Organic Semiconductor Devices Studied By Kelvin Probe Force Microscopy

With the developments of functional nano materials and organic electronics,organic semiconductor devices attract most researchers for the unique advantages of low cost,aboundant materials,available of large-scale fabrication and capable of application on flexible substrates.The mechanisms of the organic semiconductor devices remain further study since the developement is under initial stage.Charge behaviors such as charge generation,transfer,transport,recombination,and trapping are essential mechanisms for the device performances,which attribute to the structures and electronic properties of materials.Therefore,analyzing the structures and the corresponding electronic properties are pivotal for studying the mechanisms of the devices.Kelvin probe force microscopy(KPFM)is a tool that measures the topography and the corresponding electronic property of a material in the nanoscale,which is widely applied in characterizing the work functions,interfacial dipoles,energy band bendings,and charge trappings in the nano materials.The thesis studies organic field-effect transistor(OFET)based nano-floatinggate non-volatile memories and organic solar cells(OSCs)utilizing KPFM,and established several methods that analyzing charge behaviors in the organic semiconductor devices in the micro perspectives by KPFM.In the first chapter,the developments and principles of KPFM are introduced,and KPFM data are elaborated followed by an introduction of KPFM applications on studying the micro mechanisms of organic semiconductor devices.In the second chapter,a method to achieve charge injection and measurement on insulator polymer films by KPFM is established.PeakForce tapping mode is first selected to perform charge injection experiments,and C program is applied to control the probe behaviors.Charge measurements by KPFM at different conditions are analyzed and optimized parameters are obtained.Meanwhile,experiment platform is built to allow charge injections with high voltages and KPFM measurements on high surface potentials.In the third chapter,charge injections and measurements on nano-floating-gates of OFET non-volatile memories have been performed by KPFM,hence the charge behaviors in the working devices can be studied.By comparing the KPFM results with the device results,the method to study charge trapping of organic semiconductor in the micro perspective has been demonstrated.Finally,in-situ KPFM measurements on OFETs have been performed and a method to study the mechanisms in working devices has been presented.In the fourth chapter,the phase segregations of the bulk heterojunctions(BHJs)in all-polymer solar cells(all-PSCs)have been studied.By analyzing the CPD results on the BHJ,the phase segregation on the top can be distinguished;moreover,the component distribution at the BHJ bottom can be obtained,which presents a new method to study sub-surface imaging for composites.The parameter R_q(root-mean-square deviations of the profile,RMSD)is introduced in the following work to quantitatively describe the contrast in CPD images,by which the phase segregation states can be described quantitatively.Consequently,manufacturing conditions on the phase segregation of the BHJs have been studied.Finally,KPFM measurements with light illumination have been performed and charge behaviors in the BHJs under illumination are studied.