We demonstrate that a post-annealing step results in enhanced open-circuit voltage (Voc) and fill factor (FF) and lower reverse saturation current (Js) that consequently increases the power conversion efficiency (PCE) of organic bulk-heterojunction (BHJ) devices by about 40 % as a result of better contact formation, as typically assumed. Although true, we show that additional device properties are affected as well. We found that annealing induces vertical phase segregation and consequently the enrichment of donor and acceptor materials at the correct electrical contact. In addition, a de-doping process and a decrease in defect density also take place and are the major causes for device improvement after post-annealing the OPV devices. Implications for OPV basic research and manufacturing are discussed.