Study On The Optical And Electrical Characteristics Of Organic Electronic Devices Enhanced By Gold Nanocrystal Decoration

Organic optoelectronic devices decorated by metal nanocrystal have attracted much research interest in recent years due to some advantages of organic electronics including light & thin,low cost,flexibility,transparency,and so on.Meanwhile,the suitable photo-excitation of metal nanocrystal will generate surface plasmons to trap/capture photon energy on metal surface,significantly enhancing light-matter(i.e.organic semiconductors)interaction,thus,making it possible to manipulate photons in a micro/nano level.This type of optoelectronic devices have shown powerful application potentials for subwavelength optical circuit,photon harvesting and conversion,trapping,emission,optical sensing and spectroscopy fields.At present,a large number of publications have also successfully demonstrated performance enhancements of organic lightemitting diode(OLED)and solar cell(OSC)applications based on the metal plasmon resonance mechanism.It is worth noting that organic semiconductors and their photodetection applications still remain significant challenges,such as lower field-effect mobility of organic semiconductors,fewer reports for long-wave photodetection in near-infrared region/direction,ultra-sensitive broadband light detection and weak light detection.Clearly,the doping effect of metal nanocrystal on organic semiconductors can be used to improve the mobility of field-effect transistors;while,as for plasmon effect,the possibility of manipulating photons within the nanometer regime will enable new organic nanodevices that offer high optical detection performance and multiple-functionality.With the aim to address these issues above,this thesis is built on a large number of literature reviews and theoretical basis.Therefore,the work in this thesis can be summarized as follows:In the first design,size-controllable gold nanocrystals(AuNCs)are prepared and optimized via a simple and rapid vacuum-thermal-deposition method,and introduced into the organic heterojunction(NPB/C60)interface.Based on internal photoemission mechanism of AuNCs,we demonstrate the plasmon-induced sub-bandgap photodetection in near-infrared direction with the organic photodiode for the first time.The device has a photoresponsivity of 205 mA/W and a quantum conversion efficiency up to 39%.The parameter values above are much higher than the currently reported sub-bandgap photodetectors based on internal photoemission of plasmonic metals.The related studies show that the excellent device performance is mainly due to the near-field enhancement,direct hot carrier injection and transfer transition induced by localized surface plasmon resonance(LSPR)of AuNCs.Further surface-enhanced Raman spectroscopy(SERS)measurements,near-field simulations of LSPR,and energy-level mechanisms for device operation well explain the physical process for performance enhancement.In the second design,the optimized Au NCs are used to decorate porous organic-inorganic lead halide perovskites(MAPb I3-xClx)with excellent photoelectric properties,so as to design and fabricate a Metal-Semiconductor-Metal(MSM)planar photoconductor for optical sensing in UVVis-NIR spectral range.The results show that the Au NCs modified porous perovskite photoconductor achieves an ultra-low noise level(~20.0 pWHz-1/2),and obtains a significant increase of photoresponse in the UV-Vis region,while the photodetection spectrum is also extended to the sub-bandgap near-infrared region.Compared with state-of-the-arts,the photoconductor achieves a very weak light detection(~5.7 pW),ultra-fast response time(~600 ns)and ultra-high photosensitivity(~105).In addition,the device also exhibits good time-stability,uniformity and flexibility.Finally,the potential photophysical mechanism is revealed by the photoconductive energy-level model,the theoretical simulation and the photoluminescence spectrum analysis.In the third design,the as-prepared AuNCs are utilized to dope the organic semiconductor(CuPc)active layer,so as to design and fabricate high-performance organic field-effect transistor(OFETs),and the electrical and optical properties of the devices related to the decoration location of AuNCs are systematically studied.The results show that the surface modification of AuNCs for OFETs can obviously improve the device characteristics,and further optimization of CuPc thickness will realize the field-effect mobility of ~ 0.013 cm2V-1s-1,which is among the reported maximum values for CuPc-OFETs.Based on the device energy-level model and semi-quantitative theoretical calculation,the electrical performance enhancement of OFETs can be attributed to the surface modification of AuNCs to generate additional channel effect in organic active layer and promote the charge carrier transport.In addition,the photodetection performance of OFET indicates that the surface modification of AuNCs can significantly enhance its weak light detection(detectable for 0.01 ?Wcm-2 red light)and photoresponsivity(up to 23359 A/W).The near-field simulation and photophysical energy-level model explain the photoresponse mechanism well.In summary,the results presented in this thesis show powerful potentials of AuNCs for modification and enhancement of organic optoelectronic devices(photodetectors),and can serve as a prototype to guide and design higher performance photoelectric conversion devices.