a, J–V–L curves of PeLEDs fabricated on PVK/PEIE with different ETLs. PO-T2T (2,4,6-tris[3-(diphenylphosphinyl)phenyl]-1,3,5-triazine) exhibits higher electron mobility than B3PyMPM. Devices with PO-T2T as the ETL show higher current density, indicating that electron injection and transport are limited primarily by the organic ETL rather than the perovskite layer. b, Transient EL response of control and target PeLEDs under a 2.8 V pulsed bias. The target device shows a longer delay time (t target , between the voltage pulse and EL onset) compared with the control device. Because EL onset is governed by the transit of minor carriers, the prolonged t target indicates slower electron transport from the cathode to the emissive layer in the target device. c, Schematic diagram illustrating the relationship between electron transport and EL onset. In the control device, excitons form predominantly at the perovskite–ETL interface and the delay time (t control ) is limited by electron transit across the B3PyMPM layer. By contrast, in the target device, excitons form mainly at the 3D/2D perovskite heterojunction, farther from the ETL interface. Thus, t target is governed by electron transport across both the B3PyMPM and the 2D perovskite layer.
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