Effect of Morphological Changes on Presence of Trap States in P3HT:PCBM Solar Cells Studied by Cross Sectional Energy Filtered TEM and Thermally Stimulated Current Measurements; Anna L. Domanski, Ingo Lieberwirth, Esha Sengupta, Katharina Landfester, Hans-Jürgen Butt, and Rüdiger Berger, Julia Rauh, Vladimir Dyakonov, and Carsten Deibel; J. Phys. Chem.; 2013, 117, 22495 - 23499.
DOI 10.1021/jp406014m
In this contributione we investigated the effect of thermal annealing on the performance of organic solar cell based on P3HT and PCBM. We discuss our results of energy-filtered transmission electron microscopy (EFTEM) and thermally stimulated current (TSC) measurements. We applied EFTEM to cross sections of pristine and thermally annealed organic solar cells to probe the sulfur concentration as measure for the P3HT distribution within the photoactive layer, and thus resolve its phase separation. We were able to attribute signals of the TSC measurements to an increased number of P3HT rich domains present in the annealed device.
Electrical tip-sample contact in scanning conductive torsion mode; Stefan Weber, Rüdiger Berger; Appl. Phys. Lett., 2013, 102, 163105.
DOI 10.1063/1.4802725
A gentle and well-defined contact is a prerequisite for reproducible scanning probe based conductivity measurements, in particular on soft organic materials. This paper describes the nature of the mechanical and the electrical tip-sample contact in scanning conductive torsion mode microscopy (SCTMM). Spectroscopy and imaging experiments showed that SCTMM allows for a gentler tip-sample contact compared to conventional conductive scanning force microscopy.
Electrical characterization of organic solar cell materials based on scanning force microscopy; Rüdiger Berger, Anna L. Domanski, Stefan A.L. Weber; Eur. Polym. J., 2013, 49, 1907-1915.
DOI 10.1016/j.eurpolymj.2013.03.005
This paper reviews the application of electrical modes in scanning probe microscopy which is of eminent importance for understanding the electrical function and interaction of materials that are structured on the nanometer scale. Many researchers use the scanning probe microscope (SPM) for the investigation of surface topography. Here, we accentuate the use of electrical modes that are unique for the correlation of structural and electric information on a nanometer scale. Special focus is given to experiments aiming at the investigation of light-induced processes which requires the integration of an additional light source into the SPM setup.
Push-Pull Design of Bis(tridentate) Ruthenium(II) Polypyridine Chromophores as Deep Red Light Emitters in Light-Emitting Electrochemical Cells; Aaron Breivogel, Myeongjin Park, Donggu Lee, Stefanie Klassen, Angelika Kühnle, Changhee Lee, Kookheon Char, Katja Heinze; Eur. J. Inorg. Chem., 2014, 2, 288–295.
DOI 10.1002/ejic.201301226
Light-emitting electrochemical cells (LECs) with a simple device structure were prepared by using heteroleptic bis(tridentate) ruthenium(II) complexes [1](PF6)2–[3](PF6)2 as emitters. The push-pull substitution shifts the emission energy to low energy, into the NIR region. The devices emit deep red light up to a maximum emission wavelength of 755 nm [CIE (International Commission on Illumination) coordinates: x = 0.731, y = 0.269 for [3](PF6)2], which, to the best of our knowledge, is the lowest emission energy for LECs containing bis(tridentate) ruthenium(II) complexes. A device structure of ITO/PEDOT:PSS/ruthenium(II) complex/Ag was used, and the thickness of the emitting layer was measured by AFM [ITO: indium tin oxide, PEDOT: poly(3,4-ethylenedioxythiophene), PSS: poly(styrenesulfonate), AFM: atomic force microscopy]. To enhance the external quantum efficiency (EQE), cells were fabricated with and without poly(methyl methacrylate) (PMMA) as additive in the emitting layer.
Controlled Synthesis of CdSe Tetrapods with High Morphological Uniformity by the Persistent Kinetic Growth and the Halide-Mediated Phase Transformation; Jaehoon Lim, Wan Ki Bae, Ko Un Park, Lisa zur Borg, Rudolf Zentel, Seonghoon Lee, Kookheon Char; Chem. Mater., 2013, 25, 1443−1449.
