铜铟镓硒薄膜的拉曼和可见-近红外光谱表征

采用恒电位电沉积法在ITO上制备了铜铟镓硒(CIGS)前驱体薄膜,该前驱体薄膜在充氩气管式炉中经过高温硒化可得到结晶良好的CIGS薄膜。采用X-射线衍射(XRD)、拉曼光谱(Raman)、扫描电子显微镜(SEM)和紫外-可见光-近红外光谱仪分别表征了CIGS薄膜的结构、形貌、成分以及可见-近红外光谱(Vis-NIR)吸收特性。XRD结果表明前驱体薄膜高温硒化后所得的CIGS薄膜具有(112)择优取向,薄膜中CIGS晶粒的平均尺寸为24.7nm,Raman光谱表明薄膜中的CIGS是具有黄铜矿结构的四元纯相,没有其他二元三元杂相存在。Vis-NIR测量结果表明CIGS的禁带宽度随薄膜中镓含量的增加而增加,当Ga含量达5.41%时,通过吸收光谱测得CIGS的禁带宽度为1.11eV,通过理论计算得到镓铟比为Ga/(In+Ga)=16.3%,小于SEM测量所得的镓铟比Ga/(In+Ga)=21.4%,这表明还需进一步提高CIGS薄膜的结晶度。所有测量表明优化后的ITO/CIGS非常适合用来制作高质量的双面太阳能电池。该研究提出了制备低成本CIGS前驱体薄膜及高温硒化的新方法,通过这些方法在ITO上制备了均匀、致密、附着力好的CIGS薄膜。通过上述表征可知,在新工艺下制备的CIGS薄膜结晶度高,成分合理,无杂相,光吸收性质好。与磁控溅射法类似,电沉积法非常适合大面积工业化生产,该工作对CIGS的规模化生产具有重要的借鉴意义。     

Improving the performance of pure sulfide Cu(InGa)S2 solar cells via injection annealing system

In this study, we present an effective method of improving the performance of pure sulfide Cu(InGa)S₂ (CIGS) solar cells via injection annealing system. The injection annealing system can perform annealing at desired temperatures, and therefore, the CIGS thin film passed over the temperature range in which secondary phases occurs. Via the injection annealing system, secondary phase InS_x was effectively removed from the surface of the CIGS thin films at the temperatures over 550°C. This resulted in the formation of good-quality P–N junction CIGS devices, thereby improving significantly the performance of the CIGS solar cell. In addition, the open-circuit-voltage (V_OC) and fill factor (FF) of the CIGS devices increased gradually with increasing annealing temperature in the range of 550–640°C. It is speculated that the bulk defects were decreased as the annealing temperature increased. Finally, via injection annealing system, a pure sulfide CIGS solar cell with an efficiency of 12.16% was achieved.     

Multistep deposition of Cu2Si(S,Se)3 and Cu2ZnSiSe4high band gap absorber materials for thin film solar cells

Cu₂ZnSi(S,Se)₄ and Cu₂Si(S,Se)₃ are potential materials to obtain cost effective high band gap absorbers for tandem thin film solar cell devices. A method to synthesize Cu₂SiS₃, Cu₂SiSe₃and Cu₂ZnSiSe₄thin film absorbers is proposed. This method is based on a multistep process, using sequential deposition and annealing processes. X‐ray diffraction analysis performed on the final thin films have confirmed the presence of the Cu₂Si(S,Se)₃ and Cu₂ZnSiSe₄phases. Scanning electron microscopy images revealed the formation of polycrystalline layers with grains size up to 1 µm. The band gap of the ternary Cu₂SiSe₃ and Cu₂SiS₃, and quaternary Cu₂ZnSiSe₄ based thin films as determined from optical and photoluminescence measurements are found to be close to their theoretical values. (© 2015 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

Thickness optimization of Mo films for Cu（InGa）Se2 solar cell applications

Mo thin films are deposited on soda lime glass (SLG) substrates using DC magnetron sputtering. The Mo film thicknesses are varied from 0.08 μm to 1.5 μm to gain a better understanding of the growth process of the film. The residual stresses and the structural properties of these films are investigated, with attention paid particularly to the film thickness dependence of these properties. Residual stress decreases and yields a typical tensile-to-compressive stress transition with the increase of film thickness at the first stages of film growth. The stress tends to be stable with the further increase of film thickness. Using the Mo film with an optimum thickness of 1 μm as the back contact, the Cu(InGa)Se2 solar cell can reach a conversion efficiency of 13.15%.     

