Charge carrier dynamics on fast timescales

Charge recombination and transfer processes in solar cells can occur within femto-seconds. Thermalization of charges to the band edges is followed by trapping, charge transfer, diffusion, recombination and extraction to the electrodes. In addition to charge transport processes on timescales of tens of ns that are accessible with electro-optical transient measurements, we aim to study even faster processes in perovskite solar cells using all-optical pump-probe techniques, such transient absorption spectroscopy (TAS), transient PL (TRPL) and photo induced absorption (PIA) in collaboration with the group of Prof. Safa Shoaee.

Transient PL measurements allow us to track the fate of photogenerated charge carriers in the active layer through their radiative recombination on ns-timescales. These measurements can assess the lifetime of charges before their non-radiative recombination and to quantify the kinetics (rate coefficients) of different recombination processes. These rates are then fed back into our device simulations or correlated with steady-state PL measurements to gain a more comprehensive picture of solar cell operation. This provides us a better understanding of the charge carrier lifetime under different conditions and internal processes in the cells. On the other hand, TAS measures the change in optical density upon an ultrafast (fs) laser excitation and can extend the time resolution to the sub-ps range allowing to complement transient PL measurements. For perovskites, this allows us for example to study hot carrier processes and the transfer of charge carriers to the transport layers. 

References

1. Wolff, C. M. et al. Reduced Interface-Mediated Recombination for High Open-Circuit Voltages in CH 3 NH 3 PbI 3 Solar Cells. Adv. Mater. 29, 1700159 (2017).
2. Stolterfoht, M. et al. Visualization and suppression of interfacial recombination for high-efficiency large-area pin perovskite solar cells. Nat. Energy 3, 847–854 (2018).
3. Wolff, C. M. et al. Nonradiative Recombination in Perovskite Solar Cells: The Role of Interfaces. Adv. Mater. 31, 1902762 (2019).
4. Wolff, C. M. et al. Perfluorinated Self-Assembled Monolayers Enhance the Stability and Efficiency of Inverted Perovskite Solar Cells. ACS Nano 14, 1445–1456 (2020).
5. Zhang, S. et al. The Role of Bulk and Interface Recombination in High‐Efficiency Low‐Dimensional Perovskite Solar Cells. Adv. Mater. 1901090 (2019). doi:10.1002/adma.201901090