Study On Broadband Photodiodes Based On Heterojunction Of Perovskite And Narrow-Bandgap Photosensitizer

Organic-inorganic hybrid perovskite materials have drawn immerse attention due to its high light absorption efficient,direct bandgap,high carrier mobility,long exciton diffusion length and simple fabrication process,showing great research prospects in photodetection fields.In recent years,the reported perovskite photodetectors have exhibited excellent photodetection performances close to commercial silicon photodetectors.However,the light absorption spectrum of perovskites mainly lies in the ultraviolet-visible(UV-vis)range,which greatly hinders its further application in broadband photodetectors.This thesis focuses on how to construct and fabricate high-performance perovskite based broadband photodetectors,and aims at solving the main obstacles of perovskite materials toward high-performance broadband photodetectors.Starting from the aspects of broadening the absorption spectrum,improving the photo-generated carrier separation and reducing the photo-generated carrier recombination,the physical process for photoelectric conversion in photodetectors is modulated by constructing heterojunctions of perovskite and narrow-bandgap photosensitizer.The perovskite-PbS colloidal quantum dots(CQDs)and perovskite-organic bulk heterojunctions(OBHJ)heterojunctions are proposed and implemented,based on which high-detectivity broadband photodiodes have been obtained.The systematic optimization was conducted toward higher device performance,and the scaling down of the broadband photodiode is further explored,verifying the application potentials in high pixel density imaging array.The detailed research contents are as follows:1.Narrow-bandgap PbS CQDs were introduced as gain materials for enhancing the light-matter interaction of perovskite materials,forming perovskite-PbS CQDs heterojunction.This heterojunction can both broaden the absorption spectrum to ultraviolet-visible-near-infrared(UV-vis-NIR)broadband region and improve the photo-generated carrier separation,beneficial for achieving high-performance perovskite based broadband photodiodes.Utilizing the perovskite-PbS CQDs heterojunction as hybrid photosensitizer,the nip structured broadband photodiode device was fabricated by a fully solution process.The inherent mechanisms for the generation,separation and transport of photocarriers under illumination of lights with different wavelengths were discussed in depth,and the key aspects affecting the photodetection behaviors were obtained.Through the optimization of device structure and functional materials,high-performance self-powered broadband photodiodes with a photosensitive area of 500 μm×500 μm were obtained with a wide response spectrum of 300-1000 nm,a low working voltage(even can work under zero bias voltage),fast response/recovery speed(response/recovery time is 20 μs)and high on/off cycling stability.Under a bias voltage of-2 V,the photodiode exhibited a high photosensitivity of 2.08 A W-1 and specific detectivity of 1.24×1011 Jones under 520 nm light illumination,while the photosensitivity and specific detectivity under 940 nm light illumination was 100 mA W-1 and 1.24×1011 Jones,respectively.Moreover,the scaling down behaviors were further explored.When the photosensitive area reduced to 100 μm×100μm,the photodetection behaviors including dark current and light current remains constant,indicating its great application potential as photosensitive unit of high pixel density imaging array.2.OBHJs were introduced as gain materials for enhancing the light-matter interaction of perovskite materials,forming perovskite-PTB7-Th/IEICO-4F heterojunctions.This heterojunction can simultaneously broaden the absorption spectrum to ultraviolet-visible-near-infrared(UV-vis-NIR)broadband region,passivate the surface defects in perovskite films and improve the photo-generated carrier separation,beneficial for achieving high-detectivity perovskite based broadband photodiodes.Utilizing the perovskite-OBHJ heterojunction as hybrid photosensitizer,the pin structured broadband photodiode device with a photosensitive area of 500 μm×500 μm was fabricated by a fully solution process.This device can be self-powered with response/recovery time of 2.5 μs/2.5 μs.Impressively,the specific detectivity exceed 1012 Jones in the whole UV-vis-NIR region,which is one order of magnitude and three orders of magnitude higher than tihose of the device based on perovskite-PbS CQDs heterojunction.This can be attributed to the reduced dark current,better crystallization of perovskite films and more efficient photo-generated carrier separation and transport in the perovskite-OBHJ interfaces.Moreover,small-molecule electron accepter PCBM was introduced between perovskite and PTB7-Th:IEICO-4F OBHJ films,which further promotes the transport and extraction of photo-generated carriers and thus resulting in the increaseed specific detectivity close to 1013 Jones.Finally,the scaling down behaviors were also explored.When the photosensitive area reduced to 100 μm×100μm,the photodetection behaviors including dark and light currents remain almost constant,demonstrating its excellent scaling down behaviors and great application potential in high pixel density imaging array.In conclusion,the heterojunction of perovskite and narrow-bandgap photosensitizers was proposed and fabricated to overcome the limitation of perovskite photodiodes in NIR region toward high-performance perovskite based broadband photodiodes.The mechanisms for the generation,separation and transport of photocarriers together with its main affecting factors were investigated,and the mechanisms and strategies for device performance improvements were also explored and discussed in depth.Finally,high-performance perovskite based broadband photodiodes were obtained,providing some theoretical and technical basis for exploring applications of perovskite materials in high pixel density broadband imaging array.