Damp Heat Test Research Articles

Degradation of encapsulants for photovoltaic modules made of ethylene vinyl acetate studied by positron annihilation lifetime spectroscopy

The structure of ethylene vinyl acetate (EVA) encapsulants of crystalline-Si photovoltaic modules after the damp heat (DH) test was evaluated by positron annihilation lifetime spectroscopy (PALS). A reduction in free-volume hole size, which indicates the progress of deacetylation, was observed after the DH test. The difference in lifetime (Δτ) between the initial and DH-tested samples clearly increased after the DH test for 3000 h. The increase in Δτ was correlated with the acetic acid concentration in the EVA estimated by ion chromatography. The depth profile analysis by slow positron beam PALS revealed that Δτ in the near-surface region of the Si-cell side was significantly larger than that of the cover-glass side. This result indicates that deacetylation near the Si cell/EVA interface is accelerated.

Correlation study of damp heat and pressure cooker testing on backsheets

ABSTRACTFor photovoltaic (PV) modules, high‐quality backsheets are essential. To verify these qualities, hot and humid tests of up to 1000 h are standard in the PV industry. Damp heat (DH) tests are useful for evaluating material quality and infant failures, especially for materials that are sensitive to hydrolysis. However, shorter test periods are needed. Hence, the objective of this study was to investigate and correlate the aging behavior of hydrolysis‐sensitive polyester [poly(ethylene terephthalate) (PET)] based backsheets after exposure to DH and pressure cooker conditions. The material properties, such as thermal and mechanical parameters, correlated well between the pressure cooker at 120 °C and the DH test, with the qualification that differences in the failure mechanism were seen after 96 and 2000 h, respectively. Nevertheless, it should be acceptable to use pressure cooker tests (PCTs) for fast material testing. The PCT is proposed to significantly reduce the testing time for infant failures and material quality to around 4 days for PET‐based backsheets. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016, 133, 44230.

Influence of oxide scales on the corrosion behaviors of B510L hot-rolled steel strips

The influence of oxide scales on the corrosion behaviors of B510L hot-rolled steel strips was investigated in this study. Focused ion beams and scanning electron microscopy were used to observe the morphologies of oxide scales on the surface and cross sections of the hot-rolled steel. Raman spectroscopy and X-ray diffraction were used for the phase analysis of the oxide scales and corrosion products. The corrosion potential and impedance were measured by anodic polarization and electrochemical impedance spectroscopy. According to the results, oxide scales on the hot-rolled strips mainly comprise iron and iron oxides. The correlation between mass gain and test time follows a power exponential rule in the damp-heat test. The corrosion products are found to be mainly composed of γ-FeOOH, Fe3O4, α-FeOOH, and γ-Fe2O3. The contents of the corrosion products are different on the surfaces of the steels with and without oxide scales. The steel with oxide scales is found to show a higher corrosion resistance and lower corrosion rate.

Degradation mechanism of Cu(In,Ga)Se2 solar cells induced by exposure to air

The degradation mechanism of unencapsulated Cu(In,Ga)Se2 (CIGS) solar cells upon exposure to air has been investigated. Exposure to air at room temperature slightly reduces the conversion efficiency of CIGS solar cells. However, this conversion efficiency decreases significantly under damp heat testing at 85 °C and a relative humidity of 85% for 15 h. The shunt resistance and conversion efficiency are completely recovered after removing the side edges of the CIGS solar cells by mechanical scribing. This result suggests that low-resistive layers are formed on the sidewalls of the solar cells during damp heat testing. In addition, alkaline solution etching has been confirmed to be an effective way of removing the low-resistive layers. The low-resistive layers on the sidewalls are identified to be molybdenum oxides and sodium molybdate by Auger electron spectroscopy. After etching the oxides on the sidewalls, the saturation current density and ideality factor are confirmed to be improved.

High-Quality Hollow Closed-Pore Silica Antireflection Coatings Based on Styrene-Acrylate Emulsion @ Organic-Inorganic Silica Precursor.

