Study On Small Molecular Near-infrared Photoresponsive Organic Field-effect Transistors

Near-infrared?NIR?photodetectors have significant applications in the fields of fiberopticcommunication,biomedicine,imaging and etc,which can convert near-infrared light signal to electric signal.With the advantages of organic semiconductors,such as favorable photoabsorption,flexibility and easy preparation,the NIR organic photodetectors have obtained fast progress.The research of NIR organic photodetectors mainly focuses on NIR organic photodiodes?NIR-OPDs?,while the NIR photosensitive organic field-effect transistors?NIR-PhOFETs?are rarely reported.Comparing to NIR-OPDs,NIR-PhOFETs have higher photoresponsivity and lower noise.Therefore,NIR-PhOFETs are more suitable as NIR organic photodetectors.In this thesis,first the PbPc singlelayer NIR-PhOFET was fabricated and characterized,and the performance of the device was improved by substrate heating and interface inducing,then we designed and fabricated the planar heterojunction?PHJ?based devices,further improving the performance of the device.First,the PbPc single-layer NIR-PhOFET was fabricated at room temperature,only showing a photoresponsivity of 0.08 mA/W.To improve the performance of the device,the OTS?octadecyltrichlorosilane?treated and untreated PbPc single-layer devices were fabricated at different substrate temperature,and the effect of substrate temperature on device performance was studied.The study showed that substrate heating improved the performance of the PbPc single-layer device.This is because substrate heating increased both crystallinity and content of the triclinic phase in PbPc film,and then enhanced the carrier mobility and the NIR absorption efficiency of PbPc film,thus improving the performance of the device.This experiment suggested that an optimum substrate temperature was about 140°C for the devices without OTS treated,and the photoresponsivity of the corresponding device was 4.9 mA/W;while that was around 100°C for those with OTS treated,and the corresponding photoresponsivity was 12.0 mA/W.Second,the PbPc single-layer NIR-PhOFET with copper phthalocyanine?CuPc?or pentacene inducing layer were fabricated,and the relationship between inducing layer thickness and device performance was studied.The study indicated that the performance of the PbPc single-layer device was improved via interface inducing.The origin for this is that inserting interfacial inducing layer increased both crystallinity and content of the triclinic phase in PbPc film,hence enhancing the carrier mobility and the NIR absorption efficiency of PbPc film.The study also indicated that the device having an inducing layer with thickness near to that of monolayer exhibited an optimal performance,since such an inducing layer had best interface inducing effect due to least lattice dislocation and highest molecular ordering.It was demonstrated that an optimum inducing layer thickness was around 2.5 nm for CuPc,and the photoresponsivity of the corresponding device was 2.3 A/W;while that was around 2 nm for pentacene,and the corresponding photoresponsivity reached 24 A/W.At last,we designed and fabricated different PHJ based NIR-PhOFETs,and studied the performance of the devices.The results showed that the devices based on pentacene/PbPc,C₆₀/PbPc and pentacene/PbP/C₆₀ PHJ had larger photoresponsivity than the PbPc single-layer device,among which the photoresponsivity of the pentacene/PbP/C₆₀ PHJ based device reached up to 30 A/W,exceeding that of some Si-based phototrasistors.This is because the use of PHJ significantly improved the mobility and?or?the exciton dissociation efficiency of the devices.