Torsten Rabe

Visualizing a Homogeneous Blend in Bulk Heterojunction Polymer Solar Cells by Analytical Electron Microscopy

To increase efficiency of bulk heterojunctions for photovoltaic devices, the functional morphology of active layers has to be understood, requiring visualization and discrimination of materials with very similar characteristics. Here we combine high-resolution spectroscopic imaging using an analytical transmission electron microscope with nonlinear multivariate statistical analysis for classification of multispectral image data. We obtain a visual representation showing homogeneous phases of donor and acceptor, connected by a third composite phase, depending in its extent on the way the heterojunction is fabricated. For the first time we can correlate variations in nanoscale morphology determined by material contrast with measured solar cell efficiency. In particular we visualize a homogeneously blended phase, previously discussed to diminish charge separation in solar cell devices.

Tunable organic thin-film laser pumped by an inorganic violet diode laser

We demonstrate an organic thin-film distributed feedback (DFB) laser pumped by an inorganic violet laser diode (λ=406nm). The active organic medium consists of a highly efficient, modified poly(9,9-dioctylfluorene) derivative containing 12% of statistical intrachain 6,6′-(2,2′-octyloxy-1,1′-binaphthalene) binaphthyl spacer groups. The polymer is doped with 4wt% of the stilbene dye 1,4-bis(2-(4-(N,N-di(p-tolyl)amino)phenyl)vinylbenzene). This guest-host system features minimum lasing threshold energy densities below 1μJ∕cm2 for second order DFB resonator geometries and significant absorption (7.3×104cm−1) at the wavelength of the pumping diode laser. The entire setup states an extremely compact and cost effective, laser source tunable between 496 and 516nm.

Deep blue widely tunable organic solid-state laser based on a spirobifluorene derivative

We report on amplified spontaneous emission and optically pumped deep blue lasing in the organic spirobifluorene derivative 2,7-bis(biphenyl-4-yl)-2′,7′-di-tert-butyl-9,9′-spirobifluorene. Solid-state lasing is observed in thin films of this material deposited on a distributed-feedback (DFB) grating substrate. The laser wavelength can be tuned from 401.5 to 434.2 nm depending on the grating period of the Bragg reflector. The blue edge of this interval at 401.5 nm makes this laser an extremely short wavelength organic DFB laser. When pumping with a pulsed nitrogen laser at 337 nm, we observe a laser threshold energy density of 83 μJ/cm2. These results render this spiro compound an excellent candidate for blue-emitting diode lasers.

Ultrawide tuning range in doped organic solid-state lasers

We report on the tunability of 4-(Dicyanomethylene)-2-methyl-6-(julolidin-4-yl-vinyl)-4H-pyran (DCM2)-doped guest-host organic lasers. As host materials Tris-(8-hydroxy-quinoline)aluminum (Alq3), 4,4′‐N,N′-dicarbazole-bipheny1 (CBP), and N′N-di(1-naphthyl)-N,N′-diphenyl-1,1′-diphenyl-4,4′-diamine (NPD) are used. The largest tuning range was observed in the Alq3:DCM2 film with 115.3nm between 597.8 and 713.1nm. In CBP:DCM2, a tuning range of 85nm was measured, whereas in NPD:DCM2 only one laser wavelength at 624.4nm could be observed. When comparing the pump energies, we observed considerably lower threshold energy densities in Alq3:DCM2 and CBP:DCM2 compared to NPD:DCM2.

Organic solid-state ultraviolet-laser based on spiro-terphenyl

We report on organic semiconductor lasers based on spiro-terphenyl as active gain medium in a second-order distributed feedback (DFB) structure. In contrast to neat p-terphenyl, the spiro-linked compound forms vitreous amorphous films—an essential prerequisite for low loss laser structures. Spiro-terphenyl shows amplified spontaneous emission in the ultraviolet part of the spectrum at 381 nm. Thin-film lasers with DFB grating periods between 200 and 230 nm exhibit emission wavelengths between 361.9 and 393.8 nm. The minimum threshold energy density under optical pumping is as low as 8.9μJ∕cm2 (383 nm). Therefore, the novel spiro-linked compound paves the way for low-threshold, widely tunable organic semiconductor lasers advancing into the ultraviolet spectral region.

