CdS/CdTe异质结太阳电池串联电阻的理论研究

串联电阻是太阳电池中一个附加的功率损耗．对太阳电池的转换效率、填充因子输出性能等均有较大影响，尽可能的减小串联电阻是提高太阳电池效率和性能所要求的。该文从理论上分析了太阳电池中串联电阻形成的各种因素，特别是对电池TCO层的薄层横向电阻做了较为详尽的理论分析，导出了TCO层薄层横向电阻的理论计算公式，并讨论了减小串联电阻的途径。并将计算值与实验结果进行了比较，两者符合较好。     

A study of vapor CdCl2 treatment by CSS in CdS/CdTe solar cells

We report the effect of CdCl₂ vapor treatment on the photovoltaic parameters of CdS/CdTe solar cells. Vapor treatment allows combining CdCl₂ exposure time and annealing in one step. In this alternative treatment, the CdS/CdTe substrates were treated with CdCl₂ vapor in a close spaced sublimation (CSS) configuration. The substrate temperature and CdCl₂ powder source temperature were 400 °C. The treatment was done by varying the treatment time (t) from 15 to 90 min. Such solar cells are examined by measuring their current density versus voltage (J-V) characteristics. The open-circuit voltage (V_oc), short circuit current density (J_sc) and fill factor (FF) of our best cell, fabricated and normalized to the area of 1 cm², were V_oc = 663 mV, J_sc = 18.5 mA/cm² and FF = 40%, respectively, corresponding to a total area conversion efficiency of η = 5%. In cells of minor area (0.1 cm²) efficiencies of 8% have been obtained.     

Highly efficient large area (10.5%, 1376 cm) thin-film CdS/CdTe solar cell

Highly efficient large area (10.5%, 1376 cm²) thin-film CdS/CdTe solar cell sub-module has been fabricated. Very recently we also have fabricated a very large area sub-module of aperture area 5413 cm² exhibiting a conversion efficiency of 8.4%. Such a high efficiency has been achieved by depositing all the constituent films such as SnO₂: F, CdS and CdTe having greater uniformity and better quality under atmospheric pressure conditions. A post deposition treatment of CdTe surface with CdCl₂ has been optimized to improve the overall solar cell output performance significantly.     

EFFECT OF OXYGEN ON PHOTOVOLTAIC PROPERTIES OF SCREEN-PRINTED CdS/CdTe SOLAR CELL AND MODULE Module Reliability and Oxygen Deficiency

An n-CdS/p-CdTe solar cell module constructed by complete seal of solar cell formed on a glass substrate by screen-printing and sintering process, was subjected to a heavy sunshine weather-meter test. The conversion efficiency of the module began to degrade after 50 days. Chemical analyses on gases in the sealed module showed that decrease in oxygen down to 2 vol.% had caused the degradation. Inversely, when sufficient oxygen was supplied to the solar cell by breaking the seal, the efficiency recovered up to its initial value within several days even at room temperature. Estimation of reaction velocity at room temperature suggested that the present recovery phenomenon was the same as increase in p-type characteristics by heat treatment in air in other fabrication process of CdS/CdTe junction. Easiness and reversibility of transformation from oxygen deficient to sufficient states in CdTe, strongly suggest that oxygen in CdTe induces a single acceptor which was confirmed in oxygen doped ZnSe.     

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。     

EFFECTS OF PROCESSING ON CdTe/CdS MATERIALS AND DEVICES

By analyzing CdTe/CdS devices fabricated by vacuum evaporation, a self consistent picture of the effects of processing on the evolution of CdTe cells is developed which can be applied to other fabrication methods. In fabricating CdTe/CdS solar cells by evaporation, a 400°C CdCI₂ heat treatment is used which recrystallizes the CdTe and interdiffuses the CdS and CdTe layers. The interdiffuson can change the bandgap of both the CdTe and CdS which modifies the spectral response of the solar cell. After this heat treatment a contacting/doping procedure is used which converts the CdTe conductivity to p-type by diffusion from Cu from the contact. Finally, the cell is treated with Br₂CH₃,OH which improves both Voc and FF. Analogous process steps are used in most fabrication processes for CdTe/CdS solar cells.     

Reaction kinetics study of CdTe thin films during CdCl2 heat treatment

In order to investigate the effect of CdCl₂ heat treatment on the physical properties of CdTe thin films grown by a sputtering method, the CdTe thin films were coated with CdCl₂. The recrystallization, grain growth, randomization, and reaction kinetics were investigated by monitoring the phase transition of CdCl₂-heat-treated CdTe specimens during temperature ramp annealing or isothermal soaking by using in-situ time-resolved high-temperature X-ray diffraction. The results of annealing show that the recrystallizations of CdTe (1 1 1) and other planes do not occur simultaneously, but sequentially in terms of temperature. The results of isothermal soaking imply that the Avrami diffusion-controlled reaction model fits well with the experimental data. This proves that the CdCl₂ diffusion process is the dominant factor in the CdTe recrystallization mechanism.     

