Observation of anomalous time-delayed electron-lattice coupling in organic blends and hybrid perovskites

Creators: Vanacore, G. M.; Hu, J.; Rozzi, C. A.; Amato, M.; Baldini, E.; Keivanidis, P. E.; Wei, H.; Huang, J.; Zewail, A. H.

Abstract

Elucidating the role of atomic degrees of freedom in organic blends and hybrid perovskites is crucial to understanding the non-adiabatic dynamics within their energy landscape and to their implementation in photovoltaics technologies. Here, we report the direct visualization of the long-range lattice dynamics in rr-P3HT:PC_(71)BM and CH_3NH_3PbI_3 upon femtosecond optical excitation by combining ultrafast electron diffraction and time-dependent ab initio calculations. We find that both materials exhibit a common time-lagged lattice response induced by an impulsive high-intensity photoexcitation. This process, which is observed to occur on a time scale governed by carrier diffusion, is ascribed to the suppression of the incoherent coupling between the photoexcited species (excitons in the organic blends and free charges in the perovskite) and acoustic modes through a phonon-bottleneck effect. Only once excitons or free charges have significantly accumulated at the surface or interface, multiple decay channels open up and the phonon emission proceeds in a regenerative manner with a mechanism similar to a phonon 'avalanche'. The presence of this suppression mechanism creates a dynamic protection of the carriers from incoherent energy dissipation and could contribute to the high photovoltaic efficiencies observed in these materials.

Additional Information

This work was supported by the National Science Foundation and by the Air Force Office of Scientific Research in the Center for Physical Biology at Caltech funded by the Gordon and Betty Moore Foundation. The perovskite crystal growth is financially supported by the Defense Threat Reduction Agency (Award No. HDTRA1-14-1-0030). TDDFT calculations are performed under the financial support by the European Union projects FP7-NMP CRONOS (n. 280879-2) and FP7-MC-IIF MODENADYNA (n. 623413). J. Hu acknowledges the support from China 1000-Young Talents Plan. G. M. V. acknowledges the support from the program EPFL-Fellows co-funded by Marie-Sklodowska-Curie. The authors gratefully acknowledge the assistance of Dr. B. Chen, Dr. Z. Su, and Dr. X. Fu for TEM characterization, and of Dr. G. Grancini, Dr. J. Tang and Prof. A. Tagliaferri for helpful discussions. A special thanks goes to Prof. M. Chergui for the critical reading of the manuscript. Author contributions: G. M. V., J. Hu, and A. H. Z. designed research, performed the experiments and analyzed data. C. A. R. and M. A. performed DFT and time-dependent DFT calculations. E. B., G. M. V. and J. Hu developed the modeling of the results, together with C. A. R. and M. A.. P. E. K. coordinated the preparation of the organic blend films. J. Huang and H. W. grew the single crystal perovskite samples. All authors have contributed to writing the paper. The authors declare no competing financial interests.

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