Photoresponsive Field-effect Transistors Based On Pentacene

At present,photo organic field-effect transistors(PhotOFETs) with lateral structure have disadvantages,such as complicated fabricating process and low output current.In order to improve device performance,a vertical structure is introduced to shorten the channel length.In this thesis,we fabricated vertical PhotOFETs by using pentacene with high charge carrier mobility and investigated their photoresponsive characteristics.The influence of organic and inorganic dielectric layers as well as a modification of aluminium source electrode on the performance of PhotOFETs was studied and conclusions are drawn as follows:Firstly,we prepared pentacene thin films and studied their photoresponsive properties.The results indicated that the pentacene thin films had good crystallinity.The absorption and I_ph/I_Dark ratio increased with the thickness of pentacene.The pentacene thin film with the thickness of 100 nm was suitable forPhotOFETs.Secondly,we fabricated vertical PhotOFETs and investigated the influence of different dielectric materials on device performance.(1) Vertical PhotOFETs with LiF as a dielectric layer was made and it was found that the relative high energy barrier between Al and pentacene prevented holes from injecting,which caused poor photoresponsive characteristics of.Therefore,the A1 source electrode was modified by a ultra-thin film of V₂O₅,which helped the photo/dark current ratio increase from 2 to 35 as well as photosensitivity up to 70 mA/W.The AFM image of aluminium surface morphology showed the surface roughness of the Al source electrode modified by V₂O₅ decreased significantly,which improved the interface contact between pentacene and Al electrode.V₂O₅ also played a role of lowering the energy barrier and indeed enhanced the hole injection.(2) A vertical PhotOFET with 220 nm PVA as dielectric a layer had better device performance.Its photo/dark current ratio reached 3×10² and the photosensitivity was 0.1 A/W.We prepared a PVA polymer film with a mean surface roughness of 0.34 nm, indicating good morphology for PVA.(3) The vertical PhotOFET using CYEPL with a high dielectric constant as the dielectric layer showed n-type enhancement mode with photo/dark current ratio being 16 and photosensitivity being 0.6 mA/W.The reason for its poor behavior was the energetic disorder due to the localized states because of high dipolar disorder in the high dielectric constant materials as well as carrier localization enhanced by insulators with large permittivities due to the random dipole field presenting at the interface.