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

We evaluate practical power conversion efficiency limits (eta(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 (E-LLO), calculating how this limit varies with decreasing E-LLO. We also evaluate OPV external quantum efficiency as a function of wavelength from the optical transmittance and internal quantum efficiency limitations. For a given E-LLO, we numerically optimize donor bandgap and eta(lim) and show that eta(lim)>10% should be possible for hypothetical OPV systems generating free charge carriers efficiently at E-LLO similar to 0.3-0.4 eV. Current BHJ OPVs with low E-LLO values appear to be limited to cell efficiencies of similar to 5% largely as a consequence of incomplete exciton dissociation. (C) 2009 American Institute of Physics. [doi: 10.1063/1.3243986]