13·9%‐efficient CdTe polycrystalline thin‐film solar cells with an infrared transmission of ∼50%

To fabricate a high‐efficiency polycrystalline thin‐film tandem cell, the most critical work is to make a high‐efficiency top cell ( > 15%) with high bandgap (E_g = 1·5–1·8 eV) and high transmission (T > 70%) in the near‐infrared (NIR) wavelength region. The CdTe cell is one of the candidates for the top cell, because CdTe state‐of‐the‐art single‐junction devices with efficiencies of more than 16% are available, although its bandgap (1·48 eV) is slightly lower for a top cell in a current‐matched dual‐junction device. In this paper, we focus on the development of a: (1) thin, low‐bandgap Cu_xTe transparent back‐contact; and (2) modified CdTe device structure, including three novel materials: cadmium stannate transparent conducting oxide (TCO), ZnSnO_x buffer layer, and nanocrystalline CdS:O window layer developed at NREL, as well as the high‐quality CdTe film, to improve transmission in the NIR region while maintaining high device efficiency. We have achieved an NREL‐confirmed 13·9%‐efficient CdTe transparent solar cell with an infrared transmission of ∼50% and a CdTe/CIS polycrystalline mechanically stacked thin‐film tandem cell with an NREL‐confirmed efficiency of 15·3%. Copyright © 2005 John Wiley & Sons, Ltd.     

Copper (I) Oxide (Cu2O) based back contact for p‐i‐n CdTe solar cells

In this paper a promising solution for the notorious problem of manufacturing a stable low ohmic back contact of a CdTe thin film superstrate solar cell is presented without using elemental copper. Instead we have used a Cu₂O layer inserted between the CdTe absorber and metal contact (Au). In contrast to the barrier free band alignment gained by using the transitivity rules, XPS measurements show a barrier in the valence band of the Cu₂O layers directly after deposition, which results in a low performing JV curve. The contact can be improved by a short thermal treatment resulting in efficiencies superior to copper based contacts for standard CdS/CdTe hetero junction solar cells prepared on commercial glass/FTO substrates. By replacing the CdS window layer with a CdS:O buffer layer efficiencies of >15% could be achieved. Copyright © 2016 John Wiley & Sons, Ltd.     

太阳电池中CdS多晶薄膜的微结构及性能

采用化学水浴法制备了CdS多晶薄膜，通过XRD，AFM，XPS和光学透过率谱等测试手段研究了CdS多晶薄膜生长过程中的结构和性能.结果表明，随着沉积的进行，薄膜更加均匀、致密，与衬底粘附力增强，其光学能隙逐渐增大，薄膜由无定形结构向六方(002)方向优化生长，同时出现了Cd(OH)2相.在此基础上，通过建立薄膜的生长机制与性能的联系，沉积出优质CdS多晶薄膜，获得了转化效率为13.38%的CdS/CdTe小面积电池.     

CdS纳米薄膜的水浴法制备与表征

CdS是一种直接能隙半导体，其带隙约为2．42eV，是一种良好的太阳能电池窗口层材料和过渡层材料。化学水浴法沉积CdS薄膜具有工艺简单，成本低廉，成膜均匀、致密以及可大面积生产等优点。本文通过对化学水浴法沉积CdS薄膜的研究，阐述了CdS膜的生成和生长过程及其机理，并不断优化此方法中的各种工艺参数，得到了适合做铜铟镓硒薄膜太阳能电池过渡层的CdS薄膜，并对该薄膜的形貌、结构和性能进行了分析。     

A comparative study of Cd‐ and Zn‐compound buffer layers on Cu(In1−x,Gax)(Sy,Se1−y)2 thin film solar cells

