Dye-sensitized solar cells (DSSCs) have been classified as one of the most promising candidates for widespread solar energy utilization owing to their low cost and relatively high photon to electric current conversion efficiencies (η).1 The performances of these devices depend mainly on the properties of the functionalized organic photosensitizers present. Among photosensitizers, metal-free organic dyes -whose efficiencies have been markedly improved over the yearshave many advantages compared to ruthenium-based dyes. These advantages include their wide selection of compounds that can be easily modified to suit the absorption profile, be more environmentally-friendly and have lower production costs.
2One major flaw of organic dyes is the interaction of the iodine electrolyte with the TiO 2 semiconductor, resulting decreasing the efficiency of the cell. This can be prevented by introducing a bulky hydrophobic group at the periphery of the donor moieties.3 In our research, we attached 2-ethylhexyloxy groups at the triphenylamine moieties in order to minimize the electrolyte-TiO 2 interaction that in turn enhances the electron donating capacity of the donor group and increases the highest occupied molecular orbital energy level for better charge injection.4 The electronic and photovoltaic performances of two alkoxy substituted dyes with varying conjugation length (Scheme 1) are presented in this paper.The synthesis of chromophores 4 5 and 5 6 were achieved from intermediates derived from Wittig, 7 Heck-Mizoroki coupling,8 and Knoevenagel condensation 9 reactions. During the Knoevenagel reaction, it was observed that the purification of the product was hard to achieve when using catalytic amounts of piperidine. However, when stoichiometric amounts of piperidine were used, the product precipitated upon cooling and it could be easily filtered off.The UV-visible spectra of the chromophores in ethanol solution produced a typical red-shift of about 25 nm after styrene was introduced to chromophore 5 (429 nm), compared to chromophore 4 (404 nm) as shown in Figure 1. The red-shift of the absorption spectra in chromophore 5 was due to the increased separation of the donor and acceptor groups brought about by the extension of the π-conjugated moiety. This resulted in better charge-transfer Communications to the Editor characteristics than chromophore 4 based on the HOMO and LUMO isodensity plots shown in Figure 2. Contrary to expectations, the photovoltaic performance of chromophore 5 is 21% lower than chromophore 4 as shown in Table 1. The lower efficiency observed in chromophore 5 is due to the increase in dye aggregation at the semiconductor surface. This can be seen by looking at the amount of absorbed dye, chromophore 5 (100 nmol cm −2 ) was almost 3 times more absorbed compared to chromophore 4 (34 nmol cm
−2). The aggregation effect was further confirmed by the absorption spectra of the film (Fig. 1), where the chromophores produced a much broader absorption profile that indicates the presence of aggregation at the ...
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