【作者】 吴玲玲；

【导师】 王德亮；

【作者基本信息】 中国科学技术大学 ， 材料物理与化学， 2022， 博士

【摘要】 CdTe是一种稳定的Ⅱ-Ⅵ族二元化合物,禁带宽度在室温下约为1.45 eV,可见光范围内的光吸收系数可高达105 cm-1,这使它成为理想的光伏材料。CdTe太阳电池转换效率高、长期稳定性好、成本低廉等优势,使其成为最具竞争力的薄膜光伏产品之一。在过去数十年间,成功进行了大规模的商业化生产。CdTe薄膜太阳电池转换效率的理论极限为32.1%,目前实验室小面积CdTe太阳电池和电池组件的最高转换效率分别达到了 22.1%和18.6%。尽管CdTe薄膜太阳电池已经在市场应用中获得了可喜成果,但由于受到CdTe材料特性的影响,生产高效稳定器件仍然面临着诸多难题,如:不同物质异质结界面处能带结构对载流子输运的影响;载流子在异质结界面及CdTe体材中复合对电池性能的影响;CdTe中少数载流子寿命偏低导致较低的开路电压;低阻稳定背接触的制备等。本论文主要针对以上问题,进行了以下几个方面的研究:第一章,简单回顾了太阳电池的发展历史,阐述了半导体PN结太阳电池的工作原理及基本特性,讨论了 CdTe薄膜太阳电池的发展概况、基本结构、材料特性及电池中可能存在的复合过程。第二章,制备了传统CdS/CdTe电池,和以掺镁氧化锌(MgxZn1-xO,MZO)为窗口层的MZO/CdTe和MZO/CdSe/CdTe新结构太阳电池。在CdS磁控溅射过程中掺入一定的O2,得到禁带宽度更大的CdS:O,提高了电池蓝光响应。通过优化无氧条件下在MZO衬底上使用近空间升华沉积CdTe时的衬底温度和源温度,得到表面较平整、晶粒尺寸在1～2μm的CdTe薄膜。在此基础上引入了 CdSe层,以增加电池在长波区域的光吸收。通过优化CdSe层厚度,得到了最大短路电流为27.1 mA/cm2和转换效率为14.00%的MZO/CdSe/CdTe太阳电池。此外,采用热蒸发和快速热处理方法制备ZnTe:Cu薄膜,作为CdTe太阳电池的背电极缓冲层。对ZnTe:Cu缓冲层的制备工艺进行了优化,发现当ZnTe厚度超过100 nm时,会引起电池串联电阻增大,短路电流降低。采用2 nm Cu/100 nm ZnTe/3 nm Cu“三明治”结构的背接触缓冲层,可以保证Cu对CdTe的适度掺杂而避免过多的Cu扩散到CdTe中。最终制备的CdS/CdTe/ZnTe:Cu/Au电池具有较低的反向饱和电流,较高的开路电压和填充因子,转换效率最高可达16.02%。第三章,对MZO/CdTe和CdS/CdTe两种结构的电池性能进行了比较研究,发现前者由光生电流收集引起的性能损失更加严重。本研究中,转换效率为13.80%的CdS/CdTe电池光生电流收集引起的效率损失为1.53%,而转换效率为12.67%的MZO/CdTe电池效率损失高达3.89%。为了探究电池效率损失严重的原因,首先使用时间分辨光致发光光谱(time-resolved photoluminescence spectrum,TRPL)测试了两种结构太阳电池的少数载流子寿命,发现MZO/CdTe结构中CdTe的少数载流子寿命较低。较低的少子寿命源于MZO/CdTe PN结界面处的载流子严重复合,会导致电池开路电压损失较大。根据电池开路电压与J-V测试温度T的关系,得出MZO/CdTe和CdS/CdTe太阳电池的激活能大小分别为1.08和1.36eV,表明CdS/CdTe电池CdTe体材Shockley-Read-Hall(SRH)复合占主导作用,而MZO/CdTe电池则由PN结界面复合主导。此外,电池理想因子与温度T的关系表明MZO/CdTe太阳电池在室温条件下即存在隧穿增强的界面复合机制,而CdS/CdTe电池中该复合机制仅在温度低于240K时才发挥作用。电容-频率(C-f)测试以及太阳电池电容模拟(solar cell capacitance simulator,SCAPS)结果表明CdTe电池中对体材复合起主要作用的是位于CdTe禁带中央的深能级缺陷。SCAPS模拟得到的MZO/CdTe太阳电池中CdTe禁带中央的缺陷浓度和PN结界面复合速率都比CdS/CdTe电池高,与实验结果相符,验证了 MZO/CdTe电池严重的效率损失是由于电池中严重的复合阻碍了电子传输导致的。第四章,使用金属氧化物对CdTe表面进行钝化,以减少CdTe薄膜表面复合,延长少子寿命,同时对钝化机理进行了研究。使用TRPL测量了表面沉积NiO、V2O5和Al2O3三种金属氧化物后CdTe中的少数载流子寿命。结果表明3 nm V2O5或Al2O3对CdTe有钝化作用,而3 nm NiO对CdTe没有钝化作用。XPS测试表明表面的CdTe在覆盖氧化物后会被氧化或还原,化学钝化可能是引起CdTe少子寿命增加的原因。而C-V测试表明沉积在CdTe表面的V2O5和Al2O3中存在一定量的固定负电荷。固定负电荷能够在CdTe中产生诱导电场,加速载流子分离,降低复合,从而增大少子寿命。通过对TRPL结果进行数值模拟,验证了 CdTe中少子寿命增加是由于V2O5和Al2O3与CdTe接触形成的诱导电场作用,而不是化学钝化作用。通过改变TRPL测试中激发光的入射条件,以及沉积在CdTe表面的Al2O3厚度,进一步验证了该结论。第五章,总结了本论文的主要研究内容,对CdTe太阳电池未来的研究方向进行了展望。更多还原

【Abstract】 CdTe is a stable Ⅱ-Ⅵ binary compound with a band gap of 1.45 eV at room temperature and a high absorption coefficient of 105cm-1 in the visible range,making it an ideal photovoltaic material.CdTe solar cells have become one of the most competitive thin film photovoltaic because of its high conversion efficiency,excellent long-term stability and low cost.Over the past few decades,CdTe solar cells have been successfully commercialized.The theoretically predicted efficiency for CdTe thin film solar cell is 32.1%.The record power conversion efficiency(PCE)of CdTe solar cell and module have reached 22.1%and 