Cu(In,Ga)Se2太阳能电池快速热退火效应

利用光致发光（PL）分析快速热退火对Cu(In,Ga)Se2 (CIGS)电池的影响,研究退火对薄膜缺陷的影响。Cu(In,Ga)Se2电池的PL谱中总共有 7个峰,即2个可见波段峰和5个红外波段峰。退火温度较低,可减少薄膜体内缺陷,提高载流子浓度,改善薄膜质量;退火温度过高,则会引起正常格点处元素扩散,元素化学计量比改变,体内缺陷增加,吸收层带隙降低,反而会对CIGS薄膜造成破坏。     

Common origin of the circular-dichroism pattern in angle-resolved photoemission spectroscopy of SrTiO3 and Cu(x)Bi2Se3

Circular dichroism in the angular distribution of photoelectrons from SrTiO(3):Nb and Cu(x)Bi(2)Se(3) is investigated by 7-eV laser angle-resolved photoemission spectroscopy. In addition to the well-known node that occurs in the circular dichroism pattern when the incidence plane matches the mirror plane of the crystal, we show that another type of node occurs when the mirror plane of the crystal is vertical to the incidence plane and the electronic state is two-dimensional. The flower-shaped circular dichroism patterns in the angular distribution occurring around the Fermi level of SrTiO(3):Nb and around the Dirac point of Cu(x)Bi(2)Se(3) are explained on equal footings. We point out that the penetration depth of the topological states of Cu(x)Bi(2)Se(3) depends on momentum.     

铜铟镓硒薄膜光伏组件中电池与封装材料界面的光学特性对组件性能的影响

根据测试数据,分析模拟了铜铟镓硒(CIGS)薄膜光伏组件中电池的活性区域、非活性区域与封装材料之间界面的光学特性对组件的短路电流产生的影响。根据组件结构建立了光学模型,从光学模拟结果分析组件内的反射与吸收。发现电池前电极透明导电氧化物薄膜(TCO)与封装材料界面的反射不可忽视,提出通过在透明导电氧化物薄膜与封装材料之间添加减反射层,并以MgO作为膜层材料以降低活性区域的界面反射;模拟了在非活性区域一次反射光角度与二次反射的关系,由此分析了非活性区域反射面倾角、镜面反射与漫反射比例对光利用的影响。模拟结果显示,活性区域的减反层结构可降低透明导电氧化物薄膜表面的反射率1%以上,而通过在非活性面积区域制备光反射结构,理论上能够利用非活性区域光照超过50%。     

Diffusion of indium and gallium in Cu(In,Ga)Se2 thin film solar cells

The diffusion of indium and gallium in polycrystalline thin film Cu(In,Ga)Se₂ layers has been investigated. Bilayer structures of CuInSe₂ on top of CuGaSe₂ and vice versa have been fabricated in both a Cu-rich and Cu-poor process (in relation to the ideal stoichiometry). In each process molybdenum coated soda-lime glass with and without a sodium barrier was used. These bilayers were analyzed with secondary ion mass spectrometry, X-ray diffraction, scanning electron microscope and transmission electron microscope equipped with energy dispersive X-ray spectroscopy. It was found that the grain boundary diffusion was not significantly higher than the diffusion inside the grains, also for Cu-rich layers. The diffusion is suggested to mainly proceed via vacant metal sites in the lattice structure. In sodium free films a higher diffusion into the bottom layers, compared to films with sodium, was seen in all cases. This observation was explained with a larger number of vacancies, that facilitates indium and gallium diffusion, in the sodium free films. The difference in diffusion between indium in CGS layers and gallium in CIS layers, in both Cu-rich and Cu-poor processes, was small for layers with sodium.     

