【作者】 孟磊；

【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】 由于愈演愈烈的全球能源危机以及污染等问题,开发环境友好、廉价且高效率的太阳能电池变得越来越重要。Cu2Zn Sn（S,Se）4（CZTS）薄膜因其优质的性能,如较大的吸收系数（>104 cm-1）、合适的带隙（1.0¹.5 e V）以及载流子浓度（1015¹017 cm-3）,被认为是薄膜太阳能电池领域中理想的吸收层材料。目前,该类电池的最高转换效率已达到12.6%（2014年）。但是,因为CZTS基半导体薄膜中存在着一些固有瓶颈,使得该类太阳能电池在最近几年中发展十分缓慢。这主要是因为Cu²⁺与Zn²⁺离子半径非常相近,所以CZTS结构中会产生Cu/Zn之间的局部错位以及Kesterite/Stannite混合相的出现,与此同时,这种问题还会导致CuZn、ZnCu等深能级杂质缺陷且难以消除。目前,人们通常采用元素替换或合金化的方法来解决上述问题。其中,Cu2Cd Sn S4（CCTS）及Cu2Zn1-xCdxSn S4（CZCTS）合金的发展最引人注目。这是因为Cd²⁺离子半径比Cu²⁺和Zn²⁺大很多,这样就会有效的抑制由于Cu/Zn替位所造成的一系列问题。与CZTS类似,CCTS半导体薄膜的光学带隙为1.4 e V,而CZCTS合金薄膜的光学带隙由于Cd含量的不同在1.1¹.5 e V之间变化。两种薄膜的吸收系数都大于104 cm-1,是理想的吸收层材料。目前,CCTS和CZCTS太阳能电池的转换效率分别达到了3.1%和9.24%。较低的CCTS电池效率可能与吸收层和Cd S缓冲层之间的结构不匹配有关,这是因为CCTS为Stannite黄锡矿结构不同于CZTS（Kesterite锌黄锡矿）。所以,寻找光学带隙较大且能与CCTS吸收层匹配的缓冲层是该类太阳能电池发展的关键问题之一。此外,目前对CCTS的报道主要集中在制备方法的改进方面,而对其内部的本征缺陷以及与之相关的物理性质及机理方面的研究则相对较少。另一方面,CZCTS合金太阳能电池通过调控Cd/Zn比例可以由Stannite结构转变为Kesterite结构。这样就有效的避免了如CCTS中出现的电池结构不匹配的问题,可以很好的与Cd S缓冲层形成p-n结。且相对于CZTS,CZCTS与Cd S之间的晶格失配度随Cd含量的不同能够进行有效的调控,有利于减少CZCTS/Cd S界面处的缺陷。所以,CZCTS电池的转换效率相对于纯CCTS较高。但是,目前对于CZCTS吸收层的制备方法仍然沿用与CZTS相似的分子基溶液方法,这种方法存在着如双层膜结构,较差背电极接触等问题。尝试新型制备方法或对原有溶液法进行改进,是CZCTS太阳能电池的发展方向之一。针对以上问题,本论文采用理论计算与实验相结合的方法,（1）首先采用单一靶材射频磁控溅射的方法制备CCTS薄膜,并对薄膜进行了结构、形貌以及光、电学性能的表征。通过实验和第一性原理计算对CCTS不同化学计量比下的本征缺陷进行了研究,探讨了缺陷的产生机理及其对薄膜性质的影响及物理机制。（2）其次,采用与CCTS晶格失配度更小的Zn S作为缓冲层薄膜,制备p-CCTS/n-Zn S异质结,通过x射线光电子能谱（XPS）以及第一性原理计算研究了CCTS/Zn S界面的能带排列方式,并将该异质结制备成器件成功应用于紫外探测领域。（3）最后,采用在金属盐/硫脲溶液方法制备出Cu2Zn0.95Cd0.05Sn（S,Se）4（CZCTSSe）合金薄膜,并采用CZTS的电池结构制备出转换效率为5%的太阳能电池。在此基础上,将油胺加入金属盐/硫脲溶液中作为C源,在吸收层与Mo电极之间加入适度厚度的C掺杂CZCTS薄膜,使转换效率提高到7%。以下为具体的研究结果:（1）研究发现:贫Cd型CCTS的载流子浓度比准化学计量比的CCTS薄膜大2个数量级,但是其带隙则相对较小。通过第一性原理计算发现:对于贫Cd型CCTS,本征缺陷主要由铜空位（VCu）与复合缺陷对2CuCd+SnCd所组成。而对于准化学计量比的CCTS,本征缺陷则主要由VCu和CuCd组成。在这些本征缺陷中,VCu是这两种薄膜p型导电的主要原因,而2CuCd+SnCd缺陷对的存在则是贫Cd型CCTS较高载流子浓度和较低带隙的原因。（2）I-V测量显示该p-CCTS/n-Zn S异质结具有很好的整流特性。通过采用x射线光电子能谱的测量并结合第一性原理计算,说明p-CCTS/n-Zn S异质结属于I型（cliff型）能带排列。较高的导带阶（CBO）阻止了p-CCTS一侧产生的光生电子向n-Zn S方向的漂移。所以,Zn S缓冲层尽管与CCTS晶格适配度很小,但由于该异质结能带排列方式不适合对光生载流子的收集及输运,所以不能应用在CCTS太阳能电池中。但是对p-CCTS/n-Zn S器件光电响应测试结果表明:在波长330 nm出现了光电流的峰值并且在380 nm出现了很陡的电流下降,说明这种p-n异质结器件是一种典型的可见光盲区型紫外探测器,可以应用在紫外探测领域。（3）通过对CZCTSSe吸收层的硒化速度进行研究,发现缓慢硒化热处理过程更有利于提高CZCTSSe吸收层的结晶质量,减少表面孔洞,将太阳能电池效率由3%提高到5%。另一方面,在缓慢硒化的前提下,在吸收层与Mo电极界面处加入适当厚度的碳（C）掺杂CZCTS前驱体薄膜能够明显改变CZCTSSe吸收层的晶体生长机制,减少在背电极界面处的孔洞并降低电池的串联电阻。最终将CZCTSSe太阳能电池的转换效率提高到7%。除了高转换效率,这种电池还具有优异的可重复性,及在空气中的稳定性。更多还原

【Abstract】 Because of the increasingly serious global energy crisis,material researches to develop environmentally friendly,low-cost and high-efficiency solar cells have become extremely important.Recently,earth abundant Cu2 Zn Sn S4（CZTS）thin films are considered as ideal absorber materials for thin film solar cells（TFSC）due to their suitable properties such as large absorption coefficient（>104 cm-1）,direct bandgap（1.0¹.5 e V）,and suitable charge carrier concentrations（1015¹017 cm-3）.To date,the