Carrier Transport Mechanism and Band Offsets at the Interface of ZnS/<Emphasis Type="Italic">n</Emphasis>-Si (111) Heterojunctions Fabricated by Vacuum Thermal Evaporation

The electrical properties and band offset of ZnS/n-Si(111) heterojunctions with and without annealing were analyzed. The result showed that the rectifying characteristics of ZnS/n-Si(111) heterojunctions became better and the leakage current increased after annealing. This phenomenon is mostly due to the volatilization of S atoms of ZnS films and leads to defect levels appearing at the interface of the ZnS/n-Si(111) hetrojunctions. The valence band offset (ΔE _V) of the ZnS/n-Si(111) heterojunctions can be calculated to be ?0.7 ± 0.15 eV by means of photoelectron spectroscopy, indicating that the band offsets of ZnS/n-Si(111) heterojunctions show a type-II band alignment.     

Effects of different precursors on Cu2ZnSnS4 thin film solar cells prepared by sputtering method

In this study, the impacts of different precursors on Cu₂ZnSnS₄ thin film solar cells were investigated. The two kinds of precursors of (Cu+Sn)/Zn and (Cu+Sn)/ZnS were deposited on Mo-coated soda lime glasses by magnetron sputtering. Cu₂ZnSnS₄ (CZTS) films based on different precursors were fabricated by soft annealing and following two-step sulfurization in sulphur vapour. The crystal structure, phase purity, surface morphology, composition and optical properties of CZTS films from different precursors were characterized by X-ray diffraction (XRD), Raman spectroscopy, scanning electron microscope (SEM), energy dispersive spectrometry (EDS) and UV–vis–NIR spectroscopy, respectively. As a result, the CZTS thin films with smooth surface and uniform compositional ratio distribution were obtained from the precursors of (Cu+Sn)/ZnS. The best conversion efficiency of the fabricated CZTS film solar cell based on (Cu+Sn)/ZnS precursors was 3.36%.     

XPS analysis and characterization of thin films Cu2ZnSnS4 grown using a novel solution based route

A new technique to grow single phase Cu₂ZnSnS₄ (CZTS) thin films for solar cells applications using a chemical route is presented; this consist in sequential deposition of Cu₂SnS₃ (CTS) and ZnS thin films followed by annealing at 550 °C in nitrogen atmosphere, where the CTS compound is prepared in one step process by simultaneous precipitation of Cu₂S and SnS₂ performed by diffusion membranes assisted CBD (chemical bath deposition) technique and ZnS by conventional CBD technique.Measurements of X-ray diffraction (XRD), Raman spectroscopy and scanning electron microscopy (SEM) were used to identify the phases present in the CTS and CZTS films as well as to study their structural and morphological properties. Further, the oxidation states and the chemical composition homogeneity in the volume were studied by X-ray photoelectron spectroscopy (XPS) analysis. Oxidation states and results regarding structural and morphological characterization of CZTS films prepared using the novel technique are compared with those results obtained from single phase CZTS films prepared by sequential evaporation of metallic precursors in presence of elemental sulfur. XRD and Raman spectroscopy studies were used to verify that the CZTS films prepared by the novel method do not present secondary phases.     

Cu2ZnSn(S,Se)4薄膜太阳电池研究进展

Cu2ZnSn(S,Se)4(CZTS)材料具有与Cu(In,Ga)Se2(CIGS)材料相似的光学性质和半导体性质,且原料丰富,是CIGS薄膜太阳电池重要的后备材料。有关CZTS薄膜制备工艺的研究和电池器件转换效率提升的研究正成为本领域新的研究开发热点。目前,有实力的薄膜太阳电池研究队伍已经针对CZTS薄膜太阳电池开展了持续的研究,试图通过不同的CZTS吸收层制备方式和优化电池组装工艺过程,进一步提高CZTS薄膜太阳电池的光电转换效率。文章阐述了CZTS材料特性,着重介绍了目前国内外所采用的CZTS薄膜制备方法,详细讨论了各种薄膜沉积技术的优缺点。最后展望了CZTS电池的发展趋势。     

