Interface Electronic Property Of Organic Semiconductor Thin Film Devices Visualized By Kelvin Probe Force Microscopy

With the rapid development of function materials and organic electronics,organic semiconductor thin films attract more and more attentions for the advantages of low cost,wide material sources,low temperature preparation,simple process,available of large-scale flexible fabrication and good optical and electric properties.Recently,the organic semiconductor devices are booming,in which the understanding of interface electronic properties of devices is one of the important issue for the development of organic electronics.Kevin probe force microscopy(KPFM)is a noninvasive,more direct and accurate method to map simultaneously both surface morphology and electric potential images with high spatial and electrical resolution.Thus as an ideal method to study local electronic properties,it has been widely applied in characterizing the electronic properties,such as work functions,interfacial dipoles,energy bands,and charge trapping.In this paper,we use KPFM to investigate the interface effects of the organic electronic devices,the main contents are as following:1.The interface electronic properties of organic field-effect transistors based on the weak epitaxy growth have been performed by KPFM,the systems with semiconductor template layer and insulated template layer have been studied respectively,we found that the introduction of template layer will not only have induced optimization on the upper organic semiconductor thin film morphology,but also can change the interface electronic structure,which will have a significant impact on device performance.For the system with semiconductor template layer,we adopt pentacene molecules as the top layer and using two oligomers para-sexiphenyl(p-6P)and 2,5-Bis(4-biphenylyl)bithiophene(BP2T)as the bottom template layer.Although they show similar morphology but exhibit significant differences in device performance.By applying the Kelvin probe force measurements on the devices,we found this is due to the different interface electronic properties at the organic/organic heterostrutures in devices.In pentacene/p-6P,the heterostructure interfaces presents a negative interface dipole,the pentacene layer are the major charge transport channel.While in pentacene/BP2T there is a positive interface dipole,the BP2T layer undertake the major charge transport process.Moreover,in the pentacene/p-6P heterostructure,charge transfer effect leads to more holes accumulated in the pentacene layer and accordingly enhances the charge transport in the devices.For the system with insulating template layer,the performance of pentacene transistors based on the insulating template layer dotriacontane(C32)and tetratetracontane(C44)is better than single pentacene device.By KPFM characterization,we found that the introduction of C32 and C44 film would change the interface electronic structure in the devices.Compared with the negative dipole at the pentacene/SiO2 interface,there is a larger negative interface dipole at the interface between pentacene and the insulating template layer C32(C44).The larger negative interface dipole will lead to more holes accumulate in pentacene and make the device performance better.2.The surface potential of different typical model systems have been characterized by KPFM and their applications in the sub-surface structure location detection have also been studied.The mechanisms and applicability of the sub-surface imaging based on the simple model systems consisting of the insulating,conducting and semiconducting polymer and Au-Si patterned conductive substrate were investigated and discussed by analyzing the influence of interface interaction between covering material and sub-surface bulk phase material on surface potential,which provides a theoretical basis for the study of sub-surface imaging with the complex multi-component organic electronic devices.The results demonstrate that the sub-surface imaging effect is good for the system with insulating polymer and its imaging effect is independent of the insulating polymer thickness.While for the system with conducting polymer,the sub-surface structure is undetectable by KPFM due to the electrical shielding effect of the covering conducting polymer.For the system with semiconducting polymer,it is complicated and need to be analyzed according to the energy level matching between upper material and patterned conductive substrate in the structures.