The Ultrathin Optical Coating Model In The Application Of Organic Solar Cells

Small Molecular Organic Photovoltaic cells (OPVs) have a rapid development in power conversion efficiencies (PCE) and reached12%recently, due to its advantages in process, cost and mass manufacture. However, due to the small exciton-diffusion length and the large energy gap of organic semiconductors, the OPVs device normally is deficit of light absorption hence smaller PCE compared to the commercially available Si solar cells. In order to enhance the light absorption, the tandem structure, the optical spacer at the metal/organic interfaces and plasmonic nanostructures had been widely employed in OPVs. But these methods usually requires an extra functional layer and an extra process. This thesis will use the ultrathin optical coating concept to design a highly absorbing OPVs structure without adding an extra layer or an extra process.This new ultrathin optical coating has been proposed recently by Mikhail A. Kats et al., which can achieve high absorption (i.e.65-90%) in the visible light wavelength by only using5-20nm Ge film on the Au (or Ag) substrate, which is insensitive to incident angle. In this ultrathin bilayer coating, the large optical attenuation within the highly absorbing dielectrics and the concomitant non-trivial interface phase shifts induce the strong absorption resonance.The OPVs structure of transparent-electrode/highly-absorbing active material/metal in sequence itself is very similar to the ultrathin optical coating structure. Firstly, we optimize the optical constants and the thickness of the active layer, using the model, then find the eligible active materials to enhance the optical absorption of the device. As demonstrated, the device with the structure of ITO/Leadphthalocyanine(SubPc):Buckerminster fullerene (C60)(1:4, by wt%)/Al had been studied. It is found that more than90%optical absorption can be obtained in the device with a broaden wavelength range of480-620nm. Moreover, the optical absorption of the device had been optimized by further calculating the optical electric field inside the OPVs device. This work paved a new way to design the OPVs device.This thesis had also studied the electrical field distribution inside the OPVs device to enhance its PCE. By optical design, a bilayer nanocomposite of C60:LiF had been successfully applied in the OPVs device to achieve high PCE. By optical designing, a tandem structure device with the interconnect layer of C60:LiF/Ag/MoOx had been optimized to achieved the enhanced optical absorption.The whole optical calculation had been based on the transfer matrix, and programed by Matlab software. This work will provide the optimized device structure for the future OPVs devices.