Single-molecule charge transfer dynamics in dye-sensitized p-type NiO solar cells: influences of insulating Al2O3 layers

In this study, we investigated interfacial charge transfer dynamics in water-soluble perylenediimide (WS-PDI) dye sensitized p-type semiconductor NiO nanoparticle films to better understand how molecular interactions influence photoconversion processes involved in solar cells by means of ensemble-averaged and single-molecule spectroscopies. Transient absorption data showed that strong and weak electronic couplings coexist between WS-PDI molecules and NiO nanoparticles, resulting in fast (within several picoseconds) and slow (requiring tens of picoseconds to nanoseconds) hole transfer from the excited dye to NiO, followed by charge recombination occurring at pico- to microsecond time scales. The correlated analyses of single-molecule fluorescence intensity, lifetime, blinking, and anisotropy revealed the intrinsic distribution and temporal fluctuation of interfacial charge transfer reactivity, which are closely related to site-specific molecular interactions and dynamics. It was also found that a suitable insulating Al2O3 layer can weaken the electronic interaction between WS-PDI and NiO, thereby retarding charge recombination and significantly enhancing photoelectric conversion efficiency. The results presented here will provide a reliable basis for design of highly efficient p-type solar cells and other molecule/semiconductor systems for their use in optoelectronic and solar energy applications.