Semiconducting polymers for stretchable, ultra-flexible, and mechanically robust organic photovoltaics

The original vision of organic electronics comprises the use of organic conductors and semiconductors specifically designed to accommodate large strains to enable highly deformable and mechanically robust devices for organic photovoltaics, biosensors, and electronic skins. However, mechanical properties of organic materials are often overlooked; as a result, many of these materials are unable to accommodate the mechanical stresses required for their intended applications. Thus, it is important to understand the parameters that govern mechanical properties of these materials. Chapter 1 provides an introduction to the characteristics, applications, and fabrications of stretchable electronics. The idea of intrinsically stretchable electronics comprising molecularly designs of semiconducting polymers is outlined. Chapter 2 focuses on the mechanical degradation and stability of organic solar cells. The key highlights are the importance of mechanical properties and mechanical effects on the viability of organic solar cells during manufacture and in operational environment. Chapter 3 and Appendix A investigate the effects of the length of the alkyl side chains in poly(3-alkylthiophenes) on the deformability of the pure polymer films and their blends with fullerenes. Chapter 4, 5, and Appendix B provide studies on the inherent competition between good photovoltaic performance and mechanical compliance; a critical length of the alkyl side chains on the poly(3-alkylthiophene) allows for co-optimization of both photovoltaic and mechanical properties. In Chapter 6 and Appendix C, the effect of incompletely separated grades of electron acceptors on the mechanical deformability of organic solar cells is investigated in an effort to simultaneously improve the mechanical robustness of the organic solar cells and reduce the energy of production. Chapter 7 describes the plasticization of the common transparent electrode using common processing additives. Chapter 8, 9, and 10 investigate the mechanical properties of low-bandgap polymers as the function of the molecular structure and solid-state packing. Chapter 11 introduces a novel experimental method, photovoltaic mapping (PVMAP), which combines the use of non-damaging electrode and gradients in processing parameter to spatially map the photovoltaic properties of organic solar cells.