Organic Light Emitting Diodes (OLEDs) consists of one transparent conducting anode that is normally indium tin oxide, or “ITO”, one or more hole transport layers (HTLs), an emitting layer (EML), and one or more electron transport layers (ETLs), all stacked between a metal cathode and a transparent glass or plastic substrate. Since the HTLs, EML and ETLs are organic materials that are amorphous and semiconducting, these characteristics are typical of OLEDs. Presently it is feasible to produce OLEDs in a more cost-effective manner than conventional LEDs or LCDs and hence they find applications in areas like cell phone displays or even thin and flexible e-paper.
High durability is an important parameter in the physical performance of OLEDs and is associated with a high glass transition temperature and thermal and morphological stability. These properties can be investigated with thermal analysis techniques that include differential scanning calorimetry (DSC) and thermogravimetry (TG). Additionally, the evaporation behavior of OLED materials in a vacuum has attracted a lot of interest as OLEDs are created through vapor deposition in a vacuum.
OLED Example 1
Temperature-dependent mass change (TG), rate of mass change (DTG) and heat flow rate (DSC) of an OLED material measured under high vacuum using TG-DSC is shown in Figure 1.
Figure 1. Temperature-dependent mass change (TG), rate of mass change (DTG) and heat flow rate (DSC) of an OLED material measured under high vacuum
In this example, an OLED material of unknown composition was heated in a high vacuum ranging approximately between 10-5 mbar using the NETZSCH STA 449 Jupiter simultaneous thermal analyzer. The heating rate was 5 K/min and the initial sample mass was 7.32 mg. Total evaporation of the sample took place in the range from about 210°C to 280°C, with a maximum rate at 268°C as observed in the TG and DTG curves in Figure 1. Moreover, the DSC signal exhibited an endothermic effect with an enthalpy of about 0.58 kJ/g, which is the energy needed for the evaporation process.
OLED Example 2
As a second OLED example, α-NPD was studied using the NETZSCH STA 409 CD SKIMMER, which allows simultaneous measurement through TG and DSC along with mass spectroscopy (MS) of the evolved gases. The substance α-NPD (N, N’-Di[1-naphtyl]-N, N’-diphenylbenzidine) is used in OLEDs as a hole transport layer at the anode or as an electron-blocking stratum in the emitting layer.
Figure 2 shows the mass changes and DSC signal of α-NPD, the initial sample mass is 5.24 mg and is determined in a dynamic helium atmosphere having a flow rate of 100 ml/min at a heating rate of 10 K/min. The sample did not show any mass change before reaching about 350°C, as reflected by the horizontal TG curve. The DSC signal showed an endothermic peak at an extrapolated onset temperature of 278°C which is because of melting of the sample. A heat of fusion of 108 J/g was determined from the peak area. In the temperature range from 350°C to 500°C, a mass loss of 85.3% occurred, due to sample decomposition. The mass loss occurred along with an endothermic effect visible in the DSC having an enthalpy of 193 J/g.
Figure 2. Temperature-dependent mass change (TG), rate of mass change (DTG) and heat flow rate (DSC) of the sample
Figure 3 shows an overview of the TG-MS results obtained for the α-NPD sample. This 3-D graph, is one of the main highlights of the NETZSCH Proteus software generation 6.0 and displays the temperature-dependent TG-MS data in the range from 350°C to 500°C. Furthermore, the mass spectrometer signals are a function of the mass-to-charge ratio, m/z of the detected ions, where z is normally 1 and m/z thus reflects the mass number. The strongest MS intensities were seen at about 479°C, which is in very good correlation with the TG and DTG curves (see also Figure 2).
Figure 3. Temperature-dependent mass change (TG, green curve on the left) and mass spectrometer signals (ion current for different mass numbers, m/z). The mass spectrum at 479°C is highlighted in white.
Figure 4. Mass spectrum of the sample α-NPD measured at 479°C
This shows the quick movement of the evolved gases from the sample directly into the mass spectrometer of the SKIMMER system. Figure 4 also shows the mass spectrum at 479°C as a two-dimensional plot where certain exemplary strong mass numbers are marked. Such a mass spectrum can serve as a type of fingerprint that allows identification of the evolved gases, which can be done through a technique like the NIST library search supported by the NETZSCH Proteus software. In case the substance is known, as in the case of α-NPD, some fragments and molecules such as phenyl and naphthalene may still be present and these were also detected with strong intensities as in Figure 5. The mass spectrum shown in Figure 4 contains a lot of other fragments, isotopes and multiplies charged ions.
Figure 5. Temperature-dependent mass change (TG) mass spectrometer signals (mass numbers 77, 218, 127, 152 and 370) of the sample α-NPD together with its structural formula.
To summarize, thermogravimetry (TG) enables the investigation of the evaporation even in a vacuum and also the decomposition behavior of materials used in OLEDs and thus, also of their thermal stability. Differential scanning calorimetry (DSC) is used to measure caloric effects that include glass transition or melting and specific heat capacity. Detection of the evolved gases with mass spectroscopy (MS) or Fourier Transform Infrared Spectroscopy (FTIR) allows for a chemical analysis of the sample. Simultaneous Thermal Analysis (STA) coupled to evolved gas analysis can apply the TG, DSC, MS and/or FTIR methods simultaneously in a single experiment.
This information has been sourced, reviewed and adapted from materials provided by NETZSCH-Gerätebau GmbH.
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