Device Characteristics of MEH-PPV Polymer LED

organicex01.in : Device Characteristics of MEH-PPV Polymer LED

Requires: Blaze/Organic Display
Minimum Versions: Atlas 5.22.1.R

This example demonstrates the basic characteristics of an Organic Light Emitting Diode (OLED). It demonstrates:

  • OLED structure definition using Atlas syntax
  • Material parameter specification
  • Choosing the OLED physical models
  • Current density versus anode voltage characteristics
  • Current density versus electric field characteristics
  • Luminescent power of the OLED
  • Langevin recombination plot at 10V

The file starts with the SET statement which defines the thickness variables for substitution into Atlas syntax. This is followed by defining the OLED structure such as the mesh, region, electrode location and doping. This is a three layer device consisting of the MEH-PPV polymer film sandwiched between an Indium-Tin-Oxide (ITO) coated glass substrate and a metal. The ITO is the anode while the cathode is made up of metal. In addition, the polymer film is p-type doped with a uniform concentration of 1e15 cm-3.

After the device description, the MATERIAL statement is used to specify the material properties of the ITO and also the MEH-PPV material. In the first MATERIAL statement, the REAL.INDEX parameter is used to set the real refractive index (which is directly related to the absorption coefficient) of the ITO. In the second MATERIAL statement, AFFINITY specifies the polymer LUMO level and EG300 specifies the bandgap of the polymer. This gives a HOMO level of 4.9eV (i.e. HOMO level = EG300 + AFFINITY ). RST.EXCITON specifies the exciton singlet to triplet ratio of the polymer. This parameter is a key factor in determining the ultimate limit of the OLED efficiency. TAUS.EXCITON specifies the singlet exciton lifetime and LDS.EXCITON specifies the singlet exciton diffusion length.

Other material parameters of the polymer such as bandgap, density of state (Nc and Nv), relative permittivity, lifetimes, Richardson constant and real refractive index are also specified in this statement.

After the MATERIAL statements, the MOBILITY statement is used to set the polymer mobility parameters for the Poole-Frenkel-like mobility model. Parameters such as the DELTAEN.PFMOB and DELTAEP.PFMOB specify the thermal activation energy of the organic polymer at zero electric field for electrons and holes respectively, while BETAN.PFMOB and BETAP.PFMOB specify the electron and hole Poole-Frenkel factor respectively. MUN and MUP are used to set the low field electron and hole mobilities respectively.

The CONTACT statements are used to specify the workfunction of metal and ITO at the cathode and the anode respectively. The first CONTACT statement specifies a workfunction of 2.9eV (equivalent to Calcium) at the cathode. This forms an electron-injection barrier of 0.1eV (i.e. Ca workfunction - polymer affinity) at the cathode. The second CONTACT statement specifies the workfunction of ITO as 4.7eV. This forms a hole-injection barrier of 0.2eV given the polymer HOMO level of 4.9eV. The NEW.SCHOTT option specifies a heterojunction thermionic tunneling model to be used, while the AUTO.OHMIC option automatically converts the contact to ohmic when there is no barrier.

In the MODELS statements, several organic models are used such the Poole-Frenkel-like mobility model ( PFMOB ), the Langevin bimolecular recombination model ( LANGEVIN ) and the singlet exciton model ( SINGLET ). The PFMOB and LANGEVIN models account for the transport and recombination mechanism of the polymer, while the SINGLET model is used to calculate the radiative rate for luminescence due to the Langevin recombination in the OLED. The PRINT option prints the status of all models, coefficients, and constants in the runtime output window.

After specifying the structure, an initial solution is first performed with the SOLVE INIT statement. This is followed by biasing the anode to 10V. Here, the luminous wavelength L.WAVE is set in the SOLVE statement so that the luminous power can be extracted during the ramp. A PROBE statement is included to probe the maximum electric field of the device at x = 5um.

The current density versus voltage and the current density versus electric field data are then extracted via the EXTRACT statement. After extracting the data, these data are plotted with the TONYPLOT statements. For the first plot of current density versus voltage, you should select to display "anode bias" along the x axis and "anode current" along the y axis. The second plot shows the current density versus the electric field.

The third plot is the Luminescent power of the OLED. In this figure, it is seen that the device "turns-on" at about 2 Volts. Finally, the contour plot of the Langevin recombination at 10V is displayed. In this case, most of the radiative recombination is seen to be confined within the polymer layer close to the cathode.

To load and run this example, select the Load example button in DeckBuild. This will copy the input file and any support files to your current working directory. Select the run button to execute the example.