【作者】 孔慧；

【导师】 杨平雄； 褚君浩；

【作者基本信息】 华东师范大学 ， 微电子学与固体电子学， 2014， 博士

【摘要】 本文从光伏行业发展概况、CIGS薄膜电池的国内外研究现状和产业化发展现状分析入手,介绍了相关的检测设备及其工作原理,对各层材料特性、制备方法作了一定的对比分析,选择用磁控溅射CuIn0.7Gao.3Se2四元合金靶材法制备CIGS薄膜的吸收层材料和后硒化工艺,探索了一步法制备CIGS吸收层薄膜的可行性,制备的glass/Mo/CIGS/CdS/i-ZnO/AZO薄膜电池的光电转化效率为2.08%。论文获得了以下主要结果：1、磁控溅射法制备底电极Mo薄膜的工艺研究。本文采用两步法(高气压(2pa)高功率(130W)溅射约100nm,再用低气压(0.5pa)高功率(130W)溅射沉积约900-1000纳米)制备的Mo层具有良好的电阻率,鱼鳞状的结构,附着力较好,方块电阻为0.26Ω2/o,膜厚1080nm,电阻率达到9.8×10-5Ω·cm。2、详细研究了用磁控溅射四元合金CIGS靶制备吸收层时,衬底温度、工作气压、快速退火硒化等工艺改变对吸收层薄膜的晶体结构、禁带宽度和表面形貌的影响。实验证明：(1)随着衬底温度的升高,(112)峰的强度变强,半高宽变窄,薄膜的结晶质量有所改善,样品的化学计量比接近靶材的固有成分,贫铜与贫硒的温度效应不明显。(2)随着工作气压的增加,出现贫铜现象；与Sigmund提出的Cu元素的溅射产率比In和Ga的低的理论一致,但是薄膜的结晶质量随着压力的增加变差,XRD衍射峰的半高宽变宽；AFM和SEM图表明薄膜的表面晶粒逐渐减小,平均粗糙度从11.36nm降至5.12nm,表面较为平整,逐渐致密。由于小晶粒中存在更多的晶界,会增加载流子的复合,使短路电流变小,从而影响电池的效率。禁带宽度随着压力的增加,从1.0eV增加到1.32eV,实现了带隙的调节。(3)一步法制备CIGS吸收层薄膜容易出现贫硒现象,可以采用快速退火炉硒化来增加薄膜中硒含量,研究发现：硒化后的薄膜结晶性更好,表面平整,单晶纯相结构。Raman谱中发现位于约176cm-1处的最强峰为Al模式,峰形较弱且有点宽化的特征峰在215-220cm-1处,对应于黄铜矿的B2模式。综合各方面的条件,0.9Pa下常温溅射,再用10mg Se粉硒化所获得的薄膜组分均一,结晶质量较高,表面较为平整,适合制备CIGS薄膜电池。3、磁控溅射制备透明导电膜AZO窗口层的研究。分析了氧分压、衬底温度、工作气压和溅射功率变化对窗口层的方块电阻、电阻率、膜厚、透过率等的影响。结果表明：工作压力0.8Pa,溅射功率225W,衬底温度为225℃,AZO薄膜具有最低电阻率9.8×10-4Ω·cm,方块电阻为14欧姆,可见光区域(305-800nm)平均透过率达到85%,禁带宽度3.32eV。4、磁控溅射与化学水浴法制备CdS层的对比研究。通过SEM、透射光谱手段作了表面形貌、光学透过率和电学禁带宽度的对比研究。发现70℃时化学水浴法生长的CdS薄膜可见光范围内的平均透过率达到80%以上,禁带宽度为2.3eV左右,薄膜晶粒较大,表面光滑平整。磁控溅射制备CdS薄膜的平均透过率略低于80%,禁带宽度为2.3eV左右,晶粒较小。虽然磁控溅射制备CdS的电性能基本满足制备CIGS电池的需求,具备一定的可行性,但是可能污染真空室,在多层膜制备中引起交叉反应。而水浴法生长CdS薄膜所用设备简单、环境友好,还可以避免溅射法导致的膜面的损伤和不平整,性能稳定,更容易实现工业化生产。本论文选择化学水浴法制备CdS薄膜。5、制备了glass/Mo/CIGS/CdS/i-ZnO/AZO薄膜电池,通过I-V曲线测试,得到的电学性能为：25℃、AM1.5的条件下,制备的电池的单元面积是0.07cm2,电池的效率是2.08%,开路电压是294mV,短路电流是21.6mA/cm2,填充因子是32%。结合SEM截面图和量子效率曲线,分析了电池的效率损失和量子效率损失原因。主要有四点原因：(1)吸收层晶粒太小。从电池截面来看晶粒不清晰,没有明显的纵向晶界,其次吸收层的厚度仅为750nm,而高效电池的吸收层厚度约2微米。(2)缓冲层CdS的厚度太厚。高效率的CIGS电池的缓冲层的厚度是40-50nm,本文制备的缓冲层厚度约为80nm,导致电池的开路电压变小。(3)窗口层太薄,常见的窗口层厚度是750nm左右,我们所制备的窗口层的厚度大约350nm。(4)CIGS为单晶纯相结构。研究表明多晶结构的CIGS吸收层制备的电池转换效率明显优于单晶结构的电池性能。一方面是因为多晶的结构晶界清晰,整体是近2微米厚的晶粒,晶界可以吸杂,从而减少少数载流子在界面附近的湮灭现象,提升其导电性能；另一方面是多晶的CIGS薄膜的表面因为贫铜结构而生成某种缺陷层ODC,这种缺陷层导致价带偏移而形成大量的空穴势垒,可阻止CIGS晶粒内的空穴扩散,减少界面复合,提升吸收层电性能。(5)没有制作减反层MgF2。减反层可以增加光的入射率,提升转换效率。更多还原

