Development of a multi-wavelength photocurrent mapping system

A novel multi-wavelength near-field scanning photocurrent microscopy (MWNSPM) system has been developed, which directly correlates photocurrent generation at different incident wavelengths with morphology of organic photovoltaic (OPV) cells. The system is based on a near-field scanning optical microscope which generates the topographical map and acts as the nanoscale light source for the OPV cell. The light sources are frequency modulated using optical beam choppers, and the resulting photocurrent signal is demodulated in software via a Fourier transform to recover the signal due to each wavelength. The new MWNSPM system was used to image directly the photocurrent generated from an OPV device based on a 1:1 blend of poly(9,9'-dioctylfluorene-co-bis-N, N'-(4,butylphenyl)-bis-N,N'-phenyl-1,4-phenylene-diamine) and poly(9,9'-dioctylfluorene-co-benzo-thiadiazole). A spatial resolution of 170 ± 45 nm was achieved, which is in excellent agreement with the limit of the 200 nm probe aperture and considerably better than the resolution (700 nm) of comparable confocal photocurrent mapping.