The impact of MOCVD growth ambient on carrier transport,defects, and performance of CdTe/CdS heterojunction solar cells

CdTe solar cells were fabricated by depositing CdTe films on CdS/SnO₂/glass substrates in various metalorganic chemical vapor deposition growth ambient with varying Te/Cd mole ratio in the range of 0.02 to 15. The short-circuit current density (J_sc) showed a minimum at a Te/Cd ratio of 0.1 and increased on both sides of this minimum. The open-circuit voltage (V_oc) was found to be the highest for the Te-rich growth ambient (Te/Cd∼6)and was appreciably lower (600 mV as opposed to 720 mV) for the stoichiometric and the Cd-rich growth conditions. This pattern resulted in highest cell efficiency (12%) on Te-rich CdTe films. Auger electron spectroscopy revealed a high degree of atomic interdiffusion at the CdS/CdTe interface when the CdTe films were grown in the Te-rich conditions. It was found that the current transport in the cells grown in the Cd-rich ambient was controlled by the tunneling/interface recombination mechanism, but the depletion region recombination became dominant in the Te-rich cells. These observations suggest that the enhanced interdiffusion reduces interface states due to stress reduction or to the gradual transition from CdS to CdTe. The hypothesis of reduced defect density in the CdTe cells grown in the Te-rich conditions is further supported by the high effective lifetime, measured by time-resolved photoluminescence, and the reduced sensitivity of quantum efficiency to forward/light bias.     

Macroscopic Nanotemplating of Semiconductor Films with Hydrogen‐Bonded Lyotropic Liquid Crystals

Aqueous gel‐like lyotropic liquid crystals with extensive hydrogen bonding and nanoscale hydrophilic compartments have been used to define the growth of macroscopic nanotemplated CdS and CdTe thin films. These mesoporous semiconductor films contain a hexagonal array of 2.5 nm pores, 7 nm center‐to‐center, that extend in an aligned fashion perpendicular to the substrate. The CdS is deposited on a polypropylene substrate by a reaction between Cd(NO₃)₂ dissolved in the liquid crystal and H₂S transported via diffusion through the substrate. The CdTe is electrodeposited on indium‐tin‐oxide‐coated glass from TeO₂ and Cd(NO₃)₂, both of which are dissolved in the liquid‐crystal template. The porous nature of the CdTe films enables chemical transformations of the entire bulk of the film. As electrodeposited, the CdTe films are Te rich and, in contrast to a non‐templated film, the excess Te could be removed via a chemical treatment, proving the continuity of the pores in the nanotemplated films. These results suggest that liquid‐crystal lithography with hydrogen‐bonding amphiphiles may be a useful approach to create materials with nanoscale features over macroscopic dimensions.     

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.     

Growth of Thick Epitaxial CdTe Films by Close Space Sublimation

This paper reports on a detailed study of the development of the close space sublimation method, which has been widely used in the preparation of polycrystalline CdTe/CdS solar cells, as an epitaxial method for the growth of thick CdTe single crystal films over 200 μm on GaAs and Ge substrates for high-energy radiation detectors. The resulting microscopic growth phenomena in the process are also discussed in this paper. High-quality single crystalline CdTe thick films were prepared with x-ray rocking curves full width at half maximum (FWHM) values, which were ∼100 arcsec on Ge substrates and 300 arcsec on GaAs substrates. The quality of thick films on Ge(100) showed a substantial improvement with nucleation in a Te-rich growth environment. No Te inclusions in the CdTe films grown on GaAs(211)B and Ge(100) were observed with IR transmission imaging. Photoluminescence of CdTe/Ge shows a large reduction in the 1.44 eV defect energy bands compared with films grown on GaAs substrates. The film resistivity is on the order of 10¹⁰ Ω cm, and the film displayed some sensitivity to alpha particles.     

