Despite the rapid increases in the performance of intrinsically-stretchable organic solar cells (IS-OSCs), both the power conversion efficiency (PCE) and stretchability of the IS-OSCs should be further enhanced for their use in wearable electronics. Here, we realize efficient (PCE = 13.1%) and highly stretchable (strain at PCE_80% = 34%) IS-OSCs by developing a stretchable substrate-bottom electrode made of a molecular interdiffusion (MID)-assisted thermoplastic urethane-poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (TPU-PEDOT:PSS) bilayer. The interdiffused bilayer in the MID-based TPU-PEDOT:PSS affords a strongly adhesive interface with fracture energy (G_c) = 45.0 J m⁻², which is 3-times higher than the conventional stamp-transferred (ST) TPU-PEDOT:PSS (G_c = 15.1 J m⁻²). Importantly, the increased adhesion of MID-based TPU-PEDOT:PSS significantly enhances the overall IS-OSC durability. For example, the stretchability of the MID-based IS-OSCs (strain at PCE_80% = 34%) was 2-times higher than the conventional ST-based IS-OSCs (strain at PCE_80% = 17%) when the active layer was PM6:Y6-BO:N2200. The finite element simulation observes that the high adhesion between the TPU and PEDOT:PSS layers effectively dissipates mechanical stress and prevents cracking at their interface, enhancing the stretchability of the entire IS-OSCs. Therefore, our work provides a useful strategy for developing stretchable transparent electrodes and demonstrates the importance of their interfacial adhesion properties in achieving efficient and highly stretchable IS-OSCs.