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

Abstract

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 Al 2 O 3 or TiO 2 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 TiO 2 . 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 TiO 2 . 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. ■ INTRODUCTIONThe advantages of metal-free organic sensitizers, for the application in dye-sensitized solar cells (DSSCs), place them more and more into the focus of sensitizer research and development. Superior to their ruthenium-containing alternatives 1 are their extinction coefficients and the nearly unlimited possibilities of varying parameters such as the overall π-conjugation, the distance between the donor and the acceptor, and the nature of the anchoring groups by wellestablished synthetic methods.2 This allows for fine-tuning the spectral and electronic features as well as for a control over charge-transfer kinetics and electronic coupling to TiO 2 . Understanding the influence of structural variations to the sensitizer, such as the length of π-conjugation or anchoring/ acceptor group choice on the DSSC device performance, is crucial for advancing this field and technology. 3,4 Critical factors that influenced performance properties are (i) the excited-state redox potentials, which should be properly aligned with the conduction band of TiO 2 , (ii) the light-harvesting features of the sensitizer, (iii) the conjugation across the donor and anchoring groups, and (iv) the electronic coupling between the lowest unoccupied molecular orbital (LUMO) and the conduction band of the TiO 2 .5 It has been shown that a sensitive interplay between all of these fact...

Charge migration and charge transfer in molecular systems

“…The molecular-wire role of the spacer units was highlighted by extracting an attenuation factor beta ranging between 0.04 and 0.07 Å −1 , therefore revealing a very small dependence on the distance, which indicates an efficient conjugation between the donor and acceptor extremities of the dye. Another modification of the π-bridge is the insertion of a thiophene in the conjugation chain that not only broadens the spectral width, affecting light harvesting properties, but also increases the donor-acceptor distance, spatially affecting the HOMO-LUMO couplings and therefore electron injection and coupling to the titania 217 . The thiophene can be inserted within the donor, extending its π system.…”

Excited-state relaxation of the solar cell dye D49 in organic solvents and on mesoporous Al2O3 and TiO2 thin films

“…[7][8][9][10][11][12][13][14][15][16][17][18] Mesoporous thin films of large band-gap oxide materials, such as Al 2 O 3 , have been employed as reference systems for studying the dynamics of solar cell dyes under ''non-injecting'' conditions. 16,17,[19][20][21] For instance, the results of transient absorption experiments suggest that triarylamine-benzothiazole sensitisers inject electrons into Al 2 O 3 surface trap states. 17 Very recently, Cappel et al proposed a mechanism, in which photoexcited D131 indoline dyes bound to Al 2 O 3 exhibit efficient charge transfer to neighbouring D131 molecules.…”

Synthesis and photovoltaic properties of organic dyes containing N-fluoren-2-yl dithieno[3,2-b:2′,3′-d]pyrrole and different donors

“…The larger V OC observed for the dye 5c is probably arising from the two n-butyl chains on the fluorene segment, which effectively reduces the possibility of charge recombination by retarding the approach of triiodide ion (I 3 À ) at the TiO 2 surface [83]. However, small V OC realized for 5e is attributed to the relatively high recombination rate of electron present in the conduction band of TiO 2 (vide infra) [84]. The high photocurrent density, open-circuit potential and fill factor exhibited by the fluorenyldiphenylamine donor containing dye 5c produced the overall high efficiency for the device (6.81%).…”