M. Seeland, R. Rösch, B. Muhsin and H. Hoppe
Institute of Physics, Ilmenau University of Technology, Germany (August 2011)
Phone: +49 3677 69 3315, e-mail: firstname.lastname@example.org
In this application note, we report about the laterally resolved detection of luminescence emitted from the solar cell by using a CCD camera. This investigation method called ‘luminescence imaging’ meets the above mentioned requirements for characterization of polymer solar cells very well if a suitable detector with certain specifications is chosen. Luminescence spectroscopy revealed for many material combinations that photoluminescence emitted from the organic semiconductor is in the visible part of the optical spectrum, whereas electroluminescence is emitted in the near infrared.
Furthermore, due to photoluminescence quenching by efficient charge transfer after optical excitation as well as low electroluminescence efficiencies, the overall luminescence intensities are rather low. Therefore, the camera must have a high sensitivity in the range of 600 to 1100 nm and only little thermal noise. The Andor iKon-M DU934N-BR-DD camera features these specifications while having a proper resolution of 1024 x 1024 pixels.
A schematic of the luminescence imaging setup is shown in Figure 1. The polymer solar cells are mounted in a chuck with electrical contacts and placed underneath the CCD camera. For electroluminescence imaging (ELI) a constant current at positive driving voltage is applied to the solar cell under test, leading to radiative recombination within the organic active layer. For photoluminescence images (PLI) the organic semiconductor is optically excited and the subsequent radiative decay of photogenerated excitons is detected. To block the excitation light a cut-off filter is placed in front of the CCD camera. The camera takes images of the photons emitted by the organic semiconductor with typical exposure times of several seconds.