【作者】 张岩；

【导师】 张照胜；

【作者基本信息】 河北大学 ， 物理化学， 2022， 硕士

【摘要】 铜锌锡硫（CZTS）材料因其一系列优异的性能,如高吸收系数、合适的光学带隙且组成元素丰富无毒性而有望成为下一代薄膜光伏材料。近年来,CZTS在太阳能电池、光催化和热电等方面的应用受到越来越多的关注,有鉴于此,本论文第一个工作研究了CZTS的结构和性质,旨在对这种热门材料有全面而清晰的认识。CZTS太阳能电池在光电转换效率（PCE）方面取得显著的进步,从最初的0.66%增至目前的12.6%。然而这一效率远远低于单个p-n结太阳能电池效率的理论极限,限制了CZTS太阳能电池的商业化应用。非辐射复合过程是造成电荷和能量损失的主要途径,限制着CZTS光电转换效率的提升。实验上主要通过阳离子等价掺杂的方式提高CZTS的光电转换效率。实验结果表明,将Ag掺入CZTS中,抑制了替位缺陷,提高了少数载流子寿命,使PCE提高到8.28%。进一步将Ag和Cd掺入到CZTS中,PCE提高到10.8%。为了揭示掺杂改善CZTS光电转换效率的机制并为其它实验提供理论指导,本论文第二个工作研究了Ag、Cd及Ag+Cd共掺杂对CZTS非辐射电子-空穴复合动力学的影响。深能级缺陷往往作为非辐射复合中心,为了揭示CZTS中的非辐射复合中心,本论文第三个工作研究了CZTS本征缺陷形成能和电荷转变能级,并进一步研究了掺杂对缺陷形成能的影响,揭示了钝化机制并筛选出了合适的缺陷钝化剂。通过研究CZTS的晶体结构、电子结构、光学性质及声子性质,我们对CZTS有了全面和清晰的认识。CZTS中所有原子采取四配位,形成四面体,使得阴离子最外层满壳层,符合八耦律,因此整体能量比较低。从电子结构看,CZTS较理想的直接带隙有利于半导体对可见光的吸收,空穴和电子较小的有效质量有利于载流子的分离及运输,这些都提高了CZTS的光电转换率。从光学性质看,CZTS在可见光区有较高的吸收系数和较低的反射率,这些都是CZTS作为理想吸收材料的优势。从声子谱可以看出,CZTS的晶胞在Γ点未出现虚频,表明CZTS是热力学稳定结构。以上的计算结果有助于理解为什么CZTS有望成为理想的吸光材料。通过研究Ag、Cd及Ag+Cd共掺杂对CZTS非辐射电子-空穴复合动力学的影响,我们发现掺杂对CZTS带隙的影响可以忽略不计;其次,掺杂通过降低电子和空穴波函数重叠,减小了非绝热耦合;最后,掺杂通过抑制原子热运动,降低了参与电荷复合的声子频率从而延缓了量子退相干。在以上因素中,掺杂引起的非绝热耦合的变化最大,是影响非辐射电子-空穴复合的主导因素。因此,我们认为由掺杂引起的非绝热耦合减小占主导从而延缓了CZTS非辐射电子-空穴复合。通过研究CZTS中本征缺陷形成能和电荷转变能级,我们发现Cu _Zn在整个费米能级变化范围内其缺陷形成能最低且电荷转变能级较浅,这意味着在CZTS中占主导的Cu _Zn替位可以大量电离产生空穴,是形成CZTS p型半导体的主要因素;其次,Sn _Zn在带隙中引入较深的转变能级,同时Sn _Zn缺陷形成能较低可以大量存在,进而形成有害的非辐射复合中心。通过研究不同掺杂剂对缺陷形成能的影响,我们发现Cr、Fe和Mg可以有效钝化Sn _Zn深能级缺陷,抑制Sn _Zn形成有害的缺陷非辐射复合中心。因此,我们认为Cr、Fe和Mg可以作为CZTS合适的缺陷钝化剂。通过研究掺杂对CZTS非辐射电子-空穴复合的影响,我们揭示了掺杂影响CZTS光生电荷动力学的物理机制,为设计性能更加优异的CZTS光伏电池提供理论指导。通过研究不同掺杂剂对缺陷形成能的影响,我们揭示了钝化机制并筛选出合适的掺杂剂,为实验上选择合适的掺杂剂提供新的参考。更多还原

