WO2011032741A2

WO2011032741A2 - Method for manufacturing a thin-film photovoltaic cell module encompassing an array of cells and photovoltaic cell module

Info

Publication number: WO2011032741A2
Application number: PCT/EP2010/059849
Authority: WO
Inventors: Rainer Klaus Krause; Zhengwen O. Li; Roger A. Quon; Lawrence A. Clevenger; Kevin S. Petrarca; Carl Radens; Brian C. Sapp
Original assignee: International Business Machines Corporation
Priority date: 2009-09-16
Filing date: 2010-07-09
Publication date: 2011-03-24
Also published as: WO2011032741A3

Abstract

The invention relates to a method for manufacturing a thin-film photovoltaic cell module (10) encompassing an array of cells (100), comprising the steps of (i) providing the array of cells (100); (ii) determining, per cell (100), an electrical performance for one or more cells (100) of the array; (iii) identifying each cell (100) by its position in the array; (iv) determining one or more electrical paths (50, 52, 54) encompassing one or more of the cells (100) according to at least one optimization criterion; and (v) combining two or more cells (100) for realizing one or more electrical paths (50, 52, 54) by maintaining or establishing electrical connections (30) between individual cells (100) of the array according to the at least one optimization criterion.

Description

D E S C R I P T I O N

Method for Manufacturing a Thin-Film Photovoltaic Cell Module Encompassing an Array of Cells and Photovoltaic Cell Module

FIELD OF THE INVENTION

The invention relates to a method for manufacturing a thin-film photovoltaic cell module encompassing an array of cells and a photovoltaic cell module.

BACKGROUND OF THE INVENTION

It is known in the art that thin film photovoltaic solar modules consist of lamellar cells. In long lamellar cells weak spots dominate the efficiency of the cells. US 6,635,817 discloses to manufacture photovoltaic solar modules having a lattice or matrix cell structure which can be connected electrically in series and/or in parallel. As a result, the sensitivity to weak spots in the cell array is reduced.

SUMMARY OF THE INVENTION

It is an object of the invention to provide method for

manufacturing a thin-film photovoltaic cell module encompassing an array of cells which allows for an improved overall

efficiency of the thin-film photovoltaic cell module.

Another object is to provide an improved photovoltaic cell module encompassing an array of cells. These objects are achieved by the features of the independent claims. The other claims, the specification and the drawings disclose advantageous embodiments of the invention.

Proposed is a method for manufacturing a thin-film photovoltaic cell module encompassing an array of cells, wherein providing the array of cells is provided and, per cell, an electrical performance for one or more cells of the array is determined. Each cell is identified its position in the array and one or more electrical paths encompassing one or more of the cells according to at least one optimization criterion is determined. Two or more cells are combined for realizing one or more

electrical paths by maintaining or establishing electrical connections between individual cells of the array according to the at least one optimization criterion.

Advantageously it is possible to select optimized electrical paths encompassing cells with desired performance and/or quality which as a result yield an optimized performance of the array of cells. Unfavorable electrical paths with cells of weaker

performance or even defect cells can be identified and omitted. Weak spots in the array of cells can be circumvented. By using an appropriate optimization algorithm, the array of cells can be designed to yield a desired performance. Expediently it may be possible to assign cells with an insufficient performance to specific areas of the array of cells. As a result, it may be possible that the specific areas with low quality or faulty cells can reveal problems or failures in the manufacturing process of the cells.

In a favourable embodiment of the invention, the cells may be grouped in one or more groups of cells depending on at least one quality criterion. Advantageously, one or more electrical paths in two or more groups of cells may be combined for meeting the at least one optimization criterion for the array. By grouping the cells according to their quality criterion, cells can be identified by high quality, medium quality, low quality of even as inappropriate for usage, for instance. The optimization algorithm for determining an optimized set of electrical path which can fulfill a desired performance when combined in an appropriate way can be facilitated if the grouped cells are considered according to their quality criterion. By grouping cells arrays can be made with optimized performance, based on the criteria defined, having a number of categories which allow to make arrays with a predefined performance. For instance, defining three categories high, medium, low, a high performing array, a mid performing array and a low performing array can be made. Otherwise a mix would be generated with a performance determined by the lowest performing cell.. The cells in one group can be treated independently from cells in another group of cells or dependent on cells in another group of cells.

