Fabrication of CdSe sensitized SnO2 nanofiber quantum dot solar cells

In the present study, we report a cost-effective quantum dot solar cells based on a combination of electrospinning and successive-ionic-layer-adsorption and reaction (SILAR) methods. CdSe nanocrystals are deposited on electrospun SnO₂ nanofibers by SILAR method using CdCl₂ as the cadmium source and Na₂Se as selenium source. The as-prepared materials are characterized by spectroscopy and microscopy. CdSe deposited SnO₂ electrodes are also characterized by spectroscopy and microscopy. Cells are fabricated with platinum (Pt)-sputtered FTO glasses used as the counter electrodes and polysulfide solution used as the electrolyte. The efficiency of the cells is studied for different number of SILAR cycles. Current density–voltage (J–V) measurements on a cell having CdSe deposition of 7 SILAR cycles and SnO₂ coating area 0.25 cm² showed an overall power conversion efficiency of 0.29 % with a photocurrent density (J_SC) of 5.32 mA cm⁻² and open circuit voltage (V_OC) of 0.23 V under standard 1 Sun illumination of 100 mWcm⁻² (AM 1.5 G conditions). This is improved by carefully coating SnO₂ film without losing the structures. Also ZnS passivation layer is coated to obtain an improved efficiency of 0.48% with J_SC of 4.68 mAcm⁻², and V_OC of 0.43 V.     

Conductivity of structures based on doped nanocrystalline SnO2 films with gold contacts

The conductivity of nanocrystalline Pt-, Pd-, and Ni-doped tin dioxide films on insulating SiO₂ substrates is investigated in the temperature range 77–400 K. Doping allows variation of the resistance from 10⁴ to 10⁷Ω. It is established that, in contrast with a Au/single-crystal SnO₂ contact, the gold contacts for the nanocrystalline material are ohmic in the entire temperature range and their contribution to the conductance of all the structures investigated does not exceed 5%. Fiz. Tekh. Poluprovodn. 33, 205–207 (February 1999)     

Formation of two-layer composite-on-insulator structures based on porous silicon and SnO x . Study of their electrical and gas-sensing properties

The objective of this work is to fabricate and study multilayer “composite-on-insulator” sensor structures based on porous silicon and nonstoichiometric tin oxide. Two-layer structures “macroporous silicon-mesoporous silicon” on single-crystal silicon with sharp geometrical boundaries are grown. Test “composite-on-insulator” structures are fabricated. Oxide on macroporous silicon walls and a buried layer of oxidized mesoporous silicon play the role of the insulator. Nonstoichiometric tin oxide deposited onto the extended surface of oxidized macroporous silicon by chemical vapor deposition (CVD) is the sensitive layer. The gas sensitivity is studied upon exposure to NO₂ and degassing in air at room temperature. The sensitivity of the por-Si/SnO _x composite structures is higher than the sensitivity of tin-oxide film samples.     

A new type of counter electrode for dye sensitized solar cells based on solution processed SnO2 and activated carbon

A new type of nano-composite material is investigated for use as the counter electrode of a dye sensitized solar cell. The proposed material consists of SnO₂ and activated carbon, which can be deposited using a cost-effective sol–gel technique. The effects of the activated carbon concentration on the structural and optoelectronic properties were investigated using scanning electron microscopy, X-ray diffraction, UV–vis spectroscopy and four probe measurements. It was found that the addition of activated carbon reduces resistivity and optical transmittance, and suppresses fluorine doped tin oxide (FTO) growth. The nanocomposite material was integrated into a dye sensitized solar cell and evaluated using cyclic voltammetry and J–V characteristics under solar light illumination, and demonstrated power conversion efficiency ~2.35%.     

