Studies On Vertical Organic Light-emitting Transistors

Organic light-emitting transistor (OLET) integrates organic thin film transistor (OTFT) and organic light-emitting diode (OLED) into one device, using the transistor to control its luminance. It has advantages of high current density, good controlling ability and simple fabrication process. Vertical organic light-emitting transistor (VOLET) with good performance is presented with a novel configuration, which can shorten channel length and as a result to reduce operating voltage and to improve luminescence efficiency.In this dissertation, we fabricated a kind of VOLET device and characterized its optoelectronic characteristics. The influence of some main factors on the VOLET with Alq₃ as emitting layer was analyzed by inserting a hole-transporting layer and changing the thickness of the dielectric cell and source electrode, as well as by ozone treating on middle Al electrode. Basically, it is mainly included the following content:Firstly, a VOLET device ITO/Alq₃(60nm)/Al(25nm)/LiF(90nm)/Al in which Alq₃ layer acts as emitting layer was fabricated and characterized. We found that the reason for not showing good controlling ability of gate voltage on drain current was the unbalanced carriers injection from source and drain electrodes, respectively, as a result, causing the number of holes much less than that of electrons. In order to enhance hole injection, a pentacene thin film as hole-transporting layer was inserted between drain electrode and emitting layer. The highest luminescence efficiency was obtained from the device ITO/Pentacene(40nm)/Alq₃(60nm)/Al(22)/LiF(120nm)/Al because of its relatively balanced carriers injection.Secondly, we investigated the influence of dielectric layer on VOLET device performance. It was found the device with 150 nm LiF layer as insulating layer showed best performance and it operates in n-type enhancement mode. The experimental results indicated an optimized parameter for dielectric layer could ensure both high capacitance of the capacitive cell and low leakage current of the device. In addition, an organic semiconductor C₆₀ was introduced as both dielectric layer and electron-transporting layer in device ITO/Pentacene(40nm)/Alq₃(60nm)/C₆₀(30nm)/Al(25nm)/C₆₀(20nm)/Al and it was found that C₆₀ acted as both electron-transporting layer and hole-blocking layer for the high HOMO energy level of C₆₀, and electrons injected from gate electrode contributed to both I_ds and luminescence intensity. At last, the influence of middle Al electrode on VOLET was investigated by changing the thickness of source electrode and its treatment by UV-ozone. The "on/off" current ratio and "on/off" luminance ratio were 35 and 46, respectively, at the source electrode thickness of 15 nm with the mean roughness of 9 nm. Ozone treatment of 10 minutes on source electrode may help to significantly reduce the gate leakage current and to enhance capacitance of the dielectric cell, resulting in the "on/off" current ratio to 60 and the "on/off" luminance ratio to 130. The experimental results indicated that the middle Al electrode was the key factor to VOLET, the thickness and surface morphology of the central Al electrode as well as its oxidation level together determined the carrier injection efficiency and the operating capability of gate voltage.