【Abstract】 This paper is focused on the preparation and optical&electrical properties of CuInGaSe2(CIGS) solar cells. The quaternary CIGS target is used to prepare the absorbed layer by RF sputtering process from the ananlysis of photovoltaic industry overview, CIGS solar cells research stutation and status of industrial development. The structure of CIGS solar cells is glass/Mo/CIGS/CdS/i-ZnO/AZO, and the efficiency of solae cells is up to2.08%. In addition, the working principle of testing equipment and properties of each layers are discussed.Firstly the preparation of Mo electrode by magnetron sputtering is investigated. The Mo layer exhibits scaly structure, good adhesion, sheet resistance about0.26Ω/□, thickness of layer1080nm, and electrical resistivity about9.8×10-5Ω·cm by two-steps method, which contains the working pressure of2pa in the first step and0.5pa in the second step. Both of the power in two steps is130w.Secondly the effect of substrate tempreture, sputtering pressure and post-selenization process on the films’structure, optical properties and surface of the CIGS obsorbed layer fabricating by sputtering quaternary CIGS target was investigated. The XRD patterns shows that the intensity of (112) peak becomes stronger and the FWMA of (112) peak tend to be smanller with increasing the substrate tempreture, which indicate the crystallization quality become better. Besides the composition of films is closed to the composition of target, which shows temperature effect of poor Cu and poor Se is not obvious. The poor Cu state occurs as the increasing of working pressure, which is accordance with the report by Sigmund. The yield of Cu is lower than that of In and Ga. The patterns of AFM and SEM indicated that the grain size become smaller and average roughness decreases from11.36nm to5.12nm. The surface of small grain size exhibit smooth and dense, while the more grain boundaries in small grain-size films will enchance the carrier recombination, lower short-circuit current and drop the efficiency of cells. The optical bandgap of CIGS films shifts from1.0eV to1.32eV with the increasing working pressure. In general, the CIGS film prepared at1.0pa shows good properties and satisfy requirement of solar cell. The poor Se state in CIGS films prepared by one step method can be eliminated by post-selenization process. The results show that post-selenization films possess better crystalline, smoother suaface and pure phases. The peak located at176cm-1in Raman spectra is vertified as Al mode, while the broaded peak located at215-220cm-1is identified as B2mode of chalcopyrite structure. In conclusion, the CIGS films prepared by sputtering at0.9pa and post-selenization with10mg Se exhibit near stoichiometric ratio and smooth surface, which is suitable to fabricate the CIGS solae cell.The AZO windows layer prepared by magnetron sputter is studied. The effect of power, working pressure and substrate temperature on optical and electrical properties of AZO transparent conductive thin films has been studied. The result shows that the AZO thin films prepared at working pressure of0.8pa, sputtering power of225w, the substrate temperature of225℃possess lowest electrical resistivity about9.8×10-4Ω·2cm, and the average transmittance of AZO thin films in the visible region is up to85%with bandgap of3.32eV.Comparative analysis of CdS layer prepared by magnetron sputtering and chemical bath method. The test result of CdS thin films obtained by the chemical bath method in surface morphology, transmission spectrum and band gaps show that a smooth surface, a larger grain and an average transmittance reaches above80%in visible light range, which band gap is about2.3eV. However, the thin films deposited by magnetron sputtering indicate that a small grain, an average transmittance slightly less than80%and a similar band gap. Although the later films show certain feasibility, which can basically meets the requirement of the electrical properties in the CIGS solar cells. The previous method can make the absorption layers avoid the injury from sputtering method, clean the absorber layer surface, low cost, more stable performance and easily realize industrial production. Because of sputtering can bring pollution to vacuum chamber and cross reaction in the multilayer film preparation, the chemical bath method is preferred. The solar cells with the structure glass/Mo/CIGS/CdS/i-ZnO/AZO were obtained, the result indicate an efficiency of2.08%, an open circuit voltage of294mV, the short circuit current of21.6mA/cm2and the fill factor is32%on a solar cell unit area about0.07cm2, which were characterized by current-voltage measurement under AM1.5illumination at25℃. The reasons for the loss of solar cell efficiency and quantum efficiency have been analyzed though the combined with the SEM section and the quantum efficiency curve. There are four main reasons:(1) the smaller grain in the absorber layer, no obvious longitudinal grain boundary and the grain cannot clear observe form the section of thin films. Then, the thin film thickness is too thin to the higher efficiency, which only about750nm, the high efficiency of CIGS solar cells would need a1500nm thickness.(2) the CdS thin film is too thick for the cells. Buffer layer CIGS cell is generally high efficiency of the thickness is40-50nm, from the battery of the section, we prepared buffer layer thickness is about80nm, the buffer layer is too thick to affect the rate of cellvoltage.(3) The thickness of window layer also affects the conversion efficiency of the battery, the thickness of window layer was about350nm.(4) The study found photoelectric conversion efficiency of CIGS polycrystalline thin film solar cells is higher than that of CIS single crystal solar cell. The reason is:CIGS polycrystalline film crystal andcolumnar section, thin film is single grain, reducing carrier transmission boundary composite;and the formation of lean CIGS thin film on the surface of the ordered defect compounds layer (ODC), in each grain surface of polycrystalline CIGS also have ODClayer, internal the ODC and CIGS VB offset between grains by forming a hole barrier, prevent grain tograin boundary diffusion inside the hole, so that theinterface recombination is reduced; grain gettering effect on grain, the grain internal purity is higher thansingle crystal material.(5) Without antireflection layer MgF2.更多还原