用光电子能谱研究CdTe太阳电池的背接触特性

CdTe太阳电池的背电极须采用高功函数金属。通过采用光电子能谱(XPS)分析了高功函数金属Au和Ni分别作为背电极的CdTe太阳电池背接触特性,发现在背电极剥离后,Au在ZnTe/ZnTe∶Cu背接触层表面以Au单质形式存在,扩散深度较浅;Ni扩散到ZnTe/ZnTe∶Cu复合层的深度比Au大,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。在两样品中还发现,不论是Te的峰还是Zn的峰,其峰的位移变化都很小,说明两样品中Te和Zn的存在形式没有发生变化。     

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     

稀土掺杂CdTe太阳电池背接触层ZnTe的第一性原理研究

结合CdS/CdTe太阳电池背接触层的制备要求考虑, 利用基于密度泛函理论平面波超软赝势方法和广义梯度近似, 计算了未掺杂ZnTe、稀土Y、Gd掺杂ZnTe的能带和电子态密度, 得到了不同体系下系统总能和晶格常数. 研究表明, 稀土Y和Gd掺杂后ZnTe结构的稳定性均提高, 掺杂Y使ZnTe与CdTe的晶格匹配更好. 计算表明, 掺杂可使载流子发生简并, 掺Y比掺Gd电子有效质量小, 掺Y与掺Gd的载流子浓度数量级相同. 根据计算结果分析了稀土掺杂对ZnTe背接触层的影响. 关键词： ZnTe 稀土掺杂 第一性原理 太阳电池背接触层     

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.     

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     

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     

Deep level transient spectroscopy investigation of deep levels in CdS/CdTe thin film solar cells with Te:Cu back contact

Deep levels in Cds/CdTe thin film solar cells have a potent influence on the electrical property of these devices. As an essential layer in the solar cell device structure, back contact is believed to induce some deep defects in the CdTe thin film. With the help of deep level transient spectroscopy (DLTS), we study the deep levels in CdS/CdTe thin film solar cells with Te:Cu back contact. One hole trap and one electron trap are observed. The hole trap H1, localized at E_v+0.128~eV, originates from the vacancy of Cd (V_Cd. The electron trap E1, found at E_c-0.178~eV, is considered to be correlated with the interstitial Cu_i⁼ in CdTe.     

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     

沉积条件对ZnTe/ZnTe:Cu薄膜结构及CdTe电池性能的影响

用共蒸发法沉积了ZnTe/ZnTe:Cu复合多晶薄膜,通过XRD,XPS,C-V,I-V等研究了沉积温度对薄膜结构、Cu浓度分布及电池性能的影响.结果表明,沉积温度对薄膜的结构影响不明显,薄膜呈立方相,经185 ℃退火后出现了六方相.对薄膜的剖析发现,Cu浓度分布呈现先上升到一极大值而后快速下降的趋势, 100 ℃沉积的ZnTe/ZnTe:Cu薄膜,ZnTe层起到了阻止Cu扩散作用,用这种薄膜制作的太阳电池X_D较大 关键词： ZnTe多晶薄膜 沉积温度 薄膜结构 器件性能     

Performance improvement in CdTe solar cells by modifying the CdS/CdTe interface with a Cd treatment

The performance of n-CdS/p-CdTe solar cells is often degraded under light soaking or thermal stress, even though the technology of CdTe solar cells is close to a commercial level. The Cu diffusion from a Cu back contact to a CdS window layer might degrade the cell's performance. To prevent the Cu diffusion, a very-thin intrinsic CdTe layer was introduced at the n-CdS/p-CdTe interface by depositing a very-thin Cd metal layer on the CdS film and converting the Cd metal into intrinsic CdTe during p-CdTe deposition at high temperature. By the Cd treatment on CdS surface, pinholes or voids were eliminated at the CdS/CdTe and the intermixing of Te and S at the interface was much suppressed. The depletion width was much increased and the intensity of LTPL peak was increased. The analysis suggested that an intrinsic CdTe interlayer was formed and the surface recombination rate was suppressed by the intrinsic interlayer. As a result, the short circuit current of the CdTe solar cell was significantly increased due the increased current gain in the blue wavelength region. The thermal stability of the CdTe solar cell was also greatly improved and the Cu diffusion was retarded by the intrinsic CdTe interlayer at the n-CdS/p-CdTe.     

