A Study of Ta-Si-N/Ti Bilayer Diffusion Barrier for Copper/Silicon Contact Systems

Ta-Si-N (10 nm)/Ti (20 nm) bilayer film has been designed with the purpose of using as diffusion barrier in copper interconnection. Ta-Si-N/Ti bilayer diffusion barriers were deposited on the substrate of n-type (100) silicon wafer using radio-frequency (RF) magnetron sputtering, followed by in situ deposition of copper. To investigate the thermal stability of the Ta-Si-N/Ti diffusion barriers, annealing was subsequently conducted in N₂ gas for 60 min and annealing temperatures were chosen at 600°C, 650°C, 700°C, 750°C, and 800°C. X-ray diffraction (XRD) revealed that Ta-Si-N layer grown on the Ti layer exhibited an amorphous phase. The results indicated that Ta-Si-N/Ti film can prevent copper diffusion at 750°C. After annealing at 750°C, the production of Ti-Si layer can effectively decrease contact resistance between barrier and silicon.     

High voltage Cu(In,Ga)S2 solar modules

Sulfurcell (SC) has been running a pilot production for thin-film solar modules using CuInS₂-chalcopyrite (CIS) as absorber material since 2004. Since then production technology has been constantly improved with module power values exceeding 64 W, corresponding to an aperture area efficiency level of about 9%. Small area (0.5 cm2) cells cut out of such CIS modules reach maximum efficiencies close to 11%. Strong efforts have been made to develop a new sequential Cu(In,Ga)S₂ (CIGS) process suitable for production of large-scale CIGS solar modules thereby enabling module efficiencies above 10%. CIGS-based solar cells are—quite similar to CIS-based modules—prepared from sputtered metals subsequently sulfurized using rapid thermal processing in sulfur vapor. Such Cu(In,Ga)S₂ solar cells reach material record efficiencies about 13%. The cells are characterized by high open-circuit voltages up to 890 mV. Based on the results of the “Helmholtz Zentrum Berlin” (HZB), Sulfurcell has successfully scaled this process to our typical module size of 125 cm × 65 cm and is currently piloting the process for mass production. This paper will give an overview of electrical and structural parameters of world's first large-scale CIGS modules. CIGS module and cell parameters will be compared with standard CIS module and cell parameters and measured CIGS efficiency temperature coefficients will be compared with typical temperature coefficients of modules based on established PV technologies.     

Characterization of poly-Si thin-film solar cell functions and parameters with IR optical interaction techniques

Three different optical interaction techniques have been employed to characterise the electrical and material parameters of polycrystalline silicon (poly-Si) thin-film solar cells with an interdigitated mesa structure. First, Light Beam Induced Current (LBIC) in the infrared range was used to locally analyse the light collection properties. Second, electroluminescence in forward bias (EL) yielding information on band to band recombination was performed. Third, electroluminescence in reverse bias (ELR) was utilized to gain information on the intraband relaxation. The EL and ELR measurements were performed using cooled Si-CCD (Silicon-based Charge Coupled Device) and InGaAs (Indium Gallium Arsenide) detectors. The high resolution IR-LBIC measurement equipped with a 1,064 nm wavelength laser has been applied to investigate the grain boundary characteristics in the absorber layer. Additionally, the local electrical characteristics of the absorber layer (diffusion length, doping concentration and built-in potential) have been extracted by performing a bias-dependent IR-LBIC measurement based on a simple theoretical model with the assumption of relatively small diffusion length compared to the absorber layer thickness. The local/spatial distribution of the diffusion length in the absorber layer of the thin-film solar cell has been extracted. Furthermore, the temperature dependence of the photocurrent of thin-film solar cells in a temperature range of −25 to +70 °C has been locally investigated using IR-LBIC. Additionally, the temperature dependence of the reverse bias characteristics of the poly-Si thin-film solar cell is analysed and compared with that of monocrystalline Si solar cell. For the EL and ELR measurements a spectral analysis of the emitted light has been performed. From the EL results material properties like diffusion length and process induced defects have been deduced and insights on the quality of production processes like metallization and etching were gained. The complementary information from the ELR measurements provides access to additional types of defects resulting from generation centres, such as lattice disorder, crystal defects and charged coulomb centers.     