This paper reports a comparative study of Cu(In,Ga)(S,Se)₂ (CIGSSe) thin‐film solar cells with CBD‐CdS, CBD‐ZnS(O,OH) and ALD‐Zn(O,S) buffer layers. Each buffer layer was deposited on CIGSSe absorber layers which were prepared by sulfurization after selenization (SAS) process by Solar Frontier K. K. Cell efficiencies of CBD‐CdS/CIGSSe, CBD‐ZnS(O,OH)/CIGSSe and ALD‐Zn(O,S)/CIGSSe solar cells exceeded 18%, for a cell area of 0.5 cm². The solar cells underwent a heat‐light soaking (HLS) post‐treatment at 170 °C under one‐sun illumination in the air; among the three condtions, the ALD‐Zn(O,S)/CIGSSe solar cells showed the highest cell efficiency of 19.78% with the highest open‐circuit voltage of 0.718 V. Admittance spectroscopy measurements showed a shift of the N₁ defect's energy position toward shallower energy positions for ALD‐Zn(O,S)/CIGSSe solar cells after HLS post‐treatment, which is in good agreement with their higher open‐circuit voltage and smaller interface recombination than that of CBD‐ZnS(O,OH)/CIGSSe solar cells. Copyright © 2015 John Wiley & Sons, Ltd.     

太阳电池中CdS薄膜的制备及其性能的研究

分别采用化学池沉积(CBD)和真空蒸发法,在三种衬底(玻片、ITO玻片、SnO2玻片)上沉积CdS薄膜,并利用扫描电镜(SEM)、透射光谱、X射线衍射(XRD)等方法对沉积膜进行了测试分析,同时阐述了两种不同方法下CdS膜的生长沉积机制。     

Stable CdTe thin film solar cells with a MoOx back‐contact buffer layer

MoO_x thin films were employed as a buffer layer in the back contact of CdTe solar cells. A monograined CdS layer was employed as the window layer to reduce light absorption. The insertion of a MoO_x buffer layer in the back contact greatly reduced the Schottky barrier leading to increased fill factor and open‐circuit voltage. A CdTe solar cell, with an efficiency as high as 14.2%, was fabricated. The use of a MoO_x buffer layer made it possible to fabricate high‐efficient CdTe solar cell with much less Cu in the back contact, thus greatly enhancing the cell stability. Copyright © 2015 John Wiley & Sons, Ltd.     

CdTe-HgCdTe叠层太阳电池CdS窗口层透射光谱性能研究

CdS窗口层光谱透射率的提高对CdTe-HgCdTe叠层太阳电池有效利用入射太阳光并增大电池的短路电流密度有重要的影响。通过研究化学水浴法、近空间升华法和磁控溅射法制备的CdS薄膜在CdCl2退火前后的光谱平均透过率和短路电流密度损失表明:在光谱区520~820 nm,化学水浴法制备的CdS薄膜在退火前后具有最高的光谱平均透过率,对应的CdTe顶电池有最小的短路电流密度损失;在光谱区820~1150和520~1150 nm,磁控溅射法制备的CdS薄膜在退火前后均具有最高的光谱平均透过率,对应的HgCdTe底电池和CdTe-HgCdTe叠层太阳电池有最小的短路电流密度损失。在光谱区520~820、820~1150和520~1150 nm,CdCl2退火可以显著增大CdS薄膜的光谱平均透过率,降低对应CdTe顶电池、HgCdTe底电池和CdTe-HgCdTe叠层电池的短路电流密度损失。     

The effect of oxygen on interface microstructure evolution in CdS/CdTe solar cells

Microstructural changes at the CdS/CdTe solar cell interface where close‐spaced sublimation (CSS) is used as the growth technique to deposit the p‐type CdTe absorber layer are studied by systematic layer characterization at various stages during heterojunction growth. CdS layers grown by both chemical bath deposition (CBD) and CSS provide a basis for determining the effects of CdS crystallinity, grain size, and oxygen content on the subsequent CdTe layer. As‐grown CBD CdS films exhibit small grains and variations in optical properties attributed to film impurities. In contrast, CSS yields CdS films with good crystallinity, larger grains, and nearly ideal optical properties. The hexagonal nature of CSS‐grown CdS is seen to nucleate hexagonal CdTe during the initial stages of CdTe film growth. Cubic CdS deposited by CBD in contrast promotes cubic CdTe nucleation. Oxygen anneals in the latter case can aid hexagonal CdTe nucleation. Auger electron spectroscopy (AES) and transmission electron microscopy (TEM) of the CdS/CdTe interface show CdS‐dependent differences in interdiffusion at the interface. This interdiffusion appears to be determined by the oxygen level in the CdS. When low‐oxygen‐containing CSS CdS films are used, sulfur diffusion is substantial, leading to significant consumption of the CdS layer. When these same films are annealed in oxygen, the consumption is reduced. Te diffusion into the CdS layer is also observed to decrease with oxygen anneals. Optical modeling shows that Te alloying with the CdS layer can greatly reduce the short‐circuit current of CdS/CdTe devices. Copyright © 2002 John Wiley & Sons, Ltd.     

