All Inorganic Halide Perovskite Nanocrystals For Ultrasensitive Photodetectors

All-inorganic halide perovskites possess superior optoelectronic properties such as high optical absorption coefficient,tunable optical bandgap,and excellent carrier transport capability,combined with low manufacturing cost and easy solution processing,making them interesting candidates for highly innovative and disruptive nextgeneration optical signal detection.Recently,an explosion of research has occurred on the synthesis of perovskites with a large variety in morphology and dimension.In particular,perovskite colloidal nanocrystals generally have diverse morphology,quantumsize effects,controllable size uniformity,and are easy to form smooth films,each allowing for the specific photodetector application.However,there are still some challenges in perovskite nanocrystals.Firstly,the intrinsic ionicity and low lattice formation energy tend to cause structural instability and optical property degradation of nanocrystals.Secondly,the extremely fast nucleation growth rate makes kinetic modulation difficult,leading to the common hexahedral nanocrystals.Additionally,the lead-free bismuth halide perovskite is an indirect bandgap semiconductor and often behaves with severe photogenerated carrier recombination caused by the layered crystal structure,limiting its technical applications.Based on this,we aim to achieve high-performance and stable all-inorganic perovskite nanocrystal-based photodetectors.Starting from crystal structure modulation and surface inorganic salt ligand assistance engineering,we optimized the synthesis strategy to achieve morphology modulation,phase stability,and optoelectronic performance enhancement of perovskite nanocrystals.Eventually,a variety of vertically structured photodetector devices(type Ⅰ,type Ⅱ,and back-to-back p-n junction/Schottky junction)have been constructed and realized the efficient and stable application in the optical detection field.This thesis mainly covers the following three areas:1.By choosing phenacyl bromide as the Br source,the monodisperse and homogeneous polyhedral CsPbBr3 nanocrystals(12 faceted rhombic dodecahedrons and 26 faceted rhombicuboctahedrons)were obtained.In this case,phenacyl bromide could slow the reaction progress and the tertiary ammonium ion formation leading to the nucleation of quasi-spherical dots were two key steps to growing high-quality polyhedral CsPbBr3 nanocrystals.Next,the p-Si/CsPbBr3 vertical photodetectors with type-Ⅰ heterojunction structures were constructed.The results showed that the 12 faceted CsPbBr3 nanocrystals-based device ultimately achieved ultra-high performance photodetection due to the relatively superior charge extraction rate and low carrier recombination loss.Typically,the responsivity(R)and detectivity(D*)reached a high level of 8.36 A W-1 and 9.33×1013 Jones,respectively.Meanwhile,a fast response on the μs scale(90/162μs)and a high on/off ratio of 23 807 were achieved successfully.It is worth mentioning that the device showed very low noise characteristics and high sensitivity for weak light detection with obvious photoresponse and an ultralow noise equivalent power(NEP)of 2.14×10-15 W Hz-1/2.Subsequently,we proposed the charge transfer mechanism of the device and proved the significantly enhanced performance originated from not only the reduced radiative recombination loss of the 12 faceted CsPbBr3 film but also the ideal type-I band alignment of the device,which assist in effectively suppressing the recombination of electron-hole pairs and improving the carrier collection efficiency.In conclusion,our work has greatly enriched the field of CsPbBr3 based photodetection and demonstrated the important role of morphological structure modulation and device structure optimization in this field.2.By adjusting the conventional hot injection synthesis and choosing ammonium iodide(NH4I)as the additional reaction source and surface ligands,the high-quality αCsPbI3 nanocrystals with high luminescence and excellent stability were obtained.In view of this,we constructed a vertical Si/α-CsPbI3 type-Ⅱ structured photodetector.As a result,the device exhibited a rather pronounced photoelectric effect with a high on/off ratio of 303,excellent responsivity(R)and detectivity(D*)of 19.62 A W-1 and 4.24×1012 Jones.In addition,the device showed a significantly wider optical detection range from UV to NIR(405-1064 nm).More importantly,it demonstrated an extremely fast response(9/8 μs)and superior environmental stability.In conclusion,this work illustrates that room-temperature phase stable α-CsPbh nanocrystals can be prepared by optimized synthesis and surface ligand strategies,and the great potential of band alignment engineering in developing high-performance photodetectors.3.In this work,the monodispersed all-inorganic lead-free hexagonal Cs3Bi2I9 perovskite nanocrystals were obtained via a simplified one-step reaction of metal acetate salt with trimethylsilyl iodide.Then we further constructed the flexible Cs3Bi2I9 detector and the p-Si/Cs3Bi2I9/graphene vertical heterojunction photodetector to investigate their photodetection performance.As a result,the flexible device not only showed excellent detective performance but also yielded high-resolution photographs as an imaging sensor.Surprisingly,the p-Si/Cs3Bi2I9/graphene detector achieved a significant improvement in detection performance benefiting from a rational device structure design(back-to-back p-n junction and Schottky junction).It performed broad spectral response from ultraviolet to infrared light(254-1064 nm)and excellent stability in ambient air.In the meantime,the device exhibited excellent optical response performance with a high responsivity(R)of 23.6 A W-1 and a high detectivity(D*)of 1.75×1013 Jones under 650 nm illumination even in weak light intensity as low as 1 μW cm-2 and achieved ultra-fast response(31/34 μs).In addition,finite element analysis further demonstrated the excellent optoelectronic properties of Cs3Bi2I9 materials and the broad spectral response of vertical heterojunctions.In conclusion,our work implied the promising application of phase-stabilized lead-free Cs3Bi2I9 nanocrystals in the field of highperformance broadband photoelectric detection.