The cells which convert energy from light into electrical energy by photovoltaic effect are called solar cells or photovoltaic cells. Dye sensitized Solar Cells (DSSC) is a special type of solar cells. A DSSC contains a dye molecule which acts as a molecular electron pump. In simple words, the dye molecule absorbs visible light and transfers an electron into the surrounding semi-conductor. Through the connected anode and the outer circuit, this electron arrives at the cathode from where it is transferred to the reduced electrolyte. The electron is finally transferred to the oxidized dye molecule again, thus completing the circuit. General information about major components of a DSSC can be found in our previous post.
An electrolyte is among the major parts of a DSSC. Organic liquid electrolytes are commonly used in DSSCs. Components of liquid organic electrolytes are volatile, and leakage of the electrolyte can also occur. In addition, they cause corrosion of the counter electrode (CE), desorption of the dye molecules and degradation of the sealants used.
One of the alternatives to liquid organic electrolytes is using polymer-based-quasi-solid-state electrolytes (PQSSE). Quasi-solid state is special state in which a substance is neither fully liquid nor fully solid (i.e., it is a semi-solid). Electrolytes in quasi-solid states have cohesiveness like solids and diffusive properties like liquids, simultaneously. This helps them in achieving long-term stability as well as good ionic conductivity and interfacial contact property.
PQSSE are prepared by encasing the liquid electrolyte inside organic polymer gels. Polymers used in PQSSE are often called as gelators or absorbers. Therefore, the components of a PQSSE include polymer (or oligomer), organic solvent(s) and the inorganic salt(s). Depending upon the polymer type used, the PQSSE can be classified as thermoplastic PQSSE, thermosetting PQSSE and composite PQSSE.
In this post, we aim to provide a general overview of thermoplastic and thermosetting polymers used/studied as carriers of the organic electrolytes of DSSC. Composite PQSSE are not discussed here.
Thermoplastics are the plastics whose flow behavior can be changed by changing the temperature without degrading the original polymer network. Therefore, thermoplastic PQSSEs use polymer gelators which allow change in the flow properties of the electrolyte by changing the temperature while keeping the structural integrity of the polymer.
While physical cross-links help in the entrapment of the liquid electrolyte, the chemical structure of the polymer helps in keeping the anions and cations separated, thus, enhancing the ionic conductivity.
Thermoplastics which have been heavily investigated as PQSSE include:
- Polystyrene (PS)
- Poly(vinyl chloride) (PVC)
- Poly(acrylonitrile) (PAN)
- Poly(ethyleneoxide) (PEO or PEG)
- Poly(vinylpyrrolidinone) (PVP)
- Poly(vinylidene ester) (PVE)
- Poly(vinylidene fluoride (PVDF)
- Poly(methyl methacrylate) (PMMA), etc.
Thermoplastic PQSSEs are typically prepared by mixing the liquid electrolyte (composed of organic solvent, inorganic salts and additives) with the polymer. Initially, this mixture is heterogeneous which gradually turns into a gel like structure.
Both chemical and physical properties of the used polymer play a role in the overall properties of the thermoplastic PQSSE. While physical cross-links help in the entrapment of the liquid electrolyte, the chemical structure of the polymer helps in keeping the anions and cations separated, thus, enhancing the ionic conductivity.
The importance of chemical structure of the used polymer can be understood by considering the example of PEO. This polymer (and its copolymers) is the most widely investigated polymer in thermoplastic PQSSEs as a gelator. Ether groups constitute the main chain while polyhydric groups make up the side chains of PEO. Both these groups can interact with the alkali metal cations. Such interactions help in keeping the iodide anions away from the alkali cations and, hence, increase the ionic conductivity inside the DSSC. The same groups can also form hydrogen bonds with the organic solvents (like propylene carbonate) in the electrolyte which aids in the formation of a stable PQSSE with temperature dependent flow properties (i.e., it is thermoreversible).
The major problem with the thermoplastic PQSSEs is finding the optimized concentration of a given polymer at which sufficiently high electrolyte conductivity and good sealing properties are achieved.
Factors which influence the diffusivity and conductivity of the electrolyte inside the thermoplastic PQSSE include concentration and structure of the polymer, solvent, ionic conductor, additives and temperature.
Good interfacial wetting and high ionic conductivity have been noticed for thermoplastic PQSSE. Vaporization of the liquid solvent and leakage problems are reduced in such electrolytes. On the other hand, the major problem with the thermoplastic PQSSEs is finding the optimized concentration of a given polymer at which sufficiently high electrolyte conductivity and good sealing properties are achieved.