Further, one or more electrical paths in one or more groups of cells depending on the same of at least one optimization

criterion may be combined. Alternatively, one or more electrical paths in one or more groups of cells depending on different of at least one optimization criterion may be combined. In the same array it may be possible to combine both strategies for

combinations of cells, depending on a targeted result for the array .

According to an advantageous embodiment of the invention, the electrical connections between the cells can be arranged

electrically in series and/or in parallel according to the at least one optimization criterion. This allows for avoiding cells of inappropriate quality and an unwanted reduction of the overall performance of the array of cells.

According to an advantageous embodiment of the invention, the at least one quality criterion comprises efficiency of the cell. Expediently, cells with a high or at least sufficient efficiency can be distinguished from faulty cells.

According to a further advantageous embodiment of the invention, the at least one optimization criterion comprises at least one of electrical output power, electrical current, open circuit voltage. Photovoltaic modules for various applications can be provided. For instance, modules for space applications may favor a high power output whereas for modules for home appliances a high open circuit voltage may be favored over a high power and/or high current output.

In a favourable embodiment of the invention, the performance data of the cells and/or the groups of cells can be fed into an optimization procedure for determining one or more electrical paths with optimized performance. Advantageously, optimization of the array of cells can be done for a large number of cells in a reasonable time.

In a further favourable embodiment of the invention, one or more cells can be disconnected from the array according to the performance of the one or more cells. Favorably, faulty cells can be removed from the active parts of the array of cells.

As a result weak spots virtually do not interfere with adjoining cells with better performance integrated in the electrical paths .

In a further favourable embodiment of the invention, the cells can be patterned wherein an electric connection is provided between a front electrode of a first cell and a back electrode of an adjoining cell. This allows for a favorable serial

connection of adjoining cells.

In a further favourable embodiment of the invention, the cells may be patterned with a polygon cross section parallel to a carrier of the array. A polygon cross section allows for

electrical connections to cells surrounding a center cell. For instance a hexagonal cell can be surrounded by six hexagonal cells resulting in a homogeneous array of hexagonal cells.

However, other cross sections are possible such as triangles, rectangles, particularly squares, and other polygon cross sections. As an extreme value of a polygon it is also possible to have round cell cross sections.

According to a further advantageous embodiment of the invention, the back electrode can be patterned in a polygon cross section prior to deposition of an active layer of the cell. Favorably, the back electrode can be interconnected over the whole array area on the carrier of the photovoltaic module thus facilitating the contacts to the back electrode during testing of the

individual cells.

According to another aspect of the invention, a photovoltaic cell module is proposed encompassing an array of cells,

comprising cells being grouped in one or more groups of cells depending on at least one quality criterion. By grouping the cells according to at least one quality criterion it is possible to obtain separate sub arrays of cells with well defined

properties. For instance, cells can be grouped to form a high quality, a medium quality and a low quality sub array of cells. It can be avoided that low quality cells deteriorate the overall performance of the cell module.

Favorably, the photovoltaic cell module can comprise cells which have a polygon cross section, for instance a hexagon cross section, resulting in a high density of cells.

BRIEF DESCRITON OF THE DRAWINGS The present invention together with the above-mentioned and other objects and advantages may best be understood from the following detailed description of the embodiments, but not restricted to the embodiments, wherein is shown in:

Fig. 1 an example embodiment of the invention with an array of cells with hexagonal cross section with a detail of a portion of the array;

Fig. 2a, 2b an example embodiment of the invention with probing of individual cells in an array of cells (Fig. 2a) and current-voltage characteristics for a photovoltaic cell (Fig. 2b);

Fig. 3 an example embodiment of the invention showing a

distribution of cells of different performance assigned to groups of different quality criteria;

Fig. 4a-4d various examples for possible combinations of cells;

Fig. 5 an example embodiment of an optimum matrix;

Fig. 6a, 6b interconnected hexagonal cells in an array of cells

(Fig. 6a) and interconnected hexagonal cells in an array of cells surrounding a disconnected faulty cell (Fig. 6b) ;

Fig. 7 an example embodiment of the invention with serial and parallel linked cells; and

Fig. 8a, 8b an example embodiment of the invention with cells connected in parallel (Fig. 8a) and cells connected in series (Fig. 8b) .

