All Solution Process for Multilayer Structured Polymer Light-emitting Diodes
Date Issued
2010
Date
2010
Author(s)
Lai, Hsiao-Han
Abstract
Polymer light-emitting diodes (PLEDs), as a branch of organic electroluminescence (OEL), are mainly fabricated by solution based wet process. To improve device performance, multilayer structured device is absolute necessary. However, the choices of solvents are limited; this means that the “like dissolves like effect” had to be carefully avoided or the predeposited layer will be destroyed by the incoming layer.
In this thesis, we have demonstrated an all solution process to fabricate multilayer layer structured PLEDs. By carefully selecting the materials and solvents used in each layer, the interfacial mixing issue can be avoided.
The first part aimed at the optimization of multilayer structured PLEDs. Poly(9,9-dioctylfluorene-co-N-(4-butylphenyl)-diphenylamine) (TFB, dissolved in p-xylene) acts as hole injection layer, Green B (dissolved in toluene) is the emissive polymer where TiOx (diluted with 2-ethoxyethanol) can be taken as electron injection layer. The maximum current efficiencies of single-, double- and triple-layer PLEDs are 6.42, 8.29 and 10.36 cd/A, respectively. The total increment is 61.4% from single-layer to triple-layer devices. Therefore, with better charge balance, the device lifetime is increased significantly from 4.48×103 h of single-layer device to 5.75×105 h for triple-layer device.
In the second part, the effect of surface roughening via adding prism sheet is considered. The efficiencies, color shifts and spectra of PLEDs are affected by varying the thickness of the emissive layers. In our optical configuration, the output of electroluminescence intensity is given by the product of optical response of device structure and photoluminescence of Green B. We had combined the experimental results with the theoretical simulation to optimize the device structure by considering the optical design and the opto-electronic characteristics. The optical cavity was tuned by changing the thickness of TiOx to achieve the best current efficiency with the conventional prism sheet. Consequently, we demonstrated a PLED device with 72.6% enhancement in current efficiency by applying both the TiOx layer and prism sheet.
Subjects
all solution process
extraction efficiency
micro cavity
multilayer structured
polymer light-emitting diodes
spin coating
Type
thesis
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