Nanoscale mapping of wavelength-selective photovoltaic responses in H- and J-aggregates of azo dye-based solar cell films

Abstract

In the present study, nanoscale wavelength-selective photovoltaic activities in H- and J-aggregates of azo dye-based solar cell films were mapped by wavelength-dependent photoconductive noise microscopy. In this strategy, the local conductivities and charge traps in dye films were mapped by a conducting probe scanning the surface while illuminating the lights with selected wavelengths. We observed the formation of localized domains exhibiting wavelength-dependent photoconductivities. The individual domains could be identified as H- and J-aggregates, which showed dominant photoexcitations at wavelengths of 600 and 450 nm, respectively. Notably, the short-circuit currents I_sc and photoconductivities Δσ_PC of the film showed power-law dependencies with trap densities under illuminated conditions (N_{T_L}) and the trap density change by the illumination (ΔN_T), respectively, like Δσ_PC ∝ |ΔN_T|^1/2 and I_sc ∝ N_{T_L}^−3/4 for each wavelength illumination. These results revealed the carrier recombination process in cooperation with the traps which could be a major factor determining the performance of solar cells. Significantly, the J-aggregates showed lower trap densities than those of the H-aggregates, resulting in superior solar cell characteristics of the J-aggregates, such as a higher I_sc and larger open circuit voltages. Since our method allows mapping the nanoscale photovoltaic activities of solar cell film aggregates, it can be a powerful tool for both basic research and in the application of photoelectronic devices.