A life-cycle analysis on thin-film CdS/CdTe PV modules

Authors have evaluated the life cycle of a thin-film CdS/CdTe PV module to estimate the energy payback time (EPT) and the life-cycle CO₂ emissions of a residential rooftop PV system using the CdS/CdTe PV modules. The primary energy requirement for producing 1 m² of the CdS/CdTe PV module was similar to a-Si PV module at annual production scale of 100 MW. EPT was calculated at 1.7–1.1 yr, which was much shorter than the lifetime of the PV system and similar to that of a-Si PV modules. The life-cycle CO₂ emissions were also estimated at 14–9 g-C/kWh, which was less than that of electricity generated by utility companies.     

CdTe/CdS异质结特性

从理论上对CdTe／CdS太阳电池CdS/CdTe异质结的特性进行了研究和讨论，结果表明可以简单利用改变CdTe、CdS两种半导体材料的掺杂浓度来改变CdS/CdTe异质结的能带结构。针对不同的能带结构采用了不同的物理模型，得到的CdS/CdTe异质结伏安特性曲线有一折点，且折点位置随异质结能带结构的变化而变化。     

Photoluminescence studies of CdS thin films annealed in CdCl2 atmosphere

We have grown CdS films by the Close Spaced Vapor Transport technique under specific conditions: substrate temperature (T_s): 450 °C, source temperature (T_so): 725 °C, argon pressure in the chamber (P_Ar): 100, 200 and 500 mT, deposition time (t_d): 100 s. The films were studied by measuring the luminescence properties at different temperatures in the range 10–300 K. The room-temperature PL spectrum of the as-grown CdS films showed a very broad band centered at 2.26 eV and a shoulder in the low-energy side at 1.80 eV. After CdCl₂ thermal annealing at 300 K, the spectrum showed better PL characteristics: a strong band in the low-energy side at 1.67 eV and a band in the high-energy side at 2.47 eV. The analysis at lower temperatures showed that the high-energy band becomes most intense and shifts to higher energies reaching a value of 2.54 eV, very close to the energy band gap at 10 K. The low-energy band becomes broader and centered around 1.9 eV. Analysis of the PL intensity as a function of temperature in an Arrhenius representation, allows applying a theoretical model for the quenching of the PL intensity.     

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％。     

SnO_2／p接触特性对a－Si太阳电池填充因子的影响

用ＰＥＣＶＤ方法制备出高电导率（～０．２ｓｃｍ－１）、宽带隙（～２．２ｅＶ）的Ｐ型微晶化硅碳合金（ｐ－μｃ－ＳｉＣ：Ｈ）薄膜材料。利用ｐ－μＣ－ＳｉＣ：Ｈ／ｐ－ａ－Ｓｉ：Ｈ复合结构做ａ－Ｓｉ太阳电池的窗口材料，明显改善了ＳｎＯ２／ｐ之间的接触特性，从而使１０ｃｍ×１０ｃｍ单结集成型电池的填充因子从０．７０以下提高到０．７２。     

光热对薄膜CdS／Cu2S太阳电池物理参数的影响

研究了光、热和湿气对ＣｄＳ／Ｃｕ２Ｓ电池效率及电池成结、结电容、二极管特性和光谱响应特性的影响。结果表明，对未成结电池光加速成结作用，对成结电池光引起电池性能下降，这种衰降是不可逆过程。     

GROWTH OF HETEROEPITAXIAL CdTe LAYERS ON REUSABLE Si SUBSTRATES AND A LIFT-OFF TECHNIQUE FOR THIN FILM SOLAR CELL FABRICATION

Heteroepitaxial (111) and (100) oriented CdTe layers have been grown on Si substrates by conventional molecular beam epitaxy (MBE) and photo-assisted MBE (PAMBE) using stacked BaF₂-CaF₂ as a buffer to overcome the 19% lattice mismatch between Si and CdTe. Heteroepitaxial As doped p-type CdTe(lOO) layers have been grown on BaF₂-CaF₂/Si(100). The dopant activation is accomplished using an extra Cd source and laser illumination of the substrate during growth. The growth kinetics and surface reconstructions have been studied using RHEED during CdTe growth under different conditions, and the induced effects on Te-desorption, Cd-migration, and As-substitution on Te-vacancy site have been correlated. The resistivity of As doped CdTe layers is down to 20 ohm cm. The 8 K photoluminescence spectra of such a layer shows a dominant (A^°, X) peak at 1.590 eV and the As acceptor level corresponds to a shallow level with = 60 me V activation energy. A lift-off technique has been used to separate the single crystal CdTe thin films from the Si wafer by dissolving the fluoride buffer. CdS/CdTe solar cells have been fabricated in these layers.     

