衬底温度对碲化镉薄膜性质及太阳电池性能的影响

蒸汽输运法是制备高质量且大面积均匀的CdTe薄膜的一种优良的方法。采用自主研发的一套蒸汽输运沉积系统制备了CdTe多晶薄膜, 并研究了衬底温度对CdTe薄膜性质及太阳电池性能的影响。利用XRD、SEM、UV-Vis和Hall等测试手段研究了衬底温度对薄膜的结构、光学性质和电学性质的影响。结果表明, 蒸汽输运法制备的CdTe薄膜具有立方相结构, 且沿(111)方向高度择优。随着衬底温度的升高(520℃~640℃), CdTe薄膜的平均晶粒尺寸从2 μm增大到约6 μm, CdTe薄膜的载流子浓度也从1.93×10¹⁰ cm^-3提高到2.36×10¹³ cm^-3, 说明提高衬底温度能够降低CdTe薄膜的缺陷复合, 使薄膜的p型更强。实验进一步研究了衬底温度对CdTe薄膜太阳电池性能的影响, 结果表明适当提高衬底温度, 能够大幅度提高电池的效率、开路电压和填充因子, 但是过高的衬底温度又会降低电池的长波光谱响应, 导致电池转换效率的下降。经过参数优化, 在衬底温度为610℃、无背接触层小面积CdTe薄膜太阳电池的转换效率达到11.2%。     

Substrate temperature dependent structural,optical, morphology and electrical properties of RF sputtered CdTe thin films for solar cell application

In this work, we have studied the influence of substrate temperature on structural, morphology optical, and electrical properties of CdTe thin films deposited by RF magnetron sputtering. Films were analyzed by using variety of techniques such as low angle X-ray Diffraction, UV–Visible spectroscopy, Raman spectroscopy, Field emission scanning electron microscopy (FE-SEM), Energy-dispersive X-ray spectroscopy (EDAX) Hall Measurement etc. Low angle XRD analysis showed that CdTe films are polycrystalline and has cubic structure with preferred orientation is along (111) direction. Raman scattering studies revealed the presence of single phase CdTe over the entire range of substrate temperature studied. The FE-SEM analysis showed that CdTe growth process occurred predominantly by grain growth and not through the layer-by-layer mode. Compositional analysis carried out using EDAX suggests that CdTe films deposited at low substrate temperatures are Te rich and that at higher temperatures is Cd rich. Electrical resistivity of CdTe films decreases with increase in substrate temperature and whereas positive increase in Hall coefficient suggests as-deposited CdTe films are p-type. The UV–Visible spectroscopy analysis showed that the band gap increases from 1.47 to 1.51 eV when the substrate temperature increased from 50 to 300 °C. Such optimum band gap CdTe can be use as absorber material in photovoltaic applications like the CdS/CdTe and ZnO/CdTe solar cells.     

X-ray,electron microscopy and photovoltaic studies on CdTe thin films deposited normally at different substrate temperatures

X-ray diffraction, transmission electron microscopy and electron diffraction studies were conducted on CdTe thin films deposited on glass substrates kept at different substrate temperatures. Variation of the different structural parameters, such as lattice constant, crystallite size, r.m.s. strain, dislocation density and stacking fault probability with substrate temperature, was investigated in the temperature range 300 to 498 K. An increase in the lattice constant and crystallite size values and a decrease in the other parameters with increase in temperature of the substrate was observed. A photovoltage was observed for CdTe film deposited normally on glass substrates kept at higher substrate temperatures. The development of photovoltage in the film is explained in the light of the formation of crystallites of variable structure.     

Homojunction and heterojunction based on CdTe polycrystalline thin films

The reaction growth temperature plays an important role for growing CdTe thin film on a foreign substrate by CVD. If a growth temperature is more than 560 °C, the CdTe film is a p-type. The higher growth temperature, the greater is the carrier concentration. If the growth temperatures are less than 520 °C, the CdTe films are n-type. A p-type CdTe film is firstly grown at a higher temperature, and an n-type CdTe film is then grown at a lower temperature, forming a CdTe homojunction. The properties of as-deposited CdTe homojunction are measured for the first time. In addition, a p-type CdTe film is exposed in an air environment for several weeks, forming a hybrid layer of CdO and TeO₂. An ITO film is then deposited on the oxide layer, forming an n-ITO/i/p-CdTe heterojunction. At the same time, an n-ITO/p-CdTe heterojunction is also deposited by the same processes except without the oxidation treatment. The efficiency of the heterojunction solar cell with the oxidation treatment is much greater than that without the oxidation processing.     