Understanding the junction degradation mechanism in CdS/CdTe solar cells using a Cd-deficient CdTe layer

It is known that CdTe solar cells are often degraded under solar illumination. But the degradation mechanism is not fully proved because it does not appear consistently. The junction degradation in CdS/CdTe solar cells was investigated using a CdTe layer with Cd deficient composition, where Cd vacancy concentration is high. It was found that the Cu atoms easily filled the Cd vacancies in CdTe and transport to junction area from Cu back contact. PL measurement and spectral quantum efficiency measurement showed that the incorporation of Cu atoms in CdS forms a defect energy level at 1.55 eV below the conduction band in CdS. As a result, the junction built-in potential is decreased and light penetration into CdTe absorber is shielded. For reliable and stable CdTe cells, the formation of Cd vacancy in CdTe should be avoided by careful control of CdTe.     

Recombination-induced voltage-dependent photocurrent collection loss in CdTe thin film solar cell

Recently, the efficiency of CdTe thin film solar cell has been improved by using new type of window layer Mg_xZn_1-xO (MZO). However, it is hard to achieve such a high efficiency as expected. In this report a comparative study is carried out between the MZO/CdTe and CdS/CdTe solar cells to investigate the factors affecting the device performance of MZO/CdTe solar cells. The efficiency loss quantified by voltage-dependent photocurrent collection efficiency (η_C(V')) is 3.89% for MZO/CdTe and 1.53% for CdS/CdTe solar cells. The higher efficiency loss for the MZO/CdTe solar cell is induced by more severe carrier recombination at the MZO/CdTe p—n junction interface and in CdTe bulk region than that for the CdS/CdTe solar cell. Activation energy (E_a) of the reverse saturation current of the MZO/CdTe and CdS/CdTe solar cells are found to be 1.08 eV and 1.36 eV, respectively. These values indicate that for the CdS/CdTe solar cell the carrier recombination is dominated by bulk Shockley—Read—Hall (SRH) recombination and for the MZO/CdTe solar cell the carrier recombination is dominated by the p—n junction interface recombination. It is found that the tunneling-enhanced interface recombination is also involved in carrier recombination in the MZO/CdTe solar cell. This work demonstrates the poor device performance of the MZO/CdTe solar cell is induced by more severe interface and bulk recombination than that of the CdS/CdTe solar cell.     

磁控溅射Cl掺杂CdTe薄膜的孪晶结构与电学性质

CdTe用作薄膜太阳能电池吸收层需要经过氯处理才能得到高的光电转换效率,其中Cl原子的作用机理仍然没有完全被理解.实验发现Cl原子主要偏聚在CdTe晶界处,对晶界有钝化作用,而有第一性原理计算认为Cl原子掺入CdTe晶格能够引入浅能级提高光电转换效率.为了验证Cl原子掺杂是否对CdTe的光电转换效率有益,本文通过磁控溅射制备了100 ppm(ppm=1/1000000)Cl原子掺杂的CdTe(CdTe:Cl)薄膜并研究了薄膜的晶体结构与电学性质,同时对比了正常氯处理的无掺杂CdTe薄膜与CdTe:Cl薄膜之间的性质区别.实验发现Cl原子掺杂会在CdTe:Cl中形成大量仅由几个原子层构成的孪晶,电子和空穴在CdTe:Cl薄膜中没有分离的传导通道,而在氯处理后的CdTe薄膜中电子沿晶界传导,空穴沿晶粒内部传导.磁控溅射沉积的CdTe:Cl多晶薄膜属于高阻材料,退火前载流子迁移率很低,退火后载流子浓度降低到本征数量级,电阻率提高.CdTe:Cl薄膜电池效率远低于正常氯处理的无掺杂CdTe薄膜电池效率.磁控溅射制备的非平衡重掺杂CdTe:Cl多晶薄膜不适合用作薄膜太阳能电池的吸收层.     