most efficient thin filmsolar cells currently using the CZTS as absorber layers give laboratory efficiencies of 12.6 % for mixed sulfoselenide（CZTSSe）absorbers.However,owing to many bottlenecks such as disorder in structure,secondary phases（Zn S,Kesterite/Stannite mixed phase）and intrinsic defects with deep levels（CuZn,ZnCu）,the development of CZTS based solar cells is very slow in the recent years.Results of experiments and theoretical calculations indicate that these bottlenecks are mainly due to that the ionic radiuses of Cu²⁺ and Zn²⁺ are similar.Up to now,many efforts have focus on element substitution or alloying method to overcome these problems.Among them,the developments of Cu2 Cd Sn S4（CCTS）and Cu2Zn1-xCdxSn S4（CZCTS）solar cells have been attracted much attention.Like CZTS,the bandgap of CCTS is about 1.4 e V,while CZCTS can turn the badgap from 1.0 to 1.5 e V by varying Cd content.Both of thin films have high absorption coefficient（>104cm-1）,suggesting that they are suitable for photovoltaic application,and the efficiency of both solar cells is 3.1 % and 9.24 %,respectively（relative lower than that of CZTS）.For CCTS,the structure is Stannite（CZTS: Kesterite）that is not suitable to combine with Cd S buffer and form benign p-n heterojunction.Herein,it is necessary to find a novel buffer with larger bandgap and suitable structure,which can further increase the performance CCTS solar cells.Moreover,the recent reports for CCTS thin film and solar cells are mainly focus on preparation method and techniques.There exist few researches on the intrinsic defects of CCTS and related physical mechanisms.On the other hand,CZCTS alloy thin film can change the phase form Stannite to Kesterite by varying Cd content and form good p-n heterojunction with Cd S buffer.In the meanwhile,because the lattice constant of CZCTS can be changed,lattice mismatch between CZCTS and Cd S is better than CZTS/Cd S,which is good for the decrease of interface defects.Thus,the efficiency of CZCTS is higher than that of pure CCTS.Recently,the preparation methods of CZTS absorber are molecular-solution method,which always induce double-layer structure（compact/fine grains layers）and poor back contact adhesion.Therefore,replacing or optimizing the synthesis method and techniques is one of the ways to develop CZCTS solar cells in future.To cure the above problems,this doctoral thesis presentsthe following content by using theoretical and experimental methods:（1）Firstly,CCTS thin films are synthesized by radiofrequency（RF）magnetron sputtering method with one single target,which is different from reported method or techniques.We characterize the as-prepared CCTS thin films in structure,surface/cross-section morphology,optical and electrical properties.Additionally,we study the intrinsic defects of CCTS thin films with different composition by using experimental analysis and first-principles calculations,as well as discuss the formation mechanism and their effects on physical properties of CCTS thin films.