Structural Design And Performance Research Of Near-Infrared Organic Photodetectors

Organic photodetectors,with advantages such as light weight,low cost,tunable detection wavelength,high scalability,good biocompatibility,and the ability to operate at room temperature,show great value in portable and wearable electronic devices.Among them,the near-infrared organic photodetectors have broad application prospects in the fields of environmental monitoring,medical diagnosis,biochemical analysis,infrared communication,etc.,which have attracted extensive attention in academia and industry and become one of the hot spots in current research.At present,the research on near-infrared organic photodetectors is in a rapid development stage,but to achieve industrialization,there are still some challenges to overcome.Firstly,the overall performance of near-infrared organic photodetectors is still limited by high noise and low response,how to further reduce the dark current and improve the responsivity is the primary challenge for the development of near-infrared organic photodetectors.Secondly,the detection band of high-performance near-infrared organic photodetectors is mostly located within 1000 nm,and there is little exploration of near-infrared organic photodetectors with a spectral response of more than 1000 nm,so how to expand the effective response range is a key challenge for their further development and application.In addition,the active layer of near-infrared organic photodetectors is usually prepared from toxic halogenated solvents（e.g.,chloroform,chlorobenzene,and 1,2-dichlorobenzene）that are hazardous to human health and the environment,which adds to the difficulty and cost of industrialization,and is the ultimate challenge for their commercialization.To address the above challenges,this thesis firstly utilizes the active layer morphology modulation strategy to achieve the suppression of dark current and the enhancement of responsivity to obtain high-performance near-infrared organic photodetectors.Secondly,through device structure design,the performance of near-infrared organic photodetectors in the wavelength range above 1000 nm has been improved in a targeted manner.Moreover,green and environmentally friendly non-halogenated solvents were combined with asymmetric non-fullerene acceptors to fabricate high-performance near-infrared organic photodetectors treated with green solvents.These near-infrared organic photodetectors have been applied to pulse monitoring and detection in the food industry.The main contents and innovations of this thesis are as follows:1.A novel solution sequential deposition method is proposed to modulate the vertical morphology of the active layer,achieving enhanced external quantum efficiency and suppressed dark current,and ultimately obtaining a high-performance layer-by-layer near-infrared organic photodetector.Traces of the polymer acceptor PY-IT were first added to the donor layer D18-Cl to achieve the modulation of the phase separation of the D18-Cl film.Subsequently,during the deposition of acceptor L8-BO solution,the introduction of PY-IT promoted the diffusion of L8-BO in the D18-Cl layer,resulting in a more desirable vertical morphology of the active layer.Research has revealed that the D18-Cl:PY-IT/L8-BO films exhibited enhanced crystallinity compared to the D18-Cl/L8-BO films,which promoted the reduction of the trap density and thus suppressed the device dark current.Additionally,the D18-Cl:PY-IT/L8-BO device achieved enhanced exciton dissociation and charge transport,promoting the enhancement of device responsivity.Therefore,the D18-Cl:PY-IT/L8-BO device exhibits a high specific detectivity of7.35×10¹³ Jones at 805 nm,which is superior to the performance of the D18-Cl/L8-BO device(3.03×10¹³ Jones).Finally,the D18-Cl:PY-IT/L8-BO device was successfully applied in arterial pulse monitoring,demonstrating the value of the device in medical monitoring applications.2.Aiming at the performance vulnerability exhibited by near-infrared organic photodetectors in the wavelength range above 1000 nm,a hybrid pseudo-planar/bulk heterojunction structure based on an ultra-narrow bandgap material system is designed to obtain high-performance near-infrared organic photodetector with a detection wavelength of up to 1600 nm.The hybrid pseudo-planar/bulk heterojunction was constructed by sequentially depositing an N2200 polymer layer and a bulk heterojunction layer based on the PPh TQ:COTIC-4F system,utilizing the interdiffusion between the two layers.Research has found that the N2200 layer with a wide optical bandgap raises the external charge injection barrier and achieves significant suppression of dark current.The device responsivity is also improved due to the trace penetration of the PPh TQ:COTIC-4F layer in the N2200 layer.As a result,the N2200/PPh TQ:COTIC-4F device based on the hybrid pseudo-planar/bulk heterojunction structure exhibits a high specific detectivity of up to5.76×10¹⁰ and 6.20×10⁹ Jones at 1300 nm and 1600 nm,respectively,which is much higher than that of the PPh TQ:COTIC-4F device（8.50×10⁹ and 1.26×10⁹ Jones）.In addition,the N2200/PPh TQ:COTIC-4F device was applied to the rapid determination of ethanol content in mixed solutions within a measurement error of less than 2%,demonstrating the enormous potential of this device in the food and pharmaceutical industries.3.Asymmetric non-fullerene acceptor was applied to near-infrared organic photodetectors,exploiting the large dipole moments to enhance the intermolecular interactions and optimize the active layer morphology,achieving high-performance near-infrared organic photodetectors with green solvent processing.The symmetric acceptor BTP-e C9 and the asymmetric acceptor BTP-2F2Cl were used as the investigated objects to analyze the effect of the structural symmetry of the acceptors on the performance of the green solvent-treated devices.Research has found that the asymmetric acceptor BTP-2F2Cl exhibits stronger intermolecular interactions,resulting in a more densely ordered molecular stacking and enhanced crystallinity of the PM1:BTP-2F2Cl active layer treated with green solvent.As a result,the PM1:BTP-2F2Cl device treated with green solvent exhibits higher responsivity and lower dark current,and achieves a high specific detectivity of more than 10¹³ Jones at 810 nm.