Investigating the Effect of the Auxiliary Donor in Phenothiazine-Based Dye-Sensitized Solar Cells
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- Institutt for kjemi 
The growing need for green energy gives good opportunities for DSSC to complement the silicon-based solar cells of today. The advantages are many; they can be cheap, lightweight, flexible, aesthetically beautiful and transparent, making them fitable for use as windows. But there is one problem, the efficiencies of the cells are still low and development of new and better dyes is necessary. Phenothiazine based dyes are good candidates for use in DSSCs due to a huge number of possibilities and ways of changing the properties of the dyes. For this thesis 6 new phenothiazine dyes, with a donor-π-bridge-acceptor design, have been synthesized and characterized. The auxiliary donor has been varied, aiming to study the effect of this group on the light harvesting efficiency. The synthetic route from phenothiazine to the target molecules consists of six steps. First step was a bromination of phenothiazine, followed by an alkylation (both done as a part of earlier courses), giving the main building block. The next step was the introduction of the auxiliary donor group, through a Suzuki cross-coupling, experiencing low selectivity. The resulting compounds were boronylated and subjected to a second Suzuki coupling with 5-bromothiophene-2-carbaldehyde to introduce the π- bridge moiety. Finally, the target molecules were obtained through Knoevenagel condensations, experiencing high yields and short reaction times. The novel target compounds and unknown intermediates were characterised by 1H NMR, 13C NMR, COSY, HSQC, HMBC, HRMS, IR and melting points were determined. Furthermore, the final dyes, 29-34, were analyzed with UV-Vis spectroscopy, and solar cells were fabricated for determining the power conversion efficiency of the dyes. The obtained PCE values for the new dyes ranged from 1.7 to 3.4 %, compared to 4.8% for the reference dye N719. This indicates that the nature of the auxiliary donor is, indeed, very important. Also, the best performing dye, with a PEG-substituted naphthyl auxiliary donor was not superior in the UV-Vis analyses, suggesting UV-Vis is insufficient as a sole screening method, and should be complemented with other screening methods.