Diffusion Length Mapping for Dye-Sensitized Solar Cells

The diffusion length ( L ) of photogenerated carriers in the nanoporous electrode is a key parameter that summarizes the collection efficiency behavior in dye-sensitized solar cells (DSCs). At present, there are few techniques able to spatially resolve L over the active area of the device. Most of them require contact patterning and, hence, are intrinsically destructive. Here, we present the first electron diffusion length mapping system for DSCs based on steady state incident photon to collected electron ( IPCE ) conversion efficiency ( η I P C E ) analysis. The measurement is conducted by acquiring complete transmittance ( T DSC ) and η I P C E spectra from the photo electrode (PE) and counter electrode (CE) for each spatial point in a raster scan manner. L ( x , y ) is obtained by a least square fitting of the IPCE ratio spectrum ( I P C E R = η I P C E -CE η I P C E -PE ). An advanced feature is the ability to acquire η I P C E spectra using low-intensity probe illumination under weakly-absorbed background light (625 nm) with the device biased close to open circuit voltage. These homogeneous conditions permit the linearization of the free electron continuity equation and, hence, to obtain the collection efficiency expressions ( η COL-PE and η COL-CE ). The influence of the parameter’s uncertainty has been quantified by a sensitivity study of L . The result has been validated by quantitatively comparing the average value of L map with the value estimated from electrochemical impedance spectroscopy (EIS).