18.6%,respectively.Although CdTe solar cells have been a great success,there are still some problems to be solved,for example,the effects of band alignment on carrier transport at the interface of heterojunction,the carrier recombination at heterojunction interface and in CdTe bulk region,the influence of low minority carrier lifetime on open circuit voltage in a cell,preparation of stable back contact with low resistance,etc.This thesis focuses on the following aspects of CdTe solar cells:In Chapter Ⅰ,we briefly introduced the background and development history of solar cells.And the working principle and output characteristics of solar cells based on semiconductor PN junction were also discussed.Finally,the development process,cell structure,material properties and carrier recombinantion processes in the CdTe thin film solar cells were discussed.In Chapter II,the traditional CdS/CdTe solar cells,new type MgxZn1-xO/CdTe(MZO/CdTe)and MZO/CdSe/CdTe solar cells were fabricated.During the sputtering of CdS films,O2 was introduced.And therefore,a CdS:O with larger band gap was obtained,which improved the blue light response of the cells.The CdTe thin films were deposited on the MZO substrates by using close-space sublimation(CSS)in oxygenfree ambinent.The substrate and source temperature were optimized to obtain CdTe thin film with grain size of 1～2 μm.And MZO/CdTe cells with higher efficiency were obtained.The CdSe layer was also introduced to increase the light absorption of the cell in the long-wave region.By optimizing the CdSe layer thickness,an MZO/CdSe/CdTe solar cell with a maximum short-circuit current of 27.1 mA/cm2 and a conversion efficiency of 14.00%was obtained.In addition,ZnTe:Cu films were used as a buffer layer and were prepared by thermal evaporation and rapid thermal processing.The preparation process was optimized,and it was found that when the thickness of ZnTe was more than 100 nm,the series resistance of the solar cell would increase and the short-circuit current would decrease.A sandwich structure of 2 nm Cu/100 nm ZnTe/3 nm Cu could ensure moderate doping of Cu to CdTe and avoid excessive Cu diffusion into CdTe.Finally,the CdS/CdTe solar cell prepared with 100 nm ZnTe showed low reverse saturation current,resulting in high open circuit voltage,fill factor and a maximum conversion efficiency of 16.02%.In Chapter Ⅲ,the performance of MZO/CdTe and CdS/CdTe solar cells was compared.It is found that the efficiency loss of MZO/CdTe caused by voltagedependent photocurrent collection was more serious.In this study,the efficiency loss in CdS/CdTe cell with conversion efficiency of 13.80%was 1.53%,while that in MZO/CdTe cell with conversion efficiency of 12.67%was as high as 3.89%.In order to explore the reason of the serious loss of efficiency,the minority carrier lifetime of the two solar cells was detected by time-resolved Photoluminescence spectrum(TRPL).It