ZnxCd1−xS/Cu2S heterojunction solar cells—II: Junction analysis

Cu₂S/Zn_xCd_1?xS heterojunction solar cells have been fabricated by two processes namely, spray pyrolysis and evaporation. The cells have been characterised in terms of the temperature dependence of the current-voltage and capacitance-voltage characteristics. Based on the analysis of the data, an energy band model has been proposed to explain the photovoltaic performance of the heterojunction. The dominant transport mechanism in both cases has been found to be interface recombination. The observed increase in open circuit voltage on increasing ZnS concentration has been understood on the basis of a reduction in the reverse saturation current caused primarily by an increase in the barrier height at the interface. The density of interface states is not observed to be significantly affected by the lattice parameter mismatch between Cu₂S and Zn_xCd_1?xS.     

Photobreeding Heterojunction on Semiconductor Materials for Enhanced Photocatalysis

Construction of heterojunctions to photocatalysts is one of the most promising approaches to improve charge separation efficiency; however, the established constructing processes usually require high-temperature conditions and/or the adding of highly concentrated or expensive exotic species, and the improvement of effective contact and charge exchange between heterojunction components remains a problem. This work proposes an unprecedented “photobreeding” method and realizes the direct growth of Zn nanowires and Mott–Schottky heterojunctions from ZnS or viologen-coated ZnS microspheres through a photochemical reaction at room temperature without external species, while demonstrating the hypothesis proposed 140 years ago on the formation of Zn in the photochromic process of ZnS. After photobreeding of the heterojunctions, the hydrogen production efficiency of the photocatalysts increases by 2 orders of magnitude. This inexpensive, facile and efficient synthetic method will find applications in H₂ production, organic synthesis, CO₂ reduction, nitrogen fixation, and so on.     

Cu2ZnSnS4 thin film solar cells by fast coevaporation

Solar cells based on kesterite‐type Cu₂ZnSnS₄ (CZTS) were fabricated on molybdenum coated soda lime glass by evaporation using ZnS, Sn, Cu, and S sources. The coevaporation process was performed at a nominal substrate temperature of 550°C and at a sulfur partial pressure of 2–3 × 10⁻³ Pa leading to polycrystalline CZTS thin films with promising electronic properties. The CZTS absorber layers were grown copper‐rich, requiring a KCN etch step to remove excess copper sulfide. The compositional ratios as determined by energy‐dispersive X‐ray spectroscopy (EDX) after the KCN etch are Cu/(Zn + Sn): 1.0 and Zn/Sn: 1.0. A solar cell with an efficiency of 4.1% and an open‐circuit voltage of 541 mV was obtained. Copyright © 2010 John Wiley & Sons, Ltd.     

Effects of the compositional ratio distribution with sulfurization temperatures in the absorber layer on the defect and surface electrical characteristics of Cu2ZnSnS4 solar cells

Although Cu₂ZnSnS₄ (CZTS) has attracted attention as an alternative absorber material to replace CuInGaSe₂ (CIGS) in solar cells, the current level of understanding of its characteristic loss mechanisms is not sufficient for achieving high power conversion efficiency. In this study, which aimed to minimize the characteristic losses across the devices, we examined the relations between the compositional ratio distribution in the absorber layer, subsequent defect formation, and surface electrical characteristics. A high‐temperature sulfurization process was used to improve the crystallinity of the absorber layer, which increased the uniformity of the compositional ratio distribution and consequently suppressed the formation of a ZnS secondary phase on the CZTS/MoS₂ interface. Because defects and defect clusters generated in the absorber layer are shallower when the compositional ratio distribution is uniform, the electron‐hole recombination loss is reduced. These characteristics were confirmed by measuring the defect energy level using admittance spectroscopy and by analyzing the surface potential and current characteristics. These measurements revealed that improving the compositional ratio distribution suppresses the formation of deep‐level defects and reduces the rate of carrier recombination. In addition, improving the compositional ratio distribution substantially contributes to improving the series resistance and short circuit current density characteristics. Copyright © 2015 John Wiley & Sons, Ltd.     

