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ABSTRACT To obtain novel low-bandgap materials with tailored hole-transport properties and extended absorption, electron rich 3,4-ethylenedioxythiophene is introduced as a comonomer in diketopyrrolo[3,4- c]pyrrole copolymers with different aryl flanking units. The polymers are characterized by absorption and photoluminescence spectroscopy, dynamic scanning calorimetry, cyclic voltammetry, and X-ray diffraction. The charge transport properties of these new materials are studied carefully using an organic field effect transistor geometry where the charge carriers are transported over a narrow channel at the semiconductor/dielectric interface. These results are compared to bulk charge carrier mobilities using space-charge limited current (SCLC) measurements, in which the charge carrier is transported through the complete film thickness of several hundred nanometers. Finally, charge carrier mobilities are correlated with the electronic structure of the compounds. We find that in particular the thiophene-flanked copolymer PDPP[T]2-EDOT is a very promising candidate for organic photovoltaics, showing an absorption response in the near infrared region with an optical bandgap of 1.15 eV and a very high bulk hole mobility of 2.9 × 10−4 cm2 V−1 s−1 as measured by SCLC. This value is two orders of magnitudes higher than SCLC mobilities reported for other polydiketopyrrolopyrroles and is in the range of the well-known hole transporting polymer poly(3-hexylthiophene). © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016, 54, 639-648 [ABSTRACT FROM AUTHOR]

The influence of different electron extracting interlayers such as calcium (Ca), zirconium acetylacetonate (ZrAcac), and a polyfluorene derivative (PFN) in combination with an aluminum (Al) cathode is investigated on the performance of bulk-heterojuntion solar cells. Two different photoactive systems, P3HT:PC61BM and PDPP:PC71BM, are selected for this study. The electroabsorption measurements have been carried out for obtaining the built-in voltage ( Vbi) and transfer matrix simulations for the determination of parasitic absorption. The solar cell performance is influenced by different parameters such as diode turn-on voltage, leakage currents, built-in voltages, and parasitic absorption. The small diode turn-on voltage and high parasitic absorption in Ca contact devices limit the open circuit voltage and short circuit current, respectively. Likewise, high leakage currents using ZrAcac contact limit the fill factor in P3HT:PC61BM solar cell devices. However, the PFN-based devices with small parasitic absorption, smaller leakage currents, and a relatively high Vbi show maximum performance with both material systems. This work highlights the importance of choosing the suitable interlayers in device optimization and clearly demonstrates that not only the low work function of an electron extracting interlayer but also its optical properties and charge selectivity significantly influence the final solar cell performance. [ABSTRACT FROM AUTHOR]