CdTe/CdS异质结太阳电池的电特性--关于"CdTe/CdS异质结特性"一文同郭江等商榷

笔者认为郭江等关于“CdTe／CdS异质结特性”一文，对CdTe／CdS太阳电池的CdS／CdTe异质结特性的讨论时存在理论上的错误和设计思想不妥。例如，求解CdTe／CdS异质结空间电荷区电势满足的泊松方程时，所给的边界条件有错，因而所得的电势分布亦有错；关系式△Er=(Eg2-Eg1)-△Ec=0有错；CdTe／CdS异质结能带结构不妥等。据此，我们针对上述CdS／CdTe异质结太阳电池存在的问题进行了理论研究和讨论，所得到的相应研究结果，与郭江等商榷。     

CdTe多晶薄膜太阳电池的结构改进

采用共蒸发法制备了ZnTe：Cu和Cd1-xZnxTe多晶薄膜，研究了薄膜的结构和性能。获得了Cd1-xZnxTe多晶薄膜的光能隙与锌含量的关系，ZnTe：Cu多晶薄膜光能隙随着掺Cu浓度的增加，光能隙减小。分别用ZnTe／ZnTe：Cu和Cd1-xZnxTe／ZnTe：Cu复合层作为背接触层，既能修饰异质结界面，改善电池的能带结构，又能防止cu原子向电池内部扩散。获得了面积0．5cm^2，转换效率为13．38％的CdTe多晶薄膜太阳电池。     

CdS/CuInSe_2异质结薄膜太阳电池

用蒸发方法制备CdS/CuInSe_2(CdS/CIS)异质结薄膜太阳电池。在Mo/玻璃或Mo/Al_2O_3衬底上,用双源法蒸发CIS多晶材料+Cu或CIS多晶材料+Se,通过控制对黄铜矿结构的微小偏离,淀积电阻率不同的两层p-CIS层,然后用蒸发CdS方法,通过控制In的蒸发,在高阻p-CIS层上淀积两层电阻率不同的n-CdS层,并在低阻n-CdS层上蒸发Al栅和SiOx减反射膜,构成n-CdS/p-CIS异质结薄膜太阳电池。对电池的I—V特性和光谱响应特性进行了研究。     

高效率Ⅱ-Ⅵ族(CdS,CdSe,CdTe)量子点敏化太阳电池

简单总结了笔者研究组近三年在量子点敏化太阳电池方面的研究工作.通过发展一些简单可控的合成方法制备了一系列Ⅱ-Ⅵ族量子点敏化的高效率太阳电池.利用连接剂辅助化学浴沉积法,以巯基乙酸为连接剂一步水热制备了单分散CdTe/CdS或CdTe/CdS核壳结构量子点以及量子点敏化的TiO2电极,并分别获得了最高3.80％(CdTe/CdS)和2.83％(CdSe/CdS)的光电转换效率;利用旋涂法在氧化锌纳米线阵列表面依次沉积了CdS/CdSe量子点,并取得了3.45％的光电转换效率;首次利用原位电沉积法在由纳米棒和纳米颗粒共同组成的分等级TiO2微米球电极上直接沉积CdS及CdSe量子点,取得了4.8％的光电转换效率,并用强度调制光电流/光电压谱(IMPS/IMVS)对CdS、CdSe量子点敏化电池和CdS/CdSe量子点共敏化电池进行了动力学研究,该型电池的电子收集效率高达98％.     

近空间升华技术中气氛对CdTe薄膜的结构、性质的影响

发展近空间升华氩气氛沉积工艺可以降低制备成本.该文使用近空间升华技术,在不同氧浓度的氩氧气氛下沉积CdTe多晶薄膜.测量了薄膜的X射线衍射谱,计算了薄膜的织构系数Ci及其标准方差σ;测量了薄膜的透过率谱,计算了光能隙.研究了择优取向程度和光能隙随氧浓度的变化.结果表明,1)不同气氛下沉积的CdTe薄膜均为立方相结构.随氧浓度的增加;σ增加;氧浓度为6%时,σ最大;之后随氧浓度增加,σ降低,在12%达到最小,然后随氧浓度的增加而增加.在玻璃衬底上沉积的CdTe薄膜,比在CdS上沉积的具有更高程度的择优取向;2)CdTe薄膜的光能隙为1.50eV～1.51eV,氧浓度对其影响甚微.     

