Experimental a) Preparation of electrochromic layers: PoAnis-TSA was prepared by oxidation of o-methoxyaniline by (NH 4 ) 2 S 2 O 8 in a 1.0 mol L ±1 TSA/ 3.0 mol L ±1 NaCl aqueous solution, as described previously [26]. Films of this polymer were deposited onto ITO electrodes by spin coating from a DMF solution (60 g L ±1 ). The ITO½PoAnis-TSA electrodes were heated at 60 C for 1 h and the thickness was measured with an Alpha Step 100 Tencor Instrument.Films of Poly(ET2)-Hydrin-C blended on ITO plates were prepared by cyclic voltammetry (±0.2 to 1.1 V, 20 mV s ±1 , vs. Ag½AgCl) using a 21 0 ±3 mol L ±1 ET2 + 0.1 mol L ±1 (C 4 H 9 ) 4 NClO 4 acetonitrile solution on Hydrin-C (1 mm) coated ITO electrodes in a conventional three electrode cell. A platinum wire was the counter electrode. Hydrin-C was deposited onto ITO plates by casting a DMF solution of the elastomer (10 g L ±1 ).The charges of the single electrodes were measured by cyclic voltammetry, using Ag½AgCl as reference and a platinum wire as counter electrode in a 0.05 mol L ±1 CF 3 COOH + 0.1 mol L ±1 (C 4 H 9 ) 4 NClO 4 acetonitrile solution.The electrochemical polymerizations and measurements were performed with an AMEL 5000 Multi-function apparatus or with an AMEL potentiostat model 552 interfaced with an AMEL model 566 function generator and an AMEL model 731 integrator.b) Preparation of the solid electrolyte: The elastomer, Hydrin-C (Zeon Chemicals), was purified by dissolution in CHCl 3 followed by coagulation in CH 3 OH and drying under vacuum until a constant weight was achieved. The solid electrolyte film was prepared as described previously [18], dissolving 0.1 g of Hydrin-C and 0.035 g of LiClO 4 in 100 mL of THF.c) Construction of the device: Before assembling the device, the ITO½PoAnis-TSA and ITO½Poly(ET2) blend electrodes were separately polarized at 0.65 and 0.0 V (vs. Ag½AgCl), respectively, in a 0.05 mol L ±1 CF 3 COOH + 0.1 mol L ±1 (C 4 H 9 ) 4 NClO 4 acetonitrile solution. After this, the electrodes were washed with acetonitrile and dried. A thin film of the solid electrolyte was spread over the PoAnis-TSA electrode and evaporated until a plastic-like consistency was obtained. Finally, the electrochromic cell was assembled by gluing ITO½PoAnis-TSAkHydrin-C-LiClO 4 to the Poly (ET2)-Hydrin-C blend½ITO. The thickness of the solid electrolyte was controlled using a polyethylene spacer (10 mm). The device was assembled under atmospheric conditions. The spectrophotometric measurements were performed with a Perkin Elmer l9 spectrophotometer. Impedance measurements were carried out on an SI 1255 Schlumberger HF frequency response analyzer interfaced with a SI 1286 Schlumberger electrochemical interface. Sinusoidal perturbations of ±0.010 V were applied between 50 mHz and 10 kHz.Since the first report of electroluminescence (EL) in poly(p-phenylenevinylene) (PPV) [1] there has been increasing interest in conjugated polymer light-emitting diodes (LEDs) both from basic physics and applications perspectives. [2] Recent reports of lasing [3] ...
We report fluorescence measurements on poly(p-phenylene vinylene͒, PPV, and four derivatives of this polymer, all of which show strong luminescence and can be used as emissive materials in electroluminescent diodes. We measure the variation of the emission spectrum with excitation energy at low temperature, and find a threshold energy above which emission is independent of excitation energy and below which the emission energy tracks with the excitation energy. This information makes it possible to separate out the effects of spectral diffusion by exciton migration from other forms of excited-state relaxation. We find that PPV and two derivatives with asymmetric, branches side chains show little or no excited-state relaxation. In contrast, the other two derivatives ͑one with bromine and dodecyloxy attachments at the two and five positions on the phenylene, the other with hexyloxy attachments at these sites, and cyano groups at the vinylic carbons͒ show further relaxation by about 0.25 eV. We consider that emission in these two polymers is from an interchain excimer excited state. Supporting evidence for the cyano-PPV is seen in the differences between the dilute solution and solid-state fluorescence spectra.
We present excitation spectra, picosecond decay time, and quantum efficiency measurements of the photoluminescence (PL) signal in poly(p-phenylenevinylene) (PPV). In pristine PPV we observe a constant efficiency with excitation wavelength for singlet intrachain exciton generation, which we model to be close to unity. In contrast, photo-oxidized PPV samples contain a distribution of quenching centers following the profile of the absorption depth, which causes the PL efficiency to decrease with increasing excitation energy, and the PL efficiency and decay time to be no longer simply related.[S0031-9007(96)00969-6] PACS numbers: 78.55. Kz, 71.35.Cc, 78.47. + p The photophysics of conjugated polymers has been of interest for some time, [1] particularly since the discovery of electroluminescence (EL) in the polymer poly(pphenylenevinylene) (PPV) in a light-emitting diode (LED) structure [2]. Since the initial observation of green electroluminescence from PPV, LED's with various colors of emission [3-7] and improved efficiencies [8] have been reported.Much research has centered on the arylene-vinylenebased polymers, of which PPV is the simplest and most studied. However, there is still controversy surrounding the nature of the primary photoexcitation in these materials. Debate has often centered on the binding energy of the photoexcited state which can arise from both Coulombic and lattice distortion contributions. If this is small then description within a band model may be appropriate [9][10][11]; in contrast, if the electron and hole are strongly bound, they form an intrachain exciton [12][13][14][15]. We consider vide infra that an exciton model is more appropriate. However, it has also been suggested that nonemissive spatially indirect excitons ("bound polaron pairs" [12]) are the primary photoexcited species in these materials [16][17][18][19]. Luminescence in PPV is due to radiative decay of the intrachain singlet exciton, and measurements of photoluminescence (PL), provide important information about the nature of the photoexcitations. There is a discrepancy apparent between high values of PL efficiency (up to 0.27) reported recently [20] and the estimates for the quantum yield of "polaron pairs" [16 -19]. A high quantum yield of a nonemissive species such as polaron pairs also invalidates the presumed relationship [20,21] between PL quantum efficiency and lifetime. Furthermore, conventional estimates of maximum LED efficiencies of 25% of the PL quantum yield [22] may be too low if photoexcitation forms polaron pairs with high efficiency, but electrical injection does not [17,19]. A resolution of these contradictory results is essential for a greater understanding of photophysical processes in conjugated polymers, and of great importance to the development of LED's based on these materials.We have addressed these issues through measurements of photoluminescence excitation spectra, PLE, PL quantum yield, and PL transient decay on pristine and photooxidized samples of PPV. We find that the pristine material sh...
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