Effect of ZnTe/ZnTe:Cu complex back-contact on device characteristics of CdTe solar cells

ZnTe/ZnTe:Cu complex layers deposited by vacuum co-evaporation have been in- troduced to CdS/CdTe solar cells. The C-V and I-V curves have been investigated and the effects of un-doped ZnTe layer thickness as well as annealing temperatures on I-V characteristics of CdTe solar cells have been studied. The results show that the “roll over” and “cross over” phenomena of dark and light I-V curves can be eliminated by use of ZnTe/ZnTe:Cu layer and the fill factor for a typical sample has increased to 73%, where there is no high resistance transparent layer. The reasons have been discussed combined with the energy band diagram of CdTe solar cells.     

Preparation and Properties of CdTe Polycrystalline Films for Solar Cells

The structure and characteristics of CdTe thin filrns are closely dependent on the whole deposition process in close-space sublimation （CSS）. The physical mechanism of CSS was analyzed aud the temperature distribution in CSS system was measured, and the influences of the increasing-temperature process and pressure on the preliminary nucleus creation were studied. The resuits indicate ： tire samples deposited at different pressures hare a cubical structure of CdTe and the diffraction peaks of CdS and SnO2 ： F. As the atmosphere pressure increases, the crystal size of CdTe decreases, the rate of the transparency of the thin film decreases and the absorption side moves towards the short-wave direction. After a 4-minute depositing process with a substrate teraw.rature of 500℃ and a source temperature of 620 ℃, the polycostallinc thin films can be mmade , so the production of high-quality integrated cell with StrO2： F/ CdS/ CdTe/ Au structure is hopeful.     

硝酸-冰乙酸腐蚀对CdTe太阳电池性能的影响

在CdTe与金属背电极间形成稳定的低电阻接触,有助于提高CdTe太阳电池性能。采用硝酸-冰乙酸腐蚀CdTe薄膜并用真空蒸发法沉积铜背接触层,制备CdTe太阳电池。结果表明,化学腐蚀后在膜面生成了富碲层,硝酸-冰乙酸腐蚀为各向同性刻蚀。对背接触层进行优化退火处理,获得转化效率11.75%的CdTe太阳电池。     

Deposition methods and properties of polycrystalline CdS thin films

CdS thin film was used as a suitable window layer for CdS/CdTe solar cell, and the properties of CdS thin films deposited by pulsed laser deposition (PLD), chemical bath deposition (CBD) and magnetron sputtering (MS) were reported. The experimental results show that the transmittances of PLD-CdS thin films are about 85% and the band gaps are about 2.38–2.42eV. SEM results show that the surface of PLD-CdS thin film is much more compact and uniform. PLD is more suitable to prepare the CdS thin films than CBD and MS. Based on the thorough study, by using totally PLD technique, the FTO/PLD-CdS(150 nm)/CSS-CdTe solar cell (0.0707 cm²) can be prepared with an efficiency of 10.475%.     

High rate deposition of microcrystalline silicon films by high-pressure radio frequency plasma enhanced chemical vapor deposition (PECVD)

Hydrogenated microcrystalline silicon (μc-Si:H) thin films were prepared by high-pressure radio-frequency (13.56 MHz) plasma enhanced chemical vapor deposition (rf-PECVD) with a screened plasma. The deposition rate and crystallinity varying with the deposition pressure, rf power, hydrogen dilution ratio and electrodes distance were systematically studied. By optimizing the deposition parameters the device quality μc-Si:H films have been achieved with a high deposition rate of 7.8 ?/s at a high pressure. The V _oc of 560 mV and the FF of 0.70 have been achieved for a single-junction μc-Si:H p-i-n solar cell at a deposition rate of 7.8 ?/s. Supported by the National Natural Science Foundation of China (Grant No. 50662003) and the State Development Program for Basic Research of China (Grant No. G2000028208)     

CdS/Cu2O太阳能电池的制备及膜厚对光电性能的影响

采用水浴法和电沉积法制备CdS/Cu2O复合膜,组装成异质结薄膜太阳能电池。通过改变薄膜的厚度,测试了不同厚度的窗口层和吸收层对太阳能电池性能的影响。实验表明,在400 nm厚的CdS薄膜上沉积30次Cu2O薄膜,所获得的复合膜具有最大的填充因子FF(为0.42)和光电转换效率η(0.05%)。并通过实验发现,适当减少CdS窗口层的厚度,可以提高光的透射率,产生更多的光生载流子,提高了光电转换效率。适当增加Cu2O吸收层的厚度,可以提高光的吸收率,产生更多的光生载流子,提高了光电转换效率。     

