CdS/CdTe薄膜太阳电池的深能级瞬态谱和光致发光研究

用深能级瞬态谱和光致发光研究了无背接触层的CdS/CdTe薄膜太阳电池的杂质分布和深能级中心.得到了净掺杂浓度在器件中的分布.确定了两个能级位置分别在E_V+0365 eV和E_V+0282 eV的深中心,它们的浓度分别为167×10¹² cm^-3和386×10¹¹ cm^-2,俘获截面分别为143×10^-14cm²和153×10^-16cm².它们来源于以化学杂质形式存在的Au和（或）Te_Cd^-复合体,或与氩氧气氛下沉积CdTe时的氧原子相关. 关键词： 深能级瞬态谱 光致发光 CdS/CdTe太阳电池     

基于氧化镍背接触缓冲层碲化镉薄膜太阳电池的研究

采用电子束蒸发法制备了NiO薄膜,并对其作为碲化镉薄膜太阳电池背接触缓冲层材料进行了相关研究.NiO缓冲层的加入使得碲化镉太阳电池开路电压显著增大.通过X射线光电子能谱测试得到的NiO/CdTe界面能带图表明NiO和CdTe的能带匹配度很好.NiO是宽禁带P型半导体材料,在电池背接触处形成背场,减少了电子在背表面处的复合,从而提高电池开路电压.通过优化NiO薄膜厚度,制备得到转换效率为12.2%、开路电压为789 mV的碲化镉太阳电池.研究证实NiO是用来制备高转换效率、高稳定性碲化镉薄膜太阳电池的一种极有前景的缓冲层材料.     

Electronic properties and deep level transient spectroscopy of CdS/CdTe thin film solar cells

It is well known that preparing temperatures and defects are highly related to deep-level impurities. In our studies, the CdTe polycrystalline films have been prepared at various temperatures by close spaced sublimation (CSS). The different preparing temperature effects on CdS/CdTe solar cells and deep-level impurities have been investigated by I--V and C--V measurements and deep level transient spectroscopy (DLTS). By comparison, less dark saturated current density, higher carrier concentration, and better photovoltaic performance are demonstrated in a 580oC sample. Also there is less deep-level impurity recombination, because the lower hole trap concentration is present in this sample. In addition, three deep levels, E_v+0.341 eV(H4), E_v+0.226 eV(H5) and E_C-0.147 eV(E3), are found in the 580oC sample, and the possible source of deep levels is analysed and discussed.     

CdTe薄膜太阳电池背接触的研究

用近空间升华法制备了CdTe多晶薄膜，用硝酸-磷酸(NP)混合液对薄膜表面进行了腐蚀.经SEM观测，腐蚀后的CdTe薄膜晶界变宽，XRD测试发现，经NP腐蚀后，在CdTe薄膜表面生成了一层高电导的富Te层.在腐蚀后的CdTe薄膜上分别制备了Cu，Cu/ZnTe：Cu，ZnTe：Cu，ZnTe/ZnTe：Cu四种背接触层，比较了它们对太阳电池性能的影响.结果表明，用ZnTe/ZnTe：Cu复合层作为背接触层的效果较好，获得了面积为0.5cm²，转换效率为13.38%的CdTe多晶薄膜太 关键词： 硝磷酸腐蚀 背接触层 CdTe太阳电池     

化学水浴法制备大面积CdS薄膜及其光伏应用

采用化学水浴法制备了大面积CdS多晶薄膜,研究了薄膜的形貌、结构和光学性质,结果表明,大面积CdS多晶薄膜具有良好的均匀性,通过优化CdS多晶薄膜,制成了不同CdS窗口层厚度的CdTe小面积太阳电池,减薄CdS薄膜可有效提高器件的短路电流,改善器件性能.随后,在面积30cm×40cm的衬底上制备了全面积为993.6cm²的CdTe太阳电池组件,其27个集成单元的电学性质较为均匀,太阳电池组件的光电转换效率8.13%. 关键词： 化学水浴法(CBD) CdS薄膜 CdTe太阳电池 CdTe太阳电池组件     

CdTe太阳电池的不同背电极和背接触层的特性研究

用Ni替代Au来作为CdTe太阳电池的背电极，比较了Ni,Ni/Au,Au/Ni及Au背电极对电池性能的影响.发现Ni作为背电极和ZnTe/ZnTe:Cu复合层接触，电池的开路电压V_oc略有降低，填充因子FF有增有减，变化幅度不大，但因短路电流I_sc有较大的提高，转换效率η平均增长4％.测试了不同背电极的CdTe太阳电池的暗I-V和C-V特性，对背电极剥离后的样品进行了XPS测试分析.结果表明，Ni扩散到ZnTe/ZnTe:Cu复合层的深度比Au多，且大多呈离子态，与ZnTe/ZnTe:Cu复合层中的富Te离子形成Ni_xTe，提高了掺杂浓度，使电池性能获得改善. 关键词： 金属背电极 复合背接触层 转换效率 CdTe太阳电池     

