GaSe Formation at the Cu(In,Ga)Se2/Mo Interface–A Novel Approach for Flexible Solar Cells by Easy Mechanical Lift‐Off

Implementing photovoltaic devices based on high efficiency thin‐film technologies on cheap, light‐weight and flexible polymeric substrates is highly appealing to cut down costs in industrial production and to accelerate very large scale deployment of photovoltaics in the upcoming years. Lift‐off processes, which allow separating active layers from primary substrates and subsequent transfer onto an alternative substrate without modifying the upstream production process and without performance losses, are an emerging alternative to direct growth on polymeric substrates. This study concerns the feasibily of direct mechanical lift‐off process for high efficiency Cu(In,Ga)Se₂ (CIGS) thin film solar cells grown by coevaporation on glass/molybdenum substrates without performance losses. The study presents an in depth characterization (SEM,AFM,GIXRD,XPS) of samples leading to excellent lift‐off properties. They are explained by a specific gallium rich CIGS graded interface structure according to the interfacial sequence glass/Mo/MoSe₂/Ga_xSe_y/Ga‐rich‐CIGS. The interfacial layer, attributed to GaSe, has a layered structure and out performs the molybdenum diselenide layered layer which forms spontaneously at the interface Mo/CIGS. It allows a very easy lift‐off process at the interface GaSe/CIGS thanks to Van‐der‐Waals adhesion mechanism in GaSe. Key physical‐chemical parameters are identified and analyzed. After lift‐off, an efficiency of 14.3%, higher than the initial reference CIGS solar cell efficiency (13.8%) is measured.     

A comparative study of pulsed laser deposition and flash evaporation of Culn0.75Ga0.25Se2 thin films

In this paper initial results are presented for the growth and characterisation of polycrystalline Culn_0.75Ga_0.25Se₂ thin films prepared by pulsed laser deposition and flash evaporation. Analogies are drawn between these two important deposition technologies. The deposited films were characterised using a veriety of analytical techniques, including energy-dispersive analysis of X-rays and Rutherford-backscattering spectroscopy for compositional evaluation, X-ray diffreaction and Raman spectroscopy for structural evaluation, scanning electron microscopy for surface examination and the four-point and hot-point probe techniques for electrical characterisation. The comparison of films produced by these two deposition methods revealed that, in terms of their stochiometry, electrical and physical characteristics, good-quality GIGS thin films could be produced by both techniques.     

CIGS薄膜太阳能电池缓冲层CdS薄膜的制备研究

目前CdS材料的制备方法有很多种,但是最常用的是化学水浴法。本文研究了浓度、反应溶液pH值、温度、沉积时间对CdS缓冲层薄膜的影响,对CIGS薄膜太阳能电池缓冲层CdS薄膜的制备方法进行了论述。     

大面积ZnO:Al窗口层的制备与研究

采用磁控溅射方法,制备了大面积(300mm×300mm)ZnO:Al薄膜作为CIGS太阳电池的窗口层。设计以电阻率和透过率测试值的标准差乘积作为衡量大面积低阻ZnO窗口层的均匀性标准,确定4mm/s作为衬底与靶材最合适的相对行走速度,并进一步研究工作气压和溅射功率对ZnO:Al薄膜结构、电学特性和光学特性的影响,实现在400～1 200nm波长范围内平均透过率大于85%,电阻率ρ约1.0×10-3Ω.cm。试验结果表明,改善的工艺条件能得到大面积内厚度均匀、光电特性良好的低阻ZnO窗口层。     

溶液PH值对CdS薄膜结构特性的影响

采用化学水浴沉积(CBD)工艺在玻璃衬底上制 备CdS薄膜,研究溶液PH值对CdS 薄膜结构特性的影响。薄膜的厚度、组份、晶相结构和表观形貌分别由台阶仪、X射线荧光 光谱(XRF)、X射线衍射(XRD)和场发射扫描电子显微镜(FESEM)来表征。溶液的 PH值为11.26、 11.37和11.48时,CdS薄膜的晶相以六方相为主,薄膜的厚度先增大后减小; PH值为11.62、11.66时,薄膜的晶相以立方相为主,薄 膜的厚度进一步减小。同时,随着溶液PH值 增大,CdS薄膜的晶格常数也逐渐增大。两种晶相的CdS薄膜缓冲层与CIGS薄膜分别构成异 质 对形成异质结时的晶格失配分别为32.297%和1.419%,界面态密度分别为2.792×10¹⁴和8.507×10¹²,因此高效CIGS薄 膜太阳电池更需要立方相的CdS薄膜。     

