影响硫化镉/碲化镉性能的关键制备技术探讨

CdTe多晶薄膜具有接近理想光谱响应状态的禁带宽度(1.45eV)和高达10-5cm-1的光吸收系数,是较具前途的光伏材料,而CdTe薄膜太阳能电池由于具有低的制造成本和高的转换效率,使其成为最具发展前途的薄膜太阳能电池。CdS和CdTe膜层的质量对CdTe薄膜太阳能电池的性能有很大影响,如何完善CdS和CdTe的制备技术,获得高质量的膜层,引起了广泛关注。笔者综述了影响CdS和CdTe质量的关键制备因素,提出了一些新的制备方法。     

CdTe多晶薄膜制备及后处理对CdS/CdTe界面的影响

采用近空间升华法分别在玻璃、CdS及CdS1-xTex衬底上沉积了CdTe多晶薄膜,通过原子力显微镜的观察和X射线衍射的分析,比较了它们的微结构。结果表明,用CdS和CdS1-xTex多晶薄膜作为衬底沉积的CdTe多晶薄膜结构与衬底相似,具有(111)面择优取向。通过对在不同氧分压下进行后处理的CdS/CdTe薄膜的断面及光能隙的研究,发现在氮氧(4∶1)气氛下后处理的薄膜CdS层明显减薄,这样的结果有利于改善CdTe太阳电池的光谱响应,增加载流子收集。我们认为氧在退火中促进了CdS/CdTe界面互扩散,扩散的结果不仅弥补了CdS、CdTe间的晶格失配,而且降低了界面的位错密度,并获得了面积为0.52 cm2,转换效率为13.38%的CdTe多晶薄膜电池。     

溅射制备CdS薄膜及其在CdTe电池中的应用

采用射频磁控溅射方法分别在石英玻璃和TCO玻璃上制备了不同厚度的CdS薄膜,研究了CdS薄膜厚度对薄膜结构和光学性质的影响。在不同厚度的CdS/TCO衬底上,进一步制备成CdTe薄膜太阳电池。结果表明,CdS薄膜厚度的增加有利于薄膜的生长与结晶;110nm的薄膜具有禁带宽度为2.41eV的最大值;测试电池性能,得到CdS厚度为110nm的电池具有11.42%的最高转换效率。     

CdTe/CdS太阳电池光照和电子辐照研究

采用近空间升华法制备CdTe多晶薄膜,以ZnTe/ZnTeCu复合多晶薄膜作为背接触层,获得了转换效率为13.38%的CdTe/CdS太阳电池.用光强为100mW/cm2的卤钨灯对电池光照7天后,发现电池性能无明显变化.经能量为1.6MeV,辐照剂量为1013～1015电子/cm2的电子束辐照后,电池性能有不同程度的衰降,经真空150℃退火30min后,电池性能恢复到接近辐照前的水平.     

衬底温度对碲化镉薄膜性质及太阳电池性能的影响

蒸汽输运法是制备高质量且大面积均匀的CdTe薄膜的一种优良的方法。采用自主研发的一套蒸汽输运沉积系统制备了CdTe多晶薄膜, 并研究了衬底温度对CdTe薄膜性质及太阳电池性能的影响。利用XRD、SEM、UV-Vis和Hall等测试手段研究了衬底温度对薄膜的结构、光学性质和电学性质的影响。结果表明, 蒸汽输运法制备的CdTe薄膜具有立方相结构, 且沿(111)方向高度择优。随着衬底温度的升高(520℃~640℃), CdTe薄膜的平均晶粒尺寸从2 μm增大到约6 μm, CdTe薄膜的载流子浓度也从1.93×10¹⁰ cm^-3提高到2.36×10¹³ cm^-3, 说明提高衬底温度能够降低CdTe薄膜的缺陷复合, 使薄膜的p型更强。实验进一步研究了衬底温度对CdTe薄膜太阳电池性能的影响, 结果表明适当提高衬底温度, 能够大幅度提高电池的效率、开路电压和填充因子, 但是过高的衬底温度又会降低电池的长波光谱响应, 导致电池转换效率的下降。经过参数优化, 在衬底温度为610℃、无背接触层小面积CdTe薄膜太阳电池的转换效率达到11.2%。     

