Stress-strain Behavior Of Organic Single Crystal Arrays And The Effect On Transistor Performance

Flexible electronics,a technology for preparing electronic devices based on organic/inorganic materials on a flexible or stretchable substrate.This technology enables the devices to be flexible or stretchable without sacrificing the electronic performance and suitable for miscellaneous applications.Compared with traditional electronic technology,flexible electronics can be widely used in information,medical treatment,energy,etc.,such as flexible display devices,wearable devices,flexible photovoltaics,and so on.Researches on flexible devices mainly focus on organic thin film,carbon nanotubes and inorganic structures.There are few works on organic singlecrystal flexible devices and their flexible limit.Organic single crystals,with regular molecular packing and fewer structural defects,are ideal charge-transport media for flexible devices.To utilize high-quality organic single crystals in flexible devices,the diverse response(cracks,delamination)of single crystals to bending strain should be meticulously explored,as well as the associated effects on transistor performance.In this thesis,aligned organic single-crystal arrays were used as the main research object,and a series of works about the flexibility of single crystals and the performance of the corresponding flexible devices were carried out.In chapter 1,the research status of flexible field-effect transistors as well as the preparation and stress-strain behavior of organic single crystals are reviewed.In chapter 2,we investigated the morphology change induced by bending strain on an array of aligned p-type semiconductor TIPS-pentacene single crystals(thickness:40-70 nm).Under bending strain,crystals show different imperfections: cracks in tensile bending state and delamination in compressive bending state.The cracks induced by tensile bending strain are caused by both loaded stress and crystallographic planes of dense molecular packing.The delamination caused by compressive bending strain is firstly observed in single crystals.Determined by the anisotropy of single crystals,imperfections are prone to appear in crystals when the direction of the bending strain is along the crystal ribbons.In chapter 3,the flexible OFETs based on TIPS-pentacene single-crystal array were successfully fabricated,and the relationship between the transistor performance and crystal imperfections was meticulously explored in different bending states.When bending radius = 5 mm,even though the crystal surface has obvious cracks,the transistor performance shows negligible change due to the intact charge transport channel.In addition,the devices exhibit an excellent performance stability after being bent for 1,000 cycles or keeping bent for over 20 days(bending radius = 10 mm).In chapter 4,we studied the morphology change of n-type semiconductor TIPSTAP single crystals(thickness: 10-20 nm)under different bending conditions and the performance change of the according transistors.Due to the thin thickness,TIPS-TAP crystals do not crack or delaminate under bending stress.However,there exist some surface steps on the bottom surface of the crystals,which are easy to delaminate from the dielectric layer when bending,causing the change of actual charge transport area,and leading to the degradation of transistor performance.In addition,we prepared a flexible complementary inverter based on TIPS-pentacene and TIPS-TAP single-crystal arrays successfully.The inverter can work in bending condition.