Rationalizing the Physical Mechanisms behind the Amplified Spontaneous Emission Signatures in CsPbBr₃ Nanocrystal Films

Thanks to their optical and electronic properties, lead halide perovskites have become prominent materials in optoelectronics. CsPbBr₃ nanocrystals (NCs) stand out as excellent candidates for implementing lasers due to their excellent photoluminescence quantum yields and their facile and low-cost production. Elucidating their stimulated emission mechanisms is fundamental in addressing their possible limitations and to achieve more efficient perovskite lasers, specially continuous wave or electrically excited ones. Two questions remain open: why the Amplified Spontaneous Emission (ASE) band is shifted from the fluorescence one, and why the former seems to coexist with the latter. These characteristics have led to a debate about which is the mechanism behind the ASE band shift. Some reports claim that the fluorescence comes from single excitons, while the ASE has a biexcitonic origin. Others defend that both fluorescence and ASE are generated by localized single excitons, and the shift owes to reabsorption.

In this communication, we address these questions through experimental ASE measurements, combined with numerical simulations, and the use of a novel analytical expression to retrieve the optical gain from these experiments [1]. We show that the ASE behaviour in CsPbBr3 NCs thin films stems from four processes: reabsorption due to a large overlap between the absorption and fluorescence spectra, a strong contribution of excited state absorption at the fluorescence window, the excitation of differently polarized waveguide modes, and the coexistence of short- and long-lived localized single excitons. The results in this work establish guidelines with which to analyse the optical gain in perovskite samples, which can help in rationalizing the ASE signatures of both CsPbBr₃ NCs and perovskites in general, and which provide insightful information on research avenues to increase the efficiency of perovskite light-emitting devices.

Reference:

[1] S. Milanese, M. L. De Giorgi, M. Anni, M. Bodnarchuk, and L. Cerdán, Adv. Opt. Mater., 2401078 (2024)