LAMP
Laboratory for Advanced Molecular Processing

Photovoltaics

Dye-sensitized solar cells (DSCs)

Dye-sensitized solar cells (DSCs) are photoelectrochemical cells that use photo-sensitization of wide-band-gap mesoporous oxide semiconductor. These cells were invented by Michael Graetzel et al. in 1991 and are also known as Graetzel cells. DSCs are extremely promising because they are made of low-cost materials and do not need elaborate apparatus to manufacture. They comprise a dye-sensitized nanoporous titinia electrode on transparent conductive oxide (TCO) electrode, electrolytes containing iodide/triiodide redox couple filling the pore of the electrode, and a platinum counter electrode placed on the top of the titinia electrode. The current energy conversion efficiency is about 11%, as was reported by Graetzel et al.
Dye-sensitized solar cells (DSCs
Attached to the surface of the nanocrystalline film is a monolayer of the charge transfer dye. Photo excitation of the latter results in the injection of an electron into the conduction band of the oxide. The original state of the dye is subsequently restored by electron donation from the electrolyte, usually an organic solvent containing redox system, such as the iodide/triiodide couple. The regeneration of the sensitizer by iodide intercepts the recapture of the conduction band electron by the oxidized dye. The iodide is regenerated in turn by the reduction of triiodide at the counterelectrode the circuit being completed via electron migration through the external load. The voltage generated under illumination corresponds to the difference between the Fermi level of the electron in the solid and the redox potential of the electrolyte. Overall the device generates electric power from light without suffering any permanent chemical transformation.
Sealing Resins

Photoelectrodes

Dye sensitized solar cells (DSSCs) are based on the photo-injection of electrons from dye molecules into an inorganic semiconductor and holes transport by a redox mediator. Efficient light-to-energy conversion of dye sensitized solar cells (DSSCs) requires that the sensitized semiconductor electrode will have a high surface area. The high surface area is necessary because of the low absorbance of dye monolayers and the low efficiency of dye multi-layers. In the dye sensitization of large band gap semiconductors for photovoltaic solar cell application, the highest energy conversion efficiencies have been achieved with titanium dioxide (TiO2). Other semiconductors, such as tin oxide (SnO2), zinc oxide (ZnO), and niobium oxide (Nb2O5), have been employed with less success. As the key component of DSSCs, the porous electrode shows high surface area, which enables both efficient electron injection and light harvesting. Unfortunately, the porous electrode introduces the charge recombination which mainly occurs at the electrode/electrolyte interface, thus decreasing all cell parameters and its total conversion efficiency.
One of the promising methods for improving DSSCs efficiency is to modify the surface of a semiconducting photoelectrode. For example, the energy conversion efficiency of the cell has been improved by employing a thin coating layer of various oxides on the TiO2 surface. The thin coating layer, which has a higher conduction band edge than TiO2, has been reported to retard the charge recombination between injected electrons and electron acceptors such as I3- ions and using the shell material provide faster electron injection. Higher isoelectric point (IEP) of the shell materials favors strong dye absorption resulting in better light absorption and, thus, a higher photocurrent.
Iso Electric Point PH ai2o3
In the last decade, several methods have been applied to reduce recombination at the interface including dip-coating in a solution to fabricate an insulating layer on TiO2, spin-coating a solution on TiO2 followed by oxidation, chemical vapor deposition, electrochemical deposition, and mixing of the metal salts with TiO2 or SnO2 nanoparticles in solution. The essential improvement in electron injection efficiency and thereby overall performance can be obtained by proper designing or choosing of the second metal oxide with suitable band gap, optimal thickness of shell layer and isoelectric point (IEP). Another effective approach for the modification of TiO2 surface is to treat TiO2 photoelectrode with acid, UV and plasma etc.