Thermosetting polymers are those which are formed by the crosslinking reaction of monomers and cannot be reprocessed by reheating. Upon heating beyond a certain temperature (depending upon the chemistry), they start to degrade/decompose unlike thermoplastics which have a specific melting/softening temperature before the arrival of a decomposition temperature.
First thermoset used in DSSC as electrolyte carrier was made by using polyethylene glycol (PEG) + poly(ethylene glycol) diacrylate (PEGDA)Parvez et al., Solar Ener. Mat. Solar Cells
Volume 95, Issue 1, 2011, Pages 318-322
Generally, for preparing thermosetting PQSSE, monomers (and/or oligomers) of the thermosetting polymer are mixed with the electrolyte (and other additives). This process is termed as in-situ preparation method. Under suitable reaction conditions, the monomers react to form a cross-linked polymer network encasing the liquid electrolyte. Polymerization can be started by using Ultraviolet (UV) light or heat depending upon the monomer/reactants type. After the reaction has completed to a certain degree, the thermosetting polymer electrolyte becomes solid. Upon reaction completion, the thermosetting polymer electrolytes still appear like solid.
An alternative method for the preparation of thermosetting PQSSE involves adsorption/absorption of the liquid electrolytes on already prepared polymers. Such thermosetting polymers can absorb considerable amounts of the liquid electrolytes. For example, poly(acrylic acid) (PAA) modified with Poly(ethylene glycol) (PEG) (PAA-PEG) can absorb liquid electrolyte 8 to 10 times more than its own weight.
Both methods produce solid polymer encaging some electrolyte in the liquid form (i.e., they are not 100% solid). That’s why they are called thermosetting PQSSE.
Following monomers, oligomers and polymers are some of those which have been used/studied in the open literature to prepare thermosetting PQSSEs by using different polymerization methods.
- α-methacryloyl-ω-methoxyocta (oxyethylene)
- Methyl methacrylate (MMA) and 1,6-hexanediol diacrylate (HDDA)
- Polyethylene glycol (PEG) + poly(ethylene glycol) diacrylate (PEGDA)
- 2-hydroxyethyl methacrylate (HEMA) + tetra(ethylene glycol) diacrylate (TEGDA)
- Poly(ethylene oxide-co-propylene oxide) trimethacrylate (oligomer)
- Poly(acrylic acid) modified PEG and polypyrole
- Poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP)
- Poly(oxyethylene) segments + amido-acid linkers + amine termini + amide
- Polyvinyl-(acetate-co-methyl methacrylate) P(VA-co-MMA)
For in-situ preparation, there are several conditions which need to be fulfilled for getting the desired properties of a thermosetting PQSSE:
- Iodine (I2) needs to present in the electrolyte during polymerization.
- Temperature of the polymerization must be lower than the decomposition temperature of the used dye.
- No byproducts should be produced during the polymerization. Byproducts can reduce the photovoltaic performance of the PQSSE e.g., initiator decomposition products can have such an effect on the final thermoset electrolyte. Ideally, polymerization should be able to proceed without the presence of an initiator.
Poly(acrylic acid) (PAA) modified with Poly(ethylene glycol) (PEG) (PAA-PEG) can absorb liquid electrolyte 8 to 10 times than its weight.Wu et al., Adv. Mat. Vol.19, Issue22, 2007, Pages 4006-4011
The presence of I2 in the electrolyte has an inhibitory effect on the photopolymerization of monomers. A combination of photocatalysis and photopolymerization has been proposed as solution to overcome this problem.
With thermosetting PQSSEs conversion efficiencies as high as 10.6% have been reported on a lab scale. Such high PCE values were made possible by combining nanoporous polymer network with the surface of nanocrystalline TiO2. MMA and HDDA were polymerized via surface-induced polymerization on nanocrystalline TiO2 which generated a nanoporous polymeric structure on TiO2. This structure allowed selective transportation of the I2 ions of the electrolyte which in turn depressed the dark current and, consequently, increased the PCE value.
Chemical, physical and thermal stabilities of thermosetting PQSSE are better than liquid organic electrolytes and thermoplastic PQSSEs. However, the ionic conductivity of thermosetting PQSSEs is lower (due to suppressed mobility of the redox couple) than the liquid organic electrolytes and thermoplastics PQSSEs which is limiting their large scale application.