C60 Doping Research Articles

Optical properties of poly(disilanyleneoligothienylene)s and their doping characteristics

Optical absorption spectra, photoluminescence (PL), Stokes shift, and electroluminescence (EL) in poly(disilanyleneoligothienylene)s have been found to be dependent on the length of oligothienylene units. The color of EL changes from green to red with increasing length of oligothienylene from three to seven thiophene rings. Their stability is much higher than polysilanylene and increases with increasing length of oligothienylene and by C60 doping. Photoconductivity is strongly enhanced and PL is remarkably quenched upon C60 doping, which were discussed by the photo-induced electron transfer between aromatic units in these polymers and C60.

Growth and characterization of potassium-doped superfulleride thin films

Growth conditions for the formation of thin films (100–300 Å) of potassium-doped superfullerides (KxC60, x>6) are examined. Thin films of these compounds are formed by depositing C60 onto a potassium precovered single crystal quartz substrate maintained at 200 K or lower, in a proportion of K:C60>12:1, followed by annealing the surface to the K-sublimation temperature (300 K). In situ measurements of electrical and optical properties are used to identify the compounds. The formation of superfullerides is confirmed by C60 doping of these phases to check for the formation of insulating K6C60 with a characteristic absorption spectrum. The absorption spectrum of the superfullerides shows distinct features corresponding to the filling of the t1g band. The presence of two superfulleride phases is suggested, a near-metallic superfulleride KxC60 (x≈11.2) and a more insulating KxC60 (x≈8–9).

Reduction of photoscission of σ bonds in polysilanes by fullerene doping

We report here an effective reduction of photoscission of σ bonds in polysilanes by C60 doping. The temporal variation of photoluminescence in polysilanes has been measured as a function of a doping amount of C60. The results indicate that the photoscission of σ bonds is related to a photogenerated electron-hole pair rather than a hole or an electron. The energy released when the electron-hole pair recombines nonradiatively is more likely required in addition to the thermal energy to scissor σ bonds. The C60 doping does not affect the activation energy of the photoscission process but decreases the photoscission cross-section. It is also demonstrated that the photoluminescence quenching by the C60 doping is observed for polysilanes not only with aromatic side groups but also with saturated hydrocarbon side groups.

Charge Transfer in Fullerene-Conducting Polymer Compositex: Electronic and Excitonic Properties

C60 doping into conducting polymer with highly extended π-electron system in the main chain induces remarkable quenching of photoluminescence in conducting polymer and drastic enhancement of photoconductivity. These results can be explained in terms of photo-induced charge transfer between conducting polymer and C60. That is, photoexcited excitons or exciton-polarons on conducting polymer are effectively dissociated at C60 molecules transferring electrons to C60. Photoexcitation of C60 results in the transfer of hole from C60 to conducting polymer. These novel C60 doping effects have been observed not only in conducting polymers with non-degenerated ground state structures but also those with degenerated ground state structure such as di-substituted acetylene polymers with solitonic electronic systems. Highly effective photo-induced charge transfer has been also observed in conducting polymer/C60 heterojunctions, which are interpreted as donor (D)-acceptor (A) photocell. Based on this finding we have demonstrated an organic photovoltaic cell with D-A double heterojunction, Al/C60/OEP/conducting polymer/TTO, in which OEP is octaethylporphine as an light absorbing antenna molecule. Novel characteristics have also been observed in various other junction devices utilizing C60 doped conducting polymer. Granular and multiphase superconductivity has been found in C60-conducting polymer-alkali metal composites. Effect of other type of fullerenes such as C70, modified C60 and C60 polymers, and also effect of C60 doping in polysilanes and their derivatives have also been studied.

