Variable wavelength photocurrent mapping on PbS quantum dot: fullerene thin films by conductive atomic force microscopy

Mixed films of PbS quantum dots and fullerene derivatives were investigated using spectrally resolved photoconductive atomic force microscopy (pcAFM) equipped with an external source measure unit to accurately measure small currents. Combining pcAFM with a tuneable laser light source allowed us to record local internal power conversion efficiency (IPCE) spectra and thus to determine the wavelength of highest photoactivity. Samples exhibited photoactivity in the near infrared around 900 nm with an IPCE of 10% at a single measurement point. Photocurrent images with a 5 nm pixel size were recorded showing inhomogeneous photocurrent distribution uncorrelated to the topography but reflecting the heterogeneous make-up of the film. The perpetual degradation of the photocurrent due to sample oxidation under ambient conditions was also monitored.