ABSTRACT
Synthesis Residuals........................................................................... 184 7.5 Identification and Characterization of Defect States in Donor-Acceptor
Blends ........................................................................................................... 187 7.5.1 General Depiction ............................................................................. 187 7.5.2 Defect States in the Donor and Acceptor Phases and New States
in Blends ........................................................................................... 188 7.5.3 Effects of Defects on the Performance of Organic Photovoltaic
Devices ............................................................................................. 191 7.5.4 Defect States at Organic-Electrode Interfaces ................................. 192 7.5.5 Origins of Defect States: Oxygen, Structural, and Synthesis
Residuals ........................................................................................... 193 7.6 Conclusions ................................................................................................... 195 References .............................................................................................................. 196
Organic photovoltaic (OPV) cells have been improving in performance over the last few years. But bottlenecks still exist, and the efficiencies are yet to reach the Schockley-Queisser limit (~23%).1 Key bottlenecks are misalignment of energy levels, insufficient optical absorption, small exciton diffusion lengths, carrier recombination, and low charge carrier mobilities. Recent improvements in efficiencies are mainly due to the development of new materials and improved processing conditions. New materials that have been reported either show improved bandgaps that absorb more solar radiation or energy level offsets that improve cell voltage, charge dissociation, etc., or a combination of these properties. Nevertheless, irrespective of what the material is, the understanding of basic physical parameters and processes and their correlation to performance remain important to further improve the applicability and promise of this technology.