The electron-voltaic effect in CdS/Cu2S heterojunctions

An investigation of energy conversion in CdS/Cu₂S thin film heterojunctions has shown that the devices appear to operate differently in the photovoltaic and electron-voltaic modes, the response in the latter mode occurring mainly on the CdS side of the junction. With a view to the manufacture of compact long-life reliable microgenerators the electron-voltaic mode has been studied using 6–15 keV electrons, 75–400 keV electrons and β-emitting radioactive sources such as tritium, promethium and thallium. Greater efficiencies for electron-voltaic energy conversion were obtained when the cells were fabricated by a ‘dry’ barrier method rather than by a conventional ‘wet’ barrier technique. Tritium was found to be an ideal source for electron—voltaic energy conversion in the particular CdS/Cu₂S heterojunctions studied.     

Solution‐Growth Strategy for Large‐Scale “CuGaO2 Nanoplate/ZnS Microsphere” Heterostructure Arrays with Enhanced UV Adsorption and Optoelectronic Properties

Intrinsically p‐type conductivity and a wide bandgap of ≈3.6 V endow inorganic delafossite CuGaO₂ with great promise for fabricating high‐performance UV photodetectors. Nevertheless, CuGaO₂‐based optoelectronic devices hindered because the intrinsic direct transitions are symmetry forbidden in CuGaO₂. This study reports a large‐area synthesis of “CuGaO₂ nanoplate/ZnS microsphere” heterostructure arrays using a facile solution‐based strategy associated with an oil/water interfacial self‐assembly approach. It is found that a large number of ZnS microspheres with a polycrystalline structure grow on the top surface of CuGaO₂ hexagonal platelets through Ostwald ripening mechanism, forming high‐density p–n heterojunctions. A parabolic dependence between the size of ZnS microsphere and the growth time is confirmed in this growth. The UV light adsorption of the heterostructure CuGaO₂/ZnS thin film is two times higher than that of the pristine CuGaO₂ thin film. Furthermore, the as‐designed “CuGaO₂ nanoplate/ZnS microsphere” heterostructure arrays exhibit enhanced photoresponse properties. This work offers a new insight into the rational design of optoelectronic devices from the synergetic effect of p‐type 2D nanoplates as well as n‐type nanostructures such as ZnS, ZnO, CdS, and CdO.     

InP/GaAs0.51Sb0.49/InP fully self-aligned double heterojunction bipolar transistors with a C-doped base: a preliminary reliability study

A preliminary reliability study is reported for carbon-doped InP/GaAs_0.51Sb_0.49/InP NpN double heterojunction bipolar transistors (DHBTs) lattice-matched to InP substrates. These DHBTs take advantage of the staggered (“type II”) band lineup at InP/GaAs_0.51Sb_0.49 interfaces: in this system, the GaAs_0.51Sb_0.49 base conduction band edge lies 0.15–0.18 eV above the InP collector conduction band, thus enabling the implementation of InP collectors free of the collector current blocking effect encountered in conventional Ga_0.47In_0.53As base DHBTs. The structure results in very low collector offset voltages, low emitter-base turn-on voltages, and very nearly ideal base and collector current characteristics with excellent junction ideality factors. Cut-off frequencies in excess of 100 GHz have been measured, making InP/GaAsSb DHBTs very attractive for wireless communication systems. InP/GaAs_0.51Sb_0.49 heterojunctions have so far received little attention in the literature, and no reliability information is available for this promising material combination. We have found that electrical stressing at moderate bias in fully self-aligned non-passivated devices results in a rapid, and reversible, degradation of device properties which is manifested through an increase of the base current ideality factor n_B. On the other hand, the collector current remains unchanged, indicating that there is no dopant migration effect under the test conditions used here.     

