The Fabrication And Study Of Ambipolar Organic Field Effect Transistors And Organic Light Emitting Transistors

As one of the key interests of organic electronics, OFET (Organic Field EffectTransistors) has shown widely application potential in the fields such as plasticintegrated circuits and transparent display devices. Since ambipolar OFET has certainadvantages in fabricating CMOS (Complementary Metal Oxide Semiconductor)integrated circuits, it has drawn more attention in recent years. OLET (Organic LightEmitting Transistors) is a new functional organic electronic electroluminescent devicebased on the architecture of OFET. OLET is considered as one of the competitive wayfor the next generation of flexible displays since it combines the characteristics ofboth OFET and OLED. Meanwhile, it is also an ideal architecture to fulfill electricalpumped organic laser because it is easier to obtain higher current density with such adevice than that with the architecture of OLED.We are aimed to fabricate high performance ambipolar OFET and OLET. Wehave done deeply research about the processes. By optimizing the processes, we haveovercome the problems such as low carrier mobilities, high working voltages, badoperational stability and emitting characteristics. The key research work and resultsare as follows:1. Fabrication of air-stable ambipolar OFET with balance and high carriermobilities. We have chosen pentacene and PTCDI-C13as hole and electron transport layer, respectively, combined with bilayer heterojunction architecture. Theoptimization includes the substrate temperature, the thickness and evaporation rate ofboth pentacene and PTCDI-C13film. We observe that a changing growth rate ofPTCDI-C13film will result considerable enhancement for carrier mobilities and airstability. The mobility for hole and electron increase from0.09cm2/V s and0.05cm2/V s to0.11cm2/V s and0.10cm2/V s, respectively. By further modifying theSiO2substrates with PS, the devices show good air stability and remarkable high andbalanced carrier mobilities with0.41cm2V s and0.40cm2V s for hole and electron,respectively, which is the highest for air-stable ambipolar devices to our knowledge.By investigating the morphology of the PTCDI-C13films at different growth stages,we confirm that the improvement is due to the greater grains at the beginning of thegrowth process under the slower growth rate, and a different rate thereafter willenable PTCDI-C13to fill the gaps of former grains, which results less barriersbetween grain gaps and therefore higher carrier mobilities. CMOS inverters withsignal gain of38are also fabricated based on such high performance devices.2. Fabrication of low voltage ambiploar OFET. We use anodized Al2O3instead ofoxidized SiO2, modified with polystyrene (PS), as the gate insulator, together with thechanging growth rate method. The influence of different anodized voltages isinvestigated. When15V is applied as the anodized voltage, the working voltage ofambipolar OFET is reduced to5V. By modifying the SiO2substrates with PS, thedevices show carrier mobilities with0.50cm2V s and0.20cm2V s for hole andelectron, respectively. We obtain the relationship between off-current state and thethreshold voltage as well as the source and drain voltages: ΔVGS=(VTH-e-VTH-h)-|VDS|,by analyzing the transfer curves of the device quantitatively. The relationshipindicates that a wide off-current state could be reached by adjusting the thresholdvoltages properly, which proved that ambipolar OFET is able to overcome theproblem of high off-current, and to be used in low power consumption CMOSintegrated circuits. Meanwhile, the working voltage of CMOS inverters based on suchdevices is also about5V. The bias-stress study of the low voltage ambipolar OFETshows different shift phenomenons from that of unipolar OFET reported by literatures, which indicates that the cause of threshold voltage shift of ambipolar OFET is notbased on the electrochemical reactions at the interface of SiO2and organic layer butthe traps inside organic layers.3. Fabrication of high performance OLET. We fabricated tri-layer heterojunctionOLET by incorporating emitting layer Alq3:DCJTI (2%) based on the aforementionedhigh performance ambipolar OFET. The device shows power intensity and externalefficiency of0.45Wm-2and0.005%, respectively. To reduce the injection barrier ofhole between Alq3and pentacene, we investigate the influence of NPB layer, whichacts as hole transportation layer. The result shows that as the thickness of NPBincreased from0nm to11nm, the hole mobility increased from0.004cm2/V s to0.036cm2/V s linearly, which indicates that NPB is able to reduce the barrier betweenAlq3and pentacene. We also find that the doping of DCJTI (2%) into Alq3will makethe electron mobility drops to one half. However, by incorporating a Alq3layer ontothe Alq3:DCJTI (2%) layer, the negative influence will be overcome and aconsiderable enhancement for output power, which is from20pW to170pW, couldbe obtained.4. The design of architectures of organic laser transistors based on OLET. One ofwhich is to incorporate a pair of DBR reflectors on the side of gate electrode and thetop of organic layer, which ensures a vertical micro-cavity. The design results showthat a minor adjustment of the thickness of the optimized OLET, combined with theDBR reflectors fabricated in our laboratory before, will ensure the most intense partof the standing wave inside of the cavity to locate at the Alq3:DCJTI layer and obtainlight output from the top DBR reflector at610nm. Another one is based on secondorder DFB grating. The design result, which is based on the theory of coupled mode,shows that when a rectangular grating with period of402.4nm, duty cycle of0.4anddepth of20nm is incorporated onto the surface of SiO2insulator, we can obtain lightoutput with far-field divergence of0.0065°along the direction of channel width, fromthe top of the device. The threshold current density is1.23kA/cm2with thisarchitecture, which means the carrier mobilities as high as0.1cm2/V s are required.