Recombination in Polymer:Fullerene Solar Cells with OpenCircuit Voltages Approaching and Exceeding 1.0 V

Polymer:fullerene solar cells are demonstrated with power conversion effi ciencies over 7% with blends of PBDTTPD and PC 61 BM. These devices achieve open-circuit voltages ( V oc ) of 0.945 V and internal quantum effi ciencies of 88%, making them an ideal candidate for the large bandgap junction in tandem solar cells. V oc ’s above 1.0 V are obtained when the polymer is blended with multiadduct fullerenes; however, the photocurrent and fi ll factor are greatly reduced. In PBDTTPD blends with multiadduct fullerene ICBA, fullerene emission is observed in the photoluminescence and electroluminescence spectra, indicating that excitons are recombining on ICBA. Voltage-dependent, steady state and time-resolved photoluminescence measurements indicate that energy transfer occurs from PBDTTPD to ICBA and that back hole transfer from ICBA to PBDTTPD is ineffi cient. By analyzing the absorption and emission spectra from fullerene and charge transfer excitons, we estimate a driving free energy of –0.14 ± 0.06 eV is required for effi cient hole transfer. These results suggest that the driving force for hole transfer may be too small for effi cient current generation in polymer:fullerene solar cells with V oc values above 1.0 V and that non-fullerene acceptor materials with large optical gaps ( > 1.7 eV) may be required to achieve both near unity internal quantum effi ciencies and values of V oc exceeding 1.0 V.