Chemical stability of sputtered Mo/Sb2Te3 and Ni/Sb2Te3 layers in view of stable back contacts for CdTe/CdS thin film solar cells

Sb–Te phases sputtered from an Sb₂Te₃/Sb/Te target in a substrate temperature range from 293 to 523 K are characterised by X-ray diffraction (XRD) and Hall measurements. A simple thermodynamic model is introduced for estimating the chemical stability of the Ni/Sb₂Te₃ and the Mo/Sb₂Te₃ interface. These data and the results of kinetic test reactions for sputtered Ni/Sb₂Te₃, Ni/Sb–Te, Mo/Sb₂Te₃ and Mo/Sb–Te layers are compared using XRD measurements. Metal/Sb₂Te₃ thin film double-layer systems are used as a model for an innovative back contact for CdTe/CdS thin film solar cells offering an improved long-term stability.     

Influence of CdCl2 treatment on structural and optical properties of vacuum evaporated CdTe thin films

In this article we have discussed the structural, optical properties of vacuum evaporated CdTe thin films before and after CdCl₂ treatment. The CdTe thin films were prepared by vacuum evaporation. Films were prepared under the vacuum of 10⁻⁶ Torr. The structural studies have been performed by the X-ray diffraction (XRD) technique. The XRD analysis of vacuum evaporated CdTe films reveals that the structure of films is polycrystalline in nature. However, the crystallinity has been improved after the CdCl₂ treatment as shown by an increase of the diffraction peak intensities. This is due to the enhancement in the atomic mobility of CdTe. The optical properties of the CdTe thin films have been studied by the spectrophotometer in the 300–800 nm wavelength range. It is observed that the optical band gap energy is highly dependent on CdCl₂ treatments. The optical transitions in these films are found to be direct and allowed.     

多晶CdS／Cu2S异质结太阳电池光电流及转换效率的计算

根据半导体材料的性能参数，对多晶ＣｄＳ／Ｃｕ２Ｓ薄膜太阳电池在各种浓度下的光伏特性作了较深入的分析和计算。计算中考虑到耗尽区宽度的变化以及内表面复合损失对光电流ＪＬ的影响，同时还用Ｒｏｔｗａｒｆ晶界复合损失模型计算了晶粒度对光电流及光伏特性的影响。存在一个最佳Ｃｕ２Ｓ受主浓度Ｎａ＝１０＾１５ｃｍ＾－３，单晶和晶粒度Ｒ＝３μｍ的多晶电池，其转换效率分别为１３．６％和１３．３％。     

Influence of CdS window layer on 2-μm thick CdS/CdTe thin film solar cells

Influence of the CdS window layer on the PV performances of 2-μm thick CdS/CdTe solar cells has been studied as a function of the CdS thickness, d_CdS. With a reduction of d_CdS from 114 to 95 nm, J_SC increases due to an increase in blue response. While, at d_CdS<85 nm, the conversion efficiency largely decreases due to a decrease in V_OC and FF. The deterioration of the crystallinity of CdTe due to a decrease in the sulfur composition x of the CdTe_1−xS_x mixed-crystal layer is concluded to be the most possible mechanism for the large decreases in V_OC and FF.     

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。     

氢处理对太阳薄膜材料Mo/CuIn1-x GaxSe2/CdS的影响

将固态源硒化法制备的太阳薄膜材料(Mo／CuIn1-xGaxSe2／Cds)在500℃进行氢处理和氮处理，然后利用XRD、SEM和I-V测试分析了氢处理和氮处理对CuIn1-xGaxSe2薄膜结构、形貌和太阳电池性能的影响。研究表明在500℃的条件下，经过不同时间的氢处理，可以使样品的开启电压减小(特别是经过180min氢处理的样品)，提高了p-n结的性能，从而提高光电转换效率，并可以使样品表面的大颗粒逐渐减少，促进CuIn1-xGaxSe2相的形成，这和I—V性能的测试结果相符合。而氮气的处理对于样品的I—V性能几乎没有影响，这证明了上述性能的提高是由于氢的钝化作用引起的，而非普通的热处理所致。