Scientific Results


Impact of sputtered ZnO interfacial layer on the S-curve in conjugated polymer/fullerene based-inverted organic solar cells

In Thin Solid Films

The impact of crystalline structure changes of sputtered ZnO interfacial layer on performances of inverted organic solar cells (OSCs) has been investigated. We find that the structural modification of the ZnO cathode interfacial layer, obtained by thermal annealing, plays a crucial role in the origin and solving of the S-curve in conjugated polymer/fullerene photovoltaics. Our results show that the crystallization (i.e. crystallites size) of poly(3-hexylthiophene) (P3HT) evolves as a function of that of ZnO according to the annealing temperature. This evolution can directly impact the interfacial orientation and organization of the chains of P3HT at the ZnO buried interface. Such an ordered profile favors the vertical phase segregation and raises the carrier mobility, which explains the disappearance of the S-shape observed in current density-voltage device characteristics for annealing temperatures above 200 °C. These results adequately address recent research and provide an important insight into the interfacial layers of inverted OSCs.28.05.2014

Youssef Jouane, Silviu Colis, Guy Schmerber, Aziz Dinia, Paul Bazylewski, Gap Soo Chang, Yves-André Chapuis

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Determining the photocurrent of individual cells within an organic solar module by LBIC and the filtering approach: Experiments and simulations

In Solar Energy Materials and Solar Cells

Light beam induced current (LBIC) measurements of ITO-free organic photovoltaic modules with monolithic series connection are presented. Selective bias illumination is used to extract the light beam generated current from the module. A large variation of the photocurrent is observed among the cells which is unlikely to match the real generated photocurrent. In order to investigate this behavior, a novel approach (filtering approach) is presented in which the photocurrent of the measured cell is reduced while the remaining cells are kept under full illumination. This method enables the detection of the correct photocurrent of each cell and thus to identify the cell which limits the module current if all the cells comprise large parallel resistances. Dark lock-in thermography measurements of the module revealed several shunted cells. In this case the filtering approach overestimates the photocurrent. Numerical simulations of LBIC applied to modules using either selective bias illumination or forward bias voltage were carried out to understand the observed behavior in detail. The results reveal that a reliable detection of the photocurrent is impossible when several cells have rather low parallel resistances. Whereas for selective bias illumination the photocurrent response of the measured cell is an unambiguous function of the cell׳s parallel resistance this is not the case for an applied forward bias voltage.

Jens Reinhardt, Pälvi Apilo, Birger Zimmermann, Sanna Rousu, Uli Würfel

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Luminescent Properties and Energy Transfer in Pr3+ Doped and Pr3+-Yb3+ Co-doped ZnO Thin Films

In The Journal of Physical Chemistry

Embedding luminescent rare earth ions into transparent oxides such as ZnO is a well-known approach to functionalize the material by adding photon-management properties. In this paper we present a detailed study of the luminescence properties and energy level structure of Pr3+ and Yb3+ ions embedded in ZnO thin films deposited by magnetron reactive sputtering. Careful study of the photoluminescence and excitation spectra allowed identifying and locating almost all excited levels of Pr3+ and Yb3+ ions. Thus, an almost complete electronic energy level diagram of these ions in the ZnO crystal lattice can be drawn for the first time. In particular, we show that the crystal field of ZnO strongly modifies the energy level structure of the Pr3+ and Yb3+ ions creating energy mismatches between the transitions useful for cooperative down conversion. Finally, we demonstrate that light emission from Pr3+ ions can be induced both by direct excitation of the ions and indirectly by energy transfer from the ZnO matrix.

