OPTIMISATION OF THE FRONTSHEET ENCASPULANT FOR INCREASED RESISTANCE OF LIGHTWEIGHT GLASS-FREE SOLAR PV MODULES F.Lisco 1 , Alessandro Virtuani 1 , Christophe Ballif 1,2 1 École Polytechnique Fédérale de Lausanne (EPFL), Institute of Microengineering (IMT), Photovoltaics and Thin Film Electronics Laboratory (PV-Lab), Rue de la Maladière 71b, 2000 Neuchâtel, Switzerland 2 CSEM, PV-center, Jaquet-Droz 1, 2000 Neuchâtel, Switzerland ABSTRACT: The aim of this work is to propose a BIPV module design that is contemporarily lightweight, rigid and resistant to the relevant climatic and mechanical stresses (e.g. exposure to DH, UV, hail impacts, etc), supporting structure and reliability at the same time. Encapsulants represent a key part in the PV structure, acting as the bonding layer between the front or back sheet and the PV cell. Therefore, they need to provide excellent adhesion between these components, which is achieved via lamination. They also have to be transparent and provide outstanding electrical insulation and impact resistance to the module. All these properties must be retained after years of exposure to the UV from the sun or other severe weather conditions. With this work, we want to provide a “recipe” to define and qualify the optimum front sheet encapsulant for the proposed lightweight composite PV modules. Keywords: lightweight PV modules, encapsulants, frontsheet, hail resistance 1 INTRODUCTION Conventional solar photovoltaic (PV) modules made with c-Si solar cells are typically glass/foil modules with a weight of 12-16 kg/m 2 , or glass/glass modules weighting 14-20 kg/m 2 or more, depending on the glass thickness. For BIPV applications, glass/glass modules are generally preferred for the higher structural stability and for safety reasons. Lightweight PV modules based on glass-glass technology exist [1], however the standard glass sheets are being substituted by polymeric material as front sheet and a glass fiber reinforced polymer as back sheet [2]. Ensuring the reliability and long-term performance of these devices requires a careful optimization of module structure and processing methods and a careful selection of the materials. The front sheet, as one of the main constituents of lightweight modules, has to provide high transparency, allowing as much light as possible reaching the solar cells, protection for the solar cells from airborne pollutants, hail precipitation and also additional benefits, including mechanical stability, electrical insulation and protection from mechanical damage. EVA has dominated the PV industry as the encapsulant of choice; however, numerous studies from late 1990s until today, report that PV module performance reduces, due to the degradation of the encapsulant. Browning/yellowing (which reduces the light reaching the solar cells), moisture absorption and acetic acid formation (which causes corrosion of metallization), delamination and bubble formation are the typical forms of EVA degradation. Various other encapsulants like silicone, ionomer, polyvinyl butyral (PVB), and polyolefin elastomers (POE), thermoplastic polyurethane (TPU) are also known in the industry as alternatives to EVA. Most of these alternative encapsulants need a cocktail of chemicals like multiple stabilizers, additives, peroxide curing agent, UV absorbers, etc. to function properly [3]. The aim of this work is to provide a “recipe” to define and qualify the optimum frontsheet encapsulant for the proposed lightweight composite PV modules. 2 AIM OF THE WORK AND STRUCTURE OF OUR LIGHTWEITGH PV STRUCTURE We already demonstrated the possibility to produce a lightweight PV module with a weight of ~6kg/m 2 , by substituting the typical front glass with a thin polymer sheet and the standard back sheet by a composite sandwich structure [4]. These composite structures are usually composed of two skins bonded to a core, using a stiff adhesive. Such a lightweight PV module is sketched in Figure 1 [2]. With this work, we want to identify the optical, mechanical, morphological, physical and electrical properties, required to choose the optimum frontsheet encapsulant for lightweight composite PV modules. With those identified characteristics, the lightweight structure will be then able to withstand UV exposure, DH and mechanical tests (following the UV preconditioning test combined with Sequence E for module qualification as defined in IEC 61215 [5]). Figure 1: Schematic diagram of the Lightweight PV module structure. 3 EXPERIMENTAL WORK With this work crosslinking encapsulants, thermoplastic and elastomeric polyolefins will be investigated, by using Differential Scanning Calorimetry (DSC), rheology, UV- Vis spectrophotometry, and SEM analyses to study and compare their properties with the final aim to choose the candidate encapsulant to be integrated in the PV lightweight modules frontsheet. Hail test, mechanical and DH tests will be also performed to investigate the reliability of the module structure with the candidate encapsulants. These modules will be then characterised electrically (to extract IV parameters) and by means of electroluminescence (EL) to visualize the induced damages. 3.1 Encapsulant preparation Since providing high transparency is one of the main requirements for a front sheet material, optical analysis (T% and R% measurements) have been performed. To understand/simulate the behavior of these materials, 37th European Photovoltaic Solar Energy Conference and Exhibition 777