Study On Dark Current Performance Of Organic Photodetectors Based On Naphthalene Dithiadiazole Donor Material System

Photodetector,which can convert optical information into electrical signals,applicated in optical monitoring and detection widely.As an emerging candidate for next-generation photodetectors,organic photodetectors are light in weight,can be processed in solution and fabricated into flexible and large-area devices,which complemented the application scenarios of traditional inorganic photodetectors and attracted extensive attention from industry and academia in recent years.Recently,many advances have been reported on the important parametric properties of organic photodetectors,some of which can approach or even surpass the device performance of inorganic Si-and Ge-based photodetectors.With the rapid development of organic photodiodes,the current research is mainly aimed at how to improve the photoresponse performance of organic semiconductor materials and the sensitivity of devices through molecular design and device structure design,but the research on how to effectively suppress the dark current of organic photodetectors to improve the detectivity of device is not deep enough.Therefore,the thesis focused on the working mechanism of dark current in organic photodetectors and studied the methods to reduce the dark current of organic semiconductor photodetectors and improve device performance,which provided favorable conditions for constructing high-performance organic photodetectors and realizing the wide application of organic photodetectors.The organic semiconductor electronic donor material system selected in this paper contains naphthalene dithiadiazole（NT）units,and the electron acceptor material used fullerene,non-fullerene or conjugated polymer n-type semiconductor material,the bulk heterojunction film formed by blending the donor material and the electron acceptor material used as the photosensitive layer,we systematically studied the effect of the thickness of the organic semiconductor photosensitive layer film,the molecular weight of the conjugated polymer material,the charge injection barrier under reverse bias and electron blocking layer on the dark current,elucidated the mechanism affecting dark current of the photodetection device,and developed effective methods to suppress the dark current and improve the photodetection performance of devices.The main parts of this thesis are listed as follows:In chapter 2,we fabricated high-performance thick film organic photodetectors based on PNTT:PC₇₁BM.PNTT with different molecular weights were blended with fullerene derivative acceptor PC₇₁BM to study the influence of the molecular weight of the donor on the microscopic morphology and device performances,microscopic morphology based on high molecular weight donor materials tend to be face-on stacking,which is helpful to charge transport.Meanwhile,the highly ordered structure reduces the formation of defects,contributing to lower dark current and hence higher device performance.By further adjusting the thickness of the photo-sensitive layer,the dark current density of the device decreased gradually,the lowest dark current density of 3.30×10^-10A cm^-2 was obtained under-0.1 V bias at the photo-sensitive layer thickness of 400 nm,which proves the advantage of thick film processing for PNTT:PC₇₁BM.As the result,a specific detectivity of 4.00×10¹³Jones and a photoresponsivity of 0.41 A W^-1 were obtained with optimized devices thickness of 760 nm,which were the state-of-the-art performances at this wavelength when reported.The chapter 3 further explored the effect of acceptor molecular weight on NT812:N2200device performance on the basis of film thickness,and established the correlation between acceptor molecular weight,film morphology and device performance for all-polymer organic photodetectors.NT812:N2200-H microscopic morphology presents a fibrous interpenetrating network structure,which is different from agglomeration phenomenon of the low molecular weight device.The bi-continuous morphology is conducive to the transfer of charges and the photo response performance of the device.While XPS test and analysis found that the NT812:N2200-L blend film is favorable for forming vertical phase distribution,which can suppress the injection of charge under reverse bias.Therefore,the NT812:N2200-L-based devices have lower dark current density and higher specific detectivity.The optimized NT812:N2200-L device exhibited a low dark current density of 1.25×10^-10 A cm^-2 under-0.1V reverse bias,and obtained a specific detectivity of 2.42×10¹³ Jones at 720 nm.This work provided a brief but effective method for constructing of high-performance all-polymer organic photodetectors.Chapter 4 further explored the effect of acceptor fluorination on injection barrier and dark current,and established the correlation between acceptor fluorination-energy level shift-injection barrier-device performance.We chose NT812 and blended with ITIC,IT-2F and IT-4F,which own similar structures and energy level,to fabricate organic photodetectors device.Charge injection barrier were quantitatively calculated through temperature-dependent mobility tests and Scanning Kelvin probes measurement.Fluorination of end groups of the acceptor materials is accompanied by the shift of the energy level,despite the effect on the light absorption and light harvesting properties of the material is slight.The unfluorinated acceptor increase electron injection barrier,which is conducive to the blocking of electrons.As the result,with less halogen atom,the dark current density gradually decreased.The dark current density of optimized NT812:ITIC devices is 5.18×10^-10 A cm^-2 and the specific detectivity reaches2.35×10¹³ Jones,much better than NT812:IT-2F and NT812:IT-4F devices.The results provided a feasible strategy for material selection of photodetector.Chapter 5 explored the effect of electron blocking layer on organic photodetector device performance via adjusting the injection barrier.NT30:IEICO-4F blend film was selected as photo-sensitive layer and a new type of thermally cross-linked material HT2 was used as the electron blocking layer.The cross-linking property of HT2 reduced the leakage channel,the high energy level of HT2 played the role of blocking electron injection,which was beneficial to reduce the dark current density of the devices.The optimized device achieved a dark current density as low as 1.42×10^-10 A cm^-2,and the specific detectivity reached 5.74×10¹³ Jones.Based on the result,the organic light emitting diode hole transport material BSFN was introduced to dope HT2,which can improve the hole transport performance and blocked the electron injection at the same time.As the result,the device performance greatly improved attributed to the synergistic improvement of both photoresponsivity and dark current density.The dark current density of the final optimized device is as low as 3.85×10^-11 A cm^-2,the responsivity at 850 nm increased from 0.39 A W^-1 to 0.43 A W^-1,and the specific detectivity reached 1.22×10¹⁴ Jones.This is the state-of-art parameters in organic photodetectors field,and also the highest value at this wavelength,showing great potential in industrial applications.