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.

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.

Es geht auch ohne Ru: Farbstoffsensibilisierte Solarzellen, die metallfreie organische Farbstoffe enthalten, wurden in den vergangenen Jahren entscheidend verbessert. Mittlerweile haben sich verschiedene Strategien bei der Konstruktion der Farbstoffe etabliert. In diesem Aufsatz werden Struktur‐Eigenschafts‐Effizienz‐Beziehungen aus der Vielzahl publizierter Farbstoffe abgeleitet, die dabei helfen sollen, hocheffiziente Sensibilisatoren zu entwerfen. Farbstoffsensibilisierte Solarzellen (dye‐sensitized solar cells, DSSCs) haben in den letzten Jahren beachtliche Aufmerksamkeit auf sich gezogen, weil sie eine Möglichkeit zur kostengünstigen Produktion von Solarenergie darstellen. In diesem Aufsatz konzentrieren wir uns auf aktuelle Fortschritte bei der Entwicklung von funktionellen metallfreien Farbstoffen und deren Anwendung in farbstoffsensibilisierten Solarzellen mit unterschiedlichen Elektrolytsystemen. Ebenso wird die Cosensibilisierung, eine Technik zur Erweiterung des Absorptionsbereichs, als eine Möglichkeit erläutert, die Leistungsfähigkeit von Solarzellen zu verbessern. Zusätzlich berichten wir über invertierte Farbstoffe, die in Photokathoden Verwendung finden und damit einen relativ neuen Ansatz für die Herstellung von Tandemzellen darstellen. Besonderer Wert wurde darauf gelegt, Beziehungen zwischen der Molekülstruktur, den daraus resultierenden physikalischen Eigenschaften und der Leistungsfähigkeit in Solarzellen herauszuarbeiten.

Within the present work, two series of novel ruthenium(II) phthalocyanine (RuPc) complexes with one [RuPcCOPy-nT] or two [RuPc(Py-nT)(2)] dendritic oligothiophene (DOT) ligands in the axial positions are reported. The ability of Ru(II) for axial coordination in RuPcs allowed the attachment of the Pc through the metal site to the DOT-ligands bearing pyridine at the core position of the dendrons. These extended pyridine functionalized conjugated DOT-ligands (Py-nT) were chosen to cover the spectral window between 380 and 550 nm, where the RuPc does not exhibit a strong absorption, in order to improve the light-absorption of these complexes and hence enhance the efficiency of the corresponding solar cells. Good efficiencies of up to 1.6% have been achieved when blended together with a fullerene acceptor in solution-processed photovoltaic devices, providing by far the best phthalocyanine-based bulk heterojunction solar cells reported to-date.