Enhancement of dielectric barrier layer properties by sol-gel and PECVD stacks

Enhancement of dielectric barrier layer properties by Sol-Gel and PECVD stacks Carmen López-López,*1 Mari-Fe Menendez,2 Luis Andres Menendez,2 Armando Menendez,2 Pascal Sánchez,2 Maria D. Alba,3 Emilio Sánchez-Cortezon,1 Jose-Maria Delgado-Sanchez1 1 Abengoa, Campus Palmas Altas, C/ Energía Solar 1, 41014 Sevilla, Spain 2 Photonic Group – ITMA Materials Technology, Calafates 11, 33417 Avilés, Spain 3 ICMS (CSIC-US), Americo Vespuccio 49, 41092 Sevilla, Spain Corresponding Author *Carmen López-López E-mail: [email protected] Address: Abengoa, Campus Palmas Altas, C/ Energía Solar 1, 41013 Sevilla, Spain ABSTRACT Thin-film PV modules grown on flexible, light weight, thermally stable and low cost substrates such as stainless steel foil, are an attractive product for solar market applications. When metal foils are used as substrate, it is essential to deposit a dielectric barrier layer to isolate electrically and chemically the thin-film solar cells front the substrate. In this work, SiOx stacks deposited on ‘rough’ stainless steel by a combination of a new sol-gel formulation and a Plasma Enhanced Chemical Vapor Deposition (PECVD) deposition step are reported as a suitable dielectric barrier layer candidate. Using these SiOx multilayers, a smooth and homogeneous film was achieved. X-ray diffraction (XRD) analysis showed that back contact of the solar cell (based on Molybdenum) is not affected by the presence of the barrier layer. Moreover, according to X-ray photoelectron spectroscopy (XPS) and Secondary Ion Mass Spectrometry (SIMS) measurements, this approach led to excellent barrier layer properties against the diffusion of impurities from the stainless steel. A complete electrical characterization of these dielectric barrier layers was also carried out showing good electrical insulation. GRAPHICAL ABSTRACT RESEARCH HIGHLIGHTS - Dielectric barrier layer for flexible metallic substrates has been developed. - SiOx based on Sol-Gel process showed a novel chemical formulation. - The dielectric barrier layer exhibits a dielectric breakdown voltage about 1000 V for a 4 microns thick stack. - Dielectric barrier layer was capable to reduce the surface roughness by 40-50%. - This research opens new challenges for low cost thin-film manufacturing. ABBREVIATIONS PECVD, Plasma Enhanced Chemical Vapor Deposition; CTE, coefficient thermal expansion; Rt, surface roughness; TOF-SIMS, SEM, Scanning Electron Microscopy; Secondary Ion Mass Spectrometry; XRD, X-ray diffraction; XPS, X-ray photoelectron spectroscopy. KEYWORDS: SiOx barrier layer, dielectric, stainless steel, breakdown voltage, flexible, thinfilms solar cell 1. INTRODUCTION One of the challenges for photovoltaic (PV) market applications is to be able to manufacture efficient low cost optoelectronic devices based on suitable low cost materials and processes. Therefore, there is an industrial interest to transfer conventional manufacturing process from thin film PV technologies (CIGS and CZTS) based on glass to alternative low cost materials.[ 1 - 7 ] Furthermore, new substrates for thin-film PV devices will open new market opportunities and features such as flexibility, low weight, low cost, roll-to-roll manufacturing, etc. It is believed that these PV modules are the ideal candidates for PV markets such as Building Integrated Photovoltaic Applications (BIPV) and distributed generation in rooftops. Stainless steel is often identified as one of the best alternative candidate to rigid glass substrates traditionally used for thin-film PV modules.[ 8 , 9 ] However, an extra layer must be added to the stainless steel, named dielectric barrier layer, to make it compatible with the thinfilm PV modules, guaranteeing their performance and their electrical interconnection through the so-called monolithic integration process. The dielectric barrier layer and its application on the metallic substrate must be designed with the following functions: (i) inhibit diffusion of transition metallic elements (Fe, Cr, Ni) from the stainless steel substrate to the semiconductor structure of the solar cell; (ii) reduce the surface roughness of the substrate as much as possible in order to ensure low cost and to minimize the pinholes creation; (iii) mechanically stable to avoid failures during the subsequent thermal annealing like delamination or presence of cracks. This is achieved by designing the layer with a coefficient of thermal expansion (CTE) similar to those materials in contact with it; (iv) provide electrical insulation between the metallic substrate and the back contact of the solar cell. This feature is critical in order to monolithically integrate the neighboring solar cells to build the thin-film PV module.