The effect of the dynamic adsorption mode on impedance of composite structures with porous silicon

Impedance and optical properties of pressed composites based on microcrystalline and nanoporous silicon powders were studied. Porous silicon crystallites were obtained by stain etching of initial microcrystalline silicon powder. Oxygen passivation of the surface during porous-powder formation provided stability of composite characteristics. An analysis of experimental dependences of the impedance in a frequency range of 1–10⁶ Hz allowed separation of contributions of grain bulk and crystallite interfaces to the total composite conductivity. The results of the study of the time dependences of the impedance were used to determine the variation rates of electrical parameters of composite structures under dynamic adsorption-desorption influence of external reactants (H₂O and C₂H₅OH).     

Nonlinear optical response of planar and spherical CdSe nanocrystals

The nonlinear optical response of a colloidal solution of planar CdSe semiconductor nanocrystals (nanoplatelets) is studied for the first time. The nonlinear optical response of these nanoparticles is compared to that of spherical CdSe nanocrystals (quantum dots). The photoinduced nonlinearity is attributed to the optical generation of long-lived charge carriers in the nanoobjects under study. It is shown that, upon the exposure of a cell with the solution of nanoparticles to focused continuous-wave (cw) laser radiation with a wavelength of 473 nm, the nonlinear optical responses of CdSe nanoplatelets and quantum dots are somewhat different at identical optical densities at the above-indicated wavelength. The differences are supposedly associated with a higher diffusion rate of spherical nanoparticles in the solution because of their smaller size compared to that of nanoplatelets.     

Photoluminescence of CdSe nanoparticles in porous GaP

The photoluminescence properties of porous GaP films nanostructured with CdSe particles (d = 2.8 nm) are studied. It is shown that deposition of the CdSe nanoparticles onto the porous GaP films shifts the photoluminescence spectrum of the nanoparticles covered with a surface-active medium. to shorter wave-lengths. It is established that violation of integrity of the surface-active coating of the CdSe nanoparticles results in the formation of at least two fractions of nanoparticles different in dimensions. These fractions of nanoparticles are responsible for well-resolved photoluminescence bands with peaks at 508 and 560 nm.     

A left-handed composite medium based on waveguiding structures with a bianisotropic filling

A new composite medium based on waveguiding structures loaded with bianisotropic gratings of planar double split rings (PDSRs) and planar helices is considered. The microwave transmission spectra of this medium are studied experimentally. The effective parameters of the medium are determined theoretically, and its passbands are investigated. The characteristics of the medium are compared to those of infinite homogeneous bianisotropic media. A relationship between the spectral transmission characteristics and the type of resonance excitation is established. The transparency bands and overforbidden bands of the composite medium are revealed. The combinations of the medium’s effective parameters that correspond to these bands are determined. It is shown that the transparency bands where the medium is characterized simultaneously by negative permittivity and negative permeability can be identified.     

Photoconductivity of organic polymer films doped with porous silicon nanoparticles and ionic polymethine dyes

Features of electrical conductivity and photoconductivity of polyvinylbutyral films containing porous silicon nanoparticles and similar films doped with cationic and anionic polymethine dyes are studied. Sensitization of the photoelectric effect by dyes with different ionicities in films is explained by the possible photogeneration of holes and electrons from dye molecules and the intrinsic bipolar conductivity of porous silicon nanoparticles. It is assumed that the electronic conductivity in porous silicon nanoparticles is higher in comparison with p-type conductivity.     

Photoconductivity of InAs/GaAs structures with InAs nanoclusters in the near-infrared region

InAs/GaAs multilayered heterostructures containing dense arrays of the low-defect partially relaxed InAs nanoclusters larger than defect-free quantum dots are fabricated by metal-organic chemical vapor deposition in an atmospheric-pressure reactor. The structures have intense photoconductivity in the wavelength range of 1–2 μm at room temperature. The detectivity of fabricated prototypes of photodetectors is D* = 10⁹ cm Hz^1/2 W⁻¹. The relaxation time of photoconductivity at a wavelength of 1.5 μm is less than 10 ns.     