Visible Light Photoconduction of Poly(disilanyleneoligothienylene)s and Doping Effect of C60

The photoconductive properties of poly(disilanyleneoligothienylene)s [PDS(Th)m; m = 2−4], which have a disilanylene group and an oligothienylene unit in a polymer backbone, and fullerene (C60) doping effects were investigated. It was found that these polymers are photoconductive and have photocarrier generation maxima in accordance with their optical absorption spectra. The polymer PDS(Th)4 was photoconducting when irradiated in the visible light and the quantum efficiency for photocarrier generation was 2% at 480 nm (E = 6 × 105 V cm-1). The hole-drift mobility was measured and ranged between 1.1 × 10-4 cm2 V-1 s-1 at 2 × 105 V cm-1 and 2.1 × 10-4 cm2 V-1 s-1 at 6 × 105 V cm-1 at room temperature. As far as we know, this is the first observation of photocarrier generation in the visible region in silicon-based polymers. Furthermore, C60 doping enhanced the photoconductivity quantum efficiency effectively to 85% at 470 nm (E = 3 × 105 V cm-1) by a photoinduced charge transfer mechanism.

Electrical Properties of Discotic Liquid Crystal Hexahexyloxytriphenylene

Carrier transport of 2,3,6,7,10,11-hexahexyloxytriphenylene (HHOTP) in the discotic phase and the effects of C60 doping and shear stress have been studied by a time-of-flight method. The relatively high mobility of positive carriers insensitive to temperature and also C60 doping are interpreted to be the hole mobility. The hole mobility decreases on applying shear stress which deforms the columnar structure in the discotic phase. Small and temperature-dependent mobility (with activation energy of 0.3 eV) of negative carriers which are influenced by C60 doping is interpreted to be that of ionic carriers. Electronic band schemes of HHOTP in various phases (solid, discotic and isotropic) have been evaluated by absorption and photoelectron emission spectra measurements.

Enhanced Photooxidative Stability of Conjugated Polymers via C60 Doping

The effect of C60 doping on the photooxidative stability of the conjugated polymer poly[2-methoxy-5-(2‘-ethylhexyloxy)-p-phenylenevinylene] (MEH-PPV) is investigated via Fourier transform infrared and ultraviolet−visible absorption spectroscopies. The photooxidation of both MEH-PPV and MEH-PPV/C60 exhibits first-order kinetics. Analysis reveals that C60 slows the rate of photodegradation of MEH-PPV by more than an order of magnitude.

Doping effects of higher fullerene in conducting polymer

Doping effects of higher fullerenes such as C70 on electrical and optical properties of conducting polymers such as poly(3-alkylthiophene), poly(2,5-dialkoxy-p-phenylene vinylene) and poly(9,9-dialkylfluorene) have been studied and compared with those by C60 doping. Fluorescence intensity was quenched and photoconductivity was enhanced upon doping of higher fullerenes in the conducting polymers just as the case of C60 doping. However, there also exist differences in detailed aspects in quenching of photoluminescence and enhancement of photoconductivity between C60 doping and higher fullerenes dopings. These results are interpreted by taking into account the electronic energy states of C60 and higher fullerenes.

X-Band Optically Detected Magnetic Resonance (ODMR) Study of C60-Doped π-Conjugated Polymers

Abstract The X-band ODMR of 0.1–10 mol.% C60-doped 2,5-dihexoxy poly (p-phenylenevinylene) (DHOPPV) and 2,5-dibutoxy poly(p-phenylene-acetylene) (DBOPPA) is described. As invariably observed in undoped π-conjugated polymers, it includes a narrow PL-enhancing polaron resonance at g ≊ 2.002, and full- and half-field triplet exciton powder patterns. The polaron resonance is surprisingly weak in undoped DHOPPV, but its intensity increases twentyfold at 0.1 mol.% C60, and then decreases one hundredfold at 10 mol.%. In undoped DBOPPA the narrow resonance is intense and initially decreases with C60 content. C60 doping generally drastically weakens the full- and half-field triplet exciton resonances. The results are discussed in relation to the role of positive (p+) and negative (p−) polarons and bipolarons as nonradiative singlet and triplet quenching centers; the polaron resonance is then attributed to their removal by spin-dependent p+−p− recombination, which may also generate radiative singlet excitons.