Markus K. R. Fischer

Metal-free organic dyes for dye-sensitized solar cells: from structure: property relationships to design rules.

Dye-sensitized solar cells (DSSC) have attracted considerable attention in recent years as they offer the possibility of low-cost conversion of photovoltaic energy. This Review focuses on recent advances in molecular design and technological aspects of metal-free organic dyes for applications in dye-sensitized solar cells. Special attention has been paid to the design principles of these dyes and on the effect of various electrolyte systems. Cosensitization, an emerging technique to extend the absorption range, is also discussed as a way to improve the performance of the device. In addition, we report on inverted dyes for photocathodes, which constitutes a relatively new approach for the production of tandem cells. Special consideration has been paid to the correlation between the molecular structure and physical properties to their performance in DSSCs.

Highly efficient photocathodes for dye-sensitized tandem solar cells

Thin-film dye-sensitized solar cells (DSCs) based on mesoporous semiconductor electrodes are low-cost alternatives to conventional silicon devices. High-efficiency DSCs typically operate as photoanodes (n-DSCs), where photocurrents result from dye-sensitized electron injection into n-type semiconductors. Dye-sensitized photocathodes (p-DSCs) operate in an inverse mode, where dye-excitation is followed by rapid electron transfer from a p-type semiconductor to the dye (dye-sensitized hole injection). Such p-DSCs and n-DSCs can be combined to construct tandem solar cells (pn-DSCs) with a theoretical efficiency limitation well beyond that of single-junction DSCs (ref. 4). Nevertheless, the efficiencies of such tandem pn-DSCs have so far been hampered by the poor performance of the available p-DSCs (refs 3, 5-15). Here we show for the first time that p-DSCs can convert absorbed photons to electrons with yields of up to 96%, resulting in a sevenfold increase in energy conversion efficiency compared with previously reported photocathodes. The donor-acceptor dyes, studied as photocathodic sensitizers, comprise a variable-length oligothiophene bridge, which provides control over the spatial separation of the photogenerated charge carriers. As a result, charge recombination is decelerated by several orders of magnitude and tandem pn-DSCs can be constructed that exceed the efficiency of their individual components.

Dendritic oligothiophene-perylene bisimide hybrids: synthesis, optical and electrochemical properties

Novel dendritic oligothiophene-perylene bisimide hybrid (DOTPBI) systems were investigated. Perylene bisimide (PBI) derivatives with phenoxy spacers bearing dendritic oligothiophenes (DOT) up to the third generation (G3) in the bay position, with an increasing number of conjugated thiophene units have been synthesized and characterized as novel functional materials. Investigation of the optoelectronic properties revealed that the perylene core and dendritic oligothiophene units are electronically decoupled. In the DOTPBIs intramolecular fluorescence resonance energy transfer (FRET) and photoinduced electron transfer (PET) from the DOT to the PBI moiety have been investigated by various optical measurements, revealing that the PET process is facilitated with increasing DOT generation and donor strength. Electropolymerization led to interesting cross-linked donor acceptor-type conducting polymer films.

Energy migration in dendritic oligothiophene-perylene bisimides

A series of novel oligothiophene-perylene bisimide hybrid (DOTPBI) dendrimers up to the second generation (G0, G1, and G2) were investigated. Optical measurements such as nonlinear optical and time-resolved spectroscopy, including two-photon absorption, fluorescence upconversion, and excited state transient absorption were carried out. Results of these measurements revealed the ability of these molecules to undergo intramolecular fluorescence resonance energy transfer (FRET) from the dendritic oligothiophenes (DOT) to the perylene bismide (PBI) moiety. The delocalization length and the photoinduced electron transfer (PET) rate were investigated as a function of dendrimer generation. A fast energy transfer process from the DOT dendron to the PBI core was observed. For the case of the G2 dendrimer, with relatively large thiophene dendrons attached to the bay area of the perylene bisimide, the PBI core is highly twisted and its ability to self-assemble into π-π stacked aggregates is destroyed. As a result, among the three generations studied, G1, which has the best two-photon cross section and the most efficient energy transfer, is the best light harvesting material.

Core-functionalized dendritic oligothiophenes - novel donor-acceptor systems

Novel core-functionalized dendritic oligothiophenes (DOT) bearing pyridine or methylpyridinium acceptors have been synthesized up to the third generation. A bathochromic shift in the absorption of the functionalized dendritic oligothiophenes and the appearance of new absorption bands were noticed issuing from an intramolecular charge transfer process. These novel donor–acceptor systems are, due to their broad absorption, promising materials for optoelectronic applications such as organic solar cells. By electrochemical measurements HOMO/LUMO energy levels have been determined and two compounds could be identified with almost ideal energy levels for application in bulk heterojunction solar cells (BHJSC) with fullerene PC61BM as acceptor.

Charge and energy transfer processes in ruthenium(II) phthalocyanine based electron donor–acceptor materials—implications for solar cell performance

Six supramolecular electron donor–acceptor hybrids, based on a ruthenium(II) phthalocyanine [RuPc] coordinating different dendritic oligothiophene (DOT) ligands [Py-nT] (n = 3, 9, 21) in either one [RuPcCO(Py-nT)] or two [RuPc(Py-nT)2] axial positions, have been characterized by standard spectroscopic methods and their photophysical behavior has been established by using ultrafast and fast time-resolved techniques. Based on the spectrochemical and radiolytically generated [Py-nT] (i.e., one-electron reduction of [Py-nT]) and [RuPcCO(Py) or RuPc(Py)2] (i.e., one-electron oxidation of [RuPcCO(Py) or RuPc(Py)2]) features, the deactivation processes were assigned to a solvent independent energy transfer in RuPcCO(Py-3T) and RuPc(Py-3T)2 and a strongly solvent dependent charge transfer mechanism, which competes with the energy transfer and the intersystem crossing for RuPcCO(Py-9T), RuPc(Py-9T)2, RuPcCO(Py-21T) and RuPc(Py-21T)2.