Research On The Fabrication And Performance Of Organic Photodiodes Based On Narrow-bandgap Non-fullerene Acceptors

Organic semiconductors have attracted extensive attention from scientific community due to their advantages of light weight,flexibility,tunable absorption,solution processability,and easy integration into readout circuits.Organic photodiodes can be divided into organic solar cells and organic photodetectors according to their application scenarios.In recent years,the rapid development of narrow-bandgap non-fullerene acceptors has improved the utilization efficiency of near-infrared light in semi-transparent organic solar cells,making it promising for building-integrated photovoltaic glass with heat-insulated function.Besides,narrow-bandgap non-fullerene acceptors have promoted the development of organic photodetectors in infrared sensing fields such as health monitoring,digital imaging,night vision,food monitoring and optical communication.This dissertation targets for the fabrication and performance study of organic photodiodes based on narrow-bandgap non-fullerene acceptors.In Chapter 2,semi-transparent organic photodiodes based on the sequentially deposited bilayer structure were studied.The adoption of bilayer structure not only simplified the device optimization,it was also found that,as donor and acceptor were separately deposited,the power conversion efficiency（PCE）of bilayer semi-transparent organic photodiodes was improved by simply increasing the thickness of acceptor layer,which has strong near-infrared absorption but weak visible light absorption,without significantly affecting the average visible light transmittance（AVT）of device.However,in bulk heterojunction（BHJ）structure,the increase of BHJ film thickness unavoidably enhanced the donor absorption in visible region,leading to a tradeoff between PCE and AVT in BHJ semi-transparent organic photodiodes.In addition,we also used the bilayer structure for semi-transparent organic photodetectors.Eventually,the bilayer device exhibited higher PCE and AVT,as well as lower dark current density and higher responsivity in the top-illuminated direction than the BHJ device.The above results indicate that the sequentially deposited bilayer structure provides a new idea for the fabrication of high-performance semi-transparent organic solar cells and top-illuminated photodetectors.In Chapter 3,ultraviolet-visible-to-near-infrared organic photodiodes comparable to crystalline silicon-based commercial devices were developed by using a series of narrow-bandgap non-fullerene acceptors with high near-infrared absorption coefficients and optimal design of device structure and sensing layer thickness.The adoption of inverted device structure increased the charge injection barrier,and thick junction strategy reduced the pinhole and defects in sensing layers and effectively shifted the responsivity peak to 930 nm.Therefore,the above approaches not only effectively reduced the dark current but also improved the external quantum efficiency in near-infrared region.Consequently,the dark current density was reduced to 0.35 n A cm^-2,the spectral response covered 300～1000 nm,and the external quantum efficiency exceeds 60%.Owing to low dark current and high responsivity at near-infrared bands,an unexpectedly high specific detectivity of 5.1×10¹³ Jones at 930 nm was obtained,together with a linear dynamic range of 157 d B and a-3 d B cut-off frequency of 4.5 k Hz.In addition,compared with rigid silicon-based detectors,flexible organic photodiodes exhibited similar PPG waveforms and a more distinct shoulder peak,which is important for further calculation of other physiological parameters through the first derivative of the PPG waveform.This work indicates that broadband organic photodiodes have great application potentials in 2D or 3D imaging with high frame rate and multiple band selection,and wearable health monitoring.On the basis of Chapter 3,Chapter 4 further shifted the detection wavelength of organic photodiodes to 1050 nm,and proposed an efficient and generic doping compensation strategy to improve the specific detectivity of infrared organic photodiodes.It was found that the doping compensation strategy not only suppressed the nonradiative recombination near the COTIC-4F energy gap,but also improved the efficiency of charge generation and carrier collection.More importantly,this strategy reduced the trap density of sensing layer,increased the depletion width,and reduced the dark current density by 1～2 orders of magnitude.Therefore,an ultralow noise spectral density of 8×10^-15A Hz^-1/2 as well as a high specific detectivity over 10¹³ Jones in780～1070 nm was achieved at room-temperature.Such excellent performance enabled the infrared organic photodiode not only to perform non-invasive and real-time detection of pulsed waves by photoplethysmography under weak light intensity,but also to be applied to image arrays with high pixel density,which contain the sensing layer with thickness of only a few hundred nanometers,and the sensing layer does not need to be patterned.These advantages simplify the device structure and fabrication process of the image sensors,and are beneficial to prepare low-cost,high-yield photodetectors and image arrays.In addition,this strategy also had good universality to other n-type organic semiconductors.This work suggests that improving the specific detectivity by doping compensation is an effective approach to construct infrared organic photodiodes with low trap density,low noise and high detectivity.In Chapter 5,lead sulfide quantum dots（Pb S QDs）with strong absorption at 1000～1700nm were selected,and short-wave-infrared photodiodes with organic-inorganic hybrid structure was developed by combining organic bulk-heterojunction films with Pb S QDs.Compared with Pb S QDs based photodiodes,organic thin films do not require complex layer-by-layer solid-state ligand exchange,which simplifies the preparation process,avoids the introduction of surface defects during the fabrication,creates a good interfacial morphology and promotes efficient exciton dissociation and charge transport.Finally,the optimal device showed an external quantum efficiency of 32%at 1520 nm,a peak responsivity of 0.39 A W^-1,a peak specific detectivity of 2.0×10¹² Jones at 100 Hz,-3 d B bandwidth of 43 k Hz.And a linear dynamic range of 82 d B was obtained under the illumination of 1550 nm monochromatic light.Through systematic measurements and analysis on devices with different sensing layer structures,we found that the improvement of device performance was not only due to the regulation of trap density and activation energy,but also due to the increase of build-in potential and depletion width.These factors together contribute to the improvement of external quantum efficiency,and the reduction of ideality factor,1/f noise and dark current density.The organic-inorganic hybrid short-wave-infrared photodiode developed in this work has comparable sensitivity to commercial In Ga As detectors,and has important applications in the fields of lidar,optical communication,and food monitoring.