Making use of a facile and low-cost way for the preparation of a hierarchically organized novel hollow closed-pore silica antireflective coating (CHAR) with tailored optical properties and a mechanical reliability is of great interest in the field of solar photovoltaic technology. The process mainly contains two aspects: (1) a styrene-acrylate emulsion @ organic-inorganic silica precursor (SA@OISP) core/shell hierarchical nanostructure, consisting of a sacrificial styrene-acrylate (SA) primary template, was fabricated using a sol-gel method; (2) the self-assembly of the nanostructures leads to SA@OISP nanospheres forming the high-quality hollow closed-pore silica antireflection coating (CHAR) by a dip-coating process and a subsequent calcination treatment. The resulting SA@OISP nanospheres have a mean diameter of 65.2 nm and contained a SA soft core with a mean diameter of approximately 54.8 nm and an organic-inorganic silica precursor (OISP) shell with a thickness of approximately 6-10 nm. Furthermore, the prepared CHAR film exhibited a high transmittance and good ruggedness. An average transmittance (TAV) of 97.64% was obtained, and the value is close to the ideal single-layered antireflection coating (98.09%) over a broad range of wavelengths (from 380 to 1100 nm). The CHAR film showed a stable TAV, with attenuation values of less than 0.8% and 0.43% after the abrasion test and the damp heat test, respectively. The conversion efficiency of the CHAR coating cover solar modules tends to be increased by 3.75%. The promising results obtained in this study suggest that the CHAR film was considered as an essential component of the solar module and were expected to provide additional solar energy harvest under extreme outdoor climates.

Effects of UV on power degradation of photovoltaic modules in combined acceleration tests

UV exposure and other factors such as high/low temperature, humidity and mechanical stress have been reported to degrade photovoltaic (PV) module materials. By focusing on the combined effects of UV stress and moisture on PV modules, two new acceleration tests of light irradiation and damp heat (DH) were designed and conducted. The effects of UV exposure were validated through a change in irradiation time (UV dosage) and a change of the light irradiation side (glass side vs backsheet side) in the UV-preconditioned DH and cyclic sequential tests, respectively. The chemical corrosion of finger electrodes in the presence of acetic acid generated from ethylene vinyl acetate used as an encapsulant was considered to be the main origin of degradation. The module performance characterized by electroluminescence images was confirmed to correlate with the measured acetic acid concentration and Ag finger electrode resistance.

Sequential and combined acceleration tests for crystalline Si photovoltaic modules

The sequential combination test for photovoltaic modules is effective for accelerating degradation to shorten the test time and for reproducing degradation phenomena observed in modules exposed outdoors for a long time. The damp-heat (DH) test, thermal-cycle (TC) test, humidity-freeze (HF) test or dynamic mechanical load (DML) test is combined for the test modules. It was confirmed that chemical corrosion degradation or physical mechanical degradation is reproduced by the combination of the above tests. Cracks on the back sheet and delamination, often observed upon outdoor exposure, were well reproduced by the combination of DH and TC tests and TC and HF tests, respectively. Sequential DH and TC tests and DML and TC tests accelerated the degradation. These sequential tests are expected to be effective in reducing the required time of indoor testing for ensuring long-term reliability.

Development of a High Temperature Interconnect Solution as an Alternative to High Lead or Gold Content Solders

AbstractA collaborative research programme between project partners Microsemi, the National Physical Laboratory (NPL) and Gwent Electronic Materials (GEM), has successfully developed innovative materials specifically designed to offer an alternative for high Pb or Au content materials to increase the operating temperature of electronic assemblies. Currently, for electronic assemblies to operate at high temperature, they must use a high lead solder or a very expensive gold based solder to withstand these temperatures. The ELCOSINT project has developed an inexpensive lead-free alternative for joining high temperature electronics suitable for operating at temperatures above 250°C utilising standard surface mount assembly processes. This paper summarises the work undertaken by the authors to develop and better understand this new family of electrical interconnection materials. The project brought together a materials supplier (GEM – Gwent Electronic Materials), an end-user (MSL - Microsemi) and an technology research organisation (NPL – National Physical Laboratory) to jointly develop, test and implement in production, the solution based on silver-loaded silicone materials. This paper focuses on the testing and materials evaluation undertaken at NPL to determine the long term performance of these alternative materials including high temperature ageing up to 300°C, thermal cycling and damp heat testing. Details of the shear strength and electrical performance of interconnects between the substrates and components during the test regimes are given. The manufacturing process is outlined including details of the test vehicles utilised. The processing temperature for the conductive adhesive is 250°C which offers additional advantages in potential improvements in component and substrate reliability compared to soldered solutions which would typically be processed at temperatures above 300°C.