N含量对Ta-Si-N扩散阻挡层阻挡性能的影响

采用磁控反应溅射技术在p型Si(111)衬底上制备了Ta-Si-N薄膜与Cu/Ta-Si-N复合结构,并对样品进行了快速热处理.用四探针电阻测试仪、原子力显微镜、X射线衍射和扫描电镜等对样品的电阻、形貌、结构与特性进行了分析表征.实验结果表明,随着N含量的增加,Ta-Si-N薄膜的方块电阻单调增加,表面粗糙度则先减小后增大;Ta-Si-N阻挡层的阻挡性能随N含量的增加而有所增强,但当N含量过大时,阻挡性能的提升并不明显;沉积态的Ta-Si薄膜为纳米晶结构,掺入N后,薄膜成为非晶态,但在高温热处理后Ta-Si-N薄膜重新结晶,铜原子主要通过晶界扩散并与Si反应,导致阻挡层失效.     

Pilot production of large-area CuInS2-based solar modules

A pilot production of CuInS₂-based thin-film solar modules has been established in Berlin, Germany. To date, its 125 cm × 65 cm modules have produced an aperture area conversion efficiency as high as 7.6%, and avenues towards even higher performance levels have been identified. The pilot production features industrially-proven sputtering technology for material deposition and a new rapid thermal process allowing a five-minute sulfurization cycle time. The first thousand modules produced (representing a total of 45 kWp) were subjected to statistical analysis documenting the continuous enhancement of module power over the early months of pilot production. Transient effects were found that influence module I(V) characteristics cause inaccurately low module power measurements when flasher-type sun-simulators are used in testing — a phenomenon correctable by the introduction of a pre-test light-soaking procedure. In order to verify the post-light-soaking test results data from outdoor measurements taken under standard test conditions as well as performance data of outdoor PV systems were analyzed. A higher energy output on a Wh/Wp-basis found for CuInS₂-based solar modules than for polycrystalline silicon-based solar modules is presumed to be due to a lower temperature coefficient. For evaluating the long-term future potential of the new CuInS₂-based technology, a cost model is introduced to assess the economic relevance of uptime, cycle time and yield values, and to show that both efficiency and productivity are crucial for high-capacity manufacture of thin-film solar modules.     

Thin-film solar cells

The rapid progress that is being made with inorganic thin-film photovoltaic (PV) technologies, both in the laboratory and in industry, is reviewed. While amorphous silicon based PV modules have been around for more than 20 years, recent industrial developments include the first polycrystalline silicon thin-film solar cells on glass and the first tandem solar cells based on stacks of amorphous and microcrystalline silicon films (“micromorph cells”). Significant thin-film PV production levels are also being set up for cadmium telluride and copper indium diselenide.     

Thermal effects and plasma damage upon encapsulation of polymer solar cells

A barrier structure consisting of silicon oxide and silicon nitride films was deposited via plasma-enhanced chemical vapor deposition (PECVD) for the encapsulation of polymer solar cells (PSCs). The total concentration of the solution and the ratio of P3HT and PCBM on the performance of polymer solar cells were studied by UV-Vis absorption spectroscopy, atomic force microscopy and photocurrent measurement. Base on these measurements, there is a compromise between light absorption and phase separation with increasing blend concentration. The PSCs were annealed at 80, 100, 120 and 140 °C for 10-60 min to investigate the thermal effects and to estimate the best deposition temperature of the barrier layers. Nevertheless, the devices with the encapsulation of barrier layers had relatively low power conversion efficiencies (PCE) of 0.98% comparing to the devices heated in the PECVD system (1.57%) at the same condition of 80 °C for 45 min due to the plasma damage during the film deposition process. After inserting a 5-nm TiO_x layer between Al/barrier structure and active layer against the plasma damage, the annealed devices presented an average PCE of 2.26% and demonstrated over 50% of their initial value after constant exposure to ambient atmosphere and sunlight for 1500 h.     