Effect of Annealing on Structural, Optical and Electrical Properties of CdS Thin Films Grown by ILGAR

CdS thin films were deposited by ILGAR （ ion lay gas reaction） method. The effect of annealing temperature under N2 atmosphere on the structural, chemical, topographical development and optical and electrical properties of CdS thin films was investigated by XRD, SEM, XPS, UV- VIS and two-probe technique. It is found that the cubic-phase of as-deposited CdS film transforms to hexagonal phase with a perfected orientation along （002） plane at 300 ℃ . The band gap decreases with increasing annealing temperature until 300 ℃ , which is consistent with the grain growth. The fall of dark and light resistivitiy is obvious with increasing annealing temperature, corresponding to the continuous grain growth and deviation of stoichiometry at higher temperature. The smooth and uniform surface of as-deposited films becomes rougher through thermal treatment, which is related to grain growth and sublimation of CdS at a higher annealing temperature.     

An estimation method on failure stress of micro thickness Cu film-substrate structure

The failure of thin film-substrate structure occurs mainly at the thin film or the interface. However, the characterizing and estimating methods of failure stress in thin film are neither uniform nor effective because there are some complex effects of such as size, interface and stress state on the failure behavior of thin film-substrate structure. Based on the scanning electron microscope (SEM) in-situ investigation on the failure models of the Cu thin film-substrate structure and the nano scratched testing results, the failure stresses in different thicknesses of the Cu film-substrate were characterized, which were compared and confirmed by other methods, such as Stoney formula and other empiric equations. These results indicate that the novel estimating method of failure stress in thin film based on the critical wavelength of surface unstable analysis is better than other methods. The main reason is that the novel estimating method of failure stress in meso thickness film fully considered the effect factors of free surface unstable behavior and elastic anisotropy of thin film. Therefore, the novel estimating method of failure stress assists people to understand the critical interfacial strength and to set up the failure criterion of thin film-substrate structure. Supported by the National Natural Science Foundation of China (Grant No. 10772091) and National Basic Research Program of China (Grant Nos. 2004CB619304-5, 2007CB936803)     

Performance improvement of <Emphasis Type="Italic">n-i-p</Emphasis> μc-Si:H solar cells by gradient hydrogen dilution technique

High pressure radio frequency plasma enhanced chemical vapor deposition (RF-PECVD) process was adopted to investigate the effect of constant hydrogen dilution technique and gradient hydrogen dilution technique on the structural evolution of intrinsic films and the performance of n-i-p microcrystalline silicon solar cells. The experiment results demonstrated that the grain size and crystalline volume fraction along the growth direction of intrinsic films can be controlled and the performance of solar cells can be greatly improved by gradient hydrogen dilution technique. An initial active-area efficiency of 5.7% (V _oc=0.47 V, J _sc=20.2 mA/cm², FF=60%) for the μc-Si:H single-junction n-i-p solar cells and an initial active-area efficiency of 10.12% (V _oc=1.2 V, J _sc=12.05 mA/cm², FF=70%) for the a-Si:H/μc-Si:H tandem n-i-p solar cells has been achieved. Supported by the National Basic Research Program of China (“973” Program) (Grant Nos. 2006CB202602, 2006CB202603) and the Tianjin Assistant Foundation for the National Basic Research Program of China (Grant No. 07QTPTJC29500)     

电化学沉积CdS薄膜及其在CZTS薄膜太阳能电池中的应用

采用硫代硫酸钠、硫酸镉,配以有机酸NTA调节溶液pH值,首次在碱性环境中电沉积制备CdS薄膜,并将其应用到Cu2ZnSnS4（CZTS）薄膜太阳能电池中作为缓冲层．实验探讨了pH值、溶液浓度、沉积电位对薄膜晶体结构、形貌、界面等微观结构以及光学特性的影响、在pH值为9．36、Cd2＋浓度为0．025mol／L、沉积电位为-1．7V时,获得了表面均匀致密而无针孔、近化学计量原子比、禁带宽度为2．4eV的CdS薄膜,将其应用于CZTS薄膜太阳能电池中,所制备的缓冲层CdS薄膜展现了与CZTS薄膜良好的匹配性,CZTS／CdS的P—n结质量得到改善．     

