Direct Tracking of Charge Carrier Drift and Extraction from Perovskite Solar Cells by Means of Transient Electroabsorption Spectroscopy

The best perovskite solar cells currently demonstrate more than 25% efficiencies, yet many fundamental processes that determine the operation of these devices are still not fully understood. In particular, even though the device performance strongly depends on charge carrier transport across the perovskite layer to selective electrodes, information about this process is still very controversial. Here, we investigate charge carrier motion and extraction from an archetypical CH3NH3PbI3 (MAPI) perovskite solar cell. We use the ultrafast electric-field-modulated transient absorption technique, which allows us to evaluate the electric field dynamics from the time-resolved electroabsorption spectra and to visualize the motion of charge carriers with subpicosecond time resolution. We demonstrate that photogenerated holes drift across the mesoporous TiO2/perovskite layer during hundreds of picoseconds. On the other hand, their extraction into the spiroOMeTAD hole transporting layer lasts for more than 1 nanosecond, suggesting that the hole extraction is limited by the perovskite/spiro-OMeTAD interface rather than by the hole transport through the perovskite layer. Additionally, we use the ultrafast time-resolved fluorescence technique that reveals fluorescence decay during tens of picoseconds, which we attribute to the spatial separation of electrons and holes.

Automated summary

The best perovskite solar cells currently have efficiencies higher than 25%, but many fundamental processes that determine their operation are not fully understood. In this study, the researchers investigate charge carrier motion and extraction in a typical CH3NH3PbI3 (MAPI) perovskite solar cell using ultrafast techniques. They find that photogenerated holes drift across the perovskite layer for hundreds of picoseconds, while their extraction into the hole transporting layer takes over 1 nanosecond, suggesting that the hole extraction is limited by the perovskite/spiro-OMeTAD interface. The researchers also observe fluorescence decay, indicating spatial separation of electrons and holes.