The mechanism behind the reversible perovskite solar cell (PSC) behavior in the slower time domain (ms – min) is still under discussion. In particular, the role of iodine is often unclear. This situation is hindering an effective optimization of long-term stable devices. In this presentation, a straightforward approach will be presented by simply assuming charge trapping and a basic redox type process.If electrons are charge transferred (figure 1) to electron trapping sites, which are located in the perovskite itself and in the electron selective material, then iodide is oxidized in return to iodine containing species.The reverse reaction takes place if electrons are released from the traps resulting in the reduction of iodine species back to iodide.
From these assumptions the following situation derives after illumination at different externally switched potentials: At Voc the electron traps are filled and, as a compensation of charges, iodine species are increasingly generated until equilibrium with electron recombination and therefore highest Voc is reached. By switching to the initially high Isc, electrons are released externally to the hole selective electrode which leads to a reduction of iodine species by electron transfer there. The growing presence of unoccupied electron traps slows down electron transport until a lower Isc is reached in steady state. Again switching back to Voc, the still high density of traps now results in high electron recombination and therefore an initially low Voc. For potentials applied between Voc and Isc an intermediate situation is arising.
From the above, one can learn that filling of electron traps should always be related to the occurrence of iodine which due to its chemical reactivity might be of concern for long-term stability. Supporting results from potential sweep and transients measurements are reported. Simultaneously, the withdrawal of electrons at potentials below the open circuit voltage and the reversible occurrence of iodide species are monitored by time- resolved photoluminescence quenching. A novel independent determination of iodine species via changes in the optical absorption has been made possible in diffuse reflection mode.