Structural and optical characteristics of Al x Ga1- x N/AlN superlattice

AlN/Al_0.3Ga_0.7N superlattices were grown on (0001) sapphire substrate by metal-organic chemical vapor deposition (MOCVD). The superlattice period varies from 6 to 30. The layer thickness of different period stack was designed. GaN or AlGaN template was employed for growing AlN/AlGaN superlattices. Reflectivity, SEM, AFM and XRD data of the Al _x Ga_1-x N/AlN superlattices are presented. It is found that the templates used have an intensive impact on surface roughness and interfacial properties of following AlN/AlGaN superlattices. The result of atomic force microscopy indicates that AlN/AlGaN superlattices grown on GaN template exhibit quasi-two-dimensional growth mode. The resulting superlattice has a smooth surface morphology and distinct interface. No crack is observed in the area of a 2-inch wafer. Supported by the Special Funds for Major State Basic Research Project (973 Project) (Grant No. 2006CB6049), the Hi-tech Research Project (Grant Nos. 2006AA03A103, 2006AA03A118, and 2006AA03A142), the National Natural Science Foundation of China (Grant No. 60676057), and the Research Fund for the Doctoral Program of Higher Education of China (Grant No. 20050284004)     

太阳电池中CdTe多晶薄膜沉积制备及其性能

在氩氧混合气氛下近空间升华技术(CSS)制备CdTe多晶薄膜中，薄膜的结构、性质决定于整个沉积过程深入研究沉积过程中的热交换、物质输运，有助于制备薄而致密的具有进行良好光电性质CdTe薄膜．通过分析近空间沉积的物理机制，测量近空间沉积装置内温度分布，对升温过程、气压与薄膜初期成核的关系进行讨论。研究结果表明，不同气压下制备的样品，均有立方相CdTe，此外，还有CdS和SnO2：F衍射峰，CdTe晶粒随气压增加有减小趋势；随气压的增加．透过率呈下降趋势，相应的CdTe吸收边向短波方向移动。在此基础上制备出转换效率优良的结构为SnO2：F／CdS／CdTe／Au的串联集成电池。     

Protein adsorption behaviors on chitosan/poly(ɛ-caprolactone) blend films studied by quartz crystal microbalance with dissipation monitoring (QCM-D)

Chitosan/poly(ɛ-caprolactone) (PCL) blend films in different mass ratios were prepared using the chitosan/PCL mixture solutions in 80 vol.-% acetic acid by spin coating. Their surface micromorphologies were assessed by atomic force microscopy (AFM). It was found that the micromorphology of chitosan/PCL blend films was in large extent related to the mass ratio of chitosan. 25 wt% chitosan/PCL blend film presented microphase separation. The protein adsorption of bovine serum albumin (BSA) onto chitosan/PCL blend films was investigated by using quartz crystal microbalance with dissipation monitoring (QCM-D) in real time. The results suggested that the amount of adsorbed BSA on the chitosan/PCL blend films decreased with the addition of chitosan, but the structure and viscoelastic properties of the adsorbed BSA layers were greatly affected by the surface micromorphology of chitosan/PCL blend films. BSA absorbed on the 25 wt% chitosan/PCL blend film with microphase separation showed larger adsorption reversibility, and preferred to form a loose, dissipative layer in comparison with those on other chitosan/PCL blend films without microphase separation. Supported by the National Natural Science Foundation of China (Grant No. 20504018) and the National High Technology Research and Development Program of China (863 Program) (Grant No. 2007AA09Z440)     

Effects of different CdCl<Subscript>2</Subscript> annealing methods on the performance of CdS/CdTe polycrystalline thin film solar cells