Zn1?xMgxO用于 CIGS 太阳电池的研究进展

Zn1-xMgxO透过率高、带隙可调,且与CIGS太阳电池在晶格和能带结构上匹配良好,可用作CIGS太阳电池缓冲层、窗口层,因此制备高质量的Zn1-xMgxO薄膜是提高太阳电池性能的关键。文章介绍了Zn1-xMgxO薄膜的结构特性、光学特性及制备方法;从Mg含量、Zn1-xMgxO膜厚及Zn1-xMgxO/CIGS界面处缺陷密度等方面概述了Zn1-xMgxO用于CIGS太阳电池的研究进展,并比较了Zn1-xMgxO与In2S3,ZnS,CdS等其他材料作缓冲层的CIGS太阳电池性能的差别。     

Cu(In,Ga)Se2 solar cell grown on flexible polymer substrate with efficiency exceeding 17%

Development of a Cu(In,Ga)Se₂ thin film solar cell on a polyimide film with a conversion efficiency of 17.1%, measured under standard test conditions at the European Solar Test Installation (ESTI) of the Joint Research Centre (JRC) of the European Commission, Ispra, is reported. The drastic improvement from the previous record of 14.1% efficiency is attributed to a more optimized compositional grading, better structural and electronic properties of the absorber layer as well as reduced reflection losses. Basic film and device properties, which led to the improvement in the efficiency record of flexible solar cells are presented for the new process and compared to the old process. Copyright © 2011 John Wiley & Sons, Ltd.     

Surface engineering in Cu(In,Ga)Se2 solar cells

Surface modifications of three‐stage co‐evaporated Cu(In,Ga)Se₂ (CIGS) thin films are investigated by finishing the evaporation with gallium‐free (CuInSe₂, CIS) stages of various lengths. Secondary‐ion mass spectrometry shows substantial interdiffusion of indium and gallium, smearing out the Ga/(Ga + In) profile so that the addition of a CIS layer merely lowers the gallium content at the surface. For the thinnest top layer, equivalent to 20 nm of pure CIS, X‐ray photoelectron spectroscopy does not detect any compositional difference compared with the reference device. The modifications are evaluated electrically both by temperature‐dependent characterisation of actual solar‐cell devices and by modelling, using the latest version of scaps‐1d (Electronics and Information Systems, Ghent University, Belgium). The best solar‐cell device from this series is obtained for the 20 nm top layer, with an efficiency of 16.6% after antireflective coating. However, we observe a trend of decreasing open‐circuit voltage for increasingly thick top layers, and we do not find direct evidence that the lowering of the gallium concentration at the CIGS surface should generally be expected to improve the device performance. A simulated device with reduced bulk and interface defect levels achieves nearly 20% efficiency, but the trends concerning the CIS top layer remain the same. Copyright © 2011 John Wiley & Sons, Ltd.     

Accurate explicit equations for the fill factor of real solar cells—Applications to thin‐film solar cells

Even within the simplest real solar cell model, the exact value of the fill factor (FF) is only computable by numerical calculations. Here, we perform approximations to the power–voltage curve given by the one‐diode model with series and shunt resistance losses, obtaining explicit expressions for the voltage and current at the maximum power point, and thus an explicit approach for the FF. Over a broad range of possible solar cell parameters, including cells where the impact of shunt losses on the fill factor is not negligible, the approximate equations yield relative errors typically around 1%. The equations are applied to explore the dependence of FF on alternative buffer material thickness of organic solar cells, and to investigate the incidence of shunt and series resistance losses on the FF of Cu(In,Ga)Se₂ solar cells under indoor illumination conditions. Copyright © 2012 John Wiley & Sons, Ltd.     

Development of gallium gradients in three‐stage Cu(In,Ga)Se2 co‐evaporation processes

We use secondary‐ion mass spectrometry, X‐ray diffraction and scanning electron microscopy to investigate the development over time of compositional gradients in Cu(In,Ga)Se₂ thin films grown in three‐stage co‐evaporation processes and suggest a comprehensive model for the formation of the well‐known ‘notch’ structure. The model takes into account the need for compensating Cu diffusion by movement of group‐III ions in order to remain on the quasi‐binary tie line and indicates that the mobilities of In and Ga ions differ. Cu diffuses towards the back in the second stage and towards the front in the third, and this is the driving force for the movement of In and Ga. The [Ga]/[In + Ga] ratio then increases in the direction of the respective Cu movement because In has a higher mobility at process conditions than has Ga. Interdiffusion of In and Ga can be considerable in the (In,Ga)₂Se₃ film of the first stage, but seems largely to cease in Cu(In,Ga)Se₂ and shows no signs of being boosted by the presence of a Cu₂Se layer. Copyright © 2011 John Wiley & Sons, Ltd.