Polymer Light Emitting Transistors: Control over Charge Injection, Recombination, and Morphology toward High Performance

Light emitting field effect transistors (LEFETs) are a new device class that can clearly turn light on and off without additional driving circuit. Important work was focused on equalizing electrons and holes to maximize the recombination efficiency. However, most semiconducting polymers with visible electroluminescence (EL) are p-type transport materials with well separated HOMO/LUMO energy levels. Single metal contacts cannot efficiently inject both carrier species. Therefore, ambipolar light emitting materials that provide both p-type and n-type transports from identical contacts are attractive. Though outstanding EL efficiency was achieved, ambipolar LEFETs suffer from low brightness because high recombination rate was obtained in the lowest current regime. Very high intensity (8500 cd/m2) was only achieved by applying additional output coupling techniques. Moreover, ambipolar light emitting polymers with decent mobility are extremely rare; seriously limiting Red-Green-Blue colors. Therefore, device architectures that can provide more electronic control on charge transport and injection and more choices for colors were required.;We propose three approaches toward better performance; electrical control via device architecture, optical enhancement for output coupling efficiency, and morphologic control achieving long term ordering. Split-gate transistors with separated gate electrodes can control charge injection and enhance the density of weakly transporting carrier species to achieve efficient recombination. An optoelectronic gate can increase output coupling efficiency and brightness. Additionally, introducing long term ordering into bilayer LEFETs can increase mobility, hence, higher current density and emission intensity. Electrical, optical, and morphological improvements toward better performance are presented.