Properties of electrochemically deposited CdTe thin films: annealing effect

CdTe thin film have been deposited onto stainless steel and fluorine doped tin oxide coated glass substrates from aqueous acidic bath using electrodeposition technique. The different preparative parameters, such as deposition time, bath temperature, pH of the bath have been optimized by photoelectrochemical (PEC) technique get good quality photosensitive material. The deposited films are annealed at different temperature in presence of air. Annealing temperature is also optimized by PEC technique. The film annealed at 200 °C showed maximum photosensitivity. Different techniques have been used to characterize the as deposited and also annealed (at 200 °C) CdTe thin film. The X-ray diffraction (XRD) analysis showed the polycrystalline nature and a significant increase in the XRD peak intensities is observed for the CdTe films after annealing. Optical absorption shows the presence of direct transition with band gap energy 1.64 eV and after annealing it decreases to 1.50 eV. Energy dispersive analysis by X-ray study for the as-deposited and annealed films showed nearly stoichiometric compound formation. Scanning electron microscopy reveals that spherically shaped grains are more uniformly distributed over the surface of the substrate for the annealed CdTe film. Photovoltaic output characteristics and spectral response of the annealed film have been carried. The fill factor and power conversion efficiency (η) of the cell are found to be 71 and 3.89 %.     

Influence of the substrate temperature on the structure and surface roughness of Cd0.18Sb0.64Te0.18 films

Using a radiofrequency sputtering deposition technique, ternary Cd0.18Sb0.64Te0.18 thin films have been grown on glass substrates at several substrate temperatures (50–250°C). The samples have an Sb content of about 63 at %, as measured by Auger spectroscopy. The surface roughness, the structural and the electrical properties of the films were studied as a function of substrate temperature. X-ray diffraction (XRD) measurements showed that the structure of the films changes from an amorphous phase, when deposited at lower substrate temperatures, to a mixture of two crystalline phases (CdTe and Sb) for higher substrate temperatures. Atomic force microscopy shows an increase in the surface roughness with an increase in the substrate temperature, clearly showing the formation of crystalline phases with microcrystallite sizes in good agreement with those determined from XRD measurements. The amorphous-to crystalline transition is accompanied by an abrupt increase in the room temperature electrical conductivity of the films. This increase in the conductivity as well as its temperature dependence in the range of room temperature to 150°C can be understood in terms of an electrical percolation process through the conducting Sb crystallites.     

Electrical conduction and transmission electron microscopy studies of CdSe0.8Te0.2 thin films

Electrical resistance of CdSe_0.8Te_0.2 thin films were found to be dependent on various film parameters such as substrate temperature, film thickness, deposition rate and post-deposition heat treatment in different environments. A decrease in film resistivity was observed for thicker films and for those heat treated in vacuum. Films deposited at higher substrate temperatures and faster rates showed an increase in film resistivity. A spectrum of activation energies was observed in the films which fell within either of the activation energies observed in CdSe or CdTe films. Films heated in an oxygen environment showed an increase in film resistivity with a different activation energy. Transmission electron microscopy (TEM) of the films showed an improvement in crystallinity with increasing film thickness and substrate temperature, and a reduction in crystallinity with increasing deposition rate.     

Growth regimes of CdTe deposited by close-spaced sublimation for application in thin film solar cells

We have systematically investigated the growth of CdTe thin films by Close Spaced Sublimation (CSS). Thin films of CdTe were deposited onto CdS substrates held at temperatures in the range 250 to 550 °C. The effect of substrate temperature and evaporation rate on structure and surface morphology of CdTe films were investigated. Up to 450 °C substrate temperature the growth rate was almost constant and decreased exponentially for higher temperatures. The structures of the CdTe films were determined by XRD and a strong (111) orientation was observed within the temperature range 250 °C-470 °C. Above 470 °C the texture changed to mostly (311) and (220) orientations. Surface morphology and grain size of CdTe growth was determined with AFM and SEM. The morphology of the layers showed three major modes: Columnar grains with a diameter of 0.2 μm and a length of 6 μm for temperatures from 250 °C to 350 °C, pyramidal grains with a diameter of 0.5-1.5 μm up to 470°C and irregular shaped grains with a diameter of 5-10 μm for temperatures up to 550 °C. The roughness increased linearly from 15 nm to 220 nm within the substrate temperature range.     

