Bright and efficient white stacked organic light-emitting diodes by linzhengnd


									Bright and efficient white
  stacked organic light-
     emitting diodes
    J.X. Sun, X.L. Zhu, H.J. Peng, M. Wong, H.S. Kwok
 Center for Display Research and Department of Electronic and Computer
 Engineering, The Hong Kong University of Science and Technology, Clear Water
 Bay, Kowloon, Hong Kong
 (Received 27 July 2006; received in revised form 10 November 2006; accepted 27 November 2006 .
 Available online 8 January 2007)
                                 Organic Electronics
 Experiment details
 for the conventional single-unit OLED, the peak efficiency is often
 achieved at low current density level (e.g.,0.01 mA/cm2) and hence low
 brightness, remaining a drawback to solid-state lighting

 stacked OLED (SOLED) with N emissive units is showing N times
 luminous efficiency as high as that of the conventional single-unit device

 Structure of the WSOLED along with the molecular structures of organic
materials used in this work
                                               The CIE coordinates of bottom blue unit, top
                                             red unit and total WSOLED at different driving

  Normalized EL spectra of bottom blue unit and
top red unit under different driving voltages
   Results                               ∫Le(λ)×K(λ)dλ=L

          472nm       612nm              Le(λ)- radiance
                                          K(λ)- ocular response curve for cones
                                             L – luminance




EL spectra of the WSOLED under different driving voltages

                                       The peak ratio of blue to red emission in the
                                    WSOLED, together with the experimental results
                                    denoted as asters

   The experimental curves of luminous efficiency vs. current
 density of top red unit and blue bottom unit individually biased
                          for calculation
         (a) Luminance and current density vs. voltage

EL spectra of the alternative WSOLED at a driving
voltage of 17 V

                                    (b) power efficiency and external quantum efficiency vs.
                                   voltage curves of the WSOLED
 efficient white stacked organic light-emitting diodes using a novel ACL
 to connect the blue and red emissive units have been demonstrated.

 In one implementation using singlet red emitter:
 very high brightness - 40,000 cd/m2
 white light with CIE coordinates - (0.32, 0.38)
 a correlated color temperature (CCT) - 5796 K
 a color rendering index (CRI) - 67

 In another implementation using a red phosphorescent emissive unit:
 a much purer white light emission with CIE coordinates of (0.33, 0.31)
 as well as better EQE are achieved.
 this latter efficient white light was achieved at only 17 V with a
 luminance of 4450 cd/m2

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