Application Study Of Polythiophene Derivatives In Photodetectors

Recently,organic polymers have attracted much attention in the field of optoelectronics due to their unique advantages.By combining different donor units with acceptor units,researchers have developed a variety of efficient D-A copolymers with diverse optoelectronic characteristics.The molecular structure of the polymers determines the fundamental properties of the materials.Combined with rational molecular design strategies,their properties can be flexibly tuned to give an insight into the relationship of the molecular structure-material properties-device performance,which would promote the development and application of polymers in the field of optoelectronics.In this paper,three types of polythiophene derivatives have been designed through main-chain modulation,side-chain engineering and elemental substitution strategies.We investigate the molecular structures,photophysical properties and their photodetectors’performances,and explore the application of these OPDs in optical communications.1.The polythiophene ester derivatives PTFBT and PBTAT with tetrafluorobenzene and benzotriazole groups were designed based on biphenyls with ester alkyl by main chain modulation.The photophysical,electrochemical and carrier transport properties of two polymers were investigated.They both exhibits molecularπ-πstacking and high hole mobilities.The photodetectors with the structure of ITO/PEDOT:PSS/Polymer:ITIC（1:1,w:w）/C₆₀/Li F/Al were prepared using PTFBT and PBTAT as the donors and ITIC with complementary absorption spectra as the acceptor.The two devices based on PTFBT and PBTAT exhibit good photodetection performance in a broad spectral range of 300-800 nm.In particular,the PBTAT-based device achieves an external quantum efficiency（EQE）of 21%and a detection rate of1.54×10¹² Jones under 0 V.The relatively high device performance of the PBTAT-based device can be attributed to the improved hole mobility and deep HOMO level,which results in a large built-in electric field and more efficient dissociation of photo-generated exitions.In addition,we also analyzed the exciton dissociation and carrier transport dynamics in two OPDs.It is found that the PBTAT-based device shows the lower carrier nonradiative recombination and device transport resistance,which enable the device to obtain lower dark current and higher photoresponse.2.Three random copolymers（PTQ1-PTQ3）with different side chain substitutions（alkyl,phenyl and ester groups）were designed using quinoxaline-thiophene（TQ）as the conjugated backbone.All three polymers exhibit strong UV absorption properties and shallow HOMO energy levels,and their photophysical properties can be effectively controlled by the rational design of TQ side chains.Furthermore,the polymers were blended with ITIC as bulk heterojunction photosensitive layers to prepare dual-band response and self-powered OPDs.The PTQ3-based device exhibits a lower dark current density of 8.1×10^–10 A cm^–2 and a higher EQE of 33.9%,and its photodetectivity exceeds 6.0×10¹² Jones.The high performance could be attributed to the strong intra-molecular charge transfer for improving its hole mobility of PTQ3 and the effective collection of photo-generated carriers by the corresponding electrodes.Through the analysis of carrier dynamics,it is found that device P3 has small interface energy barrier（between PTQ3 and ITIC）and low dark carrier density,which results in a low carrier recombination rate in device.In addition,it’s found that these OPDs all exhibit a binary optical response phenomenon under a small external bias,which can be used for various combinations of encoded signal in optical communications.Considering the UV and red light dual-band response of the OPDs,a feasible encryption scheme for optical communication is designed.This strategy provides some application guidelines for high-performance polymer OPDs.3.Three D-A copolymers,PTPC,PTPN and PTPSi,were designed by using thiophenopyrazine（TP）as the acceptor unit and bridging C,N and Si atoms with dithiophene as the donor unit,respectively.All polymers exhibit high hole mobilities,strong intramolecular charge transfer and good molecular planarity.Density functional theory（DFT）calculations revealed that S atom on dithiophene and N atom on pyrazine can form S···N noncovalent interactions,which can form intramolecular conformational locks and enhance the molecular planarity.In addition,three types of perovskite photodetectors were prepared using PTPC,PTPN and PTPSi as hole transport layers and perovskite Cs_0.05FA_0.81MA_0.14Pb I_2.55Br_0.45 as photosensitive layer.The effects of different bridging atoms of HTL on perovskite defect passivation and device performance were investigated.The results show that the PTPN-based device presents the best performance with a dark current density is 3.25×10^-10 A cm^-2,a responsivity（R）of 0.321 A W^-1,an EQE of 88%and a detection rate of 3.42×10¹²Jones.This can be attributed to the N atom（bridging atom）with lone pair electrons in PTPN,which promotes ICT between the D unit and A unit,resulting in higher hole mobility in PTPN;At the same time,the S and N atoms in PTPN have a synergistic passivation effect on the uncoordinated Pb²⁺in perovskite,reducing defects in perovskite films and suppressing the generation of dark currents.In addition,the application of perovskite photodetectors in the field of optical communication was explored by utilizing the dual band spectral response characteristics of the detector.