Preparation and characterization of pulsed laser deposited CdTe thin films at higher FTO substrate temperature and in Ar + O2 atmosphere

Pulsed laser deposition (PLD) is one of the promising techniques for depositing cadmium telluride (CdTe) thin films. It has been reported that PLD CdTe thin films were almost deposited at the lower substrate temperatures (<300 °C) under vacuum conditions. However, the poor crystallinity of CdTe films prepared in this way renders them not conducive to the preparation of high-efficiency CdTe solar cells. To obtain high-efficiency solar cell devices, better crystallinity and more suitable grain size are needed, which requires the CdTe layer to be deposited by PLD at high substrate temperatures (>400 °C). In this paper, CdTe layers were deposited by PLD (KrF, λ = 248 nm, 10 Hz) at different higher substrate temperatures (T_s). Excellent performance of CdTe films was achieved at higher substrate temperatures (400 °C, 550 °C) under an atmosphere of Ar mixed with O₂ (1.2 Torr). X-ray diffraction analysis confirmed the formation of CdTe cubic phase with a strong (1 0 0) preferential orientation at all substrates temperatures on 60 mJ laser energy. The optical properties of CdTe were investigated, and the band gaps of CdTe films were 1.51 eV and 1.49 eV at substrate temperatures of 400 °C and 550 °C, respectively. Scanning electron microscopy (SEM) showed an average grain size of 0.3–0.6 μm. Thus, under these conditions of the atmosphere of Ar + O₂ (15 Torr) and at the relatively high T_s (500 °C), an thin-film (FTO/PLD-CdS (100 nm)/PLD-CdTe (~1.5 μm)/HgTe: Cu/Ag) solar cell with an efficiency of 6.68% was fabricated.     

Depth profiling study of in situ CdCl2 treated CdTe/CdS heterostructure with glancing angle incidence X-ray diffraction

CdTe thin films have been deposited using spray pyrolysis technique without and with in situ CdCl₂ treatment. Scanning electron microscopy studies show enhanced grain growth in the presence of CdCl₂. Glancing angle incidence X-ray diffraction is used for the micro structural study of polycrystalline CdS/CdTe heterostructure at different depths by changing the incident angle. Spraying of CdCl₂ on CdS prior to CdTe deposition promotes S diffusion throughout CdTe film and also Te diffusion into CdS. Whereas spraying of CdCl₂ in between CdTe deposition prevents S diffusion partially and Te diffusion completely. There is an associated change in the microstress of the CdTe film at different layers. The films without CdCl₂ treatment show compressive microstress varying from −98 to −158 MPa with increasing incident angle. CdCl₂ spray during CdTe deposition shows compressive microstress, which varies from −98 MPa at the interface to −19 MPa near the surface and CdCl₂ spray prior to CdTe deposition leads to a mildly tensile stress, from +40 to +20 MPa, which is very close to the standard shear stress of 10 MPa for CdTe.     

Formation of structure of the CdTe film, recrystallized on Mo/glass substrate under high temperature and mechanical pressure

Large-grained and 7 μm thick CdTe film has been fabricated on top of Mo coated soda-lime glass substrate. As a new approach the dynamic recrystallization process (DRC) was used to form the structure of films. For the characterization of the structure and composition of the films a scanning electron microscopy (SEM), energy dispersive X-ray microanalysis (EDS) and X-ray diffraction (XRD) methods were used. The changes in the structure of films were studied in accordance with the process temperature, pressure and as-deposited film thickness. Significant changes in the CdTe film were observed after DRC of deposited films at the process temperatures between 450 °C and 550 °C. EDS quantitative analysis showed that during the recrystallization the Mo and CdTe films composition remained stable for all studied samples. The XRD results showed that the increase in the process temperature caused improvement in orientation of the films along direction of (111). The DRC temperature above 550 °C reduced the orientation again. The limits of the temperature and pressure in application of soda-lime glass in DRC were found and discussed.     

Multiple thermal oxidation of silicon and the electrophysical properties of silicon oxide films and the Si-SiO2 bonding layer

Epitaxial films of CdS, CdSe and CdTe grown by evaporation in vacuum onto (110) surfaces of Ge had the sphalerite structure in parallel or nearly parallel alignment with the substrate. In the case of the CdS films it was found that there was a critical temperature of about 370°C. Annealing or growth above this temperature resulted in a domain-form transformed structure. The CdSe and CdTe films were found to have the domain-form structure as-grown at any temperature in the epitaxial range. Films of sphalerite-structure CdS grown on (110) surfaces of NaCl could also be annealed to produce the domain-form transformed structure. There were found to be two types of domain in the films with this structure. Each type of domain was faulted on one of the two {111} planes inclined at 35°19′ to the [110] film normal. In CdTe the faulting was random, giving rise only to streaks in the diffraction pattern. In CdSe and CdS, however, the faulting was partly regular, giving rise to extra spots along the 〈111〉 streaks in the diffraction pattern.In CdSe the faults could be annealed out and the domain boundaries were then found to be antiphase boundaries. The origin of the two types of domains at least in CdSe was therefore a particular form of double positioning, leading to polarity reversal between regions of the film growing from the two types of nuclei.A lath-form transformed structure was found only in some CdS films that were grown on (110) NaCl, floated off and annealed on gold grids. This structure is dealt with in the second paper in this series.     

