SnO2-based electron transporting layer materials for perovskite solar cells: A review of recent progress

In recent years, due to their high photo-to-electric power conversion efficiency(PCE)(up to 23%(certified)) and low cost, perovskite solar cells(PSCs) have attracted a great deal of attention in photovoltaics field. The high PCE can be attributed to the excellent physical properties of organic–inorganic hybrid perovskite materials, such as a long charge diffusion length and a high absorption coefficient in the visible range. There are different diffusion lengths of holes in electrons in a PSC device, and thus the electron transporting layer(ETL) plays a critical role in the performance of PSCs. An alternative for TiO2, to the most common ETL material is SnO2, which has similar physical properties to TiO2 but with much higher electron mobility, which is beneficial for electron extraction. In addition, there are many facile methods to fabricate SnO2 nanomaterials with low cost and low energy consumption. In this review paper, we focus on recent developments in SnO2 as the ETL of PSCs. The fabrication methods of SnO2 materials are briefly introduced. The influence of multiple Sn O2 types in the ETL on the performance of PSCs is then reviewed. Different methods for improving the PCE and long-term stability of PSCs based on SnO2 ETL are also summarized. The review provides a systematic and comprehensive understanding of the influence of different Sn O2 ETL types on PSC performance and potentially motivates further development of PSCs with an extension to SnO2-based PSCs.

Switching of K-Q intervalley trions fine structure and their dynamics in n-doped monolayer WS_(2)

Monolayer group VI transition metal dichalcogenides(TMDs)have recently emerged as promising candidates for photonic and opto-valleytronic applications.The optoelectronic properties of these atomically-thin semiconducting crystals are strongly governed by the tightly bound electron-hole pairs such as excitons and trions(charged excitons).The anomalous spin and valley configurations at the conduction band edges in monolayer WS_(2)give rise to even more fascinating valley many-body complexes.Here we find that the indirect Q valley in the first Brillouin zone of monolayer WS_(2)plays a critical role in the formation of a new excitonic state,which has not been well studied.By employing a high-quality h-BN encapsulated WS_(2)field-effect transistor,we are able to switch the electron concentration within K-Q valleys at conduction band edges.Consequently,a distinct emission feature could be excited at the high electron doping region.Such feature has a competing population with the K valley trion,and experiences nonlinear power-law response and lifetime dynamics under doping.Our findings open up a new avenue for the study of valley many-body physics and quantum optics in semiconducting 2D materials,as well as provide a promising way of valley manipulation for next-generation entangled photonic devices.

IMU Action Recognition Based on Machine Learning

1 Introduction With the advent of the global Internet information era,communication and artificial intelligence have been developing rapidly in information fusion.Accurate motion recognition can provide a new way for humancomputer interaction,while artificial intelligence combined with motion recognition can conduct intelligent analysis of the collected information.

Reconfigurable logic and in-sensor encryption operations in an asymmetrically tunable van der Waals heterostructure

Reconfigurable devices can be used to achieve multiple logic operation and intelligent optical sensing with low power consumption,which is promising candidates for new generation electronic and optoelectronic integrated circuits.However,the versatility is still limited and need to be extended by the device architectures design.Here,we report an asymmetrically gate two-dimensional(2D)van der Waals heterostructure with hybrid dielectric layer SiO_(2)/hexagonal boron nitride(h-BN),which enable rich function including reconfigurable logic operation and in-sensor information encryption enabled by both volatile and non-volatile optoelectrical modulation.When the partial gate is grounded,the non-volatile light assisted electrostatic doping endowed partially reconfigurable doping between n-type and p-type,which allow the switching of logic XOR and not implication(NIMP).When the global gate is grounded,additionally taking the optical signal as another input signal,logic AND and OR is realized by combined regulation of the light and localized gate voltage.Depending on the high on/off current ratio approaching 105 and reliable&switchable logic gate,in-sensor information encryption and decryption is demonstrated by manipulating the logic output.Hence,these results provide strong extension for current reconfigurable electronic and optoelectronic devices.

High responsivity and near-infrared photodetector based on graphene/MoSe_2 heterostructure

Graphene has attracted great interest in optoelectronics, owing to its high carrier mobility and broadband absorption. However, a graphene photodetector exhibits low photoresponsivity because of its weak light absorption. In this work, we designed a graphene/MoSe_2 heterostructure photodetector, which exhibits photoresponse ranging from visible to near infrared and an ultrahigh photoresponsivity up to 1.3 × 104 A·W^(-1) at 550 nm. The electron–hole pairs are excited in a few-layered MoSe2 and separated by the built-in electric field. A large number of electrons shift to graphene, while the holes remain in the MoSe_2, which creates a photogating effect.

p-/n-Type modulation of 2D transition metal dichalcogenides for electronic and optoelectronic devices

Two-dimensional layered transition metal dichalcogenides(TMDCs)have demonstrated a huge potential in the broad fields of optoelectronic devices,logic electronics,electronic integration,as well as neural networks.To take full advantage of TMDC characteristics and efficiently design the device structures,one of the most key processes is to control their p-/n-type modulation.In this review,we summarize the p-/n-type modulation of TMDCs based on diverse strategies consisting of intrinsic defect tailoring,substitutional doping,surface charge transfer,chemical intercalation,electrostatic modulation,and dielectric interface engineering.The modulation mechanisms and comparisons of these strategies are analyzed together with a discussion of their corresponding device applications in electronics and optoelectronics.Finally,challenges and outlooks for p-/n-type modulation of TMDCs are presented to provide references for future studies.

