A high-boiling-point additive that favors both poly[ (4,4-didodecyldithieno[3,2-b:2',3'-d]silole)-2,6-diyl-alt-(2,1,3-benzoxa diazole)-4,7-diyl] and PC71BM in a bulk heterojunction solar cell is used to reduce large-scale aggregation and phase separation, which increases device performance. This is in contrast to the majority of highboiling-point additives that improve performance by increasing phase separation.