Carrier injection and bipolar transport in NPB for single-layer OLEDs

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					                  Hong Kong Baptist University                                                                                                                                                                              SPIE-OP-07, Organic Light Emitting Materials and Devices XI, 6655-58

                                          Carrier injection and bipolar transport in NPB for single-layer OLEDs
                                                                                                                                                                                    Appl. Phys. Lett. 90, 213502 (2007).
                                                                                S.C. Tse, K.K. Tsung and S.K. So
                                              Department of Physics and Centre for Advanced Luminescence Materials, Hong Kong Baptist University
Introduction & Aims
• Single-layer organic-light-emitting diode (OLED) is very appealing because of simplified device structure.
                                                                                                                                                                                                                                                                                                                                 N                              N
• We demonstrate below that N’-diphenyl-N,N’-bis(1-naphthyl)(1,1’-biphenyl)-4,4’diamine (NPB), a well-recognized hole transporter, can be used in
  single-layer OLEDs. The following three issues will be addressed: a) Bipolar transport of NPB, b) Single-layer OLEDs based on NPB, and c) Nearly
  Ohmic injection from PEDOT:PSS to NPB.                                                                                                                                                                                                                                                                                 Fig.1 Chemical structure of NPB


                                                                                             Photocurrent (a.u.)
A. Bipolar transport of NPB                                                                                                290K                                                                              B. Single-layer OLEDs based on NPB                                                                (a)                        *              PEDOT : PSS
                                                                                                                                                                                           τ                                                                                                                             O       O
   Electron and hole mobilities of NPB were evaluated                                                              2
                                                                                                                           50kV/cm                                       1
                                                                                                                                                                                                                 Single-layer OLEDs was fabricated with a configuration of                                                             :
by time-of-flight (TOF) measurement. Follows are the                                                                                                                                                         ITO/ PEDOT:PSS (50nm)/ NPB (120nm)/ Ca/ Ag. Current-                                                   *        S       n*
experimental details:                                                                                              1                                                 0.1                                     voltage (JV) characteristics of the single-layer device was
                                                                                                                                                                             10               100                                                                                                                                                                      N
                            Al                      ITO                                                                                                                                                      compared to the theoretical hole space-charge-limited current
                N2 laser                                                                                                                                                                                                                                                                                                    N
               (337.1nm)      h   Organic                                                                          0                                                                                         (JSCL) of NPB:                                                                                                                       DSA-Ph
                              h    (d µm)                  CRO                                                          0              20                         40         60       80
                                                                                                                                                             Time (µs)                                                9                       F2       where µ0 and β were                                                                                            LUMO
                                                                                                                                                                                                               J SCL = µ0ε 0ε r exp(0.89β F )
                                     d                                                                                                                                                                                                                  evaluated from TOF.                                     (b)                                           2.4eV
                                                                                  Fig. 2 Electron TOF transient for NPB under an                                                                                      8                       d                                                                                                    2.7eV
                              V                                                   applied field of 50kV/cm. The film thickness was
                                                             R                    15.3µm.                                                                                                                        To confine the recombination zone, DSA-Ph was                                                                                                           Ca
                                                                                                                                                                                                                                                                                                                    PEDOT            NPB                       NPB    (2.9eV)
                   Operating pressure: < 10-3 torr                                                                 -3                                                                                        intentionally doped in the middle of the single-layer OLEDs
                                                                                                        10                                               NPB (µe)                                                                                                                                                    :PSS
                                                                                                                                                                                                             with 10% concentration. The doped device structure was ITO/                                           (5.15eV)
• Set up a electric-field with reverse bias voltage V.                                                                                                                                                       PEDOT:PSS (50nm)/ NPB (40nm)/ NPB:DSA-Ph (40nm)/

                                                                                  Mobility (cm /Vs)
• Optical generation of free carriers from ITO (electron)                                                                             NPB (µh)                                                               NPB (40nm)/ Ca/ Ag.                                                                                                     5.4eV
                                                                                                                                                                                                                                                                                                                   HOMO                                     NPB:DSA-Ph

  or semi-transparent Al (hole).                                                                                   -4
                                                                                                        10                                                                                                   Results and discussions                                                                            Fig. 4 (a) Chemical structure of PEDOT:PSS
• Current was monitored by CRO and charges collected                                                                                                                                                                                                                                                            and DSA-Ph. (b) An energy diagram of the
                                                                                                                   -6                                                Alq3 (µe)                               For applied voltage < 4V,                                                                          undoped and DSA-Ph-doped NPB single-layer
  by a counter electrode at a distance d.
• Flight time τ was determined from transient plot and                                                  10
                                                                                                                   -7                N                                                                       • JV curves of single-layer OLEDs match with the
                                                                                                                                     O                   O
  evaluate the carrier mobilities from:
                                                                                                                                                    Al       N
                                                                                                                                                                             Alq3 (µh)                         computed hole JSCL. Thus, the conduction of NPB                                      10
                                             d2                                                                    -8                               O                                                                                                                                                               Undoped
                                        µ =                                                             10

                                                                                                                                                                                                                                                                         Current density (mA/cm )
                                                                                                                                                                                                               devices are hole-dominated and bulk-limited.

