Center for Functional Nanomaterials Seminar

Organic Solar Cells (OSCs) have attracted extensive attention in recent years because of their unique advantages such as their low cost, light weight, flexibility, and compatibility with the ink printing fabrication technique. The OSCs have a sandwich architecture composed of a blended layer with polymer as the donor and low-molecular weight molecule as the acceptor between an ITO anode and a metal cathode. The typical thickness of the active layer is around 100 nm, but the conversion efficiency can be close to 9%. The further improvement of the efficiency will need more fundamental studies on materials engineering, photophysics, and device physics.
This presentation will address the following three important issues toward OSCs: (1) the origin of open circuit voltage, (2) the charge accumulation effect on open circuit voltage and (3) the magnetic field effect on internal photovoltaic processes in OSCs. It was found that the open circuit voltage is significantly dependent on the energy difference between the HOMO of the donor and the LUMO of the acceptor. Moreover, the dielectric background and charge accumulation at the interface can also affect the open circuit voltage. More importantly, we found that the photocurrent will change under external magnetic field, which forms a unique tool to study spin dependent internal photovoltaic processes. In general, the magnetic field effect on photocurrent change can be attributed to two facts. One is that a magnetic field can change the singlet and triplet ratios in excited states, and the other is that the singlets and triplets can have different contributions to photocurrent. This presentation will discuss (1) how singlets and triplets are involved in magnetic field effect on photocurrent change, (2) what the differences are between the magnetic field effects on photocurrent change for pristine polymer and polymer/molecule composite based photovoltaic devices, and (3) how the binding energies of charge transfer complexes