WO2010042170A2

WO2010042170A2 - Interconnection of adjacent devices

Info

Publication number: WO2010042170A2
Application number: PCT/US2009/005482
Authority: WO
Inventors: Nicholas J. Dartnell; Julie Baker
Original assignee: Eastman Kodak Company
Priority date: 2008-10-09
Filing date: 2009-10-06
Publication date: 2010-04-15
Also published as: WO2010042170A3; TW201025704A; GB0818531D0

Abstract

A method of electrically connecting adjacent devices is disclosed. Each device provides a conducting wall and an insulating wall of a closed channel, the two conducting walls being electrically isolated and the channel having one or more entry points. A liquid is applied to at least one entry point and capillary wicks to thereby fill the channel and provide the means of communication. The liquid is cured after entry into the channel by heating.

Description

INTERCONNECTION OF ADJACENT DEVICES

The present invention uses capillary wicking into a channel as a means of creating a manufacturable serial interconnection between adjacent devices. It has particular application to optoelectronic devices such as photovoltaic cells in a module.

BACKGROUND OF THE INVENTION

Conventional dye-sensitized solar cells as described by Gratzel consist of a transparent conducting substrate such as ITO, on top of which is a sintered and dye coated layer of titanium dioxide nanoparticles (the anode). A hole carrying electrolyte which typically contains an iodide/tri-iodide redox couple as the electron (or hole) transfer agent is placed within the pores of and on top of this layer. The solar cell sandwich is completed by putting on top of the electrolyte a catalytic conducting electrode, often made with platinum as the catalyst (the cathode). When light is shone on the cell, the dye is excited and an electron is injected into the titania structure. The excited, now positively charged dye oxidises the reduced form of the redox couple in the electrolyte to its oxidised form e.g. iodide goes to tri-iodide, and is itself reduced to the neutral ground state. The tri-iodide may now diffuse towards the platinum coated electrode. When the cell is connected to a load the electrons from the anode pass through the load to the cathode and at the cathode the oxidised form of the redox couple is reduced e.g. tri-iodide to iodide, completing the reaction. When the electrolyte is a liquid, which is often the case to improve the performance of the cell, an impermeable gasket is required to both contain the electrolyte (and prevent atmospheric ingress) and also to space apart the counter and working electrodes.

Conventionally, the common means for creating a serial interconnect for dye-sensitized solar cells (DSSC) include connections external to the cell, alternating the orientation of adjacent cells so that one substrate has both counter and working electrodes present or building the cell in a monolithic fashion, i.e. in coatable layers from the bottom up on one substrate. US 2005/0199279 discloses means of interconnecting cells using connections that are external to the cells, i.e. connection to electrodes via faces that are on the outer surfaces of the solar cell, for example through the use of a separate wire like tab. The creation of these interconnects is often achieved in a batch process not readily applicable to roll manufacturing and furthermore is either likely to increase the non-active area of the module or shadow a portion of the active area. This technology is more appropriate for solid state e.g. wafer or thin film devices.

US 6555741 and US 2005/0067006 both disclose the creation of an internal serial interconnect. In the former the interconnect is created using conductive particles of a magnitude in the same order as that of the dimensions of the internal gap to be bridged. In the latter a wire is used to form the conductive bridge between the overlapping electrodes in adjacent cells. In both cases these connections have to be applied before the two electrodes/substrates are joined and therefore to ensure a good connection the materials and the joining process for the cell and the interconnect have to be compatible, i.e. for thermal, physical treatment and dimensions etc. As used here the interconnect acts as the cell spacer/seal and therefore the materials have to be compatible with the very reactive electrolyte which can reduce the choice of conductor. The material must also be of the appropriate dimensions so as not to make the interelectrode spacing too great. A wire is not trivial to apply in a non continuous process and is bound in an adhesive which has to be additionally applied. The conductive particles are also applied in a polymeric adhesive and in either of these cases the creation of a good interconnect risks the spreading of these adhesive materials, reducing the overall quality of the join and seal.