硝磷酸腐蚀的CdTe太阳电池性能

CdTe薄膜的腐蚀是制作CdS/CdTe光伏电池的重要技术之一,本实验采用硝磷酸溶液（硝酸1%＋磷酸70%＋去离子水29%）腐蚀CdTe薄膜,通过XRD测试发现在CdTe膜上生成了碲层.随后,在腐蚀后的CdTe薄膜上分别沉积了几种结构的背接触层,并制备出相应结构的CdTe太阳能光伏电池.通过电池的光、暗I-V和C-V特性测试,以ZnTe/ZnTe：Cu/Ni为背接触的小面积太阳电池,其性能优于其它背接触的电池.实验结果表明器件性能与碲的生成和铜的扩散密切相关.     

近空间升华沉积CdTe多晶薄膜过程的研究

研究了近空间升华（CSS）沉积CdTe多晶薄膜的物理机制,测量了近空间沉积装置内的温度分布,分析了升温过程、气压与薄膜的初期成核的关系,优化了升温过程,在此基础上制备出了转换效率优良的结构为SnO2：F／CdS／CdTe／Au的串联集成太阳电池。同时结果表明：近空间升华制备TdTe多晶薄膜的物理过程主要是,Cd代升华前升温,Cd代源升华分解为Cd、Te2, Cd、Te2在衬底上化合沉积以及衬底上的CdTe反升华等过程。在正常的近空间升华过程中,CdTe的蒸汽压远小于保护气体的气压。后者对对成核的晶粒方向几乎没有影响,但它通过改变分子平均自由程来影响Cd、Te2分子的扩散,从而影响薄膜的生长速率。     

磁控溅射法制备的CdS:Al薄膜的性质研究

采用Al和CdS双靶共溅射的方法, 调控Al和CdS源的沉积速率, 制备出不同Al掺杂浓度的CdS:Al薄膜。通过XRD、SEM、AFM、紫外-可见透射光谱分析、常温霍尔测试对CdS: Al薄膜的结构、形貌、光学和电学性质进行表征。XRD结果表明, 不同Al掺杂浓度的CdS:Al薄膜均为六方纤锌矿结构的多晶薄膜, 并且在(002)方向择优生长。SEM和AFM结果表明, CdS:Al薄膜的表面均匀致密, 表面粗糙度随着Al掺杂浓度的增加略有增加。紫外-可见透射光谱分析表明, CdS:Al薄膜禁带宽度在2.42~2.46 eV 之间, 随着Al掺杂浓度的增加而略微减小。常温霍尔测试结果证明, 掺Al对CdS薄膜的电学性质影响显著, 掺Al原子浓度3.8%以上的CdS薄膜, 载流子浓度增加了3个数量级, 电阻率下降了3个数量级。掺Al后的CdS薄膜n型更强, 有利于与CdTe形成更强的内建场, 从而提高太阳电池效率。用溅射方法制备的CdS:Al薄膜的性质适合用作CdTe薄膜太阳电池的窗口层。     

CuxTe薄膜的结构及其对CdTe太阳电池性能的影响

用真空共蒸发法制备了CuxTe薄膜并将其运用于CdTe太阳电池中.对薄膜进行了X射线衍射(XRD)分析,比较了有、无CuxTe插层的CdTe太阳电池的暗态,I-V特性和C-V特性.结果表明,刚沉积的薄膜非晶结构占主导地位,只有部分Cu/Te配比较低的薄膜出现多晶结构.CuxTe插层的引入有利于消除roll over(暗态I-V曲线饱和)现象,使电池的二极管理想因子和暗饱和电流密度降低,CdTe掺杂浓度增加,有效地改善了CdTe太阳电池的性能.用CuxTe薄膜作为背接触层,获得了效率为12.5%的CDS/CdTe小面积(0.0707cm2)太阳电池.     

