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.