A fiber optic methane sensor based on wavelength adaptive vertical cavity surface emitting laser without thermoelectric cooler

Based on Tunable Diode Laser Absorption Spectroscopy (TDLAS) technique and the feature of Vertical Cavity Surface Emitting Laser (VCSEL) wavelength scanning range up to dozens of nanometer, methane detection technique based on wavelength adaptive VCSEL without Thermoelectric Cooler (TEC), is proposed, then the laser methane sensor system design is followed. The reliability experiments study under variety of environments in laboratory, such as temperature impact test, damp heat, and cyclic and dusty impact test, were carried out. And then the test data and some analysis were given. These data verified the feasibility of the wavelength adaptive technique, and these data showed the laser methane sensor can provide long working time without any calibration, and they have the advantages of high accuracy and stability.

High reliability and high density graded index polymer waveguides for optical interconnect

We fabricated graded index polymer waveguide with Polynorbornene material using Photo-addressing method. The graded index polymer waveguide can pass damp heat test at 85°C and 85%RH with low cross talk under −30dB.

A study for degradation of flexible organic photovoltaic modules via damp-heat test: By accessing individual layers of the module

The stability of slot-die coated flexible organic photovoltaic (OPV) modules with inverted structures of indium tin oxide (ITO)/ZnO/photoactive layer/poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS)/Ag was analyzed under damp heat conditions. The work primarily focused on the understanding of the degradation mechanism of organic photovoltaic modules in very severe operating conditions (85°C/85% relative humidity [rh]). The electrical change of the OPV module was measured as a function of the aging time under damp heat. A rapid drop in the efficiency of the module was observed, mainly caused by the decline of the open-circuit voltage (Voc) and fill factor (FF). We supposed that the degradation of the modules mainly progressed by the oxygen and moisture penetrating the terminals or at the edges of the barrier films. The physical modification of the layers composing the device by degradation was observed; hence, morphological and chemical analyses were conducted. The analyses revealed that Ag atoms migrated to the interlayers of the cell, resulting in the increase of shunt paths; this was the main reason underlying the reduction of Voc and FF of the OPV module under damp heat. The ZnO layer was also etched by the acidic molecules from the diffused PEDOT:PSS polymer. In addition, the performance parameters of several OPV modules were evaluated simultaneously under damp heat conditions for 1000h. The same lifetime patterns were observed although the initial efficiencies of the modules were diverse. The interpretation of the lifetime patterns and a suggestion for an objective comparison between the modules showing different initial efficiencies were also addressed.

A high reliability module with thermoelectric device by molding technology for M2M wireless sensor network

This paper presents the fabrication of a new energy harvesting module that uses a thermoelectric device (TED) by using molding technology. Through molding technology, the TED and circuit board can be properly protected and a heat-radiating fin structure can be simultaneously constructed. The output voltage per heater temperature of the TED module at 20 °C ambient temperature is 8 mV K−1, similar to the result with the aluminum heat sink which is almost the same fin size as the TED module. The accelerated environmental tests are performed on a damp heat test, which is an aging test under high temperature and high humidity, highly accelerated temperature, and humidity stress test (HAST) for the purpose of evaluating the electrical reliability in harsh environments, cold test and thermal cycle test to evaluate degrading characteristics by cycling through two temperatures. All test results indicate that the TED and circuit board can be properly protected from harsh temperature and humidity by using molding technology because the output voltage of after-tested modules is reduced by less than 5%. This study presents a novel fabrication method for a high reliability TED-installed module appropriate for Machine to Machine wireless sensor networks.

