An optimized multilayer structure of CdS layer for CdTe solar cells application

CdS layers grown by ‘dry’ (close space sublimation) and ‘wet’ (chemical bath deposition) methods are deposited and analyzed. CdS prepared with close space sublimation (CSS) has better crystal quality, electrical and optical properties than that prepared with chemical bath deposition (CBD). The performance of CdTe solar cell based on the CSS CdS layer has higher efficiency than that based on CBD CdS layer. However, the CSS CdS suffers from the pinholes. And consequently it is necessary to prepare a 150 nm thin film for CdTe/CdS solar cell. To improve the performance of CdS/CdTe solar cells, a thin multilayer structure of CdS layer (∼80 nm) is applied, which is composed of a bottom layer (CSS CdS) and a top layer (CBD CdS). That bi-layer film can allow more photons to pass through it and significantly improve the short circuit current of the CdS/CdTe solar cells.     

Carbon nanotube arrays for photovoltaic applications

Vertically aligned periodic arrays of carbon nanotubes (CNTs) are used to create topographically enhanced light-trapping photovoltaic cells. The CNTs form the back contact of the device and serve as a scaffold to support the photoactive heterojunction. Molecular beam epitaxy is used to deposit CdTe and CdS as the p/n-type materials and ion-assisted deposition is used to deposit a conformal coating of indium-tin oxide as the transparent top contact. X-ray diffraction data shows (111) texture of the CdTe. Photocurrent produced “per cm² of footprint” for the CNT-based device is 63 times that of a commercially available planar single crystal silicon device.     

Conversion of a green light emitting zinc-quinolate complex thin film to a stable and highly packed blue emitter film

Tris-(8-hydroxyquinoline) metal complex Znq₂, used as light-emitting layer in electroluminescent (EL) devices was synthesized and optical properties of as-deposited Znq₂ in thin films were studied. Interesting phenomenon was observed while studying the ageing and degradation behavior of Znq₂ films and consequently a stable form having strong blue photoluminescence (peak 465 nm) was discovered. This converted (from green to blue emitting) film has higher molecular packing density and comparable photoluminescence intensity with Znq₂ film. Here, we report on the production of this blue material under controlled conditions and its optical properties.The thin films have been deposited by thermal evaporation on quartz and silicon substrates. The optical constants (n and k) of green Znq₂ film and converted (to blue) thin films have been determined using spectroscopic ellipsometry. Environment induced effects on optical properties of films have been studied using ellipsometry, photoluminescence and UV–vis transmission measurements.     

The photoluminescence enhancement of electrospun poly(ethylene oxide) fibers with CdS and polyaniline inoculations

Blending electrospinning of cadmium sulfide (CdS) quantum dots (QD) with poly(ethylene oxide) (PEO) solution was employed to fabricate one-dimensional ultrafine fibers with an average diameter of 450 nm. This study focused on systematic investigations into the role of the matrix polymer and the optimal electrospinning parameters for enhancing the photoluminescence properties of fibrous composites. CdS QDs showed a homogeneous distribution within the composite fibers, and fluorescence spectra showed that PEO successfully passivated the interface defects and quenched the visible emission of CdS QDs. The QDs concentration and electrospinning voltage were found to play important roles in enhancing the passivation effect of PEO and adjusting the photoluminescence intensity of the composite fibers. Furthermore, the addition of polyaniline enhanced the photoluminescence intensity of the electrospun fibers, and an electron–hole mechanism was proposed.     

Effect of conjugation length on photophysical properties of a conjugated–non-conjugated multiblock copolymer

The synthesis, structural characterization and photophysical properties of copolymers built by conjugated PPV type units of variable length and an aliphatic spacer are described. The aliphatic spacer provided good film forming capability and solubility. The effective conjugation extension was determined and its influence on the bandgap was evaluated, complementing theoretical and experimental data, since the PV units were oligomeric in size but with the mobility of longer chains. Aggregation phenomena were investigated; ground state dimers or higher associated forms were detected. The electrical and photovoltaic properties were studied in devices built with the neat polymers or containing a heterojunction with fullerene (C₆₀), the photovoltaic responses were in agreement with the photoluminescence results. The most efficient photovoltaic device was based on the polymer containing the longest conjugated block, which also the most luminescent material.     