is found that the minority carrier lifetime of CdTe in MZO/CdTe structure is low.The low minority carrier lifetime is due to serious recombination of carriers at the interface of MZO/CdTe PN junction,which leads to a high open-circuit voltage loss.According to the relationship between the open-circuit voltage and the J-V test temperature T,the activation energy of MZO/CdTe and CdS/CdTe solar cells was 1.08 eV and 1.36 eV,respectively,indicating that bulk Shockley-Read-hall(SRH)recombination in CdTe films in the CdS/CdTe played a dominant role.However,the MZO/CdTe cell was dominated by PN junction interface recombination.In addition,the relationship between cell ideal factor and temperature indicated that the tunneling enhancement interface recombination existed in MZO/CdTe solar cells even at room temperature,while it only affected the performance of CdS/CdTe solar cells when the temperature was below 240 K.The results of capacitance frequency(C-F)test and solar cell capacitance simulator(SCAPS)simulation showed that the deep level defect located in the center of the forbidden band played the main role in the bulk recombination of CdTe cells.The simulated defect concentration and interface recombination velocity in MZO/CdTe solar cells were higher than those in CdS/CdTe cells,which were consistent with the experimental results.And the serious efficiency loss of MZO/CdTe cells was caused by the serious recombination in the cells that hindered electron transport.In Chapter Ⅳ,the metal oxides were used to passivate the CdTe surface,decrease the surface recombination and increase the minority carrier lifetime.The mechanism of the passivation was studied.The minority carrier lifetimes in CdTe thin films coated with NiO,V2O5 and Al2O3 were measured by TRPL.In CdTe thin films coated with a 3-nm-thick V2O5 or Al2O3 layer,the TRPL lifetime was increased compared to that of the bare CdTe or NiO-coated CdTe thin films.The CdTe on the surface was oxidized or reduced after metal oxides deposited as the XPS results indicated.Chemical passivation may be the reason for the increase of CdTe minority carrier lifetime.The C-V measurements showed fixed negative charges were exsited in V2O5 and Al2O3 films.These charges could induce electric filed in the CdTe films which helped accelerate carrier separation,reduce recombination and increase minority carrier lifetime.According to the TRPL simulation results,the mechanism for surface passivation of CdTe by V2O5 and Al2O3 was investigated.And the results revealed that the increased carrier lifetime was caused by field effect rather than the chemical passivation.The conclusion was verified by changing the incident condition of TRPL measurements and the thickness of Al2O3 deposited on the CdTe surface.In Chapter Ⅴ,the main resarches of this thesis were summarized.The future research direction of CdTe solar cells were prospected.更多还原