碲化锌复合背接触层对碲化镉太阳电池性能的影响

碲化镉太阳电池的流行结构为n-CdS/p-CdTe。通过对其能带结构的分析。采用ZnTe:Cu作背接触层。再在p-CdTe和ZnTe;Cu之间引入不掺杂的ZnTe作过渡层,以改进这种电池的结特性和载流子收集,从实验上对三种结构的CdTe太阳电池进行了对比研究。结果表明复合的ZnTe/ZnTe:Cu背接触层。的确能大幅度提高CdTe太阳电池的转换效率。这种结构的电池经电子205计量站测试转换效率已达11.6%。     

CdS／CdTe－CdS／Cu_2S异质二重结薄膜太阳电池的设计与计算

根据半导体材料的性能参数，考虑光电压Ｖ和耗尽区宽度Ｗ的变化对光电流ＪＬ的影响，较严格地计算了ＣｄＳ／ＣｄＴｅ和ＣｄＳ／Ｃｕ２Ｓ两种异质结单晶薄膜太阳电池的光伏特性曲线。然后在的条件下，对由上述两种异质结构成的二重结太阳电池的ＣｄＴｅ、Ｃｕ２Ｓ厚度进行匹配，计算各种组合下二重结太阳电池的光伏特性曲线。理论证明最佳匹配厚度Ｈｍａｘ约为９．０６μｍ，最大短路电流、开路电压、转换效率分别为１４．２２ｍＡｃｍ－２、１．３Ｖ和１４、６８％。     

真空气相沉积制备碲化镉薄膜的电学和光学特性

用真空气相9沉积法，同时蒸发Cd和Te材料，在玻璃衬底上沉积CdTe薄膜。对CdTe薄膜掺杂In，并氮气作保护气体，在不同温度和时间下对薄膜进行热处理，研究薄膜的电学特性和光学特性。结果表明，用真空气相沉积法制备的CdTe薄膜的电学、光学特性和由其他方法制备的CdTe薄膜的电学和光学特性基本一致。     

CdS/CdTe太阳电池中CdS多晶薄膜的结构和光学性能

采用XRD,AFM,XPS和光学透射谱对化学水浴法制备的CdS多晶薄膜进行了测试分析。刚沉积的CdS多晶薄膜均匀、透明、致密,主要呈现立方结构;Cd和S的原子百分比约为1 10;能隙(Eg)约为2 47eV。在不同温度下后处理会出现六方结构和3CdSO4·8H2O衍射峰,同时晶面择优取向发生了变化。通过沉积高质量的CdS薄膜,获得了效率约13 4%的CdS/CdTe小面积太阳电池。     

酞菁铜/氟代苝酰亚胺异质结性能研究

合成一种氟代n型有机半导体材料2,5-二氟代苯基苝酰亚胺(D25DFPP),制备CuPc/D25DFPP异质结,研究异质结的结构和性能,并将其引入ITO/PEDOT:PSS/CuPc/D25DFPP/Al结构光伏器件。结果表明,D25DFPP与CuPc的紫外-可见吸收光谱有较好的光吸收互补特性。通过紫外光电子能谱(UPS)研究CuPc/D25DFPP异质结的电子结构,发现在两者的界面处同时存在能带弯曲和界面偶极作用,异质结中发生较强的电荷转移,并且两者能级匹配。异质结在120℃下退火后有较大的晶粒和较强的荧光猝灭,使得退火后光伏器件的光电转换效率比未退火时提高1倍以上,达到0.82%,显示在光电转换领域有着潜在的应用前景。     

Recent technical advances in thin-film CdS/CdTe solar cells

CdS/CdTe solar cells have attracted attention recently for their potential as low-cost, high-efficiency solar cells of the future. It is because the CdTe layer (used for photoelectric conversion) has a bandgap energy of 1. 51 eV, which corresponds well to sunlight spectra, and the direct transition type energy band structure enables formation of thinner films.We have already industrialized CdS/CdTe solar cells in mass production stage using a printing-sintering process, as large-area modules for electric power generation(Higuchi , 1993, Omura , 1991), and as cells for indoor applications (primarily in calculators. Suyama , 1986). However, this solar cell has a conversion efficiency of approximately 6%.Recently, there has been considerable research into thin-film CdS/CdTe solar cells which have a thinner CdS film formed by CVD or CBD (Britt , 1993) process, and thus are photosensitive to light with wavelengths of 500 nm or less. At present stage of our art, in solar cells formed by the CSS with a CdTe film on CVD CdS, a conversion efficiency of 15. 05% has been obtained in cells with an area of 1 cm² (verified at JQA).     