近距离升华制备CdTe掺Te薄膜的结构与电性能研究

采用近距离升华(Close-Spaced-Sublimation,CSS)技术制备CdTe及掺Te薄膜.并利用XRF、XRD、SEM及Hall系统研究了其含量、结构、表面形貌和电性能.结果表明,CSS技术制备的CdTe薄膜晶形好,晶粒度较RF方法制备的薄膜增大约100倍.Te掺入CdTe薄膜后,改变了CdTe膜的结晶特性,适当掺入Te可以促进CdTe晶格的生长,并导致Cdrre膜晶格常数变大.薄膜面电阻率降低,面载流子浓度增大,以及载流子迁移率的增大,表明掺杂Te后CdTe膜的电导性能大大改善.     

Annealing effects on the chemical deposited CdS films and the electrical properties of CdS/CdTe solar cells

CdS layers grown by chemical bath deposition (CBD) are annealed in the oxygen and argon-hydrogen atmosphere respectively. It has been found that the open circuit voltage of the CdS/CdTe solar cell increases when the CBD CdS is annealed with oxygen before the deposition of CdTe by close spaced sublimation (CSS), while the performance of the solar cell decreases when the CBD CdS is annealed with argon-hydrogen. Electronic properties of the CdS films are investigated using X-ray photo-electron spectroscopy (XPS), which indicates that the Fermi level is shifting closer to the conduction band after annealing in the oxygen and consequently a higher open circuit voltage of the solar cell can be obtained.     

CdTe thin film solar cells: Interrelation of nucleation, structure, and performance

The performance of CdTe solar cells as prototype of thin film solar cells strongly depends on film morphology. The needs for high solar cell performance using thin film materials will be addressed covering nucleation and growth control of thin film materials. In order to understand the basic growth mechanisms and their impact on cell performance, we have systematically investigated the growth of CdTe thin films by Close Spaced Sublimation (CSS) using the integrated ultra-high vacuum system DAISY-SOL. CdTe thin films were deposited on TCO/CdS substrates (transparent conductive oxide) held at 270 °C to 560 °C. The properties of the films were determined before and after CdCl₂ treatment using X-ray diffraction and electron microscopy. In addition, solar cells were prepared to find correlations between material properties and cell efficiency. At low sample temperature the films tend to form compact layers with preferred (111) orientation which is lost at elevated temperatures above 450 °C. For CdS layers without (0001) texture there is in addition a low temperature regime (350 °C) with (111) texture loss. After activation treatment the (111) texture is lost for all deposited layers leading to strong recrystallisation of the grains. But the texture still depends on the previous growth history. The loss of (111) texture is evidently needed for higher performance. A clear correlation between cell efficiency and the texture of the CdTe film is observed.     

Physical properties of Bi doped CdTe thin films grown by CSVT and their influence on the CdS/CdTe solar cells PV-properties

The physical properties of Bi doped CdTe films, grown on glass substrates by the Closed Space Transport Vapour (CSVT) method, from different Bi doped CdTe powders are presented. The CdTe:Bi films were characterized using Photoluminescence, Hall effect, X-Ray diffraction, SEM and Photoconductivity measurements. Moreover, CdS/CdTe:Bi solar cells were made and their characteristics like short circuit current density (J_sc), open circuit voltage (V_OC), fill factor (FF) and efficiency (η) were determined. These devices were fabricated from Bi doped CdTe layers deposited on CdS with the same growth conditions than those used for the single CdTe:Bi layers. A correlation between the CdS/CdTe:Bi solar cell characteristics and the physical properties of the Bi doped CdTe thin films are presented and discussed.     

