Chain length effect of large organic cations on the amplified spontaneous emission and lasing properties of quasi-2D Dion–Jacobson perovskites

Quasi-two-dimensional (quasi-2D) Dion–Jacobson (DJ) perovskites modified with large organic cations exhibit superior optoelectronic properties and stability compared to their three-dimensional (3D) counterparts, making them promising for light-emitting diodes and lasing applications. While diammonium cations with linear chain structures are preferred due to their hydrogen-bonding capability and enhanced structural stability, the effect of cation chain length remains insufficiently understood. Here, we systematically investigate three diammonium spacers with varying chain lengths. Ultrafast spectroscopy reveals that longer-chain cations promote pronounced phase separation, which facilitates the formation of a high-quality n=2 phase, uniform phase distribution, efficient energy transfer, and suppressed non-radiative recombination. These effects collectively reduce the amplified spontaneous emission threshold, from 41.94 μJ/cm2 (EDA, short-chain) and 29.86 μJ/cm2 (BDA, medium-chain) to an ultralow 7.67 μJ/cm2 (PentDA, long-chain). Using this optimized gain medium, we demonstrate a single-mode vertical-cavity laser with a threshold of 6.75 μJ/cm2, a narrow linewidth of 0.49 nm, and a high-quality factor of 1093.8. Conversely, short-chain cations lead to weak phase separation and an insufficient n=2 phase content, resulting in film transient absorption characteristics dominated by excited-state absorption. This study clarifies the critical role of cation chain length in tuning the phase behavior and optical gain of quasi-2D DJ perovskites, providing guidance for their use in low-threshold laser devices.