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Alloying is an effective way to modulate material’s properties. In particular, graded alloying within a single domain of two-dimensional transition-metal chalcogenide (2D-TMC) is of great technolog...

The band alignment of CdTe/MoOx hetero-interface contact at back electronic electrode in CdTe thin film solar cell was quantitatively characterized by photon electron emission spectroscopy. The experimental results showed that depending on the CdTe surface chemical state and the stoichiometric value of x in MoOx, energy barrier with a value as low as 0.23–0.39 eV was developed at the CdTe/MoOx contact interface for a MoOx buffer layer with an x value of 2.86–3. CdTe solar cell with relatively high efficiency can be fabricated by using a MoOx as a back contact buffer, which is allowed to have a relatively large stoichiometric x range of 2.86–3. The Te-rich CdTe surface obtained by chemical etching induced a reduction reaction upon the deposition of a MoOx thin layer, leading to the formation of reduced Mo states and other defect states below the Fermi level, which assisted hole carrier transport.

Black phosphorus (BP) has drawn great attention owing to its tunable band gap depending on thickness, high mobility, and large Ion/Ioff ratio, which makes BP attractive for using in future two-dimensional electronic and optoelectronic devices. However, its instability under ambient conditions poses challenge to the research and limits its practical applications. In this work, we present a feasible approach to suppress the degradation of BP by sulfur (S) doping. The fabricated S-doped BP few-layer field-effect transistors (FETs) show more stable transistor performance under ambient conditions. After exposing to air for 21 days, the charge-carrier mobility of a representative S-doped BP FETs device decreases from 607 to 470 cm2 V–1 s–1 (remained as high as 77.4%) under ambient conditions and a large Ion/Ioff ratio of ∼103 is still retained. The atomic force microscopy analysis, including surface morphology, thickness, and roughness, also indicates the lower degradation rate of S-doped BP compared to BP. Fir...

Comparative study of GaAs and CdTe solar cell under low-intensity light irradiance was carried out to study the cell device performance in response to the changed light irradiance intensity. For highly efficient GaAs solar cell, the series Rs and the shunt Rp resistance were found to be low/high enough to have almost undetectable negative effect on the cell device performance at low-intensity light irradiance. The robust diode parameters guaranteed good cell performance at low-intensity light irradiance. An ideal logarithmic function was established to describe the variation of cell efficiency with light irradiance intensity for GaAs solar cell. For CdTe polycrystalline solar cell, the relatively large series resistance Rs and small shunt resistance Rp, compared to that of the GaAs solar cell, significantly decreased the cell fill factor. With increased light illumination intensity, both the diode ideality factor A and the reverse saturation current density J0 were increased due to the high density of interface states at the CdS/CdTe junction. The comparative study and conclusions drawn in this work provide device fabrication improvement direction for the CdTe solar cell.

Black phosphorus nanosheets (BP NSs)-decorated single-layer WS2 phototransistors were fabricated by drop-casting the liquid-phase exfoliated BP NSs atop the channel of single-layer WS2 grown by chemical vapor deposition technique. It was found that, upon the decoration of BP NSs, the BP/WS2 hybrid phototransistor exhibited an enhancement of responsivity and extension of response spectral range. The responsivity is up to 120 mA/W in the hybrid phototransistor, being about 20 times larger than that in devices based on the pristine single-layer WS2. Moreover, the hybrid device possess high response speed with the response and recovery time of ~54 and ~140 μs, respectively, and also excellent photoswitching stability.

The absorber layer in a CdTe solar cell is made of a polycrystalline CdTe thin film with a thickness of several μm. It is highly desirable that the CdTe film could be made of through-thickness CdTe grains. In such a case, the photon-generated carriers could be transported to the positive and negative contacts without encountering any grain boundaries in CdTe, thus significantly reducing the carrier scattering/recombination occurring at the grain boundary. We propose and demonstrate that one of the most important roles for the presence of oxygen during the CdTe film growth is to in situ form a low-melting liquid mixture of oxides (CdTeOx, TeOx and CdOx), Te, and CdTe on the growth front of the CdTe grain surface. This liquid mixture, having a thickness of ∼50 nm, assists material transport from the vapor through the liquid to the liquid–solid interface, where Cd, Te, and Cd–Te molecules reacted to epitaxially deposit on the CdTe growth front. Assisted by a highly crystalline mono-grained CdS window layer template, CdTe preferential growth with through-thickness grains along the normal direction of the film has been achieved. CdTe solar cells fabricated with such CdTe absorbers demonstrate much enhanced external quantum efficiency, and cells with efficiencies as high as 14–15.2% have been fabricated.