M. Balestrieri, M. Gallart, M. Ziegler, P. Bazylewski, G. Ferblantier, G. Schmerber, G. S. Chang, P. Gilliot, D. Muller, A. Slaoui, S. Colis, A. Dinia

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A deep-purple-grey thiophene–benzothiadiazole–thiophene BODIPY dye for solution-processed solar cells

In New Journal of Chemistry

In this work we explore the synthesis of an extended 4,4-difluoro-4-bora-3a,4a-diaza-s-indacene dye (BODIPY) engineered from thiophene–benzothiadiazole–thiophene modules linked in the 3,5-substitution positions. We found that this highly soluble dye absorbs up to 800 nm in solution and up to 900 nm in thin films. An effective charge transfer absorption band was found at around 479 nm. The hybrid dye emits at 778 nm with a quantum yield of about 6%. Similar electrochemical and optical gaps were determined about 1.36 eV. When deposited in thin films the dye exhibits an ambipolar nature with well-balanced hole and electron mobilities. Bulk heterojunction solar cells based upon this dye blended with [6,6]phenylC61or71butyricacid methylester (PC61BM or PC71BM) provide a power conversion efficiency of about 1.26% upon a mild thermal annealing.

Antoine Mirloup, Nicolas Leclerc,Sandra Rihn, Thomas Bura,Rony Bechara, Anne Hébraud, Patrick Lévêque, Thomas Heiser, Raymond Ziessel

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Systematic variation of the stabilizer to reduce the annealing temperature of sol–gel derived titanium oxide in inverted organic solar cells

In Organic Electronics

Solution-processed titanium oxide is used as an electron selective window layer in inverted organic solar cells. For the goal of roll-to-roll processed organic photovoltaics annealing temperatures of 150 °C and above have to be avoided. Therefore the influence of different stabilizers (complexing agents) on the required annealing temperature for the TiOx layers is investigated. A clear connection between the boiling points of the different complexing agents and the annealing temperature is observed. In total a series of four different stabilizers with boiling points ranging from 55 °C to 140 °C have been studied. By applying a low boiling complexing agent in the synthesis of the TiOx-nanoparticles the required annealing temperature could be brought down from 150 °C to 85 °C while maintaining a power conversion efficiency of 3.4% using a mixture of Poly(3-hexylthiophene-2,5-diyl) and [6,6]-phenyl-C61-butyric acid methyl ester as the photoactive layer.

M. Seßler, A. Saeed, M. Kohlstädt, U. Würfel

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Perylenediimide-Based Donor–Acceptor Dyads and Triads: Impact of Molecular Architecture on Self-Assembling Properties

In Journal of the American Chemical Society (JACS)

Perylenediimide-based donor–acceptor co-oligomers are particularly attractive in plastic electronics because of their unique electro-active properties that can be tuned by proper chemical engineering. Herein, a new class of co-oligomers has been synthesized with a dyad structure (AD) or a triad structure (DAD and ADA) in order to understand the correlations between the co-oligomer molecular architecture and the structures formed by self-assembly in thin films. The acceptor block A is a perylene tetracarboxyl diimide (PDI), whereas the donor block D is made of a combination of thiophene, fluorene, and 2,1,3-benzothiadiazole derivatives. D and A blocks are linked by a short and flexible ethylene spacer to ease self-assembling in thin films. Structural studies using small and wide X-ray diffraction and transmission electron microscopy demonstrate that AD and ADA lamellae are made of a double layer of co-oligomers with overlapping and strongly π-stacked PDI units because the sectional area of the PDI is about half that of the donor block. These structural models allow rationalizing the absence of organization for the DAD co-oligomer and therefore to draw general rules for the design of PDI-based dyads and triads with proper self-assembling properties of use in organic electronics.

Pierre-Olivier Schwartz, Laure Biniek, Elena Zaborova, Benoît Heinrich, Martin Brinkmann, Nicolas Leclerc, Stéphane Méry

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Large Scale Alignment and Charge Transport Anisotropy of pBTTT, Films Oriented by High Temperature Rubbing

In Macromolecules

A large-scale alignment method is used to orient the conjugated polymer poly(2,5-bis(3-dodecyl-2-yl)-thieno[3,2-b]thiophene) (C12-pBTTT). This fast one-step alignment method does not use any alignment layer and does not necessarily require postalignment annealing of the films. It exploits the increased plasticity of the conjugated polymer films for 50 °C ≤ T ≤ 125 °C to obtain high in-plane orientations by mechanical rubbing of the films. As visualized by HR-TEM, the in-plane alignment of C12-pBTTT chains and size of the oriented domains increase with the temperature of the film during rubbing (Tr). The domains have a preferential face-on orientation; i.e., the π-stacking direction is along the film normal and the chain direction parallel to the rubbing direction. Postrubbing annealing at T < 200 °C can further improve in-plane alignment whereas for T ≥ 200 °C, edge-on oriented C12-pBTTT crystals are formed. The anisotropy of hole transport for highly in-plane oriented face-on as well as edge-on oriented films was measured in OFET devices. Depending on the annealing conditions, this anisotropy of hole mobility varies in the range 7−70 with the highest mobilities along the rubbing direction and the highest anisotropies for the oriented face-on films.