US 2007/0178232, US 2007/0122932 and US 2007/0120097 disclose a number of routes and compositions by which a conductive feature can be printed onto a solar cell, including into a predefined trench. However none of these are designed for creating a serial interconnect but rather for printing current collecting busbars or tracks. These lines are therefore dimensionally very fine so as to reduce the degree of shading of the active area. WO2007/061448 discloses a means of filling a microfluidic channel with a metallic conductor in order to connect together two electrodes. This document does not mention the connection of solar cells. The claim involves heating to form a liquid metal that then flows into the channel and is subsequently allowed to cool. A disadvantage is clearly therefore that if the metal used is of a low enough temperature to be compatible with plastic or polymeric supports when molten, preferably below 70⁰C, then in the operation of a solar cell there is the possibility of the metal re-melting and breaking the cell-to-cell interconnection.

US 2005/0236037, US 6706963 and GB 2427963 all disclose modules that are connected in the so called 'W arrangement. This is a design where effectively both top and bottom substrates are identical. Therefore the cells on a substrate alternate with a counter electrode for one cell being adjacent to the working electrode for the next cell. This design greatly simplifies the process of creating a serial interconnect since the connection is in the plane of the substrate. However it means that for the module every other cell is a reverse orientation to the light. For these designs it is required that the counter electrode be as transmissive as possible without compromising its operation. It is likely that those cells that are illuminated through the counter electrode provide a lesser performance than those illuminated through the working electrode. US 2007/0131271, US 2007/0131272, US 6069313 and US 2005/0257826 all disclose monolithic dye sensitised solar cells. A monolithic approach is one where the solar cell is built upwards in layers from a single substrate. This therefore requires a coatable counter electrode material but also a coatable 'solid- state' electrolyte. This latter may consist of an insulating porous spacer post filled with electrolyte but in general this design is less applicable for liquid electrolytes and the requirement for many patterning steps using paste-like materials means a reliance on screen printing. A monolithic design does however mean that the counter electrode can be directly printed to connect to the working electrode of the adjacent cell making serial interconnection convenient. US 2007/0012353 and US 2006/0160261 both disclose the process of filling 'via' holes in solar cells with, for example, silver paste in the former case. This is for a serial interconnection but is not really appropriate for a DSSC or, for that matter, for a roll-to-roll type process, hi the latter case an additional solid conducting underlayer such as a metal foil is employed to facilitate an interconnection effectively externally to the cell. Again the primary embodiments are for solid state thin film devices not DSSC.

PROBLEM TO BE SOLVED BY THE INVENTION

To increase the power output from a photovoltaic device it is conventional to connect together several individual cells to create a module and possibly even to connect together modules to form an array. The creation of a module from separate solar cells can be either achieved in parallel, series or a combination of connections. Two photovoltaic cells connected in parallel provide an increased current in the module and can be analogous to creating a single cell of the combined area if for example the cells are matched and collection of current is good, e.g. there is little ohmic loss. A serial interconnection between cells increases the voltage achievable by the module and is independent of cell or module active area but relates more to the number of matched photovoltaic cells connected in series.

Since for most applications of solar cell modules, e.g. charging batteries or powering electronic devices, there exists a particular output voltage requirement, the serial interconnection of cells is of key importance. Since a serial interconnection requires connecting together the opposite polarity electrodes of adjacent cells it is not trivial to achieve, especially in a continuous or roll-to-roll manufacturing process which may be appropriate, for example, for dye sensitised solar cell DSSC technology.

The present invention aims to provide a convenient way of creating a serial interconnect between cells in a module.

SUMMARY OF THE INVENTION One possible construction process for a DSSC requires the use of a gasket to contain the liquid electrolyte in the cell. This gasket and the gasket for the adjacent cell create a channel between the two cells where the conductive substrates for the counter and working electrodes face each other. It has been demonstrated that a conductive ink can be wicked into this channel from either end, or through holes in one substrate, and cured at low temperatures that are compatible with polymeric supports, typically less than 12O⁰C to create a metallic conductor. By this means and through appropriate isolation of adjacent cells on the same substrate an internal serial interconnect has been generated using a process that is compatible with volume manufacture and for use with polymeric or other low temperature substrates. According to the present invention there is provided a method of electrically connecting adjacent devices, each device providing a conducting wall and an insulating wall of a closed channel, the two conducting walls being electrically isolated, the channel having one or more entry points, a liquid being applied to at least one entry point and capillary wicking to thereby fill the channel and provide the means of communication, the liquid being cured after entry into the channel by heating.

