Proceedings of International Conference on Hybrid and Organic Photovoltaics (HOPV18)
DOI: https://doi.org/10.29363/nanoge.hopv.2018.032
Publication date: 21st February 2018
All-inorganic colloidal caesium lead halide perovskite quantum dots (QDs) have recently emerged as promising material for optoelectronics (solar cells, light emitting diodes, lasers, et al.).[1] CsPbBr3 QDs have also been demonstrated as stable two-photon-pumped lasing medium. However, the details of the photophysics under two-photon absorption (2PA) are yet to be investigated. CsPbBr3 QDs show red-shifted photoluminescence (PL) emission and increased PL lifetime under 2PA compared with PL emission under one-photon absorption (1PA). These differences have not been thoroughly addressed by previous reports, although here are various speculations about possible explanations of the different PL emission and decay in case of one and two photon exciotation, such as re-absorption, alternative electronic relaxation pathways or size inhomogeneity.
Herein, we used ultra-fast spectroscopies to study the photophysical process of 2PA in CsPbBr3 QDs. We found that, the 2PA cross-section follows a power law dependence on QD sizes with exponent 3.3±0.2. [2] The empirically obtained power-law dependence and wavelength dependence allow us to assign the two-photon excited final state to an exciton state. [3] The quadratic dependence of the two-photon excited PL intensities on excitation intensities indicates that the 2PA process is mediated by a virtual state. The 2PA cross-section shows a stronger size-dependence compared to the 1PA cross-section. Therefore, the larger QDs can be preferably excited under 2PA. We conclude that the size inhomogeneity is the reason for the red-shifted PL spectrum and the relatively slower PL decay with two-photon excitation. Furthermore, we simulated PL emission with 2PA and 1PA based on size-dependent PL emission and the NCs size distribution from TEM. The simulations agree well with the experimental results giving support to our conclusion that the differences between the excited state dynamics after 2PA and 1PA excitation in CsPbBr3 QDs are due to the different size selection by the excitation under these two conditions. [4] Such size selection can be a general feature of the perovskite QDs and may be tuned via QDs size distribution to influence their performance within QD- based nonlinear optical materials and devices.
References
[1] M. V. Kovalenko, L. Protesescu, M. I. Bodnarchuk, Science 2017, 358, 745-750.
[2] J. Chen, K. Žídek, P. Chábera, et al., J. Phys. Chem. Lett., 2017, 8, 2316–2321
[3] S. C. Pu, M. J. Yang, C. C. Hsu, et al., Small, 2006, 2, 1308-1313
[4] J. Chen, P. Chábera, T. Pascher, et al., J. Phys. Chem. Lett., 2017, 8, 5119–5124.
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