Study of the device characteristics of a CdTe solar cell under weak light irradiance ( E irra ) is important both for the understanding of the fundamental device physics and for the commercial application, where E irra with intensity much less than one sun is often encountered for an outdoor solar cell module. In this study, CdTe solar cell performance under E irra as low as 0.015 sun was studied. Both the fill factor (FF) and the open-circuit voltage ( V oc ) were found to be critically affected by the shunt resistance at low E irra . The current shunting depends critically on the physical location of the shunting paths in the CdTe absorber layer. Space-charge limited current (SCLC) was identified to be an important contribution to the shunting current in thin film CdTe solar cell. At an E irra as low as 0.015 Sun, CdTe solar cell with a high shunt resistance maintained an efficiency of ~70 to 80% of the value tested at the standard AM1.5 E irra . The experimental results showed that polycrystalline CdTe thin film solar cell is a good photovoltaic device for electric power generation at low E irra . This study provides constructive guidelines for the future design and fabrication of CdTe solar cells.

A low electric resistive and stable back contact on p-type CdTe semiconductor is crucial for the commercial employment of high efficiency CdTe thin film solar cell. In this study, V 2 O 5 was deposited as a buffer layer between CdTe and metal electrode in the back contact of CdTe solar cells. Different back contact structures were fabricated on CdTe to study the effect of a V 2 O 5 buffer layer on cell device performance. Both the quantitative band alignment and the device performance of the CdTe solar cells with a V 2 O 5 buffer layer demonstrated that a much lower Schottky barrier was formed compared to the cells with an Au-only back contact. The defect states related to oxygen vacancy within the band gap of the V 2 O 5 played a crucial role in reducing the energy barrier for hole carrier transport. Employing a back contact structure of Cu/V 2 O 5 /Cu/Au, a CdTe solar cell with an efficiency as high as 14.0% was fabricated. Long term device stressing test demonstrated that, compared to the CdTe cells with a Cu/Au back electrode, solar cells with the insertion of a V 2 O 5 buffer layer showed much enhanced device stability.

Black phosphorus (BP), a two-dimensional (2D) material, has a direct bandgap that can be tuned by changing the layers and applied strain, which fills the lacuna left by graphene topological insulators and transition-metal dichalcogenides. Theoretically, the direct and tunable bandgap should enable broadband applications for optoelectronics with high efficiencies in the spectral range from the visible to the mid-infrared. Here, a BP broadband optical modulator is experimentally constructed and passively modulated lasers at 639 nm (red), 1.06 μm (near-infrared), and 2.1 μm (mid-infrared) are realized by using a BP optical modulator as the saturable absorber in bulk lasers. The obtained results provide a promising alternative for rare broadband optical modulators and broaden the application range of BP in photonics.

The low responsivity observed in photodetectors based on monolayer transition-metal dichalcogenides has encouraged the pursuit of approaches that can efficiently enhance the external quantum efficiency, which relies predominantly on the light absorption, the lifetime of the excess carriers, and the charge collection efficiency. Here, we demonstrate that phototransistors fabricated on large-area bilayer tungsten disulfide (WS2) grown by chemical vapor deposition exhibit remarkable performance with photoresponsivity, photogain, and detectivity of up to ∼3 × 103 A/W, 1.4 × 104, and ∼5 × 1012 Jones, respectively. These figures of merit of bilayer WS2 provide a significant advantage over monolayer WS2 due to the greatly improved carrier mobility and significantly reduced contact resistance. The photoresponsivity of bilayer WS2 phototransistor can be further improved to up to 1 × 104 A/W upon biasing a gate voltage of 60 V, without evident reduction in detectivity. Moreover, the bilayer WS2 phototransistor exhi...