In particular, an internal serial interconnect in a dye sensitised solar cell may be created by isolating adjacent cells on a common substrate, dispensing a 'conductive' ink, permitting it to capillary wick into a channel created by the overlapping substrates between adjacent cells and the gaskets used to seal each cell and thermally curing the ink to create a conductive interconnect. The curing process can be made compatible with the polymeric substrate materials.

ADVANTAGEOUS EFFECT OF THE INVENTION The method of the present invention provides a simple construction of a solar cell that is applicable to a roll-to-roll fabrication process. There is no need to pre-apply an interconnect before completing cell construction, that is the joining of the counter and working electrodes.

The liquid is simply added at an entrance to a channel and the curing step is irreversible leading to a conductive connection that is not prone to re-melting. The use of the channel isolates the connector from the electrolyte. This broadens the choice of materials that can be used. Furthermore the cell spacing function is separated from the conductivity function.

BRIEF DESCRIPTION OF THE DRAWINGS The invention will now be described by way of example with reference to the following drawings in which:

Figure 1 is a schematic three dimensional view of a module; Figure 2 is a schematic cross sectional view of the module; and Figure 3 is a graph illustrating the characteristic I- V curves of two individual cells and a module.

DETAILED DESCRIPTION OF THE INVENTION

Figure 1 is a schematic three dimensional illustration of a module in accordance with the invention. Those skilled in the art will appreciate that such a solar cell module may be fabricated by implementing any one of a number of techniques and using a variety of materials. Cells 3 in this module include an active layer situated between a first substrate 1 and second substrate 2 with each cell 3 being electrically isolated from each adjacent cell on the same substrate. Each cell is surrounded by a gasket material 4 which bridges the gap between the two substrates and which spans the cell to cell isolation means on one of the two substrates on one side of the cell and spans the cell to cell isolation means on the other of the two substrates at the other side of the cell. The gaskets 4 that surround adjacent cells create a channel 5 that provides access to the working electrode of one cell and the counter electrode of the neighbouring cell. This channel is filled with a means 9 of conductively interconnecting the cells in the module. This conductor is introduced into the channel as a liquid by means of capillary wicking, and is cured to improve the conductivity and fix the interconnect into the channel. Examples of the substrate include, but are not limited to, a plastic, a glass, a metal, a ceramic, or the like. Plastics that may be used as the substrate include, for example, polyethylene terephthalate (PET), polyethylene naphthalate (PEN), polybutylene terephthalate (PBT), a polyimide, and the like. Glasses that may be used as the substrate include, for example, borosilicate glass, quartz glass, soda glass, and the like. Metals that may be used as the substrate include, for example, titanium, nickel, stainless steel and the like. Preferably, the substrate will be a plastic.

A conductive layer is deposited on the substrate. The layer will be made of a conductive metal oxide, such as indium doped tin oxide (ITO) if a plastic substrate is to be used, hi the case of a glass, metal or ceramic substrate, a layer of fluorine doped tin oxide may be used. It is preferable that the conductive layer is substantially transparent.

The material constituting the substrate and the conductive layer must be resistant to the electrolyte. In the case in which an electrolyte containing iodine is used, copper and silver are unsuitable materials, for example, as they are readily attacked by the iodine and easily dissolve into the electrolyte.

The method used to form the conductive layer on the chosen support is not particularly limited and examples include any known film formation methods, such as sputtering methods, or CVD methods, or spray decomposition methods.

