DAMP HEAT STABILITY OF TRANSPARENT CONDUCTIVE ZINC OXIDES: ROLE OF ENCAPSULANTS AND PROTECTIVE LAYERS

The mechanisms and environmental influences that cause photovoltaic modules performance degradation are poorly understood, but it is well known that water vapour is deeply implicated in the degradation process. Indeed, some layers and interfaces of thin film modules can be moisture sensitive and depending on the processing conditions, they degrade after exposure to damp heat conditions (85°C, 85% relative humidity) [1]. Transparent conductive oxides (TCO), as used in CIGS or thin silicon film cells play a particular role linked to reliability issues. We showed recently that low-pressure chemical vapour deposition zinc oxide (LPCVD ZnO) can withstand damp heat test even without encapsulant providing doping of the ZnO is high enough, though this is unfavourable for free carrier absorption (reduction of spectral response in the infrared part) [2]. Reduction of doping leads to improved optical properties but needs therefore an optimized encapsulation strategy to avoid the deterioration of the TCO conductivity. In previous work, the degradation of LPCVD ZnO used in thin-film silicon solar cells was investigated [3]. It was shown that the decrease of the ZnO conductivity was essentially due to the humidity increasing inside the encapsulant. However other effects take part in the degradation process and remained yet unexplained. In this paper we will report on several other possible sources of degradation, which have been identified. In order to demonstrate and quantify these effects, we used various encapsulants, but without back protection (foil or glass), and we exposed the samples to different type of atmospheres. The resistivity of the ZnO was monitored using an inductive contactless and a four points probe methods. Finally, schemes to perform highly reliable laminates when using lightly doped ZnO are proposed

Monitoring water vapour penetration using a contactless technique

Some layers of thin film photovoltaic modules maybe critically sensitive to moisture. In this study we present a new tool for monitoring the effect of moisture using a particular Transparent Conductive Oxide (TCO) as a sensor. The moisture content of the encapsulant was determined by Fourier Transform Infra Red (FTIR) spectroscopic measurements. The TCO resistivity was measured using an inductive method. The different spectroscopic results show that the diffusion of water vapour in the encapsulant used in this study is in good agreement with Fick’s law and correlate well with the increase of resistivity of the TCO. However, the transport measurements bring evidence for a degradation of the TCO resistivity undetectable by conventional FTIR measurements

Efficiency of silicon thin-film photovoltaic modules with a front coloured glass

Photovoltaic electricity has already proven its ability to compete with other well established technologies for energy production. The abundant and non-toxic raw materials, the yearly increasing efficiency as well as the production cost of silicon thin-film solar cells getting lower and lower make this technology always more interesting for a wide spread use. Beside the functional features, the size, colour and glass texture of a PV module determine its appearance and aesthetics. In order to be more compliant with the built environment, photovoltaic installations have to be improved in terms of visual rendering, matching of colour of the existing roof-tops and parasitic reflections. The crystalline technology already offers various types of systems with a large choice of shapes, textures and colours as well as “semi-transparent” modules more easily integrated in the roof-tops or facade. By changing the anti-reflective coating (ARC) of a crystalline solar cell, it is possible to modify their colour [1]. However, for thin-film silicon technology the challenge is completely different, and up to now, the only way to modify the module colour is to reduce the thickness of the active layer and consequently its efficiency. Therefore new ways to enhance the visual rendering of the thin-film modules have to be explored. A study led in the frame of the ArchinSolar project [2] has shown that architects are ready to integrate PV modules with enhanced aesthetic aspect, even though there was a 10 % loss in efficiency. The present study shows how new coloured filters can be used to enhance PV modules’ appearance while minimizing power loss, to achieve a better integration in the traditional urban or rural environment

Scanning tunneling microscopy at multiple voltage biases of stable "ring-like" Ag clusters on Si(111)-(7×\times7)

Since more than twenty years it is known that deposition of Ag onto Si(111)-(7\times7) leads under certain conditions to the formation of so-called "ring-like" clusters, that are particularly stable among small clusters. In order to resolve their still unknown atomic structure, we performed voltage dependent scanning tunneling microscopy (STM) measurements providing interesting information about the electronic properties of clusters which are linked with their atomic structure. Based on a structural model of Au cluster on Si(111)-(7\times7) and our STM images, we propose an atomic arrangement for the two most stable Ag "ring-like" clusters.Comment: 9 pages and 5 figure