Growth of centimeter scale Nb_(1−x)W_(x)Se_(2) monolayer film by promoter assisted liquid phase chemical vapor deposition

Two-dimensional(2D)transition-metal dichalcogenide materials(TMDs)alloys have a wide range of applications in the field of optoelectronics due to their capacity to achieve wide modulation of the band gap with fully tunable compositions.However,it is still a challenge for growing alloys with uniform components and large lateral size due to the random distribution of the crystal nucleus locations.Here,we applied a simple but effective promoter assisted liquid phase chemical vapor deposition(CVD)method,in which the quantity ratio of promoter to metal precursor can be controlled precisely,leading to tiny amounts of transition metal oxide precursors deposition onto the substrates in a highly uniform and reproducible manner,which can effectively control the uniform distribution of element components and nucleation sites.By this method,a series of monolayer Nb_(1−x)W_(x)Se_(2)alloy films with fully tunable compositions and centimeter scale have been successfully synthesized on sapphire substrates.This controllable approach opens a new way to produce large area and uniform 2D alloy film,which has the potential for the construction of optoelectronic devices with tailored spectral responses.

Unidirectional bound states in the continuum in Weyl semimetal nanostructures

Recently studied bound states in the continuum(BICs) enable perfect localization of light and enhance light–matter interactions although systems are optically open. They have found applications in numerous areas,including optical nonlinearity, light emitters, and nano-sensors. However, their unidirectional nature in nonreciprocal devices is still elusive because such trapping states are easily destroyed when the symmetry of an optical system is broken. Herein, we propose nonreciprocal and dynamically tunable BICs for unidirectional confinement of light and symmetry-protected BICs at Γ-point by introducing antiparallel magnetism into the optical system. We demonstrate that such BICs can be achieved by using topological magnetic Weyl semimetals near zero-index frequency without any structural asymmetry, and are largely tunable via modifying the Fermi level.Our results reveal a regime of extreme light manipulation and interaction with emerging quantum materials for various practical applications.

Binary-ternary transition metal chalcogenides interlayer coupling in van der Waals type-II heterostructure for visible-infrared photodetector with efficient suppression dark currents

Ternary two-dimentional(2D)materials exhibit diverse physical properties depending on their composition,structure,and thickness.Through forming heterostructures with other binary materials that show similar structure,there can be numerous potential applications of these ternary 2D materials.In this work,we reported the structure of few-layer CrPS_(4)by X-ray diffraction,transmission electron microscope,and electron-density distribution calculation.We also demonstrated a new application of the CrPS_(4)/MoS_(2)heterobilayer:visible-infrared photodetectors with type-II staggered band alignment at room temperature.The response of the heterostructure to infrared light results from a strong interlayer coupling that reduces the energy interval in the junction area.Since the intrinsic bandgap of individual components determines wavelengths,the decrease in energy interval allows better detection of light that has a longer wavelength.We used photoluminescence(PL)spectroscopy,Kelvin probe force microscopy(KPFM)under illumination,and electrical transport measurements to verify the photoinduced charge separation between the CrPS_(4)/MoS_(2)heterostructures.At forward bias,the device functioned as a highly sensitive photodetector,as the wavelength-dependent photocurrent measurement achieved the observation of optical excitation from 532 to 1,450 nm wavelength.Moreover,the photocurrent caused by interlayer exciton reached around 1.2 nA at 1,095 nm wavelength.Our demonstration of the strong interlayer coupling in the CrPS_(4)/MoS_(2)heterostructure may further the understanding of the essential physics behind binary-ternary transition metal chalcogenides heterostructure and pave a way for their potential applications in visible-infrared devices.

Carrier mobility tuning of MoS_(2) by strain engineering in CVD growth process

Strain engineering is proposed to be an effective technology to tune the properties of two-dimensional(2D)transition metal dichalcogenides(TMDCs).Conventional strain engineering techniques(e.g.,mechanical bending,heating)cannot conserve strain due to their dependence on external action,which thereby limits the application in electronics.In addition,the theoretically predicted strain-induced tuning of electrical performance of TMDCs has not been experimentally proved yet.Here,a facile but effective approach is proposed to retain and tune the biaxial tensile strain in monolayer MoS_(2) by adjusting the process of the chemical vapor deposition(CVD).To prove the feasibility of this method,the strain formation model of CVD grown MoS_(2) is proposed which is supported by the calculated strain dependence of band gap via the density functional theory(DFT).Next,the electrical properties tuning of strained monolayer MoS_(2) is demonstrated in experiment,where the carrier mobility of MoS_(2) was increased by two orders(~0.15 to~23 cm^(2)·V^(−1)·s^(−1)).The proposed pathway of strain preservation and regulation will open up the optics application of strain engineering and the fabrication of high performance electronic devices in 2D materials.