In the drawings, like elements are referred to with equal reference numerals. The drawings are merely schematic

representations, not intended to portray specific parameters of the invention. Moreover, the drawings are intended to depict only typical embodiments of the invention and therefore should not be considered as limiting the scope of the invention.

DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS Fig. 1 displays a top view on a favorable example embodiment of a thin-film photovoltaic cell module 10 encompassing an array of cells which are generally denoted with reference numeral 100, and for illustration may be denoted with an appended alphabetic character a, b, c etc.

The cells 100 are composed in a way well known in the art of multiple layers stacked in a stack direction perpendicular to the paper plane in the drawing, comprising back and front electrodes, photoactive material, buffer layers etc. Some layers are described in more detail in Figs. 7a and 7b. The cells 100 exhibit a polygonal cross section crosswise to their stack direction, by way of example a hexagonal cross section. As can be seen in Fig. 1, a hexagonal cell 100 in the center can have six hexagonal cells 100 adjoining the center cell 100. The insert in the drawing illustrates that the hexagonal cells 100a, 100b, 100c, lOOd can be in close proximity so that the area of the array is filled with cells 100 without too much vacancies. Of course, the cells 100 may also be prepared with other cross sections such as circles, squares, triangles etc.

Fig. 2a depicts one step of the method according to the

invention and Fig. 2b depicts the response of a photovoltaic cell to illumination. An efficiency of cells lOOOx, lOOy of an array of cells 100 of a thin-film photovoltaic cell module 10 deposited on a carrier 12, e.g. glass, is measured for each individual cell lOOx, lOOy. The carrier 12 is covered with a conducting back electrode 20 which may be metal, e.g. molybdenum (Mo) or a transparent conductive oxide layer (TCO) , for

instance. A measurement device 60 is connected to the back electrode 20 and the top of the cell lOOy which is currently probed while illuminated by a light source 50. By way of example, when the cell lOOx is irradiated with e.g. sunlight, optical radiation is transformed into an electric current with an efficiency n. The efficiency n of a photovoltaic cell is generally defined by the ratio of a measured output power Pm divided by the product of an incoming power density of 1000 W/m² and the area A of the cell:

The power density of solar radiation with a clear sky at the equator at noon at March or September is defined as 1000 W/m². For instance, if the cell lOOx yields a measured output power of Pm=16 mW with the input power density E=1000 W/m² and an area of A= 10-4 m² , the cell lOOx has an efficiency of n=16%.

The cell lOOx has a short circuit current Jsc and an open circuit voltage Uoc with the power Pm = Uoc - Jsc . The parameters short circuit current Jsc and open circuit voltage Uoc describe, as generally known to a person skilled in the art, the so called I- V curve of a cell 100 which shows a virtually high resistance up to the open circuit voltage Uoc and a low resistance with a steep current rise as well known in the art. The fill factor FF is shown in Fig. 2b as a result of the voltage U _MPP and current J_MPP at the maximum power point MPP, the open circuit voltage Uoc

U

and circuit current Jsc with FF MPP MPP

U„„^■ J.

An IV-curve of a photovoltaic cell measures U_oc, I_SC/ U_mpp and I_mpp as well as the fill factor FF. With U_mpp and I_mpp the power efficiency is determined. The fill factor FF is also a measure for the efficiency n, meaning the higher this number the better performing is the cell. U_oc and I_sc multiplied with the fill factor FF is again the efficiency power. As a result, the fill factor FF is a measure how good the cell is physically operating. By way of example, a fill factor FF below 0,55 means that to much losses are present, like recombination effects of charge carriers and the like.