（2）Secondly,we chose Zn S thin film as the buffer layer for CCTS and form p-CCTS/n-Zn S p-n heterojunction,because the lattice mismatch between Zn S and CCTS is much smaller.Then study is carried out for the band alignments of CCTS/Zn S by XPS measurements and first-principle calculations.Furthermore,this p-n heterojunction is successfully applied in ultraviolet photo detection.（3）Finally,we successfully synthesize Cu2Zn0.95Cd0.05Sn（S,Se）4（CZCTSSe）solar cells by using metal salt/thiourea solution method and efficiency of 5 % is obtained.In addition,we use oleylamine as carbon source and insert rational thick carbon-doping CZCTS thin films in to CZCTSSe/Mo interface to significant increase the efficiency of CZCTSSe solar cell up to 7 %.Detailed results of all mentioned contents are as follow:（1）It is found that the carrier concentrationof the Cd-poor CCTS is two orders of magnitude higher than that of the near stoichiometric CCTS while its optical bandgap is smaller to the near stoichiometric CCTS’.It is suggested by usingfirst-principles calculations that the dominant intrinsic defects are Cu vacancy（VCu）and fully passivated defect complex of 2CuCd +SnCd in the Cd-poor CCTS,but VCu and CuCd in the near stoichiometric CCTS.The VCu is responsible forthe p-type conductivity of both CCTS films,while the 2CuCd+SnCd complex for smaller bandgap and higher carrier concentration of the Cd-poor CCTS.（2）Current-voltage measurement of the CCTS/Zn S heterojunction photodetector shows a good rectifying behavior.X-ray photoelectron spectroscopy measurements and first-principles calculations indicate that the CCTS/Zn S heterojunction has a type-I（cliff-type）band alignment.The large conduction-band offset leads to the barrier that inhibits the drifting of photo-generated electrons from p-CCTS to n-Zn S layer.Thus,Zn S buffer is not suitable for CCTS solar cells.But,spectral response characteristics of the device canbe used in ultravioletdetection.The photodetector shows a peak photocurrent at 330 nm and a sharp photocurrent edge at about 380 nm,suggesting atypical visible-blind characteristic.（3）Compared with rapid selenizing process,the slow selenizing process can further improve the crystalline quality of CZCTSSe absorber and increase the efficiency of CZCTSSe solar cells from 3 % to 5 %.On the other hand,the suitable thickness carbon-doping CZCTS films cansignificantly change the crystal growth mechanism of CZCTSSe absorber layers,eliminate the voids in the back contact,andreduce the series resistance.The efficiency of CZCTSSe solar cells fabricated by insetting the appropriate thickness carbon-doping CZCTS films is above 7 %,and shows the excellent repeatability and long-term air stability compared with the control devices.更多还原