Thermal degradation by intrinsic morphological change and extrinsic oxidation remain outstanding challenges for bulk heterojunction (BHJ)-based polymer solar cells (PSCs). Postprocessing thermocleavage of side chains on the donor conjugated polymers using ester pyrolysis is a proven method to kinetically trap the morphology via increased glass transition temperature (Tg) and improved thermal oxidation resistance. We previously showed that having a certain fraction of thermocleavable side chains (TCS) incorporated into the archetypical polymer P3HT, achieved through the copolymerization of a TCS-functionalized thiophene monomer, can offer high thermal stability to its BHJ devices without compromising significant performance metrics. This work expands the concept of using the copolymerized TCS monomer unit to balance the stability with efficiency into the state-of-the-art PSC system PffBT-T4-OD:PCBM, where the original octyl–decyl (OD) branched side chains are partially replaced with TCS in 50–70 mol % in the copolymers. Structural differences of the fully cleavable PffBT-T4-TCS polymer and P3ET polymers disclose that increasing backbone rigidity and alkyl chain length can increase the temperature of eliminating alkyl chains by up to 20 °C in the solid state. Dynamic mechanical analysis shows the cleaved PffBT-T4-TCS polymer has significantly increased thermal relaxation temperatures and high storage modulus over a large temperature range. Thermal stability testing at 100 °C in air reveals that increasing the TCS content drastically increases the polymer resistance to oxidation. PSCs made with the fully cleavable PffBT-T4-TCS polymer offer only a meager efficiency of 0.2%, while the copolymer with 60 mol % TCS can deliver a PCE of 3.2% with its BHJ device, double the previous highest reported efficiency for TCS-containing polymer-based PSCs. Importantly, the copolymer with the 60 mol % TCS-based device is stable, retaining 80% of the initial performance after accelerated aging tests (100 °C, 2 weeks). Together with our previous works, these new findings demonstrate that using partial cleavage of side chains could be a general strategy to gain both efficiency and stability for conjugated polymer-based PSCs.