Seminar Abstract:
Crystalline silicon and thin-film technologies have ruled the photovoltaic landscape for several decades with continual efficiency improvements and cost reductions. One recent trend has been to use thinner semiconductors, which reduces both materials cost and losses associated with bulk recombination. Light management for these and more advanced multi-junction structures becomes critical in order to maximize absorption in the active layer and minimize parasitic loss in the electrodes. This talk will first discuss our work using periodic arrays of vertical silicon nanowires to improve light absorption in thin films of crystalline silicon. By forming a photonic crystal structure we can control the propagation of light through the material leading to path length enhancements above the randomized scattering limit. Increased surface recombination losses due to the nanowire geometry can be minimized by coating the structure with a hole conducting polymer, which simultaneously forms a junction for charge carrier extraction and helps to passivate the surface leading to improved solar cell photocurrent. The talk will then detail how we exploit the plasmonic field enhancements in small gaps between silver nanowires self-assembled from solution to locally heat and weld them together only at junction points, forming excellent transparent electrodes. Since the heat generation is strongly dependent on the gap size, it self-limits after welding occurs, preventing further heating that could break apart nanowire networks or damage heat-sensitive materials underneath. Finite element method simulations show that at each junction the top nanowire focuses light and generates heat selectively in the bottom nanowire while electron microscopy confirms that the bottom nanowire always recrystallizes epitaxially onto the top nanowire at the junction. We have watched this welding process evolve in time using dark-field scattering and electrical resistance measurements on single junctions and connected the microscopic changes to results in macroscopic interconnected networks of silver nanowire transparent electrodes. The highly local nature of the heating enables this plasmonic welding process to be used for metal nanowire mesh electrodes on heat-sensitive substrates like Saran Wrap and polymer photovoltaics that are destroyed using a standard hotplate treatment. These studies lay the foundation for a metal nanowire top electrode self-assembled from solution that simultaneously acts as an active optical element to increase light absorption and a high-performance transparent electrode to collect current.