Wavefunction engineering for efficient photoinduced-electron transfer in CuInS <sub>2</sub> quantum dot-sensitized solar cells
Jianhui Sun, Limin An, Gaopeng Xue, Xinghui Li
Abstract
Abstract The high efficiency of quantum dot-sensitized solar cells (QDSSCs) is a benefit of the highly efficient photoinduced-electron transfer (PET) to external electrodes. Here, we investigated how the surface defects and conduction-band (CB) offsets between core and shell materials affect the PET from CuInS 2 quantum dots (QDs) by means of time-resolved femtosecond transient absorption and nanosecond photoluminescence spectroscopy. The transfer of 1S excited electrons from CuInS 2 QDs to TiO 2 films is demonstrated and we find that the surface-electron trapping can significantly reduce the efficiency of the PET. Though the electron trapping can be suppressed after ZnS surface passivation, the PET decreases significantly to a low efficiency of ∼33% from the type I CuInS 2 /ZnS core/shell QDs because of their low electron density at the surface of the QDs. The surface-electron density is increased with the strategy of wavefunction engineering by reducing the CB offset, which allows us to achieve a quasi-type II carrier confinement in CuInS 2 /CdS core/shell QDs. The PET efficiency appears to be as high as ∼95% from the CuInS 2 /CdS core/shell QDs, which is ascribed to synergistic effects of the surface passivation and enhanced delocalization of the electron wavefunction from the CuInS 2 core to the CdS shell. Finally, we demonstrate that these new mechanistic understandings of the PET processes are crucial to improving the efficiency of CuInS 2 QDSSCs.