Focussed ion beam and field emission gun–scanning electron microscopy for the investigation of voiding and interface phenomena in thin‐film solar cells

The use of focussed ion beam milling combined with high resolution scanning electron microscopy analysis as a characterisation tool for thin‐film photovoltaics is reported. CdTe solar cell cross sections are examined in high detail with as‐grown and CdCl₂‐treated devices being compared. Observed changes in microstructure of the thin‐film layers are related to the device performance. The CdCl₂ treatment is shown to cause a reduction in the CdTe defect density at regions close to the interface and induce recrystallization of the CdS layer. Furthermore, the focussed ion beam technique is shown to reveal voids formed within the device's thin‐film layers at various processing stages that have not been previously observed in working cell structures. The back‐contacting Te‐rich layer resulting from nitric–phosphoric acid etching is also observed, with the etched layer being seen to propagate down the CdTe grain boundaries. Copyright © 2011 John Wiley & Sons, Ltd.     

Recrystallization and sulfur diffusion in CdCl2-treated CdTe/CdS thin films

The role of CdCl₂ in prompting recrystallization, grain growth and interdiffusion between CdS and CdTe layers in physical vapor-deposited CdS/CdTe thin-film solar cells is presented. Several CdTe/CdS thin-film samples with different CdTe film thicknesses were treated in air at 415°C for different times with and without a surface coating of CdCl₂. The samples were characterized by scanning electron microscopy, transmission electron microscopy, energy dispersive x-ray spectroscopy, x-ray diffractometry and optical absorption. The results show that CdCl₂ treatment enhances the recrystallization and diffusion processes, leading to a compositional variation within the CdTe layer due to diffusion of sulfur from the CdS. The highest sulfur concentrations observed after 30 min treatments with CdCl₂ at 415°C are near the solubility limit for sulfur in CdTe. The compositional distributions indicated by x-ray diffraction measurements of samples with different CdTe thickness show that the S-rich CdTe_1−xS_x region lies near the CdTe/CdS interface. A multiple-step mixing process must be inferred to account for the diffraction profiles obtained. © 1997 John Wiley & Sons, Ltd.     

A study of polycrystalline Cd(Zn,Mn)Te/Cds films and interfaces

Polycrystalline films of Cd_1-x Zn _x Te (x = 0–0.4) and Cd_1-x Mn _x Te (x = 0–0.25) were grown by MBE and MOCVD, respectively, on CdS/SnO₂/glass substrates to investigate their feasibility for solar cell applications. The compositional uniformity and interface quality of the films were analyzed by x-ray diffraction, surface photovoltage, and Auger depth profile measurements to establish a correlation between growth conditions and lattice constant, atomic concentration, and bandgap of the ternary films. MBE-grown polycrystalline Cd_1-x Zn _x Te films showed a linear dependence between Zn/(Cd + Zn) beam flux ratio, Zn concentration in the film, and the bandgap. Polycrystalline Cd_1-x Zn _x Te films grown at 300° C showed good compositional uniformity in contrast to compositionally non-uniform Cd_1-x Mn _x Te films grown by MOCVD in the temperature range of 420–450° C. The MBE-grown Cd_1-x Zn _x Te interface also showed significantly less interdiffusion compared to the MOCVD-grown Cd_1-x Mn _x Te/CdS interface, where preferential exchange between Cd from the CdS layer and Mn from the Cd_1-x Mn _x Te film was observed. The compositional uniformity of MOCVD-grown polycrystalline Cd_1-x Mn _x Te films grown on CdS/SnO₂/glass substrates was found to be a strong function of the growth conditions as well as the Mn source.     

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。     

近空间升华沉积CdTe薄膜的微结构和PL谱

用近空间升华法在CdS薄膜上沉积了CdTe薄膜.研究了在两种保护气氛下所沉积的多晶CdTe薄膜在后处理后的微结构、表面形貌及光致发光(PL)谱,并研究了CdTe表面和CdS/CdTe界面的PL谱的区别,根据薄膜的微结构对碲化镉在太阳电池中的应用进行了讨论.     

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.     

Direct growth of CdTe on (100), (211), and (111) Si by metalorganic chemical vapor deposition

CdTe epilayers were grown directly on (100), (211), and (111) silicon substrates by metalorganic chemical vapor deposition (MOCVD). The crystallinity and the growth orientation of the CdTe film were dependent on the surface treatment of the Si substrate. The surface treatment consisted of exposure of the Si surface to diethyltelluride (DETe) at temperatures over 600°C prior to CdTe growth. Direct growth of CdTe on (100) Si produced polycrystalline films whereas (lll)B single crystals grew when Si was exposed to DETe prior to CdTe growth. On (211) Si, single crystal films with (133)A orientation was obtained when grown directly; but produced films with (211)A orientation when the Si surface was exposed to DETe. On the other hand, only (lll)A CdTe films were possible on (111) Si, both with and without Te source exposure, although twinning was increased after exposure. The results indicate that the exposure to a Te-source changes the initial growth stage significantly, except for the growth on (111) Si. We propose a model in which a Te atom replaces a Si atom that is bound to two Si atoms.     