基于PVK/PbS纳米晶有机无机复合薄膜电双稳器件性能研究

通过简单旋涂方法,制备了一种基于硫化铅（PbS）纳米晶与聚乙烯基咔唑（PVK）的有机/无机复合薄膜电双稳器件,并对所制备的器件进行性能测试及其电荷传输机制研究。首先采用热注入的方法制备了尺寸均一的立方形PbS纳米晶,然后将PbS纳米晶与PVK聚合物混合作为活性层材料,制备了有机/无机复合薄膜电双稳器件。该器件展示了良好的电双稳特性并且可以实现稳定的“读-写-读-擦”操作。器件的最大电流开关比能够达到10⁴。并进一步对器件在正向电压下的I-V曲线进行了理论拟合,发现在不同电流传导状态下,器件符合不同的电传导模型。进而分析了该电双稳器件中的电荷传输机制,认为在电场的作用下,发生在纳米晶与聚合物之间的电场诱导电荷转移是产生电双稳特性的主要原因。     

Effect of different ethanol/water solvent ratios on the morphology of SnO2 nanocrystals and their electrochemical properties

We report on the effect of different ethanol/water solvent ratios on the morphology of SnO₂ nanocrystals prepared by the conventional hydrothermal method and their electrochemical properties. The nanocrystals were structurally and morphologically characterized by X-ray diffraction (XRD), field-emission scanning electron microscopy (FESEM), surface area measurements, and transmission electron microscopy. The XRD patterns indicate that the sphere-like SnO₂ microcrystals have a rutile-type tetragonal structure and FESEM images show that the microspheres have a diameter of 2–5 μm. We found that the ethanol/water volume ratio plays an important role in formation of the final product. Electrochemical tests revealed that the SnO₂ microspheres had a high initial capacity of 1546 mAh g^?1 at a current density of 100 mA g^?1 and retained a reversible capacity of 439 mAh g^?1 after 30 discharge cycles.     

The effect of adsorption on the electrical properties of structures based on oxidized porous silicon

The effect of adsorption of hydrogen-sulfide and acetone polar molecules on current-voltage characteristics and time dependences of the current was studied for the 〈Pd-por-Si(O)-p⁺-Si-Al〉 structures based on oxidized porous silicon. Structures of two types were obtained—those with a Schottky barrier and those with space-charge-limited currents. The presence of a positive space charge was found near the palladium electrode; this charge defined the current-voltage characteristics in the region of V<0.2 V and varied appreciably depending on the gas used. The parameters of the layer of oxidized porous Si and the range of operating voltages corresponding to the most marked effect of a gas were determined. A maximum current variation up to 10³ times under the action of hydrogen sulfide with a concentration of ～10 ppm was obtained for the structures with the Schottky barrier with a reverse voltage close to that of reversible breakdown. The results obtained are explained by the charge exchange of traps, which is corroborated by a Fermi level shift and a Schottky barrier lowering caused by the adsorption of polar molecules.     

Preparation of CdSe nanocrystals in organic system and electroluminescence characteristics of the devices

Semiconductor nanocrystals have attracted wideinter-estsinthe last fewyears because they have high lumi-nescence efficiency and size-tunable band gap character-istics .The semiconductor nanocrystals could be func-tionalized using various surfactants to ma…     

Interface traps effect on the charge transport mechanisms in metal oxide semiconductor structures based on silicon nanocrystals

The transport phenomena in Metal-Oxide-Semiconductor (MOS) structures having silicon nanocrystals (Si-NCs) inside the dielectric layer has been investigated by high frequency Capacitance-Voltage (C-V) method and the Deep-Level Transient Spectroscopy (DLTS). For the reference samples without Si-NCs, we observe a slow electron trap for a large temperature range, which is probably a response of a series electron traps having a very close energy levels. A clear series of electron traps are evidenced in DLTS spectrum for MOS samples with Si-NCs. Their activation energies are comprised between 0.28 eV and 0.45 eV. Moreover, we observe in this DLTS spectrum, a single peak that appears at low temperature which we attributed to Si-NCs response. In MOS structure without Si-NCs, the conduction mechanism is dominated by the thermionic fast emission/capture of charge carriers from the highly doped polysilicon layer to Si-substrate through interface trap-states. However, at low temperature, the tunneling of charge carriers from highly Poly-Si to Si-substrate trough the trapping/detrapping mechanism in the Si-NCs contributed to the conduction mechanism for MOS with Si-NCs. These results are helpful to understand the principle of charge transport of MOS structures having a Si-NCs in the SiO_x = 1.5 oxide matrix.     