Carrier transport mechanisms of multilevel nonvolatile memory devices with a floating gate consisting of hybrid organic/inorganic nanocomposites

Nonvolatile memory devices based on a poly(4-vinylphenol) (PVP) layer containing Cu₂ZnSnS₄ (CZTS) nanoparticles were fabricated by using a simple spin-coating method. An energy dispersive spectrum revealed that the CZTS nanoparticles were Cu poor and Zn rich. Transmission electron microscopy images showed that the CZTS nanoparticles were randomly distributed in the PVP layer. Capacitance–voltage (C–V) curves for Al/CZTS nanoparticles embedded in PVP layer/p-Si devices at 1 MHz showed a hysteresis with flat-band voltage (V_fb) shifts, which resulted from the existence of CZTS nanoparticles acting as trap sites in the memory devices. The magnitudes of the V_fb corresponding to the memory window shifts between 1.0 and 2.5 V, as determined from the C–V data at 1 MHz, were dependent on the voltages applied to the memory device, indicative of multilevel characteristics for the memory effect. The operating mechanisms of the writing and the erasing processes for Al/CZTS nanoparticles embedded in PVP layer/p-Si devices are described on the basis of the C–V results and the energy-band diagrams.     

Charge transport mechanisms in anisotype n-TiO2/p-Si heterostructures

Anisotype n-TiO₂/p-Si heterojunctions are fabricated by the deposition of a TiO₂ film on a polished poly-Si substrate using magnetron sputtering. The electrical properties of the heterojunctions are investigated and the dominant charge transport mechanisms are established; these are multi-step tunneling recombination via surface states at the metallurgical TiO₂/Si interface at low forward biases V and tunneling at V > 0.6 V. The reverse current through the heterojunctions under study is analyzed within the tunneling mechanism.     

Electronic properties of Pb1−xHgxSSi heterojunctions

Two types of heterojunctions have been prepared by electroless deposition of Pb_1?xHg_xS (x = 0–0.33) films with α′ and β′ structures on n-type silicon single crystal substrates. Functional behaviour of the forward characteristics is explained on the basis of band to band tunnelling coupled with the recombination processes. The energy band diagram is given for both types of Pb_1?xHg_xS/Si heterojunctions in agreement with the experimental results.     

Impact of alloying duration of an electrodeposited Cu/Sn/Zn metallic stack on properties of Cu2ZnSnS4 absorbers for thin‐film solar cells

The impacts of preheating of an electrodeposited Cu/Sn/Zn (CTZ) stack precursor on structural changes of the CTZ precursor and the impact on structural and electric properties of the finally obtained Cu₂ZnSnS₄ (CZTS) films are discussed in detail. We found that preheating for relatively long durations improved the qualities of CZTS films: these films were composed of large grains and had compact and flat surface morphologies. The best solar cell with efficiency of 8.1% was obtained on the basis of a CZTS film derived from the CTZ precursor preheated for 200 min. The external quantum efficiency response of the cell indicated efficient utilization of photons with relatively long wavelength regions because of its good structural and electronic properties. On the other hand, a short circuit current density–temperature property of one of the best cells in this study suggested that the CZTS film had deep acceptor levels and/or an appreciable energy barrier to the Mo back contact. Moreover, an open circuit voltage–temperature property of the corresponding device showed activation energy of 1.18 eV, indicating preferential occurrence of CdS–CZTS interface recombination. Copyright © 2015 John Wiley & Sons, Ltd.     

