【作者】 郑根华；

【导师】 王德亮；

【作者基本信息】 中国科学技术大学 ， 凝聚态物理， 2020， 硕士

【摘要】 碲化镉(CdTe)薄膜太阳电池是目前最具有潜力的薄膜太阳电池之一,其具有成本低、温度系数小和弱光性能优异等优势,并且近年来转化效率迅速提升,具有重要的研究与应用价值。CdTe的禁带宽度为1.45 eV,处于太阳电池材料理想的禁带宽度范围内。CdTe为直接带隙半导体材料,对于可见光的吸收系数高达105 cm-1,是非常优异的太阳电池光吸收材料之一,2 μm的厚度就能够吸收99%的能量高于其禁带宽度的光子。CdTe太阳电池的实验室转化效率最高已经达到22.1%,大面积组件效率也达到了 19%。然而,理论上单结CdTe太阳电池的转化效率为29%,CdTe太阳电池依然存在许多关键问题需要研究与解决。传统硫化镉(CdS)窗口层带来510 nm以下波长光子的损失,从而造成太阳电池的短路电流降低;新型掺镁氧化锌(MZO)窗口层与CdTe界面存在能带匹配和界面形成等问题,造成太阳电池J-V曲线出现“S-kink”弯折现象。本文对改善传统CdS/CdTe太阳电池性能和对新型窗口层MZO与CdTe界面的能带调控进行了研究。第一章,介绍了太阳电池的背景与历史,简述了太阳电池的结构与工作原理,概述了 CdTe太阳电池的制备工艺与发展现状。第二章,改善传统CdS/CdTe太阳电池性能的研究。CdS因其禁带宽度为2.4 eV,对510 nm以下波长的可见光具有强烈吸收。通过减薄CdS窗口层厚度,减少了可见光在窗口层处的损耗,使得更多光子能够到达CdTe光吸收层。从不同厚度CdS的透射谱中可以看出,随着CdS厚度减薄,510 nm以下波长的透过率明显上升。实验结果显示随着CdS厚度的减薄,电池短路电流持续上升。从EQE曲线可以看出,在510 nm波长以下EQE曲线响应明显上升。但随着CdS薄膜厚度进一步减薄,电池漏电增大,从而限制了电池转换效率的继续上升。CdS薄膜禁带宽度随着掺杂氧的含量增加而增加,对CdS进行掺氧可以使得CdS禁带宽度增大,从而减少CdS窗口层对可见光中短波光子的吸收。本文通过提高CdS掺氧量至7%,制备出了转化效率高达16.4%的CdS/CdTe太阳电池。第三章,MZO/CdTe电池的界面钝化与能带调控。实验制备了含有不同氧化物钝化层的电池,发现Al2O3薄膜加重了电池J-V曲线中的“S-kink”现象,使得电池性能下降,而SnO2薄膜明显提高了电池开路电压。通过时间分辨光致发光(TRPL)测试了 Al2O3和SnO2钝化层对MZO/CdTe电池少数载流子寿命的影响。Al2O3具有明显钝化作用,SnO2钝化效果不明显,表明了钝化作用不是电池开路电压提高的根本原因。X射线光电子能谱(XPS)结果显示,SnO2与CdTe界面存在较小的导带带阶,减小了电池反向饱和电流,使得电池开路电压得到提高。Al2O3与CdTe界面存在2.83 eV的导带带阶,较大的导带带阶阻碍了载流子的传输,造成电池J-V曲线中“S-kink”现象加重,使得电池性能下降。通过适度增加SnO2薄膜厚度,电池串联电阻下降,填充因子增大,“S-kink”现象得到改善。第四章,高温衬底法制备CdTe太阳电池。通过提高CdTe薄膜生长过程中衬底温度,改善CdTe薄膜结晶质量。实验表明通过高温衬底法制备出的CdTe晶粒更大。通过不同方法制备MZO/CdTe结构电池,表明使用高温衬底法制备的MZO/CdTe电池具有更高的转换效率,EQE曲线也表明了 CdTe薄膜结晶质量变好。高温衬底法对MZO/CdSe/CdTe和MZO/CdS/CdSe/CdTe结构电池性能也有一定的改善。第五章,总结了本论文的研究内容与主要结论,对CdTe太阳电池的未来与发展进行了展望。更多还原