The active layer is a composite layer having the function of absorbing light and converting a portion of that energy into an electron and a hole and facilitating the transfer of those charged species to opposite electrodes or substrates. Generally the active layer comprises some composite structure of a hole transporting and a electron transporting material. In one example the active layer comprises a dye sensitised semiconducting oxide structure for electron transport and an electrolyte for hole transport. The oxide semiconductive film is a porous thin layer of interconnected metal oxide particles. Metal oxide particles that may be used include titanium oxide (TiO₂), tin oxide (SnO₂), tungsten oxide (WO₃), zinc oxide (ZnO), niobium oxide (Nb₂O₅) and antimony oxide (Sb₂O₅). Preferably, the metal oxide particles will be titanium dioxide (TiO₂). The method for forming the oxide semiconductive porous film is not particularly limited. It can be formed, for example, by employing methods in which a dispersion solution that is obtained by dispersing commercially available oxide semiconductor fine particles in a desired dispersion medium, or a colloid solution that can be prepared using a sol-gel method is applied, after desired additives have been added if required, using a known coating method such as a screen printing method, an inkjet method, a roll coating method, a doctor blade method, a spin coating method, a spray coating method, or the like. Sintering of the oxide semiconductive porous film may be achieved via pressure or heat, depending on the substrate chosen.

The dye that is provided in the oxide semiconductive porous film is not particularly limited, and it is possible to use ruthenium complexes or iron complexes containing bipyridine structures, terpyridine structures, and the like in a ligand; metal complexes such as porphyrin and phthalocyanine; as well as organic dyes such as, but not limited to, eosin, rhodamine, coumarin, and melocyanine, or derivatives of the above. The dye can be selected according to the application and the semiconductor that is used for the oxide semiconductive porous film. Preferably, the dye will be a ruthenium complex.

For the electrolyte solution, it is possible to use, for example, a 'polymer gel electrolyte', an organic solvent electrolyte or an ionic liquid based electrolyte (room temperature molten salt) that in each case contain a redox pair. The electrolyte is composed of a redox pair contained in a liquid solvent or a pseudo solid form (that permits ionic conduction or charge transport). The solvent for the liquid electrolyte can be a purely organic solvent or a so called ionic liquid (room temperature molten) of low volatility, or a combination of these components, and in turn the redox pair can contain a component that is considered a molten salt. The pseudo solid electrolyte can be formed by adding gelling agents to a liquid form of the electrolyte, for example by the use of polymers such as epichlorohydrin-co-ethylene oxide or poly(vinylidene fluoride-co- hexafluoropropylene) (PVDF-HFP), or sugars such as sorbitol derivatives or the addition of nanoparticles such as silica or other solids, e.g. Lithium salts. Alternatively it can be created through the addition of the redox pair to a system that is essentially solid in certain areas of its phase diagram such as plastic crystals like succinonitrile. The polymer gelled electrolyte may in addition contain plasticisers such as for example propylene and/or ethylene carbonate.

Examples of the organic solvent include acetonitrile, methoxy acetonitrile, propionitrile, propylene carbonate and diethyl carbonate. Examples of the ionic liquid include salts made of cations, such as quaternary imidazolium based cations and anions, iodide ions or bistrifiuoromethyl sulfonylimido anions, dicyanoamide anions, and the like.

The redox pair that is contained in the electrolyte is not particularly limited. For example, combinations such as iodine with iodide ions or bromine with bromide ions may be used to create the redox pair.

Additives such as tert-butylpyridine and the like may also be added to the electrolyte.

The method for forming the electrolyte layer between the working electrode and the counter electrode includes for example, a method in which the electrodes are disposed facing each other and the electrolyte is supplied between the electrodes to form the electrolyte layer. Alternatively, the electrolyte may be dropped, applied or cast onto the working electrode or counter electrode to form the electrolyte layer and the other electrode may then be stacked on top. In order to prevent leakage of the electrolyte from the space between the working electrode and the counter electrode, it is preferable to seal the gap between the electrodes with an appropriate material.

The counter electrode includes an electron conductive material. The counter electrode may also be a conductive transparent substrate. The counter electrode may also be an electron conductive material coated on an electron insulating support. Specific examples of the electron conductive material include platinum, ITO and carbon, or combinations thereof. The counter electrode acts as a catalyst for the regeneration of the redox pair in the cell.

