The Internal Field Distribution in Light Emitting Electrochemical Cells and Light Emitting Diodes: A Comparative Study
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roabsorption response is proportional to the imaginary part of the third-order susceptibility, Im χ (3 ) (hυ ) and the square of the electric field [13] −∆ T ∆α (hυ ) ∝ ∝ Im χ ( 3) ( hυ ) E 2 , T where α is the absorption coefficient, hυ is the photon energy, T is the transmission, and E is the electric field, consisting of a DC and an AC contribution. The electroabsorption signal as a function of the DC bias is measured in order to be able to distinguish between the two models mentioned above. The results obtained for the LECs show three different regions for the EA signal as a function of an external bias in reverse direction: For low DC bias voltages, the EA signal rises linearly with the DC bias, similar to a conventional LED. For voltages higher than a first threshold value the EA signal remains basically constant, suggesting that the additional DC field is now screened inside the device and drops mainly at the contacts. After a second threshold voltage is reached, however, both the electroabsorption signal and the current through the device rise sharply. EXPERIMENTAL DETAILS Prior to use methyl substituted laddertype poly(paraphenylene) (mLPPP), poly(ethylene oxide) (PEO), MW 100.000, and LiCF3SO3 (Li-triflate) were heated under vacuum. A solution of mLPPP in absolute and dry cyclohexanon was prepared, which was directly spincasted on indium tin oxide (ITO) coated glass substrates in case of the LEDs. For the LECs PEO and Li-triflate were added as an electrolyte in a weight ratio of 20:10:4. The devices were heated (T~60°) in argon atmosphere to drive out the solvent. To finish the structure aluminum cathodes were produced by thermal evaporation in a high vacuum, defining active device areas of 9 mm2. Device thicknesses were in the range of 150-200 nm. The devices were contacted with silver paste and put into a sample holder which was filled with argon. The device design is shown in Figure 1b. The absorption spectrum shown in Figure 2 was recorded using a λ9 UV-Vis spectrometer at room temperature. The experimental setup for the electroabsorption measurements used is shown in figure 1a. A xenon lamp and a monochromator were used as a lighting source. The light passes through the glass and the ITO layer, before passing through the polymer. It is reflected at the aluminum electrode, passes through the active layer a second time and is detected by the photodiode.
Figure 1. a.) Experimental setup for electroabsorption measurements b.) Device design L...Lamp,MC...Monochromator, F...Filter (optional), S...Sample, PD...Photodiode, FG...Function generator, VM...Voltmeter, AM...Amperemeter, LI...Lock-In Amplifier, PC...Personal Computer
Figure 2 Absorption spectrum of mLPPP A rectangular AC bias VAC (at frequency f) and a variable DC offset VDC were applied to the devices. For positive bias values, the aluminum electrode was wired as the anode. This direction is commonly referred to as the reverse direction (of an LED). The signal from the photodiode was recorded with a Lock-In Amplifier referenced to the fundamental
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