Long-Range π-Conjugation in Phenothiazine-containing Donor-Acceptor Dyes for Application in Dye-Sensitized Solar Cells

Four organic donor–π-bridge–acceptor dyes containing phenothiazine as a spacer and cyanoacrylic acid as an acceptor were synthesized and tested as sensitizers in dye-sensitized solar cells (DSCs). The influence of iodide- and cobalt-based redox electrolytes on the photovoltaic device performance was investigated. In these new dyes, systematic π-conjugation was extended by inserting one or two phenothiazine moieties and investigated within the context of the resulting photoinduced charge-transfer properties. A detailed investigation, including transient absorption spectroscopy and quantum chemical methods, provided important information on the role of extended π-conjugation on the photophysical properties and photovoltaic device performance. Overall, the results showed that the extension of π-conjugation by one phenothiazine unit resulted in the best device performance owing to reduced recombination rates, whereas extension by two phenothiazine units reduced dye adsorption on TiO2 probably owing to the increase in molecular size. The performance of the dyes in DSCs was found to be a complex interaction between dye structure and size.

Position-Dependent Extension of π-Conjugation in D-π-A Dye Sensitizers and the Impact on the Charge-Transfer Properties

Thomas Wesley Holcombe, Jinhyun Kim, Jacques-Edouard Moser, Mateusz Barnaba Wielopolski, Yuxiang Yu, Shaik Mohammed Zakeeruddin

A series of five organic donor-π-bridge-acceptor (D-π-A) sensitizers is investigated within the context of their photoinduced charge-transfer properties. Thereby, the focus is set on the impact of structural modifications of the molecular architecture on the π-systems of the dyes. In particular, two different modes of systematic extension of the sensitizers’ π-systems, namely, (i) within the electron donating site and (ii) within the π-bridge, are investigated by means of steady-state and time-resolved spectroscopic methods. The photophysical studies of the molecules in solution and as deposited on Al2O3 or TiO2 films reveal that different effects on the charge-transfer characteristics evolve dependent where – within the molecular structure – the modification of the π-system is performed. Hence, π-extension of the donor sites, for instance, leads to a strong red shift of the absorption features and a variation of light-harvesting properties. Modifying the π-bridges results in a spatial decoupling of the HOMO and LUMO orbitals, which goes along with changes of the electronic coupling to TiO2. Furthermore, solution studies show that the electronic structure of the dyes governs their singlet excited-state features. As shown, the results obtained from these studies then allow important predictions about the deactivation of the excited states of these molecules adsorbed on TiO2. Finally, quantum chemical methods – among others, time-dependent density functional theory calculations – provide conclusive insight into the relationship between the electronic structure of the dyes and its impact on the photoinduced charge-transfer characteristics.

American Chemical Society2013

Novel organic dyes for enhanced red to near IR response in dye sensitised solar cells

The aim of this work was to develop new, organic dyes, capable of harvesting photons in the red to near-IR region of the spectrum. A series of squaraine and tricarbocyanine dyes were produced and characterised and their optical and redox properties studied. The results of initial photovoltaic tests, followed by optimisations of cell performance, have also been presented. These clearly demonstrate the effectiveness of these dyes in solar cells. A total of 12 new squaraine dyes were successfully synthesised, seven of which were tested in "dye-sensitised" solar cells. The efficiencies ranged from 1.2%-3.7%, for the best dye [B1]. It was concluded that enhanced performance could be achieved in the presence of chenodeoxycholic acid (CDCA) and tetrabutylpyridine (TBP). These additives both help to prevent aggregation and increase the cell potential, limiting charge collection losses and enhancing performance. Three new tricarbocyanine dyes were also synthesised, two of which were used in solar cells. Unfortunately, due to the interaction of [B12] with the electrolyte, the dye proved to be unstable, as both the absorption maxima in UV-vis and fluorescence emission wavelengths were shifted to higher energies. Thus, the red response of the dye was significantly reduced and photovoltaic performance hindered. A limited photo-response of [B14] was also seen, although this dye appeared to be very stable in the presence of the electrolyte. To deduce the possible reasons for the poor performance of these dyes, their optical and electrochemical properties were investigated. Thus, cyclic voltametry was employed to identify the redox potentials of the dyes and their charge injection and recombination dynamics were studied through nanosecond time-resolved transient laser spectroscopy. It was concluded that the dyes theoretically possess the ideal characteristics for solar cell function and that the problem of [B14] lay in the purity of the dye. [B14] proved to be very difficult to purify as the dye was unstable in solution and decomposed on the columns employed in chromatography. Fortunately the dye appeared to be stable in the presence of an electrolyte when adsorbed on TiO2. The dye gave an IPCE of 12% between 800 – 900nm, which is particularly impressive, especially if the purity of the dye is considered. A new method of purification must be found however, if photovoltaic performance is to be enhanced. Finally, the effects of a compact blocking layer on device performance were studied. It was clearly demonstrated that a compact layer was essential. This was investigated and a relationship between the structure of the dye and the necessity of the blocking layer was established.

EPFL2006

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

Magdalena Anna Marszalek, Jacques-Edouard Moser, Mateusz Barnaba Wielopolski, Shaik Mohammed Zakeeruddin

High extinction coefficients and easily tunable spectral properties of π - conjugated donor-acceptor dyes are of superior advantage for the design of new metal- free 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.

E D P Sciences2013

Novel organic dyes for enhanced red to near IR response in dye sensitised solar cells

EPFL2006