Over the past decade, degradation and power loss have been observed in PV modules resulting from the stress exerted by system voltage bias. This is due in part to qualification tests and standards that do not adequately evaluate for the durability of modules to the long-term effects of high voltage bias experienced in fielded arrays. High voltage can lead to module degradation by multiple mechanisms. The extent of the voltage bias degradation is linked to the leakage current or coulombs passed from the silicon active layer through the encapsulant and glass to the grounded module frame, which can be experimentally determined; however, competing processes make the effect non-linear and history-dependent.Appropriate testing methods and stress levels are described that demonstrate module durability to system voltage potentialinduced degradation (PID) mechanisms. This information, along with outdoor testing that is in progress, is used to estimate the acceleration factors needed to evaluate the durability of modules to system voltage stress. Na-rich precipitates are observed on the cell surface after stressing the module to induce PID in damp heat with negative bias applied to the active layer.

ECN set up a cross testing project in which suppliers of electrically conductive adhesives (ECA) and conductive back-sheet (CBS) foils participated. In the component part of the project, combinations of adhesive and foil were characterised for peel strength and contact resistance. The separate components were tested on dot geometry (ECA) and surface structure (CBS). In the module manufacture and testing part, 12 combinations of ECA and CBS were used in 4-cell MWT modules. The modules were tested up to 2000 hours in damp heat and 400 thermal cycles (both tests, 2 times the requirement of IEC61215). Most combinations passed the 5% power loss criterion. One of the conductive adhesives performed well on three different types of back-sheet which is supportive of a robust technology. Other results show that it is important to thoroughly test interesting combinations of ECA and CBS at module level before adding them to the recommended bill of materials.

In this paper we present a module concept designed for better performance under shadow conditions, incorporating also lower BOS costs. This concept is a highvoltage, low-current, shadow resistant, back-contact module concept. The experimental results under partial shading conditions for mini-modules having a Voc of 39.8V show a substantial improvement in shadow performance. The concept is based on the existing ECN MWT (Metal Wrap Through) backcontact technology [1], where the standard MWT cells are replaced by series connected "mini" cells. With the flexibility of the back-foil system we are able to approach a more linear shadow response for our module.

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