The operational perovskite/organic tandem solar cells are subjected to light irradiation and driven by a higher bias voltage than single-junction solar cells, posing a severe challenge to their stabilities. Light irradiation can trigger halide phase segregation in the perovskite subcell, exacerbated under higher bias voltage through electron-phonon coupling. To address this, dimethylammonium ion (DMA+) incorporation delays perovskite crystallization by forming an intermediate phase, enhancing crystallinity, and reducing lattice structural defects. DMA+ with a larger ionic radius entering the A-site of lattice tilts the [PbX6]4- (X: I or Br) octahedral, enlarging the perovskite bandgap, shortening Pb-I bonds, and reinforcing the lattice. This mitigates halide escaping from the lattice and subsequent ion migration. Phase segregation in the perovskite subcell is significantly suppressed under high-power irradiation and bias voltage. Consequently, the perovskite subcell exhibits increased and stable quasi-Fermi-level splitting values, delivering a high open-circuit voltage of 1.34 V. Notably, 0.062-cm2 and 1.004-cm2 perovskite/organic tandem solar cells achieved remarkable efficiencies of 26.15% (certified of 25.34%) and 24.87%, respectively, exhibiting excellent operational stability of T90 ~ 1350 h.