Hartwig Tillmann

Polymer—perylene diimide heterojunction solar cells

Thin-film small molecule/polymer hybrid bilayer photovoltaic cells have been constructed, exhibiting power conversion efficiencies of 0.71% under 80 mW/cm2 white light illumination. The parameters influencing the photovoltage of these devices are explored by reversing the order of the photoactive layers while maintaining the same electrode configuration. It has been found that the properties of the organic photoactive layers play an important role in determining the direction of current flow and the photovoltage of the device. Comparison is made to analogous pure small molecule bilayer devices, and conclusions about some of the factors influencing device efficiency are drawn. It has been shown that ordering of the band offsets of the two organic materials plays an important role in determining the polarity of the photocurrent and the photovoltage of the device.

Near-infrared sensitivity enhancement of photorefractive polymer composites by pre-illumination

Among the various applications for reversible holographic storage media, a particularly interesting one is time-gated holographic imaging (TGHI). This technique could provide a noninvasive medical diagnosis tool, related to optical coherence tomography. In this technique, biological samples are illuminated within their transparency window with near-infrared light, and information about subsurface features is obtained by a detection method that distinguishes between reflected photons originating from a certain depth and those scattered from various depths. Such an application requires reversible holographic storage media with very high sensitivity in the near-infrared. Photorefractive materials, in particular certain amorphous organic systems, are in principle promising candidate media, but their sensitivity has so far been too low, mainly owing to their long response times in the near-infrared. Here we introduce an organic photorefractive material—a composite based on the poly(arylene vinylene) copolymer TPD-PPV—that exhibits favourable near-infrared characteristics. We show that pre-illumination of this material at a shorter wavelength before holographic recording improves the response time by a factor of 40. This process was found to be reversible. We demonstrate multiple holographic recording with this technique at video rate under practical conditions.

Synthesis and Properties of Novel Well-Defined Alternating PPE/PPV Copolymers

The polycondensation reactions of luminophoric dialdehydes 5 and bisphosphonates 3 provide a new type of π-conjugated polymers 7 with well-defined structure (—Ar—C≡C—Ar—C≡C—Ar—CH=CH—Ar— CH=CH—)n , which was confirmed by NMR, infrared and UV-Vis spectroscopy. High molecular weight (Mw up to 500 000 g/mol), thermostable, soluble and transparent film-forming materials were obtained. The grafting of large alkoxy side chains enhances the solubility and processability of the new compounds. The incorporation of triple bonds into the PPV backbone increases the oxidation and reduction potentials, thus making these polymers potentially good electron-transporting materials if used in light-emissive-diode devices. The polymers are photoconductive and show very good photoluminescent properties in solution as well as in the solid state. Identical absorption (λmax,abs = 445 nm) and emission (λmax,em = 490 nm) behaviors were observed for all polymers in solution (CHCl3), resulting in a fluorescence quantum yield of 70%, but the photophysical behavior in solid state was side group dependent. Polymer 7ac shows a very large Stokes shift (137 nm) and lower fluorescence quantum yield (19%), whereas 7aa, 7bb and 7ab, consisting of side groups equal to or longer than O(CH2)11CH3 are characterized by smaller Stokes shifts (around 30 nm) and comparatively higher fluorescence quantum yields (34 to 44%).

Poly(p-phenylenevinylene) derivatives: new promising materials for nonlinear all-optical waveguide switching

Several new derivatives of poly(p-phenylenevinylene) (PPV) are investigated regarding their linear and nonlinear optical material and waveguide properties, including their nonlinear photonic bandgap properties that are induced by photoablated periodic Bragg gratings. The new materials were prepared by means of the polycondensation route, which yields polymers with excellent solubilities and film-forming properties. Comparative data suggest that the new polycondensation-type MEH-PPV (completely soluble, strictly linear and fully conjugated), in particular, is the most promising polymer under investigation to fulfill the requirements for all-optical switching in planar waveguide photonic bandgap structures. UV-photobleaching techniques and photoablation in the UV, VIS, and near-infrared ranges at different pulse durations are investigated. Homogeneous submicrometer gratings that serve as Bragg reflectors have been fabricated in MEH-PPV thin films by application of these methods. The great potential of this type of materials for nonlinear all-optical switching applications that arises from their unique optical properties and their patterning behavior is discussed in detail. Numerical simulations of a switching device based on gap-soliton formation in a nonlinear periodic waveguide structure with the newly obtained material data have been carried out. We show that one can expect photonic bandgap all-optical switching in MEH-PPV planar waveguides. Device performance considering different grating parameters is discussed.

Amplified spontaneous emission and optical gain spectra from stilbenoid and phenylene vinylene derivative model compounds

We report single-pass optical gain measurement in poly(styrene) waveguides doped with stilbenoid and phenylene vinylene derivative model compounds under picosecond excitation. Using a wavelength dependent model of amplified spontaneous emission, we produce optical gain spectra for these materials. Net optical gains of g=15–20 cm−1 are deduced. The spectra also exhibit features consistent with excited state absorption and fluorescence depletion via stimulated emission. A figure of merit describing the potential of a material for use as a laser medium is presented and comparisons with other recent publications are made. This comparison indicates that organic materials can compete with inorganic semiconductors for optically pumped applications.

