Atmosphere Effect On The Photoelectric Characteristics Of Organic Field-effect Transistors

Organic field-effect transistors(OFETs)have gained widespread popularity in the field of organic optoelectronic devices due to their low cost,solution processability,and inherent compatibility with flexible substrates.The charge carrier mobility of OFETs has increased from 10-5 cm2 V-1 s-1 to more than 40 cm2 V-1 s-1.Despite the great progress achieved,air effect is still a main issue that influences the device performance of OFETs.Thus,this thesis mainly focuses on the air effect on the photoelectric properties of OFETs.It includes three parts:1.Organic phototransistors(OPTs)are OFETs with photo response characteristics.Recently,a persistent photoconductivity(PPC)behavior,i.e.photocurrent can persist for a long time after light illumination,has been widely observed in OPTs.In this part,2,8-difluoro-5,11-bis(triethylsilylethynyl)anthracene dithiophene(diF-TESADT)single crystal small molecule organic semiconductor was used to construct the OPTs.Interestingly,oxygen molecules in air atmosphere play a crucial role in determining the PPC behavior in OPTs by measuring in oxygen,nitrogen,air,and vacuum conditions respectively.According to first-principles calculations,the adsorbed oxygen molecules could indeed generate a new energy state at 1.8 eV with respect to the Fermi level of diF-TESADT.Besides,the average surface potential of diF-TESADT crystals decreased from-459 mV at initial state to?-176 mV after illumination of 10 min by scanning Kelvin probe force microscopy(SKPM)measurements,which indicated the electron trapping characteristic of this oxygen-relelated new state.In addition,it's proved that PPC behavior would be enhanced or weakend in different temperature and illumination time.Our work reveals the significant role of oxygen in PPC behavior and also provides guidelines for controlling the PPC behavior toward device applications.2.OPTs are increasingly used in ultraweak light detection and image sensors,owing to their remarkable performance,such as high gain,high responsivity,and high detectivity.In this part,it was found that the photocurrent of OPTs would continuously increase due to the trapping of photo-generated electrons under light illumination,which could be originated from electron trapping in oxygen-induced deep energy levels.Based on this phenomenon,high-performance OPTs were fabricated on SiO2/Si substrate using photo-active diF-TESADT as active layer.The maximum responsivity(R)and specific detectivity(D*)of fabricated devices can reach up to 5.7 × 108 A W-1 and 6 × 1018 Jones,after illumiating the OPT for 15 hours(the illumination intensity is 1 nW cm-2,Vg=-50 V and Vds=-40 V).These parameters showed great application potential toward high-performance ultraweak light detectorsas well as photo-controled memory transistors.Our work provides a new direction towards the construction of weak light imaging systems.3.Accurate measurement and calculation of the charge carrier mobility in OFETs are of great significance for device development and practical applications.However,the transfer curves of OFETs are usually observed to deviate from the ideal curves for standard mobility calculation.This phenomenon is described as the non-ideal behavior,which will cause an inaccurate estimation of charge carrier mobility.Our work showed that air atmosphere could alsoresult in the non-ideal behavior in OFETs.DiF-TESADT with a bandgap of-2.02 eV was used as active layer to construct the OFETs.The electrical characteristics and non-ideal behavior were studied in air and vacuum condition,respectively.The OFETs exhibit non-ideal behavior in air condition,which could be explained by the oxygen-induced deep defect levels in the energy band gap of organic semiconductors.These defect levels can act as deep electron trapping centers at a positive gate voltage,thereby inducing more holes injection.The trapped electrons can be released and recombine with the mobile holes when applying a large negative gate voltage,resulting in a decrease in the extracted charge carrier mobility.The OFETs exhibit a hole mobility of 0.7 cm2 V-1 s-1 in low gate voltage range(Vg=0?-25 V),while the value extracted in high gate voltage range((Vg=-25?-45 V)is 0.2 cm2 V-1 s-1.Finally,the non-ideal behavior of the OFETs can be effecitively improved by selectively doping at the channel interface or controlling the measurement condition,thereby achieving a device exhibiting near-ideal behavior.