Device physics of white polymer light-emitting diodes

PhD ceremony: Mr. Herman Nicolai, 16.15 uur, Academiegebouw, Broerstraat 5, Groningen

Dissertation: Device physics of white polymer light-emitting diodes

Promotor(s): prof. P.W.M. Blom

Faculty: Mathematics and Natural Sciences

The increased awareness of our energy consumption provides an incentive to investigate energy efficient light sources. A promising new light source is the organic light-emitting diode (OLED). In an OLED light emission originates from the recombination of electrons and holes in an organic semiconductor. Although OLEDs are already used in mobile phone displays, they are not yet widely used for lighting. One factor hampering the breakthrough of OLEDs is the fabrication cost. A special class of OLEDs is the polymer light-emitting diode (PLED) in which the active layer consists of a polymer (plastic) semiconductor. Polymer semiconductors can be processed from solution which enables cheap fabrication technologies such as printing. PLEDs therefore offer the potential of cost- and energy-efficient large area lighting solutions. Lighting requires the simultaneous emission of two or three colors so that the output is perceived as white. This can be achieved by the use of a copolymer in which green and red dyes are incorporated in a blue-emitting polymer, so that white light emission can be obtained using only one emissive layer. The fact that the emission of multiple colors takes place in a single layer also complicates the understanding of the device operation. In this work, the operation of a white PLED is unraveled by studying the operation of the blue-emitting PLED and by the stepwise investigation of the influence of the green and red dye. It is shown that the dyes act as charge traps and it is demonstrated the blue light emission originates from the recombination of free charges on the blue backbone polymer, while the green and red light originates from the recombination of trapped charges on the dyes. By combining these recombination mechanisms we can reproduce the device characteristics and the output spectrum of the white PLED.