Organic Thin‐Film Photovoltaic Cells Based on Oligothiophenes with Reduced Bandgap

Abstract

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).

Enhanced Photogeneration of Triplet Excitons in an Oligothiophene–Fullerene Blend

“…Related work in our group has shown that such blend layers can be used in flat heterojunction solar cells where the triplet excitons are separated at the interface to a donor material into holes on the donor and electrons on C 60. [8] We therefore believe that the concept is of general significance and might open a route to highly efficient solar cells, if similar blend systems with high triplet diffusion lengths at room temperature are found.…”

Solution-processed bulk heterojunction organic solar cells based on an oligothiophene derivative

“…4 In recent years, solution processable BHJ organic solar cells based on conjugated small molecules and fullerenes have been extensively investigated. [5][6][7][8][9][10][11] Solution processable small molecules have numerous merits, such as, high chemical stability, high carrier mobility, well defined structures, easy synthesis and purification, intrinsically monodisperse, and few deep electron traps in solid films. [12][13][14] Oligothiophenes possess the above advantages, which makes them promising donors for solution processed BHJ solar cells when blend them with PCBM.…”

“…The absorption spectrum of DCN7T film spreads from 400 to 800 nm, with a peak at 614 nm, indicating that DCN7T absorbs solar energy more effectively than P3HT, which absorbs photons in the range of 400-650 nm with a peak at 522 nm. In addition, it is important to note that a shoulder at 670 nm suggests a vibronic progression enforced by strong intermolecular interactions among the thiophene rings and the cyanovinyl-substituents in DCN7T molecule, 7,19 which is redshifted compare with that of P3HT ͑at 600 nm͒. It is obvious that the blend films of DCN7T/PCBM exhibit broader absorption spectra than that of the pure DCN7T due to the complementary absorption spectra between the DCN7T and PCBM, implying an effective solar photon harvest.…”

A high performance semitransparent organic photodetector with green color selectivity

“…23) was used as an electron blocking layer to reduce the injected dark current density from the IZO anode at a reverse bias. [12][13][14] The molecular structures and energy levels of DMQA 24 and DCV3T 25 are depicted in Fig. 1(b).…”