CdTe Films Deposited by Closed—space Sublimation

CdTe films are prepared by closed-space sublimation technology,Dependence of film crystalline on substrate materials and substrate temperature is investigated.It is found that films exhibit higher crystallinity at substrate temperature higher than 400℃.And the CdTe films deposited on CdS films with higher crystallinity have bigger crystallite and higher uniformity.Treatment with CdCl2 methanol solution promotes the crystallite growth of CdTe films during annealing.     

Influence of diffusion barriers on the nucleation and growth of CVD Cu for interconnect applications

Nucleation and growth behavior of Cu influence strongly the macroscopic properties of the resultant films. In this work the nucleation of CVD Cu on different underlayer materials is studied. It is found that nucleation on bare diffusion barrier surfaces leads to island growth and, therefore, bad wetting and adhesion. An enrichment of F, O and carbon was found at the interface between the CVD Cu film and the diffusion barrier. However CVD Cu deposited on top of Ta with a 200-Å PVD Cu layer on top results in good wetting. CVD Cu films grown on a PVD Cu layer expose a highly preferred 111 orientation. In this case SIMS analysis reveals a comparably low concentration of oxygen, carbon and flourine at the interface region between the CVD Cu and the barrier. These observations shed light on relevance of surface conditions for the CVD Cu deposition process. They significantly affect both film adhesion and crystal orientation, which are crucial for the use of CVD Cu as interconnect material.     

Growth of CdTe Films on Amorphous Substrates Using CaF<Subscript>2</Subscript> Nanorods as a Buffer Layer

Continuous biaxially textured CdTe films were grown on biaxial CaF₂ buffer layers. The CaF₂ nanorods were grown by oblique angle vapor deposition and possessed a {111} 〈121〉 biaxial texture. The CdTe film was deposited by metal organic chemical vapor deposition (MOCVD). Film morphology and the CdTe/CaF₂ interface were studied by scanning electron microscopy and transmission electron microscopy. Characterization showed that small CdTe grains formed initially from the CaF₂ surfaces. These small grains then merged into large columnar grains during growth. Analysis revealed that the crystalline orientation of the CdTe film followed the biaxial texture of the CaF₂ nanorods.     

Role of oxygen during CdTe growth for CdTe photovoltaic devices

Polycrystalline CdTe thin films for CdTe solar cell devices are typically grown using physical vapor techniques such as close‐spaced sublimation or vapor transport deposition with, in some cases, a small amount of oxygen in the process gas. In this report, we discuss the results of growing CdTe films in an environment of pure oxygen using close‐space sublimation. The results show that oxygen has a positive impact on CdTe film quality as it increases charge carrier mobility and lifetime in the absorber layer of CdTe‐based devices. This has resulted in a 10–20 mV improvement in open‐circuit voltages when growing the CdTe in pure oxygen. Copyright © 2014 John Wiley & Sons, Ltd.     

Copper migration in cdte heterojunction solar cells

CdTe solar cells were fabricated by depositing a Au/Cu contact with Cu thickness in the range of 50 to 150Å on polycrystalline CdTe/CdS/SnO₂/glass structures. The increase in Cu thickness improves ohmic contact and reduces series resistance (R_s), but the excess Cu tends to diffuse into CdTe and lower shunt resistance (R_sh) and cell performance. Light I-V and secondary ion mass spectros-copy (SIMS) measurements were performed to understand the correlations between the Cu contact thickness, the extent of Cu incorporation in the CdTe cells, and its impact on the cell performance. The CdTe/CdS/SnO₂/glass, CdTe/ CdS/GaAs, and CdTe/GaAs structures were prepared in an attempt to achieve CdTe films with different degrees of crystallinity and grain size. A large grain polycrystalline CdTe thin film solar cell was obtained for the first time by selective etching the GaAs substrate coupled with the film transfer onto a glass substrate. SIMS measurement showed that poor crystallinity and smaller grain size of the CdTe film promotes Cu diffusion and decreases the cell performance. Therefore, grain boundaries are the main conduits for Cu migration and larger CdTe grain size or alternate method of contact formation can mitigate the adverse effect of Cu and improve the cell performance.     

