Gisela L. Schulz

Correlation of π-Conjugated Oligomer Structure with Film Morphology and Organic Solar Cell Performance

The novel methyl-substituted dicyanovinyl-capped quinquethiophenes 1-3 led to highly efficient organic solar cells with power conversion efficiencies of 4.8-6.9%. X-ray analysis of single crystals and evaporated neat and blend films gave insights into the packing and morphological behavior of the novel compounds that rationalized their improved photovoltaic performance.

Dithienopyrrole-based oligothiophenes for solution-processed organic solar cells

Isomeric dicyanovinylene-terminated oligothiophenes 1 and 2 comprising a central dithienopyrrole (DTP) unit have been developed for solution-processed small molecule organic solar cells (SMOSCs) giving the highest power conversion efficiency of 4.8% for DTP-based oligomeric materials.

Understanding the effect of solvent vapor annealing on solution-processed A–D–A oligothiophene bulk-heterojunction solar cells: the role of alkyl side chains

Solution-processed bulk heterojunction solar cells consisting of the previously developed dithienopyrrole containing A–D–A oligothiophenes (A = acceptor, D = donor unit) and [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) with power conversion efficiency up to 7.1% after solvent vapor annealing (SVA) are demonstrated. The influence of the position of the alkyl side chains attached to the thiophene units on the SVA, and the usage of either PC61BM or PC71BM as acceptor, is investigated in more detail by negative secondary ion mass spectrometry (SIMS), Kelvin probe force microscopy (KPFM), photoluminescence (PL), and grazing-incidence X-ray diffraction spectroscopy (GIWAXS). It was found that besides increased crystallinity and domain sizes, the active layers consisting of two different isomers which we will refer to in the following as isomer 1 or isomer 2 had different compositions after SVA treatment. In the former, a more or less homogeneously-mixed D:A blend was observed, whereas the latter showed a vertical gradient of PCBM in the active layer and much stronger phase segregation on the surface. These findings correlate well with the differences in solar cell performance of both isomers, before and after SVA.

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.

2,2':3',2''-Terthiophene-based all-thiophene dendrons and dendrimers: synthesis, structural characterization, and properties.

The synthesis of generational dendritic oligothiophenes (DOTs) has been successfully achieved by a divergent/convergent approach that involves halogenation, boronation, and palladium-catalyzed Suzuki coupling reactions. The key point in the presented synthetic approach is the use of trimethylsilyl (TMS) protecting groups, which allow for the core-lithiation and subsequent boronation of the dendrons and for the peripheral ipso-substitution with iodine monochloride or N-bromosuccimide. In addition, the TMS protecting groups can be completely removed by using tetrabutylammonium fluoride, thus yielding only-thiophene-based dendrons and dendrimers. Due to their highly branched structure, all these synthesized DOTs are soluble in organic solvents. Chemical structures were confirmed by NMR spectroscopic, mass spectrometric, and elemental analysis. Concentration-dependent (1)H NMR spectroscopic investigations revealed that the higher generation compounds tend to aggregate in solution. Such an aggregation behavior was further confirmed by measuring with MALDI-TOF MS. Both MALDI-TOF MS and gel-permeation chromatography (GPC) analyses confirmed the monodispersity of the DOTs. Furthermore, GPC results revealed that these DOT molecules adopt a condensed globular molecular shape. Their optical and electronic properties were also investigated. The results indicated that these DOTs comprise various conjugated α-oligothiophenes with different chain lengths, which results in the higher generation compounds showing broad and featureless UV/Vis absorption spectra and ill-defined redox waves.

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.

A-D-A-Type Oligothiophenes Containing Benzothiadiazole Terminal Units for Small Molecule Organic Solar Cells

Abstract The electron-deficient, fused-heterocyclebenzo[c][1,2,5]thiadiazole (BTDA) is investigated as acceptor group in A-D-A-type oligothiophenes in order to correlate their relative acceptor strength with opto-electronic and photovoltaic properties. In this respect, two novel BDTA-capped oligothiopheneswere synthesized and characterized by optical and electrochemical measurements. They showed broad absorptions in the visible spectrum and HOMO-LUMO energies well suited for organic solar cells. The attachment of terminal BTDA acceptor units to the conjugated oligothiophene backbone resulted in a hypsochromic shift in UV-Vis absorption and larger band gap in comparison to previously reported analogous dicyanovinylene (DCV)-substituted oligothiophenes indicating that BDTA is a weaker acceptor than DCV. Vacuumprocessed m-i-p (metal-intrinsic-p-doped)-type bilayer solar cells using these co-oligomers as donor and C60 as acceptor gave moderate power conversion efficiencies of around 1.0%. Bulk-heterojunction (BHJ) solar cells prepared by solution-processing using fullerene PC61BM as acceptor generated slightly lower efficiencies of 0.9%, whichwere increased to 1.5% by using the higher fullerene PC71BM. It was found that the cell efficiencies were mostly limited by the low photocurrent densities due to moderate light absorption in the bilayer devices and low fill factors coming from inefficient charge transport in the solutionprocessed BHJ devices.