Magnetic Field-Assisted Interface Embedding Strategy to Construct 2D/3D Composite Structure for Stable Perovskite Solar Cells with Efficiency Over 24%

Perovskite solar cells (PSCs) based on 2D/3D composite structure have shown enormous potential to combine high efficiency of 3D perovskite with high stability of 2D perovskite. However, there are still substantial non-radiative losses produced from trap states at grain boundaries or on the surface of conventional 2D/3D composite structure perovskite film, which limits device performance and stability. In this work, a multifunctional magnetic field-assisted interfacial embedding strategy is developed to construct 2D/3D composite structure. The composite structure not only improves crystallinity and passivates defects of perovskite layer, but also can efficiently promote vertical hole transport and provide lateral barrier effect. Meanwhile, the composite structure also forms a good surface and internal encapsulation of 3D perovskite to inhibit water diffusion. As a result, the multifunctional effect effectively improves open-circuit voltage and fill factor, reaching maximum values of 1.246 V and 81.36%, respectively, and finally achieves power conversion efficiency (PCE) of 24.21%. The unencapsulated devices also demonstrate highly improved long-term stability and humidity stability. Furthermore, an augmented performance of 21.23% is achieved, which is the highest PCE of flexible device based on 2D/3D composite perovskite films coupled with the best mechanical stability due to the 2D/3D alternating structure.     

Highly efficient CIS solar cells and modules made by the co-evaporation process

Thin-film photovoltaic modules which use the chalcopyrite Cu(In,Ga)(Se,S)₂ (CIGS) as the light-absorbing layer have now entered the decisive industrial phase. Companies located mainly in Germany and Japan will produce more than 100 MWp CIGS modules in 2008, demonstrating that the CIGS technology has already achieved a certain maturity. Whereas key features of the technology are already well-optimized, there are several approaches to further improve the productivity of new lines. The ZSW operates a line for 30 × 30 cm² modules in which all process steps - from glass cleaning to module encapsulation - are being developed. A major goal of the development is the very fast and efficient transfer of promising new materials and processes from cells to the industrial module level. Therefore, ZSW is focusing on processes like the in-line co-evaporation method for CIS or chemical bath deposition for buffer layers to optimize the junction. We could demonstrate efficiencies close to 18% for small test cells and 14-15% for modules with modified processes. Different cell and material data from optoelectronic measurements and microscopic analysis will be presented in this contribution.     

High performance encapsulation structures utilizing Russian Doll architectures

A Russian Doll encapsulation architecture utilizing pairs of free-standing barrier films and epoxy seals separated by nitrogen spacers is presented, enabling the use of low-cost epoxy to attach two or more free-standing barrier films to a substrate with improved barrier performance. The performance of various Russian Doll encapsulations was evaluated with the calcium thin film optical transmission test, showing improved performance of the Russian doll configuration relative to a non-nested barrier/spacer architecture, and demonstrating that water vapor transmission rates (WVTR) of 0.00021 g/(m², day) or below can be achieved with low-cost materials in this architecture. This WVTR correlates to a predicted lifetime of more than 10 years for bulk heterojunction solar cell modules fabricated and tested by Konarka Technologies (Lowell, MA, USA).     

Atomic layer deposition on polymer based flexible packaging materials: Growth characteristics and diffusion barrier properties

One of the most promising areas for the industrial application of atomic layer deposition (ALD) is for gas barrier layers on polymers. In this work, a packaging material system with improved diffusion barrier properties has been developed and studied by applying ALD on flexible polymer based packaging materials. Nanometer scale metal oxide films have been applied to polymer-coated papers and their diffusion barrier properties have been studied by means of water vapor and oxygen transmission rates. The materials for the study were constructed in two stages: the paper was firstly extrusion coated with polymer film, which was then followed by the ALD deposition of oxide layer. The polymers used as extrusion coatings were polypropylene, low and high density polyethylene, polylactide and polyethylene terephthalate. Water vapor transmission rates (WVTRs) were measured according to method SCAN-P 22:68 and oxygen transmission rates (O₂TRs) according to a standard ASTM D 3985. According to the results a 10 nm oxide layer already decreased the oxygen transmission by a factor of 10 compared to uncoated material. WVTR with 40 nm ALD layer was better than the level currently required for most common dry flexible packaging applications. When the oxide layer thickness was increased to 100 nm and above, the measured WVTRs were limited by the measurement set up. Using an ALD layer allowed the polymer thickness on flexible packaging materials to be reduced. Once the ALD layer was 40 nm thick, WVTRs and O₂TRs were no longer dependent on polymer layer thickness. Thus, nanometer scale ALD oxide layers have shown their feasibility as high quality diffusion barriers on flexible packaging materials.     