Optimization of vapor post‐deposition processing for evaporated CdS/CdTe solar cells

The effects of thermal annealing in conjunction with CdCl₂ vapor heat treatment on the properties of CdTe/CdS thin films and devices deposited by physical vapor deposition are reported. Results are compared for three treatment variations: high‐temperature anneal only, high‐temperature anneal followed by CdCl₂ vapor heat treatment and CdCl₂ vapor heat treatment only. X‐ray diffraction, transmission electron microscopy and scanning electron microscopy show improved crystallographic properties of the CdTe film and reduced CdS/CdTe interdiffusion when a high‐temperature anneal is used prior to CdCl₂ treatment. The CdTe/CdS solar cells fabricated using an anneal at 550°C in argon prior to the CdCl₂ vapor heat treatment exhibited improved electrical characteristics compared to cells fabricated with no anneal step, yielding an open‐circuit voltage exceeding 850 mV. Copyright © 1999 John Wiley & Sons, Ltd.     

氩氧气氛下沉积的CdTe薄膜及太阳电池的性质

研究了在氩氧气氛下近空间升华沉积CdTe的技术．发展了在表面十分平整的玻璃衬底上沉积优质CdTe薄膜的方法,对比了在玻璃衬底和CdS薄膜上CdTe薄膜的结构特征．通过研究氧分压对CdTe薄膜择优取向的影响,证实了在恰当的近空间升华沉积过程中,两种衬底上的CdTe薄膜具有相同的结构．研究了玻璃衬底上CdTe薄膜的电学与光学性质,观察了后处理对上述薄膜性质的影响,并研制出了效率达1338%的小面积CdTe 薄膜太阳电池．     

Molecular beam epitaxy of CdTe and Hg1-xCdxTe ON GaAs (100)

Using the molecular beam epitaxial (MBE) technique, CdTe and Hg_1-xCd_xTe have been grown on Cr-doped GaAs (100) sub-strates. A single effusion cell charged with polycrystal-line CdTe is used for the growth of CdTe films. The CdTe films grown at 200 °C with a growth rate of ~ 2 μm/hr show both streaked and “Kikuchi” patterns, indicating single crystalline CdTe films are smoothly grown on the GaAs sub-strates. A sharp emission peak is observed at near band-edge (7865 Å, 1.577 eV) in the photoluminescence spectrum at 77 K. For the growth of Hg_1-xCd_xTe films, separate sources of HgTe, Cd and Te are used. Hg_0.6Cd_0.4Te films are grown at 50 °C with a growth rate of 1.7 μm/hr. The surfaces are mirror-smooth and the interfaces between the films and the substrates are very flat and smooth. As-grown Hg_0.6Cd_0.4Te films are p-type and converted into n-type by annealing in Hg pressure. Carrier concentration and Hall mobility of an annealed Hg_0.6Cd_0.4Te film are 1 × 10¹⁷ cm^?3 and 1000 cm²/V-sec at 77 K, respectively.     

Crystal defects and interdiffusion behavior of Hg1−xCdxTe/(100)CdTe epitaxial layers grown by chemical vapor transport

Crystal defects of chemical vapor transport grown Hg_1−xCd_xTe on (100) CdTe structures have been investigated using chemical etching, wavelength-dispersive spectroscopy, x-ray rocking curve, and scanning electron microscopy methods The results indicate that the origin and spatial distribution of the misfit dislocations can be attributed to both the lattice parameter misfit and the inevitable interdiffusion occurring between the substrate and the epitaxial layer. It is proposed that the interdiffusion of Hg along the [100] direction is enhanced by dislocation channels and other defect cores along or near this direction owing to defects on the initial surface of the CdTe substrate. The results indicate that the subgrain boundaries in Hg_1−xCd_xTe are caused by slight misorientation of the lattices and polygonization of the defects during epitaxial layer growth, and by the propagation of the subgrain boundaries existing in the CdTe substrate.