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C4.03 - Process Control of Interfacial Composition and Adhesion in Polymer Solar Cells 
April 22, 2014   11:15am - 11:30am

Organic materials have enabled large scale semiconductor production on flexible substrates, but are often more mechanically fragile than their inorganic counterparts with a higher tendency for adhesive and cohesive failure. We use a thin-film adhesion technique to quantify the impact of various processing, film structure and environmental variables on the cohesion and adhesive properties of organic semiconductor materials and their interfaces. Specifically, we will compare solution processing (spin, spray and slot die coating) with thermal evaporation, and demonstrate that overall solution processing leads to improved adhesion. We also show the importance of various film structure parameters, such as the polymer layer thickness, composition and molecular weight. We discuss how to tune key interfacial and film parameters, such as interface chemistry, bonding and morphology, by thermal annealing to improve the adhesion. For example, the P3HT:PCBM/PEDOT:PSS interface in an inverted polymer solar cell has an adhesive value of only ~1.5 to 2 J/m2, and can be significantly increased by post electrode deposition thermal annealing time and temperature. Using near edge X-ray absorption fine structure (NEXAFS), we precisely quantified the interfacial composition at the delaminated surfaces and correlated the increase in adhesion to changes in the interfacial structure. The over 50% increase in adhesion is caused by the development and expansion of an intermixed layer of P3HT:PCBM and PEDOT:PSS at already 45C annealing. However, thermal annealing before electrode deposition and above the crystallization temperature of PCBM (120C) should be avoided to ensure device reliability. At these conditions, micrometer sized PCBM aggregates form that not only weaken the P3HT:PCBM but also decrease the device efficiency. The structural and chemical reorganizations are correlated with glass temperature and crystallization temperature of the materials used in the structure and thus the conclusion can be generalized to other materials systems. Understanding the interlayer adhesion and developing strategies to improve the adhesion of organic semiconductors is essential to improve the overall mechanical integrity and yield general guidelines for the design and processing of reliable organic electronic devices.

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Keynote Address
Panel Discussion - Different Approaches to Commercializing Materials Research
Business Challenges to Starting a Materials-Based Company
Fred Kavli Distinguished Lectureship in Nanoscience
Application of In-situ X-ray Absorption, Emission and Powder Diffraction Studies in Nanomaterials Research - From the Design of an In-situ Experiment to Data Analysis