Influence of the Energy Levels on the Efficiency of Organic Bulk Heterojunction Solar Cells

We investigate the influence of the energy levels on the power conversion efficiency of a P3HT:PCBM and a MEH-PPV:PV bulk heterojunction cell. We calculate the upper-limit for the efficiency, and make realistic assumptions to predict efficiencies obtainable in the near future. The ideal position of the energy levels of donor and acceptor are obtained from these calculations, giving an idea how the ideal organic solar cell should look like. Also the expected efficiency gain by tuning energy levels, bandgap or absorption window can be derived from the simulations. Our calculations show that, for a bulk heterojunction cell with P3HT as donor, reaching efficiencies of 10 % is only possible by adapting the LUMO-level of the electron acceptor. Further, lowering the bandgap of the donor would only increase the efficiency if also the absorption window increases. We also demonstrate that MEH-PPV is not a good donor for organic solar cells, because of the high bandgap. The results indicate that, by changing the energy levels, organic cells could have the potential for reaching higher efficiencies , which is a necessity for future commercial applications.