【作者】 杨刚；

【导师】 姚斌； 李永峰；

【作者基本信息】 吉林大学 ， 凝聚态物理， 2017， 博士

【摘要】 太阳能被认为是最经济和环保的可再生能源,基于光伏效应的太阳能电池由于能够直接将光照转换成电能,因而受到了广泛的关注。薄膜太阳能电池具有制备工艺简单,所需原材料少,便于携带等优点而被广泛研究。铜铟镓硒（Cu In Ga Se2,CIGS）和碲化镉（Cd Te）薄膜太阳能电池已经实现了产业化生产,但是生产这两种电池需要消耗大量的稀有金属和有毒重金属元素。锌黄锡矿结构的铜锌锡硫（Cu₂Zn Sn S₄,CZTS）基材料具有合适的光学带隙和超过104 cm-1的高吸收系数,其组成元素都是地壳含量丰富且无毒的,理论预测CZTS太阳能电池转换效率超过30%,因此,被认为是替代CIGS制备薄膜太阳能电池的理想吸收层材料。CZTS基太阳能电池的效率在短短二十年内就从0.66%提升到12.6%,但与CIGS电池效率相比还有很大差距。除了材料单一相范围小、存在较多杂相和缺陷等问题外,薄膜的晶体质量以及p-n结能带匹配问题也是导致其效率较低的主要原因。针对这两个问题,我们在本文中提出了改善薄膜微结构、提高晶体质量的技术途径,并在此基础上对界面能带排列和相关CZTS基光电器件应用进行了研究,取得了如下结果:1、通过磁控溅射单一CZTS靶材、调节快速热处理气氛中S/Se元素比例,制备了大晶粒、高致密的CZTSSe薄膜。我们采取高压固态反应技术制备四元单一相CZTS靶材,然后通过磁控溅射技术和快速热处理技术制备CZTSSe吸收层薄膜。通过优化快速热处理的升温速率,得到了平整致密的吸收层薄膜;通过调节退火气氛中S粉和Se粉的比例,可以实现对薄膜中S/Se元素含量、结构以及光电性质的控制。研究结果表明贫S富Se的薄膜晶粒尺寸比较均匀,其纵向贯穿的晶粒尺寸能达到2μm,并且薄膜具有1.13 e V的光学带隙和较高的迁移率,适合作为高效率电池的吸收层。用这种薄膜制备了标准结构的电池器件,其光电转换效率为3.38%。进一步的分析表明,晶体质量的提高减少了光生载流子在CZTSSe吸收层内部的复合,但严重的界面复合导致了器件的理想因子和反向饱和电流较大,进而影响了器件的开路电压和填充因子。2、通过向CZTS前躯体溶液中添加适量的高分子聚合物,提高了薄膜晶体质量,改善了吸收层与缓冲层和背电极之间的界面性能。研究了聚乙烯吡咯烷酮（PVP）添加量对溶液法制备CZTSSe薄膜形貌及电池器件性能的影响。发现添加剂对薄膜的组分和晶体结构没有太大影响,但是能明显提高CZTSSe薄膜的致密性和晶粒尺寸。PVP添加量为5 wt%时晶粒尺寸能达到5μm以上,但是过量添加会使薄膜中出现小晶粒层,而添加比例为1 wt%时得到的薄膜表面和断面形貌都比较好。用这种薄膜制备的CZTSSe电池效率为4.34%。相比于未添加PVP的标准电池（效率为2.24%）提高了很多。电池器件界面的优化减少了光生载流子在界面处的复合。因此电池的开路电压,短路电流以及填充因子都有所提升。3、利用XPS实验测量结合第一性原理计算阐明了CZTS/Zn O异质结界面能带排列结构及其物理机制。利用磁控溅射制备了CZTS/Zn O异质结,通过X射线光电子能谱实验测量和第一性原理计算分别得到了CZTS/Zn O异质结的能带偏移。发现该异质结的能带排列为I型对齐,而且导带偏移值很小。这种能带对齐方式能够减少光生载流子的界面复合,并且不会阻碍载流子的输运。从能带排列角度说明Zn O是一种潜在的无毒缓冲层材料。4、通过调节施加偏压,实现了对p?CZTS/n?Ga N异质结光电二极管在紫外和可见区光响应的调控,并阐明了响应机制。由于宽禁带半导体Ga N和Zn O具有相似的结构和性质,我们采用射频磁控溅射方法在n-Ga N基底上沉积一层p-CZTS薄膜,制备出p-CZTS/n-Ga N异质结光电二极管,并研究了它的光响应特性。发现通过调节施加偏压可以调制其光谱响应,不加偏压和加上反向偏压时其响应波长分别位于紫外光区和可见光区,用异质结能带排列可以很好的解释观察到的实验现象。5、采用凝胶技术制备了新型的Cu₂Mg Sn S₄薄膜,阐明了硫化温度对薄膜结构及光电性质的影响规律。发现Cu+、Mg2+、Sn4+三种阳离子在晶格中的占位是随机的。由于晶体结构对称性的提高,X射线衍射结果中只出现了闪锌矿结构的特征衍射峰。CMTS薄膜具有良好的电学性质和合适的光学带隙,适合作为薄膜太阳能电池的吸收层材料。更多还原

