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Cathode - Al. Light Emitting layer. Substrate - Glass. Fig. 3 OLED Structure. Anode - ITO. Investigation of Porphyrins in a PFO Host for Organic Light Emitting Devices Brian Tuffy* and Werner J. Blau

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slide1
Cathode - Al

Light Emitting layer

Substrate - Glass

Fig. 3 OLED Structure

Anode - ITO

Investigation of Porphyrins in a PFO Host for Organic Light Emitting Devices

Brian Tuffy* and Werner J. Blau

Molecular Electronics & Nanotechnology, School of Physics, Trinity College Dublin*[email protected]

Introduction

Experimental

  • Organic Light Emitting Devices (OLEDs) will transform lighting technology due to advantages such as efficiency, brightness, colour purity, flexibility and size
  • Phosphorescent OLEDs (PhOLEDs) enhance the efficiency further by allowing the normally forbidden triplet energy levels to emit light
  • This work investigates various porphyrin molecules for their use in PhOLEDs

Simple OLED structure consists of an Indium Tin-Oxide (ITO) coated glass substrate, a thin layer of the light emitting material and an Al cathode.

  • Substrates are prepared by acid etch
  • 1.5% by wt solutions are Spincoated onto ITO
  • Thin films are left to dry in a fumehood
  • Aluminium electrodes are thermally Evaporated onto the films
  • Analysis by Current Voltage curves (IV), Photoluminescence (PL), Electroluminescence using a CCD (EL), Raman and Profilometer etc

Fig. 1 Example of a commercial PhOLED

Source: el-ink.com

Phosphorescence Vs Fluorescence

  • Spin statistics: 25% probability of singlet states and 75% probability of triplet states being formed
  • Conventional fluorescent OLED yield limited to 25%
  • In Phosphorescent materials, the population of triplet states is possible due to a spin flip of electrons/holes
  • After Internal Conversion (IC) an Inter-System Crossing (ISC) to the triplet state may occur
  • Triplet Harvesting is the conversion of fluorescent singlet states to phosphorescence triplet states
  • PhOLEDs theoretically triple the internal quantum efficiency

Thickness Measurements

  • A Veeco mechanical Profilometer is used to estimate the thickness of each layer
  • A scratch is made in the light emitting layer and the tip is scanned over each surface
  • Graph 1 shows a 35 nm layer of hemin formed from spincoating a 18 mg/ml hemin solution at 2000 RPM for 60 seconds

Fig. 2 Energy level diagram shows phosphorescence

Quantum Efficiency

External quantum efficiency ηextis measured to compare device performance

J = current density (A/m2)

Lv= Photometric Luminance (cd/m2)

λ = Peak Emission wavelength (m)

ℓ = lumens to watts conversion = 683 (Lm/W)

Graph 1: Thickness profile of OLED layers

Materials

Results

Graph 2: Current Voltage Curves of an OLED with an emitting layer of Pd-TMS porphyrin in a PFO Host (Left) and hemin protein (Right)

Structure Name PL Spectrum Details

  • Non-linear IV characteristic for hemin protein device
  • Turn on voltage of 11V
  • Non-linear rectifying IV characteristic for Pd-TMS porphyrin doped PFO device
  • High turn-on voltage of 15V due to thick emitting layer of ≈ 80 nm

Graph 3: PL and EL Spectra of Pd-TMS Porphyrin in a PFO Host

  • Photoluminescence spectrum (Red) shows the green emittion from PFO at 467 nm and the red emission from the Pd-porphyrin at 681 nm and 757 nm
  • Electroluminescence spectrum (Black) shows only the Pd-porphyrin spectrum lines
  • Difference may be due to quenching in the thin film or energy transfer from PFO to porphyrin

Note: Intensities are not relative

Future Work

  • Determine and compare quantum efficiency of devices
  • Investigate the energy transfer from PFO to the Porphyrins FRET, Dexter etc
  • Biological porphyrins such as chlorophyll a, cytochrome c and vitamin B12
  • Include electron/hole transport layers in the OLEDs to promote efficiency
  • Investigate conduction mechanisms; thermionic emission, Fowler Nordheim, space charge limited etc
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