Comparative study of CdS/CdTe cells fabricated with and without evaporated Te-layer

Thin film CdS/CdTe solar cells were fabricated by Close space Sublimation (CSS) at the Moldova State University using a Te layer at the back contact deposited by thermal evaporation. The influence of the Te layer thickness and a cadmium chloride solution treatment of the CdTe layer before and after Te layer deposition are studied. A proper comparison of the two CdS/CdTe solar cells shows that the cell with the Te layer has a higher current because of the higher blue photosensitivity, but has a lower open circuit voltage in relation to the absorber band gap.     

Comparison between the effects of CdCl2 heat treatment on CdTe films prepared by RF magnetron sputtering and close spaced sublimation methods

CdTe films were prepared on Fluorine-doped tin oxide substrate by RF magnetron sputtering and close spaced sublimation (CSS) methods, respectively. These CdTe films were then treated with a wet CdCl₂ heat process at different temperatures. The structural and optical properties of CdTe films were investigated by X-ray diffraction, scanning electron microscope and UV–Visible spectrophotometer. The results reveal that both types of CdTe films have a better crystalline and larger grain size after CdCl₂ heat treatment. However, the (422) peak has a more preferential orientation than (511) peak after CdCl₂ activation for CdTe films prepared by sputtering method, while these two peaks almost have the same intensity for CSS-prepared CdTe films. The transmittance of CdTe films prepared by CSS is apparently lower than sputtered CdTe films. Correspondingly, the efficiency of solar cells with CSS-prepared CdTe is 7.3, 2.6 % of sputtered CdTe films.     

Effects of CdCl2 in CdTe on the properties of sintered CdS/CdTe solar cells

Sintered CdS films on glass substrates with low electrical resistivity and high optical transmittance have been prepared by a coating and sintering method. All-polycrystalline CdS/CdTe solar cells with different microstructures and properties of the CdTe layer were fabricated by coating a number of CdTe slurries, which consisted of cadmium and tellurium powders, an appropriate amount of propylene glycol and various amounts of CdCl₂, on the sintered CdS films and by sintering the glass-CdS-(Cd + Te) composites at various temperatures. The presence of more than 5 wt% of CdCl₂ in the (Cd + Te) layer enhances the sintering of the CdTe film and the junction formation by a liquid-phase sintering mechanism. A low sintering temperature results in poor densification of the CdTe layer and the CdS-CdTe interface, whereas a high sintering temperature results in a deeply buried homojunction. The optimum temperature for the sintering of the CdTe layer and for junction formation decreases with increasing amount of CdCl₂. All-polycrystalline CdS/CdTe solar cells with an efficiency of 10.2% under solar irradiation have been fabricated by a coating and sintering method using cadmium and tellurium powders for the CdTe layer.     

Dependence of photovoltaic performance of solvothermally prepared CdS/CdTe solar cells on morphology and thickness of window and absorber layers

In the present work a new strategy for straightforward fabrication of CdS/CdTe solar cells, containing CdS nanowires and nanoparticles as a window layer and CdTe nanoparticles and microparticles as an absorber layer, are reported. CdS and CdTe nanostructures were synthesized by solvothermal method. X-ray diffraction analysis revealed that highly pure and crystallized CdS nanowires and nanoparticles with hexagonal structure and CdTe nanoparticles with cubic structure were obtained. Atomic force microscope and field emission scanning electron microscope images showed that CdS nanowires with length of several μm and average diameter of 35 nm, CdS nanoparticles with average particle size of 32 nm and CdTe nanoparticles with average particle size of 43 nm, were uniformly coated on the substrate by the homemade formulated pastes. Based on ultraviolet–visible absorption spectra, the band gap energies of CdS nanowires, CdS nanoparticles and CdTe nanoparticles were calculated 2.80, 2.65 and 1.64 eV, respectively. It was found that, the photovoltaic performance of the solar cells depends on thickness of CdTe and CdS films, reaching a maximum at a specific value of 6 μm and 225 nm, respectively. For such cell made of CdS nanowires and CdTe nanoparticles the V_OC, J_SC, fill factor and power conversion efficiency were calculated 0.62 V, 6.82 mA/cm², 59.7 and 2.53 %, respectively. Moreover, photovoltaic characteristics of the solar cells were dependent on CdTe and CdS morphologies. CdS/CdTe solar cell made of CdTe and CdS nanoparticles had the highest cell efficiency (i.e., 2.73 %) amongst all fabricated solar cells. The presented strategy would open up new concept for fabrication of low-cost CdS/CdTe solar cells due to employment of a simple chemical route rather than the vapor phase methods.     