Preparation of CdTe films by electrodeposition

Single-phase CdTe thin films have been prepared by depositing sequentially a layer of tellurium and a layer of cadmium on a molybdenum substrate followed by a short thermal treatment. Deposition of tellurium films was done in an aqueous solution containing TeO₂ at a current density of ≈ 1 mA/cm². An aqueous solution containing cadmium sulfate was used for cadmium deposition with a current density of ≈1 mA/cm². Solution temperature was ≈ 95°C for tellurium film deposition and was 50°C for cadmium deposition. It was found that after a heat treatment at ≈ 370°C for 10 min the deposited Te/Cd layers were converted to CdTe thin films with a cubic structure. Compositional uniformity of the films was also investigated by electron probe microanalysis.     

Study of Ar + O2 deposition pressures on properties of pulsed laser deposited CdTe thin films at high substrate temperature

Cadmium telluride (CdTe) thin films deposited by pulsed laser deposition (PLD) on fluorine–tin–oxide substrates under different pressures of argon (Ar) + oxygen (O₂) at high substrate temperature (T_s = 500 °C) was reported in this paper. In our work, the CdTe thin films were prepared successfully at high T_s by inputting Ar + O₂. As reported, PLD-CdTe thin films were almost prepared at low substrate temperatures (<300 °C) under vacuum conditions. The deposition of CdTe thin films at high T_s by PLD is rarely reported. The influence of the Ar + O₂ gas pressure on thickness, structural performance, surface morphology, optical property and band gap (E_g) had been investigated respectively by Ambios probe level meter, X-ray diffraction (XRD), scanning electron microscopy (SEM) and UV–Vis spectrometer. Strong dependence of properties on the deposition pressures was revealed. In the range of Ar + O₂ gas pressure from 5 to 12 Torr, the deposition rate and the E_g of CdTe films vary in the range of 41.9–57.66 nm/min then to 35.26 nm/min and 1.51–1.54 eV then to 1.47 eV, respectively. The XRD diagrams showed that the as-deposited films were polycrystalline, and the main phase was cubic phase. However, the preferred orientation peak disappeared when the deposition pressure was higher. SEM images indicated that the CdTe film deposited at a higher deposition pressure was more uniform and had a higher compactness and a lower pinhole density. Furthermore, based on this thorough study, FTO/PLD-CdS (100 nm)/PLD-CdTe (~1.5 μm)/HgTe:Cu/Ag solar cells with an efficiency of 6.68 % and an area of 0.64 mm² were prepared successfully.     

Structural and electrical studies of thermally evaporated nanostructured CdTe thin films

CdTe thin films were deposited on KCl and glass substrates using thermal evaporation technique under high vacuum conditions. CdTe bulk compound grown by vertical directional solidification (VDS) technique was used as the source material to deposit thin films. Powder X-ray diffraction technique was employed to identify the phase of the as grown bulk CdTe compound as well as its thin films. Surface morphology and the stoichiometry of the bulk compound and thin films was carried out by using scanning electron microscope (SEM) with an attachment of energy dispersive spectrometer(EDS). Microstructural features associated with the as deposited CdTe thin films were studied by using transmission electron microscope (TEM). The films deposited on to glass substrates at different temperatures have been used to study the I-V characteristics of the films. These parameters have been studied in detail in order to prepare good quality nanostructured thin films of CdTe compound. CdTe bulk compound grown by VDS method and its thin films prepared by thermal evaporation method found to have single phase with cubic structure. Size of the particles in the as deposited films vary between 5 and 40 nm In the present study efforts have been made to correlate the electrical and optical properties of the CdTe thin films with the corresponding microstructural features associated with them.     