The role of buffer layer between TCO and p-layer in improving series resistance and carrier recombination of a-Si:H solar cells

The properties of the window layer and transparent conducting oxide (TCO)/p interface in silicon based thin-film solar cells are important factors in determining the cell efficiency. As the potential barrier got larger at the interface, the transmission of photo-generated holes were impeded and the recombination of photo-generated electrons diffusing back toward the TCO interface were enhanced leading to a deterioration of the fill factor. In this paper different p-layers were studied. It was found that using p-type hydrogenated amorphous silicon oxide (a-SiO_x:H) layer as the window layer along with a 5 nm buffer layer which reduced the barrier at the fluorine doped tin oxide (SnO₂:F) TCO/p-layer interface, improved the cell efficiency. a-SiO_x:H was used as the buffer layer. With the buffer layer between TCO and p-type a-SiO_x:H, the potential barrier dropped from 0.506 eV to 0.472 eV. This lowered barrier results in increased short circuit current density (J_sc) and fill factor (FF). With the buffer layer, J_sc increased from 11.9 mA/cm² to 13.35 mA/cm² and FF increased from 73.22% to 74.91%.     

Surface textured molybdenum zinc oxide for light diffusion enhancement

Surface textures have been fabricated on a molybdenum doped zinc oxide (MZO) film using a shadow mask in a co-sputter process. The surface textures yielded 5.3% and 10.1% of light diffusion in the visible light region for MZO films with a thickness of 100 nm and 200 nm, respectively. Light diffusion in the near infra-red region was slightly less with 4.5% for the 100 nm MZO film and 8.9% for the 200 nm MZO film. The enhanced light diffusion will be beneficial to the light trapping efficiency of a-Si/µ-Si based thin film solar cells.     

Ultrafast deposition of silicon nitride and semiconductor silicon thin films by hot wire chemical vapor deposition

The technology of Hot Wire Chemical Vapor Deposition (HWCVD) or Catalytic Chemical Vapor Deposition (Cat-CVD) has made great progress during the last couple of years. This review discusses examples of significant progress. Specifically, silicon nitride deposition by HWCVD (HW-SiN_x) is highlighted, as well as thin film silicon single junction and multijunction junction solar cells. The application of HW-SiN_x at a deposition rate of 3 nm/s to polycrystalline Si wafer solar cells has led to cells with 15.7% efficiency and preliminary tests of our transparent and dense material obtained at record high deposition rates of 7.3 nm/s yielded 14.9% efficiency. We also present recent progress on Hot-Wire deposited thin film solar cells. The cell efficiency reached for (nanocrystalline) nc-Si:H n-i-p solar cells on textured Ag/ZnO presently is 8.6%. Such cells, used in triple junction cells together with Hot-Wire deposited proto-Si:H and plasma-deposited SiGe:H, have reached 10.9% efficiency. Further, in our research on utilizing the HWCVD technology for roll-to-roll production of flexible thin film solar cells we recently achieved experimental laboratory scale tandem modules with HWCVD active layers with initial efficiencies of 7.4% at an aperture area of 25 cm².     

Contact resistivity measurements of the buried Si-ZnO:Al interface of polycrystalline silicon thin-film solar cells on ZnO:Al

An experimental method is developed for contact resistivity measurements of a buried interface in polycrystalline silicon (poly-Si) thin-film solar cell devices on aluminum doped zinc oxide (ZnO:Al) layers. The solar cell concept comprises a glass substrate covered with a temperature-stable ZnO:Al film as transparent front contact layer, a poly-Si n⁺/p⁻/p⁺ cell, as well as a metal back contact. Glass/ZnO:Al/poly-Si/metal test stripe structures are fabricated by photolithographic techniques with the ZnO:Al stripes locally bared by laser ablation. The high-temperature treatments during poly-Si fabrication, e.g. a several hours lasting high-temperature step at 600 °C, are found to have no detrimental impact on the ZnO:Al/Si interface contact resistivity. All measured ρ_C values range well below 0.4 Ω cm² corresponding to a relative power loss ΔP below 3% for a solar cell with 500 mV open circuit voltage and 30 mA/cm² short circuit current density. By inclusion of a silicon nitride (SiN_x) diffusion barrier between ZnO:Al and poly-Si the electrical material quality of the poly-Si absorber can be significantly enhanced. Even in this case, the contact resistivity remains below 0.4 Ω cm² if the diffusion barrier has a thickness smaller than 10 nm.     