Reliability of a printed Cu busbar electrode on a conventional silicon solar cell

In this paper, we describe the reliability of the Cu busbar electrode fabricated by screen printing of resin-based Cu pastes on a crystal silicon solar cell with conventional front and rear electrodes excluding the busbar. The damp heat test (DHT) and thermal cycling test (TCT) of the cells were carried out with ethylene-vinyl acetate (EVA) and glass encapsulation, and without any encapsulation. No significant degradation was observed for either case. Oxidation was confined to the surface region even without encapsulation after 2,500 h of DHT. Cu diffusion into Si was evaluated using pseudo fill factor (pFF). No significant changes in pFF were found after heat treatment (400 °C, 1 h) in the case of resin-based Cu, where 10% pFF loss was observed when Cu was introduced by vapor deposition. From these findings, the role of resin used as a binder in Cu pastes is considered a key material parameter of Cu diffusion in Si.

Development of a pH sensor based on a nanostructured filter adding pH-sensitive fluorescent dye for detecting acetic acid in photovoltaic modules

Acetic acid formed via the hydrolysis of ethylene vinyl acetate (EVA) as an encapsulant in photovoltaic (PV) modules causes a decrease in the conversion efficiency of such modules by grid corrosion. Here, a nondestructive and simple optical method for evaluating the condition of PV modules is proposed. This method uses a dual-wavelength pH-sensitive fluorescent dye to detect acetic acid in PV modules using a change in pH. The change in pH induced by the formation of acetic acid is detected by the change in the ratio of the fluorescent intensities of two peaks of the dye. A pH-sensitive fluorescent dye showed sensitivity for small amounts of acetic acid such as that produced from EVA. Furthermore, a membrane filter dyed with a pH-sensitive fluorescent dye was confirmed to detect acetic acid in aged EVA after a damp-heat test (85 °C, 85%) for 5000 h in PV modules.

Silver nanowires/ZnO:Al bilayer structures for highly stable transparent conductive electrodes

A silver nanowires (AgNWs)/ZnO:Al (AZO) bilayer coating was coated on glass substrates at room temperature by using a polyol method and radio frequency magnetron sputtering, respectively. The microstructure, optical, electrical properties and environmental stability of the AgNWs/AZO bilayer and AZO single layer were investigated. The bilayer structures of AgNWs/AZO annealed at 250 °C could lead to an enhancement of the electrical properties and environmental stability. The sheet resistance of AgNWs/AZO bilayer decreased to 9.8 Ω/sq and the degradation percent of the sheet resistance also decreased to 20.2 % after damp heat testing for 30 days compared to AZO films with the sheet resistance of 24 Ω/sq and degradation percent of the sheet resistance of 102 %. The network distribution of AgNWs on AZO films with the columnar grain structures mitigated the negative effect of grain boundaries, which was considered to be the reason of the improvement in conductivity and environmental stability.

A Circuit-Based Approach to Simulate the Characteristics of a Silicon Photovoltaic Module With Aging

The aging of photovoltaic modules results inevitably in a decrease of their efficiency all through their lifetime utilization. An approach to simulate the evolution of electrical characteristics of a photovoltaic module with aging is presented. The photovoltaic module is modeled by an equivalent electrical circuit whose components have time-dependent characteristics determined under accelerated tests. By entering sun irradiance and temperature, I–V and P–V curves as well as efficiency evolution can be simulated over years assuming equivalent time. The methodology is applied for the case of a monocrystalline photovoltaic module modeled by a one-diode circuit and aging laws are determined with experimental results of damp heat (DH) tests 85 °C/85% RH performed by Hulkoff (2009, “Usage of Highly Accelerated Stress Test (HAST) in Solar Module Aging Procedures,” M.S. thesis, Chalmers University of Technology, Göteborg, Sweden). A power degradation rate of 0.53%/yr is found. A parametric study shows that the rundown of optical transmittance of the upper layers with aging has the most important impact by reducing the initial efficiency by 11.5% over a 25-year exposure contrary to electrical degradations which cause a decrease of 1.85% of the initial efficiency.