Mechanical Bonds and Topological Effects in Radical Dimer Stabilization

While mechanical bonding stabilizes tetrathiafulvalene (TTF) radical dimers, the question arises: what role does topology play in catenanes containing TTF units? Here, we report how topology, together with mechanical bonding, in isomeric [3]- and doubly interlocked [2]catenanes controls the formation of TTF radical dimers within their structural frameworks, including a ring-in-ring complex (formed between an organoplatinum square and a {2+2} macrocyclic polyether containing two 1,5-dioxynaphthalene (DNP) and two TTF units) that is topologically isomeric with the doubly interlocked [2]catenane. The separate TTF units in the two {1+1} macrocycles (each containing also one DNP unit) of the isomeric [3]catenane exhibit slightly different redox properties compared with those in the {2+2} macrocycle present in the [2]catenane, while comparison with its topological isomer reveals substantially different redox behavior. Although the stabilities of the mixed-valence (TTF2)^(•+) dimers are similar in the two catenanes, the radical cationic (TTF^(•+))_2 dimer in the [2]catenane occurs only fleetingly compared with its prominent existence in the [3]catenane, while both dimers are absent altogether in the ring-in-ring complex. The electrochemical behavior of these three radically configurable isomers demonstrates that a fundamental relationship exists between topology and redox properties

Insights on EVA lamination process: Where do the bubbles come from?

• The identification and control of volatile organic compounds (VOCs) released by polymers during lamination in the photovoltaic (PV) industry is of considerable importance to establish and optimize processes in order to ensure a long term reliability of the manufactured modules. • Bubble formation is one of the cause of delamination, major problems linked to PV module reliability. • Gas chromatography-mass-spectrometry (GC-MS) is used to determine and quantify the VOCs emanating from EVA at 160 C and therefore to understand the origin of the bubbles during the lamination process

Degradation in PV encapsulant strength of attachment: An interlaboratory study towards a climate-specific test

Reduced strength of attachment of the encapsulant resulting from the outdoor environment, including ultraviolet (UV) radiation, may decrease photovoltaic (PV) module lifetime by enabling widespread corrosion of internal components. To date, few studies exist showing how the adhesion of PV components varies with environmental stress. We have conducted an interlaboratory experiment to provide an understanding that will be used to develop climatic specific module tests. Factors examined in the study included the UV light source (lamp type), temperature, and humidity to be proposed for use in accelerated aging tests. A poly (ethylene-co-vinyl acetate) (EVA) formulation often used in veteran PV installations was studied using a compressive shear test - to quantify the strength of attachment at the EVA/glass interface. Replicate laminated glass/polymer/glass coupon specimens were weathered at 12 institutions using a variety of indoor chambers or field aging. Shear strength, s hear strain, and toughness were measured using a mechanical load-frame for the compressive shear test, with subsequent optical imaging and electron microscopy of the separated surfaces

Degradation in PV encapsulation transmittance: An interlaboratory study towards a climate-specific test

Reduced optical transmittance of encapsulants resulting from ultraviolet (UV) degradation has frequently been identified as a cause of decreased PV module performance through the life of service in the field. The present module safety and qualification standards, however, apply short UV doses only capable of examining design robustness or "infant mortality" failures. Essential information that might be used to screen encapsulation through product lifetime remains unknown. For example, the relative efficacy of xenon-arc and UVA-340 fluorescent sources or the typical range of activation energy for degradation is not quantified. We have conducted an interlaboratory experiment to provide the understanding that will be used towards developing a climate-and configuration-specific (UV) weathering test. Five representative, known formulations of EVA were studied in addition to one TPU material. Replicate laminated silica/polymer/silica specimens are being examined at 14 institutions using a variety of indoor chambers (including Xenon, UVA-340, and metal-halide light sources) or field aging. The solar-weighted transmittance, yellowness index, and the UV cut-off wavelength, determined from the measured hemispherical transmittance, are examined to provide understanding and guidance for the UV light source (lamp type) and temperature used in accelerated UV aging tests