When each cell 100 of an array has been tested, the efficiency n of each cell 100 is known, as well as the location of the cell 100 in the array. Expediently, the cells 100 can be grouped in groups of cells according to a measured parameter such as the short circuit current Jsc , the open circuit voltage Uoc and/or the efficiency n.

The probing of the individual cells 100 can be done by surface mapping of the photovoltaic cell module 10. The probing can be automatized based on the cell coordinates and an appropriate electric prober arrangement.

Fig. 3 shows a top view of a section of an example embodiment of a thin-film photovoltaic cell module 10 encompassing an array of cells 100 which have been grouped according to their measured efficiency into four groups 200, 300, 400, 500. By way of example, cells 100 in the group 500 may have a high efficiency n between 12% and 16%. Cells 100 in the group 400 may have a medium efficiency n between 8% and <12%, cells 100 in the group 300 may have a low efficiency n between 4% and <8% and cells 100 in the group 200 may have an insufficient efficiency below 4% and/or be faulty with a short cut or the like.

As the location of the cells 100 in the array and their actual efficiency n is known, one or more electrical paths encompassing one or more of the cells 100 according to at least one

optimization criterion can be calculated which can be optimized according to a desired optimization criterion. Two or more cells 100 can be combined for realizing such one or more electrical paths. Each cell 100 can be considered as an electrical power source.

As shown in Figs. 4a-4d a desired electrical path through an array of cells 100 can be defined by using an optimum short circuit current Jsc with resistance R_t = R_l + R₂ + ... + R_n and current

J_n1 ^■ R, + Jm ' R_^ + ...+ Jn„ ' R„

J_n =— - - (n = number of the cells considered)

(Fig. 4a) and a summarized open circuit voltage Uoc of

individual cells with (Fig. 4b) by serialization of power sources, which power sources represent the cells with voltage U₀ = U_l + U₂ + ... + U_n and resistance R₀ = R_l + R₂ + ... + R_n .

In a next step it is determined which cells should be connected in parallel based on the measured individual cell data with

Jn = J + J +-_" + J„ and — =— — ·...·— (Fig. 4c), and

R, R_t R₂ R_n

R, = ¹ - — and t/_n = ¹ - -(Fig. 4d) .

n —— - -

R_{l +} R₂ + ... + R R_{l +} R₂ + ... + R_n

Of course, the order (serial or parallel) can be changed. The formulas above only show the dependencies of voltage and current from the internal resistors like G± .

When considering a cell performance and simulating the

parameters of the cell, the resistors of the bulk material, surface, contact area etc. are of importance. T is expedient to make the material which can be manipulated the final resistors, e.g. through doping, lattice properties, contact performance etc. In theory, these resistors can then determine the resulting voltages and current. For the optimized path or decision matrix only the measured values are required, like U_oc, ^sc, U_mpp and I_mpp, which are based on the resistors of the material. Based on this analysis it is possible to determine an optimum electrical connection path through the cells 100 of the module 10. The formulas can be entered into an optimum matrix shown in a simplified way in Fig. 5 to determine the desired best values of current Jsc , voltage Uoc and/or efficiency n over all

possibilities of connections between the cells 100 in the array.

The optimum matrix can comprise rows and columns of cells categorized in groups of predefined cell qualities H, M, L, wherein H may refer to cells showing high performance, M may refer to cells with medium performance and L may refer to cells with low performance. The position of a categorized cell in the matrix represents the position of the respective cell in the array of cells. The various categories (bins) of the categorized cells are then linked together, indicated by continuous lines HI, Ml, LI (representing electrical paths) based on the optimum matrix. As described in Fig. 3 (where the cells were categorized with 500, 400, 300, 200, for instance), the matrix criteria can be based on maximum open circuit voltage Uoc , maximum short circuit current Jsc , maximum power or maximum efficiency n.

As a result of the analysis the array of cells can be divided into sub-arrays of cells with well defined properties, e.g. with high, medium and low performance.