Degradation by acetic acid for crystalline Si photovoltaic modules

The degradation of crystalline Si photovoltaic modules during damp-heat test was studied using some test modules with and without polymer film insertion by observing electrical and electroluminescence properties and by chemical analyses. Acetic acid generated by the hydrolysis decomposition of ethylene vinyl acetate used as an encapsulant is the main origin of degradation. The change in electroluminescence images is explained on the basis of the corrosion of electrodes by acetic acid. On the other hand, little change was observed at the pn junction even after damp-heat test for a long time. Therefore, carrier generation occurs even after degradation; however, such generated carriers cannot be collected owing to corrosion of electrodes. The guiding principle that module structure and module materials without saving acetic acid into the modules was obtained.

A <inline-formula> <tex-math notation="LaTeX">$1\times 8$ </tex-math></inline-formula> Splitter Chip With Integrated Fiber Grooves for Reliable Passive Alignment of Fiber Arrays

A $1\times 8$ optical splitter with integrated fiber-positioning grooves was fabricated by simultaneous formation of the waveguide cores and grooves. The waveguide cores were realized by intaglio etching of a fused-silica wafer and subsequent core filling, followed by the deposition of an overcladding layer. Fiber-positioning grooves were then formed by wet etching the silica wafer with the use of a highly selective SiN x mask. Passive alignment was accomplished by guiding the fibers into the grooves, which is favorable for automated mass production of splitter modules. The resulting $1\,\times \,8$ splitter module exhibited a small insertion loss of 59 dB. It yielded good reliability during a damp heat test at 85 °C and 85% humidity, and the insertion loss varied by <0.3 dB. This simple and reliable approach to fabricating optical splitter modules is expected to greatly reduce the time and costs associated with packaging and enable automation of the process.

Degradation behaviors of EVA encapsulant and AZO films in Cu(In,Ga)Se2 photovoltaic modules under accelerated damp heat exposure

This paper focuses on the degradation behavior of ethylene vinyl acetate (EVA) and Al-doped ZnO (AZO) films in CIGS PV modules after the damp heat test. We observed losses of the FF and efficiency in the modules after 1000h of damp heat (DH) exposure and some of them were discolored from the outside edge and the center. To clarify the degradation behavior of the non-discolored and discolored modules, a failure analysis for each position of the EVA and AZO in the CIGS module films from the edge to the center was performed. The resistivity of the AZO films in the non-discolored modules increased due to the adsorption of oxygen. In discolored modules, the resistivity of the AZO films rapidly increased at the edge region, and the morphologies of the AZO films were also changed due to the chemical degradation of EVA. These degradation behaviors of the AZO films and EVA in discolored modules were attributed to the formation of hydroxyl and carboxylic species caused by water penetration. Moreover, we reported that the transmittance and the mechanical properties of EVA after 1000h DH exposure were deteriorated by water molecules.

Long term stability of copper front side contacts for crystalline silicon solar cells

In this work the long term stability of silicon solar cells with a copper front side metallization based on a fine-line screen-printed silver seed-layer, a plated nickel diffusion barrier, a plated copper conductive layer and a silver capping is investigated in detail. Silicon nitride layers deposited by PECVD or sputtering effectively hinder copper diffusion, which might occur e.g. at contact edges. To investigate the plated nickel diffusion barrier, fast degradation of full size 156×156mm² cells at elevated temperatures on hotplates and module degradation in the climate chamber executing a damp heat test were performed. Plated nickel thickness influences the degradation speed on cell level. On module level, only the cell without diffusion barrier shows a degradation of 1.2% rel. in efficiency (IEC criteria passed) after 1500h damp heat test (85°C, 85% r.h.). The cells with diffusion barrier do not show any degradation. Comparison of cell and module results indicate that fast degradation on hotplates at cell level gives a reasonable first estimate regarding cell degradation due to copper diffusion.