Proceedings of 6th International Conference on Hybrid and Organic Photovoltaics (HOPV14)
Publication date: 1st March 2014
Electron Beam-Induced Current (EBIC) is a technique carried out in the scanning electron microscope (SEM) where the electron beam of the SEM is scanned over an electroactive (“electro-voltaic”) sample that has two contacts for passing any current that is generated in it (i.e., the electron beam generates current in analogy with a light beam in a photovoltaic cell). The measured external current can be correlated with microscopic regions in the sample. By scanning over the cross-section of a photovoltaic cell (simplest to interpret for cross-section of a planar cell but can be done also with a nanostructured cell), the regions of the cell where current is generated most efficiently (peaks in the EBIC signal) are correlated with regions of highest field throughout the cell thickness. As the generated current has to reach the contacts to be measured, the method allows one to examine two of the three main processes in a PV cell, viz. photogeneration of carrier and charge carrier separation. Actual charge transport lengths in the cell can also be measured in this way.
We apply EBIC to several organo-Pb halide perovskite absorbers in complete cells. As an example, fig. 1 shows a secondary electron image (left) and an EBIC image of the same region (right) from a CH3NH3PbI3-xClx planar solar cell together with a line scan in the EBIC image. The arrows show the peaks in the line scan which we correlate to the maximum of the electric fields at the hole conductor (left) and the electron conductor (right) showing fields formed in the perovskite at its with the two charge conductors, as in a p-i-n device. The peak closer to the HTM was consistently higher than that closer to the TiO2, indicating that electrons are extracted more efficiently than holes. Effective diffusion/migration lengths for the charges were calculated to be 1.9 ± 0.1 eV for electrons and 1.5 ± 0.2 eV for holes.
The extension of this type of analysis to other perovskite cells will be described.
Figure 1. Secondary electron (SE) and electron beam-induced current (EBIC) images of cross sections of a CH3NH3PbI3-xClx planar solar cell, together with a line scan in the EBIC image. Scale bar is 2 µm. The cell configuration is (from left side of SE image): Au (light)/ spiro (black)/perovskite (gray)/dense TiO2 (thin dark)/FTO (gray)/glass (very dark grey).
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