W. Graupner

Creation of a gradient polymer-fullerene interface in photovoltaic devices by thermally controlled interdiffusion

Efficient polymer-fullerene photovoltaic devices require close proximity of the two materials to ensure photoexcited electron transfer from the semiconducting polymer to the fullerene acceptor. We describe studies in which a bilayer system consisting of spin-cast 2-methoxy-5-(2′-ethylhexyloxy)-1,4-phenylenevinylene copolymer (MEH-PPV) and sublimed C60 is heated above the MEH-PPV glass transition temperature in an inert environment, inducing an interdiffusion of the polymer and the fullerene layers. With this process, a controlled, bulk, gradient heterojunction is created bringing the fullerene molecules within the exciton diffusion radius of the MEH-PPV throughout the film to achieve highly efficient charge separation. The interdiffused devices show a dramatic decrease in photoluminescence and concomitant increase in short circuit currents, demonstrating the improved interface.

Efficient full-colour electroluminescence and stimulated emission with polyphenylenes

Abstract Soluble and stable polyparaphenylene-type ladder polymers (LPPPs) with an extraordinarily high degree of intrachain order and exceptionally low concentration of defects belong to the class of best-defined conjugated low-dimensional organic semiconductors currently available. Parahexaphenyl, the highly stable oligomer of polyparaphenylene, can be synthesized in the form of single crystals, disordered thin films and highly ordered epitaxially grown thin films. We demonstrate the fabrication and characterization of highly efficient red-green-blue (RGB) and white light-emitting devices with these electroactive materials. These RGB devices are fabricated based on a new technique, which allows the realization of full-colour flat-panel displays. Using this new technique, we are able to produce devices having any desired emission colour in the visible and near-infrared spectrum. An efficient white-light emission is generated by an internal excitation energy (Forster-type) transfer from the blue LPPP component to a red light-emitting polymer in a polymer blend, which is used as the active layer in a light-emitting diode. We present an optically pumped methyl-substituted LPPP waveguide, laser structure, which shows a spectrally very narrow and highly directional blue-green light output.

Electroluminescence with Conjugated Polymers and Oligomers

Abstract We report on the optical properties of polymers and oligomers used in light emitting diodes (LEDs) prepared by solution processing or vacuum deposition. Special emphasis is put on the comparison of PPP and its oligomers to planarized forms of PPP.

Squeezing Organic Conjugated Molecules— What Does One Learn?**

The effects of hydrostatic pressure on conjugated polymers, in particular polyphenyls (see Figure for monomer unit) have been studied using photoluminescence, absorption, photo-induced absorption, and Raman spectroscopy. The effect of pressure on the singlet and triplet excitons and polarons allows an understanding of localized and delocalized electronic states. Changes in the intensity ratios of Raman bands that correspond to vibrations of a perpendicular and a coplanar array of phenyl rings in the chain, and comparison with calculated intensities, demonstrate the influence of pressure on the polymers conformation.

Photophysics of excitation energy transfer in highly fluorescent polymers

Abstract We report the optical properties of highly fluorescent guest host systems of two conjugated polymers. The blue emitter laddertype poly(para-phenylene) (LPPP) is blended as a host with the red emitter poly(perylene-co-diethynylbenzene) (PPDB) as a guest at sub-percent and percent level concentrations. We use transient and steady-state photoluminescence as well as near-steady-state photoinduced absorption to show that an efficient excitation energy transfer of Forster type occurs between the blue emitting host and the red emitting guest. The spectral signatures of emissive and absorptive photogenerated species in both polymers are presented. In addition, we describe analytical relations to determine the lifetime of these species from photomodulation spectroscopy.

The role of amplified spontaneous emission in the ultrafast relaxation dynamics of polymer films

Abstract We show that gain relaxation dynamics in thin films of methyl-substituted poly-(para-phenylene)-type ladder polymer, investigated by femtosecond pump–probe spectroscopy under high excitation fluence, is affected by amplified spontaneous emission (ASE), which provides for an efficient excited state population depletion mechanism. The ASE influence on the photoluminescence spectral narrowing is also investigated. A general model which accounts for both excited state population dynamics and photoluminescence spectral evolution is presented.

Ultrafast energy-transfer dynamics in a blend of electroluminescent conjugated polymers

Abstract Femtosecond pump–probe spectroscopy was applied to a guest–host system of conjugated polymers, the blue-emitting poly(para-phenylene) (m-LPPP) and the red-emitting poly(perylene-co-diethynylbenzene) (PPDB). Excitation energy transfer between m-LPPP and PPDB, monitored by the formation of the PPDB photobleaching band at 2.15 eV, takes place on a picosecond timescale. The non-exponential kinetics of the excitation energy transfer and its dependence on the acceptor (PPDB) concentration provide clear evidence for a Forster-type energy transfer.

PHOTOLUMINESCENCE AND UV-VIS ABSORPTION STUDY OF POLY(PARA-PHENYLENE)-TYPE LADDER-POLYMERS

Abstract Soluble poly(para-phenylene) (PPP)-type ladder-polymers have been shown to be suitable as active layers in light emitting diodes (LEDs). In this work we discuss the absorption spectra in the ultraviolet (UV) and visible (VIS) range as well as photoluminescence spectra of such materials. The steepness of the band edge in the absorption spectra indicates that high intrachain order and well-defined conjugation length are characteristic for these ladder-type PPPs (LPPP). By investigating the absorption and photoluminescence of LPPP dissolved in toluene and of solid LPPP films we study the influence of molecular interaction (formation of excimers, luminescence quenching through the presence of neighbouring molecules within the distance of energy transfer) on the photoluminescence quantum yield.

EXCITED-STATES IN PPP-TYPE LADDERPOLYMERS PROBED BY PHOTOINDUCED ABSORPTION

Abstract We present photoinduced absorption (PIA) measurements done on a planarized form of poly( para -phenylene) (PPP). Macromolecules of different degree of polymerization and with a chemical modification at the bridge, connecting the benzene rings of the PPP-backbone, are investigated. There is clear evidence that the quasi steady state PIA signal in the near infrared vanishes with increasing intrachain order. We discuss the stabilization of the photogenerated defects. The nature of the observed states is described based on their characteristic electronic absorptions, their lifetime and their temperature dependence.

PHOTO-EXCITED AND DOPING-INDUCED ELECTRONIC STATES IN A POLY(PARA-PHENYLENE)-TYPE LADDER POLYMER

Abstract Incorporating a poly(p-phenylene) (PPP) backbone into a planar ladder polymer yields a molecular system with well-defined configuration and conformation. This chemical modification influences the electronic properties of PPP. We discuss the absorption spectra in the ultraviolet (UV) and visible range comparing the PPP-type ladder polymer (LPPP) to PPP and poly(p-phenylene vinylene) (PPV). The photoinduced absorption (PIA) data are compared with the net change of the optical absorption upon solution doping with FeCl3. The spectral dependence of the PIA in the near infrared region (NIR), as well as the behaviour with respect to time and pumplight-intensity are evaluated. These results show that the optical properties of the LPPP are much closer to PPV than PPP.