π-Conjugated Donor-Acceptor Systems as Metal-Free Sensitizers for Dye-Sensitized Solar Cell Applications

High extinction coefficients and easily tunable spectral properties of π conjugated donor-acceptor dyes are of superior advantage for the design of new metalfree organic sensitizers for applications in dye-sensitized solar cells. Ultrafast transient absorption spectroscopy on the femtosecond and nanosecond time scales provided deep insights into the dependence of charge carrier dynamics in fully organic dye/TiO2 systems on i) the donor-acceptor distance, ii) the π-conjugation length, and iii) the coupling to TiO2 by different anchoring groups. Importantly, the observed differences in charge transfer dynamics justify the variations of photovoltaic performances of the dyes as applied in solar cell devices. This leads to the conclusion that the photoconversion efficiencies strongly depend on a delicate interplay between the dyes’ building blocks, i.e. the donor, the π-conjugated spacer and the anchor/acceptor moieties, and may easily be tuned by molecular design. Dye-sensitized solar cells (DSSCs) have attracted high interest due to their remarkable performances in converting solar energy to electricity at relatively low cost as compared with conventional siliconbased photovoltaic devices [1]. One of the key roles in DSSCs is attributed to the sensitizers, which are responsible for light absorption and the generation of electric charges. Among these, ruthenium sensitizers exhibit record solar to electricity power conversion efficiencies (PCEs) of 11-12% under AM 1.5 G irradiation [2]. Currently, however, more and more research activities focus on the design and development of new metal-free organic sensitizers with the purpose to increase the PCEs [3,4]. The high extinction coefficients of organic dyes and the ease of tuning their spectral properties by standard synthetic methods turned out to be of superior advantage. Thereby, the most efficient photovoltaic performance was achieved when employing sensitizers of a donor-π-conjugated spaceracceptor/anchoring (D-π-A) architecture. Lately, several synthetic strategies lead to the development of various donors based on coumarin, indoline, tetrahydroquinoline, triarylamine, heteroanthracene and carbazole with a cyanoacrylic acid as the acceptor/anchoring unit. DSSCs derived from this type of D-π-A systems have achieved up to 10% efficiency under AM 1.5 G irradiation [3]. Even though many structural variations of the donor moieties have been reported the impact of the π-spacer and of the acceptor/anchoring groups has been much less explored – notwithstanding the fact that these parameters show a significant influence on the spectral response and device performance. Particularly with regard to increase the PCE values of fully organic DSSCs and outdistance the ruthenium based cells, the tunability of the electronic properties by the control of the conjugation length and the anchoring modes to TiO2 is of special interest [4]. EPJ Web of Conferences