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Sub–turn-on exciton quenching due to molecular orientation and polarization in organic light-emitting devices

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Science Advances  07 Aug 2020:
Vol. 6, no. 32, eabb2659
DOI: 10.1126/sciadv.abb2659
  • Fig. 1 Comparison of polar and nonpolar ETL device performance.

    (A) Schematic of SOP-induced hole accumulation. Preferred orientation of PDMs (denoted as arrows) in the electron transport layer (ETL) leads to net polarization sheet charges on each side of the ETL (δ and δ+). These polarization charges are compensated by hole accumulation within the EML (red circles with positive signs). (B) Device schematic and molecular structures of tris-(1-phenyl-1H-benzimidazole) (TPBi; polar ETL) and 4,6-bis(3,5-di(pyridin-3-yl)phenyl)-2-methylpyrimidine (B3PyMPM; nonpolar ETL). (C) Normalized photoluminescence (PL; symbols) and luminance (solid lines) as a function of voltage for devices with TPBi (red circles) and B3PyMPM (blue squares) ETLs with thicknesses of 50 and 47 nm, respectively. (D) Internal quantum efficiency (IQE; solid lines) and normalized lock-in PL intensity (symbols) as a function of absolute current density for devices with a B3PyMPM or TPBi ETL. Normalized PL data are multiplied by ηPL = 92%, the intrinsic PL efficiency of Ir(ppy)3 at this doping concentration (26). Arrows indicate the sweep direction for the PL data, where the change in direction is due to negative photocurrent. Shaded regions in (B) and (C) are 95% confidence intervals of the mean weighted by Student’s t factor based on measurements of three to six separate pixels.

  • Fig. 2 Displacement current and lock-in PL measurements for various ETL thicknesses.

    (A) DCM sweeps (top) and lock-in PL measurements (bottom) as a function of applied voltage for different thicknesses (dETL) of the TPBi ETL. For visualization clarity, current is normalized to the value at 2 V (raw data shown in fig. S3). The onset of the roll-off in PL intensity coincides with the hole-injection voltage (Vinj; denoted with dashed arrows). (B) The same measurements for a B3PyMPM ETL, which show no discernible shift in hole-injection voltage or quenching of PL before device turn-on. (C) Vinj and hole sheet density (σh) for TPBi devices extracted from the DCM sweeps in (A). Error bars represent SDs over at least three pixels. (D) Full voltage dependence of lock-in PL data for devices with an ETL composed of TPBi or B3PyMPM. Colors correspond to the legends in (A) and (B). Shaded regions are 95% confidence intervals of the mean weighted by Student’s t factor based on measurements of three to six separate pixels.

  • Fig. 3 Device performance and PL measurements for heated depositions of TPBi.

    (A) Device structure, where substrates are heated during deposition of the EML and ETL but are kept at room temperature for all other layers. (B) EQE as a function of current density for devices deposited at 25°, 47°, 61°, and 87°C. To remove optical effects from film thickness variation, EQE values were corrected by the calculated outcoupling efficiency based on the measured dETL (table S1). Correction factors in this figure were ≤3%. Raw data are included in fig. S5. (C) DCM sweeps and (D) corresponding PL as a function of voltage for the same deposition temperatures. The right axis in (C) shows the conduction current density above device turn-on, showing that heating during deposition negligibly influences electrical characteristics. Shaded regions in (B) and (D) are 95% confidence intervals of the mean weighted by Student’s t factor based on measurements of three to six separate pixels.

  • Fig. 4 Temperature dependence of peak EQE, PL, hole-injection voltage, and charge density.

    (A) Peak EQE and normalized PL extracted near the peak EQE at J = 3 × 10−2 mA/cm2, as a function of deposition temperature. The right axis gives the percentage increase of both variables, showing good agreement. EQEs are outcoupling corrected, as in Fig. 3B (raw data in fig. S5). The 107°C device had a 70-nm-thick ETL, resulting in a correction factor of 12%. (B) Hole-injection voltage, Vinj, and injected hole sheet density at turn-on, σh, extracted from DCM as a function of deposition temperature. Solid lines are guides to the eye. Y-axis error bars are 95% confidence intervals of the mean weighted by Student’s t factor based on measurements of three to six separate pixels. a.u., arbitrary units.

  • Fig. 5 Modeling exciton quenching under electrical and optical pumping.

    (A to C) Contour plots of the percent difference between modeled and measured EQE (red) and PL (blue) at 2.5 V as a function of input parameters for the CBP:Ir(ppy)3 device in Fig. 1A. The hole distribution width (w) characterizes exponential decay (ex/w) from the EML/ETL interface. Exciton diffusivities are (A) D = 1.4 × 106 nm2/s (LD ~ 1 nm), (B) D = 2.2 × 107 nm2/s (LD ~ 4 nm), and (C) D = 1 × 108 nm2/s (LD ~ 9 nm). (D) Hole (nh, top) and exciton (nT, bottom) density of one plausible solution for the CBP:Ir(ppy)3 device in Fig. 1A. nT with and without hole quenching is shown in solid and dashed lines, respectively. Solution parameters are D = 2 × 107 nm2/s (LD ~ 3.9 nm), kTP = 5 × 10−13 cm3/s, and w = 3 nm. (E) Modeled and measured PL at turn-on (2.5 V) as a function of deposition temperature for the TPBi:Ir(ppy)3 device in Fig. 3A. The measured charge densities in Fig. 4B are the only model inputs, which vary with temperature. Temperature-independent parameters are D = 2 × 107 nm2/s (LD ~ 3.9 nm), kTP = 3 × 10−13 cm3/s, and w = 4 nm (exponential decay from HTL/EML interface).

Supplementary Materials

  • Supplementary Materials

    Sub–turn-on exciton quenching due to molecular orientation and polarization in organic light-emitting devices

    John S. Bangsund, Jack R. Van Sambeek, Nolan M. Concannon, Russell J. Holmes

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    • Figs. S1 to S8
    • Table S1

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