Marta Urdanpilleta

Functional tuning of A–D–A oligothiophenes: the effect of solvent vapor annealing on blend morphology and solar cell performance

A series of solution-processable acceptor–donor–acceptor (A–D–A) oligomers consisting of various central conjugated units, namely, carbazole, benzo[2,1-b:3,4-b′]dithiophene, 2,2′-bithiophene, dithieno[3,2-b:2′,3′-d]silole and dithieno[3,2-b:2′,3′-d]pyrrole were synthesized and developed for application in bulk-heterojunction solar cells (BHJSC). The alteration of the core moiety, while maintaining the shape of the molecular structure, enables fine-tuning of the optical energy gap and highest occupied molecular orbital (HOMO) level of the molecules. Depending on the donor strength of the core, the maximum absorption wavelength of the oligomers ranged from 488 nm to 560 nm in solution and from 530 nm to 694 nm in neat films. HOMO energy levels were shifted in a stepwise fashion from −5.8 to −5.3 eV yielding oligomers with HOMO–LUMO energy gap between 2.04 and 1.60 eV. The structural fine-tuning is further visible in the photovoltaic performance. BHJ solar cells prepared using these oligomers as donor and [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM) as acceptor demonstrated power conversion efficiencies between 1.4 and 5.9% after solvent vapor annealing. Exposure of the photoactive layer to organic solvent vapor led to re-organization of the donor material within the blend and a large enhancement of JSC and FF was observed. The role of solvent vapor annealing on the degree of crystallinity and blend morphology was further investigated by grazing incident X-ray diffraction (GIXRD) and atomic force microscopy (AFM) analysis.

STM study on the self-assembly of oligothiophene-based organic semiconductors.

Summary The self-assembly properties of a series of functionalized regioregular oligo(3-alkylthiophenes) were investigated by using scanning tunneling microscopy (STM) at the liquid–solid interface under ambient conditions. The characteristics of the 2-D crystals formed on the (0001) plane of highly ordered pyrolitic graphite (HOPG) strongly depend on the length of the π-conjugated oligomer backbone, on the functional groups attached to it, and on the alkyl substitution pattern on the individual thiophene units. Theoretical calculations were performed to analyze the geometry and electronic density of the molecular orbitals as well as to analyze the intermolecular interactions, in order to obtain models of the 2-D molecular ordering on the substrate.

The influence of alkyl side chains on molecular packing and solar cell performance of dithienopyrrole-based oligothiophenes

The synthesis of a series of dithieno[3,2-b:2′,3′-d]pyrrole (DTP) containing A–D–A oligomers for use in solution-processed bulk heterojunction solar cells is presented. This series allows investigation of the effect of alkyl chains attached to the thiophene moieties or the nitrogen of the DTP unit on the thermal and physical properties of the oligomers. The photoactive layers were probed using absorption spectroscopy, grazing incident X-ray diffraction, and atomic force microscopy. From these experiments, it was found that the hexyl chains attached to the thiophene units are instrumental in obtaining good packing and high fill factors in the solar cell devices. For the fabrication of solar cells, both PEDOT:PSS and V2O5 were investigated as hole transport layers and PDMS was employed as a solvent additive. High open-circuit voltages of up to 1.1 V, moderate short-circuit current densities, and high fill factors of up to 0.63 were obtained yielding power conversion efficiencies as high as 5.3%.

Optimization of solution-processed oligothiophene:fullerene based organic solar cells by using solvent additives.

Summary The optimization of solution-processed organic bulk-heterojunction solar cells with the acceptor-substituted quinquethiophene DCV5T-Bu 4 as donor in conjunction with PC61BM as acceptor is described. Power conversion efficiencies up to 3.0% and external quantum efficiencies up to 40% were obtained through the use of 1-chloronaphthalene as solvent additive in the fabrication of the photovoltaic devices. Furthermore, atomic force microscopy investigations of the photoactive layer gave insight into the distribution of donor and acceptor within the blend. The unique combination of solubility and thermal stability of DCV5T-Bu 4 also allows for fabrication of organic solar cells by vacuum deposition. Thus, we were able to perform a rare comparison of the device characteristics of the solution-processed DCV5T-Bu 4:PC61BM solar cell with its vacuum-processed DCV5T-Bu 4:C60 counterpart. Interestingly in this case, the efficiencies of the small-molecule organic solar cells prepared by using solution techniques are approaching those fabricated by using vacuum technology. This result is significant as vacuum-processed devices typically display much better performances in photovoltaic cells.

Assessment of gallic acid-modified fish gelatin formulations to optimize the mechanical performance of films

In this study, a response surface methodology (RSM) using a Box-Behnken design was applied to optimize the mechanical response (tensile strength, elongation at break and Youngs modulus) of fish gelatin films. These responses were analyzed as a function of glycerol content (0-10% on gelatin basis), added as a plasticizer, gallic acid content (5-15% on gelatin basis), used as crosslinker, and solution pH (4.5-10). Second order polynomial models were adjusted for the three responses, and they were found to be reliable according to the standard statistical analysis. The values of the independent factors that maximize the responses were also determined. In order to relate mechanical performance to material structure, Fourier transform infrared (FTIR) analysis was carried out and this revealed that a reaction occurs between gelatin and gallic acid through a process that releases water and provides a plasticizing effect. The performed time-, material- and cost-saving optimization of the formulation based on biodegradable compounds from abundant renewable resources enabled a sustainable approach to the development of new materials.