Center for Functional Nanomaterials Seminar

Solar cells based on conjugated polymers offer great promises since they are flexible, easy to process, and compatible with large scale roll-to-roll manufacturing. Polymer solar cells have much lower efficiencies than silicon-based solar cells. One critical limiting factor of a polymer solar cell is its small exciton diffusion length compared to the optical absorption length. I will focus on my Ph.D. work using resonance energy transfer to improve exciton harvesting in organic-inorganic hybrid solar cells.
We started by studying a simple model system consisting of an organic dye molecule Nile Red and fullerene derivative PCBM. We observed efficient energy transfer from Nile Red to PCBM. Based on our experimental results, we propose that the energy transfer from conjugated polymer to PCBM could play a crucial role in the most efficient polymer based solar cells, which has been neglected previously. We then applied this resonance energy transfer approach to planar organic-inorganic hybrid solar cells using P3HT as the energy donor and PTPTB as the energy acceptor. Due to energy transfer from P3HT to PTPTB, the 'effective' exciton diffusion length of P3HT increased by a factor of three, leading to a 3-fold increase in device photocurrent. We modeled the energy transfer in a 2-D hexagonal structure and developed a method to deposit a thin layer of energy acceptor on the surface of nanoporous TiO2 in a controllable way. This was achieved through acid-base binding effect between carboxylic acid (-COOH) groups and TiO2. As a result, a layer of interfacial dipole was formed, leading to a shift of the vacuum level. Work function measurement with Kelvin probe verified the shift of vacuum level due to the interfacial dipole layer. We observed strong correlation between the shift of vacuum level and the open circuit voltage of the device. We have shown that electropolymerization is a viable way to fill the nanopores with conjugated polymer and create multi-layer str