The gasket material can be any material that has chemical and physical resistance to the electrolyte and can bond to both substrates. Materials that can be used include Bynel or Surlyn from Dupont and Thermoset 615 from 3 M all of which can be thermally sealed to the substrates at temperatures compatible with polymeric supports.

The cell to cell isolation can be achieved by a number of routes that include but are not limited to mechanical, thermal, chemical or optical means. The conductive layer on a support could for example be scribed using a sharp blade, a hot stylus, a laser illuminating in the infrared or by masking the support and etching with an acid. If the substrate is a continuous conducting foil then one route to isolation is to mount separate sections of foil on an insulating support.

Figure 2 is a schematic cross sectional view of an exemplary embodiment of a module made in accordance with the invention as described below. This cross section provides an alternative illustration of the form of the channel 5 and serial interconnection for a solar cell module. A part of two neighbouring cells 3 in a module is shown including a portion of the gaskets 4 of these cells constituting the channel created by these gaskets. A conductor 9 is used to fill this channel and provide a serial interconnection in the module. The transparent conductive coating on the working electrode is isolated between the two cells illustrated and is situated beneath the gasket. The platinum coated conductive coating 8 on the counter electrode 1 spans the channel created by the gaskets and is isolated between cells above the opposite gasket to that for the working electrode. Each gasket surrounds a dye sensitised porous titania layer 7 and the volume enclosed contains the liquid electrolyte 6 in the cell 3.

Example

A sample of 50 Ω / square ITO-PET support was used to make the working electrode of a two cell dye sensitised solar module. Some titanium dioxide was dried in an oven at 9O⁰C overnight prior to use. This was a titanium dioxide sample which had an average particle size of 21nm (Degussa Aeroxide P25, specific surface area (BET) = 50 +/- 15 m²/g). The flexible dye sensitised solar cell module was fabricated as described below and based upon the schematic design in Figures 1 or 2. To achieve this, the working electrode was masked using 3M Scotch tape to leave the desired pattern exposed. Approximately 15μm thick mesoporous TiO₂ films were deposited onto the patterned 50 Ω / square ITO-PET by dispersing the dried TiO₂ in a mixture of dry Methyl Ethyl Ketone and Ethyl Acetate in the following ratios:

Degussa P25 TiO₂ (21nm particles) 1.35g

Methyl Ethyl Ketone 45g

Ethyl Acetate 5g

The resulting mixture was sonicated for 15 minutes before being sprayed over the entire masked area of conducting plastic substrate from a distance of approximately 25cm using a SATAminijet 3 HVLP spray gun with a lmm nozzle and 2 bar nitrogen carrier gas. The masking tape was removed and the layer was allowed to dry in an oven at 9O⁰C for one hour, before being placed between two sheets of Teflon, sandwiched between two polished stainless steel bolsters and compressed with a pressure of 15 tonnes for 15 seconds over an area of 8cm². The sintered sample were redried for one hour in the 9O⁰C oven before being sensitised by placing it in a 3XlO^"4 mol dm^"3 solution of ruthenium cis-bis- isothiocyanato bis(2,2'bipyridyl-4,4'dicarboxylic acid) overnight. This working electrode was then used to construct a dye sensitised solar cell module. Platinum coated stainless steel foil electrodes were prepared by sputter deposition under vacuum. Two separate pieces of platinum coated stainless steel were used. The ITO on the working electrode was scored with a scalpel blade so as to break the ITO coating between the two dye sensitised mesoporous layers but not separate the base areas. 3M Thermoset 615 material was used for the gasket 4 to seal the two individual cells and to join the counter and working electrodes in to the module shown schematically in Figures 1 or 2. This was achieved by making the appropriate sandwich arrangement of substrates and gaskets as schematically shown and placing the whole arrangement on a hot plate set at 110⁰C, counter electrode side down. Pressure was applied to the module over the area of the gasket to complete the seal, using the blunt end of a scalpel. The gaskets had been placed over the cell isolation lines created in the ITO layer on the working electrode substrate, the counter electrodes were placed to span each cell and to complete the channel to one side of each cell, and therefore the gaskets in adjacent cells create a channel between the two cells.

