Proceedings of nanoGe Fall Meeting 2018 (NFM18)
DOI: https://doi.org/10.29363/nanoge.nfm.2018.173
Publication date: 6th July 2018
The phenomenom of hysteresis in the current-voltage characteristics of perovskite solar cells has been discussed by numerous authors. It is agreed by many that mobile ions or ion vacancies play a significant role in the underlying processes causing this hysteresis. Calao et al. have shown that hysteresis will only occur if ion migration alters the rate of surface recombination. The latter is a loss mechanism which occurs at the interfaces of the perovskite absorber with the adjacent materials, usually electron (ETM) and hole (HTM) transporting material and it depends sensitively on the concentration of electrons and holes in the vicinity of such an interface. As Calao et al. could show there is minimal hysteresis if ETM or HTM layers passivate the corresponding interface to the absorber such that surface recombination becomes insignificant regardless of the distribution of mobile ions or ion vacancies.
We built inverted planar p-i-n perovskite solar cells and observed a high open-circuit voltage (Voc) when NiOx was employed as HTM. In contrast, Voc was significantly reduced if NiOx was replaced by PEDOT:PSS. This reduction of Voc could clearly be attributed to surface recombination occurring at the absorber/HTM interface as confirmed by photoluminescence measurements. Interestingly, the PEDOT:PSS based perovskite solar cells nevertheless did not show any hysteresis. To answer the question why the migrating ions do not cause hysteresis in these kind of devices we performed numerical simulations. We can reproduce the experimental finding when implementing interface traps at the perovskite/PEDOT:PSS interface. These states can lead to a fermi level pinning effect such that the ionic distribution has only very little influence on the concentrations of electrons and holes in the vicinity of that recombination active interface.
[1] P. Calado, A. M. Telford, D. Bryant, X. Li, J. Nelson, B. C. O’Regan, P. R.F. Barnes, Nature Communications 7 (2016), 13831
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