120nm栅长In0.7Ga0.3As/In0.52Al0.48As HEMTs 器件

利用新型的PMMA/PMGI/ZEP520/PMGI四层胶T形栅电子束光刻技术制备出120nm栅长InP基雁配In0.7Ga0.3As/In0.52Al0.48As 高电子迁移率晶体管。制作出的InP基HEMT器件获得了良好的直流和高频性能,跨导、饱和漏电流密度、阈值电压、电流增益截止频率和最大单向功率增益频率分别达到520 mS/mm, 446 mA/mm, -1.0 V, 141 GHz 及 120 GHz。文中的材料结构和所有器件制备均为本研究小组自主研究开发。     

Cu2ZnSnS4 photovoltaic cell with improved efficiency fabricated by high‐temperature annealing after CdS buffer‐layer deposition

To improve the photovoltaic properties of Cu₂ZnSnS₄ (CZTS) cells, we investigated the effect of both the thickness of the deposited CdS layers and the post‐annealing temperature following CdS deposition on the photovoltaic properties of CZTS cells using a two‐layer CZTS structure. By depositing a thin CdS layer (40 nm) followed by high temperature annealing (603 K), we observed a remarkable increase in the short‐circuit current density because of the enhancement of the external quantum efficiency in the wavelength range of 400–800 nm. The best CZTS cell exhibited a conversion efficiency of 9.4% in the active area (9.1% in the designated area). In addition, we also fabricated a CZTS cell with open‐circuit voltage of 0.80 V by appropriately tuning the composition of the CZTS layers. Copyright © 2016 John Wiley & Sons, Ltd.     

Charge trapping and interface characteristics in normally-off Al2O3/GaN-MOSFETs

Normally-off GaN-MOSFETs with Al₂O₃ gate dielectric have been fabricated and characterized. The Al₂O₃ layer is deposited by ALD and annealed under various temperatures. The saturation drain current of 330 mA/mm and the maximum transconductance of 32 mS/mm in the saturation region are not significantly modified after annealing. The subthreshold slope and the low-field mobility value are improved from 642 to 347 mV/dec and from 50 to 55 cm² V⁻¹ s⁻¹, respectively. The I_D-V_G curve shows hysteresis due to oxide trapped charge in the Al₂O₃ before annealing. The amount of hysteresis reduces with the increase of annealing temperature up to 750 °C. The Al₂O₃ layer starts to crystallize at a temperature of 850 °C and its insulating property deteriorates.     

ITO-free flexible organic light-emitting diode using ZnS/Ag/MoO3 anode incorporating a quasi-perfect Ag thin film

The multi-layer electrode (ZnS/Ag/MoO₃) was optimized by investigating the formation of a continuous Ag thin film according to the base layer. The aggregation of the Ag atom was strictly limited on the ZnS layer, which showed the best thermal stability for Ag. The thermally evaporated 7-nm-thick Ag film with surface coverage of 99.6% was achieved on the ZnS layer. We fabricated the ZnS (25 nm)/Ag (7 nm)/MoO₃ (5 nm) (Z25A7M5) multi-layer electrode, optimized through the numerical calculation. The transmittance of 83% at λ = 550 nm and sheet resistance of 9.6 Ohm/sq were recorded from the Z25A7M5 electrode. These results were mainly attributed to the uniform film-like morphology of the Ag thin film. The flexible OLEDs, based on the Z25A7M5 anode also showed feasible I–V–L characteristics compared to those of ITO-based devices.     

Highly flexible organic light-emitting diodes based on ZnS/Ag/WO3 multilayer transparent electrodes

We report our study on highly flexible organic light-emitting diodes based on ZnS/Ag/WO₃ (ZAW) multilayer transparent electrodes in which high conductivity and ductility of Ag layers allow for efficient sheet conduction and flexibility while ZnS and WO₃ layers provide a means for enhancement in optical transmission and/or carrier-injection. Devices with ZAW anodes fabricated on planarized plastic substrates not only exhibit a performance and operational stability comparable to or better than those of ITO-based devices but also show a mechanical flexibility that is far superior to that of ITO-based devices. Experimental results show that a consistent performance can be obtained in ZAW-based devices upon repeated bending down to a radius of curvature of 5 mm, below which the flexibility of the devices is limited ultimately by the delamination occurring at cathode/organic interfaces rather than by the ZAW electrodes themselves.