Cohesion and decohesion kinetics of polymer solar cells [electronic resource]
- Bruner, Christopher.
- Physical description
- 1 online resource.
- Polymer organic solar cells (OSCs) possess many desirable properties include low temperature solution processibility of photoactive materials and the utilization of flexible substrates for conformal OSC design. However, challenges remain that may inhibit their adoption and implementation. Among these are thermochemical stability, optimized power conversion efficiency, and mechanical reliability. Indeed, for organic solar cells to be used on flexible substrates, mechanical reliability of the individual layers and interfaces is pertinent. In this dissertation, quantitative micromechanical testing techniques were employed to help characterize the mechanical reliability of the layers and interfaces for polymer OSCs. From testing, it was determined that the polymer:fullerene photoactive layer consistently failed cohesively. The cohesive strength of most photoactive layers generally ranged from 0.5 to 2.0 Joules per square meter. However, by using thermal annealing, manipulating the photoactive layer structure, and selecting polymer molecular weight, cohesion values as high as 17 Joules per square meter could be achieved. Indeed, polymer molecular weight affected cohesion the most due to significant plasticity within the photoactive layer. Conversely, improved cohesion did not always result in improved device electronic performance. By optimizing cohesion and efficiency, we are able to come closer to design guidelines for mechanically robust and efficient solar cells. Finally, because OSCs must operate in the environment at temperatures as high as 65 C, we analyzed OSCs under dry, inert environmental conditions and showed how temperature affects the decohesion kinetics within the device. It was demonstrated that the decohesion rate generally accelerated with increasing test temperature. We were able to develop a viscoelastic kinetic model that was able to describe and predict decohesion for these devices. This collective work and modeling will provide greater insight into the practical limitations of OSC design and will aid in future development of OSCs with greater mechanical reliability and in-service lifetimes.
- Publication date
- Christopher Bruner.
- Submitted to the Department of Chemistry.
- Thesis (Ph.D.)--Stanford University, 2014.