ILGAR‐ZnO Window Extension Layer: an adequate substitution of the conventional CBD‐CdS buffer in Cu(In,Ga) (S,Se)2‐based solar cells with superior device performance

A novel Window Extension Layer (WEL) concept for chalcopyrite‐based thin‐film solar cell devices has been developed. The optimization of its deposition is presented. WEL means the replacement of the conventional buffer layer by a layer consisting of the same material as the window, i.e., a part of the window is directly deposited onto the absorber by a soft process, such as ILGAR (Ion Layer Gas Reaction). This sequential cyclic technique has been applied to Cu(In,Ga)(S,Se)₂ absorber substrates. The ILGAR procedure was optimized with respect to the efficiency of the resulting Mo/Cu(In,Ga)(S,Se)₂/WEL/ZnO solar cells. The devices were characterized by J–V (under AM 1.5 and without illumination) as well as by quantum efficiency measurements. Devices with ZnO WEL yield total area (0.5 cm²) efficiencies of 14.6% (best cell) without any anti‐reflecting coating. The efficiencies are superior to those of the corresponding devices with CBD (Chemical Bath Deposition)‐CdS buffer (14.1%, best cell). Thus, in contrast to other ZnO buffers, ILGAR‐ZnO achieves record efficiencies exceeding those of CBD‐CdS buffered reference cells. Copyright © 2001 John Wiley & Sons, Ltd.     

Dependence of the efficiency of a CdS/CdTe solar cell on the absorbing layer’s thickness

On the basis of the continuity equation, the spatial distribution of photogenerated excess electrons in the neutral region of the CdTe layer in a CdS/CdTe heterostructure is analyzed taking into account recombination at the rear surface of the layer. It is demonstrated that, owing to diffusion, excess electrons penetrate deep into the CdTe layer at distances far exceeding the effective penetration length for solar radiation. Calculations of the short-circuit current indicate that, for electron lifetimes of 10^?10–10^?9 s, typical of thin-film CdS/CdTe solar cells, recombination losses are insignificant if the CdTe layer’s thickness amounts to 3–4 μm but increase dramatically if the thickness is below 1–1.5 μm. In order to eliminate recombination losses in more efficient solar cells where the electron lifetime is ≥10^?8 s the absorbing CdTe layer needs to be much thicker.     

A novel CdCl2 treatment for glass/SnO2/CBD-CdS/CdTe solar cell

Residual CdCl₂ in chemical bath deposited (CBD) CdS layer was utilized to observe grain growth in CdTe layer for glass/SnO₂/CBD-CdS/CdTe structures. The above as-deposited composite films were subjected to rapid thermal annealing (RTA) for observing grain growth and subsequent cell fabrication. The films were characterized by studying their microstructural and compositional properties. Interfacial mixing behavior was studied by secondary ion mass spectroscopy (SIMS) measurements which showed a slight interfacial diffusion of the CdS layer into the CdTe layer. Performance of a photovoltaic (PV) cell structure with non-optimized thickness of the CdTe and CdS layers obtained by this technique was studied. Carrier life time was obtained from V_oc decay measurement. Photoinduced charge separation observed in this glass/SnO₂/CBD–CdS/CdTe structure was associated with an increase in the dielectric constant and a decrease in the device resistance.     