Laser threshold reduction in an all-spiro guest–host system

We report on stimulated emission in an all-spiro guest–host (G–H) system. Different doping concentrations of the guest molecule 2,2′,7,7′-tetrakis(2,2-diphenylvinyl)spiro-9,9′-bifluorene in the host material 2,7-bis(biphenyl-4-yl)-2′, 7′-di-tert-butyl-9,9′-spirobifluorene were investigated for amplified spontaneous emission (ASE) and distributed feedback (DFB) lasing. The ASE maximum could be shifted over 20nm by variation of the doping concentration. DFB lasing is observed in the pure host, the pure guest material, and in the G–H system. The laser wavelength can be tuned from 401.5 to 529.3nm by changing the grating period of the Bragg reflector and the doping concentration. A minimum threshold energy density of 6μJ∕cm2 was observed in second-order DFB structures for a doping concentration of 1.1%. In first-order DFB operation the threshold value could be further lowered to 320nJ∕cm2. These results render this material system an excellent candidate for stable and widely tunable lasers in the visible spec...

Electrically Tunable Polymer DFB Laser

IC A IO N In recent years, there is a trend in optics shifting from hard static objects, such as lenses and fi bers, to softer objects based on fl exible polymers or fl uids. Soft matter based optics can be tuned by external mechanical devices, but can also be coupled to electroactive polymer “artifi cial muscles”, for instance to provide for voltage-controlled, monolithically integrated optical elements. Soft electromechanically active optics could enable a whole new paradigm for compact and effi cient optical devices. We demonstrate the compact monolithic integration of a fl exible polymer distributed feedback (DFB) dye laser with an electroactive dielectric elastomer actuator. This combination enables direct voltage control of the DFB grating period through compression, such that an actuation voltage up to 3.25 kV leads to a continuous wavelength shift from 604 nm to 557 nm, a change of 7.8%. The solid glass lens is the fi rst choice for beam shaping. Researchers have shown that lens-shaped water droplets can be manipulated electrically for fast tuning of focal length, [ 1 , 2 ]

Quasi-continuous-wave operation of an organic thin-film distributed feedback laser

The operation of organic thin-film distributed feedback lasers at repetition rates up to 5MHz is studied. The active organic medium consists of a highly efficient, modified poly(9,9-dioctylfluorene) derivative containing 12% of statistical intrachain 6,6′ -(2,2′-octyloxy-1,1′-binaphthalene) binaphthyl spacer groups. The threshold energy density of these pulsed optically pumped lasers remains virtually unaltered at elevated repetition rates up to 5MHz. Therefore, we conclude that on this time scale photoinduced absorption which might affect the waveguide loss or the overall quantum efficiency is negligible in our active polymer. These results state an organic solid-state laser operating at repetition rates in the megahertz range which for many applications can be considered as quasi-cw.

Low loss contacts for organic semiconductor lasers

For the realization of electrically driven organic semiconductor lasers low loss contacts for charge injection are essential. The authors show that the addition of thin, highly transparent conducting layers (thickness on the order of 10–20nm) of aluminum doped zinc oxide to a planar waveguide structure formed by a 145nm thick polymer active layer leaves the threshold for the onset of amplified spontaneous emission almost unchanged. This finding paves the way towards electrically driven organic lasers without the need for unrealistically thick organic spacers to keep the waveguide mode away from the contacts.

Organic solid-state lasers based on sexiphenyl as active chromophore

We report on various sexiphenyl derivatives as gain media in organic solid-state lasers. The molecules involved in this research are simple p-sexiphenyl, the laser dye molecule 2,5,2””’,5””’-tetra-t-butyl-p-sexiphenyl (TBS) and the spirolinked sexiphenyl-derivative 2,7-bis(biphenyl-4-yl)-2’,7’-di-tert-butyl-9,9’-spirobifluorene. It appears that the morphology of vacuum-deposited thin films is highly dependent on the sterical dimensions of the respective molecules. Whereas thin films based on simple p-sexiphenyl comprise large clusters which significantly deteriorate their waveguiding properties; films formed by TBS, and the spiroderivative show a dramatically improved morphology with reduced surface roughness. Therefore amplified spontaneous emission (ASE) and lasing are demonstrated in both of the last but not in films based on p-sexiphenyl. Second-order distributed-feedback lasers with TBS as the active medium have been prepared with an emission between 390 and 435 nm depending on the grating period of ...