Organic Light Emitting Diodes for White Light Emission

Abstract

National Physical Laboratory New Delhi has taken up a program for developing WOLEDs for general lighting applications. In this effort a 1"x1" proto type of a multilayer phosphorescent efficient WOLED has been demonstrated (Fig.2). In this review, we like to highlight on the development of white organic LEDs for general lighting. 2. Basic OLED Structure and Operation principles White organic light emitting diodes are thin-film multilayer devices in which active charge transport and light emitting materials are sandwiched between two thin film electrodes, and at least one of the two electrodes must be transparent to light. Generally high work function (∼4.8 eV), low sheet resistant (20 /□) and optically transparent indium tin oxide (ITO) is used as an anode, while the cathode is a low work function metal such as Ca, Mg, Al or their alloys Mg:Ag, Li:Al. An organic layer with good electron transport and hole blocking properties is typically used between the cathode and the emissive layer. The device structure of an OLED is given in Fig. 3. When an electric field is applied across the electrodes, electrons and holes are injected into states of the lowest unoccupied molecular orbital (LUMO) and the highest occupied molecular orbital (HOMO), respectively and transported through the organic layer. Inside the semiconductor electrons and holes recombine to form excited state of the molecule. Light emission from the organic material occurs when the molecule relaxes from the excited state to the ground state. Highly efficient OLEDs which are being developed at present, contains many layers with different functionality like hole injection layer(HIL), hole transport layer (HTL),electron blocking layer(EBL), emissive layer(EML), hole blocking layer(HBL), electron transport layer(ETL) and electron injection layer(EIL) etc apart from electrodes. A schematic diagram of multilayer structure is shown in Fig. 4.

Disruptive characteristics and lifetime issues of OLEDs

“…It is, therefore, questionable that LEDs are highly effi cient because at least 50% of energy was dissipated from the 14 chips to the package. 28,53 OLEDs, on the contrary, belong to the cold emission type, which produce less heat and reduce the generation of waste heat.…”

Review— On the Development of Phosphors for Luminescent Materials: Synthesis, Characterization, Applications and Evolution of Phosphors as White-Light-Emitting Diodes

“…It is also employed to calculate color change with two or more sources (color consistency). 63,64 Under excitation, luminous substances emit radiation (in the infrared to the ultraviolet range). The incident energy is then reemitted as electromagnetic radiation in the form of high-energy electrons, photons, or electric fields.…”

“…Therefore, the color temperature of illumination is frequently used to describe its color. 63,64 Color rendering index (CRI).-When fluorescent lighting became popular during the 1950s and the 1960s, there was an obligation to evaluate and communicate how well innovative lamp color rendering differentiates from conventional incandescent and other source information. 63 The CIE testing procedure also known as the CRI was first utilized in 1965.…”

“…63,64 Color rendering index (CRI).-When fluorescent lighting became popular during the 1950s and the 1960s, there was an obligation to evaluate and communicate how well innovative lamp color rendering differentiates from conventional incandescent and other source information. 63 The CIE testing procedure also known as the CRI was first utilized in 1965. CRI is a measure of the average fidelity or how closely a light source renders colors in comparison with a reference material.…”

“…A CRI of 80 or higher is typically required to achieve illuminationquality white light. [63][64][65] The optimum illuminance for white illumination with a CRI of 80-90 was 350-400 lm W −1 . Most conventional light sources have colors that range between 2850 K and 6500 K for a black body.…”