초록
<P><B>Abstract</B><P>The inefficient charge transport and large exciton binding energy of quasi‐2D perovskites pose challenges to the emission efficiency and roll‐off issues for perovskite light‐emitting diodes (PeLEDs) despite excellent stability compared to 3D counterparts. Herein, alkyldiammonium cations with different molecular sizes, namely 1,4‐butanediamine (BDA), 1,6‐hexanediamine (HDA) and 1,8‐octanediamine (ODA), are employed into quasi‐2D perovskites, to simultaneously modulate the injection efficiency and recombination dynamics. The size increase of the bulky cation leads to increased excitonic recombination and also larger Auger recombination rate. Besides, the larger size assists the formation of randomly distributed 2D perovskite nanoplates, which results in less efficient injection and deteriorates the electroluminescent performance. Moderate exciton binding energy, suppressed 2D phases and balanced carrier injection of HDA‐based PeLEDs contribute to a peak external quantum efficiency of 21.9%, among the highest in quasi‐2D perovskite based near‐infrared devices. Besides, the HDA‐PeLED shows an ultralong operational half‐lifetime <I>T</I>50 up to 479 h at 20 mA cm<SUP>‒2</SUP>, and sustains the initial performance after a record‐level 30 000 cycles of ON-OFF switching, attributed to the suppressed migration of iodide anions into adjacent layers and the electrochemical reaction in HDA‐PeLEDs. This work provides a potential direction of cation design for efficient and stable quasi‐2D‐PeLEDs.</P></P>