Cells based on dyes

   Historical background

   The photoeffects in electrochemical systems were first observed by Becquerel in his investigation on the solar illumination on metal electrodes in 1839. Thompson and Stora reported that pure metal electrodes were also sensitive to light when coated with a dye or immersed in a dye solution [1].
However it was not until 1883 that the first solar cell was built, by Charles Fritts, who coated the semiconductor selenium with an extremely thin layer of gold to form the junctions. The device was only around 1% efficient [1].

   The modern age of solar power technology arrived in 1954 when Bell Laboratories, experimenting with semiconductors, accidentally found that silicon doped with certain impurities was very sensitive to light [1].
Since 1968, after the discovery of dye sensitization of the photocurrents on semiconducting electrodes, dyes have also been used in electrochemical energy converting cells [1].
In this period born the photo-electrochemical (PEC) cells in which dyes are used for photoeffected electron-transfer reaction [1].

   Anciently a photochemical cell is composed with inert metal electrodes immersed in two redox couple [i.e. A/B (light sensitive) and Y/Z] and the best performance is obtained if one couple (e.g. A/B) is highly reversible and the other (Y/Z) is highly irreversible. As a result, the reaction occurs in the light and dark are as follows [1]:

   Dye molecules, having redox property as well as light sensitivity, can be used in comprehensive solar cell as a redox couple among the two which is mentioned in model primitive cell [1].
Scientist have studied both homogeneous and heterogeneous type cells, and pointed out that the power output of heterogeneous cell is about 10 times higher than the homogeneous cell under similar experimental conditions. the advantages of the heterogeneous cell are very significant [1].
From the survey of PEC cell with different dyes, it is observed that photochemical solar energy conversion and storage depend on the structures, reduction potentials and absorption maxima of dyes used in solar cell [1].
To convert the broad solar spectrum, mixed dyes instead of single dye may be used in solar cell. An enhancement in power output of solar cell consisting of mixed dyes is observed when compared to the solar cell with single dye [1].
The solar energy conversion capability of mixture of dyes in liquid collector is controlled by dye-dye interactions, photodegradation and susceptibility to photo-oxidation [1].

   PEC cells with semiconductor photoanode

   PEC cells which involve the use of semiconductors, immersed in suitable electrolytes, require absorption of photons by semiconductor with energy greater than the band gap so that promotion of electrons from valence to conduction band is possible [1]. The processes which occur in a liquid junction PEC cell are as follows [1]:

  1. Absorption of photons by semiconductor promotes electrons (e−) from the valence band to the conduction band leaving an excess of holes (h+) in the valence band.

  2. Holes, at the energy level of the valence band, oxidize the reduced electroactive species (D) i.e. reductants of a solution containing redox couple (D/D+) at the interface of the photoanode.

  3. Electrons at the energy level of the conduction band, move away from the interface, through the external circuit to an inert cathode and reduce oxidants of the solution at its interface with the electrolyte.

   Liquid junction devices have several advantages such as the simplicity of fabricating a junction by immersing a semiconductor in an electrolyte solution or the use of cheap polycrystalline semiconductor photoelectrode [1]. PEC cells can have n-type semiconductor electrode like SnO2, In2O3 and ZnO [1].

   From PEC cells to Grätzel's cells

   Separing charge generation from charge transport in PEC cells based on semiconductors, using a sensitizing dye, lead to present day DSSCs, such as Grätzel's cells. Light is absorbed by the sensitizer, which is adsorbed to the surface of a wide band-gap semiconductor. Charge separation takes place via photo-induced electron injection from the excited dye into the conduction band of the semiconductor [2].

   During the first years of sensitization solar cell research, many different dyes were tested and both, p- and n-type materials, were studied as substrates for electron or hole injection [13].
The effect of a redox system was investigated in detail as were questions of dye stability. All dyes investigated at that time gradually degraded so inhibiting practical applications [13].
In 1968 the dye sensitization solar cells had approximately 0.5% energy conversion efficiency [13].

   Later, in 1980, the study on zinc oxide demonstrated an energy efficiency of 2.5%, also for light incident within the absorption spectrum of the sensitizer. The redox system used was iodide/triiodide. Ruthenium complexes were also investigated during that period but were not found to be particularly interesting. Apparently the electrolyte was not properly selected to allow the complex a suitable degree of interaction with the oxide. During this period copper complexes were also investigated as sensitizers for sintered oxide materials [13].

   A significant improvement in the field of dye sensitization solar cells came, as is well known, in 1989, through the efforts of Grätzel and his research group. The significant advance made by Grätzel’s group to improve energy conversion efficiency from lower than 2.5 to 7% was principally due to the following circumstances: progress made with the preparation of the nano-structured TiO2 was combined with the use of a ruthenium complex, which was adequately bonded to the TiO2 material. The same kind of ruthenium complex (but not identical) had been used before in 1980 and even earlier for sensitization but the selected aqueous electrolyte did not allow for an efficient adsorption to the oxide substrate [13].

   Grätzel’s group took account of that and selected organic electrolytes for sensitization solar cells [13].

   The jump in solar energy conversion efficiency motivated significant optimism with respect to the feasibility of cheap and long-term stable dye sensitization solar cells [2,13].