                                                                                                                                                                                                                                                                                                                    DSA-Ph-doped                                           4
  (Figure 2)                                τ ⋅V                                                                                                                                                                                                                                                         2                                                            10

                                                                                                                                                                                                                                                                                                                                                                               luminance (cd/m )

• Mobilities follow the Poole-Frenkel dependence:                                                                      0                                 400                  800                            • NPB has a nearly Ohmic hole injection from
                                                                                                                                                                                         1/2                   PEDOT:PSS.                                                                           10                  d                                                  3
                                                                                                                            [Electric field (V/cm)]                                                                                                                                                               ulate
                    µ = µ0 exp(β F )                                                                                                                                                                                                                                                                     0    Sim J CL                                                10
                                                                                                                                                                                                                                                                                                    10         hole

     where µ0 is zero-field mobility and β is PF slope.                               Fig. 3 Electron and hole mobilities of NPB and                                                                                                         4V,
                                                                                                                                                                                                                      For applied voltage > 4V,
                                                                                      Alq3 at 290K. The inset is Alq3 structure. [1]                                                                                                                                                                 -1                                                                    2
                                                                                                                                                                                                                      • JV curves of NPB devices acquire                                            10                                                                10
                                                                                                                                                                                                                        different shape due to electron injection                                    -2
C. Nearly Ohmic injection from PEDOT:PSS to NPB                                                                                                                                                                         from the Ca cathode.                                                         -3                                                               10

    In order to probe the contact condition between PEDOT:PSS                                                                DISCLC measurement                                                                                                                                                     10
                                                                                                                                                                                                                      • The turning point of the JV curve is at
and NPB, admittance spectroscopy (AS) and dark-injection space-                                                                                                          PEDOT:PSS              Ag                      ~4V and coincides with the starting                                         10
charge-limited current (DISCLC) transient were performed on                                                                                                                                                             voltage of LV curves, at which the                                               0           2           4            6         8      10
                                                                                                                                                                 ITO                 NPB
hole-only devices with configuration of ITO/ HIL/ NPB /Ag,                                                                                                                    h
                                                                                                                                                                                                                        detectable light emission can be observed.                                                           Voltage (V)
                                                                                                                                                                              h     (d µm)
                                                                                                                                                                                    (4.11µm)             CRO
where HIL can be ITO or PEDOT:PSS.                                                                                                                                            h                                       • The turning point of the JV curve                             Fig. 5 JVL characteristics of NPB devices. The filled (■)
                                                                                                                                                                 V                                                                                                                    and opened (□) symbols represent the undoped and DSA-
                                                                                                                                                                                                                        corresponds to the onset of electron                          Ph-doped devices, respectively. The solid line is the
   When HIL = PEDOT:PSS, a well-defined minimum was
                                                                                                                                                                                                         R              injection from the Ca cathode.                                simulated hole JSCL of NPB. [2]
observed from the capacitance plot in AS measurement.
Meanwhile, in DI experiment, the DI signal exhibits an ideal                                                                                                     Operating pressure: < 10-3 torr
characteristic with a distinct τDI. The above results indicate that                                                                                      4
                                                                                                                             Current density (mA/cm )

the contact between PEDOT:PSS and NPB is very close to Ohmic.

                                                                                                                                                                                                                       • NPB, an important organic material, was found to possess bipolar transporting
                                                                                                                                                                              τDI                                        abilities by TOF measurement.
AS measurement                                      1.3
                            Ag                                   Hole injection layer (HIL)                                                                                                                            • To demonstrate the bipolar property, NPB was employed as the host material in
        PEDOT:PSS                                                      ITO
                                                    1.2                                                                                                  2                                                               single-layer OLEDs, which attained an appreciable performance.
  ITO              NPB                                                 PEDOT:PSS
                 (3.7 µm)                                                                                                                                                     Hole injection layer (HIL)
                                                    1.1                                                                                                                                                                • Intentional doping was applied to NPB device to confine the recombination
                                         C / Cgeo

           h                                                                                                                                                                        ITO
                                                                                                                                                                                    PEDOT:PSS                            zone. The device performance has a notable improvement upon doping.
                                                    1.0                                                                                                                                                                • Clear AS and DISCLC results indicate that PEDOT:PSS forms an quasi-Ohmic
          Vac               Vdc                                                                                                                          0
         Impedance analyzer                                                                                                                                          0        20   40               60        80         contact with NPB.
                                                          Vdc = 30 V                                                                                                          Time (µs)
  Fig. 7     Experimentally determined
                                                                                                                                                                                                                     Acknowledgments                                                                     References
                                                    0.8                                                                              Fig. 8 Room temperature DI signals of ITO/                                                                                                                              [1]    Appl. Phys. Lett. 89, 262102 (2006).
  frequency dependent capacitances of                     10
                                                                                                                   5                                                                                                  Support of this research by Research Committee of HKBU
                                                                                                                                     HIL/ NPB (4.11µm)/ Ag devices using
  ITO/ HIL/ NPB (3.7µm)/ Ag devices                                                                                                                                                                                   under Grant #FRG/05-06/II-41, and the Research Grant                                   [2]    Appl. Phys. Lett. 90, 213502 (2007).
                                                                 Frequency (Hz)                                                      different HIL materials. The applied field
  using different HIL materials. [3]                                                                                                                                                                                  Council of Hong Kong under Grants #HKBU/211205E                                        [3]    Org. Electron. 7, 474 (2006).
                                                                                                                                     strength was 0.09MV/cm. [4]
                                                                                                                                                                                                                      and #HKBU/211707E are gratefully acknowledged.                                         [4]    J. Appl. Phys. 100, 063708 (2006).