LMU_photoresponse_microscopy research team at the Ludwig-Maximilians-Universität (LMU) in Munich, Germany, for the first time has succeeded in functionally characterising the active layer in organic thin-film solar cells using laser light for localised excitation of the material.
Defects in the regular arrangement of the atoms in the active layer of a solar cells act as temporary traps for charge carriers, which can reduce the usable current. The method developed by LMU now enables researchers to map the spatial distribution of these defects in organic thin films, which has never been done before. “We use a gate electrode to fill traps in an organic field effect transistor with charge carriers, explains Dr Bert Nickel, lecturer in LMU’s department of physics and head of the research team. “Subsequently, a laser is used to release carriers from these traps. Thus, the photo response current indicates the number of traps in the film.”
Nickel agrees that the research breakthrough could catalyse the advancement of organic thin-film solar cells. “We are able to map the spatial distributions of traps in an organic film,” he says. “Before, it was not known whether the traps are homogeneously distributed or if the cluster due to imperfections in the film structure.” Thanks to new mapping method, researchers are now able to systematically test how the preparation process impacts the trap distribution. The objective, of course, is to eliminate the traps.
According to Nickel, the biggest challenge in this research project thus far was to experimentally disentangle the various contributions to the photoresponse. “This could only be achieved by the combination of gating, light modulation and local illumination,” the expert reports.
Going forward, Nickel and his colleagues want to apply their new mapping method to organic heterojunctions, the key element in organic solar cells.
LMU’s research findings are detailed in the paper “Mapping of Trap Densities and Hotspots in Pentacene Thin-Film Transistors by Frequency-Resolved Scanning Photoresponse Microscopy,” published in the journal Advanced Materials.
Written by Sandra Henderson, Research Editor, Solar Novus Today