High-performance UV Photodetectors Based On Organic Polymers

The research of high-performance UV photodetectors has received great attention because of the wide application of ultraviolet（UV）photodetectors in fields such as solar astronomy,environmental monitoring,missile warning,and biomedicine.Currently,most commercial UV photodetectors are inorganic photodetectors,which have excellent performance in terms of photosensitivity,operating speed,and detection capability.However,as the size of photodetectors continues to shrink,UV photodetectors used for applications need to be lightweight,biocompatible,and flexible.Unfortunately,the inherent properties of inorganic materials cannot overcome the disadvantages of high cost and complex preparation processes.But organic semiconductors have great potential in the manufacture of low-cost,flexible,and large-area photodetectors due to their own advantages such as high efficiency,low cost,and flexibility.This thesis aims to research high-performance organic UV photodiodes.On one hand,the main problem with organic UV photodiodes is that the organic active layer is very thin（about a few hundred nanometers）,and the thinner active layer leads to low light absorption.On the other hand,the charge transport of photodiodes is through the slower path of overlapping（perpendicular）electron orbitals between chains of organic materials,so photodetectors have slower and lower photoresponsivity.At the same time,UV photodetectors are manufactured with lower detection rates and smaller switching ratios.Based on the above questions,our research content is as follows:（1）DPPT-TT organic polymer is used as a photosensitive material in this thesis.First,the performance of the basic photoconductor,photodiode and phototransistor structure is compared,so as to determine the research route of the photodiode structure.Then,based on the diode structure,the device fabrication process was optimized,and the effects of the annealing temperature,time and film thickness of the organic active layer on the device performance were studied.By controlling the variables,the optimum spin-coating parameter of the device is 1500rpm/s,and the optimum annealing parameter is 150℃/1h.（2）Based on the above structure and preparation process,a hole blocking layer（HBL）and an electron blocking layer（EBL）are added in this paper to solve the problem of low external quantum efficiency and large dark current of the ultraviolet photodetector.The carrier barrier reduces the dark current and improves the switching ratio.Moreover,the accumulation of electrons by using the blocking effect of the hole barrier layer can induce carriers to achieve the tunneling effect.After adding the Mo O₃ electron-blocking layer,the dark current of the detector was reduced by 50%.By optimizing the HBL material and comparing three different HBL layers of Hf O₂/Ti O₂/Zn O,we found that using Hf O₂ as the hole-blocking layer had the best performance.The responsivity R increased from 0.15 A/W to 5.5 A/W,an increase of about 30 times,and the detectivity D^*increased from the order of 10¹⁰ to the order of 10¹¹.However,the largest improvement was external quantum efficiency（EQE）,which increased from 50%to 1800%.（3）On the basis of the previous studies,this thesis further studies the problems in the light absorption rate and charge transport of organic ultraviolet photodetectors.First,the thin active layer results in low light absorption,and the DPPT-TT thin film only has an absorption rate of 18%at a wavelength of 365 nm.We formed a light trapping structure by using a top electrode and a step structure,which increased the absorption rate of the device at 365 nm by 40%.Second,charge transfer in photodiodes is through the overlapping（perpendicular）of electronic orbitals between organic materials,which is a slower path and can only achieve slower and lower light response photodetectors due to high recombination rates.Based on this problem,we studied the orientation of polymer chains.By depositing different heights of steps at the bottom of the device,the orientation of the polymer chain was matched with the direction of carrier transfer,thereby increasing the migration rate of the organic polymer and reducing charge recombination during the charge extraction process.The optimized device performance had a spectral sensitivity R of 383 A/W,an increase of about 70 times,the specific detection rate D^*of 2.79 x10¹² Jones,and the external quantum efficiency EQE of 140000%under 365 nm UV irradiation.Compared with the previously reported organic-based photodetectors,the photodetector studied in this thesis has excellent photodetection performance.It can be seen that ultra-thin organic UV photodetectors have great potential in low-cost UV/VIS detection and imaging applications.