Abstracts for DuPont Oral & Poster Presentations at the SNEC 12th International Photovoltaic Power Generation Conference

Oral Presentations

Developing high-performance double printing PV metallization silver pastes
Speaker: Youyong Xu, Research scientist
Abstract: Double printing (DP) has been a very important technology to achieve high-aspect ratio fingers and high efficiency solar cells. However, with the fast development of single printing (SP) technologies, the efficiency benefit of DP over SP has become smaller (+0.05%-0.1%). One of DP’s advantages is its higher EL yield than other printing techniques, which is valuable for high efficiency cells and pure cell makers. To further demonstrate the values of DP, fine-line printing with narrower screen opening (20-22um) is being adopted, which posed bigger challenges to the fine-line paste transfer. By selecting the right combination of organic vehicle and silver powders, DuPont new generation paste PVD1X and PVD2X showed much better fine-line printability than last generation. With the proper selection of organic ingredients, surface tension and rheology properties of the pastes are tuned to achieve excellent line-width control. the laydown of the new generation DP paste can be tuned by recipe itself and screen combinations, which is valuable to lowering the cell cost. With the combination of new glass frits and silver powders, the new DP packages show better ohmic contact performances than last generation, which demonstrated >0.05% eff. gain. When coupled with customized cell diffusion profiles, more efficiency gain could be obtained. The other special feature of DP is the capability for A+B floating package, which can provide obvious Voc benefit and high adhesion for the 2nd layer. With the improved glass frit combinations, we have a new A+B package which showed higher efficiency and +0.5N higher adhesion.

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Environmental stresses and field analysis of materials for distributed PV applications
Speaker: Hongjie Hu, Technical leader 
Abstract: Distributed PV is a promising PV application segment in China relative to large scale ground solar farms. Distributed PV including small scale (<20MW) PV installations on commercial and residential rooftops, over water and sewage treatment plant, etc. 7GW of distributed PV had been installed in China in 1H of 2017 and expected to reach 18GW in full year. While the installation capacity of distributed PV is only 4.2GW in 2016 and 1.4GW in 2015 in China. However, along with the high speed development, some basic analyses about environmental stresses and field failure mechanisms/root causes were ignored. A field investigation of DuPont about ~200 installations has revealed higher module visual failure rate for distributed PV (37%) by comparing to conventional ground PV (21.6%), while backsheets failure rate of distributed PV is ~3 times higher than that of conventional PV. Solar cell corrosion and hot spot, EVA browning and delamination, and backsheet yellowing, cracking and thickness erosion were found on distributed PV and count into statistical analysis. To figure out failure mechanisms, solar farm power generation, field module lab destructive analysis and environmental stresses analysis were conducted and their relations were correlated. Erosion rates of different backsheets’ outer layer materials were compared and worn out years were calculated. Corrosion behaviors of paint coated steel tile and film laminated metal plate were described, which are also important for 25yrs operation of PV installations.

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Multistress Sequential Accelerated Testing and Correlation to Field Performance
Speaker: William Gambogi, Technical Fellow
Abstract: During their service life, PV modules are exposed to a wide range of stress factors that can operate simultaneously and sequentially and include high/low temperature, temperature cycling, UV/visible irradiance, mechanical load, and internal electrical potential.  The assessment of performance for a single stress condition does not adequately predict the impact of synergistic effects associated with these multiple stresses and the impact of materials degradation on performance.  We have developed new test protocols and applied the critical durability stressors to PV modules using sequential exposure conditions and compared the resulting performance to fielded modules with the module structure and materials.  The sequential tests apply high temperature and humidity, UV/visible irradiance, thermal cycling, and mechanical stress to assess durability.  Higher stress levels are applied to provide higher acceleration factors and complete assessment within a time acceptable for new product development.  Where possible, assessment to carried out on full size modules to better compare to the field.  Comparisons are made to modules from the field and failure mechanisms and timing are compared.  The impact of changes to the bill of materials used in fabricating the modules is investigated.  Commercial module performance under these sequential test protocols is also described.  Materials and module characterization and comparison of the accelerated test and fielded modules provides insights into the degradation mechanisms responsible.  Examples of failure mechanisms observed in the field and predicted by these test protocols are provided.  Recommendations for reducing or avoiding these failure mechanisms is discussed.

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