Research On The Performance Of Organic Photodetectors Based On ZnO Interfacial Layer

As a material with high transmittance,stable properties and environmental protection,zinc oxide is widely used in the research of organic photovoltaic devices and organic light-emitting diodes.In these devices,the main function of the zinc oxide modified layer is to enhance the electron transport performance.Therefore,reducing the defects in the zinc oxide film is the key to improving the performance of the device.Therefore,in the preparation of the zinc oxide modified layer,methods such as ultraviolet,high temperature and surface coating are widely used.Unlike the above devices,organic photodetectors,as a device for detecting light signals,especially weak light signals,need a significantly low dark current,the key to improve the performance of the detector.The defects in the modified layer of the device can trap carriers,which can effectively reduce the injection of external charges,thereby reducing the dark current.Therefore,using the defects in the zinc oxide modified layer may improve the performance of organic photodetectors.Different zinc oxide films have different surface defect state densities.Even for zinc oxide nanoparticles,different processing methods will change surface defect state densities dramatically,which will significantly affect the performance of organic photodetectors.This work demonstrated a high-performance organic photodetector based on the zinc oxide nanoparticle modified layer.In addition,the effect of the defect state density of the zinc oxide film on the device performance was studied.The working mechanism of the device and how to optimize the device proposed.The content of this thesis mainly includes three parts:（1）Between the ITO and the active layer,a modified layer of zinc oxide nanoparticles was introduced to prepare an organic photodetector of ITO/Zn O/P3HT:PC₆₁BM/Al.While effectively reducing the dark current of the device,the device exhibits a higher light response.Under a bias voltage of-0.5 V,the dark current density of the device is reduced to 2.64×10^-5 m A/cm².Compared with the control device,the dark current density of the device with the zinc oxide nanoparticle modification layer is reduced by more than 3 orders of magnitude.The signal-to-noise ratio（SNR）has reached 4.04×10⁶;the external quantum efficiency（EQE）has increased to more than 1980%;the responsivity is 9.51 A/W;the detection rate is 1.03×10¹⁴ Jones;and the response time is15μs（rising time）or 10μs（falling time）;the linear dynamic range is 123.2d B（468 nm）or 109.8d B（530 nm）or 112.5d B（625 nm）,which is comparable to silicon-based inorganic photodetectors（about 120d B）.（2）In order to deeply study the effect of the surface defects on the performance of the device,the UV treatment was used to reduce the defect state density in the zinc oxide nanoparticle layer.The Fourier infrared and photoluminescence spectroscopy measurements prove that the UV treatment can effectively reduce the density of defect states in the zinc oxide film.The I-V characteristics were performed on the zinc oxide nanoparticle film based devices before and after UV treatment,and the results show that the dark current of the device after UV treatment increased significantly,which indicates that the reduction of the defect state density caused by UV treatment will significantly reduce the performance of the device.In order to further study the relationship between the density of defect states and the performance of the device,an amorphous zinc oxide film was prepared by magnetron sputtering.The Fourier infrared and photoluminescence spectroscopy results show that the density of defect states in the amorphous zinc oxide film is low.The resulted device has a high dark current density of1.72×10^-3m A/cm² under a bias of-0.3 V.This dark current value is significantly higher than that of the zinc oxide nanoparticles based device.This implies that the amorphous zinc oxide modified layer does not limit the injection of external charges.The maximum EQE of the device does not exceed 50%,which is much lower than that of the device prepared from zinc oxide nanoparticles with a higher density of defect states.The above studies have proved that the higher density of defect states in the zinc oxide nanoparticle film is an important factor in improving the performance of the device.The introduction of defects can trap the charges injected from the electrode,reducing the dark current of the device.Under illumination,the photo-generated charge carriers fill up the defects,and at the same time,the carriers accumulate at the interface between the active layer and the modified layer,which leads to the bending of the energy band and the tunneling injection.（3）In order to improve the performance of the photodetector based on the amorphous zinc oxide modified layer,ethoxylated polyethyleneimine（PEIE）is used to modify the zinc oxide surface.When PEIE forms a film onto the amorphous zinc oxide film,a dipole moment will be formed under a voltage bias applied on the device,which will impede the charge injection between the ITO electrode and the active layer.After adding the PEIE modification layer,the dark current density of the device was reduced from 1.72×10^-3 m A/cm² to 5.81×10^-4 m A/cm² under-0.3 V bias,and the photocurrent was increased from 8.80 m A/cm² to 17.23 m A/cm²,the signal-to-noise ratio increased from 5.12×10³ to 2.97×10⁴,and the corresponding EQE never exceeded 50%to a maximum of 259.02%.Therefore,the device realizes the tunneling injection under illumination with adding the PEIE modified layer.The rise time?and fall time?are10μs and 120μs,respectively.The linear dynamic range is 80d B（468 nm）or 80.4d B（530 nm）or 87.3d B（625 nm）.