Design And Fabrication Of Two-dimentional Materials/silicon Heterojunctions For Applications In Optoelectronic Devices

Owing to the extraordinary optical,electrical,magnetic and mechanical properties,two-dimensional(2D)materials,led by graphene,have aroused universal attentions and exhibit much application potentials in optoelectronics,communication,energy,catalysis and so on.For instance,by utilizing the high mobility and high transmittance,graphene can be applied in solar cell devices,acting as the transparent electrode or constructing junctions with other semiconductor materials.On the other hand,according to the layer-dependent optical bandgap,transition metal dichalcogenides(TMDs)with tunable electrical properties can be employed in high-performance optoelectronic devices.However,most reported 2D materials-based photodetectors can hardly be equipped with excellent responsivity(R)and detectivity(D*),simultaneously.As to photoconductive detectors,which possess the high R but usually with an unsatisfied D*,as to heterojunction detectors,which possess the high D* but usually with an unsatisfied R.On the other hand,it is difficult for the Si-based organic-inorganic hybrid solar cells to have both the high conversion efficiency and good performance stability simultaneously.Moreover,during the device manufacturing process,the expensive materials cost and complex processing techniques could obviously limit their large-scale commercial applications.Herein,aiming at improving the performance,reducing the cost and simplifying processing,focusing on the research of the design and fabrication of 2D materials/Si heterojunction,we made the following four achievements:1.We designed a structured three-dimensional(3D)heterojunction of RGO-Mo S2/pyramid Si via a solution-processed method.Owing to the improved light absorption by pyramid structure and enhanced charge separation/transportation by inserted RGO,the assembled photodetector exhibited excellent performance in self-driven mode with large responsivity of 21.8 A W-1,extremely high detectivity up to 3.8×1015 Jones(Jones = cm Hz1/2 W-1).These device parameters represent best results for Mo S2-based self-driven photodetectors,and the detectivity value sets a new record for all 2D metal dichalcogenides(2DMD)-based detectors reported thus far.Meantime,by taking advantage of the narrowed bandgap of Mo S2 by imperfect crystallinity,our device realized an ultra-broad spectrum response ranging from 350 nm(ultraviolet)to 4.3 ?m(mid-wave infrared).The detectivities under 1310 and 1550 nm are more than three orders of magnitude higher than prior reports.The design of novel 3D heterojunction could be extended to other 2D materials,opening up the opportunities for a host of high-performance optoelectronic devices.2.Similarly,the three-dimensional WS2/Si nanopillars heterojunction was designed and prepared through an in-situ solution-process.By changing the diameter of the used Si nanopillars,the morphology of WS2 grown on Si nanopillars can be highly controlled.Thanks to the light harvesting enhancement of the structured junction interface and a balanced distribution and morphology of WS2 on Si NPs,the assembled device shows outstanding detection ability to the NIR(near-infrared)light.An extremely high detectivity of 2.55×1013 Jones was obtained,which is the new record for the WS2-based devices thus far.Meantime,the excellent responsivity of 610 m A W-1 and fast response speed of 25.5 ?s are also competitive to those of previous devices.Moreover,a new up-conversion photodetection model was demonstrated by coupling the detector with a standard OLED device,which can respond to the invisible NIR signals with a visible green light directly.3.Based on traditional PS spheres assisted RIE etching method,we developed the technology to produce Si nanopillars with arbitrary slant angels by slanting the Si substrates during the RIE process.Owing to the isotropic etching of RIE,the angles could arbitrarily range from 0° to 90°.Both absorption test and FDTD(finite-different time-domain)simulations confirmed that,when the diameters of Si nanopillars was fixed at 600 nm and the hight was fixed at 5 ?m,the absorption of Si nanopillars with a 30° slant angle was ~10% larger than that of vertical Si nanopillars.In a preliminary experiment,photodetectors and solar cells performances based on slant Si nanopillars/graphene heterojunctions also presented obvious improvements.Additionally,through depositing Au film then removing the PS spheres,the Si nanoholes with slant angles also could be prepared.This kind of one dimensional Si structure with slant angles could be widely used in Si-based electronics4.We designed the graphene/PEDOT:PSS/Si organic-inorganic hybrid solar cells.A CVD-grown graphene film was employed as the top transparent electrode,which not only can effectively avoid the light blocking of conventional metal grids electrode,but also can protect the organic layer from been destroyed to guarantee the performance stability.To further improve the light-harvesting ability,the polymethyl methacrylate(PMMA),which was used in wet transfer of graphene,was retained and applied to construct the double layer antireflection coating with PEDOT:PSS.After optimizing the thickness of PMMA layer and PEDOT:PSS layer,the number of graphene layers,respectively,a champion device with PCE of 13.01% was obtained.Moreover,the devices exhibited good stability without any encapsulation in air.