Organic Light Emitting Diodes

More documents

Recommendations

Info

Figure 12: Photoluminiscence and electroluminiscence in conjugated polymers. a) Irradiation can excite an electron from the LUMO in the HOMO and two new energy states are generated. Both are filled with an electron of opposite spin (singlet exited state). Relaxation to the ground state leads to the emission of light of smaller frequency. b) In order to show electroluminescence, radical ions have to be produced in the polymer by the application of an electric field. When radical ions of opposite charge combine, so-called excitons (singlet or triplet excited state) are formed and the decomposition of this neutral excited state (recombination) leads to radiative emission. 3 OLED device structure An OLED device consists of one or more semiconducting organic thin films sandwiched between two electrodes – one of which must be transparent. (Fig. 13). 3.1 Basic PLED Figure 13: basic (two-layer) OLED structure The energy level diagram of a typical single layer PLED is shown in Figure 14. The device utilizes ~100 nm of PPV with an ITO anode and calcium cathode [3,14]. Figure 14: PLED device operation (energy diagram) 10
When a forward bias is applied, electrons are injected from the cathode into the LUMO of the polymer and holes are injected from the anode into the HOMO of the polymer. Thus, the electrons must overcome the barrier (electron injection barrier) between the Ca Fermi level and the LUMO level of the polymer. Low work function metals such as Mg or Ca are typically used to minimize this barrier and provide an ohmic contact. A good energy match between cathode and LUMO means that not much energy is lost when electrons are injected. Similarly, to ensure ohmic injection of holes from the ITO Fermi level into the HOMO of the polymer, the ITO may be treated in various ways (e.g., exposure to an ultraviolet-ozone cleaning) to lower its Fermi level. (barrier is here called hole injection barrier.) 3.2 Basic SMOLED The p-n diode structure proves to be a key feature for the OLED device. The basic structure of SMOLED consists of two layers of organic thin films - a hole transport layer – HTL (p-layer by LED) and an electron transport layer - ETL (n-layer by LED) - sandwiched between an anode and a cathode (Fig. 15). These two organic layers, each on the order of about 500 Å thick, provide the appropriate media for transporting charge carriers, toward the interface formed between the two layers [15]. Figure 15: A basic OLED device structure consists of a hole-transport layer and an electron transport layer sandwiched by a cathode and anode. One of the most basic SMOLED device structures uses an organic material called NPB (naphthyl substituted benzidine derivative) as HTL and Alq3 as ETL. In this typical structure we use indium tin oxide (ITO) as the transparent anode and magnesium-doped silver (Mg:Ag) as the cathode. When voltage is applied, charge injection of electrons through the cathode and hole through the anode occurs. Electrons are transported to the LUMO of the ETL, and holes to the HOMO of the HTL. Recombination of these charges occurs across the barriers, with holes primarily moving into alq3 (Fig. 16). Excitons formed in Alq3 in this case emit green fluorescence. See Flash movie [16]. Figure 16: SMOLED device operation (energy diagram) 11
Page 1 and 2: UNIVERSITY OF LJUBLJANA FACULTY OF
Page 3 and 4: 1 Introduction The basic idea of or
Page 5 and 6: Figure 4: Bonding and anti-bonding
Page 7 and 8: that uo and - o both minimize the e
Page 9: Figure 11: Band theory model in pol
Page 13 and 14: Figure 18: Schematic energy level d
Page 15 and 16: Nearby Future a) Big screens - The
Page 17: 7 References [1] C.W.Tang and S.A.

Organic Light Emitting Diodes

Create successful ePaper yourself

Delete template?

Save as template?