Dye Wide band gap semiconductor Electrolyte (Redox couple)



   The ideal sensitizer for a single junction dye sensitized solar cell should absorb all light below a threshold wavelength of about 920 nm. In addition, it must also carry attachment groups such as carboxylate or phosphonate to firmly graft it to the semiconductor oxide surface. Upon excitation it should inject electrons into the solid with a quantum yield of unity.
The energy level of the excited state should be well matched to the lower bound of the conduction band of the oxide to minimize energetic losses during the electron transfer reaction. Its redox potential should be sufficiently high that it can be regenerated via electron donation from the redox electrolyte or the hole conductor. Finally, it should be stable enough to sustain about 108 turnover cycles corresponding to about 20 years of exposure to natural light.
Much of the research in dye chemistry is devoted to the identification and synthesis of dyes matching these requirements, while retaining stability in the photo-electrochemical environment. The attachment group of the dye ensures that it spontaneously assembles as a molecular layer upon exposing the oxide film to a dye solution. This molecular dispersion ensures a high probability that, once a photon is absorbed, the excited state of the dye molecule will relax by electron injection to the semiconductor conduction band [2].

   The best photovoltaic performance both in terms of conversion yield and long-term stability has so far been achieved with polypyridyl complexes of ruthenium and osmium. Sensitizers having the general structure ML2(X)2, where L stands for 2,2'-bipyridyl-4,4'-dicarboxylic acid M is Ru or Os and X presents a halide, cyanide, thiocyanate, acetyl acetonate, thiacarbamate or water substituent, are particularly promising. Thus, the ruthenium complex cis-RuL2(NCS)2, known as N3 dye, has become the paradigm of heterogeneous charge transfer sensitizer for mesoporous solar cells. The fully protonated N3 has absorption maxima at 518 and 380 nm, the extinction coefficients being 1.3 and 1.33 × 104 M-1 cm-1, respectively. The complex emits at 750 nm the lifetime being 60 ns.
   The optical transition has metal-to-ligand charge transfer (MLCT) character: excitation oft the dye involves transfer of an electron from the metal to the π*; orbital of the surface anchoring carboxylated bipyridyl ligand from where it is released within femto- to picoseconds into the conduction band of TiO2 generating electric charges with unit quantum yield.

Fig. 1. Photo induced heterogeneus electron transfer cycle

   Discovered in 1993 the photovoltaic performance of N3 has been unmatched for 8 years by virtually hundreds of other complexes that have been synthesized and tested. However in 2001 the “black dye” tri(cyanato)- 2,2'2''-terpyridyl-4,4'4''-tricarboxylate) Ru(II) achieved a record 10.4% (air mass 1.5) solar to power conversion efficiency in full sunlight. This record has been broken only very recently by using the N3 dye in conjunction with guanidinium thiocyanate, a self-assembly facilitating additive allowing to increase substantially the open-circuit voltage of the solar cell [2].

   Wide band gab semiconductor

   The DSSC performance depends on the nature and size of the TiO2 particles. TiO2 particles must be prepared with good dispersion and high crystallinity. Since anatase is better than rutile and brookite for solar energy conversion, the anatase phase is favorable in the preparation of TiO2 particles, and the other two phases, rutile and brookite, should be avoided. The TiO2 nanoparticles, which are depleted of electrons in the dark, serve to transport electrons to a transparent conducting oxide (TCO; F:SnO2) substrate—the collecting electrode. Because of the small particle size (~20 nm), the surface area (roughness factor) of the film is more than a thousand times that of a flat electrode of the same size. The TiO2 network is the recipient of injected electrons from optically excited dye molecules and provides the conductive pathway from the site of electron injection to the collecting electrode [5].

   Electrolyte (Redox couple)

       When considering the critical parameters which controlled the development of the dye sensitization solar cell we may pin down maybe three essential factors: one is the molecular electronic quality of sensitizing molecules. The second is the quality of the redox system and the third is the surface area of the oxide material, which can be covered by a sensitizer. However, with the effort to increase the surface area of the oxide substrate to a maximum extent, the electrolyte started to penetrate the oxide substrate and to reach the front FTO-contact. Because of this electrolyte penetration, the electrical field largely disappeared from the electrolyte/nano-TiO2 interface, but sensitization still continued to work. This was simply due to the largely irreversible properties of the iodide/triiodide systems. Electrons can easily be donated but the reverse reaction to reduce triiodide is very sluggish. Empirically, it turned out that the iodide/triiodide system is much superior to other redox systems of a similar redox potential (Fe(CN)63-/4-, quinone/hydroquinone, Fe2+/3+) so that all realistic wet dye sensitization solar cells today operate with iodide/triiodide only [13].

   The interfacial properties of the TiO2 nano-particles in contact with the iodide/triiodide redox system and improved by a pyridinium compounds allow the safeguarding and collection of injected majority carriers. This contact has to remain highly vectorial in its properties for electron exchange with iodide/triiodide. This can be experimentally demonstrated, since the deposition of small islands of platinum on the front contact will immediately lead to a break down of solar cell efficiency. Platinum islands can catalyze the reverse reaction of electrons with triiodide, making the iodide/triiodide system more reversible and this apparently leads to a significant break down of solar cell efficiency. From these observations it may be concluded that the irreversibility of charge transfer at the nano-particles and at the FTO front contact of the dye sensitization solar cell critically determine solar cell efficiency [13].

   Charge carriers recombination is sufficiently slow, even in the absence of an electric field, that it can be intercepted efficiently by the reaction of the electron donor iodide, with the oxidized dye. The iodide reduces the Ru(III) complex produced at the photoanode, regenerating the original Ru(II) oxidation state. The iodide is oxidized to triiodide in this process [13].

   The increase in the open-circuit voltage and fill factor by 4-tert-butylpyridine is due to the suppression of the dark current at the semiconductor electrolyte junction. The dark current arises from the reduction of triiodide by conduction band electrons, which occurs despite the fact that the TiO2 surface is covered by a monolayer of RuL2(NCS)2. The triiodide, due to its relatively small size, either crosses the dye layer or has access to nanometersized pores into which the RuL2(NCS)2 cannot penetrate, where the surface of TiO2 is bare and exposed to redox electrolyte. The effect of 4-tert-butylpyridine is to decrease the rate of the reduction of triiodide. The decrease in the rate constants for triiodide reduction should lead to an increase in the open-circuit voltage of the cell [13].