Distributed feedback laser action from polymeric waveguides doped with oligo phenylene vinylene model compounds

We report lasing studies of poly(styrene) waveguides doped with amino- and cyano-substituted oligo phenylene vinylene (distyryl benzene) model compounds under picosecond excitation. Optical feedback is provided by distributed Bragg gratings formed in the film by interference patterns in the pump beam. We demonstrate broad tunability of laser emission in these materials and simultaneous lasing at two wavelengths separated by 23 nm. Tuning ranges of the model compounds are compared with previous experiments.

Spectroscopic and travelling-wave lasing characterisation of Gilch-type and Horner-type MEH-PPV

Abstract Two different types of poly(2-methoxy-5-(2′-ethyl-hexyloxy)-1,4-phenylene-vinylene) (MEH-PPV) (i) a strictly linear, well-defined material of moderate molar mass prepared through polycondensation (Horner-type) and (ii) a commercially available MEH-PPV-sample (Gilch-type, high-molecular and defect-rich) resulting from a dehydrohalogenation process, are carefully analysed by GPC and 13 C / 1 H -NMR spectroscopy. These studies show that for the Gilch-type polymer there is a marked deficiency of regular vinylene groups (≈30%), and therefore, a lack of long-range polyconjugation. The samples are characterized by thin-film optical constants measurements, absorption and fluorescence spectroscopic studies, and thin-film wave-guided travelling-wave laser action investigations. The influence of film-forming solvents (chlorobenzene (CB), tetrahydrofuran (THF)) and of film heating on the absorption, emission and laser behaviour is investigated. The fluorescence quantum yields of neat films (≈25% for Horner-type MEH-PPV and ≈36% for Gilch-type MEH-PPV) and toluene solutions (≈38% for Horner-type MEH-PPV and ≈32% for Gilch-type MEH-PPV) are similar. The exciton delocalisation (chromophore size) is found to be about four repeat units for both types of MEH-PPV. Remarkably good laser performance with low pump laser threshold ( 2 ) was achieved for both polymers independent of the synthesis pathway, the deviations in the molecular fine structure and the film-forming solvents. However, the preparation of high quality optical films is much easier for the Horner-type MEH-PPV than for the Gilch-type MEH-PPV making the Horner-type MEH-PPV easier to use for thin-film lasers and photonic devices.

Amplified spontaneous emission in neat films of arylene-vinylene polymers

Amplified spontaneous emission (travelling-wave lasing) was achieved for a set of 18 poly(2-methoxy-5-(2′-ethyl-hexyloxy)-1,4-phenylene-vinylene) (MEH-PPV) related arylene-vinylene copolymers, a part of which substituted with additional phenyl groups at the vinyl double bond. Wave-guiding neat thin films on glass substrates were used. The samples were transversally pumped with single second harmonic pulses of a mode-locked ruby laser (wavelength 347.15 nm). Travelling-wave emission occurred in the wavelength region between 485 and 650 nm. Repeat unit based absorption cross-section spectra, normalized fluorescence quantum distributions, and amplified spontaneous emission spectra are presented. Ground-state absorption cross-sections at the wavelengths of peak amplified emission are extracted from effective gain length measurements. Effective stimulated emission cross-sections are derived from pump pulse energy densities necessary for optical narrowing.

Optically written solid-state lasers with broadly tunable mode emission based on improved poly (2,5-dialkoxy-phenylene-vinylene)

We report an optical written distributed feedback (DFB) structure with broadly and continuously tunable mode emission (Δλ=30 nm) based on polycondensation-type poly[2methoxy-5-(2-ethylhexyloxy)-1,4-phenylene-vinylene] (MEH–PPV). In a Lloyd-mirror configuration we realize first-order feedback by dynamic photoinduced in-plane gratings, which can also be imprinted permanently. This technique is a versatile fabrication tool for polymeric DFB laser structures while also providing an efficient method to probe the lasing performance of organic solid-state laser materials. Furthermore, features of the photoinduced absorption spectra of improved MEH–PPV indicate favorable material properties towards further homopolymer solid-state laser applications.

Effective stimulated emission and excited state absorption measurements in the phenylene–vinylene oligomer (1,4-bis-(alpha-cyanostyryl)-2,5-dimethoxybenzene))

Abstract The pump probe technique was used to measure the effective stimulated emission and excited state absorption cross-sections of the phenylene–vinylene oligomer (1,4-bis-(alpha-cyanostyryl)-2,5-dimethoxybenzene) dissolved in toluene for different probe wavelengths. The effective stimulated emission cross-section was calculated from σ SE-eff = σ SE − σ ESA and found to be positive. The possible laser tuning ranges have been determined. The spectral and temporal behavior of the optical transitions were investigated. The dynamics of these optical transitions permitted direct measurements of the lifetime of the lowest triplet state, and was found to be of the order of 11 ms.