High Performance Organic Photodetector Based On PEIE And ZnO Co-modified ITO

Organic polymer photodetectors have attracted extensive attention because of their low cost,simple preparation process,flexible processing and adjustable working wavelength.In order to improve the exciton dissociation efficiency,organic polymer photodetectors usually adopt the bulk heterojunction（BHJ）structure of polymer donor and fullerene receptor.However,the donor acceptor phase separation of bulk heterojunction structure will lead to the direct contact of the donor and acceptor with the electrodes on both sides at the same time,so that electrons are injected into the active layer from the low work function cathode or hole from the high work function anode,and transmitted through the device through the donor or acceptor channel respectively,resulting in the increase of dark current of the device,which will significantly reduce the detection rate and linear dynamic range of the device.The introduction of electrode modified layer can solve this problem.At present,the reported modified layer materials include small molecular materials,polymers and metal oxide nanoparticles.They can be used as electron barrier or hole barrier to effectively reduce the injection of external charge from the electrode to the active layer,so as to reduce the dark current of the device.Among them,polyethyleneimine ethoxylated（PEIE）and sol gel ZnO were added between ITO and P3HT:PC₆₁BM active layers respectively,and the device showed a specific detection rate of 10¹² Jones.Although the two devices have similar device structure and show similar detection rate,the working mechanism is different.The former obtains higher photocurrent by using the electric dipole layer of PEIE layer,while the latter realizes lower dark current through the surface defects of ZnO.Therefore,they show different device performance:the external quantum efficiency（EQE）of PEIE device is about 2000%,and the linear dynamic range（LDR）is about 60 d B,while the LDR of ZnO device is more than 100 d B,and the EQE is relatively low,about 200%.The work of this paper mainly focuses on the combination of PEIE and ZnO modified layers,through the collaborative modification of ITO,give full play to their respective advantages and avoid their respective weaknesses,so as to realize an organic photodetector with excellent performance.The research content of this paper mainly includes the following three parts:（1）A high performance organic photodetector based on PEIE and ZnO blended ITO modified layer is realized.The structure of OPD is ITO/PEIE:ZnO/P3HT:PC₆₁BM/Al.Under applied bias of-0.3 V:The dark current density of OPD is 2.13×10^-4 m A/cm²,the photo current density is 51.65 m A/cm²,and the current light/dark ratio of the blend device reaches 2.42×10⁵,the external quantum efficiency reaches 917.44%,the spectral responsivity reaches 4.07 A/W,and the detectivity reaches 1.56×10¹³ Jones.The device has good transient response,with rise time of 24μs and fall time of 78μs,-3 d B bandwidth of 1.53 MHz and linear dynamic range of 113.26 d B（460 nm）and 109.30d B（520 nm）.Compared with the ZnO modified ITO device,the current light/dark ratio is9.26×10⁴,the spectral responsivity is 1.57 A/W,the EQE is 354.14%,and the detectivity is 4.01×10¹² Jones,the performance is improved.Compared with the PEIE device,the current light/dark ratio is 1.26×10⁴,-3 d B bandwidth of 1.24 MHz and linear dynamic range only 61.83 d B（460 nm）and 77.93 d B（520 nm）,the performance is significantly improved.（2）A high performance organic photodetector based on PEIE and ZnO bilayer ITO modified layer is realized.The OPD structure is ITO/PEIE/ZnO/P3HT:PC₆₁BM/Al.Compared with PEIE:ZnO modified ITO device,the performance is further improved.Under applied bias of-0.3 V,the dark current density of the device reduced from 2.13×10^-4 m A/cm² to 7.49×10^-5 m A/cm²,the photo current density increased from 51.65 m A/cm²to 83.25 m A/cm²,and the current light/dark ratio of the bilayer device reached 3.91×10⁵,the external quantum efficiency reaches 1703.20%,the spectral responsivity reaches7.55 A/W,and the detectivity reaches 4.88×10¹³ Jones.The transient response of the device is further improved,with rise time of 20μs and fall time of 46μs,-3 d B bandwidth of 1.76 MHz,and linear dynamic range of 117.61 d B（520 nm）and 117.99 d B（620 nm）.The parameters of the device are excellent.（3）The working mechanism of PEIE and ZnO co-modifying ITO to improve the performance of organic photodetectors was studied.Firstly,devices based on ZnO/PEIE and PEIE/ZnO/PEIE modified layers are prepared.The dark current comparison results of the two devices show that the direct contact between PEIE and ITO and the direct contact between ZnO and active layer are very important to improve the performance of the device.The ultraviolet electron spectroscopy of ITO/PEIE shows that PEIE can reduce the work function of ITO,and the electric dipole layer formed by PEIE on the surface of ITO can effectively block the dark current.The change of Fourier infrared spectrum of ZnO layer before and after UVO treatment and the change of dark current density of corresponding devices show that UVO can reduce the defect state density of ZnO,and the decrease of defect state density will lead to the increase of dark current of devices.This shows that the defect of ZnO is the key to realize the low dark current of the device.Therefore,the working mechanism of PEIE and ZnO co modifying ITO devices is as follows:in the dark state,the electric dipole layer formed by PEIE and the defects of ZnO jointly block the charge injection of ITO,resulting in low dark current;In the light state,the defects in ZnO are filled with photogenerated carriers.At the same time,the photogenerated holes after the dissociation of the photoactive layer are blocked by the deep band gap of ZnO and accumulate.The accumulated electric field leads to the energy band bending of the active layer.Finally,electrons are tunneled and injected from ITO to realize a photoconductive photodetector with high light response.Compared with the double-layer modified ZnO device,the double-layer modified PEO device has more active interface and better electrical properties than the double-layer modified ZnO device.This strategy of co-modifying ITO provides a new idea for the preparation of high-performance organic photodetectors.36 figures,1 table,and 117 references.