SnO_2纳米线基场效应晶体管的电学性能研究

基于气-液-固生长机理和原位掺杂技术,成功制备了2%(质量分数)Sb掺杂SnO_2纳米线,所制器件可以在低压环境下工作且不受栅漏电的影响;将该纳米线作为场效应晶体管的沟道,分析了不同比例的Sb掺杂对晶体管电学性能参数的影响。结果显示:Sb质量分数由0.5%增加到2.5%时,随着Sb掺杂量的增大,晶体管的稳定性增强,但栅电压的调控能力减弱,晶体管功耗增大且对空穴和电子的迁移能力减弱。     

Characterization of defects in colloidal CdSe nanocrystals by the modified thermostimulated luminescence technique

The temperature dependencies of the luminescence spectra of 5-nm-diameter CdSe semiconductor nanocrystals synthesized by colloidal-chemistry methods are investigated. The two bands observed in these spectra around 2.01 and 1.37 eV correspond to band-to-band transitions and luminescence of defect states, respectively. A model explaining the temperature behavior of the luminescence band intensities both upon cooling and heating is put forward. A new modification of spectrally resolved thermostimulated luminescence technique making it possible to determine the activation energies and the character of traps responsible for the temperature dependence of the luminescence intensities is suggested. This technique is used to obtain the activation energies of the emission and capture of electrons at traps (190 and 205 meV, respectively) and to determine the depth of the electron level (57 meV) responsible for luminescence in the 1.37-eV region.     

Charging effects in CdSe nanocrystals embedded in SiO2 matrix produced by rf magnetron sputtering

Charging effects in CdSe nanocrystals embedded in SiO₂ matrix fabricated by rf magnetron co-sputtering technique were electrically characterized by means of capacitance-voltage (C-V) combined with current-voltage (I-V). The presence of CdSe nanocrystals was demonstrated by X-ray diffraction technique. The average size of nanocrystals was found to be approximately 3 nm. The carriers transport in the CdSe/SiO₂ structure was shown to be a combination of Fowler-Nordheim tunnelling and Poole-Frenkel mechanisms. A memory effect was demonstrated and a retention time was measured.     

Photoconductivity of Si/Ge multilayer structures with Ge quantum dots pseudomorphic to the Si matrix

Longitudinal photoconductivity spectra of Si/Ge multilayer structures with Ge quantum dots grown pseudomorphically to the Si matrix are studied. Lines of optical transitions between hole levels of quantum dots and Si electronic states are observed. This allowed us to construct a detailed energy-level diagram of electron-hole levels of the structure. It is shown that hole levels of pseudomorphic Ge quantum dots are well described by the simplest “quantum box” model using actual sizes of Ge islands. The possibility of controlling the position of the long-wavelength photosensitivity edge by varying the growth parameters of Si/Ge structures with Ge quantum dots is determined.     

Multilayer photosensitive structures based on porous silicon and rare-earth-element compounds: Study of spectral characteristics

The spectral characteristics of the specular reflectance, photosensitivity, and photoluminescence (PL) of multilayer structures based on porous silicon with rare-earth-element (REE) ions are investigated. It is shown that the photosensitivity of these structures in the wavelength range of 0.4–1.0 μm is higher than in structures free of REEs. The structures with Er³⁺ ions exhibit a luminescence response at room temperature in the spectral range from 1.1 to 1.7 μm. The PL spectrum of the erbium impurity is characterized by a fine line structure, which is determined by the splitting of the ⁴ I _15/2 multiplet of the Er³⁺ ion. It is shown that the structures with a porous layer on the working surface have a much lower reflectance in the entire spectral range under study (0.2–1.0 μm).