Investigation of the effect of CdCl2 processing on vacuum deposited CdS/CdTe thin film solar cells by DLTS

Thin film CdS/CdTe solar cells have been prepared by conventional vacuum deposition technique. Deep level transient spectroscopy (DLTS), temperature and frequency dependent capacitance-voltage (C-V) measurements were utilised to investigate the performance limiting defect states in the CdTe layer subjected to the post deposition treatments such as CdCl₂-dipping and/or annealing in air. Five hole traps, all of which have been previously reported in the literature, were identified in as-grown CdTe at 0.19, 0.20, 0.22, 0.30 and 0.40 eV above the valence band. A single hole trap level has been evidenced at 0.45 eV after both post deposition heat and CdCl₂ treatments.     

A study of deformation-produced deep levels inn-GaAs using deep level transient capacitance spectroscopy

Deformation-produced deep levels, both of electron and hole traps, have been studied using deep level transient capacitance spectroscopy (DLTS) for an undopedn-type GaAs (HB grown) compressed at 440°C. Concentrations of two grown-in electron trap levels (E _c −0.65eV andE _c −0.74eV) and one grown-in hole trap level (E _v +∼0.4eV) increase with plastic deformation, while that of a grown-in electron trap level (E _c −∼0.3eV) decreases in an early stage of deformation. While no new peak appeared in the electron trap DLTS spectrum after plastic deformation, in the hole trap DLTS spectrum a broad spectrum, seemingly composed of many peaks, newly appeared in a middle temperature range, which may be attributed to electronic energy levels of dislocations with various characters.     

Synthesis of the wheat-like CdSe/CdTe thin film heterojunction and their photovoltaic applications

We report on the fabrication of wheat-like CdSe/CdTe thin film heterojunction solar cells made using a simple electrochemical deposition method and close-spaced sublimation technology on indium tin oxide (ITO) substrates. Structural, optical, and electrical properties of the wheat-like CdSe/CdTe thin film junctions were characterized by X-ray diffraction (XRD), field emission scanning electron microscope (FESEM), energy dispersive spectrometry (EDS), ultraviolet–visible (UV–vis) absorption spectrum and Keithley 2400 analysis. A significant red-shift of the absorption edge is observed in this heterojunction. The heterostructure is composed of the wheat-like CdSe array and CdTe thin film, showing optical properties typical of type II heterostructures that are suited for photovoltaic applications. A photocurrent density of 8.34 mA/cm² has been obtained under visible light illumination of 100 mW/cm². This study demonstrates that the electrochemical deposition and the close-spaced sublimation technology, which are easily adapted to other chemical systems, are promising techniques for large-scale fabrication of low-cost heterojunction solar cells.     

注硅半绝缘GaAs的深能级

用深能级瞬态谱(DLTS)详细研究了硅离子注入Liquid-encapsulated Czochralski(缩写为LEC)半绝缘GaAs的深中心。结果表明,在注硅并经高温退火的有源区中观测到4个多子(电子)陷阱,E₀₁,E₀₂,E₀₃和E₀₄。它们的电子表观激活能分别为0.298,0.341,0.555和0.821eV。其中E₀₄与EL2有关,但不是EL2缺陷。E₀₄的电子 关键词：     

Wet chemical etched CdTe thin film solar cells

Wet chemical etching process on as-deposited CdTe surface using nitric-phosphoric (NP) acid improved the efficiency of CdS/CdTe solar cells from 10.1% to 13.8%. Nitric-phosphoric (NP) acid solution etched native oxide (TeO₂) layer and resolved excess cadmium on CdTe surface. After the heat treatment process activated the CdCl₂, CdO layer which was believed to be a diffusion barrier of chlorine did not grow on the etched CdTe surface and new (V_Cd^2?–2Cl_Te¹⁺)⁰ complexes was located at E_V + 0.045 eV. New (V_Cd^2?–2Cl_Te¹⁺)⁰ complexes acted as a shallow acceptors and induced to improve V_oc and J_sc. The surface of CdTe thin film has been studied using Scanning Electron Microscope (SEM), X-ray Diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and low temperature Photoluminescence (PL).     

