Research ArticleCONDENSED MATTER PHYSICS

High-performance organic light-emitting diodes comprising ultrastable glass layers

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Science Advances  25 May 2018:
Vol. 4, no. 5, eaar8332
DOI: 10.1126/sciadv.aar8332
  • Fig. 1 Schematic device structure and performance of the studied device at different substrate temperatures.

    (A) As a first study, a single OLED sample set is prepared using the green emitter Ir(ppy)2(acac) [8 weight % (wt %)] (G0) while evaporating the EML and ETL layers at six different substrate temperatures. The rest of the layers were deposited at RT. (B) j-V and L-V characteristics of the G0 devices deposited at three different substrate temperatures: 32°C (RT), 69°C, and 90°C. Each curve is the mean of two to six devices. (C) EQE versus luminance characteristics for the same three temperatures for the G0 devices. The shadowed area represents the error bars, which are the SD with 95% confidence interval weighted with the Student’s t factor for small amount of samples. The inset shows the corresponding integrated electroluminescence (EL) spectra of the devices obtained at j = 15.4 mA/cm2 in arbitrary units (a.u.). (D) EQE (red, left axis) and LE (blue, right axis) at 100 cd/m2 in dependence of the deposition temperature. Lines are guides to the eyes. The green highlighted region corresponds to the range from 0.84 to 0.9 Tg, where Tg is the glass transition temperature of TPBi expressed in kelvin.

  • Fig. 2 Performance characteristics of the devices R1, G1, and B1 with three different phosphorescent emitters comparing two deposition temperatures.

    EQE (A to C), integrated electroluminescence spectra obtained at j = 15.4 mA/cm2 (D to F), and PL transients of the complete OLEDs (G to I) for the devices R1, G1, and B1, which only differ in the emitter with respect to the G0 stack. (A, D, and G) OLED with the red emitter [Ir(MDQ)2(acac)], (B, E, and H) OLED with a second green emitter Ir(ppy)3, and (C, F, and I) OLED with the blue emitter (FIrpic). For each emitter, two deposition temperatures are studied: RT (31°C) and 66°C. The arrows in (A) to (C) with the label indicate the EQE improvement at a luminance of 100 cd/m2.

  • Fig. 3 Device lifetimes for G0, R1, and G1 at different luminance levels in dependence of the deposition temperature.

    The lifetime LT70 is defined as the time it takes for the initial luminance to drop to 70%. Lower luminance values L lead to higher lifetimes. Samples measured at equal current densities are grouped by dashed ellipses. (A) For G0, we find enhanced LT70 at the temperature close to 0.85 Tg at a luminance L = 10,000 cd/m2 (inset). (B) Devices R1 and G1 show highest lifetimes over the full luminance range for the substrate temperature of Tsub = 66°C. Device B1 could not be measured because of the unstable blue emitter FIrpic.

  • Fig. 4 Anisotropy coefficient and outcoupling efficiency for different deposition temperatures.

    The anisotropy coefficient was determined by angular-resolved PL measurements (inset) of the p-polarized light from 50-nm-thick layers of TPBi:Ir(ppy)2(acac) 8 wt %. On the basis of optical thin-film simulations of dipole emitters in stratified layers, the outcoupling efficiency is calculated. Both quantities stay constant until 0.84 Tg. Reaching the substrate temperature of 100°C, the anisotropy coefficient is increased from 0.30 to 0.35, changing from preferentially horizontal aligned transition dipole moments to a more vertical arrangement. This leads to an absolute drop in outcoupling efficiency of approximately 2%.

  • Fig. 5 Thermal characterization as a function of the deposition temperature of TPBi single layers.

    The fictive temperature (red, left axis) as a thermal stability parameter and the onset of the glass transition (blue, right axis) as a kinetic stability parameter of 60- to 80-nm films of TPBi as a function of the substrate temperature. These parameters are extracted from the heat capacity curves performed using quasi-adiabatic fast scanning calorimetry, as described in Materials and Methods and fig. S5.

  • Table 1 Summary of EQEs and lifetimes (LT70).

    The EQEs are obtained at 100 cd/m2, and the lifetime values are obtained at 1000 cd/m2. RT refers to the tool’s standard temperature, which is close to 30°C. The 0.85 Tg criteria refer to the temperature closest to the optimal growth condition of the TPBi layers.

    Deposition temperatureG0R1G1B1
    EQE (%)RT19.410.117.91.6
    0.85 Tg24.011.621.84.2
    LT70 (hours)RT14.810.259.0
    0.80–0.85 Tg74.222.3110.0

Supplementary Materials

  • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/4/5/eaar8332/DC1

    table S1. Overview of the device lifetimes at different substrate temperatures and luminance values.

    fig. S1. PL transients of G0.

    fig. S2. Normalized forward emission spectra of G0, R1, G1, and B1.

    fig. S3. Optoelectronic characterization of devices R1, G1, and B1.

    fig. S4. Exemplary lifetime and voltage characteristics over aging time for two OLEDs of the R1 series.

    fig. S5. Calorimetric trace of TPBi layers deposited at different temperatures.

  • Supplementary Materials

    This PDF file includes:

    • table S1. Overview of the device lifetimes at different substrate temperatures and luminance values.
    • fig. S1. PL transients of G0.
    • fig. S2. Normalized forward emission spectra of G0, R1, G1, and B1.
    • fig. S3. Optoelectronic characterization of devices R1, G1, and B1.
    • fig. S4. Exemplary lifetime and voltage characteristics over aging time for two OLEDs of the R1 series.
    • fig. S5. Calorimetric trace of TPBi layers deposited at different temperatures.

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