Daily experiences with organic (carbon based) materials, for example polyvinyl chloride insulation on cables and polyethylene shopping bags suggests that they should be electrical insulators.
This is true for the majority of organic materials however there are a small subset of organic materials with a particular electronic structure that are semiconducting and so can be used to make optoelectronic devices, such as light emitting diodes.
Tang and Van Slyke demonstrated the first organic light emitting diode (OLED) with practical efficiency and brightness whilst working at Eastman Kodak in 1986.1 Since then, OLEDs have been optimized extensively and are currently found in high end television and smartphone displays because of their superior display performance compared to LCDs.
Figure 1a shows a schematic of a typical OLED structure. Electrons and holes are injected into the organic electron and hole transport layers, they recombine in the central emission layer and then the energy is transferred to a dopant molecule via resonant transfer.
The color of the OLED emission is controlled by the choice of dopant molecule which is typically a small molecule with high photoluminescence quantum yield.
Figure 1. Schematic of (a) layers in a typical bottom emitting OLED stack and mechanism of emission in (b) fluorescent doped OLEDs and (c) phosphorescent doped OLEDs.
The dopant molecule was purely organic and the light was emitted through fluorescence in first generation OLEDs. However, due to spin statistics, 75% of the excitons generated when holes and electrons recombine in the OLED are in the non-emissive triplet state (T1) and the maximum efficiency of the OLED is consequently limited to 25% (Figure 1b).
The green and red OLEDs used in modern television and smartphone displays are high efficiency second generation PhOLEDs. However, a stable, high efficiency blue phosphorescent dopant emitter has yet to be found and so further research is required to achieve this.
Characterization and development of new OLED materials and structures necessitate both electroluminescence characterization of the completed device and photoluminescence spectroscopy of the individual components. In this article, the dual functionality of the FS5 Spectrofluorometer for OLED characterization is demonstrated through the investigation of the emission properties of a PhOLED doped with the Iridium based emitter Ir(MDQ)2(acac) using time-resolved and steady state photoluminescence and electroluminescence spectroscopy.
Figure 2. FS5 Spectrofluorometer which can be equipped with a range of source meters, function generators, and light sources for steady state and time-resolved photoluminescence and electroluminescence spectroscopy.
Figure 3. Chemical structure of the phosphorescent emitter Ir(MDQ)2(acac).2
Materials and Methods
Vacuum sublimation was used to fabricate a phosphorescent OLED using Ir(MDQ)2(acac) as the dopant and encapsulated with an optical epoxy and a glass coverslip to prevent degradation. Photoluminescence and electroluminescence measurements were undertaken using an FS5 Spectrofluorometer equipped with a PMT-900 detector, an arbitrary function generator and phosphorescence lifetime electronics (multichannel scaling) for steady state and time-resolved electroluminescence.
The DC offset of the AFG was used to apply a bias to the OLED and measure the stead state electroluminescence spectra. For the time-resolved electroluminescence, the AFG applied a train of short voltage pulses to the OLED and the decay was measured using single photon counting multichannel scaling (MCS).
A Xenon lamp was used to measure photoluminescence for steady state spectra and a 445 nm pulsed diode laser with variable pulse width (VPL-445) was used for time-resolved decays.
Results and Discussion
Figure 4 shows the electroluminescence spectrum of the OLED, measured at a current density of 10 mA cm3. The peak emission wavelength of the OLED is 616 m which results in a bright orange emission, seen in the figure insert. The built-in chromaticity wizard of the FS5’s Fluoracleâ operating software calculates the chromaticity coordinates of the emission were to be 0.63, 0.37 in CIE 1931 color space.
Figure 4. Electroluminescence spectrum of the OLED at a current density of 10 mA cm2. Insert: image of the working OLED with two pixels illuminated.
Figure 5 shows the influence of charge carrier density on the electroluminescence spectrum as investigated by measuring electroluminescence spectra across four decades of current density. There is a pronounced shoulder at 675 nm at the lowest current density (blue). The shoulder feature is at a longer wavelength so must arise from states that are lower in energy than the primary emission peak.
The emission from the shoulder is pronounced because at low current densities the energetically low lying states will be preferentially populated. Figure 5 shows the relative intensity of the shoulder decreases with increasing current, this is because as the current density increases the low lying states become completely filled and additional charge carriers populate the higher energy states instead.
Figure 5. Variation of the electroluminescence spectrum of the OLED with current density and comparison to the optically excited photoluminescence spectrum.
The OLED was also optically excited at 450 nm so that the photoluminescence spectrum could be measured (shown by the dashed black line). The photoluminescence spectrum matches perfectly with the electroluminescence spectra at current densities of both 0.1 mA cm2 and 1 mA cm2. This shows that the excited state that is responsible for the photoluminescence emission is the same as the one accessed electrically. It also suggests that the number of charge carrier density during optical excitation is equivalent to the carrier density when the OLED is driven at current densities in the range of ~ 0.1 mA cm2 to 1 mA cm2.
Figure 6. Measurement of the triplet lifetime of Ir(MDQ)2(acac) using (a) time-resolved electroluminescence and (b) time-resolved photoluminescence spectroscopy. Lifetimes were fit with monoexponential decays using Fluoracle. Electrical excitation: 1 μs 4 V voltage pulses with a repetition rate of 10 kHz applied to the OLED using an AFG. Optical excitation: 445 nm 100 ns pulses with a repetition rate of 10 kHz from a VPL-445 pulsed diode laser. Decays were measured using MCS.
The lifetime of the emissive excited state in the OLED is an important parameter for investigating loss processes in the OLED such as triplet-polaron quenching and triplet-triplet annihilation. This was measured using time-resolved electroluminescence and photoluminescence spectroscopy. Figure 6a shows the electroluminescence decay following a 4 V 1 ms voltage pulse and was fit using a monoexponential decay, using Fluoracle to reveal a lifetime of 989 ns.
From the long lifetime it is clear that this is photoluminescence and it corresponds to the lifetime of the triplet state of Ir(MDQ2)(acac) dopant. The emission is seen to arise wholly from the triplet state with no observable prompt emission from the singlet state.
This measurement was repeated using a 100 ns width pulse from a pulsed diode laser. The photoluminescence decay is very similar to the electroluminescence, with a monoexponential fit showing a near identical triplet lifetime of 991 ns.
Electroluminescence and photoluminescence spectroscopy were used to investigate the properties of a phosphorescent light emitting diode. The emission spectrum of the OLED was measured and showed chromaticity coordinates of 0.63, 0.37 in CIE 1931 color space and a peak emission wavelength at 616 nm.
Time-resolved electroluminescence and photoluminescence measured the lifetime of the triplet state in the OLED as 990 ns. This article demonstrates how the functionality of the FS5 Spectrofluorometer can be extended to include device characterization and offers a complete spectroscopic solution for OLED development and research.
Thanks to Dr. Kou Youshida and Prof. Ifor Samuel of the Organic Semiconductor Optoelectronics research group at the University of St Andrews for fabricating the OLED used in this article.
References and Further Reading
- C. W. Tang, and S. A. Van Slyke, Organic electroluminescence diodes. Appl. Phys. Lett. 51 913-915 (1987)
- J. P. Duan, P. P. Sun, C. H. Cheng, New Iridium Complexes as Highly Efficient Orange–Red Emitters in Organic Light- Emitting Diodes, Adv. Mater. 15 224-228 (2003)
This information has been sourced, reviewed and adapted from materials provided by Edinburgh Instruments.
For more information on this source, please visit Edinburgh Instruments.