[3] Numerous examples consisting of Al2O3, ZnO, SiO2 or enameled layer, deposited by sputtering, sol-gel, PECVD or spray techniques have been previously investigated.[ 10 - 14 ] Nevertheless, few of them report on the electrical properties of these layers and the effect of surface roughness on the dielectric function. The aim of this work was to develop and integrate dielectric barrier layers on commercially available stainless steel foils with high surface roughness and low cost. In the present work, the feasibility of dielectric barrier layers based on SiOx stacks has been studied. These SiOx stacks were grown by a combination of sol-gel and Plasma Enhanced Chemical Vapor Deposition (PECVD). It is well documented that one of the most common failures in this barrier layer is the mechanical stability, during and after the post-thermal annealing which is the process responsible for the formation of the CIGS or CZTS semiconductor.[8] Moreover it is demonstrated that, these mechanical failures can be solved by the insertion of SiOx multilayers between the back contact film (Molybdenum) and the stainless steel substrates. The processed samples were 100x100 mm2, which shows the scalability of the proposed process technology. 2. EXPERIMENTAL 2.1. Material selection In the recent years, several related works have shown that flexible metals such a have Al,[ 15 ] Ti,[ 16 ] and steel [10,9] can yield performances close to soda lime glass (SLG). Steel is a competitive material against Ti or Al in terms of both cost and physical properties (mechanical, thermal and coefficient thermal expansion). Moreover, steel meets requirements of a PV substrate since it is very good barrier to oxygen and water, and robustness against external shocks thanks to its good weight/stiffness ratio. Table 1 presents some technical data for potential candidates to substitute glass, such as density, CTE and maximum surface roughness [2]. Table 1. Density, coefficient of thermal expansion (CTE), and surface roughness (Rt) of soda lime glass (SLG) and alternative foils. SLG Ti Kapton Stainless Steel Density [g/cm3] 4.5 4.5 8.2 7.7 Tmax [ºC] 600 > 600 < 500 > 600 CTE [10-6 K-1] 9 8.6 17 10 – 11 Rt [nm] 10 - 15 1400 540 1600 To replace soda lime glass ensuring similar CTE and low cost, the selected substrate was stainless steel AISI430, 0.2 mm thickness and 100x100mm2 active area (average roughness (Ra=0.23±0.04µm) and peak-to-valley roughness (Rt=1.51±0.33µm)). The material proposed in this work as dielectric barrier multilayer is SiOx, which CTE (1-9·10-6 K-1) is similar to the AISI430 stainless steel substrate (10-11·10-6 K-1) and Mo back contact (5-6·10-6 K-1) of the solar cell. [2] Moreover, the SiOx, due to its strong ionic interatomic bonding, allows the impurity blocking. Finally, its dielectric character provides suitable electrical insulation between the stainless steel substrate and the electrode of the solar cells. 2.2. Processing In order to analyze the barrier layer properties of the SiOx stacks, two alternative structures based on PECVD and sol-gel have been deposited on the AISI430 stainless steel substrate, shown in Figure 1: a SiOx bilayer structure [Steel/PECVD/sol-gel] [BL- 01] (Figure 1A), and a three-layer SiOx structure [Steel /sol-gel/PECVD/sol-gel] [BL- 02] (Figure 1B). A full description of the sample preparation is presented below. Afterwards the Mo layer to serve as the solar cell back contact was deposited by DC- sputtering on both structures. This Mo layer is needed to evaluate the electrical insulation provided by the dielectric barrier layer. Figure 1. Barrier multilayer scheme integrated in thin-film solar cell structure. A) BL- 01: Steel/PECVD/sol-gel, B) BL-02: Steel/sol-gel/PECVD/sol-gel. Before film deposition, the stainless steel surface was cleaned with soapy water, acetone and dryed with nitrogen to remove any grease contamination which may have hindered the SiOx adhesion. A novel SiOx sol-gel process [ 17 ] based on acid catalysis was formulated to reduce the appearance of cracks during high temperature (ca. 550 ºC) thermal annealing. The coating sols were prepared from a 2 mL of