  Ternary organic solar cells (TOSC) are currently under intensive
investigation, recently reaching a record efficiency of 17.1%. The origin of
the device open-circuit voltage (VOC), already a multifaceted issue in binary
OSC, is even more complex in TOSCs. Herein, we investigate two ternary systems
with one donor (D) and two acceptor materials (A1, A2) including fullerene and
non-fullerene acceptors. By varying the ratio between the two acceptors, we
find the VOC to be gradually tuned between those of the two binary systems,
D:A1 and D:A2. To investigate the origin of this change, we employ ultra-violet
photoemission spectroscopy (UPS) depth profiling, which is used to estimate the
photovoltaic gap in the ternary systems. Our results reveal an excellent
agreement between the estimated photovoltaic gap and the VOC for all mixing
ratios, suggesting that the energetic alignment between the blend components
varies depending on the ratio D:A1:A2. Furthermore, our results indicate that
the sum of radiative and non-radiative losses in these ternary systems is
independent of the blend composition. Finally, we demonstrate the superiority
of UPS over X-ray photoemission spectroscopy (XPS) depth profiling in resolving
compositional profiles for material combinations with very similar chemical,
but dissimilar electronic structures, as common in TOSCs.
