Influence of MoO$_{\mathrm{x}}$ interlayer on the maximum achievable open-circuit voltage in organic photovoltaic cells

Transition metal oxides including molybdenum oxide (MoO$_{\mathrm{x}})$ are characterized by large work functions and deep energy levels relative to the organic semiconductors used in photovoltaic cells (OPVs). These materials have been used in OPVs as interlayers between the indium-tin-oxide anode and the active layers to increase the open-circuit voltage (V$_{\mathrm{OC}})$ and power conversion efficiency. We examine the role of MoO$_{\mathrm{x}}$ in determining the maximum achievable V$_{\mathrm{OC}}$ in planar heterojunction OPVs based on the donor-acceptor pairing of boron subphthalocyanine chloride (SubPc) and C$_{\mathrm{60}}$. While causing minor changes in V$_{\mathrm{OC}}$ at room temperature, the inclusion of MoO$_{\mathrm{x}}$ significantly changes the temperature dependence of V$_{\mathrm{OC}}$. Devices containing no interlayer show a maximum V$_{\mathrm{OC\thinspace }}$of 1.2 V, while devices containing MoO$_{\mathrm{x}}$ show no saturation in V$_{\mathrm{OC}}$, reaching a value of \textgreater 1.4 V at 110 K. We propose that the MoO$_{\mathrm{x}}$-SubPc interface forms a dissociating Schottky junction that provides an additional contribution to V$_{\mathrm{OC}}$ at low temperature. Separate measurements of photoluminescence confirm that excitons in SubPc can be quenched by MoO$_{\mathrm{x}}$. Charge transfer at this interface is by hole extraction from SubPc to MoO$_{\mathrm{x}}$, and this mechanism favors donors with a deep highest occupied molecular orbital (HOMO) energy level. Consistent with this expectation, the temperature dependence of V$_{\mathrm{OC}}$ for devices constructed using a donor with a shallower HOMO level, e.g. copper phthalocyanine, is independent of the presence of MoO$_{\mathrm{x}}$.