Two-dimensional transition metal dichalcogenides for lead halide perovskites-based photodetectors: band alignment investigation for the case of CsPbBr3/MoSe2

The distinguished electronic and optical properties of lead halide perovskites(LHPs)make them good candidates for active layer in optoelectronic devices.Integrating LHPs and two-dimensional(2 D)transition metal dichalcogenides(TMDs)provides opportunities for achieving increased performance in heterostructured LHPs/TMDs based optoelectronic devices.The electronic structures of LHPs/TMDs heterostructures,such as the band offsets and interfacial interaction,are of fundamental and technological interest.Here CsPbBr3 and MoSe2 are taken as prototypes of LHPs and 2 D TMDs to investigate the band alignment and interfacial coupling between them.Our GGA-PBE and HSE06 calculations reveal an intrinsic type-II band alignment between CsPbBr3 and MoSe2.This type-II band alignment suggests that the performance of CsPbBr3-based photodetectors can be improved by incorporating MoSe2 monolayer.Furthermore,the absence of deep defect states at CsPbBr3/MoSe2 interfaces is also beneficial to the better performance of photodetectors based on CsPbBr3/MoSe2 heterostructure.This work not only offers insights into the improved performance of photodetectors based on LHPs/TMDs heterostructures but it also provides guidelines for designing high-efficiency optoelectronic devices based on LHPs/TMDs heterostructures.

Investigation into epitaxial growth optimization of a novel AlGaN/GaN HEMT structure for application in UV photodetectors

In this work,a novel ultraviolet(UV)photodetector(PD)based on AlGaN/u-GaN/p-GaN/u-GaN heterojunction high electron mobility transistor(HEMT)has been developed.This HEMT epilayer is grown using the metal-organic chemical vapor deposition(MOCVD)technique,and the growth parameters,including the AlGaN growth temperature,preheating temperature of the p-GaN layer,and NH3/N2 flow rate,are optimized to improve the quality of the epilayer.The optimized epilayer exhibits a flat surface with a root mean square value of 0.146 nm and low dislocation density.The p-GaN thickness in epitaxial wafers has a significant influence on electrical and UV photoresponse.With a p-GaN of 1µm,the UV PD demonstrates a significant switching ratio and transconductance of 107 and 127.3 mS mm^(-1),respectively.Acting as a UV PD,it also exhibits a high light on/off ratio(I_(light)/I_(dark))of 6.35×10^(5),a high responsivity(R)of 48.11 A W^(-1),and a detectivity(D*)of 6.85×10^(12)Jones under 365-nm UV illumination with light power density of 86.972 mW cm^(-2).The high-performance HEMT and UV detectors,which incorporate p-GaN etchless technology,have been refined through advancements in epitaxial growth and structural design.These improvements solidify the groundwork for large-scale manufacturing of UV communication systems and laser diodes.

Enhanced rectification, transport property and photocurrent generation of multilayer ReSe2/MoS2 p-n heterojunctions

Van der Waals （vdW） heterojunctions are equipped to avert dangling bonds due to weak, inter-layer vdW force, and ensure strong in-plane covalent bonding for two-dimensional layered structures. We fabricated four heterojunctions devices of different layers based on p-type distorted 1T-MX2 ReSe2 and n-type hexagonal MoS2 nanoflakes, and measured their electronic and optoelectronic properties. The device showed a high rectification coefficient of 500 for the diode, a high ON/OFF ratio and higher electron mobility for the field-effect transistor （FET） compared with the individual components, and a high current responsivity （Rλ） and external quantum efficiency （EQE） of 6.75 A/W and 1,266%, respectively, for the photodetector.

Highly sensitive infrared polarized photodetector enabled by out-ofplane PSN architecture composing of p-MoTe_(2),semimetal-MoTe_(2)and n-SnSe_(2)