Manufacturing and application of CIS solar modules

The pilot production of Cu(In,Ga)Se₂ (CIS) modules at Würth Solar has progressed steadily, and the pilot line could be transferred successfully into a continuous operation reaching maximum capacity in 2005 of 1.5 MW_p. Best modules on the standard size of 60 cm × 120 cm reached 85 W_p, which corresponds to 13% aperture area efficiency. The average module efficiency has been steadily improved reaching values between 11% and 12% in the year 2005. The overall process yield of the pilot line could be increased and stabilised at high values well above 80%.In April 2005 the Würth Group has decided to invest in a new production line with a starting capacity of 15 MW_p/a. This capacity will be available at the end of 2006. The new building at the new location in Schwäbisch Hall/Germany will be ready in mid 2006.The long-time reliability of Würth Solar CIS modules could be proven by passing successfully the certified test according to EN61646 and by stable operation in the field for several years. Additionally, outdoor results with CIS modules in various applications show high energy ratings which are at least as good as the best c-Si systems. Furthermore, various CIS module types have been developed for building integration and other applications.     

薄膜太阳能电池的研究进展

综述了目前国际上研究得最多的几种薄膜太阳能电池材料的研究现状和各自的最新进展.包括硅基类(非晶硅、多晶硅、微晶硅)、无机化合物类(碲化镉、铜铟硒、砷化镓)、有机类、染料敏化(二氧化钛、氧化锌)等,并从材料、工艺和转换效率等方面比较和讨论了它们各自性能的优劣,最后展望了这些薄膜太阳能电池材料未来的研究方向及应用前景.     

CIGS thin-film solar cells on steel substrates

Steel foil is an attractive candidate for use as a flexible substrate material for Cu(In_x,Ga_1 − x)Se₂ solar cells (CIGS). It is stable at the high temperatures involved during CIGS processing and is also commercially available. Stainless chromium (Cr) steel is more expensive than Cr-free steel sheets, but the latter are not stable against corrosion. We processed CIGS solar cells on both types of substrates. The main problem arising here is the diffusion of detrimental elements from the substrate into the CIGS absorber layer. The diffusion of iron (Fe) and other substrate elements into the CIGS layer was investigated by Secondary Ion Mass Spectrometry (SIMS). The influence of the impurities on the solar cell parameters was determined by current voltage (JV) and external quantum efficiency (EQE) measurements. A direct correlation between the Fe content in the CIGS layer and the solar cell efficiency was found. The diffusion of Fe could be strongly reduced by a diffusion barrier layer. Thus we could process CIGS solar cells with a conversion efficiency of 12.8% even on Cr-free steel substrate.     

Sulphurization of single-phase Cu11In9 precursors for CuInS2 solar cells

Using Rutherford backscattering (RBS), X-ray diffraction (XRD), and scanning electron microscopy (SEM), the sulphurization of single-phase Cu₁₁In₉ precursors to be employed as light absorbing CuInS₂ (CIS) layers in CIS-CdS heterojunction thin-film solar cells has been investigated. The Cu₁₁In₉ precursor films were produced by DC-sputtering from a single-phase Cu₁₁In₉ target. The sulphurization at 500 or 300 °C was performed by adding different amounts of elemental sulphur with heating rate and sulphurization time as additional parameters. During sulphurization at 500 °C, up to 50% of the indium initially present in the precursor is lost. We relate the In-loss to the volatile In₂S compound, the formation of which is favoured by the phase transition of Cu₁₁In₉ to Cu₁₆In₉ at 307 °C. Consequently, the In-loss can be suppressed by employing a sulphurization temperature of 300 °C. At this temperature, a prolonged sulphurization time and a large sulphur excess are necessary in order to obtain stoichiometric CIS beneath a CuS_x surface phase.     

A new approach to the manufacture of chalcogenide thin film solar cells

Glass beads of 0.2 mm diameter are covered with molybdenum, copper and indium. The copper/indium layers are transformed into copper indium disulfide (CIS) by exposing the glass beads to a hydrogensulfide/argon mixture at temperatures of around 500 °C. The CIS covered glass beads serve as the basis material for the formation of solar cells. The main advantage of this approach is the separation between absorber and cell/module formation. In this paper the different process steps necessary for cell manufacturing are described. Some properties of solar cells made out of CIS covered glass beads are presented.