Growth mechanism of thermally processed Cu(In,Ga)S2 precursors for printed Cu(In,Ga)(S,Se)2 solar cells

We investigate a process used for the selenisation of particle‐based precursors to prepare low‐cost Cu(In,Ga)(S,Se)₂ (CIGS) solar cells. It is suitable for high throughput with a short optimum selenisation duration of 3–5 min and employs a rapid thermal annealing system with elemental selenium vapour. Homogeneous crack‐free Cu(In,Ga)S₂ precursor films of up to 1 µm are obtained via doctor blading. The high selenium vapour pressure in the selenisation reaction chamber results in the formation of a compact Cu(In,Ga)(S,Se)₂ layer on top of a carbon‐rich underlayer. In order to investigate the phase development in the film, the selenisation process was interrupted at different stages and the samples were monitored via XRD and surface‐sensitive Raman measurements. We find the formation of a polycrystalline Cu(In,Ga)Se₂ phase already after 1 s at the target temperature of 550 °C. Furthermore, the effect of initial precursor thickness on solar cell parameters is discussed. Complete solar cells are prepared by conventional methods, leading to conversion efficiencies well above 8%. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Junction configurations and their impacts on Cu(In,Ga)Se2 based solar cells performances

One dimension solar cells simulator package (SCAPS) is used to study the possibility of carrying out thin CIGS solar cells with high and stable efficiency. In the first step, we modified the conventional ZnO:B/i-ZnO/CdS/SDL/CIGS/Mo structure by substituting the SDL layer with the P?+?layer, having a wide bandgap from 1 to l.12?eV. Then, we simulated the J-V characteristics of this new structure and showed how the electrical parameters are affected. Conversion efficiency of 18.46% is founded by using 1.1?μm of P?+?layer thickness. Secondly, we analyze the effect of increase thickness and doping density of CIGS, CdS and P?+?layers on the electric parameters of this new structure. We show that only the short-circuit current density (J_SC) and efficiency are improved, reaching respectively 34.68?mA/cm² and 18.85%, with increasing of the acceptors density. Finally, we introduced 10?nm of various electron reflectors at the CIGS/Mo interface in the new structure to reduce the recombination of minority carriers at the back contact. High conversion efficiency of 23.34% and better stability are obtained when wide band-gap BSF is used.     

Resonant photoemission from SmS(100)

A strong, sharp resonance enhancement of 4f photoemission has been observed on SmS(100) surfaces for photon energies in the region of the 4d-4f transitions at about 126 eV. The discrete final state reached via the excitation hν+4d¹⁰4f⁶→4d⁹4f⁷ autoionizes primarely via a super Coster-Kronig transition of the type 4d⁹4f⁷→4d¹⁰4f⁵ + unbound electron. Other decay channels, e.g, Sm 5p emission, as well as a surface induced binding energy shift in the Sm³⁺ final state are identified and discussed.     

Performance improvement of CdS/Cu（In,Ga）Se2 solar cells after rapid thermal annealing

In this paper, we investigated the effect of rapid thermal annealing (RTA) on solar cell performance. An opto-electric conversion efficiency of 11.75% (Voc=0.64 V, Jsc = 25.88mA/cm2 , FF=72.08%) was obtained under AM 1.5G when the cell was annealed at 300℃ for 30s. The annealed solar cell showed an average absolute efficiency 1.5% higher than that of the as-deposited one. For the microstructure analysis and the physical phase confirmation, X-ray diffraction (XRD), Raman spectra, front surface reflection (FSR), internal quantum efficiency (IQE), and X-ray photoelectron spectroscopy (XPS) were respectively applied to distinguish the causes inducing the efficiency variation. All experimental results implied that the RTA eliminated recombination centers at the p-n junction, reduced the surface optical losses, enhanced the blue response of the CdS buffer layer, and improved the ohmic contact between Mo and Cu(In, Ga)Se2 (CIGS) layers. This leaded to the improved performance of CIGS solar cell.     

Resonant photoemission spectroscopy study of electronic structures of LiMn2O4

The valence-band resonant photoemission spectra (RPES) of LiMn₂O₄ have been measured throughout the Mn3p absorption edge. Based on the RPES data, the contribution of Mn3d states to the valence band of LiMn₂O₄ has been described and, consequently, the detailed hybridization between O2p and Mn3d states in the valence-band was determined.     

