Theoretical Study and in Situ Raman Characterization of the Electronic Processes in Organice Field-effect Transistor

Author:Wang Cong

Supervisor:ma zuo guang


Degree Year:2019





Organic semiconductors have great potential applications in low-cost flexible electronic devices,but current organic electronics theories can not describe the charge transfer process in organic semiconductors accurately.In the present research of organic semiconductor materials and devices,we usually refer the concept of energy bands in inorganic semiconductor,and explain the experimental phenomena or design the device structure with molecular orbital theory.However,organic semiconductors are different from inorganic materials in terms of microscopic molecular structure,interfacial effect,carrier transport,and macroscopic properties,such as dielectric constants and mobility.On the other hand,the macroscopic device performance often differs greatly from the material characterization results due to the inhomogeneous molecular microstructure and complex interfacial effects.Conventional diffraction and microscopy techniques are unable to study the charge-injection/transport at a molecular scale.Therefore,developing new characterization methods which are capable to observe the carrier processes of OFETs under operation directly is crucial for understanding the real device physics more deeply.In this thesis,we combined the Raman spectroscopy with the electrical measurement of OFETs to realize the visualization of the potential distribution in the channel and the tracking of device operation states,and explored the physical principle and universality of the method further.There are four main aspects in this thesis:1.We studied the response of the normal Raman scatting of pentacene to voltage,which is due to the molecule polarization under electric field,and the potential distribution in the transistor channel is qualitatively observed from the in situ Raman mapping.It is found that the resonance Raman intensity of pentacene is highly sensitive to voltage due to the change of vibration coupling caused the modulation of molecular energy levels,and the pinch point of the channel can be observed indirectly from the in situ resonance Raman mapping.2.Surface-enhanced Raman scattering mechanisms of pentacene on silver substrates were studied by experiments and theoretical calculations.It is found that the charge transfer excitation from silver nanoparticles to pentacene is the main mechanism of Raman enhancement.Both experiments and calculations show that the variation of Fermi level or molecular orbital level under external voltage will cause a significant change in Raman intensity,which means a high sensitivity of surface-enhanced Raman scattering to external voltage.3.Silver nanoparticles were introduced into pentacene thin films and in situ Raman mappings were measured using the response of surface-enhanced Raman scattering(SERS)to potential.Channel pinch-off point could be observed directly from the two-dimensional SERS mappings,and the threshold voltage calculated from the pinch-off length is very close to the result of electrical measurement.The charge storage and release process of silver nanoparticles can be also traced from the variations of the Raman shift and intensity in the in situ SERS spectrum.By changing the depth of silver nanoparticles in the active layer,the potential distribution in the direction perpendicular to the channel was further detected,and the visualization of the three-dimensional distribution of potential and carrier concentration was realized.4.The response of Raman scattering to voltage when gold and silver substrates are combined with perylene imide and pentacene respectively is investigated.The energy level requirements and the selection principle of SERS substrates for voltage sensitive surface enhanced Raman scattering are summarized.It is found that Raman scattering in bulk heterojunctions is also regulated by the device operating voltage,and its mechanism may be related to photo-induced local charges.