Structure Optimization And Performance Study Of Organic Photodetectors And Near-infrared Organic Light-emitting Diodes

As the basic and applied research of organic optoelectronic materials and devices continues to development,organic photodetectors and organic light-emitting diodes have gradually attracted the widespread attention of many international and domestic research institutions and enterprises.These two organic optoelectronic devices with organic semiconductor materials as the photoactive layer have the characteristics of light-weight,low-cost,solution-processable,compatible with flexible substrates,and easy to prepare in large area and biocompatible,which are the advantages that the commercially produced silicon-based semiconductor devices do not have.However,to achieve large-scale production and applications,there is an urgent need to improve the overall performance of organic optoelectronic devices（especially organic photodetectors）.In view of the importance of device structure innovation to realize high performance organic optoelectronic devices,this dissertation focuses on the effective technical methods and approaches to optimize organic photodetectors and organic light-emitting diodes,starting from interface layer,active layer,and emitting layer three device structures.The main research aspects are as follows:1.Suppression of noise from various sources is one of the keys to obtaining high specific detection rate for organic photodetectors,while the dark current in the device is the most important noise source.This work starts from exploring the relationship between the charge-blocking layer and the dark current in organic photodetectors.In contrast to previous results,we find that the increase in dark current is closely related to the introduction of additional interfacial trap states in the charge blocking layer.Because of the interfacial trap states play a role in assisting carrier injection into the active layer.The experimental measurements verified a clear correlation between the density of trap states and dark current density.Meanwhile,the organic photodetector with a metal/active layer/metal sandwich structure exhibits a similar external quantum efficiency at the same reverse bias,but with the lowest dark current density,compared to the control device using a charge-blocking layer.Therefore,the highest specific detectivity can be achieved 1.23×10¹³ Jones（at 730 nm）.In conclusion,the interfacial trap state can significantly weaken the charge-blocking ability of the charge-blocking layer,and the key to obtaining organic photodetectors with low dark current density while ensuring that the metal electrode-active layer injection barrier is sufficiently large is to avoid using charge-blocking layers with high trap density.2.Selective response photodetectors have many practical applications,but the realization of selectivity often relies on optical filters systems.We designed and fabricated a self-filtering double-sided response selective organic photodetector with two different response ranges using double-sided transparent device structure.The device uses a planar heterojunction structure with a（semi-）transparent electrode as the incident light window.The device exhibits a responsivity of 51 m A/W in the 300 to 650 nm range and 11 m A/W in the 650 to 850 nm range under top illumination conditions.Similarly,the devices show a responsivity of 2 m A/W for the short-wavelength region and 131 m A/W for the long-wavelength region when under bottom illumination condition.Hence,our individual device not only works in either visible or near-infrared range but also provides narrowband detection with spectral widths down to 100 nm in the near-infrared range,it can be applied to selective narrow-spectrum responses for numerous applications.The working mechanism of spectrally selective response characteristics is clarified by comparison with bulk heterojunction structure,combined with the qualitative analysis of the external quantum efficiency spectra of the device based on the optical transmission matrix model.Moreover,we demonstrate that the device design concept is also applicable to other organic optoelectronic material systems.Finally,we built a simple visible-infrared optical communication system and successfully demultiplexing the intermixed optical signals by using our double-sided selective response devices,which brings new inspiration for device structures and applications of organic photodetectors.3.Near-infrared organic light emitting diodes can be used for a variety of applications such as optical communications,night vision,and biological imaging.The electron acceptor of acceptor-donor-acceptor（A-D-A）structure has good planarity,which were easy to aggregate and makes produced adverse effects such as excited state quenching and increased leakage current.Therefore,we used a hole-transporting polymer（TFB）to doped A-D-A emitting materials to reduce the self-aggregation of A-D-A molecules.In addition,the hole-transport and electron-blocking properties of the device are greatly enhanced due to the wetting of the substrate by TFB.As a result,the TFB:BTA3-based light-emitting device has an external quantum efficiency of up to 2.52%,peaking at 719 nm,and can reach a maximum radiance of107.5 W/（sr m²）,which is comparable to the best reported phosphorescent organic light-emitting diodes.At very low drive voltages,the power efficiency of NIR organic light-emitting diodes can reach 2%.Our findings further improve the performance of fluorescent organic light-emitting diodes and open a new path for the research of cheap,biocompatible,and efficient NIR organic light-emitting diodes.