In this paper, the effects of different CdCl₂ annealing methods, including vapor annealing and dip-coating annealing, on the performance of CdS/CdTe polycrystalline thin-film solar cells are studied. After annealing, the samples are lightly etched with 1% bromine in methanol to remove surface oxides. Both annealing methods give CdTe polycrystalline thin films with good crystallinity and complete structure. For solar cells containing the annealed CdTe films, cell efficiency first increases and then decreases as the concentration of CdCl₂ solution used for dip-coating annealing increases, and the optimized CdCl₂ concentration is 12%. The uniformity of the performance of all cells is analyzed by calculating the relative standard deviation for each parameter. The uniformity of cell performance can be improved dramatically by dip-coating annealing instead of vapor annealing. Most notably, an appropriate concentration of CdCl₂ (12%) acts as a protective layer that is conducive to realizing uniform high-performance CdS/CdTe solar cells. According to the location of depletion regions, the CdTe films treated by dip-coating annealing show a relatively low doping concentration, except for the sample treated with a CdCl₂ concentration of 6%, which is consistent with the changes of short-circuit current density of the cells. It is believed that these results can be applied to the large-scale production of CdTe polycrystalline thin-film solar cells.     

Preparation of TiO<Subscript>2</Subscript> thin film by the LPD method on functionalized organic self-assembled monolayers

In this paper, uniform titania (TiO₂) films have been formed at 50° on silanol SAMs by the liquid-phase deposition (LPD) method at a temperature below 100°C. OTS (Octadecyltrichloro-Silane) self-assembled monolayers (SAMs) on glass wafers were used as substrates for the deposition of titanium dioxide thin films. This functionalized organic surface has shown to be effective for promoting the growth of films from titanic aqueous solutions by the LPD method at a low temperature below 100°C. The crystal phase composition, microstructure and topography of the as-prepared films were characterized by various techniques, including X-ray diffraction (XRD), scanning electron microscopy (SEM), and atomic force microscopy (AFM). The results indicate that the as-prepared thin films are purely crystallized anatase TiO₂ constituted by nanorods after being annealed at 500°. The pH values, concentration of reactants, and deposition temperatures play important roles in the growth of TiO₂ thin films. Support by the National Natural Science Foundation of China (Grant No. 50672055) and National Key Technology R&D Program (Grant No. 2006BAF02A28)     

Fabrication of metal suspending nanostructures by nanoimprint lithography (NIL) and isotropic reactive ion etching (RIE)

We report herein a rational approach for fabricating metal suspending nanostructures by nanoimprint lithography (NIL) and isotropic reactive ion etching (RIE). The approach comprises three principal steps: (1) mold fabrication, (2) structure replication by NIL, and (3) suspending nanostructures creation by isotropic RIE. Using this approach, suspending nanostructures with Au, Au/Ti or Ti/Au bilayers, and Au/Ti/Au sandwiched structures are demonstrated. For Au nanostructures, straight suspending nanostructures can be obtained when the thickness of Au film is up to 50 nm for nano-bridge and 90 nm for nano-finger patterns. When the thickness of Au is below 50 nm for nano-bridge and 90 nm for nano-finger, the Au suspending nanostructures bend upward as a result of the mismatch of thermal expansion between the thin Au films and Si substrate. This leads to residual stresses in the thin Au films. For Au/Ti or Ti/Au bilayers nanostructures, the cantilevers bend toward Au film, since Au has a larger thermal expansion coefficient than that of Ti. While in the case of sandwich structures, straight suspending nanostructures are obtained, this may be due to the balance of residual stress between the thin films. Supported by the National Natural Science Foundation of China (Grant No. 20573002) and the Major State Basic Research Development Program of China (973Pprogram) (Grant No. 2001CB6105)     

Microstructures and fatigue-free properties of the La<Superscript>3+</Superscript> and Nd<Superscript>3+</Superscript> doped Bi<Subscript>4</Subscript>Ti<Subscript>3</Subscript>O<Subscript>12</Subscript> thin films prepared by modified sol-gel technique

Ferroelectric Bi_3.25La_0.75Ti₃O₁₂ (BLT) and Bi_3.15Nd_0.85Ti₃O₁₂ (BNT) thin films were fabricated on Pt/TiO₂/SiO₂/Si (100) substrates by a modified sol-gel technique. X-ray diffraction indicated that these films were of single phase with random polycrystalline orientations. The surface morphologies of the films were observed by scanning electron microscope, showing uniform, dense films with grain size of 50–100 nm. Well-saturated hysteresis loops of the films were obtained in metal-ferroelectric-metal type capacitors with Cu top electrodes at an applied voltage of 400 kV/cm, giving the remanent polarization (2P _r) and coercive field (2E _c) values of the films of 25.1 μC/cm² and 203 kV/cm for BLT, and 44.2 μC/cm² and 296 kV/cm for BNT, respectively. Moreover, these capacitors did not show fatigue behaviors after up to 1.75×10¹⁰ switching cycles at the test frequency of 1 MHz, suggesting a fatigue-free character. The influences of the La³⁺ and Nd³⁺ doping on the properties of the films were comparatively discussed. Supported by the National Key Basic Research and Development Program of China (Grant No. 2006CB932305) and the Natural Science Foundation of Hubei Province, China (Grant No. 2004ABA082)     