Leveraging the unique physical properties,two-dimensional(2D)materials have circumvented the disadvantages of conventional epitaxial semiconductors and held great promise for potential optoelectronic applications.So far,two main detector architectures including photodiode based on a van der Waals P-N junction or Schottky junction and phototransistor based on individual 2D materials or hybrids have been well developed.However,a trade-off between responsivity and speed always exists in those technologies thus hindering the overall performance improvement.Here,we propose a new device concept by sandwiching the 2D anisotropic semimetal between p-type and n-type semiconductors in the out-of-plane direction,called PSN architecture,realizing the improvement of each parameter including broad spectral coverage,fast speed,high sensitivity,power-free and polarization-sensitive.We stack the p-type 2H-MoTe_(2),Weyl semimetal 1T-MoTe_(2)and n-type SnSe_(2)layer-by-layer constructing vertical sandwich structure where the top and bottom layers contribute to the internal built-in electric field,the intermediate layer can facilitate the exciton dissociation and act as infrared polarized light sensitizers.As a result,this PSN device exhibits broadband photo-response from 405 to 1,550 nm without external bias supply.At optical communication band(1,310 nm),operating at self-driven mode and room temperature,the responsivity and detectivity can reach up to 64.2 mA·W^(–1)and 2.2×10^(11)Jones,respectively,along with fast speed on the order of millisecond.Moreover,the device simultaneously exhibits exceptional detection capability for infrared polarized light,demonstrating the anisotropic photocurrent ratio of 1.55 at 1,310 nm and 2.02 at 1,550 nm,which is attributed to the strong in-plane optical anisotropy of middle 1T-MoTe_(2)layer.This work develops a new photodetector scheme with novel PSN architecture toward broadband,self-power,polarized light sensing and imaging modules.

Optoelectronics based on 2D TMDs and heterostructures

2D materials including graphene and TMDs have proven interesting physical properties and promising optoelectronic applications.We reviewed the growth,characterization and optoelectronics based on 2D TMDs and their heterostructures,and demonstrated their unique and high quality of performances.For example,we observed the large mobility,fast response and high photo-responsivity in Mo S;,WS;and WSe;phototransistors,as well as the novel performances in vd W heterostructures such as the strong interlayer coupling,am-bipolar and rectifying behaviour,and the obvious photovoltaic effect.It is being possible that 2D family materials could play an increasingly important role in the future nano- and opto-electronics,more even than traditional semiconductors such as silicon.

基于极性可重构WSe_(2)肖特基异质结的非对称逻辑整流器和光电探测器

基于二维材料范德华异质结的自驱动光电探测器是逻辑光电子器件和智能图像传感器的重要组成部分.本文通过机械剥离和干法转移制备了一种底部Au接触的PtSe_(2)/WSe_(2)/Au非对称肖特基光电二极管.栅极可调的Au/WSe_(2)肖特基势垒大小、弱费米钉扎效应、高半金属PtSe_(2)导电率以及良好的PtSe_(2)/WSe_(2)层间耦合效应使得该二极管产生极性可重构现象,可实现栅极可调正负整流行为,且整流比变化范围在10−2到104之间,达到6个量级.我们利用此特性验证了半波逻辑整流器功能.此外,此自驱动器件的最大光响应度达316 mA W^(−1),最大光开关比达105,光电转换效率为4.62%,响应时间仅为830/950μs.光电流微区扫描结果表明,器件的光电流主要分布在Au/WSe_(2)界面边缘,证实该器件为非对称肖特基光电二极管.该器件还实现了高分辨率的可见光单点成像.上述研究结果表明,本工作为制备高性能半波整流器、超快自驱动光电探测器和高分辨图像传感器提供了一种简便有效的策略.

Deep insights into interface engineering by buffer layer for efficient perovskite solar cells: a firstprinciples study

Recent years have seen swift increase in the power conversion efficiency of perovskite solar cells(PSCs)Interface engineering is a promising route for further improving the performance of PSCs.Here we perform firstprinciples calculations to explore the effect of four candidate buffer materials(MACl,MAI,PbCl2and PbI2)on the electronic structures of the interface between MAPbI3absorber and TiO2.We find that MAX(X=Cl,I)as buffer layers will introduce a high electron barrier and enhance the electronhole recombination.Additionally,MAX does not passivate the surface states well.The conduction band minimum of PbI2is much lower than that of MAPbI3absorber,which significantly limits the band bending of the absorber and open-circuit voltage of solar cells.On the other side,suitable bandedge energy level positions,small lattice mismatch with TiO2surfaces,and excellent surface passivation make PbCl2a promising buffer material for absorber/electron-transport-layer interface engineering in PSCs.Our results in this work thus provide deep understanding on the effects of interface engineering with a buffer layer,which shall be useful for improving the performance of PSCs and related optoelectronics.