MoOx thin films were employed as a buffer layer in the back contact of CdTe solar cells. A monograined CdS layer was employed as the window layer to reduce light absorption. The insertion of a MoOx buffer layer in the back contact greatly reduced the Schottky barrier leading to increased fill factor and open-circuit voltage. A CdTe solar cell, with an efficiency as high as 14.2%, was fabricated. The use of a MoOx buffer layer made it possible to fabricate high-efficient CdTe solar cell with much less Cu in the back contact, thus greatly enhancing the cell stability. Copyright © 2015 John Wiley & Sons, Ltd.

High-crystalline-quality CdS thin films with a mono-grained layer of submicron grain size were successfully fabricated. CdS thin films prepared by chemical bath deposition were re-crystallized under different chemical atmospheres. The microstructure of the CdS films, the hetero-junction interface CdS/CdTe, and therefore the solar cell performance were critically dependent on the film process history of the window CdS layers. Heat treatment of a CdS precursor film coated with a CdCl2 layer and under a high CdCl2 vapor pressure reduced over-oxidation at the grain surface and promoted in-plane grain coalescence along the CdS/FTO (F-doped SnO2) interface. A high-crystalline-quality, mono-grain CdS layer ensured homogenous intermixing of CdS and CdTe at the junction interface. A short-circuit current as high as 25.1 mA cm−2 was obtained for a mono-grain-CdS/CdTe solar cell. The corresponding solar cell efficiency is 14.6%.

CdTe thin film solar cell with an absorber layer as thin as 0.5 μm was fabricated. An efficiency of 7.9 % was obtained for a 1-μm-thick CdTe solar cell. The experimental results presented in this study demonstrated that 1-μm-thick absorber layer is thick enough to fabricate CdTe solar cell with a decent efficiency. Formation of mono-grain CdS layer with grain size in submicron was fabricated. Heat treatment of a CdS precursor layer coated with a CdCl2 layer and under a high CdCl2 vapor pressure reduced oxide formation at the grain surface and promoted grain coalescence. High crystalline quality mono-grain CdS layer ensured homogenous intermixing of CdS and CdTe at the CdS/CdTe interface. A saturated junction leakage current in the order of 10-10 A/cm2 and a short-circuit current as high as 25.1 cm/cm2 were obtained for a mono-grain-CdS/CdTe solar cell. The corresponding solar cell efficiency is 14.6%.

We demonstrate a passively Q-switched Yb3+-dopedScBO3 bulk laser using a black phosphorous (BP) saturable absorber, a two-dimensional semiconductor. The response spectra of BP show that it is suitable as a universal switcher in the spectral range from the visible to midinfrared band. Considering the saturable absorption properties of BP and emission properties of Yb3+-doped crystals, the passively Q-switched bulk laser pulses were realized with the Yb3+:ScBO3 crystal as a gain material and a fabricated BP sample as a Q-switcher. Because of the large energy storage capacity of Yb3+:ScBO3, the maximum output energy is obtained to be 1.4 μJ, which is comparable with the previous reported maximum energy of graphene Q-switched lasers. The obtained results identify the potential capability of BP as a pulse modulator in bulk lasers, and BP plays an increasingly important role in a wide range of its applications, including photonics and optoelectronics.

Cu is widely used in the back contact formation of CdTe thin film solar cells. However, Cu is easily to diffuse from the back contact into the CdTe absorber layer and even to the cell junction interface CdS/CdTe. This phenomenon is generally believed to be the main factor affecting the CdTe solar cell stability. In this study Cu was intentionally doped in CdS thin film to study its effect on the microstructural, optical and electrical properties of the CdS material. Upon Cu doping, the VCd− and the surface-state-related photoluminescence emissions were dramatically decreased/quenched. The presence of Cu atom hindered the recrystallization/coalescence of the nano-sized grains in the as-deposited CdS film during the air and the CdCl2 annealing. CdTe thin film solar cell fabricated with Cu-doped CdS window layers demonstrated much decreased fill factor, which was induced by the increased space-charge recombination near the p-n junction and the worsened junction crystalline quality. Temperature dependent current-voltage curve measurement indicated that the doped Cu in the CdS window layer was not stable at both room and higher temperatures.