The dyed but isolated two cells of the module were then placed on a flat surface and 20μl of Advanced Nano Products Silverjet DGP- 40LT-15C ink was deposited at one end of the channel 5 created by the gaskets 4 of the two cells 3. The ink readily wicks the length of this channel thereby filling it. The module is then placed in an oven at 70°C for 20 minutes, to cure the conductive ink.

Finally each side of the module was filled with an ionic liquid electrolyte comprising:

0.1 M LiI

0.6M DMPII (l,2,dimethyl-3-propyl-imidazolium iodide) 0.05M I₂ 0.5M N-methylbenzimidazole

Solvent = MPN (Methoxypropionitrile)

Following fabrication, the dye sensitised solar cell module was characterised by placing it under a source that artificially replicated the solar spectrum in the visible region to provide an illumination of O.lOSun. Both the individual cells and the module could be measured.

The data in Figure 3 demonstrate that modules fabricated using the processes discussed above give acceptable results. When each side of the module was tested individually, good current and voltage was achieved and when the complete module was tested, double the voltage was achieved which was expected as the individual cells were connected in series.

Cell Name/# Light Level / Sun Efficiency/% Voc/V Isc/μAmps FF

CeII B 0.09 2.3 0.68 1771 0.64

CeII A 0.09 2.5 0.67 1769 0.69

Module 0.09 2.1 1.35 1749 0.58 Each cell is nominally Ix4cm and the efficiency of the module is based upon the active area. This example demonstrates that capillary filling a channel with a low temperature curing conductive ink can be used as a manufacturable way of creating a working PV module. The ink can be dispensed or inkjet printed at the start of each channel or through holes distributed along the length of one or more of the walls of the channel and cells can therefore be connected in a volume manufacture applicable process. A successful interconnect of two electrodes was even found to occur at room temperatures with this material. However the speed of curing and quality of the conductor generated can be improved with temperature. This is more the case as the length of the channel is increased.

The above example has described the method used for connecting adjacent solar cells. It should be understood that the invention is not limited to such an application. The method can equally well be used for the connection of other devices that are required to be electrically connected, such as coatable batteries.

The liquid used can be either a conductive ink or a liquid that needs treating to become conductive. In either case a post filling heat treatment causes the conductor to be retained in place.

The present invention offers a method for making connections between adjacent devices using a flowable material, including liquids containing dispersions of solid particles. It is particularly suited to making connections between devices in opto-electronic systems. The method can work on any surface on which is defined a channel. The method can be used for serial connection and parallel connection of adjacent devices. The invention has been described in detail with reference to preferred embodiments thereof. It will be understood by those skilled in the art that variations and modifications can be effected within the scope of the invention. Parts List

first substrate second substrate cells gasket material channel liquid electrolyte dye sensitised porous titania layer conductive coating conductor

Claims

CLAIMS:

1. A method of electrically connecting adjacent devices, each device providing a conducting wall and an insulating wall of a closed channel, the two conducting walls being electrically isolated, the channel having one or more entry points, a liquid being applied to at least one entry point and capillary wicking to thereby fill the channel and provide the means of communication, the liquid being cured after entry into the channel by heating.

2. The method of claim 1 wherein the liquid is a conductive ink.

3. The method of claim 1 wherein the liquid is treated to become conductive.

4. The method of claim 2 wherein the liquid is heated to temperatures between 30°C and 120°C.

5. The method of claim 4 wherein the liquid comprises metal particles within a suspension

6. The method of claim 1 wherein at least one entry point is at an end of the channel.

7. The method of claim 1 wherein at least one entry point is through one or more of the walls of the channel.

8. The method of claim 1 for the connection of solar cells.

9. The method of claim 1 for the connection of dye sensitised solar cells.

10. An optoelectronic system comprising one or more devices connected by the method of claim 1.