tetraethylorthosilicate (TEOS), 8 mL of Methyltriethoxysilane (MTES) and 3 mL of 2,4,N,N’-Dimethylformamide (2,4,N,N’- DMF) which were mixed at room temperature under vigorous stirring during 15 min. After, 5 mL of polyethylene glycol 400 (PEG), 2mL of deionized water and 250 L of H3PO4 were added in different steps waiting 15 min between each one. The so-obtained solution was finally aged for 1 h before its use. After the addition of H3PO4, the temperature of the solution increased about 5 to 8 ºC and became transparent. To achieve the suitable solution for being spin-coated, the molar ratios were adjusted as follows: 4.9 MTES/TEOS, 2.4 DI H2O/(MTES+TEOS), 0.8 N,N’- DMF/(MTES+TEOS) and 0.3 PEG-400/(MTES+TEOS). The resulting sol-gel precursor had a pH between 5.0 and 5.5 and a viscosity of 14 and 18 cP (22 ºC). Then, 2 mL of precursor solution was deposited on the steel substrate by spin-coating (Spin coater SUS Microtec Delta 6RCTT) working at 1750 rpm for 20 seconds. A dense SiOx film was formed by sintering the sol-gel film in a hot plate (Titan 5P) following this thermal profile: 60 ºC for 15 minutes in order to remove the solvents, 150 ºC for 60 minutes, 300 ºC for 30 minutes and 550 ºC for 30 minutes, finally the coated substrates were taken out form the hot plate after cooling at room temperature. The PECVD SiOx layer was deposited by radiofrequency (13.56 MHz) in a cluster tool from Elettrorava (model V0714) with a ratio of SiH4 to N2O of 6:80. The applied power density was 20.8 mW/cm2 at 500 mTorr pressure and substrate temperature was 480 ºC, which resulted in a deposition rate of 6.25 Å/s. The final thicknesses of the SiOx stacks for both BL-01 and BL-02 were 3 and 5 µm ca, respectively. Previous experiments showed that thinner SiOx stacks presented a higher probability for pinhole formation whereas thicker SiOx stacks leaded to mechanical failures such as delamination or cracks formation.[2, 18 , 19 ] To measure the dielectric properties and the impurity diffusion through the SiOx stack a Molybdenum bilayer structure, suggested by Scofield et al, [ 20 ] was deposited by magnetron sputtering, with a total thickness of 800 nm to ensure good mechanical and electrical properties of the back contact. 2.3. Characterization Surface roughness of all samples was characterized with a mechanical profilometer (model Ambios XP1, Ambios Technology), following the ISO 4287 protocol. Scotch adhesion tests were performed based on ISO 2409 protocol. Depth profiling of the barrier layers and potential diffusion of metallic elements from the metallic substrate was performed by Secondary Ion Mass Spectrometry (TOF-SIMS) from a CAMECA system, using 5 keV primary Ar+ ions. Cross sections and top surface of the barrier layers were studied with a Scanning Electron Microscope (SEM), from Hitachi S4800. The film structure was analysed by X-ray diffraction (XRD) using a Philips Panalytical X’Pert X-ray diffractometer. X-ray photoelectron spectroscopy (XPS) is an ideal technique for analyzing the surface composition and the diffusion process of stainless steel elements such as Fe, Ni or Cr. XPS equipment used in this work was a VG ESCALAB210 with a monochromatic X-ray source Aluminum Kα line with 1486.6 eV. Depth profiles analyses were obtained by sputtering the surface with an Ar+ ion source. Electrical properties of the SiOx stacks were measured using an Agilent 34401A source meter coupled to a DC power supply. [ 21 ] 3. RESULTS AND DISCUSSIONS 3.1. Adhesion, morphology and microstructure The BL-01 and BL-02 SiOx stacks were initially kept at 85 ºC during 1000 h, and then submitted to periodic thermal cycles from -25 ºC to 85 ºC during 300 hours in order to stress them thermally and mechanically. After these thermal cycles, no evidence of cracks was found in any of the stacks. Furthermore, these layers showed a pencil hardness of 6HB, and the Scotch test was successfully conducted.