In Fig. 6a and Fig. 6b an example embodiment of an array of polygon, e.g. hexagonal, shaped cells 100 of a cell module 10 is indicated wherein the cells 100 are all interconnected with their adjoining cells 100 by bridges 40. After determining the efficiencies n of the individual cells 100, by way of example one cell lOOf is detected to be faulty and is consequently grouped in a group 200 (Fig. 3) where faulty cells 100 are collected. The other cells 100 have shown a sufficient

efficiency n. The faulty cell lOOf can easily be separated from the array of cells 100 by disconnecting the bridges 40 between the faulty cell lOOf and the other cells 100. If this is done for each faulty cell (or each cell 100 grouped in a group 200 where faulty or insufficient cells are grouped) , the faulty cell lOOf does not contribute to the electrical performance of the array of cells 100 of the module 10 when the optimized

electrical paths are determined. In this way only a small spot is "cut out" from the array and electrically deactivated, and bridges 40 on only cells 100 with satisfying efficiencies n or other desired parameters are maintained and not removed, so that those cells 100 can be integrated in the electrical path or paths of the array of cells 100.

Of course, in an alternative embodiment, the cells 100 are not connected by bridges 40 in the beginning, but bridges 40 are generated in the manufacturing process of the thin film

photovoltaic module 10 after all cells 100 have been tested. In this case, no bridges 40 will be generated to or from the faulty cell lOOf with the same result of eliminating the faulty cell lOOf from possible electrical paths.

Fig. 7 depicts as an example electrical paths 50, 52, 54 of cells 100 which are connected electrically in series and in parallel. A first electrical path 50 leads through the cells starting from 100a to lOOw all connected in series. In cell lOOu an electrical path 52 branches off and in the cell lOOv

adjoining to cell lOOu another electrical path 54 branches off which is in parallel to the electrical path 52 and the section of the electrical path 50 from cell lOOv to cell lOOw.

It is clear from the drawing that by combining cells 100 in series and in parallel - by leaving out faulty ones - a high current density as well as a high efficiency n for the can be achieved by an optimization process by determining the best electrical path in the array, i.e. the best combination of the known properties of the cells 100 and by locally eliminating (or avoiding) electrical connections (bridges 40) to faulty cells 100. The combination of the cells 100 can be done for the same of at least one optimization criterion or for different ones, as desired .

Figs. 8a and 8b illustrate simplified example embodiments of cells 100 of a thin-film photovoltaic cell module 10

encompassing an array of cells 100. A layer forming the back electrode 20 is deposited on a substrate (not shown) and

patterned, e.g. as a polygon, wherein the back electrodes 20a, 20b, 20c assigned to different cells 100a, 100b, 100c are electrically interconnected. The back electrode 20 may be composed of Mo or the like. An active layer 22 is deposited on the back electrode 20 which is responsive to electromagnetic radiation such as light, which by way of example may be a CIGS layer (CIGS = copper, indium, gallium, sulfur, and selenium) well known in the art in various compositions. A buffer layer 24, e.g. ZnO or the like, covers the active layer 22. On top of the buffer layer 24, a transparent conductive oxide (TCO) layer 26 and a transparent conductive electrode 28 are deposited.

These layers 26, 28 may comprise Zn-Al-0 for instance. However, other compositions can be used as well as additional

intermediate layers and electrodes known in the art.

The first layer 26 is employed as a contact to the active layer 22 and optimized to match the physical properties of the active layer 22. The second layer 28 links the cells 100 and has to match the first layer 26 as well as the physical properties of the back contact 20. Additionally, the upper layer 28 must provide good contact properties for external electrical contacts like soldering etc.

Preferably, a three-step patterning process is applied: in a first step the back electrode 20 is patterned, for instance with a polygon cross section. Each polygon forms the back electrode 20 of a cell 100. In a second step, the active layer 22 and the buffer layer 24 are prepared and patterned, wherein an offset 32 can be provided between a compound of the two layers of active layer 22 and buffer layer 24 and the back electrode 20, so that the active layer 22 and buffer layer 24 of one cell 100 overlay the adjacent polygon of the back electrode 20 of the adjacent cell 100. In a third step the transparent front electrode is prepared and patterned. In case there are electrically

conductive bridges formed 40 (Figs. 5a, 5b) between the cells 100, these can be opened preferably by laser irradiation.

