Color emission

By changing the device architecture e.g. by building multi- instead of single layer structures the physical and chemical processes in the LED can be greatly altered. For that reason the fundamental properties of the LED, such as threshold voltage, efficiency, emission color, brightness, and lifetime can be optimized in multilayer structures [43J. 

More precise control over the emission color may be achieved by employing equimolar quantities of a fc/.v-phosphonium salt, e.g., 67 and a dialdehyde containing a flexible unit sueh as 66. This approach has been exploited by a number ol researchers 98-102[. The emission wavelength of these materials is in the blue te blue-green region of the spectrum (470-495 nm). 

According to Branchini et al. (2004), luciferase modulates the emission color by controlling the resonance-based charge delocalization of the anionic keto-form of oxyluciferin in the excited state. They proposed the structure C5 as the yellow-green light emitter, and the structure C6 as the red light emitter. 

The process for actually measuring emissive color is somewhat different and more challenging. First, we must obtain an emission spectrum by means of a spectrofluorimeter. We cam now integrate I d> to obtain the energy and then specify this in terms of x and y. There are special methods which have been developed to do so wherein seleeted wavelengths are used, depending upon the nature of the emission spectrum. We will not delve further into this method other than to state that it does exist. 

Fig. 9 (a) Molecular structures of novel ESIPT dyes, 2,5,-bis[5-(4-t-butylphenyl)-[l,3,4]oxadia-zol-2-yl]-phenol (SOX), and 2,5-bis[5-(4-t-butylphenyl)-[l,3,4]oxadiazol-2-yl]-benzene-l,4,-diol (DOX). (b) Emission colors in the Commission Internationale de L Eclariage (CEE) chromaticity diagram. The inner oval and the filled circle at coordinate (x,y) of (0.33, 0.33) indicate the white region and the ideal color, respectively. Note that PS and PVK denote polystyrene and poly (N-vinylcarbazole) film (reprint from ref. [91], Copyright 2005 Wiley-VCH)... 

In view of chemistry, most of the research has been focusing on the chemical modification of p-HBDI [113] analogs at the C(l) position, such that the emission color can be tuned via the substituent effect [114], Nevertheless, studies reveal a strong cutoff between the properties of wild-type GFP (or certain GFP mutants)... 

A modified SILAR system has been used to grow CdSe in CdS/CdSe core shell semiconductor nanocrystals.12 A cadmium precursor solution, with CdO dissolved with oleic acid in octadecane, was injected onto the substrate, and the Se solution (Se powder dissolved with tributylphosphine in octadecane) was similarly injected. The temperature of the reaction solution was 185 °C. A CdS outer layer in the CdS/CdSe/CdS colloidal quantum wells was deposited by alternating injections of cadmium and sulfur both in octadecane solutions at 230-240 °C. These structures showed high PL quantum yields (20-40%), relatively narrow emission bands, and tunable emission colors from about 520 to 650 nm depending on the number of CdSe monolayers. 

The recently synthesized PPV 68, in which the oxadiazole group is separated from the PPV backbone by an oxygen atom, is a very soluble material with optical band gap of 2.36 eV and yellowish-orange emission color (chromaticity coordinates by the Commission Internationale de l Eclairage, CIE a 0.50, y = 0.47 591 nm) [124]. An extremely high-... 

The conjugation length and the emission color of PPV-type materials can be also controlled by using short oligo(phenylene vinylene) units as pendant substituents in nonconjugated polymer chain. The advantage of such an approach is the possibility to use well-established... 

It was shown that adding low oxidation potential material to PFs can stabilize the emission color and increase the device efficiency [321]. However, using low-molecular-weight organic dopants causes several problems such as phase separation and crystallization. These problems can be partially solved by using polymer blends. Cimrova and Vyprachticky [334] reported... 

These observations inspired Inganas and coworkers [107,497,498] to exploit the principle of steric hindrance to design PTs with emission colors spanning the full visible spectrum. A wide range of 3-substituted and 3,4-disubstituted thiophenes has been synthesized and successfully polymerized by FeCl3 in chloroform, affording polymers 411-424 [466, 498-501] (Chart 2.100). 

The combination of thiophene and thiophene-51,S -dioxide units in a copolymer allows tuning the emission color from green to pure red [407,549], However, the PLEDs fabricated with these materials showed a rather low 4>el< 0.01% that further decreased with an increasing number of thiophene units. Similar results (significant decrease of the PL QE) were observed for thiophene thiophene-5,5-dioxide copolymers containing 3,6-dimethoxyfluorene (449 [303]) and carbazole units (450 [550]) (d>PL = 20-25% in solution). 

Blending with dialkoxy-PPV 14 in a device (ITO/PEDOT/polymer blend layer/LiF/Ca) substantially improved the EL efficiency (by about two orders of magnitude). A moderately efficient energy transfer from the higher band-gap PPV (AEL = 650 nm) to PT 468 (AEL = 830 nm) allowed fine-tuning of the emission color by changing the component ratio (Figure 2.32) [569],... 

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