If the residual Pd is a cluster acting as the catalytic site, then the reduced PCBM generated after electron transfer from PFBT under light illumination should still be able to give electrons to the Pd cluster for catalysis as well, similarly as Pd clusters can get electrons from the PFBT polymer. However, the photocatalysis was completely inhibited in presence of PCBM, unless an extra Pd or Pt source is introduced afterwards (Figure , S16). This cannot be well explained by the unreachable Pd cluster by PCBM due to a well-embedded Pd cluster in PFBT. Moreover, we should still be able to observe hydrogen production anyway, because the Pd cluster could still get electrons after competing with PCBM from reduced or excited PFBT for catalysis due to fast electron transfer between PFBT to Pd (fs-ns) .