Structural mechanisms of optimization of the photoelectric properties of CdS/CdTe thin-film heterostructures

Comparative studies of the effect of chloride treatment of CdS/CdTe thin-film heterostructures on the output characteristics of ITO/CdS/CdTe/Cu/Au solar cells and the crystal structure of their base CdTe layer are carried out. Structural mechanisms determining variation in the efficiency of photoelectric processes in ITO/CdS/CdTe/Cu/Au thin-film solar cells produced by varying the thickness of the CdCl₂ layer during the chloride treatment are suggested. It is shown for the first time by X-ray diffractometry that the metastable hexagonal CdTe phase transforms into a stable cubic modification during the chloride treatment. This circumstance provides a substantial improvement in the photoelectric properties of CdS/CdTe thin-film heterostructures.     

CdTe/CdS thin film solar cells grown in substrate configuration

The ability to grow efficient CdTe/CdS solar cells in substrate configuration would not only allow for the use of non‐transparent and flexible substrates but also enable a better control of junction formation. Yet, the problems of barrier formation at the back contact as well as the formation of a p–n junction with reduced recombination losses have to be solved. In this work, CdTe/CdS solar cells in substrate configuration were developed, and the results on different combinations of back contact materials are presented. The Cu content in the electrical back contact was found to be a crucial parameter for the optimal CdCl₂‐treatment procedure. For Cu‐free cells, two activation treatments were applied, whereas Cu‐containing cells were only treated once after the CdTe deposition. A recrystallization behavior of the CdTe layer upon its activation similar to superstrate configuration was found; however, no CdTe–CdS intermixing could be observed when the layers were treated consecutively. Remarkably high V_OC and fill factor of 768 mV and 68.6%, respectively, were achieved using a combination of MoO₃, Te, and Cu as back contact buffer layer resulting in 11.3% conversion efficiency. With a Cu‐free MoO₃/Te buffer material, a V_OC of 733 mV, a fill factor of 62.3%, and an efficiency of 10.0% were obtained. Copyright © 2012 John Wiley & Sons, Ltd.     

CuxS背接触层制备及在碲化镉薄膜太阳电池中应用研究

本文采用化学水浴法沉积CuxS薄膜,通过改变Cu元素比例研究其对碲化镉电池效率的影响。研究表明化学水浴法沉积的CuxS是非晶的,采用适当退火条件可以使其晶化,随着退火温度的提高,薄膜变得致密且结晶明显。CuxS薄膜厚度对电池性能有很大的影响,结果表明,随着CuxS薄膜厚度增加,电池性能先增加后减少。薄膜厚度为75nm时,CdS/CdTe电池性能最佳,达到了最高转化效率(η)为12.19%,填充因子(FF)为68.82%,开路电压(Voc)为820mV。     

CdTe/ZnTe/GaAs Heterostructures for Single-Crystal CdTe Solar Cells

Amorphous CdS/single-crystal CdTe solar cells were grown on GaAs substrates by metalorganic chemical vapor deposition. The structures of the films and the electrical properties of the devices were characterized. Highly conducting arsenic-doped ZnTe was grown on GaAs(100) substrates as the buffer layer for CdTe growth. By use of a ～30-nm ZnTe buffer layer, a p-CdTe film with a doping level of ～5×10¹⁶ cm^?3 was achieved. The hole concentration of p-CdTe increased with increasing VI/II ratio under a high As concentration during growth. From temperature-dependent Hall transport measurements, the ionization energy of the As acceptor in the p-CdTe was estimated to be approximately 88 meV. Ohmic behavior of the junctions between CdTe/ZnTe and ZnTe/GaAs was also confirmed. The solar cell performance of this structure, for example an open circuit voltage of 0.63 V, could be improved if the crystal quality of the CdTe film is optimized and the dislocation density of the CdTe film is minimized.     

