Polarized UV absorption spectra of highly oriented conjugated polymers provide information that is crucial to understanding the electronic structure of these semiconducting macromolecules. Polarized absorption spectra were obtained for soluble derivatives of poly(phenylene vinylene) and poly(paraphenylene), oriented by the use of gel-processing blends of conjugated polymers in a polyethylene (PE) matrix. The conjugated polymer chains are highly aligned and exhibit strongly polarized luminescence. The polarized UV (2 eV - 6.2 eV) absorption spectra of chain-extended MEH-PPV and of a soluble polyfluorene derivative (HEH-PF) each reveal a series of absorption bands polarized parallel to the molecular axis and a single absorption band polarized perpendicular to the axis. The 3.7 eV peak in MEH-PPV is polarized parallel to the molecular axis, indicating that it is an intrinsic feature of the PPV electronic structure. In HEH-PF, the perpendicular-polarized band is at 5.3 eV, as predicted by band-structure calculations in the literature. These data were used to provide a deeper understanding of the electronic structure of phenylene-based conjugated polymers.
E. K. Miller, C. Y. Yang and A. J. Heeger, Physical Review B 59, 4661 (1999).
and E. K. Miller, C. Y. Yang and A. J. Heeger, Physical Review (in press; 15 Sept. 1999)

Ultrafast photoinduced IRAV absorption measurements have enabled, for the first time in any semiconductor, direct determination of the initial quantum efficiency (QE) for photogeneration of charge carriers While considerable progress has been made toward understanding the properties of luminescent conjugated polymers, there has been a long-standing controversy on the mechanism of photogeneration of charge carriers: Are charged photocarriers (polarons) primary excitations, or do they result from secondary processes that involve exciton annihilation? Experiments at IPOS have now addressed the carrier photogeneration mechanism and determined the initial quantum efficiency (o) by means of transient excited-state absorption (photoinduced absorption, PIA) measurements, pumped in the visible and probed with 100 fs temporal resolution in the 6-10 mm spectral region which spans the infrared active vibrational (IRAV) modes. The method is based on the one-to-one correspondence between the steady-state and transient photoinduced IRAV absorption in pristine conjugated polymers and the linear IRAV absorption of chemically doped polymers; the strength of the IRAV modes is proportional to the density of carriers on the polymer chain. Thus, monitoring the strength of the IRAV absorption provides a direct ultrafast probe to the charge carrier density at times typical of carrier thermalization in disordered semiconductors. Photoinduced IRAV absorption in MEH-PPV/C60 confirmed charge transfer in less than 100 fs. Ultrafast electron transfer from the photogenerated excited state of PPV (and its soluble derivatives) to C60 occurs because the lowest energy unoccupied state in C60 lies within the energy gap and because energy can be conserved in the charge transfer process by promoting the hole left behind to a higher energy state within the relatively broad p -band of the semiconducting polymer. Because of the ultrafast charge transfer, the QE for charge separation and charge carrier generation in MEH-PPV/C60 approaches unity, consistent with the quenching of the photoluminescence and the enhancement of the photoconductivity in blends containing C60. The photocarrier QE in the pristine polymer was determined from the ratio of the photoinduced IRAV signals from MEH-PPV and MEH-PPV/C60. In zero external field, o » 0.1 in MEH-PPV, comparable to the QE for photoluminescence (PL). Finally, by comparing the photoinduced IRAV signals in PPV (where there are no side chains) and MEH-PPV (where the side-chains introduced for improved solubility reduce the strength of the interchain hopping interaction), the sensitivity of the rate of carrier recombination on the strength of interchain interactions was demonstrated.
D. Moses, A. Dogariu and A. J. Heeger, Chemical Physics Letters (in press).

