Study On Performance Modulation And Equivalent Circuit Modeling Of PbS Colloidal Quantum Dots Based Short-Wavelength Infrared Photodiodes

PbS colloidal quantum dots（PbS CQDs）based short-wavelength infrared（SWIR）technology is one of the most important trends for low-cost,high-performance and large-scale SWIR imaging array owing to its excellent optical and electrical properties including tunable absorption spectrum,high absorption coefficient and modulable doping type and concentration as well as ease of direct integration onto CMOS readout circuits by a solution process.Until now,the photodetection performances of solution-processed PbS CQDs photodetectors have been close to those of InGaAs technology,and PbS QDs SWIR photodetectors are more advantageous in terms of low cost,small pixel size,large resolution,full wafer processing,and wide spectral detection range from the visible short-wave infrared.However,the photodetection wavelength of reported PbS CQDs photodiodes is usually in the near-infrared region,accompanying with few studies on PbS CQDs based SWIR photodiodes.This thesis focuses on realization of high-performance PbS CQDs based SWIR photodiodes toward high pixel density SWIR imaging array aiming at solving the main problems of high reverse bias dark current and low photoresponsivity in the SWIR region.Through systematical investigation and optimization of the PbS CQDs active layer,charge transport layer and electrode materials,the reverse bias dark current has been greatly suppressed accompanying with simultaneous improvement of photoresponsivity in the SWIR region.On the basis of the achieved device performances,the equivalent circuit model as well as parameter extraction methods are proposed,which are in good agreement with the experimental results.The detailed research contents are as follows:（1）Stating from the functional layers of photodiodes,the reverse bias dark current is greatly suppressed by systematical investigation and optimization of active layer,charge transport layer and electrodes.For the active layer,the large-diameter PbS CQDs film with matched band alignments and low defect density is achieved by optimizing the purification times,film washing solvent and oxidation time;For the charge transport layer,the ZnO electron transport layer with matched band alignments and high carrier concentration is achieved by optimizing the spin-coating and annealing atmosphere and temperature;For the electrodes,ITO electrode is modified with PEIE layer to lower the work function,beneficial for effective suppression of reverse hole injection current.On the basis of the above strategies,the reverse bias dark current of PbS CQDs SWIR photodiodes have been effectively suppressed with a low reverse bias dark current density of 2.99×10-9 A cm^-2 at a bias voltage of-0.2 V,which is among the reported lowest reverse bias dark current density values of reported PbS CQDs SWIR photodiodes.（2）Interfacial charge transport layer is crucial for generation separation and transport of photogenerated carriers in the photodiodes.In this thesis,a simple and efficient strategy i.e.adoption of 2Methoxyethanol instead of anhydrous ethanol as the precursor solvent is proposed to fabricate ZnO electron transport layer of PbS CQDs SWIR photodiodes.The resulted device exhibits significantly enhanced photoresponsivity in the whole wavelength range from 420 to 1550 nm especially in the SWIR region As a result,the photodiode device shows excellent figures of merit including low reverse bias dark current of 1.28× 10^-8 Acm^-2 at a bias voltage of-0.5 V,high responsivity of 0.470A W1 to 1413 nm SWIR light,high specific detectivity of 2.22×10¹² Jones to 1413 nm SWIR light,fast response/recovery time of 6.8 μs/8.6 μs,and high light on/off switching stability.Through sysmatical and detailed device characterizations,improved charge extraction and reduced interfacial recombination at PbS/ZnO interfaces as well as higher SWIR optical transmission are responsible for the enhanced photoresponsivity in the SWIR region.（3）In order to better understand the operation mechanisms and the analysis of following practical applications,the equivalent circuit model for the PbS CQDs photodiodes is proposed.Firstly,the influence principles of the reverse saturation current,diode ideality factor,serial re-sistance and parallel resistance on the electrical behaviors of photodiodes are analyzed in detail.Then,the parameter extraction strategies for reverse saturation current J0,diode ideality factor ni,serial resistance RS and parallel resistance Rp are proposed by combining the dark current-voltage curves,impedance characteristics and open voltage and short current under illumination.The simulation results are in good agreement with the experimental results,which further confirm the availability and accuracy of the proposed equivalent circuit model and parameter extraction methods.Furthermore,the C-V and C-f characteristics of PbS CQDs photodiodes demonstrate the resistive behaviors at forward bias voltage and capacitive behaviors at reverse bias voltage,consistent with the equivalent circuit models.