DOI 10.1021/cm3035592
We report scalable controlled synthesis of CdSe tetrapods with high morphological uniformity based on the continuous precursor injection (CPI) approach with halide ligands. The CPI approach involves the successive injection of precursors into the seed solution at a controlled rate so that the reaction condition remains in the kinetic growth regime. To initiate the successful development of tetrapod structure, the controlled amount of halide ligands are added during the reaction, which triggered the formation of wurtzite arms on {111}-facets of the zincblende seeds. The formation of the wurtzite phase is responsible for the halide-mediated displacement of oleate ligands, destabilizing the embryonic CdSe phase on the zincblende seeds enabling the phase transformation to more stable wurtzite phase on the hexagonal {111}-facets. On the basis of these halide-mediated phase transformation of CdSe nanocrystals and the persistent kinetic growth induced by the CPI approach, well-defined CdSe tetrapods with controlled arm length and diameter have been produced in large quantity.
Photoinduced Charge Separation of Self-Organized Semiconducting Superstructures Composed of a Functional Polymer–TiO2 Hybrid; Lisa zur Borg, Anna L. Domanski, Rüdiger Berger, Rudolf Zentel; Macromol. Chem. Phys., 2013, 214, 975–984.
DOI 10.1002/macp.201300087
The incorporation of TiO2 nanoparticles into long (several micrometers) fibers of a gel-forming poly(para-phenylene vinylene) (PPV) derivative is described. For that purpose, a PPV-block-dopamine polymer is synthesized, which is used to coat TiO2 nanoparticles leading to well-dispersed nanoparticles and fluorescence quenching of the polymer. Independently, the homopolymer gels in various organic solvents are observed to form long fibers. By mixing the PPV homopolymer with the TiO2-PPV hybrid, we are able to incorporate the nanoparticles into the fibrous network, as shown in transmission electron microscopy (TEM) images. Photoinduced charge separation is measured by Kelvin probe force microscopy. Upon illuminating the polymer fibers, positive charges are observed, even at distances of around 300 nm. From these results, charge transport along the polymer fibers is inferred.
The effect of band gap alignment on the hole transport from semiconducting block copolymers to quantum dots; Lisa zur Borg, Donggu Lee, Jaehoon Lim, Wan Ki Bae, Myeongjin Park, Seonghoon Lee, Changhee Lee, Kookheon Char, Rudolf Zentel; J. Mater. Chem.C, 2013, 1, 1722-1726.
DOI 10.1039/C3TC00837A
Semiconducting hole transporting block copolymers were chemically modified to adjust their energy levels to that of CdSe/CdS/CdZnS red quantum dots. Hybrids with optimized energy levels could be used to build strongly improved quantum dot based LEDs (QLEDs).
Light-induced charge separation in a donor–chromophore–acceptor nanocomposite poly[TPA-Ru(tpy)2]@ZnO; Lisa zur Borg, Anna L. Domanski, Aaron Breivogel, Mareike Bürger, Rüdiger Berger, Katja Heinze, Rudolf Zentel; J. Mater. Chem.C, 2013, 1, 1223-1230.
DOI 10.1039/C2TC00535B
The synthesis and characterisation of a new donor–chromophore–acceptor system based on poly(vinyltriphenylamine) as the electron donor and a glycine-functionalised bis(2,2′;6′,2′′-terpyridine)ruthenium(II) complex acting both as a chromophore and as an anchor group attached to ZnO nanorods as the electron acceptor are described. The TPA-containing block copolymer was synthesised by Reversible Addition Fragmentation Chain Transfer (RAFT) polymerisation and the ruthenium complex glycine conjugates prepared by Solid Phase Peptide Synthesis (SPPS) were attached via post-polymerisation esterification. GPC, NMR, IR and UV-Visible spectroscopy were used to characterise the multifunctional chromophore–donor polymer. Zinc oxide nanorods were functionalised with the block copolymer by multisite adsorption via the glycine COOH groups. The functionalised nanoparticles were well dispersible in organic solvents. Photoluminescence studies showed a complete quenching of the phosphorescence of the ruthenium chromophore (3MLCT state). Kelvin probe force microscopy (KPFM) was used to confirm that under continuous excitation of the RuII complex (1MLCT) the polymer corona develops a positive charge and thus efficient charge separation between ZnO and the polymer is achieved.