portable device for applying conducting paste on thin films in accordance with present invention.
DETAIL DESCRIPTION OF THE PRESENT INVENTION WITH REFERENCE TO THE ACCOMPANYING DRAWINGS

[0031] The invention makes a disclosure regarding a technology pertaining to a portable device to apply conductor paste on thin film sample employed for devising high efficiency silicon solar cells.

[0032] The device forms the pattern at one touch by means of guided support and also controls the flow of silver conductor paste to prevent reach of excess paste to the substrate, thereby avoiding smearing of pads. The device involving technology of screen printing comprises a box filled with conductor paste, beneath which a screen is located that controls the flow of paste through mesh opening.

[0033] The push button along with movable arms actuates the small squeegee cum flood bar assembly, which moves across the screen and gives the conducting pattern in one touch. In addition to this the device is provided with an air tight cap which prevents the paste from drying. Here, the conducting paste can be applied very quickly.

[0034] Now, reference may be made to figure 3A and 3B, wherein the portable device of the instant invention can be best seen.

[0035] The device comprises a screen (1) bearing the pattern that needs to be applied on the sample. A tank (2) containing conducting silver paste is placed above the screen. Further, there is provision of a squeegee/flood bar assembly (3) which is positioned in the rail of said tank as shown in the fig. 3B. The rail is horizontally positioned in parallel to the tank.

[0036] The squeegee/flood bar assembly is connected to assembly of first and second arm (5,7). Said first arm (5) is connected to the second arm (7) by means of hinged joint (6). The assembly of arms activates the squeegee/flood bar assembly as the hinge joint (6) travels along a rail (4) mounted vertically at one end of the tank.

[0037] The vertical rail (4) is integrally connected to a plate (4’) orthogonally constituting a part substantially having a L-section. A push button (8) is mounted though the plate (4’), under which a compressed spring (11) is provided, wherein bottom tip of the button rests on the second arm (7) as illustrated in the figure 3A and 3B.

[0038] Upon pressing the push button the squeegee gets activated and traverses a distance of about 10mm along the length of the refilling tank. This causes release of the conducting paste for application thereof in the required pattern on the screen (1) through said tank.

[0039] The compressed spring ensures that the force applied on push button is controlled and also helps in restoring the position of push button after application of conducting paste. The flood bar connected on the opposite end of the assembly (3) collects back the excess paste upon release of push button.

[0040] Thus, the squeegee is positioned on one side and the flood bar is on other side of said assembly.

[0041] In conjunction with the above, there is provision of a top cover (10) that enables protection of the internal assembly from accidental stresses.

[0042] Further, a bottom cover (9) is provided for protection of the internal assembly from accidental stresses, which also prevents spilling of the conductor paste.

[0043] Now, advantageous features of the invention can be listed hereinbelow:-

- Simple construction
- Compact
- Prevents smearing of pads
- Faster to form pattern
- Enables accurate study of the electrical conductivity parameters of thin films.

[0044] It is to be noted that the present invention is susceptible to modifications, adaptations and changes by those skilled in the art. Such variant embodiments employing the concepts and features of this invention are intended to be within the scope of the present invention, which is further set forth under the following claims:-