However, other techniques like etching or mechanical techniques are possible.

Fig. 8a shows three cells 100a, 100b, 100c electrically

connected in parallel. In this case, an offset between the active layer 22 and buffer layer with respect to the back electrode 20 is not necessary. Fig. 7b shows three cells lOOd, lOOe, lOOf connected electrically in series. The series

connection is established by providing a small offset 32 with respect to the stack direction 150 between the back electrode 20 and active layer 22 and buffer layer 24 so that the top

electrode 28 of each cell lOOd, lOOe, lOOf contacts a connecting bridge 30 along the stack direction 150.

For instance, the connecting bridge 30d of the cell lOOd

provides an electrical connection between the top electrode 28d of the first cell lOOd and the back electrode 20e of the

adjoining cell lOOe and so forth.

Claims

C L A I M S

A method for manufacturing a thin-film photovoltaic cell module (10) encompassing an array of cells (100),

comprising the steps of

(i) providing the array of cells (100);

(ii) determining, per cell (100), an electrical

performance for one or more cells (100) of the array;

(iii) identifying each cell (100) by its position in the array;

(iv) determining one or more electrical paths (50, 52, 54) encompassing one or more of the cells (100) according to at least one optimization criterion; and

(v) combining two or more cells (100) for realizing one or more electrical paths (50, 52, 54) by maintaining or establishing electrical connections (30) between individual cells (100) of the array according to the at least one optimization criterion.

The method according to claim 1, comprising the step of grouping the cells (100) in one or more groups (300, 400, 500) of cells (100) depending on at least one quality criterion .

The method according to claim 2, comprising the step of combining one or more electrical paths (50, 52, 54) in two or more groups (300, 400, 500) of cells (100) for meeting the at least one optimization criterion for the array.

The method according to claim 2 or 3, comprising the step of combining one or more electrical paths (50, 52, 54) in one or more groups (300, 400, 500) of cells (100) depending on the same of at least one optimization criterion.

5. The method according to claim 2 or 3, comprising the step of combining one or more electrical paths (50, 52, 54) in one or more groups (300, 400, 500) of cells (100) depending on different of at least one optimization criterion.

6. The method according to any preceding claim, wherein the electrical connections (40) between the cells (100) are arranged electrically in series and/or in parallel

according to the at least one optimization criterion.

7. The method according to any preceding claim, wherein the at least one quality criterion comprises efficiency of the cell (100) .

8. The method according to any preceding claim, wherein the at least one optimization criterion comprises at least one of electrical output power, electrical current, open circuit voltage .

9. The method according to any preceding claim, wherein the performance data of the cells (100) and/or the groups (300, 400, 500) of cells (100) are fed into an optimization procedure for determining one or more electrical paths (50, 52, 54) with optimized performance.

10. The method according to any preceding claim, comprising the step of electrically disconnecting one or more cells (200) from the array if the performance of the one or more cells (100) is below one or more minimum limits.

11. The method according to any preceding claim, comprising the step of patterning the cells (100) wherein an electric connection (30) is provided between a front electrode (28) of a first cell (100a) and a back electrode (20) of an adjoining cell (100b).

12. The method according to any preceding claim, comprising the step of manufacturing the cells (100) with a polygon cross section parallel to a carrier (12) of the array.

13. The method according to claims 11 and 12, wherein the back electrode (20) is patterned in a polygon cross section prior to deposition of an active layer of the cell (100) .

14. A photovoltaic cell module (10) encompassing an array of cells (100), fabricated by a method according to any preceding claim, comprising cells (100) being grouped in one or more groups (300, 400, 500) of cells (100) depending on at least one quality criterion.

15. The photovoltaic cell module (10) according to claim 14, wherein the cells (100) have a polygon cross section.