Fabrication-Method-Dependent Excited State Dynamics in CH3NH3PbI3 Perovskite Films

In-Sik Kim,Dong-Yu Kim,Seong-Jin Son,In-Wook Hwang,Cheol Jo,Do-Kyeong Ko,Rira Kang

doi:10.1038/s41598-017-16654-1

Abstract

Understanding the excited-state dynamics in perovskite photovoltaics is necessary for progress in these materials, but changes in dynamics depending on the fabrication processes used for perovskite photoactive layers remain poorly characterised. Here we report a comparative study on femtosecond transient absorption (TA) in CH3NH3PbI3 perovskite films fabricated by various solution-processing methods. The grain sizes and the number of voids between grains on each film varied according to the film synthesis method. At the low excitation fluence of 0.37 μJ cm−2, fast signal drops in TA dyanmics within 1.5 ps were observed in all perovskite films, but the signal drop magnitudes differed becuase of the variations in charge migration to trap states and band gap renormalisation. For high excitation fluences, the buil-up time of the TA signal was increased by the activated hot-phonon bottleneck, while the signal decay rate was accelerated by fluence-dependent high-order charge recombination. These fluence-dependent dynamics changed for different perovskite fabrication methords, indicating that the dynamics were affected by morphological features such as grain sizes and defects.

Highlights

One effective methodological approach to enhance the performance of solution-processed PeSCs is the fabrication optimisation of the perovskite photoactive layer, achieved by controlling the composition and/or condition of the precursor solution, the types of solvents and additives, and the processing environment
Because the signal drops in phase II can arise from red shifts in the PB band peak caused by the band gap renormalisation (BGR) accompanying the initial hot-carrier dynamics, the dynamics in phase II are the result of the simultaneous contributions of trap states and the BGR
We conducted transient absorption (TA) spectroscopy on MAPbI3 perovskite films fabricated by the CHP, CBdrp, and IFF methods and demonstrated that the carrier dynamics of the perovskite films depended on the sample fabrication methods

Summary

Results and Discussion

The films show negative TA signals at energies below the band gaps, indicating that carriers are excited to transiently unoccupied states formed by band gap renormalisation (BGR)[20]. As the pump-probe delays are increased, the short-wavelength tails of the PB bands are changed from positive to negative values; the PB bands are slightly red-shifted and the negative TA signals below the band gap disappear These features reflect the instantaneous evolution of excited energy states during hot-carrier cooling and BGR20–22. Just after phase II, the relative intensities of the peak TA values differ for the CHP (0.88 ± 0.025), CBdrp (0.80 ± 0.025), and IFF (0.81 ± 0.029) films, as determined statistically from 20 data points obtained at the end of phase II These differences are attributed to the presence of trap states[23], which can induce charge transfer from the band edge of excited states to below-gap trap states. The hot-phonon bottleneck is activated in proportion to the initial carrier density; ,the carrier density of the CHP film is larger than those of the other films even at the same excitation fluence

Perovskite fabricated by CHP CBdrp IFF

Conclusion

Experimental Section

Author Contributions

Additional Information