Design And Device Performance Of Hot-exciton Red Light Materials Containing Naphtho[2,3-c][1,2,5]thiadiazole Group

The organic light-emitting diode（OLED）technology has gradually become the mainstream display technology that can replace traditional display technologies thanks to its unique advantages,such as fast response,self-emission,and flexibility,after more than thirty years of development.Organic light-emitting materials,as the core technology of OLED,have attracted widespread attention from the academic and commercial sectors.The commercially successful OLED materials are mainly efficient metal complex phosphorescent materials,with metal complexes such as Ir and Pt as the main components.These materials are highly toxic,expensive,and highly dependent on resource reserves.Therefore,developing low-cost,high-efficiency pure organic small molecule fluorescent materials is crucial for the sustainable development of the entire OLED industry.Traditional organic small molecule fluorescent materials are limited by spin statistical rules in the electroluminescence process,utilizing only 25%of the electrically generated excitons,namely singlet excitons,resulting in an external quantum efficiency（EQE）not exceeding 5%.Therefore,effectively using the remaining 75%of the electrically generated triplet excitons for emission is crucial in improving the efficiency of electroluminescent devices.Various emission mechanisms focusing on triplet exciton utilization have recently been proposed,including triplet-triplet annihilation（TTA）,thermally activated delayed fluorescence（TADF）,and hot-exciton mechanisms.Among them,the hot-exciton mechanism,with its unique rapid reversed intersystem crossing（RISC）rate and the utilization of excitons breaking the spin statistical limit,can achieve high-efficiency and stable electroluminescent devices.Narrow-bandgap red and near-infrared light-emitting materials have critical practical applications in full-colour displays,white light illumination,bioimaging,night vision,and other fields.However,the development of organic red and near-infrared materials has been relatively slow compared to green and blue light materials.The main reason for this is the limitation of the energy gap law:as the emission energy gap narrows,the overlap of the vibrational wave functions between the ground state and excited state increases,enhancing non-radiative transitions in the excited state,thereby reducing the material’s photoluminescent quantum yield（PLQY）.To overcome this limitation,we have selected appropriate donor-acceptor（D-A）units for charge transfer（CT）-dominated hybrid local and charge-transfer（HLCT）excited state materials to achieve both the emission colour and high luminescence efficiency.This work focuses on the study of hot-exciton red light materials with HLCT excited state properties,aiming to improve the comprehensive performance of the materials in terms of optimizing colour,PLQY,exciton utilization efficiency,and carrier mobility.Furthermore,through a combination of theoretical simulations and experiments,the study investigates the impact of material molecular structure on its excited state properties,aggregation state properties,and electroluminescent properties,ultimately achieving high-performance and stable red to near-infrared electroluminescent devices.The main research content and conclusions are as follows:（1）Introduction of auxiliary acceptor cyano-group to optimize red light colour.Using naphtho[2,3-c][1,2,5]thiadiazole（NZ）as the central acceptor core,chemical modifications were made at positions 4 and 9.Using phenyl dimethylacridan as the donor and benzene ring as the end-capping group,a molecule DMACNZP with CT-dominant HLCT excited state properties was constructed.Meanwhile,an auxiliary acceptor cyano-group was introduced on the end-capping benzene ring to construct DMACNZC,investigating the effect of cyano-group introduction on the material and device performance.The study found that introducing the auxiliary acceptor cyano-group led to a slight enhancement of the excited state CT component,achieving a 16nm redshift in the emission colour at the single-molecule level.The formation of hydrogen-bond-locked dimericπ-πstacking in the crystal was the main reason for the60 nm redshift in the emission of the aggregated state thin film.It improved the electron injection performance in non-doped OLEDs,while the exciton utilization efficiency increased to 61%～92%.The roll-off was reduced to 7.1%(difference between EQE at1000 cd m^-2 and maximum EQE),achieving stable electroluminescence.（2）Construction of LE-dominant HLCT states to enhance PLQY.Firstly,the benzene end-capping of DMACNZP was replaced with a m-terphenyl end-capping to increase conjugation,obtaining the material DMACNZ3Ph.Crystal structure analysis results showed that the meta-triphenyl benzene group increased not only intra-molecular conjugation but also introduced intermolecular C-H···πinteractions,helping to suppress molecular vibrations,weaken non-radiative transitions,and enhance the material’s PLQY.Furthermore,three different donors were used to study the effect of donor electron-donating ability on the HLCT state type and excited state structure.Compared to the three strong donors triphenylamine,spirofluorene,and acridine,the weak donor phenanthroimidazole combined with the acceptor NZ（PPINZ3Ph）achieved higher PLQY（56.73%）and optimized electroluminescence performance（EQE=6.93%）while retaining appropriate CT components.Additionally,due to the weakened CT component,PPINZ3Ph had closer T₂ and T₃ levels,making it easier to build more efficient hot-exciton channels to increase RISC efficiency and rate.（3）Further reduction of S₁ state CT component to construct a weakly hybridized excited state,achieving high efficiency and high carrier mobility red fluorescence electroluminescent devices.The m-terphenyl group of PPINZ3Ph was replaced with an indolocarbazole group（ICZ）to construct the material ICZNZPPI,which exhibited a nearly complete disappearance of solvent-induced redshift,forming a weakly hybridized state.Theoretical calculations showed that its S₁ state was locally dominant.At the same time,S₂ was CT-dominant,but this did not affect its electroluminescence efficiency,indicating that the S₂ state could build effective hot-exciton channels.Furthermore,introducing the electron transport group perylene bisimide（PBI）and cyano-group to construct the materials ICZNZPBI and ICZNZPPICN improved material carrier mobility and increased electroluminescence efficiency.The electron mobility of material ICZNZPBI reached 1.63×10^-4 cm² V^-1 s^-1 at a relatively high level.Still,material ICZNZPPICN achieved a balanced hole and electron transport,with a maximum EQE of 13.52%in doped OLED and a maximum brightness of 19197 cd m^-2,making it one of the best-performing OLEDs in the same wavelength range reported to date.（4）Further enhancing CT state components to optimize colour,achieving high-efficiency deep-red and near-infrared electroluminescent devices.Using PPICN as an auxiliary group,strong donors triphenylamine and phenylphenothiazine were introduced at the other end of the NZ group to construct two materials,TPANZPPICN and PXZNZPPICN,with aggregation-induced emission（AIE）properties and CT-dominant HLCT excited state properties.Both TPANZPPICN and PXZNZPPICN non-doped OLEDs achieved efficient and stable electroluminescence,with maximum EQEs of 2.69%at 692 nm for deep-red light and 0.70%at 756 nm for near-infrared light.The doped OLED of TPANZPPICN achieved a nearly 8%maximum EQE at 660 nm for deep-red light,maintaining at 4.72%at 1000 cd m^-2,with a maximum brightness of11213 cd m^-2,making it one of the best-performing deep-red light devices.