GB2471262A

GB2471262A - Dye sensitized solar cells

Info

Publication number: GB2471262A
Application number: GB0906328A
Authority: GB
Inventors: Donald James Highgate; Rachel Louise Smith; Nicholas Baynes; Kris Hyde
Original assignee: ITM Power Ltd
Current assignee: ITM Power Research Ltd
Priority date: 2009-04-15
Filing date: 2009-04-15
Publication date: 2010-12-29
Anticipated expiration: 2029-04-15
Also published as: GB2471262B; GB0906328D0

Abstract

A solid polymer membrane of a DSSC is cured between the device electrodes and an ionic moiety is added before or after in situ polymerisation of the membrane. Potassium iodide may be added to a monomer mixture used for forming the membrane prior to polymerisation. The potassium iodide oxidation process is much slower than the polymerisation process and the polymerisation reaction is therefore not inhibited by the presence of iodine ions. Alternatively, iodine electrolyte material may be infused into a membrane after it has been cured.

Description

Photovoltaic Cell

Field of the invention

This investion realtes to a photovoltaic cell.

Background of the invention

A photovoltaic electrochemical cell converts light energy into electrical energy, the "photovoltaic effect" being the process through which light energy is converted into electrical energy. Photovoltaic cells are typically solid state devices, usually semiconductors such as silicon. Usually one or more photosensitive junctions are irradiated, simultaneously generating a voltage and a current. A potentially lower cost alternative to the solid state devices are dye-sensitized cells. Dye-sensitized solar cells offer a promising route to low cost solar energy. A standard dye-sensitized cell uses a liquid electrolyte to complete the electrical circuit between the convoluted surface of the dye-sensitized electrode and the flat counter electrode. A liquid is normally necessary to fill all the microscopic voids in the dye-sensitized electrode and thereby provide an effective ion conduction path to each dye molecule, where the light is absorbed and energy converted into electronic form. However cells made with the conventional liquid electrolyte suffer from a risk of leakage, variable thickness and pooling' in flexible cells, and, in some formulations, loss of volatile components by diffusion and evaporation, all of which can be detrimental to the reliability and longevity of the cell.

WO-A-03/023890 describes a composite MEA formed by an in-situ polymerisation process, and the materials used. PCT/GB 04/03570 described the use of these materials in photovoltaic electrochemical cells. This publication further described a method of forming both MEAs and membranes having improved photovoltaic properties.

Application 0900568.7 describes the use of encapsulation methods as a route to incorporating components which inhibit polymerisation, and example is to provide an iodide/-tn-iodide system encapsulated for later release into a solid electrolyte to allow in-situ curing of the MEA.

Summary of the invention

The use of hydrophilic polymers in conjunction with a conventional ionic-liquid based PV electrolyte enables the production of improved PV cells, made via the one-shot production process as described in WO-A-03/023890. Additionally the use of hycirophilic ionic materials as described in WO-A-03/023890 may give additional advantages. In either case, once this mixture is cured there are no volatile components, and the electrolyte is a solid, both important requirements for long term stability.

lTMs range of solid electrolytes can be cured in-situ from relatively low viscosity mixtures of monomers, into transparent solid electrolytes. These mixtures cure by a free-radical polymerization which can be initiated by gamma irradiation, thermal initiators or photo initiators. By incorporating suitable ionic moieties we have produced materials which function as redox mediators in dye sensitised solar cells (DSSCs), as an example Grätzel DSSCs use an iodide/tn-iodide system, although the application is not limited to these chemistries. Before curing, the electrolyte monomer mixture has similar viscosity to that of standard DSSC electrolytes. Incorporation of suitable components may be by a variety of routes: 1. All electrolyte components cured in situ.

a. As a homogenous mixture b. With one or more components encapsulated ready for later release 2. Infusing a liquid electrolyte or absorbing components into the cured polymer/ionomer.

3. Attaching the electrolyte strongly or loosely to the polymer/ionomer, either during or after curing.

The invention described in this application is compatible with a range of dye stuffs and semiconductors as used by Grätzel, G24i and Dyesol.

Description of the invention:

The following examples are given for illustrative purposes; they are not intended to limit the scope of the application.

