Jan Schöneboom

An Improved Perylene Sensitizer for Solar Cell Applications

1,6-Dithiophenol-substituted perylene organic sensitizer 1 was synthesized, and its photovoltaic properties in dye-sensitized solar cells were assessed. When anchored onto TiO2 film, the dye exhibits an unprecedented incident monochromatic photon-to-current conversion efficiency of 87 % and yields a power conversion efficiency of 6.8 % under standard AM 1.5 solar conditions.

The influence of local electric fields on photoinduced absorption in dye-sensitized solar cells.

The dye-sensitized solar cell (DSC) challenges conventional photovoltaics with its potential for low-cost production and its flexibility in terms of color and design. Transient absorption spectroscopy is widely used to unravel the working mechanism of DSCs. A surprising, unexplained feature observed in these studies is an apparent bleach of the ground-state absorption of the dye, under conditions where the dye is in the ground state. Here, we demonstrate that this feature can be attributed to a change of the local electric field affecting the absorption spectrum of the dye, an effect related to the Stark effect first reported in 1913. We present a method for measuring the effect of an externally applied electric field on the absorption of dye monolayers adsorbed on flat TiO(2) substrates. The measured signal has the shape of the first derivative of the absorption spectra of the dyes and reverses sign along with the reversion of the direction of the change in dipole moment upon excitation relative to the TiO(2) surface. A very similar signal is observed in photoinduced absorption spectra of dye-sensitized TiO(2) electrodes under solar cell conditions, demonstrating that the electric field across the dye molecules changes upon illumination. This result has important implications for the analysis of transient absorption spectra of DSCs and other molecular optoelectronic devices and challenges the interpretation of many previously published results.

Effect of molecular packing on the exciton diffusion length in organic solar cells

The efficiency of photocurrent generation in bilayer organic solar cells is shown to increase when molecular order is improved. This effect is studied in cells using pure cis and trans isomers of 3,4,9,10-perylene tetracarboxylic bisbenzimidazole. X-ray diffraction studies show that the π-π stacking direction lies in the substrate plane for both isomers and that the trans isomer exhibits improved molecular order in the out-of-plane direction. The improved stacking leads to an increased exciton diffusion length and increased external quantum and power conversion efficiencies. These results provide insight into the effect of molecular structure and packing on the exciton diffusion length.

Rainbow Perylene Monoimides: Easy Control of Optical Properties

Perylene dyes have been widely used as photoreceptors in organic photovoltaics because of their outstanding photo-, thermal and chemical stability as well as their excellent photophysical properties. Herein we describe a novel generation of perylene dyes based on N-(2,6-diisopropylphenyl)-perylene-3,4-dicarboximide. The optical properties of these novel perylenes can be finely tuned via the substituents in the 1-, 6- and 9-positions of the perylene core. The facile synthesis, tunable orbital and absorption properties, and electrochemical potentials help us to design efficient perylene sensitizers for solar-cell applications.

Perylenes as sensitizers in hybrid solar cells: how molecular size influences performance

Dye-sensitized solar cells (DSCs), one kind of hybrid solar cells, are being intensively developed due to their high efficiency and low cost. One of the main factors to improve the efficiency is the minimization of the recombination of holes and electrons at the TiO2/dye/electrolyte interface. To suppress the charge recombination, dye arrangement on the TiO2 surface plays the pivotal role in DSCs. Herein we report three perylene sensitizers of various molecular sizes, which are derived from the introduction of different groups in the 1,6-positions of the perylene core. The same donor (di-p-tert-octylphenylamino) and acceptor (anhydride) moieties in these perylene sensitizers render them highly similar spectroscopic and electrochemical properties, which can be used to compare the effect of the dye-loading on the TiO2 surface, namely, the photovoltaic performance as a function of the sensitizer size. These results will help in better understanding the complex relationship between the molecular size and the device performance.

Double Donor-Thiophene Dendron-Perylene Monoimide: Efficient Light-Harvesting Metal-Free Chromophore for Solid-State Dye-Sensitized Solar Cells

Dye-sensitized solar cells (DSCs) based on a stable largeband nanostructured semiconductor, such as titanium dioxide, are low cost and easily processable alternatives to conventional silicon wafers, and as such have lately drawn much attention. In particular, the solid-state DSCs show great potential owing to their increased stability compared to liquid DSCs. The reason for this stability is the exchange of the liquid electrolyte, which often bears the problem of leakage and electrode corrosion, for a solid hole-conducting material, mainly 2,2’,7,7’-tetrakis(N,N-para-dimethoxyphenylamino)-9,9’-spirobifluorene (spiro-MeOTAD). However, compared to the parent liquid DSCs, solid-state DSCs have shown much lower efficiencies. Whilst ruthenium-based sensitizers, and now a first porphyrin sensitizer, have shown efficiencies up to approximately 11 % in liquid cells, solidstate DSCs only reach values of up to 6 %. Most of the more efficient sensitizers are ruthenium-based, which have drawbacks such as cost, sustainability, and limited ease of band-gap manipulation. One very stable and metal-free alternative are sensitizers based on perylene monoimides, which are known for their excellent chemical, photochemical, and thermal stability as well as high absorptivity and acceptor ability. Another outstanding class of chromophores are thiophenes, in particular oligothiophenes, for their highly variable optical properties according to their architecture, extraordinary charge transport properties, and extinction. Both perylenes and thiophenes have found wide application in optoelectronic devices. Herein, we present a donor–acceptor perylene monoimide with a branched terthiophene spacer group and a triphenylamine donor moiety (1 a, Scheme 1) as well as a naphthalene analogue (1 b, Scheme 1). As reported by Thomas et al. and Fischer et al., the combination of a triphenylamine donor and a branched oligothiophene spacer in combination with a 2-cyanoacrylate acceptor gave good efficiencies of up to 6.15 % and 6.8 % in liquid DSCs and up to 2.6 % in a solid-state DSC. Furthermore, three moieties—triphenylamine, oligothiophene, and perylene monoimide—have recently caused a stir in a p-DSC (NiO), showing a sevenfold increase in energy conversion efficiencies compared to preceding sensitizers. However, we have designed our sensitizers for n-DSCs (TiO2), in which the perylene sensitizer 1 a in particular shows an outstanding efficiency of 3.8 % under 1.5 AM illumination (1 sun). To the best of our knowledge, this is an unprecedented performance for a perylene sensitizer, the best perylene sensitizer for solid-state DSCs so far being ID176 with an efficiency of 3.2 %. Both compounds were prepared by the initial introduction of the terthiophene spacer group by Suzuki coupling with the brominated perylene (or naphthalene) imide, successive Suzuki coupling with the triphenylamine donor, and finally saponification and imidization with glycine to yield the final product (Scheme 1). As described above, both sensitizers consist of an acceptor unit with a carboxylic acid anchor in the imide structure, a branched terthiophene (a a connection and a b connection of the thiophene units) and a triphenylamine donor. In [a] H. Wonneberger, Dr. C. Li, Prof. Dr. K. M llen Max-Planck Institute for Polymer Research Ackermannweg 10, 55128 Mainz (Germany) Fax: (+49) 6131-379-100 E-mail : lichen@mpip-mainz.mpg.de muellen@mpip-mainz.mpg.de [b] Dr. C.-Q. Ma, Prof. Dr. P. B uerle Institute of Organic Chemistry II and Advanced Materials University of Ulm Albert-Einstein-Allee 11, 89081 Ulm (Germany) [c] Dr. N. Pschirer, Dr. I. Bruder, Dr. J. Schçneboom, Dr. P. Erk BASF SE 67056 Ludwigshafen (Germany) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/asia.201000895.