Low‐Temperature‐Processed CdS as the Electron Selective Layer in an Organometal Halide Perovskite Photovoltaic Device

Organic–inorganic halide perovskites have recently been crowned as the leading next‐generation photovoltaic material due to their high efficiency and simple fabrication process. Herein, a low‐temperature‐processed CdS thin film (commonly used as a buffer layer in commercial CdTe or CIGSe solar cells) is reported as an electron selective layer in perovskite devices based on the following reasons: First, the photoelectric property of CdS thin film is investigated, illustrating the possibility of CdS as the electron selective layer in the application of methylammonium lead (II) iodide perovskite devices. More specifically, CdS semiconductor film presents a higher mobility compared with traditional TiO₂ thin film, which benefits the electron extraction and transmission; second, it is found that the perovskite thin film spun‐coating on the CdS substrates grows with an obvious tendency along the direction toward the thickness of thin film, which reduces the chance of recombination of electrons and hole, beneficial to their separation. It is also revealed that the perovskite‐device‐based CdS electron selective layer has a higher stability compared with that of TiO₂ due to the difference of substrates wetting.     

Synthesis and optical characterization of nanocrystalline CdTe thin films

From several years the study of binary compounds has been intensified in order to find new materials for solar photocells. The development of thin film solar cells is an active area of research at this time. Much attention has been paid to the development of low cost, high efficiency thin film solar cells. CdTe is one of the suitable candidates for the production of thin film solar cells due to its ideal band gap, high absorption coefficient. The present work deals with thickness dependent study of CdTe thin films. Nanocrystalline CdTe bulk powder was synthesized by wet chemical route at pH≈11.2 using cadmium chloride and potassium telluride as starting materials. The product sample was characterized by transmission electron microscope, X-ray diffraction and scanning electron microscope. The structural characteristics studied by X-ray diffraction showed that the films are polycrystalline in nature. CdTe thin films with thickness 40, 60, 80 and 100 nm were prepared on glass substrates by using thermal evaporation onto glass substrate under a vacuum of 10⁻⁶ Torr. The optical constants (absorption coefficient, optical band gap, refractive index, extinction coefficient, real and imaginary part of dielectric constant) of CdTe thin films was studied as a function of photon energy in the wavelength region 400–2000 nm. Analysis of the optical absorption data shows that the rule of direct transitions predominates. It has been found that the absorption coefficient, refractive index (n) and extinction coefficient (k) decreases while the values of optical band gap increase with an increase in thickness from 40 to 100 nm, which can be explained qualitatively by a thickness dependence of the grain size through decrease in grain boundary barrier height with grain size.     

肖特基势垒对CdS/CdTe薄膜电池J-V暗性能的影响

采用数学模拟方法分析了不同背接触势垒高度(φ_b) 对于CdS/CdTe薄膜电池的J-V(电流密度-电压)方程的影响, 得出了势垒高度与roll-over的变化对应关系. 采用相应Cu/Mo背电极的CdS/CdTe薄膜电池在220-300 K的变温J-V曲线的数值分析与理论分析相对照, 分析了背势垒对于J-V曲线拟合参数的影响. 修正了φ_b 与反向饱和电流(J_b0)关系式, 理论与实验符合得非常好. 关键词： CdS/CdTe薄膜 伏安特性 肖特基势垒 roll-over     

渐变带隙氢化非晶硅锗薄膜太阳能电池的优化设计

利用一维微电子-光电子结构分析软件(AMPS-1D)在AM1.5G(100 mW/cm2)、室温条件下模拟和比较了有、无渐变带隙氢化非晶硅锗(a-SiGe:H)薄膜太阳能电池的各项性能.计算结果表明:渐变带隙结构电池具有较高的开路电压(V oc)和较好的填充因子(FF),转换效率(E ff)比非渐变带隙电池提高了0.477%.研究了氢化非晶硅(a-Si:H)、氢化非晶碳化硅(a-SiC:H)和氢化纳米晶硅(nc-Si:H)三种不同材料的窗口层对a-SiGe:H薄膜太阳能电池性能的影响.结果显示:在以nc-Si:H为窗口层的电池能带中,费米能级E F已经进入价带,使得窗口层电导率及电池开路电压有所提高,又由于ITO与p-nc-Si:H的接触势垒较低,使得接触处的电场降低,更有利于载流子的收集.另一方面,窗口层与a-SiGe:H薄膜之间存在较大的带隙差,在p/i界面由于能带补偿作用形成了价带势垒(带阶)?E v,阻碍了空穴的迁移,因此我们在p/i界面引入缓冲层,使得能带补偿作用得到释放,更有利于空穴的迁移和收集,得到优化后单结渐变带隙a-SiGe:H薄膜结构太阳能电池的转换效率达到了9.104%.     

