(394f) Nanostructured Plasmonic Cathodes for Enhanced Performance of Organic Solar Cells

Organic solar cells (OSCs) offer significant potential to lower the manufacturing costs of solar energy harvesting devices and enable large area, lightweight, and highly flexible photovoltaics.  A central challenge for OSCs is the trade-off between an active layer of sufficient film thickness to absorb as much light as possible while minimizing charge recombination.  This balance between absorption and recombination typically results in using thin active layers that do not take full advantage of the incident light.  Developing light manipulation techniques to maximize absorption is a key component for pushing OSCs to their peak potential.  Integrating plasmonic nanostructures into these devices is one promising way to enhance absorption and is typically achieved by deploying Au or Ag plasmonic nanoparticles in various layers of an OSC.  Additionally, replacing ITO as an electrode is desirable due to the high cost and limited supply of indium, as well as the lack of mechanical robustness and flexibility of ITO itself.  In this work, we designed and fabricated plasmonic nanohole arrays to serve as the cathode for high efficiency OSCs replacing ITO and increasing forward light scattering and light trapping in the active layer while reducing reflectance.  We used Finite-Difference Time-Domain (FDTD) electromagnetic simulations to design the nanohole size and pitch to maximize both the intensity of plasmon-induced electric fields and light absorption near the bandgap of the active layer.  Once the most favorable nanostructure design was chosen, FDTD simulations and transfer matrix optical modeling were used to determine the optimal layer thicknesses of the active and interfacial layers.  We used a blend of [6,6]-phenyl C71-butyric acid methyl ester (PC₇₁BM) and the low-bandgap semiconducting polymer thieno[3,4-b]thiophene/benzodithiophene (PTB7) to make the bulk heterojunction active layer and made both inverted and conventional devices.  Plasmonically enhanced devices were fabricated on bare glass and on bare flexible PET and compared to control devices on ITO-coated glass and ITO-coated PET.  The effects of these plasmonic electrodes to enhance light absorption, enable omnidirectional light harvesting, and increase robustness of the flexible devices will be discussed.  These nanostructured plasmonic electrodes were fabricated using nanoimprint lithography in a process that is scalable to roll-to-roll manufacturing.  To our knowledge, this is the first time nanopatterned plasmonic electrodes have been applied to high efficiency and flexible OSCs and the learnings presented here can be utilized to fabricate high performance electrodes for widespread photovoltaic systems.