The Role of Exciton Ionization Processes in Bulk Heterojunction Organic Photovoltaic Cells

To realize efficient photoconversion in organic semiconductors, photogenerated excitons must be dissociated into their constituent electronic charges. In an organic photovoltaic (OPV) cell, this is most often accomplished using an electron donor-acceptor (D-A) interface. Interestingly, recent work on MoO_x/C₆₀ Schottky OPVs has demonstrated that excitons in C₆₀ may also undergo efficient bulk-ionization and generate photocurrent as a result of the large built-in field created by the MoO_x/C₆₀ interface. Here, it is demonstrated that bulk ionization processes also contribute to the short-circuit current density (J_SC) and open-circuit voltage (V_OC) in bulk heterojunction (BHJ) OPVs with fullerene-rich compositions. Temperature-dependent measurements of device performance are used to distinguish dissociation by bulk-ionization from charge transfer at the D-A interface. In optimized fullerene-rich BHJs based on the D-A pairing of boron subphthalocyanine chloride (SubPc)-C₆₀, bulk-ionization is found to be responsible for ≈16% of the total photocurrent, and >30% of the photocurrent originating from C₆₀. The presence of bulk-ionization in C₆₀ also impacts the temperature dependence of V_OC, with fullerene-rich SubPc:C₆₀ BHJ OPVs showing a larger V_OC than evenly mixed BHJs. The prevalence of bulk-ionization processes in efficient, fullerene-rich BHJs underscores the need to include these effects when engineering device design and morphology in OPVs.