Electrical properties of the CdTe back contact: A new chemically etching process based on nitric acid/acetic acid mixtures

The performance of the back contact is one of the major issues of CdTe solar cell research. Standard nitric-phosphoric (NP) acid chemical etching before metallization is widely used to improve contact formation. However, previous studies of this traditional etching method indicated a blocking Schottky barrier at the back contact, and a roll-over phenomenon was found in the J-V curves of the CdTe solar cells. In this work, a new etching solution, i.e. a nitric-acetic (NA) acid was employed. The etching rate was slow and a Te-rich layer was formed on the surface, which was less than 1 nm. The CdTe solar cell with this new etching method showed no roll-over phenomenon and displayed a good ohmic back contact performance. XPS analysis demonstrated that the back contact barrier height was close to those of CdTe with standard NP etching. A possible mechanism was presented for the improvement of back contact properties.     

碲化镉薄膜太阳能电池电学特性参数分析

用inline方式全部近空间升华方法制备n-CdS/p-CdTe取得了～11%的转换效率(AM1.5). 把其中n-CdS层采用磁控溅射方法取得了～10%的转换效率(AM1.5). 基于其电流密度-电压(J-V)曲线和外量子效率曲线, 分析了其拟合关键参数对于电池性能的影响程度, 并从理论分析上把目前器件性能参数与当今前沿性能参数以及其理论值进行比较, 指出了如何提高电池转换效率(η)的方法: 提高开路电压(V_oc)、短路电流(J_sc)和填充因子(FF). 关键词： 碲化镉电池 电流密度-电压曲线 外量子效率曲线 电学特性     

Influence of 120 MeV Si9+ ion irradiation on ZnTe semiconductor thin films

ABSTRACT

ZnTe (Zinc Telluride) is a potential semiconducting material for many optoelectronic devices like solar cells and back contact material for CdTe-based solar cells. In the present study, ZnTe thin films were prepared by thermal evaporation technique and then irradiated with 120?MeV Si⁹⁺ ions at different fluences. These films are characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and UV–Visible spectroscopy techniques. XRD study confirms increased crystallinity and grain growth for post-irradiated ZnTe thin films for fluences, up to 1?×?10¹¹ ions cm^?2. However, the grain size and crystallinity decreased for higher fluence-exposed samples. SEM images confirm the observed structural properties. Modification of the surface morphology of the film due to the ion irradiation with different fluences is studied. Optical band gap of film is decreased from 2.31?eV (pristine) to 2.17?eV after irradiation of Si⁹⁺ ions.     

Influence of P<Superscript>+</Superscript>-ZnTe back surface contact on photovoltaic performance of ZnTe based solar cells