Numerical simulation of ultra-thin CdTe solar cells with a buffer layer of MoOx in the backwall configuration

The software of Solar Cell Capacitance Simulator (SCAPS) is used to investigate the performance of ultra-thin CdTe solar cells in the backwall configuration (glass/ITO/MoO_x/CdTe/CdS/SnO₂/Ag). The backwall structure utilizes ultra-thin CdTe absorber layer instead of CdS film facing light illumination, which eliminates the absorption of CdS in short-wavelength region and improves the blue response of CdTe. A buffer layer of MoO_x is added to modify the contact between CdTe and ITO, reducing the valence band barrier height and simultaneously forming an electron reflector, which can reduce electron-hole recombination at this contact. When the thickness of MoO_x is 2 nm, the simulation results show that an efficiency can reach up to 25.5% with high ITO work function and ideal interface recombination velocity.     

Development of ZnTe film with high copper doping efficiency for solar cells

Since a hole barrier was formed in back contact due to mismatch of work function, the back contact material for CdTe cell has been a significant research direction. The ZnTe:Cu is an ideal back contact material, which reduces the valence band discontinuity and can be used as the electron back reflection layer to inhibit interface recombination. The conductivity of ZnTe:Cu film is improved by applying RF-coupled DC sputtering and post-deposition heat treatment. The doping efficiency is computed as the ratio of free hole density and copper concentration, which can be correlated with performance for CdTe-based solar cell. The higher doping efficiency means that more copper atoms substitute for Zn sites in ZnTe lattices and less mobilized copper atoms remain which can enter into the CdTe absorber layer. Copper atoms are suspected as dominant element for CdTe-based cell degradation. After optimizing the ZnTe:Cu films, a systematic study is carried out to incorporate ZnTe:Cu film into CdTe solar cell. The EQE spectrum is kept relatively stable over the long wavelength range without decreasing. It is proved that the conduction band barrier of device with ZnTe:Cu/Au contact material has an effect on the EQE response, which works as free electron barrier and reduces the recombination rate of free carrier. According to the dark J—V data or the light J—V data in the linear region, the current indicates that the intercept gives the diode reverse saturation current. The results of ideality factor indicate that the dominant recombination occurs in the space charge region. In addition, the space charge density and depletion width of solar cell can be estimated by C—V profiling.     

Degradation sources of CdTe thin film PV: CdCl2 residue and shunting pinholes

The present work considers two observable phenomena through the experimental fabrication and electrical characterization of the rf-sputtered CdS/CdTe thin film solar cells that extremely reduce the overall conversion efficiency of the device: CdCl₂ residue on the surface of the semiconductor and shunting pinholes. The former happens through nonuniform treatment of the As-deposited solar cells before annealing at high temperature and the latter occurs by shunting pinholes when the cell surface is shunted by defects, wire-like pathways or scratches on the metallic back contact caused from the external contacts. Such physical problems may be quite common in the experimental activities and reduce the performance down to 4–5 % which leads to dismantle the device despite its precise fabrication. We present our electrical characterization on the samples that received wet CdCl₂ surface treatment (uniform or nonuniform) and are damaged by the pinholes.     

Deep level transient spectroscopy in Hg1−xCdxTe

We report the application of Deep Level Transient Spectroscopy (DLTS) in Hg_1-xCd_xTe, demonstrating for the first time the utilization of DLTS techniques in a narrow band-gap semiconductor, E_g < 0.40 eV. DLTS measurements performed on an n⁺-p diode with E_g (x=0.21, T=30 K) =0.096 eV have identified an electron trap with an energy of E_v + 0.043 eV and a hole trap at E_v + 0.035 eV. Measurements of trap densities, capture cross sections, and the close proximity of the electron and hole trap locations within the band-gap suggest that DLTS may be observing both the electron and hole capture at a single Shockley-Read recombination center. The trapping parameters measured by DLTS predict minority carrier lifetime versus temperature data to be comparable with the experimentally measured values.     

CdTe太阳电池中起因于Cu的深能级

在CdTe太阳电池中,易引入并形成Cu深能级中心. 本文采用深能级瞬态谱测试法研究了ZnTe背接触和石墨背接触CdTe太阳电池的部分深能级中心. 研究中运用密度泛函相关理论,分析闪锌矿结构CdTe,Cd空位体系和掺Cu体系的电子态密度,计算得出T_d场和C_3v场下Cu²⁺ d轨道的分裂情况. 计算结果表明,CdTe太阳电池中的E_v+0206 eV和E_v+0122 eV两个深中心来源于Cu替代Cd原子. 计算结果还表明,掺入Cu可降低CdTe体系能量. 关键词： 深能级瞬态谱 第一性原理 CdTe Cu杂质