The electronic-coupling interaction between noble-metal cocatalysts and host semiconductor nanocrystals has been found to be effective for the utilization of the solar energy. However, electron transfer (ET) mechanism between noble metals and self-assemblies has not been elucidated clearly. Here, we revealed a mechanism of back-electron-transfer-enhanced photocatalysis, which contributed to the visible-light photocatalytic improvement of perylene diimide (PDI) assembly for phenolic degradation and hydrogen generation. By compared with the energy level of PDI and Pd quantum dots (QDs), it seemed to be disadvantageous for ET from PDI assembly to Pd QDs, but surface photovoltage spectra showed that this can come true under visible-light irradiation, indicative of formation of a new Schottky barrier resulting from other light-induced transition species. Absorption spectra showed that an additional electronic state existed above the conduction band of PDI assembly originating from PDI radical anions. Such active species provided the light-driven dissipative structure with stable energy and electron flow through two opposite types of ET pathway: one is ET from light-excited PDI anions to Pd QDs and the other is direct electron injection from plasma Pd QDs accumulated with high-energy electrons to PDI neutral molecules. It has been found that continuous ET from PDI assembly to Pd QDs caused the plasma resonance of the Pd QDs to be higher in energy to overcome interfacial energy barrier. And then back-ET from Pd QDs to neutral PDI molecules occurred and led to formation of more PDI radical anions that were important for the improvement of the photocatalytic activity. These findings provide a new strategy for the development of highly efficient visible-light photocatalysts based on self-assembly for energy production.