For polymer solar cells (PSCs) with conventional configuration, the vertical composition profile of donor:acceptor in active layer is detrimental for charge carrier transporting/collection and leads to decreased device performance. A cross-linkable donor polymer as the underlying morphology-inducing layer (MIL) to tune the vertical composition distribution of donor:acceptor in the active layer for improved PSC device performance is reported. With poly(thieno[3,4-b]-thiophene/benzodithiophene):[6,6]-phenyl C-71-butyric acid methyl ester (PTB7:PC71BM) as the active layer, the MIL material, PTB7-TV, is developed by attaching cross-linkable vinyl groups to the side chain of PTB7. PSC device with PTB7-TV layer exhibits a power conversion efficiency (PCE) of 8.55% and short-circuit current density (J(SC)) of 15.75 mA cm(-2), in comparison to PCE of 7.41% and J(SC) of 13.73 mA cm(-2) of the controlled device. The enhanced device performance is ascribed to the much improved vertical composition profile and reduced phase separation domain size in the active layer. These results demonstrate that cross-linked MIL is an effective strategy to improve photovoltaic performance of conventional PSC devices.