【Abstract】 Cadmium telluride(CdTe)thin-film solar cell is one of the most promising thin-film solar cells.It has the advantages of low cost,small temperature coefficient and excellent weak light performance.In recent years,the cell efficiency has been increased rapidly.The band gap of CdTe thin films is 1.45 eV,which is within the range of the ideal band gap of solar cell materials.CdTe is a direct bandgap semiconductor material,and its absorption coefficient for visible light is as high as 105 cm-1.2-μm-thick CdTe film can absorb 99%of the photons with energy higher than its band gap.CdTe is one of the excellent solar cell materials.The highest laboratory efficiency of CdTe solar cells has been reached 22.1%,and the efficiency of large-area modules is as high as 19%.However,the theoretical limit efficiency of single-junction CdTe solar cell is 29%,and there are still many key issues that need to be studied and solved.The traditional cadmium sulfide(CdS)window layer has photon loss of wavelengths below 510 nm,which leads to a reduction in the short-circuit current of CdS/CdTe solar cell.Magnesium-doped zinc oxide(MZO)window layer has band mismatch with CdTe,and causes serious "S-kink" in the solar cell J-V curves.In this thesis,we studied the improvement of the performance of traditional CdS/CdTe solar cells and the band alignment at the MZO/CdTe interface.In Chapter Ⅰ,we introduced the background and history of solar cells,briefly described the structure and working principle of solar cells.We also summarized the preparation and development of CdTe thin film solar cells.In Chapter Ⅱ,we optimized the performance of traditional CdS/CdTe thin film solar cells.CdS thin film can strongly absorb the visible light below 510 nm,due to its band gap of 2.42 eV.By reducing the thickness of the CdS window layer,the loss of visible light in the window layer was reduced.More photons can reach the CdTe layer.From the transmission spectrum of the CdS films with different thicknesses,it can be seen that the transmittance of wavelengths below 510 nm was increased significantly as the thickness of CdS was decreased.The experimental results show that as the thickness of CdS was decreased,the short-circuit current of cells was increased.It can be seen from the EQE curves that the EQE response were increasesd significantly below the 510 nm wavelength.However,as the thickness of the CdS film becomes thinner,cell current leakage became more serious,and the cell efficiency was no longer increased.The band gap of the CdS film can be increased as the content of doped oxygen is increased.Doping the CdS with oxygen can increase the band gap of the CdS films,thereby reducing the absorption of short-wave photons in the visible light range.By increasing the amount of oxygen to 7%,we got the highly efficient CdS/CdTe solar cell with an efficiency as high as 16.4%.In Chapter Ⅲ,we studied the band alignment at MZO/CdTe interface.Time-resolved photoluminescence spectroscopy spectra show that the minority carrier lifetime of the MZO/Al2O3/CdTe structure is much longer than the MZO/CdTe structure,which indicates that Al2O3 has strong passivation effect on CdTe.However,CdTe solar cells with Al2O3 as the passivation layer exhibited serious "S-kink" in the J-V curves.The large conduction band offset as high as 2.83 eV between Al2O3 and CdTe was found to be the main reason of the "S-kink" for the solar cells with Al2O3 as passivation layer.The conduction band offset between SnO2 and CdTe is only 0.21 eV.The conduction band offset between SnO2 and CdTe can be adjusted by controlling the thickness of the SnO2 films.With the increase of SnO2 film thickness,the VOC was decreased and FF was increased.The "S-kink" was weakened by increasing the thickness of the SnO2 films.In Chapter Ⅳ,we fabricated CdTe solar cells by using high temperature substrate method.By increasing the substrate temperature during the CdTe film growth,the crystal quality of the CdTe film was improved.Experiments showed that large CdTe grains can be prepared by high temperature substrate method.The results show that the MZO/CdTe cell prepared by using the high temperature substrate method has higher efficiency.The EQE curve also shows that the crystalline quality of the CdTe thin film becomes better.The high-temperature substrate method also improves the performance of MZO/CdSe/CdTe and MZO/CdS/CdSe/CdTe solar cells.In Chapter V,we summarized the research work and the main conclusions of this thesis,and prospected the future and development of CdTe thin film solar cells.更多还原