Organische Solarzellen. Energie der Zukunft

Photovoltaic solar cells are of increasing importance in the use of regenerative energies due to the high supply of solar radiation. Therefore beside the established inorganic solar cells more low costs solar cells are developed which contain organic materials as active compounds for energy conversion. The article describes construction and function of dye‐sensitized solar cells and organic solid state solar cells. For comparison and understanding of these cells it is necessary to mention also some aspects of photosynthesis and inorganic solar cells. Altogether an insight in solar energy conversion systems under consideration of current developments is presented.     

Redox-Active Ligand Assisted Catalytic Water Oxidation by a RuIV=O Intermediate

Water splitting is one of the most promising solutions for storing solar energy in a chemical bond. Water oxidation is still the bottleneck step because of its inherent difficulty and the limited understanding of the O−O bond formation mechanism. Molecular catalysts provide a platform for understanding this process in depth and have received wide attention since the first Ru-based catalyst was reported in 1982. Ru^V=O is considered a key intermediate to initiate the O−O bond formation through either a water nucleophilic attack (WNA) pathway or a bimolecular coupling (I2M) pathway. Herein, we report a Ru-based catalyst that displays water oxidation reactivity with Ru^IV=(O) with the help of a redox-active ligand at pH 7.0. The results of electrochemical studies and DFT calculations disclose that ligand oxidation could significantly improve the reactivity of Ru^IV=O toward water oxidation. Under these conditions, sustained water oxidation catalysis occurs at reasonable rates with low overpotential (ca. 183 mV).     

A Ruthenium Complex–Porphyrin–Fullerene‐Linked Molecular Pentad as an Integrative Photosynthetic Model

A ruthenium complex, porphyrin sensitizer, fullerene acceptor molecular pentad has been synthesized and a long‐lived hole–electron pair was achieved in aqueous solution by photoinduced multistep electron transfer: Upon irradiation by visible light, the excited‐state of a zinc porphyrin (¹ZnP*) was quenched by fullerene (C₆₀) to afford a radical ion pair, ^1,3(ZnP^.+‐C₆₀^.−). This was followed by the subsequent electron transfer from a water oxidation catalyst unit (Ru^II) to ZnP^.+ to give the long‐lived charge‐separated state, Ru^III‐ZnP‐C₆₀^.−, with a lifetime of 14 μs. The ZnP worked as a visible‐light‐harvesting antenna, while the C₆₀ acted as an excellent electron acceptor. As a consequence, visible‐light‐driven water oxidation by this integrated photosynthetic model compound was achieved in the presence of sacrificial oxidant and redox mediator.     

Time‐Resolved Interception of Multiple‐Charge Accumulation in a Sensitizer–Acceptor Dyad

Biomimetic models that contain elements of photosynthesis are fundamental in the development of synthetic systems that can use sunlight to produce fuel. The critical task consists of running several rounds of light‐induced charge separation, which is required to accumulate enough redox equivalents at the catalytic sites for the target chemistry to occur. Long‐lived first charge‐separated state and distinct electronic signatures for the sequential charge accumulated species are essential features to be able to track these events on a spectroscopic ground. Herein, we use a double‐excitation nanosecond pump–pump–probe experiment to interrogate two successive rounds of photo‐induced electron transfer on a molecular dyad containing a naphthalene diimide (NDI) linked to a [Ru(bpy)₃]²⁺ (bpy=bipyridine) chromophore by using a reversible electron donor. We report an unprecedented long‐lived two‐electron charge accumulation (t =200 μs).     

Electrochemically Gated Long‐Distance Charge Transport in Photosystem I

The transport of electrons along photosynthetic and respiratory chains involves a series of enzymatic reactions that are coupled through redox mediators, including proteins and small molecules. The use of native and synthetic redox probes is key to understanding charge transport mechanisms and to the design of bioelectronic sensors and solar energy conversion devices. However, redox probes have limited tunability to exchange charge at the desired electrochemical potentials (energy levels) and at different protein sites. Herein, we take advantage of electrochemical scanning tunneling microscopy (ECSTM) to control the Fermi level and nanometric position of the ECSTM probe in order to study electron transport in individual photosystem I (PSI) complexes. Current–distance measurements at different potentiostatic conditions indicate that PSI supports long‐distance transport that is electrochemically gated near the redox potential of P700, with current extending farther under hole injection conditions.