The use of non-fullerene acceptors in organic photovoltaic (OPV) devices could lead to enhanced efficiencies due to increased open-circuit voltage (V-OC) and improved absorption of solar light. Here we systematically investigate planar heterojunction devices comprising peripherally substituted subphthalocyanines as acceptors and correlate the device performance with the heterojunction energetics. As a result of a balance between V-OC and the photocurrent, tuning of the interface energy gap is necessary to optimize the power conversion efficiency in these devices. In addition, we explore the role of the charge transport layers in the device architecture. It is found that non-fullerene acceptors require adjusted buffer layers with aligned electron transport levels to enable efficient charge extraction, while the insertion of an exciton-blocking layer at the anode interface further boosts photocurrent generation. These adjustments result in a planar-heterojunction OPV device with an efficiency of 6.9% and a V-OC above 1 V.