Enhanced photocurrent generation by folding-driven H-aggregate formation

Abstract

A novel bis(merocyanine) dye has been synthesized and its folding and aggregation behavior has been elucidated in dependence on the solvent polarity. Extended cofacially p-stacked H-aggregates could be prepared in nonpolar solvents based on the preorganized folded dimer structure of the dye exhibiting a more than 120 nm hypsochromic shift of the absorption band. Furthermore, incorporation of such H-aggregate domains in bulk heterojunction (BHJ) solar cells by careful adjustment of processing conditions afforded a nearly twofold increase in the photocurrent generation. Our detailed investigations on the external quantum efficiencies of these blends reveal that the increase of the shortcircuit current density J SC stems from the H-aggregated dye manifold in the photoactive layer of the solar cell devices. Results and discussion SynthesisThe target bis(merocyanine) dye 4 was synthesized according to the route depicted in Scheme 1. In the rst step, the literature

Spacer‐Modulated Differentiation Between Self‐Assembly and Folding Pathways for Bichromophoric Merocyanine Dyes

“…The solvent‐dependent spectral changes of 1 f (C5 spacer), 1 g (3‐oxopentyl spacer) and 1 h (C6 spacer) resemble those of C12‐tethered dye 1 k (see Figure S2c,d,e and Figure 2 c). The solvent‐dependent spectral changes of bis(merocyanine) dyes 1 e – k are accompanied by a clear quasi‐isosbestic point, respectively, as observed previously for dye 1 d with m ‐xylylene spacer,15 revealing a chemical equilibrium between two species, that is, the folded and the unfolded conformers of these molecules…”

“…We have previously shown that, due to the accessibility of broad solvent permittivity range, tetrahydrofuran (THF; ε r =7.58)/methylcyclohexane (MCH; ε r =2.02) solvent mixtures are highly suitable to study merocyanine dye aggregation 15. 22 In nonpolar MCH aggregation of dipolar merocyanines is favored, while the interaction of dipole moments of dyes with polar THF stabilizes the monomeric species with respect to the aggregate 23.…”

“…Our recent research activities on merocyanine dye aggregates suggest that these dyes are particularly suited for such investigations due to their following outstanding features: i) This class of dyes show very distinct spectral changes in the UV‐visible spectral range upon aggregation (enabling easy monitoring),13 and ii) the binding strength between merocyanines is quite high due to pronounced electrostatic support (dipole–dipole interaction) to the attractive forces evoked by dispersion forces between aromatic scaffolds that ensure aggregation either by folding or self‐assembly 14. Moreover, in a recent communication we could demonstrate that an optimized processing procedure taking advantage of a folding‐driven self‐assembly sequence of bis(merocyanine) dye 1 d (see Scheme ) afforded bulk heterojunction solar cells with improved photocurrents, pinpointing the impact of supramolecular order on the functional properties of a material 15. These promising preconditions motivated us towards a fundamental study devoted to self‐organization behavior of molecular architectures in which two merocyanine dyes are tethered by spacer units of different length and stiffness.…”

Folding‐Induced Modulation of Excited‐State Dynamics in an Oligophenylene–Ethynylene‐Tethered Spiral Perylene Bisimide Aggregate

“…To exclusively achieve the desired intramolecular aggregate by folding, THF had to be used for dimer F2 , as self‐assembly into larger aggregate structures was observed in MCH (Figure S2 in the Supporting Information); a similar case to this has also been observed for a bis(merocyanine) dye 8. 12 The monomeric reference compound F1 was almost insoluble in MCH, so it had to be dissolved in THF as well for comparison with the larger oligomer analogues. The absorption of F1 exhibited the characteristic features of a PBI monomer consisting of well‐resolved S 0 –S 1 vibronic structures at 425–550 nm without appreciable solvent dependence,13 whereas clear differences were observed in the absorption spectra of F2 and F8 depending on the number of building blocks and the solvent.…”

In-situ modification of PEDOT:PSS work function using alkyl alcohols as secondary processing solvents and their impact on merocyanine based bulk heterojunction solar cells

“…and characterization data of all MC dyes were123 reported previously[12,[24][25][26][27]. PC 61 BM (P99.5%) was obtained124 from Nano-C.…”