Characteristics of nanocrystalline CdS films fabricated by sonochemical, microwave and solution growth methods for solar cell applications

Nanocrystalline CdS was synthesized by sonochemical and microwave methods. Characterization of nano-CdS was carried out by optical absorption, X-ray diffraction, TEM, SEM and EDX. A hyperbolic band model was used to calculate a shift in energy over that of the band gap of bulk CdS. Colloidal films comprising of nanocrystalline CdS were fabricated by the dip coating method. A comparison of optical absorption of CdS films on indium tin oxide (ITO)/glass by sonochemical and microwave methods was made with solution grown CdS films on ITO/glass. A blue shift in energy level at the nanoscale is demonstrated by optical absorption, and X-ray diffraction of nano-CdS shows a cubic structure. Electron microscopy studies with an FE-SEM and TEM show a particle size of 15 nm and diffraction patterns show a crystalline nature. Overall reduction in optical absorption due to blue shift is expected to result in higher performance, especially in short-circuit currents in CdS/CdTe solar cells.     

Preparation and characterization of thin films of electrodeposited CdTe semiconductors

Thin films of CdTe semiconductors were prepared by electrodeposition technique in aqueous solutions. The deposition mechanism was investigated by cyclic voltammetry. The potential regions for the formation of the n-CdTe and p-CdTe films were determined. The structure, composition and morphology characteristics of as-deposited thin films of CdTe grown on SnO₂/glass and CdS/SnO₂/glass were investigated by XRD, EDAX and SEM techniques. The optical properties were measured to determine the absorption coefficient and band gap values. The as-deposited CdTe films grown on SnO₂/glass contained free Te while those grown on CdS/SnO₂/Glass did not contain this phase. The CdTe has the cubic structure with strong (111) orientation. The EDAX analysis showed a nearly stiochiometric Cd:Te ratio. The band gap has a value of 1.48 eV, which is in a good accordance with those reported in the literature. The effect of annealing at 350 and 400°C after CdCl₂ treatment on the structure and morphology was also examined.     

Rectification in CdS/TCO bilayers

The origin of process-induced rectification in CdS/ITO and CdS/SnO₂ bilayers has been investigated. Both pre-treatment of the transparent conducting oxide (TCO) substrates and post-growth treatment of the bilayers were explored for both oxidising and reducing conditions. In/CdS/TCO structures were used for I-V testing, and the CdS layers were verified as being pinhole-free using a test employing a rectifying Au/CdS contact. Whilst neither pre-oxidation nor reduction of any TCO substrate failed to induce rectification in CdS/TCO, oxidation of CdS always induced rectification, regardless of the substrate type. This was attributed to oxidation of CdS (confirmed by Auger electron spectroscopy), and the results are consistent with a band diagram postulated for the CdO/CdS/ITO structure. Recommendations are made for the fabrication of CdTe/CdS/TCO solar cells.     

Growth and process optimization of CdTe and CdZnTe polycrystalline films for high efficiency solar cells

Polycrystalline CdTe solar cells with efficiencies of approximately 10% were achieved by metal organic chemical vapor deposition growth of CdTe on glass/SnO₂/CdS substrates. An in situ pre-heat treatment of the CdS substrate at 450 °C in an H₂ ambient was found to be essential for high performance devices because it removes oxygen-related defects on the CdS surface. This heat treatment also results in a cadmium-deficient CdS surface which may, in part, limit the CdTe cell efficiency to 10% owing to cadmium vacancy related interface defects. The CdCl₂ treatment used in CdTe cell processing was found to promote grain growth, reduce series resistance and interface state density, and change to dominant current transport mechanism from thermally assisted tunneling and recombination via interface states to recombination in the depletion region. These beneficial effects resulted in an increase in the CdTe/CdS cell efficiency from around 2% to approximately 9%. In addition to the CdTe cells, polycrystalline 1.7 eV CdZnTe films were grown by molecular beam epitaxy for tandem cell applications. CdZnTe/CdS cells processed using the standard CdTe cell fabrication procedure resulted in 4.4% efficiency, high series resistance, and a band gap shift to 1.55 eV. Formation of ZnO at and near the CdZnTe surface was found to be the source of high contact resistance. A saturated dichromate etch instead of the Br₂:CH₃OH etch prior to contact deposition was found to solve the contact resistance problem. The CdCl₂ treatment was identified to be the cause of the observed band gap shift owing to the preferred formation of ZnCl₂. A model for the band gap shift along with a possible solution using an overpressure of ZnCl₂ in the annealing ambient is proposed. Development of a sintering aid which promotes grain growth and preserves the optimum 1.7 eV band gap is shown to be the key successful wide gap CdZnTe cells.     

