Hi,
I study perovskite.
Through the project, I studied the OPV cell, and the nyquist plot was obtained through impedance measurement.
In the case of perovskite, the nyquist plot has two semi-circles.
But OPV cell has one semi-circle.
Is this a common case?
why does one semi-circle appear?
What do the low and high frequency regions mean respectively?
thanks.
From the material point of view perovskite active layer, as opposite to the organic active layer, is polycrystalline. Due to band banding at grain boundaries, the latter possess different conductivity in comparison to the bulk of crystallites. Therefore, polycrystalline films often reveal two semi-circles that originated from the bulk of crystallites and grain boundaries.
Moreover, I would recommend plotting in the log-log scale. There is a chance that you will see even more semi-circles. They may originate from charging and discharging bulk and interface traps, polarization effects due to ions migration or charge injection at forwarding bais.
In order to have a more general picture, I recommend plotting real and imaginary parts of impedance and electric modulus vs. frequency in the log-log scale. This approach provides more chances to distinguish between different features of the measured impedance spectra.
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