Buffer-less Cu(In,Ga)Se2 solar cells by band offset control using novel transparent electrode

Cu(In,Ga)Se₂ (CIGS) solar cells without buffer layers have been demonstrated. Currently, CdS, Zn(O,S,OH), ZnS, or InS buffer layers are used in high efficiency CIGS solar cells to suppress interface recombination. One of the important parameters to reduce the recombination is the conduction band offset (CBO) between the buffer and CIGS layers. In this study, we have proposed the use of a novel transparent conductive oxide (TCO) which can control the CBO to reduce interface recombination and eliminate the buffer layers. The device simulation was used to verify the effect of CBO control theoretically. Then, the novel TCO material of ZnO_1?xS_x:Al prepared by co-sputtering of ZnO:Al₂O₃ and ZnS targets was fabricated to verify the CBO effect experimentally. The efficiency of a CIGS solar cell with a ZnO:Al/CIGS/Mo/soda-lime glass structure, i.e. buffer-less structure using a conventional TCO, was significantly low because of severe shunting. In contrast, the use of ZnO_1-xS_x:Al instead of ZnO:Al increased the shunt resistance of the CIGS solar cell, resulting in higher open-circuit voltage and efficiency. The result is the first proof of the concept of the buffer-less CIGS solar cells.     

Numerical analysis of CdS-CIGS interface configuration on the performances of Cu(In,Ga)Se2 solar cells

One dimension solar Cell Simulation package (SCAPS) is used to analyze the impact of the CdS-CIGS interface configuration on the performances of CIGS solar cells. We simulated the current-voltage characteristic of two models of the cell: one with a donor type defect (OVC model) and the other with acceptor type defect (P+ model) at the CdS-CIGS interface. The advantages and disadvantages of these CIGS surface configuration on the electrical parameters were discussed according to their thicknesses, defect density and carrier lifetime. The simulation results show that the model with the P+ layer has poor performance when its thickness and defect density increase, due to a huge distortion on the J-V characteristic. On the other hand, the OVC layer plays a fundamental role in the performance of CIGS solar cells. Better performances are obtained with the OVC model when the density of donor defect is in the range 10¹³ - 10¹⁵ cm⁻³, the charge carriers lifetime in the range 0.02 - 1 ns, and the thickness of the OVC layer in the range 200 - 400 nm.     

Compositional investigation of potassium doped Cu(In,Ga)Se2 solar cells with efficiencies up to 20.8%

We report on the interaction between intentional potassium doping of thin film Cu(In,Ga)Se₂ (CIGS) solar cells, CIGS absorber composition, and device efficiency. Up to now high efficiency CIGS solar cells could not be produced with a gallium/(gallium + indium) ratio higher than 35%. The new doping process step does not only increase solar cell conversion efficiencies up to 20.8%, but also allows a shift in the CIGS absorber composition towards higher gallium content whilst maintaining this high efficiencies level. We find that the saturation of the open circuit voltages for higher gallium content that is normally observed can partially be overcome by the new doping procedure. This observation leads us to the conclusion that even on this high performance level CIGS solar cells still hold a potential for further development beyond the record values reported here. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Photovoltaic performance of flexible Cu(In,Ga)Se2 thin-film solar cells with varying Cr impurity barrier thickness

We report the effect of Cr impurity barrier on Cu(In,Ga)Se₂ (CIGS) thin-film solar cells prepared on flexible substrates. The Cr films with varying the thickness (t_Cr) were deposited on stainless steel substrates using direct-current magnetron sputtering. The solar cell performance was improved by increasing t_Cr since the diffusion of Fe impurities from the substrate to CIGS was suppressed. Although the elemental composition, grain size, and strain of CIGS film showed little change with varying Fe content, the fill factor and the short-circuit current density increased as decreasing Fe. The Fe increased the series resistance, shunt paths, and saturation current density. The reduction of Fe caused a steeper bandgap grading in CIGS which enhances current collection due to higher electric fields in bulk CIGS. CIGS solar cells with 1000 nm-thick Cr barrier showed the best conversion efficiency of 9.05%.