Substrate Evolution to Microstructural and Optoelectrical Properties of Evaporated CdS Thin Films Correlated with Elemental Composition

A typical high-e fficiency solar cell device needs the best lattice matching between different constituent layers to mitigate the open-circuit voltage loss. In the present work, the physical properties of CdS thin films are investigated where films with 100 nm thickness were fabricated on the different types of substrates viz. soda–lime glass, indium-doped tin oxide(ITO)-and fl uorine-doped tin oxide(FTO)-coated glass substrates, and silicon wafer using electron beam evaporation. The X-ray diffraction patterns confirmed that deposited thin films showed cubic phase and had(111) as predominant orientation where the structural parameters were observed to be varied with nature of substrates. The ohmic behaviour of the CdS films was disclosed by current–voltage characteristics, whereas the scanning electron microscopy micrograph revealed the uniform deposition of the CdS films with the presence of round-shaped grains. The elemental analysis confirmed the CdS films deposition where the Cd/S weight percentage ratio was changed with nature of substrates. The direct energy band gap was observed in the 1.63–2.50 eV range for the films grown on different substrates. The investigated properties of thin CdS layers demonstrated that the selection of substrate(in terms of nature) during device fabrication plays a crucial role.     

Optical Parameters of Spray-Deposited CdS<Subscript>1−<Emphasis Type="Italic">y</Emphasis></Subscript>Te<Subscript><Emphasis Type="Italic">y</Emphasis></Subscript> Thin Films

CdS _x Te_1?x and CdS_1?y Te _y solid solutions are usually formed in the interfacial region in CdS/CdTe solar cells during the deposition of the CdTe layer and/or the processing steps of the device. In this work, indium-doped CdS_1?y Te _y thin films were prepared by first producing CdS:In thin films by the spray pyrolysis technique on glass substrates, then annealing the films in nitrogen atmosphere in the presence of elemental tellurium. The films were characterized by scanning electron microscopy, energy dispersive x-ray spectroscopy, and transmittance measurements. The transmittance was used to deduce the reflectance from which the optical parameters were computed. The extinction coefficient, refractive index, the real and imaginary parts of the dielectric constant, optical conductivity, and energy loss were computed, and their dependence on the composition was investigated. In addition, the dispersion of the refractive index was analyzed by the single oscillator model, and dispersion parameters were investigated.     

Cascade structure of TiO2/ZnO/CdS film for quantum dot sensitized solar cells

Quantum dot sensitized solar cells based on cascade structure of TiO₂/ZnO/CdS electrode and polysulfide electrolyte were fabricated. The ZnO layer was deposited on screen-printed TiO₂ layer by ultrasonic spray pyrolysis method. The structure, morphology and impedance of TiO₂/ZnO film photoanode and the photovoltaic performance of TiO₂/ZnO/CdS cell were investigated. It is found that the short circuit current density and conversion efficiency are significantly improved by the introduction of ZnO layer into TiO₂/CdS film. A power conversion efficiency of about 1.56% has been obtained for TiO₂/ZnO/CdS cell, which is about 57% higher than that for TiO₂/CdS cell (0.99%). The formation of an inherent energy barrier between TiO₂ and CdS films and the passivation of surface traps on the TiO₂ film caused by the introduction of ZnO layer, which reduces the charge recombination and favors the electron transport, should be mainly responsible for the performance enhancement of TiO₂/ZnO/CdS cell.     

溶胶-凝胶法制备掺铁钛酸铋铁电薄膜光学性能的研究

通过溶胶-凝胶法制备了不同掺杂浓度的Fe-BTO铁电薄膜以减小其光学带隙,研究不同Fe掺杂浓度对BTO铁电薄膜铁电光伏效应的影响。结果表明,使用溶胶-凝胶法对BTO铁电薄膜掺杂不同浓度的Fe,所制备的薄膜结晶度较好、网状结构明显、空间分布均匀,晶粒大小均一;通过溶胶-凝胶法制备Fe-BTO铁电薄膜在Fe掺杂浓度x=0.9附近可以明显减小其禁带宽度。     

Photoelectrochemical investigations of cadmium sulphide (CdS) thin film electrodes prepared by spray pyrolysis

Polycrystalline cadmium sulphide (CdS) thin films have been prepared by spraying a mixture of an equimolar aqueous solutions of cadmium chloride and thiourea on preheated fluorine doped tin oxide (FTO) coated glass substrates at different substrate temperatures. The cell configurations n-CdS/1 M (NaOH + Na₂S + S)/C were used for studying the capacitance-voltage (C-V) characteristics in dark, current-voltage (I-V) characteristics in dark and under illumination, photovoltaic power output and spectral response characteristics of the as deposited thin films. Photoelectrochemical study shows that as deposited CdS thin films exhibits n-type of conductivity. The spectral response characteristics of the films at room temperature show a prominent sharp peak at 500 nm leading to optical bandgap energy of 2.48 eV. It is found that fill factor and efficiency are maximum for photoelectrode deposited at 300 °C. This is due to low resistance; high flat band potential, maximum open circuit voltage as well as maximum short-circuit current. The measured values of efficiency (η) and fill factor (FF) are found to be 0.17% and 0.38 respectively for film deposited at 300 °C.     

