All solution-processed, hybrid light emitting field-effect transistors

Abstract

Light emitting fi eld-effect transistors (LEFETs) are an emerging class of multifunctional optoelectronic device. LEFETs combine the switching abilities of a fi eld-effect transistor (FET) with the light emitting ability of an OLED. [1]This is particularly the case for unipolar LEFETs as the switching ability is often lower for ambipolar LEFETs where the gate voltage is less able to switch off the current fl ow. This combination of functionality has the potential to simplify circuitry for new applications such as pixels for displays, sensors, and optoelectronic devices in communications.[2-4]Furthermore, it can provide a platform for studying the optoelectronic properties of organic semiconducting materials. So far, single layer organic LEFETs have been fabricated using polymers, [5,6]evaporated small organic molecules [7,8]and single crystals. [9,10]In these devices there is often a trade-off between the charge mobilities (both electron and holes) and the external quantum effi ciency (EQE). For example, tetracene has a high mobility but low EQE in an LEFET whereas MEHPPV has a lower mobility but a higher effi ciency. [11]A strong disadvantage of these devices is that the highest effi ciencies are generally measured at the lowest brightnesses. These problems have been partially addressed in second generation LEFETs by using two layer devices, one being a high-mobility charge transport layer and the other comprised of an emissive material. These devices have yielded much higher brightnesses, >5000 cd m- 2 , at similar EQEs (0.13%). [12]The use of dendrimer-based materials as emissive layers has yielded similar effi ciencies with the advantage of harvesting the emission from both singlet and triplet excitations. [13]A further feature of the bilayer strategy is that with the correct choice of materials it is possible to tune the colour by varying the voltage, a feature that could be useful for commercialization of LEFETs. [14]However, the majority of hetero-structure (bi-layer) LEFETs operate in the unipolar p-channel regime in which the holes move across the transistor channel and electrons are injected into emissive layer by the low work-function metal electrode, with the emission occurring close to this latter electrode. n-type hetero-structure LEFETs are much more diffi cult to achieve with organic semiconducting materials due to the shortage of high-mobility n-type organic semiconductors [15]and quenching of the electroluminescence by the n-type charge transporting layer.