Example 1: Homogenous curing in situ Grätzel DSSCs contain both iodide (I) and tn-iodide (13). The tn-iodide is normally produced by adding iodine (12) to a solution containing iodide. Iodine and tn-iodide are inhibitors of free radical polymerization, the process by which ITM's class of hydrophilic ionic materials are cured. Therefore the iodide and tn-iodide can not simply be added to the monomer mixture prior to curing. Suitable materials can be produced via two routes, an encapsulation method or via an oxidation process. This example details the oxidation process.

The oxidation process works by adding just iodide (for example in the form of potassium iodide) to a liquid acidic monomer mixture, tn-iodide is slowly generated by reaction with dissolved oxygen, which must diffuse in from the atmosphere.

31 + h Oz + 2H -> 13 + H20 The slowness of this reaction means that there is time for the mixture to be cured before significant tn-iodide is generated, and thus the problem of inhibition is solved.

When cured in-situ (incorporating either the encapsulation or oxidation process to produce tn-iodide) the solid polymer electrolyte makes intimate contact with the surface of both electrodes: the dye-sensitized nanoporous semi conductor, and the catalyst-coated counter electrode.

Although free-radical polymerization can be initiated in a variety of ways, the use of an oxidation method of creating tn-iodide means it is necessary to cure as quickly as possible.

A low rate of initiation would be unable to keep pace with generation of the tn-iodide inhibitor. Thus photo initiation is the preferred option.

For good longevity, the transparent top sheet of a DSSC is UV blocking. A cut-off of 400nm blocks <5% of solar energy (about 7% of absorbable photons) but greatly enhances the stability of organic materials in the cell, particularly the dye and the electrolyte. However, because of this cut-off, many standard UV initiators and light sources are not suitable, the initiator needs to respond to visible light, Diphenyl (2,4,6-trimethlybenzoyl)-phosphine oxide (TPO) is a preferred initiator. Suitable lamps for curing include high pressure mercury discharge lamps with enhanced visible output such as UV00000489 bulb from Uvitron, or LED sources such as the Firefly from Phoseon.

Example 1:

8.38g of 2-Acnylamido-2-methyl-1-propanesulfonic acid (AMPSA) was placed in a beaker with 4.20g of deionised water and stirred for ten minutes at room temperature. This resulted in a white slurry. The beaker was transferred to an ice bath and after one minute 3.09g of a 50% solution of NaOH in water was added drop-wise. This produced a clear solution. The beaker was removed from the ice bath and the following ingredients added whilst stirring. 2.02g of 2-hydroxyethyl methacrylate (HEMA), 0.60g of TPO, 0.60g of Allyl methacrylate (AM). After stirring for another 20 minutes a clear solution resulted. This mixture cures readily to an elastic solid in a few minutes with suitable visible light initiation.

However, it does not yet contain any iodide. It should not be stored with iodide present as it will gradually oxidise to tn-iodide, making curing more difficult.

To produce a working dye-sensitized solar cell electrolyte an aqueous solution of potassium iodide is added immediately before curing. In this case, 0.32g of a 50% aqueous solution of Ki was added to 2g of the monomer mixture. This was sealed in a plastic bag and placed under a curing lamp. A clear elastic solid was produced in 6 minutes. By the next day the solid polymer had turned brown due to the oxidation of iodide to tn-iodide. The final composition of the mixture just before curing is: 36% water, 9% HEMA, 1.6% AMPSA (acid form), 41% AMPSA sodium salt, 2.7% TPO, 2.7% AM, 7% KI.

Example 2:

The following materials were placed in an amber bottle with a magnetic stirrer bead.

0.268g Lii, 0.068g guanidine thiocyanate, 0.097g TPO, 0.258 AM, 0.767g HEMA, 0.754g Methylmethacrylate (MMA), 2.81g 1-propyi-3-methylimidazolium iodide(PMII). After stirring for an hour at room temperature a clear solution was produced.