Synthesis and characterization of novel oxygenated CdSe window layer for CdTe thin film solar cells

Significant increases in the solar conversion performance of thin film CdS/CdTe solar cells have been realized in recent years by the inclusion of wider gap materials, notably CdS:O, in place of the CdS window layer. Similarly, use of CdSe in combination with CdTe has given yet further increases in current density due to the formation of CdTe_1-xSe_x and the resulting enhanced long wavelength collection. Here we report the synthesis and properties of oxygenated CdSe films and their influence on device performance when included into CdTe solar cells. The CdSe:O films were made by pulsed laser deposition in oxygen at pressures of 0.01 and 0.03 Pa. Optical transmission measurements indicated that oxygenation acts to increase transparency in the 400–750 nm range, and that increased oxygen content acts to increase the apparent absorption threshold slightly. The higher transmission acts to increase the spectral response of CdTe solar cells in the range 350–500 nm compared to CdSe- and CdS-containing control devices. Moreover, the already known advantage of enhanced long wavelength response (850–910 nm) over that of CdS/CdTe is retained for both CdSe and CdSe:O window layers. Hence the use of CdSe:O as a window layer is demonstrated to increase the current harvesting of these solar cells by increasing both their long and short wavelength performances. These gains in the short circuit current outweigh small losses in fill factor and open circuit voltage making CdSe:O a valuable possible means to increase the performance of CdTe solar cells.     

CIGS薄膜太阳能电池缓冲层CdS薄膜的制备研究

目前CdS材料的制备方法有很多种,但是最常用的是化学水浴法。本文研究了浓度、反应溶液pH值、温度、沉积时间对CdS缓冲层薄膜的影响,对CIGS薄膜太阳能电池缓冲层CdS薄膜的制备方法进行了论述。     

Recent progress in thin-film cadmium telluride solar cells

Cadmium telluride (CdTe) with a room-temperature bandgap energy of 1.45 eV has been shown to be the most promising low-cost, thin-film photovoltaic material for terrestrial applications. Significant progress has been made during the past several years, and thin-film CdTe solar cells of > 1 cm² area with conversion efficiencies higher than 12% have been prepared by several techniques. Thin-film CdTe photovoltaic modules with 10% efficiency have also been produced. They are of the heterojuntion configuration using a transparent conducting semiconductor (TCS) as the window and p-CdTe as the absorber. In this paper, the potential window materials for thin-film CdTe solar cells are discussed. Thus far, cadmium sulphide (CdS) with a bandgap energy of 2.42 eV at room temperature has been found to be best suited for efficient CdTe solar cells. the deposition techniques for p-CdTe films capable of producing efficient solar cells, including close-spaced sublimation (CSS), electrodeposition, screen printing and spraying, are briefly reviewed, and the characteristics of the resulting solar cells are discussed. It is concluded that the efficiency of thin-film CdTe solar cells can be increased to 18-19% in the near-term, leading to 15-16.5% efficient modules.     

Preparation and characterization of pulsed laser deposited CdS/CdSe bi-layer films for CdTe solar cell application

CdS/CdSe bi-layer film was prepared by pulsed laser deposition with different substrate temperatures as an improved window layer for CdTe solar cells. The total thickness of each CdS/CdSe bi-layer film was about 70 nm, which could contribute to comparatively high transmittance of photons and, therefore, improving the photocurrent. Substrate temperature influenced the properties of the CdS/CdSe bi-layer films and the study showed that the bi-layer film prepared at 400 °C achieved the best optical transmittance and crystallinity. The crystal structure and optical transmittance of CdS/CdSe/CdTe stack before and after CdCl₂ annealing treatment were investigated by utilizing X-ray diffraction and UV/Vis spectrophotometer, respectively. It showed that further CdCl₂ annealing treatment improved the inter-diffusion of Se into CdTe, facilitating the formation of a CdTe_1−xSe_x alloy in the absorber layer. Comparing with CdTe, the alloy actually showed a smaller band gap which produced an obvious red shift of the absorption edge in long wavelength region. CdSe window layer was consumed by the inter-diffusion, while enhanced the short wavelength response in the range of 300–500 nm. The device based on CdS/CdSe window layer realized a J_SC enhancement due to the improved collection within both short and long wavelength regions accompany with a V_OC enhancement when compared to CdS/CdTe solar cell. The CdTe cell with CdS/CdSe bi-layer window deposited at 200 °C showed an efficiency of 13.47% with V_OC of 791 mV and J_SC of 27.40 mA/cm².