# The BCL Method for DSSC: Basis and Applications +
## 8 Improvement of the Performance of N3 with a Mono Carboxylate Ligand  
The development and the improvement of new dyes for sensitized solar cell in order to increase their efficiency require the knowledge of several types of processes occurring during the operation of the cell [25]. The electron transfer between the dye and the semiconductor is one the key process participating in the photocatalytic cycle and involves electron transfer from the dye to the semiconductor through the anchor ligand, which acts as a molecular bridge, transferring electronic density among both species. In principle, the anchoring ligand accomplishes two functions: it acts as physical linker between the dye and the surface of the semiconductor, as well as participating in the electron injection process through the carboxylic group. The structure of a typical dye sensitized solar cell follows the general type of donor-linker-acceptor architecture; in this case, the donor is located on the ruthenium complex, the linker is the dicarboxy-bipyridine ligand, and the acceptor is titanium dioxide. Interaction occurs when the ruthenium dye absorbs energy from sunlight spectra reaching a transient excited state, through which it is able to transfer energy to the acceptor group, composed mainly by empty orbitals with a metallic character. According to the BCL scheme, the energy absorbed by the ruthenium dye is delivered to the anchor ligand; this process is followed by an intersystem crossing to reach a triplet state, where the electronic density is concentrated on the carboxylic moiety and becomes available for injection. In this sense carboxyl groups can act as discharge points for the energy concentrated on the anchor ligand to be transferred to the surface of the acceptor. For the case of ruthenium-based DSSC, two types of anchor ligands (Figure 35) that have been successfully employed are 4,ddicarboxibipyridine (dcbp) and tricarboxylicriidine (tctp). When these types of ligands form ruthenium dyes that additionally incorporate thiocianate as donor group, high values of photoconversion efficiency have been observed. The use of these types of organic molecules as anchor ligands has been very exhaustive and has given rise to a series of ruthenium complexes with thiocyanate as donor booster, that have been very successful in terms of efficiency, measured under standard conditions. In particular, the homoleptic [Ru(SCN)\({}_{2}\)(dcbp)\({}_{2}\)]\({}^{+2}\), also labeled as N3, has been extensively studied, and its photoconvertion efficiency measured under standard conditions is 7.1%. Phenylpiridine (pp) ligand (Figure 36) has been reported to be a useful ancillary ligand to replace tycocianate and provide a new family of ruthenium dyes with a high-efficiency yield. This ligand accomplishes two important functions: to act as an electron donor and stabilize the oxidized dye after electron injection, and to cause a red shift of the absorption spectra. The participation of the pp ligand in the molecular orbital composition is mainly at the HOMO level. The mono carboxylate version (mcpp) displays rather different behavior [26], since the symmetry of the ligand is broken with the presence of  
Figure 35: Dcbp, tctp, and N3 dye. the carboxy moiety on the pyridine ring, causing its decomposition in two fragments: one located in the HOMO and the other in the LUMO, through which electron injection occurs. Analogous with mcpp, bicyclic monocarboxylated ligands derived from bipyridine can be visualized. Nevertheless, it is interesting to note that the countertype of this complex--the monocarboxylated version, mcbp--has not yet been synthesized, although there attempts have been carried out [27]. From a molecular design point of view, the study of this type of ligand is attractive in order to determine if the energy available to be injected into the acceptor group is affected by the number of carboxyl groups present in the anchor ligand [28].