The role of exciton ionization processes in bulk heterojunction organic photovoltaic cells

Dissociating photogenerated excitons into their constituent charges is essential for efficient photoconversion in organic semiconductors. Organic photovoltaics cells (OPV) widely adopt a heterojunction architecture where dissociation is facilitated by charge transfer at a donor-acceptor (D-A) interface. Interestingly, recent work on MoO$_{\mathrm{x}}$/C$_{\mathrm{60}}$ Schottky OPVs has demonstrated that excitons in C$_{\mathrm{60}}$ may also undergo bulk-ionization to generate photocurrent, driven by the built-in field at the MoO$_{\mathrm{x}}$/C$_{\mathrm{60}}$ interface. Here, we show that bulk-ionization processes also contribute to the photocurrent in bulk heterojunction (BHJ) OPVs with fullerene-rich compositions. The short-circuit current density (J$_{\mathrm{SC}})$ in a MoO$_{\mathrm{x}}$/C$_{\mathrm{60}}$ Schottky OPVs shows almost no dependence on temperature down to 80 K. This characteristic of bulk-ionization allows the use of temperature-dependent measurements of J$_{\mathrm{SC}}$ to distinguish dissociation by bulk-ionization from charge transfer at a D-A interface. For BHJ OPVs constructed using the D-A pairing of boron subphthalocyanine chloride (SubPc)-C$_{\mathrm{60}}$, bulk-ionization is found to contribute \textgreater 10{\%} of the total photocurrent and \textgreater 30{\%} of the photocurrent from C$_{\mathrm{60}}$. We further find that fullerene-rich SubPc-C$_{\mathrm{60}}$ BHJ OPVs show a larger open-circuit voltage (V$_{\mathrm{OC}})$ than evenly mixed BHJs due to the presence of bulk-ionization. This talk will examine the dependence of J$_{\mathrm{SC}}$ and V$_{\mathrm{OC}}$ on the relative fraction of dissociation by charge transfer and bulk-ionization processes.