Study on the thermal stability and electrical properties of the high-k dielectrics (ZrO2)x(SiO2)1?x

(ZrO₂) _x (SiO₂)_1−x (Zr-Si-O) films with different compositions were deposited on p-Si(100) substrates by using pulsed laser deposition technique. X-ray photoelectron spectra (XPS) showed that these films remained amorphous after annealing at 800°C with RTA process in N₂ for 60 s. The XPS spectra indicated that Zr-Si-O films with x=0.5 suffered no obvious phase separation after annealing at 800°C, and no interface layer was formed between Zr-Si-O film and Si substrate. While Zr-Si-O films with x >0.5 suffered phase separation to precipitate ZrO₂ after annealing under the same condition, and SiO₂ was formed at the interface. To get a good interface between Zr-Si-O films and Si substrate, Zr-Si-O films with bi-layer structure (ZrO₂)_0.7(SiO₂)_0.3/(ZrO₂)_0.5(SiO₂)_0.5/Si was deposited. The electrical properties showed that the bi-layer Zr-Si-O film is of the lowest equivalent oxide thickness and good interface with Si substrate. Supported by the National Nature Science Foundation of China (Grant No. 60636010) and the National Basic Research Program of China (“973” Program) (Grant No. 2004CB619004)     

Design and fabrication of erbium doped photosensitive fibers

A kind of erbium doped photosensitive fiber (EDPF) was proposed and fabricated, whose core was made of double layers named photosensitive layer and erbium doped layer. The double-layer core design can overcome difficulties in fabrication of EDPF with single core design, i.e. the conflict between the high consistency rare earth doping and high consistency germanium doping. A sample was fabricated through the modified chemical vapor deposition method combined with solution doping technique. The peak absorption coefficient was 48.80 dB/m at 1.53 μm, the background loss was lower than 0.1 dB/m, and the reflectivity of the fiber Brag gratings (FBG) written directly on the sample fiber was up to 97.3% by UV-writing technology. Moreover, a C band tunable fiber laser was fabricated using the sample fiber, in which a uniform FBG was written directly on EDPFs as a reflector. A single wavelength lasing with a maximum wavelength tuning range of 1555.2–1558.0 nm was achieved experimentally. Within this tuning range, the full-width at half maximum (FWHM) of the laser output was smaller than 0.015 nm and the side mode suppression ratio (SMSR) was better than 50 dB. Supported by the National High Technology Research and Development Program of China (863 Project) (Grant No. 2007AA01Z258), the National Natural Science Foundation of China (Grant No. 60771008), Program for New Century Excellent Talents in University (Grant No. NCET-06-0076), Beijing Natural Science Foundation (Grant No. 4052023), and the Beijing Jiaotong University Foundation (Grant No. 2006XM003)     

Photocatalytic performance of TiO<Subscript>2</Subscript> thin films connected with Cu micro-grid

Aiming at reducing the recombination of photo-induced carriers in semiconductor photocatalytic process, we prepared TiO₂ thin film with its surface modified by a connected Cu micro-grid via a microsphere lithography strategy, which showed higher photocatalytic activity than the pure TiO₂ film. The improvement of photocatalytic activity of Cu micro-grid to the TiO₂ film is due to the charge carrier separation and electron transfer by the conducting metal grid. The photocatalytic activity was improved as metal loading increased, which obtained the best performance at a certain loading amount, and then decreased at higher loading amount. This phenomenon was attributed to the metal’s bulk effect which could be explained by the relationship between the energetic positions and the metal cluster size. Supported by the National Natural Science Foundation of China (Grant Nos. 50672003, 50872005) and the National Basic Research Program of China (“973” Project) (Grant No. 2007CB613302) and the Fok Ying Tong Education Foundation (Grant No. 111050)