Surface morphology and photoluminescence properties of ZnO thin films obtained by PLD

ZnO thin films on Si(111) substrate were deposited by laser ablation of Zn target in oxygen reactive atmosphere, Nd-YAG laser with wavelength of 1 064 nm was used as laser source. XRD and FESEM microscopy were applied to characterize the structure and surface morphology of the deposited ZnO films. The optical properties of the ZnO thin films were characterized by photoluminescence. The UV and deep level (yellow-green) light were observed from the films. The UV light is the intrinsic property and deep level light is attributed to the existence of antisite defects (Ozn). The intensity of UV and deep level light depends strongly on the surface morphology and is explained by the surface roughness of ZnO film. A strongly UV emission can be obtained from ZnO film with surface roughness in nanometer range.     

SrMoO4薄膜的化学溶液制备技术和发光性质研究

利用化学溶液技术制备了具有单一白钨矿结构的SrMoO4多晶薄膜.用X射线衍射仪(XRD)分析了薄膜的晶相结构,用红外光谱(IR)对薄膜的均一性进行了表征,用原子力显微镜(AFM)对薄膜的表面形貌进行了观察.采用荧光光谱仪测试了所制SrMoO4薄膜在不同温度下的光致发光特性.研究结果表明,在276 nm的紫外光激发下,钼酸锶薄膜室温条件下显示出良好的光致发光特性,呈现宽带(～300 nm)的发光特征.另外,光致发光光谱的峰值呈现出明显的温度漂移特性,从511 nm(11 K)变化到484 nm(293 K).此外,本文还讨论了SrMoO4薄膜在闪烁材料和紫外成像薄膜材料方面的可能应用.     

Formation and properties of nano- and micro-structured conducting polymer host–guest composites

Process of formation of polyaniline (PANI) at the surface of SiC and CdS nanoparticles or submicron- and micron-sized particles of poly(vinylidene fluoride) (PVDF), polycarbonate and polyamides-11, 12 and properties of the prepared composites are considered. Beginning of the formation of the PANI shell at the particle surface was evaluated. This important result opens the possibility to control properties of the final hybrid composite. In case of CdS nanoparticles PANI was synthesized in the form of nanofibers embedding these nanoparticles. Films of the PANI–polymer composites showed the conductivity of up to ～0.4 S/cm. The planar heterojunction of the compression molded PVDF/PANI–DBSA film with bulk CdS displayed photovoltaic activity.     

Growth of CdS thin films and nanowires for flexible photoelectrochemical cells

In this study, cadmium sulfide (CdS) nanocrystal thin films and nanowires have been deposited onto mechanically flexible substrates via dc-electrodeposition, which is a very suitable technique for large area manufacturing. For the first time with this study, flexible CdS nanocrystal thin films were integrated into photoelectrochemical (PEC) cells and their performances were compared with CdS nanowires. It has been demonstrated that PEC performance of both nanocrystal thin films and nanowires were a strong function of production conditions such as deposition time and voltage. The maximum power conversion efficiency of the CdS nanocrystal thin films obtained in this study was 0.3%. On the other hand, higher efficiencies (about 1.4%) were observed for the CdS nanowires. UV-vis analysis confirmed that both transmittance and band gap energies of the CdS nanowires were lower than that of CdS nanocrystal thin films. X-ray diffraction analysis revealed that both nanocrystal thin films and nanowires have a preferred orientation at 26° (2θ), which can be attributed to the CdS (0 0 2) structure.     

Synthesis and characterization of electrospun polymer nanofibers incorporated with CdTe nanoparticles

CdTe nanoparticles were incorporated in polyvinyl alcohol (PVA) nanofibers by electrospinning. The hybrid nanofibers were characterized morphologically and optically by scanning electron microscopy, transmission electron microscopy, and photoluminescence (PL) measurements. The average diameter of the hybrid nanofibers was about 150 nm and the CdTe nanoparticles were present both inside them and on their surfaces. Compared with the PL of the CdTe nanoparticles dispersed in solution, the PL peak of the CdTe nanoparticles in the CdTe/PVA hybrid nanofibers was shifted from 525 to 426 nm. This blue shift of the PL peak results from the different quantum confinement effects, due to the different environment surrounding the CdTe nanoparticles, in the case of those dispersed in solution and those embedded in the PVA nanofibers.     