Laure Biniek, Nicolas Leclerc, Thomas Heiser, Rony Bechara, and Martin Brinkmann

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Triazatruxene-Diketopyrrolopyrrole Dumbbell-Shaped Molecules as Photoactive Electron Donor for High-Efficiency Solution Processed Organic Solar Cells

In Advanced Energy Materials

Stepwise construction of functional triazatruxene platforms allow linking to preorganized diketopyrrolopyrrole bearing thienyl subunits. These dyes exhibit strong absorption until 750 nm in thin films and show favorable redox activity allowing producing efficient organic solar cells when blended in solution with PC71BM. A maximum power conversion efficiency of 5.3% was obtained.

Thomas Bura , Nicolas Leclerc, Rony Bechara, Patrick Lévêque, Thomas Heiser, and Raymond Ziessel

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Inverted ITO- and PEDOT:PSS-free polymer solar cells with high power conversion efficiency

In Solar Energy Materials and Solar Cells

Highest published power conversion efficiencies of organic solar cells have mostly been achieved on substrates bearing a transparent indium tin oxide (ITO) electrode. However, the incorporation of ITO is not suited for future industrial production processes of organic solar cells, which will rely on a high-throughput of flexible substrates in order to achieve low cost of the final product. In this manuscript we present an alternative transparent electrode consisting of a layer stack of aluminum doped zinc oxide and a thin silver layer. Substrates with these electrodes have a transparency of above 75% in the wavelength range in which the photoactive layer absorbs light. Solar cells with a bulk-heterojunction of PTB7 and PC71BM in an inverted device architecture achieved a power conversion efficiency of 6.1%, which is the highest reported value for polymer solar cells free from both ITO and PEDOT:PSS. The sheet resistance of the novel electrodes increased only marginally after repeated bending which shows their full compatibility with future reel-to-reel processes or flexible products.

Markus Kohlstädt, Maria Grein, Patrick Reinecke , Thomas Kroyer, Birger Zimmermann, Uli Würfel

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Impact of the arrangement of functional moieties within small molecular systems for solution processable bulk heterojunction solar cells

In New Journal of Chemistry

A series of molecules based on thienofluorene derivatives as electron donating (D) and benzothiadiazole as electron accepting (A) moieties have been assembled into DAD and ADA architectures in order to investigate the influence of the way of assembling the D and A units along the conjugated backbone, on the photovoltaic performances of bulk heterojunction solar cells. It was found that the major difference in going from ADA to DAD architecture is the change in molecular organization (nematic to crystalline), which increases the charge transport mobility and probably also affects the structural organization in blends with PCBM that ultimately leads to higher photovoltaic performances.

Pierre-Olivier Schwartz, Elena Zaborova, Rony Bechara, Patrick Lévêque, Thomas Heiser, Stéphane Méry, Nicolas Leclerc

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High-Performance Solution-Processed Solar Cells and Ambipolar Behavior in Organic Field-Effect Transistors with Thienyl-BODIPY Scaffoldings

In JACS – Journal of the American Chemistry Society

Green-absorbing dipyrromethene dyes engineered from bis-vinyl-thienyl modules are planar molecules, exhibiting strong absorption in the 713–724 nm range and displaying comparable electron and hole mobilities in thin films (maximum value 1 × 10–3 cm2/(V·s)). Bulk heterojunction solar cells assembled with these dyes and a fullerene derivative (PC61BM) at a low ratio give a power conversion efficiency as high as 4.7%, with short-circuit current values of 14.2 mA/cm2, open-circuit voltage of 0.7 V, and a broad external quantum efficiency ranging from 350 to 920 nm with a maximum value of 60%.

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