Guanggen Zeng

Interface Study of ITO/ZnO and ITO/SnO2 Complex Transparent Conductive Layers and Their Effect on CdTe Solar Cells

Transparent ITO/ZnO and ITO/SnO2 complex conductive layers were prepared by DC- and RF-magnetron sputtering. Their structure and optical and electronic performances were studied by XRD, UV/Vis Spectroscopy, and four-probe technology. The interface characteristic and band offset of the ITO/ZnO, ITO/SnO2, and ITO/CdS were investigated by Ultraviolet Photoelectron Spectroscopy (UPS) and X-ray Photoelectron Spectroscopy (XPS), and the energy band diagrams have also been determined. The results show that ITO/ZnO and ITO/SnO2 films have good optical and electrical properties. The energy barrier those at the interface of ITO/ZnO and ITO/SnO2 layers are almost 0.4 and 0.44 eV, which are lower than in ITO/CdS heterojunctions (0.9 eV), which is beneficial for the transfer and collection of electrons in CdTe solar cells and reduces the minority carrier recombination at the interface, compared to CdS/ITO. The effects of their use in CdTe solar cells were studied by AMPS-1D software simulation using experiment values obtained from ZnO, ITO, and SnO2. From the simulation, we confirmed the increase of , FF, , and by the introduction of ITO/ZnO and ITO/SnO2 layers in CdTe solar cells.

Synthesis and Characterization of CZTS Thin Films by Sol-Gel Method without Sulfurization

One process of layer-by-layer sol-gel deposition without sulfurization was developed. The CZTS films with 1.2 μm and the sulfur ratio of ~48% were prepared and their characteristics were measured. The as-deposited and annealed films are of Kesterite structure. The as-deposited films do not present obvious electric conduction type. However, the annealed 9-LAY-ANN film is p-type conduction and has sheet resistance of 4.08 kΩ/□ and resistivity of 4.896 × 10−1 Ω·cm. The optic energy gap is 1.50 eV for as-deposited films and is 1.46 eV after being annealed. The region deposited by using Lo-Con solution is more compact than that by the Hi-Con solution from SEM morphology images.

Characterization of Cu1.4Te Thin Films for CdTe Solar Cells

The copper telluride thin films were prepared by a coevaporation technique. The single-phase Cu1.4Te thin films could be obtained after annealing, and annealing temperature higher than 220°C could induce the presence of cuprous telluride coexisting phase. Cu1.4Te thin films also demonstrate the high carrier concentration and high reflectance for potential photovoltaic applications from the UV-visible-IR transmittance and reflectance spectra, and Hall measurements. With contacts such as Cu1.4Te and Cu1.4Te/CuTe, cell efficiencies comparable to those with conventional back contacts have been achieved. Temperature cycle tests show that the Cu1.4Te contact buffer has also improved cell stability.

The Structure and Stability of Molybdenum Ditelluride Thin Films

Molybdenum-tellurium alloy thin films were fabricated by electron beam evaporation and the films were annealed in different conditions in N2 ambient. The hexagonal molybdenum ditelluride thin films with well crystallization annealed at 470°C or higher were obtained by solid state reactions. Thermal stability measurements indicate the formation of MoTe2 took place at about 350°C, and a subtle weight-loss was in the range between 30°C and 500°C. The evolution of the chemistry for Mo-Te thin films was performed to investigate the growth of the MoTe2 thin films free of any secondary phase. And the effect of other postdeposition treatments on the film characteristics was also investigated.

Effect of Annealing on the Properties of Antimony Telluride Thin Films and Their Applications in CdTe Solar Cells

Antimony telluride alloy thin films were deposited at room temperature by using the vacuum coevaporation method. The films were annealed at different temperatures in N2 ambient, and then the compositional, structural, and electrical properties of antimony telluride thin films were characterized by X-ray fluorescence, X-ray diffraction, differential thermal analysis, and Hall measurements. The results indicate that single phase antimony telluride existed when the annealing temperature was higher than 488 K. All thin films exhibited p-type conductivity with high carrier concentrations. Cell performance was greatly improved when the antimony telluride thin films were used as the back contact layer for CdTe thin film solar cells. The dark current voltage and capacitance voltage measurements were performed to investigate the formation of the back contacts for the cells with or without Sb2Te3 buffer layers. CdTe solar cells with the buffer layers can reduce the series resistance and eliminate the reverse junction between CdTe and metal electrodes.

Effect of Deposition Temperature on the Properties of CdTe Thin Films Prepared by Close-Spaced Sublimation

CdTe films have been prepared by close-spaced sublimation under different conditions. When CdTe is deposited at a low temperature, grains with a single predominant growth plane (111) adhere to the substrate and gather into scattered particles. With increasing deposition temperature, the number of CdTe grains on the substrate increases very quickly, the grains begin to form a quasi-continuous film, and (220), (311), (400), and (331) growth planes of CdTe begin to appear. When the deposition temperature and time are increased further, the CdTe grains begin to accumulate, the pin-holes formed initially begin to disappear, and grain boundaries with traces of layer growth can be observed. Different transmittance of samples of different thickness is clearly apparent but changes of the band gap, Eg, of the CdTe films is negligible. XPS results suggest CdTeO3 is generated on the CdTe film surface as the deposition time is increased.

Correlation of Interfacial Transportation Properties of CdS/CdTe Heterojunction and Performance of CdTe Polycrystalline Thin-Film Solar Cells

The light and dark output performances of CdS/CdTe solar cells made by close-spaced sublimation (CSS) were investigated to elucidate the transportation properties of carriers at CdS/CdTe heterojunction interface. It has been found that the interfacial transportation properties were relatively sensitive to variations of the characteristics of heterojunction due to the series resistance and shunting effects. For the high quality cell with 12.1% efficiency, narrow depletion region of ~1.1 microns and large electric field intensity of ~1.3 V/μm allow the sufficient energy-band bending close to CdS layer at CdS/CdTe heterojunction, which changes the carrier transportation mechanism from emission to diffusion and leads to the optimal rectifying characteristics with small dark saturation current density ~6.4 × 10−10 A/cm2. As a result, the schematic diagram of heterojunction band structure corresponding to various performances of solar cells has also been presented.

Optoelectrical Properties and Back Contact Characteristic of VSe 2 Thin Films: Optoelectrical Properties and Back Contact Characteristic of VSe 2 Thin Films

采用电子束蒸发法制备VSe 2 薄膜并进行退火处理, 通过XRD、SEM、透过谱、Hall效应、电导率–温度关系等表征了薄膜的结构、形貌、光学和电学性质, 用半导体特性测试仪研究了VSe 2 薄膜的背接触特性。结果表明: VSe 2 薄膜在一定的退火温度下结晶并呈稳定的六方相, VSe 2 薄膜为p型直接禁带跃迁材料, 光能隙约2.35 eV。将VSe 2 作为背接触层应用于CdTe多晶薄膜太阳电池, 消除了roll-over现象, 有效提高了器件性能。采用电子束蒸发法制备VSe 2 薄膜并进行退火处理, 通过XRD、SEM、透过谱、Hall效应、电导率–温度关系等表征了薄膜的结构、形貌、光学和电学性质, 用半导体特性测试仪研究了VSe 2 薄膜的背接触特性。结果表明: VSe 2 薄膜在一定的退火温度下结晶并呈稳定的六方相, VSe 2 薄膜为p型直接禁带跃迁材料, 光能隙约2.35 eV。将VSe 2 作为背接触层应用于CdTe多晶薄膜太阳电池, 消除了roll-over现象, 有效提高了器件性能。

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%.