Organic Light-Emitting Diodes—A Review

Abstract

Light-emitting diodes (LEDs) have become the integral part of almost all electrical and electronic systems and gadgets. LEDs of gallium arsenide (GaAs) emitting infrared light were patented by Gary Pitman and Bob Biard of Texas instruments way back in 1961. Since then, efforts have been made continuously in the direction of developing efficient LEDs in the visible and ultraviolet range also. The literature survey shows that during 1970–1979 there was a significant development in red, green, yellow, orange, and blue LEDs. Out of these, blue LED was very expensive and could not be commercialized until 1994. Although inorganic LEDs in the visible region are in use for the past 3–4 decades as replacement of incandescent bulbs, elements in seven segment displays, large RGB displays, calculator, watches, etc., but the main problems are that (i) the processing of basic inorganic semiconductor material is not environment-friendly due to requirement of high purity (ii) non-flexibility (iii) high cost of processing. Therefore, the researchers all over the world focused their research on developing systems and devices in such a way that not only environment is protected (green systems and green devices), but also requirements like flexibility, low-power consumption, and cost effectiveness be taken care of. Conducting polymer-based organic LEDs (OLEDs) and other devices is the alternative for futuristic devices. In fact the work started extensively after Alan G Mac Diarmid, Alan J. Heeger, and Hideki Shirkawa received noble prize in the year 2000 for their research on conducting polymer (polyacetylene). The present article deals with introduction and working of both inorganic and organic LEDs. The emphasis has been given on organic LEDs. Present state of art has been given in detail since its inception. Starting from a single layer OLED to present day multilayer layer OLEDs with different color-to-color tuning have been discussed in detail. The fabrication, relevant characterization techniques and analysis have also been discussed in detail. A systematic development in terms of improvement inefficiency, luminance, stability, and low-power consumption is given which has been possible due to (i) incorporation on some nanomaterials-like carbon nanotubes, quantum dots, graphene (ii) improving and optimizing the physical conditions of growth which include annealing temperature and its duration (iii) incorporation of electron transport layer (ETL), hole transport layer (HTL), electron blocking layer (EBL), hole blocking layer (HBL) in right sequence in the device structure (iv) improving the morphology of the spin-coated films by controlling spin speed and spin duration (v) concentration of conducting polymer in the organic solvent (vi) controlling thickness of the emissive layer, etc. Some light has also been thrown on future aspects and applications of OLEDs.