A drop of this monomer mixture was placed on a piece of titanium sheet 20mm x 10mm covered with dye-sensitized nanoporous titania. A piece of transparent plastic 20mm x 6mm coated with platinised transparent conductive oxide was placed on top of this so they overlapped by 9mm. The monomer mixture was allowed to wick into the porous photo anode for a minute. The top sheet was gently pressed down with a fine metal tip which was left in place whilst the device was illuminated by a curing lamp (Uvitron Intelliray 400 with UV00000489 visible-enhanced bulb) for ten minutes. This cured the mixture to a solid polymer which adhered the two electrodes together. Two drops of acetone with 50mM dissolved iodine were placed in a small glass bottle. This was allowed to dry for a minute, leaving about 0.2mg of iodine residue. The PV device was sealed in this bottle and placed in a 6OQC oven for 18 hours to infuse.

This device was tested under a solar simulator producing an AM1.5 spectrum with intensity of 1000W/rn2. The open circuit voltage was 0.50V, short circuit current density 89 A/rn2, peak power output was 22.6W/rn2, indicating a fill factor of 51% and a power conversion efficiency of 2.26%.

Example 3:

The following materials were placed in an amber bottle with a magnetic stirrer bead. 0.llg guanidine thiocyanate, 0.13g TPO, 0.36g Ethylene glycol dimethacrylate (EGDM), 1.70g HEMA, 4.70g PMII. After stirring for an hour at room temperature a clear solution was produced.

A drop of this monomer mixture was placed on a piece of titanium sheet 20mm x 10mm covered with dye-sensitized nanoporous titania. A piece of transparent plastic 20mm x 6mm coated with platinised transparent conductive oxide was placed on top of this so they overlapped by 8mm. The monomer mixture was allowed to wick into the porous photo anode for a minute. The top sheet was gently pressed down with a fine metal tip which was left in place whilst the device was illuminated by a curing lamp (Uvitron Intelliray 400 with UV00000489 visible-enhanced bulb) for ten minutes. This cured the mixture to a solid polymer which adhered the two electrodes together. Two drops of acetone with 50mM dissolved iodine were placed in a small glass bottle. This was allowed to dry for a minute, leaving about 0.2mg of iodine residue. The PV device was sealed in this bottle and placed in a 60C oven for 18 hours to infuse.

This device was tested under a halogen lamp with illumination intensity roughly equivalent to a solar spectrum at 380W/rn2. The open circuit voltage was 0.59V, short circuit current density 1.6 A/rn2, peak power output was 0.61W/rn2, indicating a fill factor of 65% and a power conversion efficiency of 0.2%.

Example 4:

A mixture was made in an opaque bottle containing: 52.5% I-iEMA, 42.5% water, 5% ethylene glycol dimethacrylate and 1% TPO. A drop of this monomer mixture was placed on a piece of titanium sheet covered with dye-sensitized nanoporous titania. A piece of transparent plastic coated with platinised transparent conductive oxide was placed on top of this so they overlapped. The device was illuminated by a curing lamp (Uvitron Intelliray 400 with UV00000489 visible-enhanced bulb) for ten minutes.

The cured cells were placed in a container and covered in a commercially purchased DSSC liquid electrolyte. These were then place in an oven at 9CC for 4 hours, allowing the electrolyte to diffuse into the polymer structure. These were then tested under a halogen bulb producing [unknown intensity] and produced a current density of 195 j.iA/cm2.

Claims

CLAIMS1. A photovoltaic cell comprising a membrane electrode assembly obtainable by a method comprising an in situ polymerisation between electrodes of monomers to form a polymer, and the incorporation of an ionic moiety such that the membrane electrode assembly can function as a photovoltaic cell.
2. A cell according to claim 1, wherein the ionic component is homogeneous with the monomers, and wherein the ionic component is polymerised in situ.
3. A cell according to claim 1, wherein the method comprises infusing an ionic moiety into the polymer, wherein the ionic moiety is a liquid electrolyte.
4. A cell according to any preceding claim, wherein the ionic moiety that is incorporated produces material that can function as a redox mediator in the cell.
5. A cell according to claim 4, wherein the redox mediator is iodide or tn-iodide.
6. A cell according to any preceding claim, wherein the cell is a dye-sensitised solar cell.
7. A cell according to any preceding claim, wherein the polymer is hydrophilic. *..S * S ********.S * * * ** * S S
S...S S.. * S5 * * S S.. *S *. S *