Semiconducting (conjugated) polymers have properties that are advantageous for photonic applications; they have high fluorescence efficiencies (> 60 %), emit at wavelengths that span the entire visible spectrum, are mechanically flexible, and can be deposited as uniform thin films by casting from solution. Since the fabrication of the first polymer light-emitting diode (LED) in 1990, there has been extensive research on polymer LEDs and many improvements have been made. Single color displays fabricated with arrays of polymer LEDs will soon be commercially available. Full color displays will require pure red, green, and blue emission. Obtaining pure emission colors from conjugated polymers or small organic molecules is difficult because their emission spectra typically have a full width at half maximum (FWHM) of 50-200 nm. Efficient, pure red-emitting polymer LEDs are particularly hard to make because the human eye is more sensitive to orange emission than red; if the spectrum falls even slightly in the orange, the perceived color is "orangish-red". Red LEDs can be made by filtering out orange emission or by using polymers or dyes whose emission starts in the red and extends into the infrared, but these LEDs are inefficient because only part of their emission is useful. In contrast to organic chromophores, rare earth ions have very sharp emission spectra (FWHM < 4 nm). In a recently completed study, IPOS researchers showed that pure red emission can be achieved in polymer LEDs by transferring energy (Förster transfer) from blue-emitting conjugated polymers to europium complexes. Similar methods were then used to make red LEDs from small organic molecules. Blends of Eu complexes in poly[2-(6'-cyano-6'-methyl-heptyloxy)-1,4-phenylene] (CN-PPP) have an emission spectral linewidth (FWHM) of only 3.5 nm, a photoluminescence (PL) efficiency of 27 % and an electroluminescence (EL) efficiency of 1.1 %. These blends could be useful as a source of pure red light for full color displays or for photonic devices that require monochromatic light.
M. D. McGehee, T. Bergstedt, C. Zhang, A. P. Saab, M. B. O'Regan, G. C. Bazan, V. I. Srdanov and A. J. Heeger, Advanced Materials (in press).

The discovery (at IPOS) of Amplified Spontaneous Emission (ASE) in conjugated polymers has generated interest in these materials; research has focussed on making lasers using conjugated polymers as the active layers. Although optically pumped lasing has been reported using a number of organic systems in a variety of resonators, electrically pumped lasers have not yet been demonstrated. Research has focussed on making better cavities to lower the thresholds for lasing, and on reducing the optical losses in thin film waveguides to reduce the lasing/ASE thresholds. IPOS research has demonstrated that blends of conjugated polymers can be used to lower the optical losses and the ASE (and lasing) thresholds of optically pumped conjugated polymers. Förster energy transfer from the host polymer to the guest polymer leads to reduced self-absorption as a result of the red shift of the emission relative to the absorption in the blend. In recent studies, the dependences of energy transfer on the concentration of the guest polymer and on the photoluminescence quantum efficiencies in the blend were explored, and the thresholds for optically pumped ASE and lasing were measured in thin film planar waveguides of the host, the guest and the host-guest blends. The blends exhibited significantly reduced self-absorption losses; ~3 cm^-1 compared to ~ 85 cm^-1 in pure host. As a result, the ASE thresholds are reduced from 5000 W/cm² in the host to 200 W/cm² in the blends. The corresponding lasing thresholds in distributed feedback (DFB) resonant structures are as low as 100 W/cm²; the lowest observed to date with any conjugated polymer system. The optical (self-absorption) losses measured in films of the blend materials were well correlated with the thresholds for ASE and lasing.
R. Gupta; M. Stevenson, J. Y. Park and A.J. Heeger (manuscript in preparation).

Blends of conjugated polymers with ultrahigh molecular weight polyethylene (UHMW-PE) containing up to 30%(w/w) of poly(2-butyl, 5-(2'-ethyl-hexyl)-1,4-phenylene vinylene), (BuEH-PPV) have been prepared by gel processing. The microstructure of both pristine and oriented (tensile drawn) films were studied using optical microscopy (365 nm ultraviolet illumination), X-ray diffraction and transmission electron microscopy. The results show that in pristine films, phase separation results in bicontinuous interpenetrating networks. After tensile drawing to a draw ratio =100, the structure develops into nematically oriented microfibers comprising oriented macromolecules interspersed within similar (partially) crystalline microfibers of UHMW-PE. The driving force for forming the microfiber structure is the tensile drawing; entanglement of the BuEH-PPV and UHMW-PE components in the network structure causes the two components to align and restructure, resulting in the stretched and oriented microfibers in the oriented film. The mechanism relies on the nature of the phase separation into bicontinuous networks in the pristine film. The order in the conjugated polymer microfibers is nematic; the BuEH-PPV macromolecules are chain extended and oriented, but with no observed interchain order. The microfibers of the UHMW-PE component, on the other hand, exhibit a relatively high degree of interchain order. The photoluminescence emission from the oriented blends (30%w/w BuEH-PPV) is polarized with a polarization ratio of 50:1 (parallel to perpendicular with respect to the draw axis).
C.Y.Yang, A.J.Heeger and Y. Cao, Polymer (in press).