Electrolytes

The iodide/triiodide redox couple shows good solubility, narrow absorbance for the visible light, a suitable redox potential, rapid dye-regeneration, and a very slow recombination rate with electrons in the TiO2 conduction band. However, the liquid electrolyte of the I3-/I- redox couple has left room for further improvement as categorized in Figure 1. First of all, the presence of liquid solvents may result in the substantial problems with the long-term operation and robust sealing, and drag on large-scale outdoor applications of the DSCs. Second, the standard potential of I3-/I- redox couple is 0.35 V (vs. NHE) and the oxidation potentials of the standard sensitizers are around 1.1 V. Thus, the driving force for reduction of the oxidized dye is as large as 0.75 V and it is considered as the largest internal potential loss in the DSC. We expect that the photovoltaic conversion efficiency attains over 15% if half of this internal potential loss could be gained. Third, iodide and triiodide ions are very corrosive to metals so that the stability of the catalyzed counter electrode could not be guaranteed. The compatibility of the iodide/triiodide redox couple with the other compartments in the DSC should be concerned to assure its durability.
Current Issues In Electrolyte
We now concentrate upon resolving the leakage and volatilization of the liquid electrolyte. Recently, solid-state and quasi-solid-state electrolytes such as polymer gel electrolytes, organic and inorganic hole-transporting materials, ionic liquids, and polymer electrolytes have been suggested in order to replace the liquid electrolyte and guarantee long-term stability. Furthermore, it is noteworthy that these electrolytes will endow the DSC with complete structural flexibility to utilize the low-cost roll-to-roll fabrication.
Solid And Ouasi Solid State Electrolytes
Of the alternative electrolytes, hole-transporting materials and polymer electrolytes are categorized into the solid-state electrolyte. The DSC with the solid-state electrolyte could completely avoid any leakage and long-term instability; however, it shows the photoelectric conversion efficiency of less than 5% due to the imperfect filling of the porous TiO2 film, the low internal charge-transport, and the poor contact with the counter electrodes. On the other hand, the polymer gel electrolyte could mitigate the leakage to retain solvent molecules in the polymer matrices, and be prepared by immobilization and plasticization of organic solvents. Owing to its sol-gel characteristic, the polymer gel electrolyte shows high ionic conductivity comparable to the liquid electrolyte, reasonable permeability into the inner structure of the TiO2 film, and negligible vapor pressure.
The long-term loss of the solvent from a polymeric solution, i.e. syneresis, can be further suppressed by high polymer content and high polymer-solvent affinity. Since the polymer content and the ionic conductivity in the polymer gel electrolytes are competitive to each other, a low polymer content less than 20wt% has been prevalently used in the past to attain reasonable ionic conductivities in the DSC. However, the well-known Flory-Huggins theory indicates that the vapor pressure of a polymeric solution is inversely proportional to the polymer content, and the polymer gel electrolyte in which polymer content is low cannot completely resolve the problems regarding the leakage and long-term stability. Besides, polymer matrices can hold solvent tightly by physical interaction, viz. polymer-solvent affinity. Polymers with different functional groups and crystallinities show different degrees of the affinity to their solvents, thereby it should be taken into account when developing new polymer gel electrolyte. Nowadays, we use a polymer with very low crystallinity and proper functional groups to fabricate quasi-solid-state DSCs with high performance and long-term stability.
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Counter electrodes

The counter electrode (CE) is one of the most important components in DSCs. CEs act as an electron carrier from the external circuit to the redox electrolyte. The iodide ions present in the redox electrolyte reduce the oxidized dye and the resultant triiodide ions are reduced back to iodide at CEs. Commonly used TCO substrates such as tin-doped indium oxide (ITO) or fluorine-doped tin oxide (FTO) exhibit slow electron transfer kinetics for triiodide reduction reaction and hence, catalytic layer is coated on TCO substrate and used as a counter electrode. Materials used for the good counter electrode should have a good catalytic effect on the reduction of triiodide, high surface area, and high electric conductivity.
Counter Electrode
The best performing material for DSC counter electrodes is Pt, which shows excellent catalytic activity for the reduction of triiodide at film thicknesses of only a few nanometers. Thermal Pt on FTO glass, prepared by the decomposition of H2PtCl6 at 400 C,and demonstrating charge transfer resistance values as low as 0.07 ohm cm2. However, Pt is very expensive, not stable for long periods in the electrolyte solution (dissolution and deactivation of Pt) and expensive, so the substitution of the noble metal with a stable and cheaper catalyst should improve the stability of the cell lowering the production costs.
Carbonaceous materials are quite attractive to replace platinum due to their high electronic conductivity, corrosion resistance towards iodine, high reactivity for triiodide reduction and low cost. Several carbonaceous materials such as carbon nanotubes, activated carbon, graphite, and carbon black had been employed as catalyst for counter electrode.
Carbon