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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]

In the quest for indium-tin oxide (ITO)-free photovoltaics and for building integrated as well as automobile roof applications, novel transparent electrodes for both front and back electrodes are required. Here we report the fabrication and integration of submicrometer transparent silver (Ag) and gold (Au) metal network electrodes, which are invisible to the naked eye, in organic photovoltaic devices. We exploit the idea of using the spontaneous cracking of a polymer layer as template to prepare the metal network. The main challenge is to apply the cracked template approach on top of soluble organic layers and to lift off the template without damaging the photoactive layer. We demonstrate that Ag or Au back electrodes can be fabricated maintaining a transmittance of 80 % for the whole visible range. These electrodes exhibit ultralow haze of approximately 5 % and an excellent figure of merit value. Moreover, the ITO-free semitransparent polymer solar cell incorporating the Ag/Ag network electrodes exhibits 57 % transmittance above 650 nm. [ABSTRACT FROM AUTHOR]

The question of designing high electron mobility polymers by increasing the planarization using diffusive nonbonding heteroatom interactions in diketopyrrolopyrrole polymers is addressed in this. For this, three different diketopyrrolo[3,4- c]pyrrole (DPP) derivatives with thienyl-, 2-pyridinyl-, and phenyl-flanked cores are copolymerized with an electron-rich thiophene unit as well as an electron-deficient 3,4-difluorothiophene unit as comonomer to obtain diverse polymeric DPPs which vary systematically in their structures. The crystallinity differs significantly with clear trends on varying both flanking unit and comonomer. The optical gap and energy levels depend more on the nature of the flanking aryl units rather than on fluorination. Additionally, the charge transport properties are compared using different methods to differentiate between interface or orientation effects and bulk charge carrier transport. In organic field effect transistor devices with very high electron as well as hole mobilities (up to 0.6 cm2 V−1 s−1) are obtained and fluorination leads to a more pronounced n-type nature in all polymers, resulting in ambipolar behavior in otherwise p-type materials. In contrast, space-charge limited current measurements show a strong influence of the flanking units only on electron mobilities. Especially, the elegant synthetic strategy of combining pyridyl flanking units with difluorothiophene as the comonomer culminates in a record bulk electron mobility of 4.3 × 10−3 cm2 V−1 s−1 in polymers. [ABSTRACT FROM AUTHOR]

This study correlates the device performance of organic solar cells and the electronic charge transport within polymer/fullerene films, directly to the optical order of the polymer. The optical order was measured by spectroscopic ellipsometry and evaluated by our previously derived model. We were able to determine the in-depth distribution of higher and lower ordered poly(3-hexylthiophene) (P3HT) domains within an [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) matrix. The over the film thickness integrated volume fraction of highly ordered P3HT domains could be directly correlated to the corresponding solar cell device performance. We are able to describe various thermally annealing conditions between room-temperature and 200 °C. [ABSTRACT FROM AUTHOR]

ABSTRACT A series of well-defined poly(3-hexylthiophene)s (P3HT) of different molecular weight (MW) and high regioregularity was investigated for charge transport properties in as-cast and melt-crystallized films. The semicrystalline structure of the P3HT was characterized by X-ray scattering and Atomic force microscopy. Crystallization by cooling from the melt led to a substantial increase in crystallinity and a stronger alignment of the crystals in comparison to as-cast films. The increase in crystallinity went along with an increase in hole mobility of up to an order of magnitude as measured by the space charge limited current method. Additionally, the hole mobility depended on the long period of P3HT lamellae and consequently on the MW. In compliance with the long period, the charge carrier mobility first increased with the MW before decreasing again at the onset of chain folding. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2013, 51, 943-951 [ABSTRACT FROM AUTHOR]