【Abstract】 Solar energy is the most economic and environmentally friendly renewable energy resources.Solar cells based on photo-voltaic effect can convert sunlight directly into electricity directly.Various kinds of solar cells have received widespread attention.Thin film solar cells have been intensively studied due to the advantages of simple preparation technology,low cost and easy to carry.Cu In Ga Se2（CIGS）and Cd Te based thin film solar cells have realized industrialization production,but the consumption of rare or toxic elements limit their further application.The kesterite Cu₂ Zn Sn S₄（CZTS）has a suitable bandgap and a high absorption coefficient larger than 104 cm-1.The constituent elements are non-toxic and earth-abundant.Theoretical limit conversion efficiency of CZTS-based solar cell is over 30%.Therefore,it is an ideal alternative absorption material for solar cells applications.The power conversion efficiency（PCE）of CZTS-based solar cells updated rapidly from 0.66% to 12.6% in the past two decades.However,there is still a considerable gap compared with CIGS cells.In addition to the small range of single phase and the existence of more second phases and defects,there are also two main reasons for the low efficiency,i.e.the poor crystal quality of the films and the mismatching of energy band at the heterojunction interface.In view of these,we mainly researched the technical way to improve the crystalline quality and the micro-structure of CZTS-based films in this thesis.In addition,we also studied the band alignment and the related photoelectric applications.The results are as follows:1.The CZTSSe thin films were prepared by sputtering a single quaternary target followed by a rapid thermal process,the effects of the S/Se element ratio in the annealing atmosphere on the films were studied.CZTSSe absorbed layers with smooth surfaces and densely packed columnar grains were obtained by optimizing the heating curve.The different Se supply during the annealing was used to adjust theS/（S+Se）ratios in the films.The structure,optical and electrical properties can also be controlled.A Se-rich CZTSSe thin film possesses more uniform grain size,the grain size can reach about 2 μm,almost equal to the thickness of the films.This film has an optimized bandgap about 1.13 e V and relatively high hall mobility,indicating that high efficiency solar cells can be obtained by this Se-rich CZTSSe film.A CZTSSe thin film solar cell with standard structure shows a conversion efficiency of 3.38%.Analysis of the J-V characteristic shows that severe interface recombination is the major reason for the large reverse saturation current and diode ideality factor.The recombination in the bulk can be reduced due to the improved micro-structure and crystalline quality.2.We proposed that the morphology and crystallinity of solution based CZTSSe films can be improved by introducing polymer into the precursor solution.The influence of different amount of Polyvinylpyrrolidone（PVP）additive on the morphology and crystallinity were investigated.It is found that the polymer additive has no effect on the element composition and the crystal structures of the kesterite films.However,the voids on the surface of CZTSSe films can be reduced and the grain size improved dramatically after the addition of PVP.Grain size can reach more than 5 μm when the mass fractions of PVP is 5 wt%,but such an excessive additive will bring a thick small grain layer in the film.This can be eliminated by reducing the amount of PVP,when the additive amount is 1 wt%,high quality CZTSSe film without fine-grain sub-layer were obtained.The CZTSSe thin film solar cell fabricated with 1 wt% PVP shows a PCE of 4.34%,which demonstrated a promotion compared with the control device（PCE of 2.24%）without PVP addition.The enhancement of the PCE can be attribute to the improvement of the interfacial properties which reduces the recombination.3.It is important to determine the band alignments at the interface for achieving high efficiency cells.We have fabricated a CZTS/Zn O heterojunction using Cd-free buffer layer Zn O.The band alignments at the interface of CZTS/Zn O heterojunction were determined by X-ray photoelectron spectroscopy combined with first-principles calculations.The results implying that the CZTS/ZnO heterojunction has a type-Iband alignment with a small conduction band offset（CBO）which will not block the transport of electrons.We can infer that Zn O may be a suitable nontoxic buffer layer material for high efficiency CZTS solar cells.4.Wide-bandgap semiconductor gallium nitride（Ga N）has the same structure with Zn O and similar wide direct bandgap（3.4 e V）.CZTS film was deposited on an n-Ga N/sapphire substrate using sputtering method to form a p-CZTS/n-Ga N heterojunction photodiode.The spectral response measurements indicate that the response wavelength of the photodiode can be tuned from ultraviolet to visible regions via applying zero and reverse bias.Alternative visible and ultraviolet light response spectra were realized in this device.Band alignment at the interface of the p-CZTS/n-Ga N heterojunction was proposed to interpret this phenomenon.5.Cu₂ Mg Sn S₄（CMTS）thin films were successfully grown on glass substrates by sol–gel spin coating technique.The effects of the annealing temperature on structural,optical,and electrical characteristics were investigated.It is found that cations（Cu+,Mg2+,and Sn4+）share the same position and have a random distribution,the crystal structure shows a high symmetry.Thus,only diffractions of zinc-blende structure were detected.Furthermore,a suitable bandgap around 1.5 e V and the stable p-type conductivity imply that the CMTS thin film seems to be an ideal absorption material for cheap and environmentally friendly solar cells applications.更多还原