Effect of interdiffusion and impurities on thin film CdTe/CdS photovoltaic junctions

We have used low temperature photoluminescence (PL) to study thin film CdTe/CdS solar cell structures. The devices were produced by close space sublimation (CSS) and have undergone a post-growth treatment, a vital step in increasing device efficiency. The treatment consisted of evaporating a thin layer of CdCl₂ onto the back CdTe surface and heat treating in air at 400 °C for between 10 and 120 min. This produced a range of device efficiencies from 2% to 9%. The efficiency improvements are the result of a complex interaction between the CdCl₂, impurities and sulfur interdiffusion. The structures were prepared for PL by a chemical bevel etching technique which allows the luminescence emission to be studied as a function of depth throughout the sample. The main features in the PL spectra have been identified as being due to the Cl-A center and the Te-dislocation-related Y luminescence band. Using PL we have quantified the S diffusion into the CdTe which has a maximum of 20% at the interface in the most efficient samples. We have also obtained the profiles of recombination and non-radiative recombination centers in the device. We observe correlations between impurity centers and device efficiency which can help explain the effects of the CdCl₂ treatment on the optoelectronic properties of the CdTe/CdS junction.     

Study of in situ CdCl2 treatment on CSS deposited CdTe films and CdS/CdTe solar cells

Effect of in situ CdCl₂ treatment on the morphological, structural and electrical properties of CdTe films as well as on solar cell characteristics of CdS/CdTe junction has been investigated. XRD measurements show that the presence of CdCl₂ vapours induces 111 oriented growth in the CdTe films. CdCl₂ concentration required for this oriented growth is found to be directly proportional to the substrate temperature. SEM measurements show enhanced grain growth in the presence of CdCl₂. Spectral response of the CdCl₂ treated CdS/CdTe solar cells shows an enhanced CdS diffusion in to the CdTe, which results in an improved spectral response in UV range and a consequent reduction in the interface states density. A drastic reduction in the deep levels due to the CdCl₂ treatment, as seen in the photo-capacitance studies, has results in CdS/CdTe solar cells having efficiency >8%.     

Effects of junction formation conditions on the photovoltaic properties of sintered CdS/CdTe solar cells

Microstructures and properties of sintered CdS films on glass substrates and sintered CdTe films on polycrystal CdS substrates have been investigated. The CdS films, which contained 9 wt % CdCl₂ as a sintering aid and were sintered at 650° C for 1 h in nitrogen, are transparent and have an average grain size of 15m and an electrical resistivity of 0.5cm. The CdTe films, which were coated on the sintered CdS substrate and were sintered above 610° C for 1 h in nitrogen, have a dense structure with an average grain size larger than 5m. All polycrystal CdS/CdTe solar cells were fabricated by this successive coating and sintering method. The sintering temperature of CdTe films on the sintered CdS films was varied from 585 to 700° C. Compositional interfaces and p-n juctions are formed during sintering. The highest solar efficiency (7.18%) was found in a solar cell made by sintering the composite layer of glass-CdS-CdTe at 625° C for 1 h. A fabrication temperature below 610° C resulted in poor solar cell efficiencies due to the porous structure of the CdTe films and above 650° C also resulted in poor efficiencies due to the formation of a CdS_1-x Te_x layer at the interface and a large p-n junction depth.