CPM Seminar

Electrode work function tuning in organic solar cells

Ayse Turak

Department of Engineering Physics
McMaster University

Photovoltaic devices based on organic semiconductors offer a promising avenue for renewable energy sources. The potential for mechanically flexible, light-weight solar cells that can be produced on a large scale at low-costs has led to substantial research in optimizing organic thin films for this purpose. The indium-tin-oxide/active layer interface is critical to the performance of organic solar cell devices. Though PEDOT:PSS is widely used as an interlayer, poor energy level matching with the active polymer blend can lead to poor long term performance. The possibility of tailoring the work function to match the energy level of the active organic layer is of great interest in the fabrication of organic devices to form barrier-free Ohmic contacts, where the work function no longer controls device performance. This talk describe a recent study on submonolayer films of LiF nanoparticles, deposited on the electrode surface with the assistance of polymeric micelle reactors. This approach enables particle deposition with controlled nanoscale surface coverage. Incorporation of the solution-processed bilayer electrodes into a conventional P3HT:PCBM device shows significant improvement in device performance, mostly likely related to the increase in surface work function from the use of LiF nanoparticles, as measured by Scanning Kelvin probe microscopy. As thermally evaporated LiF typically lowers the surface work function, the tunable behavior of the solution-processed LiF nanoparticles on ITO is attributed to a lateral depolarization effect, with the effective dipole direction switching as full surface coverage is obtained. The results strongly indicate that engineering of the interfaces is a useful tool for future device optimization.