Structural and optical characterizations of CdTe on CdS grown by hot-wall vacuum evaporation

We report on characterizations of polycrystalline CdTe on CdS grown by hot-wall vacuum evaporation. The CdTe film grown on CdS/SnO₂/glass was compared with the other two CdTe films, which were grown directly on #7059 glass and on SnO₂-coated glass. The grown CdTe/CdS film is composed of a grain size 3–15 μm and have a close-packed structure compared to other CdTe films. A weak excitonic peak at 1.59 eV as well as two kinds of donor–acceptor pair emission bands has been observed in PL spectrum of CdTe/CdS film. On the other hand, the excitonic peak cannot be detected in other CdTe films.     

Can we improve the record efficiency of CdS/CdTe solar cells?

Polycrystalline thin film CdTe continues to be a leading material for the development of cost effective and reliable photovoltaic systems. The two key properties of this material are its near ideal band gap for photovoltaic conversion efficiency of 1.45 eV, and its high optical absorption coefficient. Thin film CdTe solar cells are typically hetero-junctions with CdS being the n-type partner, or window layer. Efficiencies as high as 16.5% have been achieved.In this paper we make a physical analysis of the typical CdS/CdTe superstrate solar cell, and we show that present record efficiencies are very close to the practical efficiency limit for a CdS/CdTe hetero-junction cell. We show that a current estimate for the maximum efficiency of hetero-junction CdS/CdTe solar cells is around 17.5%, in contrast to old theoretical predictions, which calculate about 30% efficiencies for ideal homo-junction CdTe solar cells. This analysis explains why the record efficiency for this kind of cells has been stable for the last 10 years, going up by less than 1% from 15.8% to only 16.5%.     

Optimized chemical bath deposited CdS layers for the improvement of CdTe solar cells

CdS layers grown by chemical bath deposition (CBD) are treated in different ways to improve the performance of CdS/CdTe solar cells. It has been found that the open circuit voltage of the CdS/CdTe solar cell increases when the CBD CdS is annealed with CdCl₂ before the deposition of CdTe by close spaced sublimation (CSS). A thin CBD CdS (∼80 nm) with bi-layer structure can significantly improve the short circuit current of the CdS/CdTe solar cells.     

Solar Renewable Energy News International Conference (SREN-2005)

Polycrystalline thin film CdTe continues to be a leading material for the development of cost effective and reliable photovoltaics. The two key properties of this material are its band gap (1.5 eV), close to the ideal for photovoltaic conversion efficiency (1.45 eV), and its high optical absorption coefficient. Thin film CdTe solar cells are typically hetero-junctions with CdS being the n-type partner, or window layer. Efficiencies as high as 16.5% have been achieved, but still there is some potential for increasing them.We make an analysis of the typical CdS/CdTe superstrate solar cell, and from it we establish critical issues and different lines of research in order to improve the current efficiencies. We also show that present record efficiencies are very close to the practical efficiency limit for a CdS/CdTe hetero-junction cell.     

Thin film CdS/CdTe solar cells: Research perspectives

Polycrystalline thin film CdTe continues to be a leading material for the development of cost effective and reliable photovoltaics. The two key properties of this material are its band gap (1.5 eV), close to the ideal for photovoltaic conversion efficiency (1.45 eV), and its high optical absorption coefficient. Thin film CdTe solar cells are typically hetero-junctions with CdS being the n-type partner, or window layer. Efficiencies as high as 16.5% have been achieved, but still there is some potential for increasing them.We make an analysis of the typical CdS/CdTe superstrate solar cell, and from it we establish critical issues and different lines of research in order to improve the current efficiencies. We also show that present record efficiencies are very close to the practical efficiency limit for a CdS/CdTe hetero-junction cell.