Preparation and properties of CdS thin films grown by ILGAR method

CdS thin films were deposited by the ion layer gas reaction (ILGAR) method. Structural, chemical, topographical development as well as optical and electrical properties of as-deposited and annealed thin films were investigated by XRD,SEM, XPS, AFM and UV-VIS. The results showed that the thin films are uniform, compact and good in adhesion to the substrates, and the growth of the films is 2.8 nm/cycle. The evolution of structure undergoes from the cubic structure to the hexagonal one with a preferred orientation along the (002) plane after annealing at 673 K. An amount of C, O and C1 impudries can be reduced by increasing the drying temperature or by annealing in N2 atmosphere. It was found that the band gap of the CdS films shifts to higher wavelength after annealing or increasing film thickness. The electrical resistivity decreases with increasing annealing temperature and film thickness.     

Electrostatic self-assembly as a means to create organic photovoltaic devices

Recently, there has been a significant amount of work done on making photovoltaic devices (solar cells) from thin films of conjugated polymers and other organic systems. The advantages over conventional inorganic systems include the potential to create lightweight, flexible, and inexpensive structures. The challenge, however, has been to create more highly efficient devices. To date, the primary photovoltaic device mechanism that has been utilized is that of photoinduced charge transfer between an electron donor and acceptor. In this study, similar photovoltaic devices are fabricated using a water-based electrostatic self-assembly procedure, as opposed to the more conventional spin-coating and/or vacuum evaporation techniques. In this process, layers of oppositely charged species are sequentially adsorbed onto a substrate from an aqueous solution and a film is built up due to the electrostatic attraction between the layers. The technique affords molecular level control over the architecture and gives bilayer thickness values of the order of tens of angstroms. By repeating this process a desired number of times and utilizing different cations and anions, complex architectures can be created with very accurate control over the thickness and the interfaces. We have examined a number of systems built from a variety of components including a cationic PPV precursor, functionalized C₆₀, and numerous other polyelectrolytes. We report on the device characteristics of these films and on the overall applicability of this technique to the fabrication of photovoltaic devices.     

All spray pyrolysis deposited CdS sensitized ZnO films for quantum dot-sensitized solar cells

Sensitized-type solar cells based on ZnO photoanode and CdS quantum dots (QDs) as sensitizers have been fabricated. Both ZnO films and CdS QDs are prepared using ultrasonic spray pyrolysis (USP) deposition technique. This method allows a facile and rapid deposition and integration between CdS QDs and ZnO films without the need for post thermal treatment. The photovoltaic performances of the cells are investigated. The results show that the performance of the cell based on all USP deposited CdS sensitized ZnO photoanode achieves maximally a short circuit current density of 6.99 mA cm⁻² and a power conversion efficiency of 1.54%.     

Laser irradiation effects on the optical properties of layer-by-layer PPV/Congo Red thin films

In this paper we investigate the optical properties of thin films produced through the layer-by-layer (LbL) technique with a poly (p-phenylene vinylene) (PPV) precursor and the Congo Red (CR) azodye. Some significant changes are observed when the PPV/CR film is irradiated with polarized laser. An increase of about 10 times in the PPV emission intensity was observed, with an irradiation time of 100 min. Photoinduced birefringence measurements revealed that a high birefringence was produced in the region of the film submitted to photo-irradiation, while a photobleaching phenomenon was observed in the same region. However, such phenomena were not detected in measurements performed at low temperatures. The results indicate that a degradation process of the CR molecules is occurring with photo-irradiation, and the changes observed in the photoluminescence spectra are correlated with the combined effects of degradation and energy transfer between the PPV light-emitting conjugated segments and CR azochromophores.     

用于柔性电子器件的有机/无机薄膜封装技术研究进展

有机/无机薄膜封装技术被广泛用于有机发光二极管(OLED)、量子点显示及有机光伏等领域,是一种新型的柔性封装技术。综述近年来有机/无机薄膜封装技术的发展趋势,首先概述了传统硬质盖板封装方式与薄膜封装方式的发展及其优缺点。其次,系统地总结了有机/无机薄膜的制备方法,如原子层沉积、等离子体化学气相沉积等,详细阐述了不同制备方法的原理及其应用。再次,讨论了薄膜的微观缺陷、内应力,以及材料界面工程对有机/无机薄膜封装性能的影响,分析总结了有机/无机封装薄膜制备的技术要点,如采用基底表面预处理、引入中性层、调节层间应力等方式获得优质的封装薄膜。最后,探究了有机/无机封装薄膜的内在阻隔机理,提出气体在有机/无机薄膜中的传输方式以努森扩散为主,并总结了提高薄膜封装的策略,即延长气体扩散路径、“主动”引入阻隔基团及薄膜表面改性。提出了未来薄膜封装技术面临的问题,拟为柔性电子器件封装技术的发展提供一定参考。