Organic solar cells based on a new oligothiophene derivative (see figure) and fullerene C60 exhibit power efficiencies of up to 3.4 %. α,α′‐Bis(2,2‐dicyanovinyl)‐quinquethiophene (DCV5T) features a reduced optical gap with high absorption coefficients between 450 and 650 nm. Dicyanovinyl substituents on the oligothiophene rings lower the bandgap and increase the ionization energy of these oligomers, enabling the formation of photoactive heterojunctions with C60 and yielding open‐circuit photovoltages as high as 1 V.

The best polymeric solar cells reported so far are based on a so‐called bulk heterojunction of a polythiophene as donor and a soluble fullerene derivative as acceptor. However, these cells still suffer from an unsatisfying photovoltage, typically below 0.7 V. Here, we show that we can achieve higher photovoltages using a new terthiophene end‐capped with electron withdrawing dicyanovinyl groups (DCV3T) that increase both the ionization energy and even more strongly the electron affinity of the compound. The new material is tested in cells using a photoactive heterojunction to separate the excitons generated in the oligomer and a p‐doped wide‐gap transport layer. The solar cells show an open circuit voltage of up to 1.04 V and a broad spectral sensitivity band ranging from 420 nm to 650 nm. Solar cells based on such oligothiophenes are promising candidates for stacked organic solar cells tailored to the sun‐spectrum. Moreover, we present first examples of a new concept for organic solar cells: By blending DCV3T with fullerene C60, an enhanced generation of triplet excitons on the oligomer can be achieved via a back and forth transfer of excitons (ping‐pong‐effect).

A power conversion efficiency of 3.4% with an open-circuit voltage of 1 V was recently demonstrated in a thin film solar cell utilizing fullerene C 60 as acceptor and a new acceptor-substituted oligothiophene with an optical gap of 1.77 eV as donor ͓K. Schulze et al., Adv. Mater. ͑Weinheim, Ger.͒ 18, 2872 ͑2006͔͒. This prompted us to systematically study the energy-and electron transfer processes at the oligothiophene:fullerene heterojunction for a homologous series of these oligothiophenes. Cyclic voltammetry and ultraviolet photoelectron spectroscopy data show that the heterojunction is modified due to tuning of the highest occupied molecular orbital energy for different oligothiophene chain lengths, while the lowest unoccupied molecular orbital energy remains essentially fixed due to the presence of electron-withdrawing end groups ͑dicyanovinyl͒ attached to the oligothiophene. Use of photoinduced absorption ͑PA͒ allows the study of the electron transfer process at the heterojunction to C 60. Quantum-chemical calculations performed at the density functional theory and/or time-dependent density functional theory level and cation absorption spectra of diluted DCVnT provide an unambiguous identification of the transitions observed in the PA spectra. Upon increasing the effective energy gap of the donor-acceptor pair by increasing the ionization energy of the donor, photoinduced electron transfer is eventually replaced with energy transfer, which alters the photovoltaic operation conditions. The optimum open-circuit voltage of a solar cell is thus a trade-off between efficient charge separation at the interface and maximized effective gap. It appears that the open-circuit voltages of 1.0-1.1 V in our solar cell devices have reached an optimum since higher voltages result in a loss in charge separation efficiency.

A new class of π-conjugated polycyclic hydrocarbons that promises interesting electronic properties is presented. The synthesis and extension of the S,N-heteroacene series consisting of only five-membered heterocyclic rings up to a very long, stable, and still soluble decacene SN10 is realized by multiple Pd-catalyzed aminations of halogenated thiophene precursors as key reactions. These novel heteroacenes were characterized by optical spectroscopy and electrochemistry providing interesting structure-property relationships. Nearly complete bond-length equalization in the inner part of the conjugated backbone and an unusual herringbone packing in the solid state underline the structural features of these novel systems.

The authors compare organic solar cells using two different transparent conductive oxides as anode: indium tin oxide (ITO) and three kinds of aluminum doped zinc oxide (ZAO). These anodes with different work functions are used for small molecule photovoltaic devices based on an oligothiophene derivative as donor and fullerene C-60 as acceptor molecule. It turns out that cells on ITO and ZAO have virtually identical properties. In particular, the authors demonstrate that the work function of the anode does not influence the V-oc of the photovoltaic device due to the use of doped transport layers