Study On High Performance Polymer Photodetectors

Visible-near infrared photodetectors?PDs?,which can be used to detect and measure the radiation in the corresponding portion of the electromagnetic spectrum,has long been a major focus of basic science research and various applications in broad areas,such as digital imaging,bio-diagnostics,optical communication,environmental monitoring,remote sensing,and night vision.Unlike conventional PDs which consist of p-n junction,polymer photodetectors?PPDs?rely on intrinsic semiconductor as light absorber and are Schottky-type device in nature.Therefore,the dark current density of PPDs is closely related to the contact properties between the semiconducting polymer and metal electrode,and can be potentially smaller as the intrinsic charge concentration is far smaller than that of doped inorganic semiconductors.Furthermore,for the same reason,PDs can be operated at room temperature,avoiding the use of cooling system.Through years' efforts,the key device performance parameters of PPDs,such as sensitivity?detectivity?,response speed and linear dynamic range?LDR?have been significantly improved,some of which are comparable to or even better than that of their inorganic counterparts.Despite of these tremendous progresses,there are few reports on the realization of PPDs with high detectivity,fast response time and low dark current.This is a difficult task since high external quantum efficiency?EQE,also known as photo response?,a minimized dark current,as well as short transit time,are need to be satisfied at the same time.In the past several years,solvent vapor annealing?SVA?has been proved to be very efficient in controlling the film morphology of the photoactive layer of polymer solar cells,thus become routine technologies for enhanced device performance.However,This method is rarely implied to the fabrication of polymer photodetectors.In the second chapter,we demostrated that the performance of PPDs from the blend of PCDTBT:PC₇₁BM can be substantially enhanced via tetrahydrofuran solvent vapor annealing,resulting in an increase in photoresponse from 90 mA/W to 185 mA/W,while the dark current density of the detector remained nearly unchanged.As a result,the specific detecivity of the detector increased from 1.86×10¹²Jones to 5.94×10¹²Jones.In the third chapter,we report visible-near infrared photodetectors with excellent overall device parameters from conjugated polymers with different optical bandgaps by optimal design in device structure and optimizing the thickness of the photoactive layer in nanoscale.With the realization of suppressed dark current(⁰.2 nA cm^-2)and preserving their high external quantum efficiency?over 40 % in most of the spectral range?,the best visible-near infrared photodetectors shows high detectivity?D*?of 1.7×10¹³ Jones at a wavelength of 710 nm or 2.2×10¹² Jones at a wavelength of 840 nm.Their performance parameters are among the best results of polymer PDs reported to date and even better than that of commercialized inorganic silicon based PDs.We found that inverted device structure,in which ohmic contact for charge extraction and high Schottky barrier for charge injection are simultaneously formed,is ideal for high performance polymer photodetectors.This lays foundation for future work towards all solution-processed,low-cost,large-area polymer photodectors with superior device parameters.In principle,the approach can be applicable to many other material systems toward the realization of highly sensitive PPDs.However,in practice,the inverted structure may not work very well for some of materials,mainly owing to wetting properties between the PFN/photoactive layer interfaces.To retain sufficiently low Schottky barrier heights in a normal device structure,alternatively,hole transporting layer with low lying Fermi level or electron transporting layer with high lying Fermi level can be incorporated as additional layer to suppress charge injection in reverse bias.To investigate the effects of an extra layer with low lying Fermi level,we introduce a thin layer of poly?N-vinylcarbazole??PVK?,which possess an effective work function of 5.4 eV,as anode buffer layer in the devices and employ a promising narrow bandgap polymer,diketopyrrolopyrrole-containing polymer?PDPP3T?.With the incorporation of PVK layer,the obtained photodetectors show a high specific detectivity D* greater than 2.0×10¹² Jones at-1 V over a broad spectral range?500 nm to 900 nm?and a maximal D* of 2.2×10¹² Jones at a wavelength of 840 nm,which is much higher than that of the control device with PEDOT:PSS as the anode buffer layer(maximal D*⁵.2×10¹¹ Jones).There performance parameters are among the best results of polymer PDs reported to date and one order magnitude higher compared with commercialized inorganic silicon based PDs,making these devices attractive alternative for practical applications.