Study On The Properties Of Furan-fused BN Polycyclic Hydrocarbons And Their Optoelectronic Devices

BN-doped PAHs are one class of the PAHs compounds that have attracted much attention in recent years.The introduction of B atoms and N atoms makes these compounds showing novel electronic structures and photoelectric properties.In addition,the molecular energy level structure and the distribution of electron clouds of BN-doped PAHs compounds can be adjusted by introducing different electron pushing-drawing substituents.They could be widely used in OLEDs,OPDs and other optoelectronic devices.In this paper,the bis-BN phenanthrene is used as the main body of the molecule,and a series of furan-fused BN polycyclic hydrocarbons（FBN）derivatives are synthesized by introducing five kinds of aryl substituents on the B atom,while two nitrogen atoms are connected by ethylene.Through investigation of their photophysical properties,we have prepared and studied the photoelectric properties of deep-blue OLEDs and ultraviolet photodetectors（UV-OPDs）based on these derivatives,The results of the paper are as follows:1.Five furan-fused BN polycyclic hydrocarbons（FBN-X,X is-H,-CH₃,-Br,-OCH₃,and-CN）derivatives were characterized by UV-Vis absorption spectra,PL spectra,CV texts,TRPL spectra,thermogravimetric analysis（TGA）and fluorescence quantum yield（PLQY）.Combined with DFT,and TD-DFT calculations,the photophysical properties of FBN-X derivatives have been analyzed,and the energy-level structures and charge transfer properties have been explored.In addition,two representative single crystals of FBN-OCH₃ and FBN-CN derivatives were prepared by solution volatilization,and the crystal structures was briefly analyzed by X-ray diffraction method.2.The influence of aryl substituents on the excited states of the five derivatives was explored.Based on solvent effect and Lippert-Mataga model analysis,it is found that the excited state characteristics of FBN-H and FBN-OCH₃ derivatives show the efficient HLCT state hybridized by locally-excited state（LE）and the charge transfer state（CT）.Through contact angle and SEM tests,the films of derivatives prepared by the solution method all present good wettability and uniform and dense surface morphology.The FBN-X compounds were further used as the light-emitting layers to prepare deep-blue OLEDs with a configuration of ITO/PEDOT:PSS/FBN-X/TPBi/Li F/Al.The luminescent performances and impedance spectra of these devices were tested and analyzed.Among them,the FBN-OCH₃device shows the premium EL performances with a turn-on voltage of 5.0 V,a maximum luminance of 598 cd/m²,the CE of 2.92 cd/A and PE of 1.14 lm/W at the current density of 5.04 m A/cm².The main reason could be attributed to the strong electron donating ability of-OCH₃ substituent which affects the dispersion degree of conjugated electron cloud.The efficient hybrid HLCT state could promote the radioactive recombination of excitons in the excited state of molecules.3.Using the FBN-X compounds as photosensitive layers,we fabricated the single-active-layer UV-OPDs with a device structure of ITO/PEDOT:PSS/FBN-X/BCP/C₆₀/Li F/Al.The FBN-CN based device shows better UV light detection performances.It is ascribed to the introduction of strong electron-drawing group-CN resulting in the formation of donor-acceptor（D-A）mechanism in the molecule,which is conducive to the dissociation of excitons.In addition,the single-carrier type devices were prepared to characterize the carrier mobility of FBN-X compounds based on SCLC theory,which belong on P-type materials.Furthermore,the N-type PC₇₁BM acceptor was doped with FBN-X compounds as the active layers to prepare heterojunction type UV-OPDs with a device structure of ITO/PEDOT:PSS/FBN-X:PC₇₁BM/C₆₀/Li F/Al.Under the irradiation of 320 nm UV light（5 m W/cm²）,the device with FBN-CN:PC₇₁BM photosensitive layer shows a photocurrent of 45.29μA,a photoresponsibility of 11.3×10^-2 A/W,a photodetection rate of 3.52×10¹² cm·Hz^1/2/W and an EQE of 43.7%,respectively.Compared with the single-active-layer devices,these performances were increased by 78.7%,79.4%,79.6%and 79.1%,respectively.This is mainly attributed to that the increase of interfaces between donators and acceptors in bulk heterojunction domains facilitate the intermolecular charge transfer in photosensitive layer,which weakens the molecularπ-π*stacking and increases the excitons’dissociation rate and the carrier’s mobility,resulting in a strong photocurrent.Through the tests and calculations of photocurrent density J versus light intensity E,open circuit voltage V_oc versus light intensity,dynamic linear range（LDR）,impedance spectrum and depletion region of the device,we analyzed the optimal device with FBN-CN:PC₇₁BM.The reduction of trap-assisted recombination rate and the increase of recombination resistance value will suppress the dark current and improve the light response.Meanwhile,a wider depletion region in the active layer is conducive to the dissociation of photogenerated excitons and the increase of carriers’concentration,which effectively improves the photoresponsibility and photodetection rate for the device as a result.