[ 22 ] These results showed that the combination of this novel sol-gel formulation and the PECVD process, were a promising dielectric layer candidate since it passed all mechanical tests. Previous works with similar stack structure could not withstand a thermal treatment, which lead to mechanical failures. [10, 23 ] The SEM images of the samples showed dense, compact, continuous and uniform layers without any cracks in any of the proposed structures (Figure S1). Cross-section SEM images indicated that no tubular microstructure was present in any stack. Low density tubular growth is expected in magnetron sputtering processes and leads to various types of electrical failures such as pinholes or chemical contamination of the semiconductor due to an easy diffusion with metallic impurities from the substrate. Batchelor et al [ 24 ] reported that the photovoltaic performance of solar cells is strongly dependent on the surface roughness of the substrate. In order to avoid pinholes and shunts and to promote a uniform deposition of the subsequent layers, the native roughness of the substrate should be reduced by the barrier layer. [ 25 ] The evolution of the surface roughness average (Ra) value was studied (Figure 2) by mechanical profilometry, examples of roughness profiles before and after barrier layer deposition are displayed in Figure S2. It can be observed that both BL-01 and BL-02 stacks are able to reduce the initial substrate roughness over 40% and 50% ca, respectively. Regarding the roughness evolution during the different deposition steps, the novel solgel formulation is the key to improve the surface levelling since it reduces the roughness. Nevertheless, the application of the SiOx layer by PECVD technique is also crucial since it leads to a dense SiOx layer that filled up the possible defects forming during the sol-gel annealing stage, such as scratches or small grooves. The SiOx layer deposited by PECVD is also needed to obtain the optimum barrier layer thickness for Corresponding Author *Carmen López-López Abengoa, Campus Palmas Altas, C/ Energía Solar 1, 41013 Sevilla, Spain AKNOWLEDGEMENTS The research leading to these results has received funding from the European Union’s Research Fund for Coal and Steel (RFCS) research program under grant agreement nº RFSR-CT-2014-00014 5. 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Alba,3 Emilio Sánchez-Cortezon,1 Jose-Maria Delgado-Sanchez1 RESULTS AND DISCUSSIONS 1. Adhesion, morphology and microstructure SEM cross sectional images of BL-01 [Steel/PECVD/sol-gel] and BL-02 [Steel /solgel/PECVD/sol-gel] were performed to check the layered structure of both SiOx stacks. It can be seen in figure S1, the thickness of both structures was ca. 3 and 5 µm, respectively which fulfill the thickness requirements of the barrier layer. Figure S1. Cross section of barrier layers processed observed by SEM. BL-01 (right) and BL-02 (left). The surface topography of the stainless steel before and after barrier layer deposition was studied and it is displayed in Figure S2. One can observe the presence of streaks on the uncoated steel surface (Figure S2.A) due to the rolling process. Such surface topography was drastically smothered when the steel was coated with the SiOx stack (Figure S2.B). Figure S2. 3D surface topography images and surface profiles of Steel A) before coated and B) after barrier layer deposition. Figure S3 shows XRD patterns of the Mo layer deposited on glass and steel coated with BL-02 structure. From these XRD it is clear that the insertion of the barrier layer did not influence the deposition of the Mo layer, since the pattern of Mo grown on the BL-02 structure had the same diffraction peaks as those of the Mo on the glass. Figure S3. XRD patterns of Glass/Mo (top) and Steel/BL-02/Mo (bottom) 2. Dielectric performance of the barrier layer To assess the impact of mechanical stress on the sample quality, selected samples were subjected to both manual compression and tensile bending (Figure S4.A). Each sample was placed surface down to induce compressive strain and surface up to apply tensile strain (Figure S4.B). Figure S4.C shows the breakdown voltage measured for BL-01 and BL-02 samples after applying different bending radii. Although the bending strain was increased, the breakdown voltage of both structures remained almost unchanged. The optical microscopy results showed no evidence of cracks or surface deformation after the stress test was found. This characterization was also performance after 500 cycles at the minimum bending radius leading to the same conclusion: this process did not provoke any observable change in the electrical properties of the barrier layers. Figure S4. A) Images illustrating application of bending force for different bending radius. B) Schematic illustration of the measurement configuration for the bending test. C) Effect of the applied bending radius on the Breakdown voltage under tensile (Grey dots) and compressive (red dots) strain conditions.