【Abstract】 Copper Zinc tin sulfide（CZTS）is expected to become the next generation of thin-film photovoltaic material due to its excellent properties,such as high absorption coefficient,suitable optical bandgap,abundant and non-toxic constituent elements.In recent years,the applications of CZTS in solar cells,photocatalysis and thermoelectricity have attracted more and more attention.In view of this,the first work of this paper studies the structure and properties of CZTS,aiming to have a comprehensive and clear understanding of this popular material.CZTS-based solar cells have made significant improvement in photoelectric conversion efficiency（PCE）,increasing from an initial 0.66%to the current record of 12.62%.However,this effciency is far below the theoretical limit for single p-n junction solar cell,which limits the commercialization of CZTS solar cells.Nonradiative recombination process constitutes a major pathway for charge and energy loss,which limits the improvement of PCE of CZTS.The PCE of CZTS is improved mainly by cationic equivalent doping in experiment.The experimental results show that incorporation of Ag into CZTS inhibits the antisiites and increases the lifetime of minority carrier,which increases the PCE to 8.28%.Furthermore,incorporation of Ag and Cd into CZTS increases the PCE to 10.8%.In order to reveal the mechanism by which doping improves the PCE of CZTS and provide theoretical guidance for other experiments,the second work of this paper studies the effects of Ag,Cd and Ag+Cd co-doping on CZTS nonradiative electron-hole recombination dynamics.Deep-level defects are usually nonradiative recombination centers.In order to reveal the nonradiative recombination centers in CZTS,the third work of this paper studies the formation energy and charge transition levels of intrinsic defects of CZTS and further studies the influence of doping on the formation energy of defects,reveals the passivation mechanism and screens out suitable defect passivation agents.By studying CZTS crystal structure,electronic structure,optical properties and phonon properties,we have a comprehensive and clear understanding of CZTS.All atoms in CZTS adopt tetra-atomic coordination to form tetrahedron,which makes the outermost shell of anion full and conforms to the octet rule,so the overall energy is relatively low.From the point of view of electronic structure,the ideal direct bandgap of CZTS is conducive to the absorption of visible light,and the small effective mass of holes and electrons is conducive to the separation and transportation of carriers,which improve the PCE of CZTS.From the perspective of optical properties,CZTS has high absorption coefficient and low reflectivity in the visible region,which are the advantages of CZTS as an ideal absorption material.Phonon spectrum shows that CZTS cell has no virtual frequency at the pointΓ,indicating that CZTS is thermodynamically stable structure.The above results are helpful to understand why CZTS is expected to be an ideal light-absorbing material.By studying the effects of Ag,Cd and Ag+Cd co-doping on nonradiative electron-hole recombination dynamics of CZTS,we find that the effect of doping on CZTS bandgap is negligible.Secondly,doping reduces the nonadiabatic coupling by reducing the overlap of electron and hole wave functions.Finally,doping delays quantum decoherence by inhibiting atomic thermal motion and reducing phonon frequencies involved in charge recombination.Among the above factors,the change of nonadiabatic coupling caused by doping is most significant,which is the leading factor affecting the nonradiative electron-hole recombination.Therefore,we believe that the reduction of nonadiabatic coupling caused by doping is dominant and delays CZTS nonradiative electron-hole recombination.By studying the intrinsic defect formation energy and charge transition level in CZTS,we found that the defect formation energy of Cu _Znis the lowest in the whole Fermi level range and the charge transition level is shallower,which means that the dominant Cu _Zncan produce a large number of holes by ionization and is the main factor that makes CZTS become p-type semiconductor.Secondly,Sn _Znintroduces deep levels in the bandgap,and the defect formation energy of Sn_Znis low enough to exist in large quantities,thus forming harmful nonradiative recombination centers.By studying the effect of different dopants on defect formation energy,we found that Cr,Fe and Mg can effectively passivate deep level of Sn _Znand inhibit the formation of harmful defect nonradiative recombination centers.Therefore,we believe that Cr,Fe and Mg can be suitable defect passivation agents for CZTS.By studying the effect of doping on nonradiative electron-hole recombination of CZTS,we reveal the physical mechanism of doping on photocharge dynamics of CZTS,and provide theoretical guidance for the design of CZTS photovoltaic cells with better performance.By studying the influence of different dopants on defect formation energy,we reveal the passivation mechanism and screen out suitable dopants,which provides a new reference for selecting suitable dopants in experiment.更多还原