Growth of cubic and hexagonal CdTe thin films by pulsed laser deposition

The paper reports the growth of cadmium telluride (CdTe) thin films by pulsed laser deposition (PLD) using excimer laser (KrF, λ=248 nm, 10 Hz) on corning 7059 glass and SnO₂-coated glass (SnO₂/glass) substrates at different substrate temperatures (T_s) and at different laser energy pulses. Single crystal target CdTe was used for deposition of thin films. With 30 min deposition time, 1.8- to ∼3-μm-thick films were obtained up to 200 °C substrate temperature. However, the film re-evaporates from the substrate surface at temperatures >275 °C. Atomic force microscopy (AFM) shows an average grain size ∼0.3 μm. X-ray diffraction analysis confirms the formation of CdTe cubic phase at all pulse energies except at 200 mJ. At 200 mJ laser energy, the films show hexagonal phase. Optical properties of CdTe were also investigated and the band gap of CdTe films were found as 1.54 eV for hexagonal phase and ∼1.6 eV for cubic phase.     

Preparation and characterization of polycrystalline CdZnTe films for large-area, high-sensitivity X-ray detectors

This article reviews the preparation and characterization of polycrystalline CdTe and CdZnTe films to be used in large-area, high-sensitivity X-ray panel detectors intended for medical diagnostics. The films, deposited by closed-spaced sublimation, are expected to exhibit excellent efficiency at low X-ray doses because of their high sensitivity. The detectors constructed using these films incorporated a novel hybrid technique, in which zinc-doped CdTe was pre-deposited onto a ceramic substrate and then connected to a TFT circuit substrate. The sensor substrate material was specially selected to avoid both incident X-ray attenuation in the substrate and micro-cracks in the film. Zinc doping, which was used to grow the CdTe film, also served to form the heterojunction diode structure that suppressed leakage current. Moreover, the quality of a polycrystalline CdZnTe film deposited on a 9×9 substrate was characterized, revealing its applicability to large-area X-ray detectors. Further investigation and improvements are in progress.     

静电自组装制备CdTe量子点纳米薄膜

以巯基丙酸为稳定剂, 在水相中合成了表面带负电荷、具有良好的分散性、平均粒径为5nm的CdTe量子点. 通过CdTe量子点与阳离子聚电解质聚二烯丙基二甲基氯化铵(PDDA)和阴离子聚电解质聚苯乙烯磺酸钠(PSS)之间的静电相互作用, 在石英基片表面通过层层静电自组装方法制备了多层CdTe量子点纳米薄膜. 以荧光分光光度计、UV-Vis、XPS、AFM等测试手段对所得的CdTe量子点纳米薄膜进行了表征. 研究结果表明, CdTe量子点自组装多层薄膜的UV-Vis吸光度与组装层数基本呈线性关系, 薄膜成膜质量良好. 自组装薄膜基本上规整并均匀地覆盖在石英基底表面, 但薄膜中存在部分CdTe量子点聚集现象. 通过在相邻的两层CdTe量子点之间引入基本结构单元为PDDA/PSS/PDDA的聚电解质复合层, 可有效提高CdTe量子点纳米薄膜的成膜质量. 所得的CdTe量子点纳米薄膜具有良好的荧光光致发光性.     

Role of substrate temperature on the structural,optoelectronic and morphological properties of (400) oriented indium tin oxide thin films deposited using RF sputtering technique

RF sputtering process has been used to deposit highly transparent and conducting films of tin-doped indium oxide onto quartz substrates keeping the RF power constant at 250 W. The electrical, optical and structural properties have been investigated as a function of substrate temperature. XRD has shown that deposited films are polycrystalline and have (400) preferred orientation. Indium tin oxide layers with low resistivity values and high transmittance in the visible region have been deposited. Detailed Analyses based on X-ray diffraction, optical and electrical results are attempted to gain more insight into the factors that are governed by the influence of varying substrate temperature in this investigation. AFM pictures showed uniform surface morphology with very low surface roughness values. It has been observed that ITO films deposited in this study, keeping the substrate temperature at 150 °C, can provide the required optimum electrical and optical properties rendering them useful for developing many optoelectronic devices at a moderate temperature.     

Carbon incorporation in silicon-carbon films grown at different substrate temperatures

Hydrogenated microcrystalline silicon-carbon thin films have been deposited by plasma enhanced chemical vapour deposition technique at the substrate temperatures of 250 °C and 400 °C varying the radio frequency (RF) power in the 10-100 W range. The effects of substrate temperature and RF power on the structural, compositional, optical, and electrical properties have been investigated. The increase of substrate temperature or RF power leads to a decrease of crystallinity degree and an enhancement of carbon content. Optical absorption in the UV-visible region and electrical conductivity are affected in a different way by the RF power and substrate temperature variations. Silicon grain nucleation of films deposited at the temperature of 250 °C on commercial doped tin oxide substrate has been explored, for different RF power, by means of X-ray diffraction measurements.