Active channel field-effect transistors: Towards high performance light emitting transistors

Abstract

This thesis describes the study of the development and device physics of organic field-effect transistors and organic light-emitting field-effect transistors. Using chemical and physical properties of organic semiconductors such as light emission, multi-functional devices, or “Active Channel Field-Effect Transistors” can be created. This thesis first describes the creation of basic ambipolar FETs before building on this knowledge to develop ambipolar as well as hole-dominated light emitting field-effect transistors. Ambipolar FETs using a diketopyrrolopyrrole-based copolymer were first investigated in this thesis. When gold electrodes were used, the devices could transport only holes. The charge injection was altered by changing the contacts to aluminium. The devices then were able to conduct both electrons and holes with maximum electron and hole mobilities of order 10−4 cm2 V−1 s−1 and 10−3 cm2 V−1 s−1 respectively. Light-emitting field-effect transistors were first studied using the model emissive polymer Super Yellow. Charge injection was studied by altering the electron-injecting contact. Materials used included the low work function metals Ca, Ba and Sm, the inorganic salt Cs2CO3 and the organic material TPBi. Ambipolar devices were created using a neat Super Yellow layer. Electron and hole mobilities in these devices were both of the order of 10−4 cm2 V−1 s−1. The maximum external quantum efficiency was 1.2 ± 0.2 % with a Sm electron-injecting contact, however this occurred at a brightness of less than 1 cd m−2. A maximum brightness of 43 ± 9 cd m−2 was obtained using a Cs2CO3-Ag electron-injecting contact in electron accumulation mode. At this brightness, the external quantum efficiency was 0.19 ± 0.04 %. Unipolar devices were also fabricated by adding high mobility charge transport layer of PBTTT underneath the Super Yellow. Since PBTTT had a hole mobility that was orders of magnitude greater than the charge carrier mobilities of Super Yellow, charges were transported in the PBTTT, and recombined within the Super Yellow layer. Super Yellow acted as an emissive layer only. Using this bilayer device architecture, a maximum brightness of 100 ± 4 cd m−2 was obtained with a Ca electron-injecting contact. An external quantum efficiency of 0.04 ± 0.01 % was achieved at the maximum brightness. In order to improve the external quantum efficiency of LEFETs, phosphorescent materials were chosen for use as the emissive layer. A co-evaporated layer of the iridium complex Ir(ppy)3 in a host of CBP was initially used in the devices. Charge injection was studied by using either a Ba or a TPBi/Ba electron-injecting contact. Devices were first fabricated using PBTTT as a hole-dominated charge transport layer. Using a TPBi/Ba electron-injecting contact, the maximum brightness achieved was 200 ± 40 cd m−2 with an external quantum efficiency of 0.18 ± 0.01 % at the maximum brightness. Devices were then fabricated using a charge transport layer of DPP-DTT to create ambipolar devices. Electron and hole mobilities in these devices were both of order 10−2 cm2 V−1 s−1. The maximum external quantum efficiency was 0.4 ± 0.2 % at the maximum brightness of 500 ± 200 cd m−2 in hole accumulation mode using an electron-injecting contact of TPBi/Ba. In order to avoid using a co-deposited emissive layer, a solution-processed iridium-cored dendrimer layer that could be deposited without a host material was used. Like in the previous work, a charge transport layer of PBTTT was deposited underneath the emissive layer. Charge injection in the devices was altered using a change in the device geometry. Non-planar source and drain contacts were used, where one contact was deposited directly on top of the charge transport layer and the other contact on top of the emissive layer. Planar contacts were also used as control devices. There was clear evidence for transport of both holes and electrons when planar contacts were used, with light emission occurring at both contacts. The maximum mobility, brightness and external quantum efficiency was achieved using the non-planar contact geometry. Holes could be directly injected into the charge transport layer, rather than via the emissive layer. The hole mobility in these devices was 3.5 × 10−3 ± 6 × 10−4 cm2 V−1 s−1. The maximum brightness was 140 ± 50 cd m−2 with an external quantum efficiency of 0.11 ± 0.02 % at the maximum brightness. This thesis describes work on the study of charge injection into organic field-effect transistors by changing either the material used for the electron-injecting contact, or by changing the device geometry. This was achieved in devices that were either dominated by holes, or showed evidence of both electron and hole transport within the device. By changing the charge injection into the device, it was possible to improve the charge carrier mobilities, the brightness or the external quantum efficiency. Improving one or more of these things brings light emitting transistors towards becoming a viable technology.