Practical efficiency limits in organic photovoltaic cells: Functional dependence of fill factor and external quantum efficiency

We evaluate practical power conversion efficiency limits lim in bulk-heterojunction organic photovoltaic BHJ OPV cells and how the field dependence of exciton dissociation affects cell efficiencies. We treat the fill factor limit as a function of the donor-acceptor lowest unoccupied molecular orbital offset energy ELLO , calculating how this limit varies with decreasing ELLO. We also evaluate OPV external quantum efficiency as a function of wavelength from the optical transmittance and internal quantum efficiency limitations. For a given ELLO, we numerically optimize donor bandgap and lim and show that lim 10% should be possible for hypothetical OPV systems generating free charge carriers efficiently at ELLO 0.3–0.4 eV. Current BHJ OPVs with low ELLO values appear to be limited to cell efficiencies of 5% largely as a consequence of incomplete exciton dissociation. © 2009 American Institute of Physics. doi:10.1063/1.3243986