In order to improve photovoltaic performance of solar cells based on ZnTe thin films two device structures have been proposed and its photovoltaic parameters have been numerically simulated using Solar Cell Capacitance Simulator software. The first one is the ZnO/CdS/ZnTe conventional structure and the second one is the ZnO/CdS/ZnTe/P⁺-ZnTe structure with a P⁺-ZnTe layer inserted at the back surface of ZnTe active layer to produce a back surface field effect which could reduce back carrier recombination and thus increase the photovoltaic conversion efficiency of cells. The effect of ZnO, CdS and ZnTe layer thicknesses and the P⁺-ZnTe added layer and its thickness have been optimized for producing maximum working parameters such as: open-circuit voltage V_oc, short-circuit current density J_sc, fill factor FF, photovoltaic conversion efficiency η. The solar cell with ZnTe/P⁺-ZnTe junction showed remarkably higher conversion efficiency over the conventional solar cell based on ZnTe layer and the conversion efficiency of the ZnO/CdS/ZnTe/P⁺-ZnTe solar cell was found to be dependent on ZnTe and P⁺-ZnTe layer thicknesses. The optimization of ZnTe, CdS and ZnTe layers and the inserting of P⁺-ZnTe back surface layer results in an enhancement of the energy conversion efficiency since its maximum has increased from 10% for ZnO, CdS and ZnTe layer thicknesses of 0.05, 0.08 and 2 µm, respectively to 13.37% when ZnO, CdS, ZnTe and P⁺-ZnTe layer thicknesses are closed to 0.03, 0.03, 0.5 and 0.1 µm, respectively. Furthermore, the highest calculated output parameters have been J_sc?=?9.35 mA/cm², V_oc?=?1.81 V, η?=?13.37% and FF?=?79.05% achieved with ZnO, CdS, ZnTe, and P⁺-ZnTe layer thicknesses about 0.03, 0.03, 0.5 and 0.1 µm, respectively. Finally, the spectral response in the long-wavelength region for ZnO/CdS/ZnTe solar cells has decreased at the increase of back surface recombination velocity. However, it has exhibited a red shift and showed no dependence of back surface recombination velocity for ZnO/CdS/ZnTe/P?+?-ZnTe solar cells.     

Short-circuit current improvement in CdTe solar cells by combining a ZnO buffer layer and a solution back contact

Conventional CdTe solar cells have a CdS window layer, in which an absorption loss of photons with more than 2.4 eV occurs through the CdS layer. A thinner CdS layer was applied to enhance light transmission and a ZnO buffer layer with a band gap of 3.3 eV was introduced to suppress shunting through the thinner CdS window layer. A 100-nm thick ZnO layer sputter-deposited at 300 °C had uniform coverage on a transparent conductive oxide (TCO) after a subsequent high-temperature process. The ZnO layer was effective in preventing shunting through the CdS window layer so that the open-circuit voltage and fill factor of the CdTe solar cells were recovered and the short-circuit current was enhanced over that of the conventional CdTe solar cell. In the ZnO/CdS/CdTe configuration, the short-circuit current was further improved throughout the visible wavelength region by replacing the Cu-metal contact with a Cu solution contact. As a result the short-circuit current from 21.7 to 26.1 mA/cm² and the conversion efficiency of the CdTe solar cell increased from 12 to 15% without antireflective coating. Our result indicates that the Cu solution back contact is a critical factor for achieving a higher cell efficiency in addition to ZnO buffer layer.     

免掺杂、非对称异质接触晶体硅太阳电池的研究进展

免掺杂、非对称异质接触的新型太阳电池由于近几年的飞速发展,理论转化效率已达到28%,具有较大的发展空间,引起了人们的重视.由于传统晶硅太阳电池产业存在生产设备成本高、原材料易燃易爆等诸多限制,市场对太阳电池产业低成本、绿色无污染的期待越来越高,极大地增加了免掺杂、非对称异质接触的新型太阳电池研究和开发的必要性.为了进一步加快免掺杂、非对称异质接触晶体硅太阳电池的研究进度,本文对其发展现状进行了综述,着重讨论了过渡金属氧化物(TMO)载流子选择性运输的基本原理、制备技术以及空穴传输层、电子传输层和钝化层对基于TMO构建的免掺杂、非对称异质接触(DASH)太阳电池性能的影响,以期对电池的工作机理、材料选择有更深刻的认识,为新型高效的DASH太阳电池制备提供指导.     

Determination of localized states in porous silicon

We determined the density of state distribution near the Fermi level in porous silicon from the analysis of the current–voltage (J–V) and the current–thickness (J–T) characteristics in the space-charge-limited-current (SCLC) regime. The distribution exhibits a minimum density at the Fermi level, which is similar to the U-shape-trap-distribution observed in crystalline Si–SiO₂ interface or in amorphous Si. Theoretical analysis well explains both the J–V and the J–L characteristics, which implies that the current flow is entirely controlled by localized states situated at the quasi-Fermi level.